Utilization of Biomass Fly Ash for Improving Quality of Organic Dye-Contaminated Water

Abstract

Development of innovative methodologies to convert biomass ash into useful materials is essential to sustain the growing use of biomass for energy production. Herein, a simple chemical modification approach is employed to functionalize biomass fly ash (BFA) with 3-aminopropyltriethoxy silane (APTES) to develop an inexpensive and efficient adsorbent for water remediation. The amine-functionalized BFA (BFA–APTES) was fully characterized by employing a range of characterization techniques. Adsorption behavior of BFA–APTES was evaluated against two anionic dyes, namely, alizarin red S (ARS) and bromothymol blue (BTB). In the course of experimental data analysis, the computation tools of data fitting for linear and nonlinear form of Langmuir, Freundlich, and the modified Langmuir–Freundlich adsorption isotherms were used with the aid of Matlab R2019b. In order to highlight the misuse of linearization of adsorption models, the sum of the squares of residues (SSE) values obtained from nonlinear models are compared with R² values obtained from the linear regression. The accuracy of the data fitting was checked by the use of SSE as an error function instead of the coefficient of determination, R². The dye adsorption capacity of BFA–APTES was also compared with the nonfunctionalized BFA. The maximum adsorption capacities of BFA–APTES for ARS and BTB dye molecules were calculated to be around 13.42 and 15.44 mg/g, respectively. This value is approximately 2–3 times higher than the pristine BFA. A reasonable agreement between the calculated and experimental values of q_e obtained from the nonlinear form of kinetic models verified the importance of using equations in their original form. The experimentally calculated thermodynamic parameters including molar standard Gibbs free energy (Δ_adG_m⁰) and molar standard enthalpy change (Δ_adH_m) reflected that the process of adsorption of dye molecules on the BFA–APTES adsorbent was spontaneous and exothermic in nature. Moreover, the used BFA–APTES adsorbent could be regenerated and reused for several cycles with significant dye adsorption capacity. The remediation capability of the BFA–APTES adsorbent against ARS dye was also demonstrated by packing a small column filled with the BFA–APTES adsorbent and passing a solution of ARS through it. Overall, we provide a simple and scalable route to convert BFA into an efficient adsorbent for water remediation applications.

1. Introduction

Human population has increased by more than 50% in the past 5 decades, and according to an estimate by World Population Clock, there are about 7.78 billion humans living on planet Earth as of May 2020. Increase in population increases the demand of resources that are necessary to sustain the ever-improving standard of living. Besides increase in the demand of several other resources, energy demand is on a continuous rise. Electricity constitutes an important part of the global energy demand. According to International Energy Agency (IEA), the total gross electricity production was 26,700 TWh in 2018, which is about 3.8% higher than gross energy production during 2017. Despite much debated negative environmental effects associated with the use of coal, it is still the largest contributor to the global electricity generation. Recently, plant biomass is gaining popularity as the thermal source of electricity because of its low cost and availability in large quantities particularly as agricultural residues. In addition, the plant biomass is considered as a renewable and carbon neutral source of electricity because the CO₂ produced during the combustion of plant biomass in thermal power stations can be reused by the plants in the process of photosynthesis to generate more biomass.^1,2

Combustion of coal and biomass in thermal power stations results in the generation of huge amounts of ash residues. It has been conservatively estimated that about 750 million tons of coal ash and about 480 million tons of biomass ash are globally produced every year.³⁻⁸ Major portion of these ash residues ends up in landfills with a potential of inflicting serious environmental risks. Finding recycling technologies to mitigate the environmental impacts of ash residues is a formidable challenge and an interesting opportunity at the same time. With the widespread use of coal as the fuel in thermal power stations, bulk of the research work on management and utilization of ash residues is focused on the ash produced from the combustion of coal. The ash residues produced during the combustion of coal and biomass show markedly different physiochemical natures. Coal ash generally consists of spherical-shaped particles. The chemical composition of coal fly ash depends on the type of coal (bituminous, sub-bituminous, or lignite) used during the combustion process. Based on the chemical composition of the coal ash, the American Society of Testing and Material (ASTM C618) categorizes the coal ash residues into two main classes: Class F and Class C. In the case of Class F ash residues, the combined SiO₂, Fe₂O₃, and Al₂O₃ contents constitute >70%, and these residues are low in lime content. The SiO₂, Fe₂O₃, and Al₂O₃ contents of Class C ash residues are between 50 and 70%, and these residues are rich in lime content.⁹ On the other hand, the biomass ash residues are generally classified as fly ash and bottom ash. The fly ash consists of fine particles that rise with the flue gases, while the bottom ash consists of relative larger particles and are collected at the bottom of the furnace. Both types of biomass ash residues consist of irregular-shaped particles. SiO₂ is the major component of the bottom ash along with the varying concentrations of other constituents including Al₂O₃, Fe₂O₃, CaO, K₂O, Na₂O, and MgO. Fly ash on the other hand can consist of large percentages of plant nutrients including K along with the constituents found in the bottom ash. The chemical composition of fly ash residues shows relatively larger variations in its chemical constituents. The variations in chemical compositions of fly ash residues stem from the fact that biomass thermal power station can use a mixture of different types of agricultural biomasses (e.g., wheat straw, rice husk, and corn stakes to name a few) with different percentage contributions of each type in the actual feed.^8,10

