Modified coal fly ash for textile dye removal from industrial wastewater

Abstract

Complex dyes have an adverse impact on the environment since they have a high aqueous solubility and are very challenging to be removed using conventional approaches. A series of batch tests were performed to determine the optimum parameters for the direct blue 78 dye adsorption process as a function of chemically treated coal fly ash (TCFA) Effect of fly ash dose, contact time and dye initial concentration were studied using an industrial wastewater. A pilot plant, consisting of three stages; mixing, sedimentation and filtration was designed and implemented to investigate the ability of TCFA to remove COD, BOD₅, TDS and TSS from textile wastewater under optimal conditions. A series of fly ash physico-chemical analysis including particles size analysis, zeta potential, XRD, EDX, BET, SEM and FTIR were investigated. The results showed that the particles size and zeta potential for raw coal fly ash were 1.2 µm and −20.70 mv. Chemical treatment process using hydrochloric acid resulted reducing the particles size of fly ash from 1.2 µm to 0.95 µm. Zeta potential value increased to + 7.20 mv. Batch adsorption process showed that the removal efficiency decreased as the primary dye concentration increased. It increased when fly ash loading and contact time increased. Dyes removal was achieved up to 99.7% using TCFA loading 3gL⁻¹ for initial dye concentration10 mgL⁻¹. Studies on equilibrium revealed that the equilibrium data was closely fitted to the Freundlich and Langmuir isotherm models. Kinetic study of adsorption process was studied in detial. Based on high correlation coefficient (R² = 0.999), the results were followed pseudo second-order model.

Keywords

Energy adsorption modified fly ash physico-chemical analysis textile wastewater treatment

Introduction

Synthetic dyes are widely used in a variety of industries, such as paper, pulp, tannery, and textiles, due to their low cost of manufacture, vibrant colors, and resistance to environmental influences.¹ The increased use of such dyes has resulted in huge amounts of contaminated liquid wastes, including complexes consisting from both organic and inorganic materials.² For textile industry, it was estimated that, the manufacturing of 1 kg of colored fabrics requires about, 0.08–0.10 m³ of water,³ and approximately 3 × 10³ m³ of water is used to produce 20 × 10³ Kg of textiles everyday.⁴ These discharges are improved with venomous dyes and chemicals, and all their quality indicators have higher values than permissible standard limits. Therefore, these effluents required a high efficiency treatment technology before discharged into the water bodies.⁵ Therefore, with varying degrees of success, various approaches for dyes elimination from industrial wastewater have been investigated. These methods include coagulation,⁶ fenton oxidation,⁷ photo-degradation,⁸ nanofiltration,^9,10 photocatalysis,¹¹ adsorption¹² and cation-exchange membrane.^13–15 These treatment approaches have also revealed a number of drawbacks, such as lower validity in the removal of various contaminants, high costs, large reagent demands, and the generation of toxic waste products that need additional safe disposal.

Adsorption is regarded as the most adaptable dyes elimination technology because of its simplicity, higher removal efficiency, nearly minimal secondary pollution and capability to treat the dyes in highly concentrated solutions.^16,17 The most significant use of the adsorption method is the removal of colorants from industrial effluent using suitable adsorbents.^18–20 Adsorption is a desirable and efficient alternative method of treating industrial wastewater, particularly if the adsorbent is inexpensive, widely accessible, and reusable. Industrial or agricultural wastes have been used as adsorbents in a number of studies over the past several years, and the results indicate that these wastes have varying degrees of adsorption capability.^21,22

The magnetic separation technology combined with adsorption processes played a significant role for dyes removal. Magnetic materials such as Fe₃O₄ nanoparticles were examined and showed a high removal efficiency but the major commercial drawback is that magnetic particles cannot be applied at large-scale processes due to the high cost.¹⁸ Furthermore, activated carbon is a typical commercial adsorbent because it has a large surface area, and high adsorption capacity.^23,24 However, it has a limited application owing to its cost. Great efforts have been made to improve a non-conventional economical adsorbent^25–28 using various materials. Numerous solid waste products generated from various industrial processes were investigated in order to develop a more cost-effective adsorbent than activated carbon. These materials included sawdust, solid municipal waste, red mud, zeolites, coal fly ash, and waste from the sugar industry.

The disposal of solid wastes has become one of the most troublesome environmental problems in recent years. The by-product known as coal fly ash (CFA) is made primarily of unburned carbons, iron oxides, and aluminosilicate following the combustion of coal.²⁹ Coal fly ash was identified as a type of industrial solid waste that is typically released in significant quantities in landfills.³⁰ As a result, researchers and ecologists have worked very hard to examine all alternative potential uses to ensure proper disposal, management, and utilization.³¹ Although CFA is a potential adsorbent despite having a number of advantageous qualities, including a desired particle size, a large specific area with better porosity structure, and a wealth of different metal oxy-ide components.³² In recent years, converting of solid wastes (by-products) into an economical adsorbent and improve its adsorption characteristics such as particles surface area, pores volume and surface morphology using physical or/and chemical methods has become a preferable solution for industrial wastewater treatment using adsorption technology.³³ In this respect, the treated fly ash has been successfully applied as an economical adsorbent for dyes and heavy metals removal.^33,34

