Abstract
This review explores the bioremediation potential of amidoximes, organic ligands with the -C(=NOH)-NH₂ group, valued for their strong chelating and catalytic properties. Amidoximes are synthesized efficiently through nucleophilic addition of hydroxylamine to nitriles, using sustainable methods like microwave-assisted or solvent-free synthesis. These compounds form stable complexes with heavy metals such as U(VI), Cu(II), and Pb(II), enabling selective removal from wastewater, soil, and seawater, thus reducing bioaccumulation and toxicity. Characterization techniques, including FT-IR (showing C=N bands at 1680–1620 cm⁻1), Nuclear Magnetic Resonance (NMR), X-ray diffraction, UV-Visible spectroscopy, and solubility studies, confirm their structural integrity, metal-binding mechanisms, coordination geometries, and tautomeric forms. In bioremediation, amidoxime-based materials, such as poly(amidoxime)-grafted polymers, demonstrate high adsorption capacities (e.g., 886.73 mg/g for U(VI)) and catalyze organic pollutant degradation via Fenton-like reactions. They also enhance microbial activity by serving as enzyme cofactors. Their tunable properties allow customization for specific pollutants, and integration with nanomaterials or microbial systems improves efficiency. Amidoxime-based materials offer high selectivity, reusability, and alignment with green chemistry principles, making them sustainable alternatives to conventional remediation methods. Applications include uranium extraction from seawater and radionuclide capture, addressing critical environmental challenges. However, scalability and long-term environmental impacts pose challenges requiring further research. This review underscores amidoximes’ chemical versatility and sustainability in tackling heavy metal and organic pollution, emphasizing the need for continued exploration to optimize their practical deployment in global pollution management, particularly in advancing bioremediation strategies.
Introduction
Amidoxime: Synthesis, bioremediation potential
Bioremediation is an increasingly crucial field in environmental science, offering sustainable solutions to pollution challenges. This introduction will explore the potential of amidoxime complexes in bioremediation, a topic integrating chemistry, biology, and environmental engineering. We will explore into the mechanisms by which these complexes can contribute to the degradation of pollutants, their advantages over conventional methods, and the future prospects of their application. In recent years, research has expanded beyond conventional metal sequestration to explore the potential of amidoxime-based complexes in bioremediation. These complexes exhibit properties that align well with bioremediation objectives: high metal-binding affinity, chemical stability in aqueous environments, and potential for integration with microbial or enzymatic systems. The bioremediation potential of amidoxime complexes lies not only in their ability to immobilize or remove toxic elements but also in their potential to mediate biocatalytic transformations and enhance microbial tolerance to pollutants.1–5
The pervasive nature of environmental pollution, inspired by industrialization and anthropogenic practices present a major risk to ecosystems and human health. Contaminants like heavy metals, organic dyes, pesticides, and petroleum hydrocarbons are discharged into the surroundings contaminating air, water, and soil. Traditional remediation methods commonly face drawbacks including high expense and the risk of secondary pollution, and inefficiency in treating diverse pollutants. This necessitates the development of innovative and eco-friendly approaches. Bioremediation, which employs microorganisms and their metabolic pathways, plants, or enzymes to neutralize or remove pollutants, has emerged as a promising alternative. It offers an affordable, sustainable, and environmentally benign way out for various types of contamination. Table 1 shows the role of microorganisms in amidoxime-enhanced pollutant removal processes.
Amidoxime-based ligands and their binding affinities for metal ions.
Amidoximes are organic ligands characterized by the presence of the -C(NH₂)=NOH functional group, which are enriched with high affinity with respect to metal ions, especially actinides, lanthanides, and transition metals. Their bifunctional chelating nature allows them to coordinate metal centers through both the oxime and amino nitrogen atoms; this structure enables them to readily coordinate with various metal ions to form stable complexes which makes them especially important in environmental uses where selective binding of metal ions is essential. The introduction of amidoxime groups onto organic polymers or small molecules significantly improves their ion exchange, adsorption, and coordination capabilities.
The synthesis of amidoximes typically involves the reaction of nitriles with hydroxylamine. Their distinctive chemical structure grants them remarkable properties, including high chelating ability, thermal stability, and tunable electronic properties. These properties have led to their use in diverse areas, for example; polymer chemistry, pharmaceuticals, and particularly in the selective adsorption and extraction of metal ions. Consequently, amidoxime complexes have been investigated for various applications, including nuclear waste treatment, water purification, and removal of industrial effluents.
The utility of amidoxime complexes in bioremediation is rooted in their coordination chemistry. Depending on the nature of the metal ion oxidation state and the ligand environment, amidoxime can form mono-, bi-, or polynuclear complexes with distinct geometries. These geometries influence the redox potential, solubility, and biological compatibility of the complexes, making them suitable for specific remediation tasks.6–10 For example, amidoxime-uranium complexes have shown promise in uranium recovery and detoxification from contaminated aquatic systems. Similarly, amidoxime complexes of transition metals such as Fe(III), Cu(II), and Zn(II) have been studied for their catalytic roles in redox reactions involved in pollutant degradation.
The significance of amidoxime complexes in bioremediation stems from their unique interaction with pollutants. One key aspect is their ability to chelate heavy metal ions. Toxic metals such as lead, cadmium, mercury, and chromium are persistent environmental contaminants that bioaccumulate and are highly toxic. Amidoxime complexes can selectively bind to these metal ions, forming stable, nontoxic complexes that can then be removed from the environment or rendered less bioavailable. This chelation can occur through various mechanisms, Incorporating coordination bonds between the metal ion and the nitrogen and oxygen atoms of the amidoxime group. The stability and selectivity of these complexes are crucial for effective heavy metal remediation, preventing their further dispersion and uptake by living organisms.
Beyond heavy metal chelation, the bioremediation potential of amidoxime complexes extends to the decomposition of organic pollutants. While the direct biodegradation of amidoxime complexes, themselves is an area of ongoing studies, their role often lies in facilitating microbial degradation or acting as catalysts. For instance, some amidoxime complexes can interact with enzymes participate in the breakdown of organic contaminants, potentially enhancing their activity or stability.8,10–13 Moreover, the ability of amidoximes to complex with transition metals, which are often cofactors in enzymatic reactions, could indirectly influence microbial metabolic pathways. This suggests a multifaceted role for amidoxime complexes in bioremediation, encompassing both direct pollutant removal and the enhancement of natural degradation processes. Figure 1 shows the microbial processes for metal removal.
Diagram showing microbial processes for metal removal (Source: author).
The advantages of employing amidoxime complexes in bioremediation are numerous. Firstly, their high selectivity for specific pollutants, particularly heavy metals, minimizes interference with beneficial environmental components. This targeted approach enhances remediation efficiency and reduces the risk of secondary pollution. Secondly, the adjustable characteristic of amidoxime complexes allows for the design of specialized materials customized to specific contaminants and environmental conditions. Modifications to the amidoxime structure can alter their binding affinity, solubility, and stability, optimizing their performance for diverse applications. Thirdly, the potential for integration with existing bioremediation techniques offers a synergistic approach. Amidoxime complexes could be immobilized on solid supports, incorporated into microbial cultures, or used in conjunction with phytoremediation, leading to enhanced overall remediation efficacy.14–17
Importantly, amidoxime complexes demonstrate tunable properties that can be optimized for specific remediation challenges. Through structural modifications of the ligand or by changing the metal ion, researchers have tailor the complex's affinity, selectivity, and stability under diverse environmental conditions. This versatility is advantageous when dealing with complex mixtures of pollutants, where selective sequestration or transformation is necessary. Moreover, the integration of these complexes into nanomaterials or membrane systems has enhance their deployment in field-scale remediation efforts.
The coordination of amidoximes with metal ions also imparts redox activity and catalytic properties that can be leveraged in environmental remediation. For instance, iron-amidoxime complexes have shown peroxidase-like behavior, enabling them to participate in Fenton-like oxidative decomposition of organic pollutants.8,13,18,19 These reactions can be particularly useful in advanced oxidation processes (AOPs), which are employed to decompose refractory pollutants in industrial wastewaters. Similarly, copper- and manganese-based amidoxime complexes have been investigated for their superoxide dismutase-mimetic activity, providing antioxidative protection in bioremediation systems exposed to oxidative stress.
From a green chemistry perspective, amidoxime complexes also align with sustainability goals. They are often synthesized under mild conditions, using environmentally benign solvents and reagents. Additionally, their high reusability and regeneration potential reduce the environmental footprint of remediation processes. When incorporated into biosorbents or biofilters, amidoxime complexes contribute to the development of low-waste, circular approaches to pollution control. The environmental applications of amidoxime complexes are not limited to heavy metal removal. There is growing interest in their use for capturing radionuclides, such as plutonium, thorium, and neptunium, from nuclear waste streams and natural waters. These radionuclides pose long-term risks due to their radiotoxicity and persistence in the environment. Amidoxime-based materials, particularly those incorporated into fibrous matrices or hydrogels, offer an effective means of extracting these ions through selective and reversible binding. In addition, hybrid systems integrating amidoxime complexes with microbial consortia are being developed, or enzymes opens new avenues for catalytic transformation of organopollutants, such as dyes, pesticides, and pharmaceuticals.20,21
Synthesis of amidoximes
Amidoximes are a group of bifunctional organic compounds possessed by the presence of the -C(=NOH)-NH₂ functional group. Owing to their strong binding capacity for metal ions and their chelating behavior, amidoximes have gained significant attention in environmental sciences, particularly in bioremediation applications. Understanding their synthesis is therefore fundamental, both to tailor their properties for specific contaminant interactions and to improve their environmental stability and efficiency.