With the growing understanding of the physiochemical natures of the ash residues, the scientific community across the world is investing serious efforts in finding new avenues for recycling of ash residues. Because of their pozzolanic and cementitious properties, coal ash residues have become an attractive material for cement and construction industry. Recent efforts for expanding the application profile of coal fly ash have revealed the potential of coal fly ash residues as functional materials for application in environmental remediation and catalysis. Several studies are available that highlight the efficiency of coal fly ash and materials derived from it (e.g., organocomposites and porous materials including zeolites, mesoporous silica) as adsorbents for removal of dyes, toxic metal ions, radioactive pollutants, organic pollutants such as benzene, toluene, and o- and p-xylene, gaseous pollutants such as CO₂, H₂S, and SO₂ as well as support material for catalysts, photocatalysts, and electrocatalysts.^6,11−20 Because of their distinct chemical nature, biomass ash residues do not show pozzolanic and cementitious properties; however, the potential of biomass ash as part of cement mortar²¹⁻²⁶ is widely investigated. Besides, the potential of biomass fly ash (BFA) for other applications is also being explored. Reports from Guo et al. and Rafael López et al. have recently demonstrated the potential of BFA for capturing of CO₂.^27,28 In a related effort, Fernández-Delgado Juárez et al. successfully applied wood ash for removal of CO₂ and H₂S and purification of biogas.²⁹ In another study, BFA was applied as the adsorbent to remove lignin from effluents generated during the kraft pulping process. The fly ash adsorbent was also found to reduce the chemical oxygen demand and turbidity of effluents.³⁰ In the same vein, Novais et al. have demonstrated the efficiency of BFA-derived geopolymer spheres and monoliths for removal of methylene blue from aqueous solutions.^31,32

It is worth mentioning here that majority of the reported work do not consider rationally controlling the surface chemical properties of ash residues to modulate their remediation properties. In the context of application of BFA for capturing of CO₂, attempts have been made to introduce amine groups by the simple wet impregnation method.^33,34 The wet impregnation method is a convenient way of introducing certain functional groups in ash residues; however, the application of materials developed through this method is limited to capturing of gaseous pollutants. Surface chemical properties of materials can be conveniently and robustly controlled by anchoring appropriate organosilanes on their surface, a process designated as silanization.³⁵⁻³⁷ In some studies, organosilanes have been applied to modify the surface chemical nature of coal fly ash for their better integration as fillers in composite materials.³⁸⁻⁴⁰ However, to the best of our knowledge, no attempts have been made to apply the process of silanization for imparting specific functional group on the surface of BFA and apply them for remediation of dye-contaminated aqueous solutions. Herein, we report a convenient silanization method for anchoring amine groups on the surface of BFA by using 3-aminopropyltriethoxysilane (APTES). The resulting amine-functionalized BFA was applied as the adsorbent for remediation of dye-contaminated water solutions. The remediation characteristics of amine-functionalized fly ash were compared with the pristine fly ash. Special attention is given to emphasize the importance of application of isotherm models in the nonlinear form and correct statistical method (usage of SSE instead of R²) as a corrective measure for adsorption studies because linearization may cause malpractice.

2. Results and Discussion

Scheme 1 depicts the surface chemistry of BFA surface functionalization during activation and APTES modification. According to BET analysis, the specific surface areas of the A-BFA and BFA–APTES were calculated to be 95.72 and 8.15 m² g^–1, while pore volumes were 0.071 and 0.015 cm³ g^–1 and pore diameters were 1.697 and 1.061 nm, respectively.

Scheme 1. Schematic Illustration of the Surface Functionalization of BFA with APTES.

The scanning electron microscopy (SEM) imaging was used to reveal the morphology of A-BFA before and after functionalization with APTES (Figure 1). A-BFA and BFA–APTES displayed heterogeneous surface characteristics with macropores (diameter >50 nm) and particles with a range of different sizes (from 20 to 500 nm) and morphologies. Even some of the particles were found to be mesoporous in nature. Micropore size was not affected by surface treatment (Figure 1a,d), while there was almost one order of magnitude decrease in the particle size (Figure 1c,f). This drastic reduction in particle size may influence the kinetic parameters of adsorption.⁴¹

Figure 1.

SEM images revealing particles of variety of different sizes and morphologies in A-BFA (a–c) and BFA–APTES (d–f).

The surface chemical composition of A-BFA before and after silanization was monitored by X-ray photoelectron spectroscopy (XPS) analysis (Figure 2). The survey scan of A-BFA and BFA–APTES showed signals at 150 and 100 eV, which correspond to the binding energies of Si 2s and Si 2p orbitals of the Si component of BFA. The signals for the C 1s and O 1s orbitals of the carbon and oxygen contents of A-BFA and BFA–APTES were evidenced by the signals at 284 and 530 eV. The C 1s signal in the survey scan of A-BFA can be attributed to the unburnt carbon component of BFA. The appearance of the N 1s orbital signal at 400 eV in the XPS survey scan of BFA–APTES validated the functionalization of BFA with APTES. The survey scan of A-BFA did not show any signal for nitrogen, confirming that the N 1s signal in the survey scan of BFA–APTES is originating from the amino groups decorated on the surface of BFA during the process of silanization with APTES.

Figure 2.

XPS survey scans of (a) A-BFA and (b) APTES-functionalized BFA.

Thermogravimetric analysis (TGA) was performed to quantify the extent of surface functionalization of A-BFA with APTES (Figure 3). Both ash residues were dried in an oven before subjecting to TGA. The 7.2% total weight loss observed in the case of A-BFA can be attributed to the loss of any absorbed water molecules and the unburnt carbon content of A-BFA. The TGA thermogram of BFA–APTES showed a total weight loss of 9.4%, which is 2.2% higher than the weight loss observed in the case of A-BFA. The additional weight loss observed in the case of BFA–APTES can be considered as the amount of APTES functionalized on the surface of A-BFA. From this analysis, it can be inferred that every 100 g of the BFA–APTES sample contains 2.2 g of APTES functionality in it. Furthermore, it can be concluded that each portion of 100 g of BFA–APTES offers 0.033 moles of −NH₂ groups. The percentage weight loss after functionalization with silane monolayers is generally in the range of 1–2 wt %. Comparing with our previously reported work,^42,43 the weight loss of 2.2 wt % observed in this study is reasonably reliable. Combining TGA and BET (8.15 m²/g) data suggests that 2.7 mg (1.22 × 10^–5 mole) of APTES is spread per m² of the adsorbent, which translates into 7.35 × 10¹⁸ molecules of APTES grafted per m² of BFA–APTES.