It was reported that fly ash can be used as an economical adsorbent for toxic dyes removal with efficiency 99%, however the required adsorption time was 60 min which is higher than the contact time required for commercial adsorbents.²³ Therefore, several studies have been conducted to activate the fly ash using chemical methods to improve its adsorption capacity and decrease the adsorption contact time. It was found that, the treatment of fly ash using acidic solution increases its surface area from 13 m²g⁻¹ to 58 m²g⁻¹.³⁴ Although several studies have been conducted to examine the ability of CFA for dye removal, the research in direct dye adsorption technique onto CFA remains limited. The novelty of this research is to improve the adsorption characteristics of CFA (increasing its adsorption capacity with short contact time) by a chemical treatment methods to (i) enhance its surface area, (ii) improve its surface morphology and (iii) use it efficiently as a cost-effective adsorbent for textile industrial wastewater treatment. Adsorption isotherms for the direct dyes were assumed to study the effects of adsorption period, primary dye concentration and adsorbent dosage

Materials and methods

Adsorbent material

Coal fly ash (CFA) used in this research was taken from a coal-based cement factory. The coal fly ash was separated by sieving means and only particles have an average size 1.2 µm were used in this research. Raw coal fly ash (RCFA) sample (100 g) was taken into a 300 mL distilled water, the solution was stirred 4 h with speed 150 r.p.m at 30 °C for washing, filtrated using a Whatman filter paper and dried at 100 °C for 10 h using a thermal furnace. However, the TCFA was prepared by dipping 50 g of RCFA onto HCl solution with concentration 30%, the solution was stirred for 4 h with speed 150 r.p.m and then heated up to 100 °C for 12 h. After the modification process, the mixture was filtrated through a Whatman filter paper, washed with distilled water, and left to dry at 110 °C for 24 h. The obtained solids particles then subjected to physical and chemical measurements. Figure 1 shows the applied modification method for TCFA preparation.

Sources of wastewater

Synthetic wastewater

For batch adsorption tests, the direct blue 78 dye (DB78), with relative molecular mass 1060, λ_max = 602 nm, and maximum solubility of 10 gL⁻¹ at 25 °C, was chosen. Figure 2 shows the dye chemical structure. To determine the ideal adsorption conditions, including TCFA dosage and contact time, three dye solutions were synthetic with varying primary concentrations (10, 20 and 30 mgL⁻¹⁾ were prepared.³⁵

Industrial wastewater

In order to evaluate the optimum conditions and investigate its effect on chemical oxygen demand (COD), total dissolved solids (TDS), total suspended solids (TSS) and biological oxygen demand (BOD₅), a lab scale treatment model consists of three stages (mixing, sedimentation and filtration) was designed as shown in Figure 3. The influent wastewater to the pilot plant was obtained from a wastewater collection sump in a textile factory- investment industrial zone – Port Said government - Egypt. The studies in this part were conducted over two weeks (six samples per week), these samples were taken every day at two times during the day 9.00 a.m., 12.00 p.m. at a daily interval.

Characterization of adsorbent material and wastewater

Coal fly ash suspension was diluted in a distilled water and ultrasonically treated at 5% (w/v) for 20 min. The USC-1400 model is unique (40 kHz of ultrasound frequency).

The measurements were performed using the Malvern 3000 Zetasizer Nano-ZS (Malvern Instruments, UK). Diffusion of particles with Brownian motion and uses dynamic light scattering are applied for size distribution. Additionally, a dispersion of particles is given an electric field by means of laser doppler micro-electrophoresis, which causes the particles to move at a pace proportionate to their zeta potential. The particle size was determined using the Smoluchowski algorithm.

By using an Empyrean diffractometer system (Bruker D8 diffractometer, Germany). XRD analysis was conducted, the phases of coal fly ash samples were studied. Copper was used as the anode material, operating at 45 KV, 30 mA, step size 0.02 and goniometer range 240 mm across the range of 5–90°. The temperature used for these measurements is 25 °C.

Scanning electron microscopy (TESCAN MIRA-High Resolution scanning electron microscope, Tescan Essence Company, Brno, Czech Republic) and an energy dispersive X-ray (EDX) were used to analyze the samples’ elements (Oxford instru-ment nano analysis detector, UK).

Surface area analysers (Autosorb-l-C-8, Quantachrome, USA.) were used to determine the surface area of materials samples in the presence of N₂ adsorption at −195.65 °C. The samples were degasified at 200 °C for 4 h before to adsorption tests. The BET surface area for the TCFA was calculated by using the adsorption data with the BET (Brunauer-Emmett-Teller) equation.

SEM analysis was applied to observe the surface morphology and porous micro-structure of TCFA samples. TESCAN MIRA-High Resolution scanning electron microscope, Tescan Essence company, Brno, Czech Republic was used foe SEM analysis.

For the materials used, FTIR analyses were carried out utilising (VERTEX 80v vacuum FTIR Spec-trometer, Bruker corporation, Germany).