The most widely used synthetic pathway to amidoximes involves the nucleophilic addition of hydroxylamine hydrochloride to nitriles. This reaction proceeds under mild to moderately basic conditions and is widely favored due to its operational simplicity and the availability of both starting materials. The synthesis of amidoxime can be represented by the following simple reaction equation:
In this reaction, a nitrile (RCN) undergoes nucleophilic attack by hydroxylamine hydrochloride (NH₂OH. HCl), forming the amidoxime product. Typically, hydroxylamine hydrochloride is used in combination with a base such as sodium carbonate, sodium hydroxide, or pyridine to liberate the free nucleophile.
The choice of solvent and temperature shows an important role in measuring the reaction yield. Common solvents include methanol, ethanol, or aqueous mixtures, with reaction temperatures extending from room temperature to 60–80 °C depending on the reactivity of the nitrile substrate. The reagents used are nitrile compounds, hydroxylamine hydrochloride with sodium carbonate or sodium hydroxide in a suitable solvent gives amidoxime compound.11,22 This route offers excellent atom economy and can be adapted for a wide variety of aromatic and aliphatic nitriles, making it ideal for generating libraries of amidoximes for screening in bioremediation studies.
Another developed synthetic route for amidoxime is solvent-free methods and microwave
These methods are particularly relevant for the design of amidoxime-based polymers or functionalized materials intended for environmental applications, where sustainable synthesis is paramount. The reactivity of the nitrile precursor significantly determines the structure and characteristics of the resulting amidoxime. Aromatic nitriles such as benzonitrile, phthalonitrile, and isonicotinonitrile yield aromatic amidoximes that exhibit π-conjugation and enhanced stability. Aliphatic nitriles, on the other hand, produce amidoximes with flexible backbones, potentially advantageous for polymer grafting or surface anchoring in adsorbent materials.13,27,28 Moreover, substituents on the aromatic ring or in the alkyl chain can modulate the electron density at the nitrile carbon, altering both the reaction kinetics and the chemical behavior of the amidoxime. For instance, electron-withdrawing groups (e.g., -NO₂, -CN) accelerate the reaction by increasing electrophilicity, whereas electron-donating groups (e.g., -CH₃, -OH) tend to slow it down. Pyridine amidoxime is very useful in uranium binding while bis(amidoxime) ligand is used for polymeric scaffolds. The ability to tailor the electronic environment through judicious choice of nitrile precursors is central to optimizing amidoximes for specific metal-binding or degradation activities in bioremediation settings.10,13,29–32
Beyond simple mono-amidoximes, several synthetic strategies allow for multi-functionalization or polymer grafting, which are especially useful for applications such as heavy metal adsorption or radionuclide capture from aqueous media. Nitriles bearing multiple cyano groups, such as dicyanobenzene or acrylonitrile polymers, can be simultaneously converted into bis- or polyamidoximes. These compounds are highly sought after in the fabrication of chelating resins or adsorbent fibers. For example, Polyacrylonitrile (PAN) fibers treated with hydroxylamine yield amidoxime-grafted polymers, widely used in uranium recovery from seawater.33–36 This class of amidoximes features prominently in bioremediation as a result of their large surface area, tunable porosity, and the cooperative effect of multiple binding sites. The synthesis of amidoximes represents a versatile and adaptable process that enables the production of structurally diverse molecules suitable for bioremediation applications. The direct conversion of nitriles using hydroxylamine hydrochloride under mild conditions remains the cornerstone of amidoxime synthesis, while innovations such as green solvents, microwave irradiation, and polymer functionalization continue to expand the scope and efficiency of these compounds.
Synthesis of amidoxime metal complexes
Amidoximes are a class of organic compounds with the functional group -C(=NOH)-NH₂, which imparts excellent chelating ability. The structural versatility and strong affinity for transition and post-transition metals make amidoximes particularly attractive ligands for complexation reactions. These complexes have found considerable attention in coordination chemistry due to their roles in catalysis, sensing, medicinal chemistry, and most recently, environmental remediation through bioremediation pathways. This section details the synthetic methodologies employed to prepare metal complexes of amidoximes, including choice of ligands, metal precursors, reaction conditions, and characterization techniques.
The synthesis of amidoxime metal complexes typically involves the reaction of a metal salt or precursor with an amidoxime ligand in a suitable solvent.8,10,13,37 Amidoxime ligands may be monodentate, bidentate, or polydentate, depending on the availability of donor atoms and the steric/electronic environment of the molecule. The reaction often proceeds under mild to moderate conditions and can be facilitated by stirring, heating, or refluxing. Most amidoxime ligands are synthesized through nucleophilic addition of hydroxylamine to nitriles:
Where R is can be aliphatic, aromatic, or heterocyclic substituent.
These reactions are typically carried out in an alcoholic solvent such as ethanol, methanol, or aqueous media with a mild base such as sodium carbonate, sodium hydroxide or triethylamine to neutralized the acid byproducts and facilitate nucleophilic attack reaction, often under reflux conditions. These ligands are isolated by filtration or solvent evaporation and purified by recrystallization.8,22,38–40 Once the amidoxime ligand is prepared and the metal salt selected, the complexation reaction is initiated. This typically involves dissolving both components in a compatible solvent such as ethanol, methanol, water, or dimethylformamide (DMF), and allowing them to react under stirring, reflux, or even room temperature conditions.
The nature of the metal salt used often influences the geometry, stability, and solubility of the resulting complex. The coordination reaction of amidoxime metal complex metal-to-ligand ratio is usually 1:2 or 1:3, depending on coordination number.10,41–43 These salts are selected based on their solubility, coordination tendencies, and environmental relevance, the reaction times vary from 2 to 24 h. For bioremediation applications, metal complexes involving uranium, copper, cobalt, and iron are of particular interest due to their roles in radionuclide capture and redox-mediated pollutant degradation.44–47
In some reaction cases, the pH control is crucial, especially for uranyl or lanthanide complexes. A slightly acidic to neutral pH (5–7) condition is necessary to avoid hydrolysis of metal salts such as U(IV) or deprotonation of the amidoxime moiety. The resulting complexes generally obtained are colored solids gotten from the crystalline precipitates from solvent evaporation, the precipitate is filtered by suction, washed with solvent and dried in a desiccator over calcium chloride. The significance of these metal complexes in bioremediation lies in their stability and binding capacity. Amidoxime groups can strongly chelate metals such as uranium, cobalt, and copper, making them ideal for extracting toxic or radioactive metals from wastewater, contaminated soils, and seawater. Also, polymer-supported amidoximes (e.g., on polyacrylonitrile fibers) form crosslinked networks that can be deployed in environmental cleanup operations. The synthesized complexes may also serve as a catalysts in the degradation of organic contaminants, further enhancing their environmental value.8,10,13,29,39,48,49
Characterization
The successful application and optimization of amidoxime-based bioremediation systems necessitate a thorough understanding of their structural integrity, chemical transformations, and interactions with target pollutants and biological components. This understanding is primarily achieved through robust characterization techniques. Among these, Solubility, Fourier Transform Infrared (FT-IR) spectroscopy, Ultraviolet-Visible (UV-Vis) spectroscopy, Nuclear Magnetic Resonance (NMR) spectroscopy and X-ray diffraction stand out as indispensable tools. These techniques provide complementary understanding of the molecular structure, electronic transitions, and chemical environment of amidoximes, enabling researchers to confirm synthesis, monitor reaction pathways, assess stability, and elucidate mechanisms of pollutant removal.
FT-IR spectroscopy
Fourier Transform Infrared (FT-IR) spectroscopy is an analytical technique used to identify functional groups present in a molecule and to study molecular vibrations. It operates on the basis that molecules take up infrared radiation at distinct wavelengths that match their vibrational modes. When applied to amidoximes, FT-IR provides crucial fingerprints for their structural confirmation and for monitoring their behavior during bioremediation processes. The characteristic functional groups of amidoximes, namely the C=N (imine) bond and the N-OH (oxime) group, exhibit distinct absorption bands in the infrared region. Typically, the C=N stretching vibration appears in the 1680–1620 cm−1 range, while the N-OH stretching vibration is observed around 3600–3200 cm−1 (often broad due to hydrogen bonding). The O-H bending vibration from the oxime group can be found near 1400 cm−1.50–52 Additionally, N-O stretching vibrations usually show in the 980–930 cm−1 band. The presence and shifts of these bands are critical for confirming the successful synthesis of amidoximes and for assessing their purity. For instance, if an amidoxime is synthesized from a nitrile, the disappearance of the C≡N stretching band (approximately 2260–2220 cm−1) and the accompanying appearance of the C=N and N-OH bands confirm the conversion. The exact positions of these bands can vary depending on substitution on the amidoxime scaffold, the presence of electron-withdrawing or electron-donating groups, and the molecular geometry.11,22,26
FT-IR spectroscopy plays a crucial role in identifying characteristic functional groups and examining metal-ligand interactions in amidoximes and their complexes. The binding of amidoximes with metal ions leads to notable shifts in vibrational frequencies, which are diagnostic of bonding modes. During bioremediation, amidoximes form stable chelates with heavy metals (e.g., UO₂2⁺, Cu2⁺, Pb2⁺, Cd2⁺), thereby aiding in their immobilization or removal. The FT-IR spectra of these metal complexes typically display:
A downfield shift of the C=N stretching band (to ∼1600–1630 cm⁻1), indicative of coordination through the nitrogen of the oxime group.
Changes in the N-O stretching band, often shifting or splitting due to metal-oxygen bonding.
Decreased intensity or merging of the O-H/N-H bands, often resulting from deprotonation upon coordination or from hydrogen bonding alterations in the ligand framework.