Figure 3.

TGA of A-BFA and APTES-functionalized BFA (BFA–APTES).

After successful functionalization of BFA with APTES, supported by the XPS surface chemical analysis and TGA, the adsorption properties of A-BFA and BFA–APTES were investigated against two anionic dyes, namely, alizarin red S (ARS) and bromothymol blue (BTB) (Figure 4). ARS, anthraquinone dye, is used as a staining agent in textile industries.⁴⁴ In addition, this dye is also used to stain biological specimens, such as mineralized bones in vertebrate groups and small invertebrate embryos.⁴⁵ ARS cannot be completely degraded by general chemical, physical, and biological processes.⁴⁶ This resistance to degradation is attributed to the complex structures of the aromatic rings that afford high physicochemical, thermal, and optical stability.⁴⁷ BTB, a triphenylmethane dye, is also frequently used for dyeing in the textile industry.⁴⁸ It has the potential of causing damage to lungs and mucous membranes.⁴⁹ Prolonged exposures to BTB-contaminated water may even lead to organ damage.⁵⁰ A review of the recent literature shows that BTB has been employed in the development of pH sensors as well.^51,52 These considerations laid the basis of our choice of ARS and BTB as model dye contaminants.

Figure 4.

Chemical structures of anionic dyes: (a) ARS (b) BTB.

2.1. Effect of pH

The nature and magnitude of overall electrostatic charge on the interacting adsorbent and adsorbate system determine the capacity and efficiency of the process of remediation. In case of materials bearing functional groups (e.g., amines, carboxylic acid, sulfonic acid, phosphonic acid, or zwitterionic groups) that can undergo protonation or deportation, a change in the pH of the surrounding medium can lead to a change in the nature and magnitude of the electrostatic charge on the material.⁵³⁻⁵⁵ Consequently, the change in pH of the medium can impact the capacity and efficiency of the process of remediation, which makes it necessary to evaluate the impact of pH of the medium on the capacity and efficiency of the process of remediation. The effect of pH on the adsorption capacity of A-BFA and BFA–APTES as adsorbent toward the employed dyes was determined by performing adsorption studies in the pH range of 2–12 (Figure 5). The pH of dye solutions was adjusted by using 0.1 M aqueous sodium hydroxide and 0.1 M aqueous hydrochloric acid solutions. Twenty milligrams of A-BFA and BFA–APTES adsorbents was separately added to 10 mL of dye solutions (15 ppm for ARS and 10 ppm for BTB) and shaken at room temperature for an optimized period of time (t = 10 min for ARS, and t = 5 min for BTB). The adsorption capacity of BFA–APTES was found to increase as the pH of the solutions was increased from 2, and the maximum adsorption against both the dyes was obtained at pH 4 (percentage removal: 91% for ARS and 96% for BTB). However, a further increase in the pH of the medium resulted in a decrease in the adsorption capacity. For instance, the adsorption of BTB on BFA–APTES showed an abrupt reduction in adsorption with increasing pH, from 96% (pH 4) to 5% removal (pH 8) (Figure 5b). The high adsorption capacity of BFA–APTES at acidic pH can be attributed to the higher magnitude of positive charge on its surface because of the protonation of surface amino groups. Hence, the positive charge on the surface of BFA–APTES at acidic pH facilitated in enhancing its electrostatic interaction with the polar groups of the anionic dyes ARS (pK_a = 5.49 and 10.85) and BTB (pK_a = 7.3). A slightly lower percentage removal observed at pH 2 compared to pH 4 can be attributed to a higher extent of protonation of sulfonic acid groups resulting in the reduction in net negative change on the dye molecules that led to a reduced electrostatic interaction between BFA–APTES and the dye molecules. Decrease in the adsorption capacities at pH 6 and above can be related to the decrease in the extent of protonation of amino groups, leading to the decrease in the magnitude of surface positive change on BFA–APTES and increase in the extent of the deprotonation of sulfonic acid groups that leads to an increased negative charge on the dye molecules. The deprotonation of functionalities at higher pH results in an increase in the electrostatic repulsion between the exposed lone pair of electrons of amino groups on BFA–APTES surface and negative charge on the dye molecules, which manifests in the form of decrease in the adsorption capacity of BFA–APTES. Performing the adsorption studies at pH 4 would protonate all the functional groups of the anionic dyes. This suggests that the adsorption process seems to be primarily driven by the extent interaction between the protonated surface amino groups of BFA–APTES and polar groups present in the dye molecules.

Figure 5.

Effect of pH on the %age removal of (a) ARS (t = 10 min, amount of adsorbents = 20 mg, at room temperature); (b) BTB (t = 5 min, amount of adsorbents = 20 mg, at room temperature).

The adsorption behavior of A-BFA against both dyes was completely different. A-BFA is microporous and contain many −OH groups on the surface. It is well known that the free vibrating −OH groups are strong adsorption sites, particularly with lone pair adsorbates (such as SO₃, O= and Br groups).⁵⁶ Each −OH group acts as a specific adsorption site for aromatic groups of dye molecules resulting in some degree of adsorption (40–45%) at pH between 2 and 6 for both dyes. However, small adsorption of ARS at higher pH (>7) may be because of nonspecific adsorption into mesopores. There is also the capillary effect. Because BTB is a relatively large molecule, it might not reach the mesopores; hence, a very limited adsorption was observed at higher pH values. A comparison of the adsorption capacities of BFA–APTES and A-BFA clearly highlights superior adsorption capacity for BFA–APTES (Figure 5), which stems from the lack of any amino groups on the surface of nonfunctionalized BFA.