Adsorption studies

Adsorption was performed with a single component synthetic wastewater and batch adsorption system were used to determine the optimal adsorption parameters. Treated coal fly ash doses were (0.05–0.5) g in 100 mL solutions with varying primary concentration (10, 20 and 30) mgL⁻¹ was mixed at 150 r.p.m for adsorption time (0–0.5) hr using (Gallenkamp magnetic stirrer) at temperature (25 ± 2 °C). The solutions were filtrated after adsorption process using a Whatman filter paper and then subjected to further analyses. The effect of the selected optimal conditions on COD, BOD₅, TDS, TSS and dyes removal efficiency was investigated using the pilot plant. The utilized TCFA doses were 2, 3 and 4 gL⁻¹ for industrial wastewater with initial dye concentration ranges (0–10), (10–20) and (20–30) mgL⁻¹ respectively. The applied contact time was 0.5 h with mixing speed 150 r.p.m and sedimentation time equal to 15 min. After sedimentation stage, the wastewater was filtrated through a sand filter consists of two layers (40 cm sand layer and 20 cm gravel layer).

Results and discussion

Characterization of adsorbent

Particles size distribution measurements

In order to determine the effect of modification process on fly ash particles size, the particles size distribution analysis was conducted for RCFA and TCFA as shown in Figure 4. It was observed from analysis results that, 100% of RCFA particles have a size range of 0.85 µm - 1.95 µm with average particles size 1.2 µm. For TCFA particles, 13.65% of particles has a size range of 0.1 µm - 0.3 µm with average particles size 0.25 µm and 86.35% of particles has a size range of 0.55 µm - 1.2 µm with average particles size 0.95 µm. This significant decrease in particles size after modification process can be attributed to the erosion of fly ash particles surface due to chemical treatment using hydrochloric acid. It was reported that the adsorption capacity would be improved by using fly ash with a small particle size and a high accessible surface area.^36,37

X-ray diffraction analysis

The effect of modification process on chemical stability of coal fly ash particles was conducted using XRD analysis. As shown in Figure 5 (a), the RCFA contains the minerals quartz, mullite and calcium oxide, as well as certain amorphous phases. These minerals form some sharp and crystalline peaks. Raw coal fly ash was chemically stable due to the presence of a glassy layer covering its surface. This glassy layer can be noticed from the precence of the amorphous phase in XRD diagram. As a result of the chemical modification process, this glassy surface layer were eroded (amorphous phas was disappeared) due to the chemical reaction between hydrocloric acid and TCFA particles. Furthermore, the intensity of some crystalline peaks were reduced (some peaks have been dissappeared) as shown in Figure 5 (b). This erosion leads to improve porous structure and surface morphology which certainly enhancing the adsorption behavior process. These results are close to those recorded for fly ash investigated elsewhere [33].

Energy dispersive x-ray (EDX) analysis

Elemntal analysis was conducted to study the composition of fly ash after chemical treament. Energy dispersive X-ray analysis was used to determine the composition of RCFA and TCFA as shown in Figure 6. It was found that there is a change in C, O, Si, Ca, K and Cl composition as shown in Table 1. For C, O, Si and Cl contents, they were increased from 17.4%, 47.89%, 0.18% and 0.33% to 19.1%, 48.75%, 1.31% and 0.64% respectively. Other trace elements content such as Na, Mg, P and K were reduced after the chemical midification process. The increase in C % after acidic treatemnt can be attributeed to dissolving Ca, K, Na, and Mg. The outer surface of the fly ash particles was eroded by this reaction, which significantly changed the elemental compositions and surface area

Table 1.

EDX analysis results for RCFA and TCFA.

Constituents	C	O	Ca	K	Na	Mg	Cl	P	Si
RCFA Weight %	17.42	47.89	30.66	1.57	0.48	1.25	0.33	0.22	0.18
TCFA Weight %	19.10	48.75	27.77	0.29	0.28	0.54	0.64	1.04	1.31

Zeta-potential analysis

The net surface charge for RCFA and TCFA were determined after acidic treatment as shown in Figure 7. It was observed that the raw coal fly ash particles have a negative net surface charge −20.7 mv. This charge will hinder the electrostatic interaction between anionic dyes molecules and fly ash particles and negatively affected on adsorption behavior. After acidic modification process, a remarkable change was observed. The net surface charge of fly ash particles reaches + 7.20 mv. This charge will stimulate the electrostatic interaction between DB78 dye molecules and TCFA particles and positively affected on adsorption behavior. This transformation of fly ash particles surface charge can be attributed to the accumulation of H⁺ on TCFA surface after the chemical treatment using hydrochloric acid.

Analysis of surface area

The BET surface area analysis was done to examine how the modification method affected the fly ash particles. The results show a larger increase in CFA surface area; for RCFA and TCFA, it rises from 9.61 m²g⁻¹ to 60.42 m²g⁻¹. The chemical reaction between CFA components and hydrochloric acid, which affected on the CFA particles outer surface (surface was eroded) and restores their pore's structure, greatly increased the CFA sorption capacity, as shown in Figure 8. This reaction not only changed the surface morphology of CFA, but it also produced a significant amount of porous structure. Physical and chemical characteristics of RCFA and TCFA are displayed in Table 2.

Table 2.

Physico-chemical properties of raw fly ash and chemically treated fly ash.