For example, in uranyl-amidoxime complexes, the formation of chelate rings is confirmed by strong bands near 910–920 cm⁻1 (asymmetric U=O stretching) along with changes in the amidoxime moiety vibrations, confirming bidentate coordination through the N and O atoms.10,37,49
Amidoximes can adopt several coordination modes, including: Monodentate binding through the oxime nitrogen or oxygen. Bidentate chelation involving both the oxime N and O atoms. Tridentate modes when additional donor sites (e.g., NH₂ or other functional groups) are available. Bridging ligands in polynuclear metal complexes.
FT-IR spectra provide evidence of these modes through comparative analysis of free ligands and their complexes. Shifts in both stretching and bending modes enable researchers to differentiate between end-on and side-on binding geometries, that are closely linked to the stability and performance of amidoxime complexes in pollutant binding. In practical bioremediation applications, amidoxime-functionalized materials (e.g., grafted polymers, resins, hydrogels) are employed to capture toxic metals and radionuclides from aqueous environments.13,29,35,53,54 FT-IR spectroscopy is frequently employed to track: surface modification and functionalization steps, metal ion binding by observing shifts in amidoxime-related bands and regeneration or desorption processes through the return of bands to their original positions post-washing or chelator treatment.13,55 This dynamic monitoring enables optimization of material reuse cycles, an essential factor for cost-effective and sustainable bioremediation technologies.
In bioremediation applications, amidoximes are often immobilized onto various support materials (e.g., polymers, activated carbon, silica) to enhance their stability, reusability, and separation from the treated medium. FT-IR spectroscopy is invaluable for characterizing these composite materials. Through comparison of the FT-IR spectra of the unmodified support material, the amidoxime, and the immobilized amidoxime, researchers will be able to identify new peaks corresponding to the amidoxime functional groups on the support, indicating successful grafting or adsorption. Shifts in the characteristic amidoxime peaks upon immobilization can also provide insights into the type of interaction between the amidoxime and the support surface (e.g., hydrogen bonding, covalent attachment). Some studies show that, a typical characterization workflow might involve using FT-IR to confirm the successful grafting of amidoximes onto a polymer support.35,56,57
Furthermore, FT-IR is instrumental in monitoring the bioremediation process itself. For example, when amidoximes are used for heavy metal adsorption, changes in the N−OH and C=N stretching frequencies can indicate complexation with metal ions. The non-bonding electron pairs on the nitrogen and oxygen atoms of the amidoxime moiety serve as primary coordination sites for metal ions. Upon metal binding, the electron density around these atoms’ changes, leading to observable shifts in their vibrational frequencies. Similarly, in the decomposition of organic pollutants, FT-IR can track the disappearance of pollutant-specific functional groups and the emergence of new peaks corresponding to degradation products, providing evidence for the effectiveness of the amidoxime-mediated process. The technique can also help in assessing the stability of amidoximes in different environmental conditions (e.g., pH changes, presence of microbial activity) by observing any degradation or transformation of the amidoxime structure over time.58,59 The nondestructive nature and relatively rapid analysis time of FT-IR make it a highly practical tool to enable real-time observation and ensure quality control in bioremediation studies.
UV-visible spectroscopy
UV-Vis spectroscopy complements FT-IR by providing insights into the electronic structures of amidoxime compounds and their metal complexes. It is particularly useful for analyzing the nature of metal-ligand interactions, ligand-field transitions, and charge transfer phenomena relevant to redox activity and photo reactivity in bioremediation contexts. It provides a broad understanding of amidoximes in bioremediation. It is highly sensitive for quantitative analysis of concentrations and monitoring electronic transitions, making it ideal for tracking pollutant removal in solution and complexation.
UV-Visible spectroscopy is a commonly employed analytical method that detects light absorption within the ultraviolet (200–400 nm) and visible (400–800 nm) regions of the electromagnetic spectrum.60,61 This absorption occurs when electrons are elevated from ground-state orbitals to excited-state orbitals within a molecule. For amidoximes, UV-Visible spectroscopy provides valuable information regarding their electronic structure, concentration, and interactions with other species in solution, which is especially significant in relation to bioremediation. Free amidoxime ligands typically exhibit absorption bands in the 200–300 nm region, associated with π→π* and n→π* transitions of the conjugated C=N–OH system.11,13,62,63 The intensity and position of these bands depend on:
The nature of substituents on the amidoxime ring (e.g., electron-donating or withdrawing groups). Solvent effects and pH, which can shift equilibrium between protonated and deprotonated forms.
These bands serve as baseline references for evaluating spectral changes upon metal coordination. Upon coordination to metal ions, amidoxime ligands undergo changes in their electronic environment, resulting in spectral shifts and the emergence of new absorption bands. These are typically classified as:
Ligand-to-metal charge transfer (LMCT) bands, usually in the 250–400 nm region, indicating electron donation from amidoxime donor atoms (N or O) to the metal center. d–d transitions for first-row transition metal complexes (e.g., Cu2⁺, Ni2⁺, Co2⁺), typically appearing in the 500–800 nm range, depending on the metal oxidation state and geometry. Metal-to-ligand charge transfer (MLCT) in cases where amidoxime groups are conjugated with electron-accepting moieties.
The identification of these transitions allows for the estimation of ligand field strength, coordination number, and complex geometry. For example, copper(II) amidoxime complexes may show bands near 600–700 nm corresponding to d-d transitions, which broaden or shift with changes in ligand denticity or geometry (e.g., square planar vs. octahedral).64,65 The intensity and wavelength of LMCT bands provide qualitative information about binding strength, which is critical for predicting metal selectivity in mixed-pollutant environments.
Amidoximes typically contain π-electron systems (e.g., C=N bond, aromatic rings if present) and non-bonding electrons on oxygen and nitrogen atoms, which can undergo π→π∗ and n→π∗ electronic excitation upon absorption of UV or visible light. The characteristic absorption maxima (λmax) and molar absorptivities (ɛ) are unique to each amidoxime structure and can be used for identification and quantification.13,66 For instance, simple aliphatic amidoximes might show absorption in the far UV region, while amidoximes with conjugated systems or aromatic substituents will exhibit stronger and red-shifted absorptions in the near UV or even visible region. Variations in solution pH can also affect the UV-Visible spectrum of amidoximes due to protonation or deprotonation of the oxime group, leading to shifts in λmax and changes in absorbance intensity. This pH-dependent behavior can be exploited to determine the pKa values of amidoximes, which is crucial for understanding their speciation and reactivity in different environmental conditions relevant to bioremediation.
One of the primary applications of UV-Visible spectroscopy in bioremediation studies involving amidoximes is the quantitative determination of their concentration in solution. By establishing a calibration curve (absorbance versus concentration) at a specific λmax, researchers can accurately measure the amount of amidoxime present in a sample. This is essential for monitoring the synthesis yield, assessing the stability of amidoxime solutions ove r time, and tracking their release from immobilized materials. More importantly, UV-Visible spectroscopy performs an important role in monitoring the removal effectiveness of pollutants. If the pollutant itself has a characteristic UV-Visible absorption (e.g., organic dyes, certain heavy metal complexes), the decrease in its absorbance intensity over time can be directly correlated with its removal by the amidoxime. This allows for the calculation of removal percentages, adsorption capacities, and kinetic parameters of the bioremediation process. Subsequently, UV-Visible could be employed to quantify the amount of amidoxime loaded onto the support and to monitor the elimination of a targeted dye from an aqueous medium. More also, UV-Visible spectroscopy can provide insights into the mechanism of interaction between amidoximes and pollutants. For example, when amidoximes binds to heavy metal ions it leads to the formation of metal amidoxime complexes which can lead to changes in the UV-Visible band of both the amidoxime and the metal ion. These changes, such as shifts in λmax, changes in absorbance intensity, or the appearance of new absorption bands, are indicative of complex formation and can serve to establish stoichiometric ratios and stability constants of the complexes.67–69 Similarly, in the case of degradation of organic contaminants, the disappearance of the pollutant's characteristic peaks and the appearance of new peaks corresponding to degradation intermediates or products can be tracked using UV-Visible, providing evidence for the transformation pathway. The simplicity, speed, and relatively low cost of UV-Visible spectroscopy make it an indispensable tool for routine analysis and preliminary investigations in amidoxime-based bioremediation research.
NMR spectroscopy
NMR spectroscopy is possibly one of the most effective spectroscopic methods for revealing detailed molecular structures, conformation, and dynamics of organic compounds. It provides a comprehensive understanding of amidoximes in bioremediation. NMR, with its unparalleled capacity to elucidate detailed molecular structures and confirm connectivity, is crucial for unambiguous structural assignment, purity assessment, and mechanistic studies of transformations. It relies on the principle that atomic nuclei with a non-zero spin (e.g., 1H,13C, 15N, 19F) behave like tiny magnets and can take up radiofrequency energy in the presence of a strong magnetic field. The specific wavelengths at which these nuclei resonate are highly sensitive to their local electronic environment, providing a wealth of structural information. For amidoximes, NMR spectroscopy, particularly 1H NMR and 13C NMR, is indispensable for confirming their synthesis, determining their purity, and understanding their transformations in complex bioremediation systems.70–72 NMR spectroscopy would be applied to confirm the structure of the synthesized amidoxime, identify any impurities, and potentially examine the pathway of dye decomposition by analyzing the chemical shifts of the amidoxime and dye components before and after the bioremediation process.