2.2. Effect of Amount of Adsorbent

The effect of amount of adsorbent on the removal capacity of dye was investigated by varying the amounts of A-BFA and BFA–APTES adsorbents (5, 10, 15, 20, 25, and 30 mg). For this purpose, specific amounts of adsorbents were added in 10 mL dye solutions (15 ppm for ARS and 10 ppm for BTB) at room temperature with optimized pH and contact time (pH = 4, t = 10 min for and pH = 4, t = 5 min for BTB). For both dyes, BFA–APTES exhibited higher adsorption capacity as compared to the A-BFA (Figure 6). For 20 mg of adsorbent, the %age removal of ARS dye was found to be 91%, when BFA–APTES was used as the absorbent, which is ∼2.5 times higher than the % age removal of 37%, when nonfunctionalized BFA was used as the adsorbent. Similarly, employing 20 mg of adsorbent, the BFA–APTES adsorbent showed ∼2.9 times higher %age removal toward BTB dye (96% removal) when compared to the nonfunctionalized BFA (33% removal) employed as an adsorbent. Compared to the nonfunctionalized BFA, the superior adsorption capacity of BFA–APTES can be attributed to the presence of amino groups present on the surface of BFA–APTES. Although a slight increase in the percentage adsorptions was observed at adsorbent amounts higher than 20 mg, the subsequent adsorption studies were carried out using 20 mg of adsorbents for the sake of simplicity and process efficiency.

Figure 6.

Effect of the amount of adsorbent on the % age removal of (a) ARS (pH = 4, t = 10 min, at room temperature) and (b) BTB (pH = 4, t = 5 min, at room temperature). The solid lines are drawn to highlight the trend.

2.3. Effect of Contact Time

In order to determine the optimum time required for the maximum uptake of dye by the BFA–APTES adsorbent, the effect of contact time on the adsorption of dyes was studied for different time intervals ranging from 5 to 30 min (Figure 7). Twenty milligrams of A-BFA and BFA–APTES adsorbents were added to 10 mL of aqueous solutions of dyes (15 ppm for ARS and 10 ppm for BTB), and the suspension was shaken at room temperature for a set interval of time. With the increase in contact time, an increase in the uptake of dyes was observed for both adsorbents. Percentage adsorption of ARS was observed to be around 91% for BFA–APTES and 37% for BFA after a contact time of 10 min, whereas the percentage adsorption of BTB reached 96% for BFA–APTES and 33% for A-BFA after 5 min of exposure to the respective adsorbents. A further increase in the contact time did not result in any appreciable increase in the removal percentage, and for the sake of process simplicity and efficiency, all the subsequent adsorption studies were performed using contact times of 10 min for ARS and 5 min for BTB.

Figure 7.

Effect of contact time on the % age removal of (a) ARS (adsorbents = 20 mg, pH = 4 at room temperature); (b) BTB (amount of adsorbents = 20 mg, pH = 4 at room temperature). The solid lines are drawn to highlight the trend.

2.4. Effect of Initial Concentration of Dyes

A given amount of adsorbent has the capacity to adsorb only a certain amount of adsorbate species. Thus, the initial concentration of the adsorbate solution plays an important in the adsorption process. To investigate the effect of initial concentration of dyes on the removal capability of adsorbents, the adsorption experiments were carried out at different initial concentrations of dyes (ARS and BTB). Twenty milligrams of A-BFA and BFA–APTES adsorbents were added to 10 mL of dyes solutions (10, 20, 30, 40, and 50 mg L^–1), and the suspensions were shaken at room temperature for an optimized period of time (t = 10 min for ARS, and t = 5 min for BTB). As apparent from Figure 8, the % age removal of ARS and BTB by both the adsorbents decreased with increasing the initial concentration of dyes. This trend can be attributed to the saturation of the active sites for adsorption on the adsorbent surface at higher concentrations of dyes.

Figure 8.

Effect of initial concentration of (a) ARS (adsorbents = 20 mg, pH = 4, contact time = 10 min, at room temperature); (b) BTB (amount of adsorbents = 20 mg, pH = 4, contact time = 5 min, at room temperature) on the % removal by A-BFA and BFA–APTES adsorbents. The solid lines are drawn to highlight the trend.

2.5. Effect of Temperature

Influence of temperature on the adsorption properties of A-BFA and BFA–APTES as absorbents was investigated at different temperatures (298, 308, 318, 328, and 338 K). Both the adsorbents (20 mg adsorbent, pH = 4, t = 10 min for ARS, and 20 mg adsorbent, pH = 4, t = 5 min for BTB) were added to 10 mL of aqueous dye solutions (15 ppm for ARS and 10 ppm for BTB). As it is evident from Figure 9, an increase in temperature within the temperature range studied did not have a significant impact on the adsorption capacities of BTB on both A-BFA and BFA–APTES and ARS on BFA–APTES. Weakening of electrostatic interactions between dye molecules and active adsorbent sites at higher temperature could be the main contributor toward the slight decrease in adsorption capacity observed at higher temperatures.⁵⁷ A slight increase observed in the case of adsorption of ARS on A-BFA can be because of a better dispersion of the adsorbent and the availability of more binding sites at higher temperature.⁵⁸ The change in the adsorption of ARS on BFA–APTES remains within the experimental error limits.

Figure 9.

Effect of temperature on the parentage removal of (a) ARS (amount of adsorbents = 20 mg, pH = 4, contact time = 10 min); (b) BTB (amount of adsorbents = 20 mg, pH = 4, contact time = 5 min). The solid lines are drawn to highlight the trend.