Parameter	Value
Raw fly ash surface area S_BET	9.61 m²g⁻¹
Unburned carbon for RCFA (LOI)	36.16%
Total area on pores (RCFA)	3.915 m²g⁻¹
Raw fly ash pore volume	0.0149 cm³g⁻¹
Raw fly ash median pore width	25.64 nm
Treated fly ash surface area S_BET	60.42 m²g⁻¹
Unburned carbon for TCFA (LOI)	41.65%

Surface morphology

SEM analysis was applied to investigate the effect of acidic treatment process on CFA surface morphology. As shown in figure 9, coal fly ash particles have aggregated in irregular surfaces with a significant number of macropores and crevices with different size. The particles have unique size distribution and distinct shapes. As seen in SEM image Figure 9 (a), RCFA particles have a rough surface with a constrained porosity structure. The surface of the fly ash exhibited a significant change after acidic treatment. With a substantial improvement in the porous structure, the surface changed from being slippery (smooth) to eroded (rough). The particles were relatively transformed into smaller size due to chemical degradation in the presence of acidic solution as observed in SEM image Figure 9 (b). Surface analysis of RCFA and TCFA using SEM technique are in a good agreement with BET surface area findings. The TCFA is fully loaded, and all of its surface pores and outer surfaces are saturated with dye molecules after adsorption process. This can be observed from SEM image Figure 9 (c).

Chemical structure using FTIR analysis

TCFA, exhausted TCFA (after adsorption process), and DB78 dye samples were examined by FTIR analysis so as to determine the main bands of minerals and organics. The results were compared with existing literature. Figure 10, shows the FTIR spectrum, reveals the existence of nine primary peaks with higher transmittance (%) at various wave numbers (cm⁻¹).

The bending of the Si-O-Si and O-Si-O bonds is indicated by the first peak having a wave number 445.5 cm⁻¹. For the followed peaks (2, 3 and 4) having wave number 565.1, 605 and 711.6 cm⁻¹ respectively, shows the stretching of the Si ̶ O ̶ Si and Al ̶ O ̶ Si bands. For number 5, having wave number 871.7 cm⁻¹, indicates the loss of CaCo₃. The 6 and 7 peaks having wave number 1024.1 and 1400.1 cm⁻¹ respectively, specifies the rises of Si ̶ O ̶ Si bond. For peak 8, having wave number 1639.3 cm⁻¹ indicates organic matter (C = O carboxylate group).³⁸ The last peak (9) having wave number 3337 cm⁻¹ (specifies the strongly hydrogen bond Si-OH group and adsorbed molecules of water H ̶ O ̶ H bonds.³⁹ Table 3. Shows the results of chemical structure analysis for RCFA and TCFA.

Table 3.

Chemical structure measurements results for treated and exhausted CFA.

CFAwave number (cm⁻¹)(before adsorption)	CFAwave number (cm⁻¹)(after adsorption)	Band range (cm⁻¹)	Indication	Refs.
445.8	451.8	< 550	O-Si-O and Si-O-Si bonds	[39]
564.5	566.6	500-800	Al-O-Si and Si-O-Si Symmetric stretching	[39]
602.9	604.1
711.3	711.6
871	872	800-1000	Loss of gel CaCO₃	[39]
1022	1025.7	1000-1600	Gains Si-O-Si bands typical of quartz	[39]
1405.5	1407	1000-1600	Gains Si-O-Si bands typical of quartz	[39]
1633.8	1641.4	-------	Carboxylate group (C = O) of organic matter	[38]
---------	1796	1600-3500	Extending (-OH), bending (H-O-H)	[38]
3370.2	3397.32	1600-3500	Extending (-OH), bending (H-O-H)	[38]

The FTIR spectra showed that, a noticeable difference was observed when compared TCFA with exhausted fly ash and DB78 dye FTIR spectra. That's means additional bonds were resulted due to the accumulation of dyes molecules on TCFA surface. From these results it was observed that, the adsorption mechanism is chemical adsorption.

Synthetic wastewater treatment

Effect of TCFA loadings

Figure 11, illustrates the results of an investigation related to the dependence of DB78 dye removal efficiency on TCFA dose using a dose range of 0.5 to 5 gL⁻¹. The findings showed that as TCFA quantity was increased, removal efficiency increased until it reached its maximum value, after which it appeared to roughly stabilize. Three synthetic dye solutions with varying primary concentrations varied from 10 to 30 mgL⁻¹ at a pH of 8.60 were also used in the experiments. According to the results, a TCFA amount of 2 gL⁻¹ with an equilibrium loading of 5 mg/g for a solution with a primary concentration of 10 mgL⁻¹ can be used to achieve the desired elimination efficiency (99%). With a TCFA amount of 3 gL⁻¹ and an equilibrium loading of 6.5 mgg⁻¹, the maximum removal efficiency for a solution with a primary dye concentration of 20 mgL⁻¹ was 97.4%. Furthermore, with a TCFA amount of 4 gL⁻¹ and an equilibrium loading of 7.3 mgg⁻¹, the maximum removal efficiency for a solution with primary dye concentration of 30 mgL⁻¹ was 97.5%. The ability of TCFA to eliminate DB78 dye can be attributed to the electrostatic interaction between TCFA particles having a positive surface charge ( + 7.2 mv) and anionic dyes molecules.