In 1H NMR spectroscopy, the spectra shifts, integration values, and splitting patterns of proton signals provide explicit details about the number, type, and coordination of hydrogen atoms in the amidoxime molecule.8,11,73 For instance, the proton of the N-OH group in amidoximes typically appears as a broad singlet in the range of 8–12 ppm, although its exact position can be highly variable and sensitive to concentration, solvent, and temperature due to hydrogen bonding. The protons on the carbon atom adjacent to the C=N bond will also exhibit characteristic chemical shifts. For example, in acetamidoxime (CH3C(NH2)=NOH), the methyl protons would appear as a singlet, and the NH2 protons would also appear as a broad signal. The ability to distinguish between different proton environments allows for unambiguous confirmation of the amidoxime structure and differentiation from starting materials or side products. Moreover, the presence of geometric isomers (E/Z isomers) around the C=N double bond is common for amidoximes, and these isomers often give rise to distinct sets of NMR signals, allowing for their identification and quantification.8,22,74
13C NMR spectroscopy complements 1H NMR by providing information about the carbon skeleton of the amidoxime. The carbon atom of the C=N bond is highly deshielded and typically resonates in the range of 150–165 ppm, which is a characteristic signal for amidoximes.8,11,13 Other carbon atoms in the molecule will also exhibit specific chemical shifts depending on their electronic environment and connectivity. The combination of 1H and 13C NMR data, often supported by two-dimensional NMR techniques like COSY (Correlation Spectroscopy) and HSQC (Heteronuclear Single Quantum Coherence), allows for the complete assignment of all proton and carbon signals and the unequivocal determination of the amidoxime's structure.75–77
In the context of bioremediation, NMR spectroscopy is invaluable for monitoring the fate of amidoximes and pollutants. When amidoximes are used for heavy metal sequestration, NMR serves as a tool for examining the coordination of metal ions to the amidoxime ligand. Paramagnetic metal ions can induce significant shifts and broadening of NMR signals of the surrounding protons and carbons, providing insights into the binding sites and coordination geometry. For diamagnetic metal ions, subtle chemical shift changes can still indicate complex formation. Furthermore, NMR can track the decomposition of organic pollutants by amidoxime-based systems.13,29,38,78 By analyzing the NMR spectra of the reaction mixture over time, researchers can observe the disappearance of pollutant signals and the appearance of new signals corresponding to degradation products, thereby elucidating the reaction mechanism and identifying intermediates. This is particularly useful for complex organic molecules where multiple degradation pathways might exist. The high resolution and quantitative nature of NMR make it an indispensable tool for mechanistic studies, kinetic analysis, and evaluating the prolonged stability and reusability of amidoxime-based adsorbents in bioremediation. However, NMR typically requires samples to be in solution, which can be a limitation for solid-state immobilized amidoxime materials, although solid-state NMR techniques are emerging to address this.6,79
Solubility
Solubility characterization is an indispensable tool in the study and application of amidoximes for bioremediation, it serves a key function in bioremediation potential, particularly in the removal of toxic metals, radionuclides, and organic pollutants, relies heavily on their physicochemical properties, which are in turn governed by their solubility profiles. Amidoximes, a group of organic compounds have gained a progressive interest in the area of environmental remediation as a result of their strong chelating capacities and structural versatility. Solubility test is an essential tool for elucidation of which contribute to the significant of the coordination environment, molecular conformation and interaction potential of amidoximes with environmental contaminants and also governs the environmental transport and functional deployment of amidoxime compounds.
Solubility serves as a major determinant of the environmental mobility, bioavailability, and reactivity of amidoxime compounds. Water solubility is especially crucial for bioremediation applications, as it governs the diffusion of amidoximes in aqueous environments where pollutants are typically encountered.13,29,80,81 Additionally, solubility in organic solvents is relevant for synthesis, formulation, and modification steps during material preparation. Amidoximes typically exhibit moderate polarity, which allows solubility in polar protic and aprotic solvents, such as methanol, ethanol, DMSO, and acetonitrile.11,22,82 However, their solubility in water varies widely depending on structural modifications (e.g., alkyl, aryl, or heterocyclic substituents) and pH conditions. The protonation state of the amidoxime moiety also significantly affects its solubility and reactivity.8,10,11,37,38,49,66,83
In their free ligand forms, amidoximes show variable solubility profiles. Simple aliphatic amidoximes, such as acetamidoxime, are generally water-soluble due to hydrophilicity of their functional groups. However, aromatic amidoximes, such as benzamidoxime or substituted phenylamidoximes, demonstrate limited aqueous solubility due to increased hydrophobicity from the aryl rings.10,11,13,66 Amidoximes containing heterocyclic systems (e.g., pyridine or quinoline) may show enhanced solubility water and organic solvents depending on the position and type of nitrogen-containing substituents. These solubility trends are important in designing amidoxime derivatives that balance hydrophilicity for environmental dispersion with sufficient stability and adsorption capacity for pollutant capture.7,13,29,84
The formation of metal complexes generally alters the solubility characteristics of amidoximes. Metal coordination can result in reduced water solubility due to lattice energy contributions and increased molecular weight, but it can also enhance solubility under acidic or basic conditions depending on the metal-ligand charge distribution. The formation of complex with transition metals such as Fe(III), Cu(II), Zn(II), and U(VI) has been widely reported. For instance, amidoxime-based chelating resins used in uranium extraction from seawater typically show improved performance under basic conditions, where the ligand-metal complex is sufficiently soluble to maintain effective uptake kinetics.8–10,13,22,48,66,83,85 Conversely, in soil or groundwater bioremediation, where pH and ionic strength vary, solubility-tuned formulations are required for efficient pollutant mobilization and capture.
Standard solubility measurements involve determining saturation concentrations in various solvents at room temperature or controlled thermal conditions. The shake-flask method, dynamic solubility testing, and turbidimetric titration are commonly employed. UV-Visible and high-performance liquid chromatography (HPLC) quantification methods are employed to assess the dissolved concentration. For bioremediation relevance, solubility tests under simulated environmental conditions (e.g., synthetic groundwater, seawater, variable pH) are highly informative. Solubility data is often complemented by thermodynamic parameters such as solvation free energy and solubility product constants (Ksp), particularly for metal complexes.86,87 These parameters help model the behavior of amidoxime materials in situ, thereby informing their deployment in dynamic environmental systems.
X-ray diffraction
X-ray diffraction analysis in an essential instrument for the elucidation of characterization in the study and application of amidoximes for bioremediation, it contributes significantly to the understanding of the coordination environment, molecular conformation, and interaction potential of amidoximes with environmental contaminants. X-ray diffraction (XRD), particularly single-crystal X-ray diffraction and powder X-ray diffraction (PXRD), continues to be the benchmark for elucidating the molecular and crystalline structure of amidoximes and their metal complexes. Understanding the crystal structure provides insights into the geometry of coordination, ligand conformation, hydrogen bonding interactions, packing motifs, and structural variability factors that directly impact the efficacy of amidoximes in pollutant binding and sequestration.
Single-crystal X-ray diffraction offers atomistic resolution, revealing the exact coordination mode of the amidoxime functional group, which typically binds metals through the oxime oxygen, the amine nitrogen, or both, in the formation of bidentate chelates.8,10 Powder X-ray diffraction is essential for characterizing bulk materials, including polymers and composite sorbents, where single crystals are unattainable.88,89 X-ray diffraction studies have confirmed diverse coordination behaviors for amidoximes. In most metal complexes, amidoximes act as neutral or monoanionic bidentate ligands, forming five- or six-membered chelate rings. For example, in metal-ligand complexes with Co(II), Ni(II), or Cu(II), amidoximes often coordinate through both the N and O atoms, forming planar or slightly deformed octahedral geometries.
In actinide chemistry, especially uranyl(VI) complexes, amidoximes preferentially coordinate at equatorial positions through the oxime O and amino N, contributing to high thermodynamic stability and selectivity.8,10,13,37 X-ray diffraction data has also revealed extensive hydrogen bonding networks in amidoxime crystal lattices, contributing to their stability and influencing solubility. These structural details are essential in understanding the selectivity, binding affinity, and reusability of amidoxime-based sorbents and chelators in bioremediation contexts. Powder X-ray diffraction is particularly useful in the characterization of amidoxime-functionalized polymers, such as grafted polyacrylonitrile fibers, amidoxime-modified cellulose, or amidoxime-modified hydrogels. Powder X-ray diffraction patterns reveal the degree of crystallinity, amorphous content, and changes upon metal binding. For instance, uranium-loaded amidoxime fibers often show peak broadening or shifts in the 2θ values, indicating complexation-induced structural rearrangements.
Changes in crystallinity are directly related to the sorption behavior, swelling properties, and mechanical stability of these materials. Powder X-ray diffraction also helps identify phase transitions, hydration states, and possible crystal forms, all of which affect performance during bioremediation cycles.90,91 Several recent works have shown the utility of X-ray diffraction in validating the binding mechanism of amidoximes with environmental pollutants:
Uranium sequestration: Single-crystal X-ray diffraction of uranyl-amidoxime compounds has revealed hexadentate coordination environments, contributing to the development of selective sorbents for nuclear waste remediation and uranium recovery from seawater.
10
Heavy metal binding: Structures of Cu(II), Zn(II), and Pb(II) complexes with substituted amidoximes have illustrated square planar or tetrahedral geometries, with implications for the design of metal specific sensors or extractants.8,10,92 Crystal engineering: By analyzing intermolecular interactions through X-ray diffraction, recent studies have tailored amidoxime ligands with specific supramolecular features (e.g., π–π stacking, hydrogen-bonding networks) that enhance sorption kinetics and reusability.7,13,31,93
The above examples, highlighted the centrality of X-ray diffraction in rational design and functional evaluation of amidoxime-based materials for bioremediation applications. An integrated approach combining solubility profiling and X-ray diffraction analysis enables an in-depth knowledge of amidoxime e compounds for environmental deployment. For example, a material with favorable crystallinity and coordination geometry, but poor aqueous solubility, may underperform in field conditions. Conversely, a highly soluble compound without structural rigidity may suffer from leaching or instability.