2.6. Adsorption Isotherms

Figure 10 shows both experimental data and nonlinear curve fits for two-parameter and three-parameter models. A comparison of the three isotherms based on SSE values (Table 1) showed that the adsorption of ARS and BTB on BFA–APTES and A-BFA is better estimated by the modified Langmuir–Freundlich (MLF) isotherm. However, R² values of the linearized Langmuir isotherm are 0.981 and 0.998, while SSE values are 4.792 and 2.548 for ARS and BTB adsorption on BFA–APTES, respectively. Consequently, comparison between Tables 1 and 2 clearly shows that the use of the R² value can be misleading in determining the correct isotherm. Besides, adsorption data of large dye molecules cannot fit to Langmuir because impediments exist between pores and adsorbate; therefore, the value of n is usually less than 1.⁵⁹ The maximum adsorption capacities of BFA–APTES for ARS and BTB dye molecules were calculated as 13.42 and 15.44 mg/g, respectively. Meanwhile, the maximum adsorption capacities of A-BFA were 2–3 times less than BFA–APTES for both dyes (Table 1).

Figure 10.

Adsorption isotherms and nonlinear curve fits of Langmuir (black solid line), Freundlich (blue dash line), and MLF (black dots) for ARS-BFA APTES (red circle solid), ARS-A BFA (dark blue hexagon solid), BTB–BFA APTES (orange triangle up solid), and BTB–ABFA (green tilted square solid), respectively.

Table 1. Adsorption Parameters Obtained from Nonlinear Langmuir, Freundlich, and MLF Isotherm Models at Room Temperature for the Adsorption of ARS and BTB on A-BFA and BFA–APTES.

alizarin red S				bromothymol blue
	parameters	A-BFA	BFA–APTES		parameters	A-BFA	BFA–APTES
Langmuir	qmax (mg/g)	4.739	13.417	Langmuir	qmax (mg/g)	4.509	10.097
	RL	0.495	0.439		RL	0.603	0.008
	SSE	2.047	4.792		SSE	1.532	2.549
Freundlich	Kf (mg/g)	1.211	2.888	Freundlich	Kf (mg/g)	0.681	7.226
	1/n	0.309	0.385		1/n	0.429	0.128
	SSE	2.511	7.001		SSE	1.611	0.267
MLF	KMLF (L/g)	0.008	0.0011	MLF	KMLF (L/g)	0.060	0.946
	qmon (mg/g)	3.753	9.793		qmon (mg/g)	4.300	15.442
	1/n	0.262	0.261		1/n	0.931	0.274
	SSE	1.326	0.766		SSE	1.525	0.252

Table 2. Adsorption Parameters Obtained from Linearized Langmuir, Freundlich, and MLF Isotherm Models at Room Temperature for the Adsorption of ARS and BTB on A-BFA and BFA–APTES.

alizarin red S				bromothymol blue
	parameters	BFA	BFA–APTES		parameters	BFA	BFA–APTES
Langmuir	qmax (mg/g)	3.762	13.791	Langmuir	qmax (mg/g)	4.590	10.761
	RL	0.364	0.511		RL	0.640	0.042
	R2	0.720	0.981		R2	0.712	0.998
Freundlich	Kf (mg/g)	0.601	1.663	Freundlich	Kf (mg/g)	0.595	6.761
	1/n	0.518	0.582		1/n	0.467	0.117
	R2	0.863	0.782		R2	0.563	0.954
MLF	KMLF (L/mg)	0.191	0.790	MLF	KMLF (L/g)	1.378	28.506
	qmon (mg/g)	407.498	4911.640		qmon (mg/g)	0.737	10.462
	n	4.633	2.678		n	0.584	0.992
	R2	0.963	0.839		R2	0.442	0.997

For the sake of comparison, adsorption characteristics of the adsorbents reported in this study were compared with the related adsorbents reported in the literature. Table 3 gives a simple comparison of the adsorption ability of fly ash-derived adsorbent materials for the adsorption of dyes. It is evident from Table 3 that the adsorbents reported in this study exhibited higher adsorption efficiency compared to similar adsorbents, while the adsorption capacities of the absorbents reported in this study were comparable to some of the adsorbents reported in the literature.

Table 3. Comparison of Adsorption Characteristics of Adsorbents Derived from Various Fly Ash-Based Adsorbents.

sr. no	adsorbents	dyes	adsorption capacity qmax (mg/g)	time	references
1	coal fly ash	reactive red 23, reactive blue 171, acid black 1, and acid blue 193	2.102, 1.860, 10.331, and 10.937	60 min	(60)
2	coal fly ash	methylene blue	15.04	120 min	(61)
3	magnetic chitosan-fly ash (CS-FA/Fe3O4)	reactive orange 16 (RO16)	66.9	55 min	(62)
4	coal fly ash (CFA)	methylene blue	6.409	60 min	(63)
5	fly ash (FA) modified by Ca(OH)2/Na2FeO4	methyl orange	14.76	40 min	(64)
6	fly ash/NiFe2O4 composite	congo red	23.33	180 min	(65)
7	fly ash geopolymer monoliths	methylene blue	15.4	30 h	(66)
8	magnesium oxide (MgO)/fly ash composite (FAMgO)	RB5 azo dye	48.78	90 min	(67)
9	BFA–APTES	bromothymol blue	15.44	5 min	current Study
10	BFA–APTES	alizarin red S	13.42	10 min	current Study

The adsorption density (Γ, mg/m²) or the amount of dyes adsorbed on the active surface of adsorbents at a particular concentration was calculated by using following equation⁶⁸

1

where V is the volume of liquid phase (L), ΔC is the difference between the initial and final concentrations of adsorbates in aqueous solution, m is the mass of adsorbent (g), and S is the surface area (m²/g). Using this relation, the adsorption density or the amount of dyes adsorbed on the active surface of adsorbents at a particular residual concentration can be calculated. The adsorption density of ARS and BTB per unit surface area of APTES–BFA was found to be 0.306 and 0.608 mg/m². Interestingly, these grafting densities translate into 5.11 × 10¹⁷ and 6.56 × 10¹⁷ molecules of ARS and BTB per m² of the adsorbent (BFA–APTES). The number of dye molecules per unit area are slightly less than the number of APTES molecules that were estimated to be grafted on the surface (7.35 × 10¹⁸ molecules) which can be because of the steric bulk of the dye molecules that need more surface area while adsorbing on the surface. With this data, we can conclude that the surface of the adsorbent is reasonably saturated.