Effect of adsorption time

For three initial concentrations of the DB78 dye, 10, 20, and 30 mgL⁻¹, respectively, at pH 8.6, the influence of adsorption time on dye elimination efficiency was examined. The results are presented in Figure 12. In every case, the balance was attained in 30 min. Elimination was quick in the beginning and increased gradually as contact grew up. Equilibrium was reached after 30 min because no appreciable variation in dye elimination was observed anywhere during that period. This indicates that during DB78 dye molecule adsorption, the dye molecules first quickly entered the boundary layer through mass transfer, then slowly diffused from the boundary layer film onto the adsorbent surface due to the presence of open sites, and eventually diffused into the pores of TCFA.

Effect of dye primary concentration

To investigate the impact of primary dye concentration on percentage of dye removal, the direct blue 78 (DB78) dye solutions with initial concentrations of 10, 20, and 30 mgL⁻¹ were tested. According to Figure 13, the removal efficiency for an adsorbent dose of 1 ^g−1L was (95.8, 90.1, and 88.3), for a dose of 2 gL⁻¹ was (99.7, 95.7, and 93.5) %, for a dose of 3 gL⁻¹ was (99.7, 97.4 and 97.2) %, and for a dose of 4 gL⁻¹ was (99.7, 98.1 and 97.6) %. Accordingly, the primary concentration of dye solutions is (10, 20 and 30) mgL⁻¹.

Adsorption equilibrium study

The most significant aspects of adsorption research are the investigation of adsorption isotherms because they show how the adsorbent and adsorbate interact. Additionally, it conveys information regarding the adsorbent's capacity for adsorption. According to Figure 14, The features and mechanism of the adsorbate and adsorbent that interact are frequently investigated using the adsorption isotherm.⁴⁰ Two parametric (Langmuir and Freundlich model) isotherm expressions were applied in the current investigation to investigate the adsorption equilibrium data. Each model's specifics were covered in the following manner:

According to the Freundlich isotherm, adsorption cannot occur beyond monolayer coverage and the capability of a molecule to adsorb in a specified site is independent of the occupation of neighboring. Furthermore, every site can occupy only one adsorbate molecule.⁴¹ This isotherm in the liquid phase is provided by equation (1):

Q_{e} = K_{f} C_{e}^{C}

(1)

Where c = 1 / n_F, n_f is the Freundlich exponent and k_F (mgg⁻¹) is the Freundlich fixed value. The application of equilibrium data in accordance with this isotherm is shown in Figure 15. For a solution with primary concentrations varied from 10 to 30 mgL⁻¹, the k_f values were 7.50, 7.04, and 6.6, respectively, while the C values were 0.3, 0.5, and 0.6.

The Langmuir isotherm, which was successfully used in a number of adsorption procedures, theorizes that sorption takes place within the adsorbent at many homogeneous locations. The physical simplicity of the isotherm is predicated on the following presumptions ²⁶: Monolayer is the only place where adsorption can occur. There is a single solute molecule limit per location. All of the sites on the adsorbent surface are energetically equivalent. The following equation Eq. (2) gives the Langmuir isotherm in its linear form:

(\frac{C_{e}}{q e}) = (\frac{1}{Q_{0} b}) + (\frac{C_{e}}{Q_{0}})

(2)

Where q_e is the equilibrium loading (mgg⁻¹), C_e is the equilibrium concentration (mgL⁻¹), Q₀ is the adsorbent loading (mgg⁻¹), and b is the adsorption energy (Langmuir fixed value mg⁻¹).⁴² Figure 16, depicts the use of equilibrium data according to the Langmuir isotherm. TCFA loading values (Q₀) were (15.3, 35.0, and 53.8) mg/g, respectively, and the Langmuir constant (b) values were (8.2, 0.5, and 0.24) mg⁻¹ for solutions with primary concentrations varied from 10 to 30) mgL⁻¹.

Adsorption isothermal models revealed equilibrium results, which are reported in Table 4. Freundlich and Langmuir isotherms were closely followed, however the later isotherm was more appropriate for the findings. These outcomes are comparable to those seen in the direct dye adsorption in.³⁸

Table 4.

Adsorption isothermal models’ parameters values.

Primary concentration(mgL⁻¹)	Langmuir isothermal			Freundlich isothermal
Primary concentration(mgL⁻¹)	Q₀(mgg⁻¹)	b (1/mg)	R²	K_f (mgg⁻¹)	C	R²
10	15.30	8.2	0.99	11.60	2.85	0.90
20	35.0	0.52	0.94	10.40	1.92	0.94
30	53.8	0.23	0.96	9.90	1.64	0.97

Table 5.

Kinetic models parameters.

Kinetic model parameters	PFO			PSO
Kinetic model parameters	K₁(min⁻¹)	q_e(mgg⁻¹)	R²	K₂(gmg⁻¹ min⁻¹)	q_e(mgg⁻¹)	R²
10 mgL⁻¹	0.068	4.33	0.993	0.031	10.42	0.999
20 mgL⁻¹	0.074	11.63	0.975	0.01	20.49	0.995
30 mgL⁻¹	0.066	18.84	0.981	0.004	31.54	0.983

Adsorption kinetics

The kinetic studies were carried out by testing and validating the samples every 10 min until the remains dye concentration reached constant value in order to study the adsorption mechanism. With primary dye concentrations range from 10 to 30 mgL⁻¹, the kinetic statistics for adsorption process of dye onto TCFA were investigated using the well-known kinetic models, the pseudo first-order model (PFO) and pseudo second-order model (PSO). Figure 17 displays the kinetic models.