X-ray diffraction provides the structural basis for understanding solubility trends: tighter hydrogen bonding and extensive π-stacking generally reduce solubility, while disordered or less crystalline materials may exhibit higher water uptake.94,95 Furthermore, changes in X-ray diffraction patterns upon solubilization or complexation inform stability, reversibility, and performance under cycling conditions. Such integrative data allows for predictive modeling of amidoxime behavior in environmental matrices, informing the development of tunable formulations such as nanocomposites, hydrogels, resins, or biodegradable carriers optimized for field deployment.
The effective development and deployment of amidoxime-based materials for bioremediation critically depend on thorough and accurate characterization. X-ray diffraction, solubility, FT-IR, UV-Visible, and NMR spectroscopies are foundational tools in this endeavor, each contributing distinct yet complementary information. The integrated application of these spectroscopic techniques enables researchers to synthesize, characterize, and optimize amidoxime materials with precision, paving the way for their enhanced performance and broader implementation in addressing pressing environmental pollution challenges. As studies in bioremediation continues to advance, the sophisticated application of these characterization methods will remain paramount in fully harnessing the capabilities of amidoximes as versatile agents for environmental clean-up. With the increasing need for effective and eco-friendly remediation approaches, the careful characterization of amidoxime-based systems will continue to significantly contribute to the evolution of future sorbents, sensors, and catalysts for pollutant capture and detoxification.
Amidoxime complexes in the adsorption of heavy metals
The persistence, bioaccumulation, and toxicity of metals like cadmium, lead, mercury, chromium, and arsenic make heavy metal pollution a major environmental issue. Conventional remediation methods often suffer from inefficiencies, high cost, or poor selectivity. In this context, amidoxime-based materials have shown great potential for heavy metal removal as a result of their unique ability to form stable coordination compounds with metal ions. The amidoxime functional group (-C(=NOH)-NH₂), characterized by both oxime and amine functionalities, exhibits strong chelating ability, particularly toward strong Lewis's acids, including transition metals and actinides. This section explores the chemistry and applications of amidoxime complexes in heavy metal adsorption, highlighting the mechanisms of interaction, structural features, and material types employed. The review also discusses recent advancements in amidoxime-modified adsorbents and evaluates their performance metrics and environmental applicability. Table 2 shows adsorption performance of amidoxime-modified materials under various experimental conditions.
Summary of amidoxime-based adsorption processes for pollutant removal.
Coordination chemistry of amidoximes with heavy metals
Amidoximes are bidentate ligands that typically coordinate to metal ions through the oxygen atom of the oxime group and the nitrogen atom of the amino group.8,10,13,22,35,37,49,85,96 The nucleophilicity of both donor atoms facilitates the formation of five- or six-membered chelate rings upon complexation. This geometry enhances thermodynamic stability and selectivity, crucial for efficient heavy metal adsorption. The electron-donating characteristics of amidoximes make them suitable for coordinating with a variety of metal cations, especially those with vacant d-orbitals. For instance, amidoximes form stable complexes with divalent and trivalent metal ions such as Cu(II), Ni(II), Zn(II), Pb(II), and Fe(III). The precise coordination geometry depends on the metal center, ligand substitution pattern, and parameters of the solution, including pH and ionic strength.8,17,35,97–99
Hard and soft acid–base considerations
The principles of hard and soft acids and bases (HSAB), introduced by Pearson, offer a fundamental framework for understanding the selective binding behavior of amidoxime complexes toward various heavy metals. This model classifies acids and bases based on their polarizability and charge density, which significantly influences the coordination affinity and stability of metal–ligand complexes. In the context of amidoxime-modified materials used for heavy metal adsorption, Hard and soft acids and bases theory plays an essential role in detail explanation of metal ion selectivity, interaction strength, and complexation efficiency. Based on Pearson's HSAB theory, amidoximes act as borderline to hard bases, making them particularly effective for binding hard acid metals like Cr(III), Fe(III), and U(VI).8,10,13,100 However, they can also coordinate moderately well with borderline acids such as Ni(II), Co(II), and Zn(II). This versatility explains their broad-spectrum utility in environmental remediation applications involving multiple contaminants.
Amidoxime ligands contain both nitrogen and oxygen donor atoms specifically, a nucleophilic amino group (-NH₂) and an oxime group (C=NOH). These heteroatoms act as Lewis bases and contribute to the chelation of metal ions. Importantly, the nature of these donor atoms positions amidoximes as intermediate or borderline bases, capable of interacting with both hard and soft metal centers, depending on the coordination environment and pH of the system. Toxic metal ions differ in their classification under Hard and Soft Acids and Bases theory.101,102 For instance, hard metal ions such as Fe3⁺, Cr3⁺, and Al3⁺ possess high charge densities and favor coordination with hard bases such as oxygen donors. In contrast, soft acids like Hg2⁺, Cd2⁺, and Pb2⁺ are more polarizable and prefer softer donor atoms like sulfur or nitrogen. Borderline metals such as Cu2⁺, Zn2⁺, and Ni2⁺ can coordinate with either hard or soft bases depending on local electronic and steric factors.
Amidoximes show exceptional adaptability because their donor sites can effectively bind to a broad spectrum of metal ions across the Hard and Soft Acids and Bases spectrum. The oxygen atom of the oxime moiety generally acts as a hard base, favoring coordination with hard or borderline metal ions. Meanwhile, the adjacent nitrogen can participate in binding with softer acids, especially when the amidoxime is part of a delocalized chelating system. This dual binding character enhances the selectivity and uptake efficiency of amidoxime-containing adsorbents in complex aqueous systems.8,10,13,31,102 Several researchers have reported that amidoxime-functionalized materials exhibit higher adsorption affinities for uranyl ions (UO₂2⁺), a borderline acid, owing to the intense bidentate bonding through the oxime oxygen and amino nitrogen. Similar observations have been made for Cu2⁺, Zn2⁺, and Ni2⁺, where coordination involves both donor atoms, forming stable five- or six-membered chelate rings. The chelation enhances metal binding through entropic and enthalpic contributions, particularly when the geometry of the ligand complements the preferred coordination geometry of the metal ion.8,19,96
Understanding Hard and Soft Acids and Bases interactions also aids in tailoring amidoxime-based adsorbents for specific environmental remediation applications. For example, by modifying the ligand environment to include additional soft donor atoms (e.g., sulfur), the selectivity toward soft heavy metal pollutants like Hg2⁺ can be improved. Alternatively, increasing the density of oxime groups enhances the material's affinity for hard and borderline metals such as Fe3⁺ or Zn2⁺. The use of Hard and Soft Acids and Bases theory to amidoxime complexes reveals valuable insights into their coordination behavior and metal ion selectivity. This framework not only explains existing adsorption trends but also guides the rational design of novel amidoxime-based adsorbents for targeted toxic metal removal in bioremediation and water purification technologies.
Amidoxime-functionalized adsorbents for heavy metal removal
Amidoximated polymers and fibers
One of the most studied classes of amidoxime-containing materials involves the chemical modification of PAN fibers. Through hydroxylamine treatment, polyacrylonitrile's groups are converted into amidoxime groups, creating chelating resins with high metal-binding capacity. Amidoxime-modified polyacrylonitrile fibers have been widely tested for the adsorption of uranium, lead, cadmium, and copper from aqueous systems.35,103,104 In most cases, the adsorption process follows Langmuir or Freundlich isotherms, indicating monolayer adsorption with site heterogeneity. The adsorption occurs through chelation, driven by the formation of amidoxime-metal complexes on the fiber surface. These interactions are reinforced by hydrogen bonding and electrostatic forces in some cases. The adsorption capacity varies depending on the degree of amidoxime-modification, fiber surface area, and pH. For instance, amidoxime-modified polyacrylonitrile shows high affinity for Pb(II) at neutral to slightly acidic pH, reaching adsorption capacities of up to 200 mg/g in optimized conditions. Table 3 shows amidoxime-based ligands and their binding affinities for metal.13,63,99,105,106
Bioremedation efficiency of amidoxime-functionalized materials.
Amidoxime-modified hydrogels and gels
Hydrogels based on natural or synthetic polymers functionalized with amidoxime groups have earned attention for their tunable swelling behavior and ease of metal ion diffusion. For example, amidoxime-modified polyvinyl alcohol (PVA), chitosan, and cellulose hydrogels have been synthesized and evaluated for the adsorption of Cu(II), Zn(II), and Cr(VI). These materials exhibit excellent regeneration potential and high-water uptake, allowing fast migration of metal ions into the gel matrix. Amidoxime groups inside the hydrogel network provide strong metal chelation.29,35,97,107,108 Based on the studies reported, the challenges involved is their mechanical stability and recyclability remain areas of improvement, especially under real-world environmental conditions involving fluctuating pH, salinity, or competitive ions. Table 4 shows bioremediation efficiency of amidoxime-functionalised materials with their pH and regeneration potential.
Role of microorganisms in amidoxime-enhanced pollutant removal processes.
Functionalized mesoporous and nanostructured materials
Incorporating amidoximes into mesoporous silica, graphene oxide, and other nanomaterials enhances available surface and exposure of active sites, thus enhancing adsorption kinetics.
Amidoxime-grafted mesoporous silica (MCM-41, SBA-15 Amidoxime-functionalized magnetic nanoparticles (e.g., Fe₃O₄ cores) facilitate easy separation of spent adsorbents using external magnetic fields.111,112 Carbon nanotubes and graphene oxide modified with amidoximes exhibit synergistic effects between π–π stacking and chelation for enhanced adsorption.113–115
Factors influencing heavy metal adsorption
pH dependance
The pH of the solution is very crucial in both the protonation state of the amidoxime group and the speciation of metal ions. At low pH, amidoxime groups may become protonated, reducing their ability to chelate metal ions. Conversely, at high pH, metal hydroxides may precipitate, complicating adsorption measurement.