Table 4 gives a comparison of the amount of adsorbent adsorbed per unit surface area of different adsorbent materials reported in the literature and the adsorbent reported in this study.

Table 4. Comparison of Adsorption Densities of Various Adsorbents Reported in the Literature.

sr. no.	adsorbents	adsorbates	adsorption density Γ	references
1	mesoporous alumina	As(III), As(V)	1.94 × 10–6, 5.85×10–6 mol/m2	(68)
2	mesoporous Alumina	ammonia	0.44 mg/m2	(69)
3	γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite	remazol navy RGB azo dye	1.30 mg/m2	(70)
4	sepiolite	Co(II)	1:17 × 10–6 mol/m2	(71)
5	BFA–APTES	bromothymol blue	0.608 mg/m2	current study
6	BFA–APTES	alizarin red S	0.306 mg/m2	current study

2.7. Adsorption Kinetics

Adsorption rate constants were calculated by measuring instantaneous adsorption capacity (q_t) as a function of time. A comparison of linear and nonlinear forms of pseudo-first- and pseudo-second-order kinetics models is graphically demonstrated in Figure 11 for ARS adsorption on BFA–APTES, while the kinetic rate constants, SSE, and correlation coefficient (R²) of both models for all the samples are summarized in Tables 5 and 6. It is clear that the nonlinear analysis produced the best curve fits, providing estimations that were the closest to the experimental data. A reasonable agreement between the calculated and experimental values of q_e obtained from the nonlinear form of kinetic models verifies the importance of using equations in their original form.⁵⁹ Adsorption of both dyes on BFA–APTES has the fastest rates (k₂ = 1.395 g/mg min for BTB and k₂ = 0.518 g/mg min for ARS).

Figure 11.

Comparison of adsorption kinetics estimated by the nonlinear curve fits (black solid line for pseudo-first-order and dashed blue line for pseudo-second-order kinetics) and linear curve fits (green dotted line for pseudo-first-order and orange dashdotted line for pseudo-second-order kinetics) for the adsorption of ARS–BFA APTES (red circle solid).

Table 5. Nonlinear Pseudo-First-Order and Pseudo-Second-Order Kinetics Analysis Results for the Adsorption of ARS and BTB on A-BFA and BFA–APTES.

alizarin red S				bromothymol blue
	parameters	A-BFA	BFA–APTES		parameters	A-BFA	BFA–APTES
pseudo-first-order	qe(mg/g) experimental	1.339	5.296	pseudo-first-order	qe(mg/g) experimental	4.316	10.140
	qe (mg/g) calculated	1.964	5.194		qe (mg/g) calculated	3.998	10.033
	k1 (min–1)	0.325	0.369		k1 (min–1)	0.604	1.246
	SSE	0.271	0.033		SSE	0.032	0.042
pseudo-second-order	qe (mg/g) experimental	1.339	5.296	pseudo-second-order	qe (mg/g) experimental	4.316	10.140
	qe (mg/g) calculated	2.023	5.516		qe (mg/g) calculated	4.107	10.088
	k2 (g/mg min)	0.518	0.148		k2 (g/mg min)	0.544	1.395
	SSE	0.339	2.896		SSE	0.076	0.049

Table 6. Linearized Pseudo-First-Order and Pseudo-Second-Order Kinetics Analysis Results for the Adsorption of ARS and BTB on A-BFA and BFA–APTES.

alizarin red S				bromothymol blue
	parameters	A-BFA	BFA–APTES		parameters	A-BFA	BFA–APTES
pseudo-first-order	qe (mg/g) experimental	1.339	5.296	pseudo-first-order	qe (mg/g) experimental	4.316	10.140
	qe (mg/g) calculated	0.491	0.312		qe (mg/g) calculated	0.644	0.033
	k1 (min–1)	0.020	0.020		k1 (min–1)	0.020	0.152
	R2	0.05	0.27		R2	0.19	0.34
pseudo-second-order	qe (mg/g) experimental	1.339	5.296	pseudo-second-order	qe (mg/g) experimental	4.316	10.140
	qe (mg/g) calculated	1.114	5.520		qe (mg/g) calculated	4.440	10.181
	k2 (g/mg min)	0.122	0.153		k2 (g/mg min)	0.223	0.474
	R2	0.94	0.99		R2	0.98	0.99

The difference between the constants (k₁, k₂, and q_e) obtained from linear and nonlinear kinetic models appear to be very clear between Tables 5 and 6.

After this step, kinetic constants with the lowest SSE value will be used in our calculations. We assume that adsorption kinetics follows the Arrhenius type behavior.

2

and

3

The adsorption molar enthalpy (Δ_adH_m⁰) is calculated for both dyes on A-BFA and BFA–APTES from the nonlinear curve fitted plots of reaction rate constant (k) versus temperature (Table 7).

Table 7. Thermodynamic Parameters for the Adsorption of ARS and BTB on BFA and BFA-APTES.

alizarin red S			bromothymol blue
parameters	A-BFA	BFA–APTES	parameters	A-BFA	BFA–APTES
ΔG° (kJ/mol)	5.839	–1.300	ΔG° (kJ/mol)	3.787	–11.647
ΔH° (kJ/mol)	–21.948	–13.393	ΔH° (kJ/mol)	–10.585	–87.817
ΔS° (kJ/mol.K)	–0.093	–0.042	ΔS° (kJ/mol.K)	–0.048	–0.256

2.8. Adsorption Thermodynamics

The thermodynamic parameters such as adsorption molar Gibbs free energy (Δ_adG_m⁰), adsorption molar enthalpy (Δ_adH_m), and adsorption molar entropy (Δ_adS_m⁰) were studied to investigate the effect of temperature on the adsorption process at pH 4.^72,73 The magnitude of Δ_adG_m was calculated from following equation

4

where K is the equilibrium constant (K = k_ads/k_des), T is the absolute temperature (K), and R is the universal gas constant (8.314 J/mol K).