For adsorption kinetics investigation, the PFO linear equation was employed. This equation is:

l n (q_{e} - q_{t}) = l n q_{e} – k_{1} t

(3)

where qe is the TCFA equilibrium loading (mgg⁻¹), qt is the TCFA loading at time t (mgg⁻¹) and K1 (min⁻¹) is the PFO model's rate constant. Furthermore, by applying PSO, the adsorption kinetics can be investigated. The PSO linear equation is written as:

(\frac{t}{q_{t}}) = (\frac{1}{k_{2} q_{e}^{2}}) + (\frac{1}{q_{e}}) t

(4)

Where k₂ (gmg⁻¹min⁻¹) is the PSO adsorption equilibrium rate constant. where the amounts of adsorbed adsorbate at equilibrium and at time t, respectively, are denoted by q_e (mgg⁻¹) and q_t (mgg⁻¹).⁴³

The linear graphs of the PFO and PSO models for TCFA are displayed in Figure 17. Table 5 contains a list of kinetic parameters. The adsorption properties of TCFA follow PSO rather than PFO based on the low correlation coefficient for PFO and the high value for PSO. These findings suggested that PSO may accurately anticipate the kinetic process for a primary dye concentration of 10 mgL⁻¹. In comparison to PFO, the value of q_e = 10.42 mgg⁻¹ computed by PSO was closer to the actual q_e = 9.82 mgg⁻¹. PSO can forecast the kinetic process the best for a primary dye concentration of 20 mgL⁻¹. In comparison to PFO, the value of q_e = 20.49 mgg⁻¹ computed by PSO was closer to the actual q_e = 19.64 mgg⁻¹. Consequently, chemisorption is the adsorption mechanism.

Industrial wastewater treatment

These studies were carried out in three runs. The first run lasted for 3 days (the starting period of the filter), the second run lasted about 6 days (first week analysis) and the third run lasted about 6 days (second week analysis). Industrial wastewater was applied in a three stages treatment process, firstly it mixed with an optimal dose of TCFA till reaching the equilibrium conditions. In this period, particles of fly ash showed a high performance in removal of some adsorbates such as complex dyes, degradable organics and others dissolved solids. These adsorbates moved from the liquid boundary layer to the particles surface and then transported from the outer surface into pores and micropores. This adsorbates movement from wastewater to particles outer and inner surface is achieved due to the effect of driving force driven from the difference between adsorbates concentration in liquid and particles surface. When the adsorption equilibrium is reached, the wastewater is applied into a sedimentation stage in order to remove the exhausted TCFA particles from treated wastewater. Finally, the treated wastewater is applied into a filtration stage using a sand filter for removing of TCFA fine particles having a low sedimentation ability and to improve the elimination of other suspended solids.

The studies were conducted in order to describe the performance of the pilot plant with addition of TCFA to each liter of industrial wastewater. The working properties of the pilot plant on dye concentration, COD, BOD₅, TDS and TSS were determined for influent and effluent. Table 6 shows the characteristics of raw industrial wastewater.

Table 6.

Analysis results of industrial textile wastewater.

Pollutant	Second run analysis (six samples)						Third run analysis (six samples)
	1	2	3	4	5	6	1	2	3	4	5	6
Dye concentration (mgL⁻¹)	8.3	25.4	22.9	27	18.2	11.3	19.4	7.7	31	16.7	17.2	25.1
BOD₅ (mgL⁻¹)	89	164	119	181	96	110	156	95	206	172	143	188
COD (mgL⁻¹)	775	1490	1325	2226	1150	910	1330	781	1602	1118	1155	1470
TDS (mgL⁻¹)	986	2970	3660	3045	3482	1860	3040	1165	2631	2840	1711	2450
TSS (mgL⁻¹)	508	180	632	516	650	235	650	161	470	620	240	495
Temperature (°C)	28	30	29	30	31	30	30	29	29	31	28	30

Effect of treatment process on COD removal efficiency

From the second run analysis, it was found that the COD values for influent samples were varied from 775 mgL⁻¹ to 2226 mgL⁻¹ with average value equal to 1312 mgL⁻¹. For effluent samples the COD values were varied from 82 mgL⁻¹ to 442 mgL⁻¹ with average value equal to 207 mgL⁻¹. the average achieved removal efficiency for COD was 85.4% with average TCFA dose 3.3 gL⁻¹. From the third run analysis, it was found that the COD values for influent samples were varied from 781 mgL⁻¹ to 1602 mgL⁻¹ with average value equal to 1241 mgL⁻¹. For effluent samples the COD values were varied from 79 mgL⁻¹ to 291 mgL⁻¹ with average value equal to 213 mgL⁻¹. the average achieved removal efficiency for COD was 83.3% with average TCFA dose 3.2 gL⁻¹. Figure 18, shows the effect of treatment process on COD values and removal efficiency.