Competing ions and selectivity
In multicomponent systems, amidoxime-functionalized adsorbents often show preferential binding based on metal ion charge density and ionic radius.13,29,116 Selectivity coefficients are typically determined through batch experiments and reflect affinity orderings such as:
Selectivity is also enhanced by designing amidoxime derivatives with additional donor atoms (e.g., carboxylates, thiols) or by embedding them into selective frameworks such as covalent organic frameworks or metal organic frameworks.
Kinetics and isotherms
Adsorption of heavy metals onto amidoxime-containing materials generally follows pseudo-second-order kinetics, suggesting that chemisorption is the rate-limiting step. Isotherm models such as Langmuir and Freundlich are frequently applied to determine monolayer capacity and adsorption intensity.117,118
Typical values:
Langmuir capacity for Pb(II): 150–250 mg/g (PAN-based adsorbents) Langmuir capacity for Cd(II): 80–120 mg/g (hydrogels, fibers) Freundlich constants (n > 1): indicating favorable adsorption Table 5 shows the summary of amidoxime-based adsorption processes for pollutant removal with their kinetics and isotherm models.
Adsorption performance of amidoxime-modified materials under various experimental conditions.
Regeneration and reusability
An important requirement for practical application is the reusability of amidoxime-based adsorbents. Desorption is typically achieved using acidic solutions (e.g., HCl, HNO₃) or chelating agents such as EDTA. Most amidoxime-functionalized materials can be reused for 3–5 cycles with moderate loss of capacity. Meanwhile, repeated regeneration can degrade the amidoxime group or leach structural components and optimization of desorption protocols is necessary to maintain performance.13,19,114,119
Industrial wastewater treatment
Amidoxime-modified activated carbon and fiber mats have been successfully used to treat electroplating and mining effluents rich in Pb(II), Zn(II), and Cu(II).120,121 These systems demonstrate high selectivity and reduced fouling in the presence of organics.
Soil and groundwater remediation
Solid amidoxime-based composites have been deployed inside reactive permeable barriers for the in-situ elimination of Cr(VI) and Ni(II) from contaminated groundwater. The immobilized systems ensure minimal leaching and sustained performance over weeks.
Hybrid membrane adsorbers
Functional membranes with embedded amidoxime groups offer rapid filtration and simultaneous adsorption.13,53,122 These have been explored for point-of-use water purification systems and show promise due to ease of scale-up.
Amidoxime complexes in adsorption of dyes
Synthetic dyes are extensively employed in industries such as textiles, paper, leather, plastics, and cosmetics. The discharge of dye-contaminated wastewater release into natural water bodies is a major environmental hazard because of their chemical stability, strong visibility even at low concentrations, and potential toxicity. Most dyes are non-biodegradable and resist conventional wastewater treatments.123–125 Thus, enhanced treatment technologies emphasizing selective, efficient, and regenerable adsorbents have become critical.
Among different materials explored for dye adsorption, amidoxime-functionalized adsorbents have recently attracted significant attention owing to their strong binding capacity toward various organic pollutants, including anionic and cationic dyes. The amidoxime functional group (-C(=NOH)-NH₂) presents a combination of hydrogen-bond donors and acceptors, ionizable sites, and electron-donating nitrogen and oxygen atoms, making it highly reactive toward various pollutants through electrostatic, hydrogen bonding, π–π, and chelation interactions.
Structural features and binding affinities
Amidoximes are bifunctional chelating ligands that possess strong interaction with electron-deficient and aromatic compounds as a result of the presence of both amino and oxime groups. These groups provide nucleophilic centers capable of hydrogen bonding and electrostatic interactions with dye molecules. Depending on pH, amidoximes can also be ionized, enhancing their affinity toward charged dye species.13,74,126,127
The electron-rich nitrogen and oxygen atoms in amidoximes can also engage in coordination bonding with metal ions, creating amidoxime-metal complexes that act as functional sites for dye binding. These complexes exhibit tailored surface properties, modified charges, and enhanced dye uptake due to the synergistic effect between metal centers and the amidoxime framework.
Complexation of amidoxime ligands with transition or post-transition metals (e.g., Cu2⁺, Fe3⁺, Zn2⁺, Mn2⁺) modifies the electronic distribution and morphology of the resulting adsorbent. The metal centers serve as additional binding sites for dye molecules, often enhancing selectivity through: Lewis's acid-base interactions
Amidoxime-based adsorbent materials
PAN is the most common polymer substrate for amidoxime modification. Nitrile groups in polyacrylonitrile are converted into amidoxime groups through hydroxylamine treatment under alkaline or neutral conditions.35,40,130,131 Polyacrylonitrile, amidoxime polymers have been used for adsorption of methylene blue (MB), malachite green (MG), and acid red dyes.120,132–134 These adsorbents show high capacity, particularly effective against cationic dyes because of electrostatic attraction and hydrogen bonding. In a study using amidoxime-functionalized Polyacrylonitrile fibers, methylene blue adsorption reached a q_max of ∼300 mg/g at pH 7.5. The fibers were regenerable over five cycles with >85% retention of capacity. Graphene oxide modified with amidoxime groups exhibited dual affinity for cationic dyes (methylene blue) and anionic dyes (acid orange 7), showing adsorptive abilities up to 250 mg/g. FT-IR and XPS confirmed hydrogen bonding and π–π interactions as dominant mechanisms.
The composite materials were employed to eliminate rhodamine B from aqueous solutions. The incorporation of amidoxime ligands enhanced the magnetic particle's surface charge and hydrophilicity, achieving >90% dye removal within 30 min.
Adsorption mechanisms
Amidoxime-containing adsorbents interact with dyes through multiple mechanisms such as:
Amidoxime groups can undergo protonation or deprotonation based on the pH level, enabling selective adsorption of anionic or cationic dyes. At low pH, positively charged amidoximes adsorb anionic dyes. At high pH, negatively charged amidoximes prefer cationic dyes like methylene blue.
Hydrogen bonding
Amino and oxime groups donate and accept hydrogen bonds, forming stable complexes with dye molecules containing -OH, -NH₂, or sulfonate groups.135,136 Conjugated dye molecules interact with aromatic or electron-rich frameworks such as amidoxime-modified graphene or polymer backbones. In amidoxime-metal complexes, the metal center may act as a coordination site for dye ligands, improving selectivity for specific functional groups in dye structures.
Effect of pH
The pH of the dye solution profoundly influence adsorption. Best pH values vary with dye type; for cationic dyes, its better adsorption is at alkaline pH (6–8) while anionic dyes enhanced uptake at acidic pH (3–5). pH also influences the adsorbent's surface charge and the ionization state of dye molecules.137–139
Isotherms and kinetics
Langmuir and Freundlich isotherms are commonly employed to characterize dye adsorption onto amidoxime adsorbents.
Maximum adsorption capacities (q_max) for cationic dyes range from 100–400 mg/g, depending on material type and surface modification.
Pseudo-second-order kinetic behavior indicates that chemisorption occurs through valence forces and may involve ion exchange.
Thermodynamics
Adsorption is often spontaneous (negative ΔG⁰) and endothermic (positive ΔH⁰), indicating that increased temperature improves dye uptake due to enhanced diffusion and surface interactions.
Reusability and regeneration
The efficiency and sustainability of adsorbents in water treatment applications rely not only on their adsorption capacity but also on their ability to be reused and regenerated with minimal loss of performance. For amidoxime complexes used in dye removal, reusability is a key advantage, enabling repeated use over multiple adsorption-desorption cycles. This section explores the principles, mechanisms, and performance of regeneration methods applied to amidoxime-based dye adsorbents, assessing their durability, economic feasibility, and operational implications.
One of the most compelling economic advantages of amidoxime-based materials is their excellent reusability. Unlike traditional adsorbents like activated carbon, which often require thermal regeneration or are disposed of after single use, amidoxime complexes can be easily regenerated using mild reagents, such as dilute acids, bases, or salts. Studies show that amidoxime-based adsorbents can hold over 80% of their original adsorption capability after five or more cycles.13,29,53,119,140
The implications of this reusability are significant:
Reduced operational expenses through extended material lifespan Lower secondary waste disposal costs Minimized need for frequent procurement and replacement
Thus, even if the initial synthesis cost is marginally higher than unmodified biosorbents, the overall lifecycle cost per treatment cycle is substantially lower.
Structural stability of amidoxime complexes
Amidoxime groups exhibit excellent chemical stability due to their electron delocalization and resistance to hydrolysis under moderate pH and temperature. When coordinated with metal ions, amidoxime complexes form robust chelate structures that resist degradation.8,10,13,18,35,83,85,96,141–143 This intrinsic stability is critical for the preservation of adsorption sites during repeated cycles of dye uptake and release. Moreover, the nature of the polymeric or inorganic matrix such as PAN, chitosan, graphene oxide, or silica plays a role in ensuring that the functional groups remain intact and available for reactivation during regeneration. Crosslinked or nanostructured supports often provide additional protection to amidoxime functionalities, preventing leaching or deactivation during washing and reuse.
Desorption mechanisms and eluents
Effective regeneration of dye-saturated amidoxime complexes depends on reversing the adsorption process without damaging the adsorbent structure. Desorption techniques target the interactions that facilitated adsorption electrostatic forces, hydrogen bonding, π–π stacking, or chelation and aim to disrupt these by modifying the environment.