The adsorption molar enthalpy Δ_adH_m⁰ and Δ_adS_m were obtained from Van’t Hoff relation

5

The calculated thermodynamic parameters are summarized in Table 7. A study of temperature dependence of the adsorption process gives information on the spontaneity of the adsorbent–adsorbate interaction.⁷⁴ The ΔG⁰ values in the range of 0 to −20 and −80 to −400 kJ mol^–1 indicate physical and chemical adsorption processes, respectively.⁷⁵ The negative values of Δ_adG_m⁰ observed in our study suggested a spontaneous nature of the process of adsorption of dyes on BFA–APTES, whereas a positive Δ_adG_m was calculated for A-BFA, which indicated unfavorable adsorption on this adsorbent. The negative values of Δ_adH_m⁰ indicated the exothermic nature of adsorption processes.

It is worth mentioning here that the temperature range of 298–338 K was applied in this study to demonstrate the adsorption behavior of the reported adsorbent under the conditions that can be conveniently applied at large scales. In addition, the 298–338 K temperature range is widely employed by other researchers, which would make it convenient to draw comparisons between different studies.⁷⁶⁻⁷⁹

2.9. Regeneration and Reusability

Regeneration of the adsorbent after the adsorption process is important for its reuse especially for increasing the economic feasibility of the process at commercial scale. Different eluting agents including NaOH, HCl, methanol, acetic acid, and a mixture of methanol and acetic acid were employed to regenerate the adsorbent for reuse.⁸⁰ The desorption of dyes was carried out by separately washing the ARS (t = 10 min, T = 298 K) and BTB (t = 5 min, T = 298 K) loaded adsorbents with different eluting agents, 10 mL of 1 M NaOH, 1 M HCl, methanol, 1 M acetic acid, and a mixture of acetic acid and methanol (1:1). The concentrations of desorbed dye in solutions were spectrophotometrically quantified. The best desorption performance was achieved with 1 M NaOH solution for ARS (89%) and BTB (79%) (Figure 12). The superior desorption performance of NaOH highlights that the interaction between the BFA–APTES adsorbent and dyes (ARS and BTB) is electrostatic in nature with high dependency on the pH of the medium. This observation complements our results provided in the previous section of this study dealing with the effect of pH on adsorption capacity of the reported adsorbent.

Figure 12.

Effect of different eluting agents on desorption of dyes (a) ARS (amount of adsorbent = 20 mg, t = 10 min, at room temperature) and (b) BTB (amount of adsorbents = 20 mg, t = 5 min, at room temperature). The solid lines are drawn to highlight the trend.

After successful demonstration of regeneration of adsorbents, their reusability was also studied. For the regeneration of adsorbents, the dye-loaded A-BFA and BFA–APTES were separately dispersed in 10 mL of 1 M NaOH solution under optimized conditions (for ARS t = 10 min, at room temperature and for BTB t = 5 min, at room temperature). After desorption, adsorbents were washed with deionized water, dried at 80 °C, and reused for adsorption in the next cycle. Meanwhile, the concentrations of the released dyes in the supernatants were determined spectrophotometrically. To validate the reusability of A-BFA and BFA–APTES, several cycles of consecutive adsorption–desorption were carried out. The results displayed in Figure 13 showed that the BFA–APTES showed more than 60% removal for up to 5 cycles for ARS and for up to 6 cycles for BTB.

Figure 13.

Reusability of A-BFA and BFA–APTES for the adsorption of (a) ARS (amount of adsorbent = 20 mg, pH = 4, t = 10 min, at room temperature) and (b) BTB (amount of adsorbent = 20 mg, pH = 4, t = 5 min, at room temperature). The solid lines are guide to the eye.

As it can be noticed from this study, BFA by its nature is a poor adsorbent; however, it can be modified to achieve a better performance.⁸¹ Silanization is a low-cost and effective covalent coating method to modify material surfaces. A number of silane-coupling agents are commercially available, which can conveniently introduce a variety of functional groups (e.g., amino group and carboxyl group) on diverse surfaces presenting hydroxyl groups.⁸² APTES, the silane employed in this study, is the most widely used silane employed for the surface functionalization to achieve materials displaying amino groups on their surfaces.⁸³ Thus, in the present work, the development of the adsorbent involves inexpensive raw materials (BFA and APTES) and simple functionalization process. The abundance of these materials, low cost, modest processing, and reasonable adsorption ability make our reported adsorbent (APTES–BFA) an attractive platform from sustainability as well as the economic point of view. It is worth mentioning here that considering the low cost related to the materials and the process applied in this study, reusability might be of limited importance; however, we believe that the reusability experiment presented here is of high scientific interest for the scientific community which is working on the development of cost-effective adsorbents for remediation applications.