Effect of treatment process on BOD₅ reduction

From the second run analysis, it was found that the BOD₅ values for influent samples were varied from 89 mgL⁻¹ to 181 mgL⁻¹with average value equal to 126 mgL⁻¹. For effluent samples the BOD₅ values were varied from 23 mgL⁻¹ to 53 mgL⁻¹with average value equal to 37.7 mgL⁻¹. the average achieved removal efficiency for BOD₅ was 70.2% with average TCFA dose 3.3 gL⁻¹. From the third run analysis, it was found that the BOD₅ values for influent samples were varied from 206 mgL⁻¹ to 65 mgL⁻¹ with average value equal to 159.7 mgL⁻¹. For effluent samples the BOD₅ values were varied from 66 mgL⁻¹ to 22 mgL⁻¹ with average value equal to 47 mgL⁻¹. the average achieved removal efficiency for BOD₅ was 71.2% with average TCFA dose 3.2 gL⁻¹. Figure 19, shows the effect of treatment process on BOD₅ values and removal efficiency.

Effect of treatment process on TDS reduction

From the second run analysis, it was found that the TDS values for influent samples were varied from 986 mgL⁻¹ to 3660 mgL⁻¹ with average value equal to 2667 mgL⁻¹. For effluent samples the TDS values were varied from 106 mgL⁻¹ to 733 mgL⁻¹ with average value equal to 424 mgL⁻¹. the average achieved removal efficiency for TDS was 85.3% with average TCFA dose 3.3 gL⁻¹. From the third run analysis, it was found that the TDS values for influent samples were varied from 1165 mgL⁻¹ to 3045 mgL⁻¹ with average value equal to 2307 mgL⁻¹. For effluent samples the TDS values were varied from 164 mgL⁻¹ to 744 mgL⁻¹with average value equal to 447 mgL⁻¹. the average achieved removal efficiency for TDS was 80% with average TCFA dose 3.2 gL⁻¹. Figure 20, shows the effect of treatment process on TDS values and removal efficiency.

Effect of treatment process on TSS reduction

From the second run analysis, it was found that the TSS values for influent samples were varied from 180 mgL⁻¹ to 650 mgL⁻¹ with average value equal to 453 mgL⁻¹. For effluent samples the TSS values were varied from 53 mgL⁻¹ to 200 mgL⁻¹ with average value equal to 117.8 mgL⁻¹. the average achieved removal efficiency for TSS was 73.7% with average TCFA dose 3.3 gL⁻¹. From the third run analysis, it was found that the TSS values for influent samples were varied from 161 mgL⁻¹ to 650 mgL⁻¹ with average value equal to 439.3 mgL⁻¹. For effluent samples the TSS values were varied from 48 mgL⁻¹ to 171 mgL⁻¹ with average value equal to 116 mgL⁻¹. the average achieved removal efficiency for TSS was 72.3% with average TCFA dose 3.2 gL⁻¹. Figure 21, shows the effect of treatment process on TSS values and removal efficiency.

Effect of treatment process on dye concentration

From the second run analysis, it was found that the concentration values for influent samples were varied from 8.3 mgL⁻¹ to 27 mgL⁻¹ with average value equal to 18.9 mgL⁻¹. For effluent samples the concentration values were varied from 0.78 mgL⁻¹ to 4.12 mgL⁻¹ with average value equal to 2.5 mgL⁻¹. the average achieved removal efficiency for dyes was 87.7% with average TCFA dose 3.3 gL⁻¹. From the third run analysis, it was found that the concentration values for influent samples were varied from 7.7 mgL⁻¹ to 31 mgL⁻¹ with average value equal to 19.5 mgL⁻¹. For effluent samples the concentration values were varied from 0.28 mgL⁻¹ to 5.06 mgL⁻¹with average value equal to 2.9 mgL⁻¹. the average achieved removal efficiency for dyes was 86.6% with average TCFA dose 3.2 gL⁻¹. Figure 22, shows the effect of treatment process on dyes concentration values and removal efficiency.

It is obvious that the BOD₅, COD, TDS, TSS, and color concentrations in textile wastewater can be reduced by the usage of TCFA. Based on the methods used for pollutants adsorption onto absorbents, the adsorptive removal of these contaminants might be determined. Intrinsic adsorption and coulombic interaction are the two basic concepts that can explain the adsorption mechanism. The electrostatic energy of interactions between the adsorbent and adsorbate produces the coulombic effect. It can also be found in the adsorption of cationic and anionic species. Adsorbent particles size has an impact on how the intrinsic adsorption of the adsorbents occur and although intrinsic adsorption mechanism was affected by adsorbent surface areas. Additionally, elements may interact, influencing the adsorption capacity. The ratio of an adsorbent's surface area to mass is known as its specific surface area. It was revealed that the surface area has a significant impact on an adsorbent's capacity. As a result, an increase in specific surface area also increases adsorption capacity. This adsorption capacity's trend might be expressed as BOD_5, COD, TSS, or TDS. Based on solubility and diffusion mechanisms, this trend might be justified. The amount of dissolved oxygen required to degrade the waste's carbonaceous component using microorganisms is known as BOD⁵. This indicates that BOD involves dissolved wastewater organic components. The majority of dissolved contaminants diffuse more quickly than suspended particles, hence they will be adsorbed in shorter time.⁴⁴