Common elution strategies include:
pH adjustment: Acidic or basic solutions (e.g., HCl, NaOH) alter the surface charge and protonation state of amidoxime groups, weakening interactions with dye molecules. Solvent washing: Organic solvents like ethanol, methanol, or acetone disrupt π–π and hydrophobic interactions, especially useful for desorbing aromatic dyes. Salt solutions: High ionic strength buffers (e.g., NaCl or CaCl₂) promote ion exchange and weaken electrostatic dye binding.
Desorption efficiency typically exceeds 80–90% in the first few cycles, particularly for cationic dyes such as methylene blue or rhodamine B. Anionic dyes may require more aggressive regeneration due to stronger hydrogen bonding.144,145
Regeneration performance across multiple cycles
Quite a number of studies have confirmed the excellent reusability of amidoxime-based adsorbents over multiple cycles:
Amidoxime-modified polyacrylonitrile retained more than 85% of their initial methylene blue adsorption capability after five reusability cycles using ethanol and HCl eluent systems.104,119,146 Magnetic Fe₃O₄ in amidoxime nanocomposites exhibited minimal loss in rhodamine B uptake over six cycles, with regeneration via mild NaOH treatment. Amidoxime-functionalized hydrogels showed nearly complete recovery of Congo red using ethanol and NaCl solution, maintaining dye removal efficiency above 80% after four cycles.108,147
These examples demonstrate that amidoxime complexes can be regenerated effectively with little degradation in structural or functional integrity. Several factors, such as the type of dye, play a role in determining performance, regeneration method, and robustness of the base material.
Material and process optimization for enhanced reusability
To enhance regeneration performance and sustainability, several design strategies have been proposed:
Reinforcement of adsorbent matrices with crosslinkers or inorganic fillers to reduce mechanical degradation. Surface passivation to prevent fouling by organic matter or competitive adsorbates. Hydrophilic functionalization to improve solvent accessibility and facilitate desorption. Low-energy elution protocols using mild pH shifts or green solvents (e.g., citric acid) for eco-friendly operation.
Process optimization also involves minimizing regeneration time, solvent use, and thermal energy. Rapid regeneration (<30 min) under ambient conditions enhances the practicality of amidoxime complexes in continuous flow or batch systems.
Environmental and economic considerations
From an environmental perspective, reusability reduces waste generation and extends the service life of materials, aligning with circular economy principles. Moreover, fewer resources are consumed in producing new adsorbent batches.
Economically, regenerable amidoxime-based adsorbents offer cost-effective alternatives to single-use systems such as activated carbon or chemical coagulants. The reduced need for disposal and frequent replacement renders them suitable for large-scale industrial uses.
Environmental and industrial relevance
Amidoxime-modified materials have shown remarkable potential in addressing environmental pollution, particularly dye contamination in industrial wastewater. Synthetic dyes used in textile, paper, leather, and food processing industries often exhibit resistant properties, with high stability against light, temperature, and microbial degradation. Their persistence in aquatic environments leads to esthetic degradation, reduced photosynthetic activity, and potential mutagenic and carcinogenic effects on aquatic life and humans. Amidoxime complexes, due to their chemical versatility and high binding affinity, present a sustainable and selective method for dye removal, offering both environmental remediation and industrial applicability.13,16,29,35,85
One significant advantage of amidoxime complexes is their dual interaction capabilities. The amidoxime group, with both nucleophilic nitrogen and electrophilic oxime functionalities, allows for efficient removal of a broad spectrum of dyes cationic, anionic, and nonionic. This broad-spectrum adsorption is particularly relevant in real industrial wastewater streams, which often contain a complex mixture of dye classes. Their adaptability under different pH and ionic strength conditions enhances their applicability in variable industrial effluents. Amidoxime-functionalized polymers, such as modified PAN, cellulose, or chitosan, are ideal for industrial use due to their scalability, ease of processing, and mechanical robustness. Moreover, the introduction of metal ions (e.g., Fe3⁺, Cu2⁺, Zn2⁺) into the amidoxime matrix improves selective dye affinity, opening the possibility of tailored adsorbents for specific effluents. In high-throughput operations such as textile dyeing, the ability to selectively target and remove dyes like methylene blue, rhodamine B, Congo red, and malachite green enhances water recycling and reduces discharge loads.
From an environmental standpoint, amidoxime-based adsorbents contribute significantly to reducing secondary pollution.13,29,84,104,114,119,148 Unlike coagulants or flocculants, which generate large volumes of sludge requiring further treatment, amidoxime materials are capable of regeneration and repeated use across several cycles minimizing solid waste. The desorption process typically involves low concentrations of acid or base or mild solvents, which are less hazardous and more manageable in terms of disposal or recycling. Their compatibility with integrated water treatment systems such as adsorption membrane hybrid systems or adsorption photocatalysis setups further extends their relevance in industrial-scale treatment. These systems allow for enhanced contaminant removal while maintaining compact footprints, a desirable feature in space-limited industrial sites.
Moreover, amidoxime-based materials are being explored for use in zero liquid discharge (ZLD) operations, where dye-saturated adsorbents can be processed to recover concentrated dye solutions for potential reuse in dyeing processes.13,29,114,149 This aligns with the goals of industrial ecology and circular economy, promoting resource efficiency and waste minimization. Additionally, the relatively low toxicity of amidoxime compounds and their polymeric supports makes them safer alternatives to traditional adsorbents derived from hazardous materials. This is especially significant in industries that emphasize green production practices and sustainability certifications.
In industrial contexts, cost-effectiveness remains a key factor. Although functionalized materials may have a higher upfront cost compared to conventional adsorbents, their high adsorption capacity, selectivity, and regenerability reduce long-term operating costs. The development of low-cost amidoxime precursors and the possibility of using industrial waste polymers as substrates further enhance their economic feasibility. Encouraging results have been observed in preliminary studies and semi-industrial trials, particularly in textile zones of countries like India, Bangladesh, and China. These studies confirm that amidoxime-based materials offer operational advantages such as rapid adsorption kinetics, compatibility with flow systems, and low maintenance requirements.
The environmental and industrial relevance of amidoxime complexes stems from their high dye removal efficiency, eco-compatibility, operational flexibility, and alignment with sustainable manufacturing principles. Their continued development is expected to serve a key function in achieving cleaner industrial effluent standards and broader environmental conservation goals.
Cost effectiveness
In spite of their significant potential, implementing amidoxime complexes in bioremediation is not without challenges. The economic feasibility of any remediation strategy is a major determinant of its practical adoption, especially in industrial and municipal wastewater treatment systems. Amidoxime-modified materials are gaining recognition as effective options for bioremediation as a result of their high efficiency, selectivity, and environmental compatibility.13,16,29,35 One significant impediment is the cost-effective synthesis and large-scale production of these complexes, especially for widespread environmental application. Evaluating cost factors related to synthesis, operation, regeneration, and overall lifecycle allows for a realistic assessment of their practical feasibility in dye and heavy metal adsorption applications. Research into sustainable and economically feasible synthesis routes is crucial. Furthermore, understanding the long-term environmental fate and ecological hazard of the amidoxime complexes themselves and their degradation products is paramount. Thorough environmental toxicity studies are necessary to ensure that the remediation process does not introduce new environmental concerns.
Raw material and synthesis costs
Amidoxime compounds are typically synthesized through the conversion of nitrile containing precursors such as PAN, acrylonitrile-based copolymers, or cyanated natural polymers using hydroxylamine under acidic or neutral conditions.97,150 These precursors are often inexpensive and commercially available in bulk quantities, making them accessible for industrial processing. Hydroxylamine hydrochloride, the key reagent in conversion to amidoxime, is relatively low-cost and available in technical grades suitable for large-scale use. Natural and waste-derived materials like chitosan, starch, cellulose, and lignin are increasingly being studied as base materials for amidoxime group incorporation. These biomaterials are biodegradable, nontoxic, and often sourced as byproducts from agriculture or food processing industries. Their availability at low or zero cost significantly enhances the affordability of amidoxime-based biosorbents. For example, amidoxime-modified rice husk or sawdust presents a sustainable and economical alternative to synthetic polymer backbones.35,151–153
While the modification steps especially amidoxime group incorporation and crosslinking require controlled reaction conditions and sometimes involve solvents or catalysts, most studies efforts have aimed to develop green and low-energy synthesis routes. Water-based amidoxime functionalization, microwave-assisted synthesis, and solvent-free mechanochemical modifications are promising low-cost methods that reduce both material and energy input.
Operational efficiency and low material dosage
The high adsorption ability of amidoxime-functionalized materials reduces the amount of adsorbent required for effective remediation. For instance, amidoxime-modified fibers, nanoparticles, or hydrogels have demonstrated dye adsorption capacities ranging from 150 to over 400 mg/g, and metal ion uptake values often exceed 200 mg/g for ions like U(VI), Cu(II), and Pb(II).29,35,84,121 Such high efficiency translates to low dosage requirements, thereby minimizing per-treatment material costs. Additionally, the fast adsorption kinetics often observed with amidoxime materials due to favorable diffusion and surface accessibility further reduce operational costs by decreasing residence time and reactor size requirements. This is particularly beneficial in continuous-flow or packed-bed systems, where space and time are directly tied to cost effective output.
Compatibility with existing infrastructure
Amidoxime materials can be incorporated into multiple treatment platforms, beads for packed-bed columns, fibers for filter media, or membranes for crossflow systems allowing seamless integration into existing treatment of industrial effluents units.154,155 This eliminates the need for large-scale capital investment, which is often a prohibitive barrier for new remediation technologies. Furthermore, the modular and versatile nature of amidoxime complexes makes them well-suited for small to medium scale applications. (e.g., in textile dyeing units or tanneries), where cost constraints are critical. Their use in distribution and mobile treatment units provides affordable options for regions lacking centralized wastewater infrastructure.
Cost savings through resource recovery
In addition to pollutant removal, amidoxime complexes offer the potential for resource recovery, particularly in the adsorption of valuable metal ions like uranium, gold, and rare earth elements. This dual benefit of remediation and resource recovery improves cost-effectiveness by introducing economic returns. For dye adsorption, certain systems enable the desorption and reuse of dyes, especially in textile industries where the recovery of expensive dye molecules may offset treatment costs. Furthermore, amidoxime materials can be used selectively to capture high-value dyes while allowing nontoxic compounds to pass through.156–158
Economic comparison with other adsorbents
Comparing amidoxime-grafted materials to conventional adsorbents like activated carbon, ion exchange resins, or synthetic membranes, the grated materials often present lower long-term costs due to:
Higher adsorption capacities Easy and low-cost regeneration Greater durability and fewer replacements Broader applicability to various pollutants
Moreover, in contrast to some nanomaterials or functionalized resins that are costly to produce and regenerate, amidoxime complexes provide a balance between performance and affordability. Amidoxime materials align well with the circular economy model, which emphasizes reuse, recovery, and reduced resource consumption. Their ability to adsorb pollutants, regenerate efficiently, and integrate into recovery pathways.
Limitations and considerations
Amidoxime complexes have demonstrated strong promise in dye and heavy metals adsorption, yet several limitations persist within the current body of studies. One key limitation is the absence of long-term stability and durability data. Many studies focus on initial adsorption efficiency,108,159,160 but fewer evaluate the material's performance over multiple regeneration cycles.122,161–163 This raises concerns about regeneration-induced degradation and long-term reusability, which are critical for sustainable deployment.
Another limitation lies in the variability of experimental conditions. Differences in pH, temperature, dye concentration, and ionic strength across studies hinder direct comparison and broader generalization of results. Furthermore, the majority of adsorption studies are conducted under controlled laboratory settings, which might not fully represent the complex matrices of industrial wastewater. This gap limits the practical applicability and scalability of amidoxime-based adsorbents.
In addition, toxicity and environmental safety of amidoxime-grafted materials, especially after prolonged use or degradation, remain underexplored. Understanding potential leaching of metal ions or breakdown products is essential before real-world implementation. Several critical considerations should also guide future research. The pH-dependance of amidoxime group protonation, potential competition from co-existing ions, and selectivity for specific dye classes (e.g., anionic vs. cationic) all play significant roles in adsorption efficiency. 164 Moreover, the cost and complexity of synthesis, as well as the feasibility of regeneration at scale, must be weighed when assessing the industrial viability of these materials.
Despite their advantages, other challenges include:
Some regeneration methods may not fully desorb deeply bound dyes or dye aggregates. Aggressive desorption agents can slowly degrade the amidoxime group or weaken the metal-ligand complex.8,13 In practical scenarios involving mixed pollutants or high dye loads, competitive binding may affect regeneration performance. Fouling by organic matter or biofilms in real wastewater can block active sites over time.
Therefore, a balance must be maintained between regeneration efficiency and preservation of structural integrity. Addressing these limitations and integrating these considerations will be essential for advancing amidoxime-based adsorbents toward practical, environmentally responsible dye remediation technologies.
Future directions
The future prospects for amidoxime complexes in bioremediation are bright, with ongoing research focusing on several key areas. The development of novel amidoxime derivatives with enhanced bioremediation capabilities is a primary focus. This includes designing complexes with improved selectivity, higher adsorption capacities, and greater resistance to environmental degradation. Despite their promising capabilities, amidoxime-based are confronted with several obstacles that must be overcome to guarantee widespread adoption in environmental and industrial applications. The primary main difficulty is maintaining long-term structural durability of amidoxime-functionalized materials under harsh operational conditions. Repeated exposure to extreme pH, mechanical abrasion, or high dye loads can lead to surface fouling, degradation of functional groups, or leaching of metal centers in amidoxime-metal complexes.13,29,113,120 This compromises not only adsorption efficiency but also environmental safety, particularly if toxic metals are released into treated water.
Another present limitation is the feasibility of large-scale production and associated synthesis costs. While lab scale synthesis of amidoxime ligands and functionalized materials is well-documented, translating this into cost-effective industrial production remains limited. The use of expensive or specialized solvents, reagents like hydroxylamine, and post-synthesis modifications increase production costs. Moreover, the time-consuming grafting processes and purification steps may not align with rapid production cycles needed in industrial settings. Selectivity is also a concern. Amidoxime materials can interact with a broad spectrum of contaminants, such as heavy metals which may compete with dyes for binding sites in mixed-contaminant streams.19,157 This non-specificity can reduce adsorption efficiency for target dye molecules, particularly in complex wastewaters from textile and dye manufacturing plants. Developing tailored adsorbents with higher affinity for specific dye classes or introducing molecular imprinting techniques could address this issue.
Another critical limitation is decline in reusability. Although amidoxime adsorbents are reusable, their performance tends to decline after several regeneration cycles due to partial degradation of active sites or incomplete desorption.13,29,165,166 In large-scale or continuous systems, adsorbents must withstand dozens of cycles with minimal capacity loss, a target that many current materials still struggle to meet. From an environmental regulatory standpoint, there is also a need to understand the long-term ecological impact of spent amidoxime materials. While they are more environmentally benign than many conventional alternatives, questions remain about their fate in landfill or incineration processes and their breakdown products under natural conditions.
Addressing these challenges will require future research and development to concentrate on the following core areas:
Green synthesis methods: Development of solvent-free or aqueous-phase functionalization techniques, using bio-based amidoxime precursors, would reduce costs and environmental impact.157,167,168 Hybrid materials: Integrating amidoxime groups with other functional domains such as photocatalytic TiO₂, MOFs, or zwitterionic moieties could enhance multifunctionality, allowing for both adsorption and degradation or dye recovery. Nanostructured designs: Using nanofibers, aerogels, or hollow nanospheres to increase surface area and porosity will improve adsorption kinetics and overall efficiency. Molecular imprinting and specificity tuning: Designing amidoxime materials with selective binding sites for particular dye structures would improve performance in multicomponent wastewater.13,19,120,169 Life-cycle assessment (LCA): Comprehensive evaluation of the environmental footprint of amidoxime adsorbents, including production, use, regeneration, and disposal phases, is essential for commercial feasibility and regulatory approval.13,19,29,120,170 Field validation: More pilot-scale and real effluent trials are required to connect laboratory findings with performance and industrial implementation. Testing in textile dyeing units, tanneries, and pigment manufacturing plants would provide critical performance data.
While amidoxime-based materials have shown significant promise in heavy metal adsorption, several research directions can enhance their practical impact:
Structure-property tuning: Systematic design of amidoxime ligands with tailored donor sets and backbone rigidity to optimize selectivity. Green synthesis: Developing eco-friendly routes to synthesize amidoxime-functionalized materials using biopolymers or solvent-free processes. Composite formulations: Integrating amidoximes into multi-functional adsorbents capable of removing both metals and organic contaminants simultaneously.32,171–173 Field validation: Large-scale testing in industrial, agricultural, and urban settings to validate performance under complex environmental conditions.
This review is designed to explore the key aspects of the bioremediation prospects of amidoxime complexes, with a focus on their coordination chemistry, synthesis strategies, mechanisms of pollutant removal, and integration into biological systems. It will explore both the theoretical and experimental advances in this field, highlighting the potential benefits and drawbacks associated with using amidoximes complexes in various environmental settings. In doing so, it aims to identify critical areas of missing knowledge and propose directions for future studies directions that could facilitate the transition from laboratory investigations to practical applications. Summarily, while challenges remain, the future of amidoxime complexes in dye and heavy metal adsorption is promising. With advances in material science, process engineering, and sustainable chemistry, these systems can be refined for broader, more efficient, and greener applications in dye-contaminated wastewater treatment.
Conclusion
Amidoxime complexes contribute significantly to the adsorption and sequestration of heavy metals from contaminated environments. Their ability to chelate metals, adaptable structures, and compatibility with various material platforms make them powerful assets for bioremediation applications. Through continued research into coordination chemistry, material development, and environmental application, amidoxime-based systems have the potential to offer scalable, selective, and sustainable solutions to the worldwide issue of heavy metal contamination. The reusability and regeneration of amidoxime complexes offer a major advantage for their use in dye adsorption, especially in sustainable water treatment technologies.13,29,35,85 Their chemical stability, great desorption effectiveness, and minimal loss in adsorption performance over multiple cycles position them as cost-effective and environmentally friendly solutions. Future efforts should focus on improving desorption methods, developing low-energy regeneration systems, and testing under realistic wastewater conditions to ensure industrial scalability.
Amidoxime complexes represent a novel and versatile class of materials with significant potential in bioremediation, the introduction of amidoxime complexes into the field of bioremediation represents a significant advancement in environmental pollution control. Their unique chemical properties, particularly their strong chelating ability with heavy metals and their potential to influence the degradation of organic pollutants, make them ideal candidates for addressing current environmental contamination and challenges. While challenges remain in their large-scale application and understanding their full environmental impact, continuous research and emerging technologies are gradually facilitating their broader use. As we continue to seek sustainable and efficient solutions to global environmental challenges, the bioremediation potential of amidoxime complexes stands out as a promising and evolving area of scientific inquiry with the capacity to contribute significantly to a cleaner and healthier planet. With ongoing research, the integration of amidoxime combining with the insights from chemistry, microbiology, and environmental engineering, will serve as a critical factor in maximizing the potential of these fascinating compounds enabling them effective and sustainable.
Footnotes
Funding
The authors received no financial support for the research, authorship, and/or publication of this article.
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.