Conclusion

The results of CFA chemical modification process using HCl indicated a remarkable increase in its BET surface area. The surface area was increased from 9.6 m²g⁻¹ to 60.4 m²g⁻¹ and also LOI was increased from 36% to 41.5% for RCFA and TCFA respectively. Surface morphology and pore structure were significantly enhanced using acidic treatment. Treated coal fly ash with average particles size 0.95 µm was used efficiently as a low-cost adsorbent for direct blue 78 dye removal from industrial wastewater. It was observed that the removal efficiency of DB78 dye reached 90–100% with TCFA loading (26.5 mg/g) using batch adsorption. Adsorption capacity of TCFA was investigated for solutions with initial concentration 10, 20, 30 mgL⁻¹ with contact time 30 min and TCFA doses 2, 3, 4 gL⁻¹. The adsorption isothermal studies were also conducted, and the results showed that the isotherm class was presented by L shape. By applying Langmuir and Freundlich isotherm models, it was noticed that the adsorption of DB78 dye using TCFA can be fitted to Langmuir model (R² = 0.99). A pilot plant model consists of three stages (mixing, sedimentation and filtration) was used for textile wastewater treatment to study the effect of this treatment process on COD, BOD₅, TDS and TSS removal. The model results showed a decrease in COD, TDS, TSS and dye concentration values from 2226 mgL⁻¹, 3045 mgL⁻¹, 453 mgL⁻¹, and 19 mgL⁻¹ for influent wastewater to 442 mgL⁻¹, 523 mgL⁻¹, 117 mgL⁻¹, and 2.5 mgL⁻¹ respectively.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The researchers would like to acknowledge the assistance provided by the Science and Technology Development Fund (STDF) for funding the project, No. 41902 (Center of Excellence in Membrane- based Water Desalination Technology for Testing and Characterization.

ORCID iD

M Bassyouni

Ahmed Eteba is a teaching assistant in the Civil Engineering Department. He received the bachelor's degree and MSc degree from Port Said University. He has several publications in wastewater treatment technologies using adsorption and membrane technology.

M Bassyouni is a Professor of Chemical Engineering at Port Said University (PSU). He is the Director of Energy Research and Studies Centre at Port Said University. He is the Co-Founder & Chairman of Centre of Excellence for Membrane Testing and Characterization (CEMTC). He is the coordinator of International Ranking Unit at PSU. He completed a postdoctoral at TU-Clausthal, Germany in polymer processing. He holds a PhD in Chemical Engineering, Cairo University and master's degree from TU-Hamburg Harburg. He is a member of editorial board of several journals. He has experience in polymer composites and their applications with emphasis on nanocomposites, bio-composites, and using recyclable materials in favour of environmental sustainability and water treatment. He is the co-author of four books in polymers and petroleum engineering. He serves as a reviewer for several reputable journals. He received the academic of scientific research and technology award for scientific creativity in the field of materials science and its applications (2014). He received award for distinguished scientific contributions in 2015, 2016, and 2017 (KAU). He received a ward of distinguished international publications in 2018–2022 from PSU. He has received the award of the Federation of Arab Scientific Research Councils in the field of water harvesting (2020). He received the award of high citations from PSU (2022). He is a member of Industry Technology Research Council, Ministry of Scientific Research, Egypt. He is a member of the National Committee for New and Advanced Materials, Ministry of Scientific Research.

Mamdouh Saleh is a Professor in the Civil Engineering Department, specializing in sanitary and environmental engineering with an emphasis on wastewater treatment since 2009. His research focuses on using A-B waste activated sludge system for wastewater treatment. He was born in Port Said, Egypt in 1961. He was graduated from Faculty of Engineering, Suez Canal University in 1985. He received his MSc degree in civil engineering from Suez Canal University, 1990. He received the PhD degree in sanitary and environmental engineering in 1994, and completed via a cooperative research program between Suez Canal University, Port Said, Egypt and Aachen University, Aachen, Germany. He has several publications in wastewater treatment techniques.

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Modified coal fly ash for textile dye removal from industrial wastewater

Abstract

Keywords

Introduction

Materials and methods

Adsorbent material

Sources of wastewater

Synthetic wastewater

Industrial wastewater

Characterization of adsorbent material and wastewater

Adsorption studies

Results and discussion

Characterization of adsorbent

Particles size distribution measurements

X-ray diffraction analysis

Energy dispersive x-ray (EDX) analysis

Zeta-potential analysis

Analysis of surface area

Surface morphology

Chemical structure using FTIR analysis

Synthetic wastewater treatment

Effect of TCFA loadings

Effect of adsorption time

Effect of dye primary concentration

Adsorption equilibrium study

Adsorption kinetics

Industrial wastewater treatment

Effect of treatment process on COD removal efficiency

Effect of treatment process on BOD5 reduction

Effect of treatment process on TDS reduction

Effect of treatment process on TSS reduction

Effect of treatment process on dye concentration

Conclusion

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References

Effect of treatment process on BOD₅ reduction