2.10. Column Studies

BFA–APTES was chosen as the model adsorbent in batch experiments because of its aforementioned superior adsorption characteristics. A small column of the BFA–APTES adsorbent is packed to demonstrate its remediation properties in column setting. A simple glass tube (pasture pipette) with a diameter of 6 mm and a length of 22 mm is used for this purpose. A small plug of glass wool was placed at the bottom of the column (Figure 14) to prevent washing out of the adsorbent. Each adsorbent (500 mg) (A-BFA and BFA–APTES) was added to two separate columns followed by the gentle tapping of the columns to consolidate the adsorbents and remove any air bubble. ARS dye solution was chosen as an adsorbate because of its ease of visual following with 200 ppm concentration (Figure 14). It was observed that the column packed with the BFA–APTES adsorbent could remove 99% of dye (ARS) from the 15 mL of 200 ppm dye solution passed through it. The water coming out of the column packed with the BFA–APTES adsorbent was clear without any visible trace of dye in it that was also confirmed spectrophotometrically (please refer to the video provided in Supporting Information). On the other hand, red-colored water can be clearly seen coming out of the column packed with A-BFA. It is worth mentioning here that the aqueous solution of ARS dye is yellowish in color under acidic and neutral conditions. Because of the pH responsiveness, ARS shows red color under basic conditions. The basic nature of A-BFA led to the change in color of ARS dye to red while it passed through the column packed with the A-BFA.

Figure 14.

Photograph of columns packed with the A-BFA and BFA–APTES adsorbents. 200 ppm aqueous solution of ARS dye was passed through both the columns. The water coming out of the columns was collected in glass vial for spectrophotometric estimation of the dye content.

3. Conclusions

In summary, we presented silanization as a facile route for the functionalization of pristine BFA to prepare efficient adsorbents for the adsorptive removal of anionic dyes (ARS and BTB). We simply employed APTES to prepare amine-functionalized BFA (BFA–APTES). The effectiveness of the employed strategy for the fabrication of BFA–APTES and the physiochemical properties of the resulting material were established by employing XPS, SEM, and TGA. The prepared BFA–APTES was applied as an adsorbent for the adsorptive removal of anionic dyes (ARS and BTB). The adsorption behavior of A-BFA and BFA–APTES was quantified and compared with linear and nonlinear versions of isotherm and kinetic models. Our comparative calculations have shown that linearizing adsorption equations can be misleading and incomplete. The nonlinear analysis on experimental data is the most accurate calculation method, and it is simple. The BFA–APTES absorbent displayed superior adsorption capacities toward both the dyes tested in this study. The superior adsorption capacities in the case of BFA–APTES were attributed to the successful introduction of amine groups on the surface of BFA during the silanization process. The adsorption behavior was found to follow the MLF adsorption model. The thermodynamic parameters including Δ_adG_m⁰ and Δ_adH_m suggested that the process of adsorption was spontaneous and exothermic. The developed BFA–APTES adsorbent also exhibited superior recyclability and could be reused for several cycles after desorption of the adsorbed dyes. Besides studying the remediation in batch setting, we also packed columns using A-BFA and amine-functionalized BFA. Our column studies further revealed the superior performance of BFA–APTES for efficient removal of dye from aqueous solution. Based on the data presented in this study, we believe that this simple and low-cost modification approach opens up new opportunities for the fabrication of BFA-based functional materials as efficient adsorbents for remediation applications.

4. Experimental Section

4.1. Materials and Methods

BFA was obtained from a biomass thermal power station installed at Bulleh Shah Packaging (Ltd.), Kasur, Pakistan. Wheat and corn stalks constitute the major part of the biomass fuel. Toluene (99%), APTES (≥98%), BTB (95%), ethanol (>99%), acetic acid (99.7%), and methanol (>99%) were purchased from Sigma-Aldrich, Germany. ARS was purchased from Eyer Chemical reagents, China.

4.2. Activation of BFA

BFA was washed several times with distilled water to remove any water-soluble contents. After washing, BFA was dried in an oven at 100 °C for 24 h. The activation of BFA was achieved by stirring its suspension in 1 M aqueous HCl solution for 24 h. The activated fly ash was recovered by gravity filtration and washed several times with water to remove any residue HCl. The activated BFA (A-BFA) was then dried in an oven at 100 °C for 24 h. The acid treatment of fly ash improves the adsorption characteristics by removing soluble impurities present in FA and exposing the surface −OH groups.

4.3. Synthesis of Amine-Functionalized BFA (BFA–APTES)

APTES functionalization of BFA (BFA–APTES) was performed using a slightly modified version of the reported procedures.^{42,43,84−86} APTES (4 mL) was added in 36 mL of toluene; 2 g of activated BFA was added in this solution, and the suspension was refluxed for 24 h. The product was then centrifuged, washed three times with toluene, and dried in an oven at 100 °C overnight.

4.4. Structural Characterization

SEM was performed using FEI Nova Nano SEM 450 equipped with the Oxford EDX detector. XPS was performed using Thermo Scientific K-Alpha. The Mg Kα (1253.6 eV) X-ray source was operated at 300 W. A pass energy of 117.40 eV was used for the survey scans. The spectra were recorded using a 60° take off angle relative to the surface normal. TGA was carried out on a TGA Q50 V6.2 Build 187 thermogravimetric analyzer. Samples were heated at 10 °C min^–1 from ambient temperature to 800 °C under nitrogen flow. The UV/vis absorption spectra were recorded using a Shimadzu UV-1800 spectrophotometer. The N₂ adsorption–desorption measurements were performed by using a Quantachrome Nova 2200e. Prior to the measurements, A-BFA and BFA–APTES were degassed overnight under vacuum at 363 and 333 K, respectively.

4.5. Batch Adsorption Studies

The dye adsorption capacities of A-BFA and BFA–APTES were studied by a batch method, which permits a convenient evaluation of parameters that influence the adsorption process, such as chemical modification, pH of dye solutions, initial concentration of dyes, amount of adsorbents, contact time, and temperature. The effect of pH was investigated by preparing a series of dye solutions with pH ranging from 2.0 to 12.0 at a concentration of 15 mg L^–1. The pH of the dye solution was adjusted with 0.1 M HCl or 0.1 M NaOH aqueous solutions. The effect of dye concentration was monitored by using different initial concentrations of dye solutions (10, 20, 30, 40, and 50 mg L^–1) at the pH optimal for adsorption. The impact of the dosage amount was determined by adding different amounts (5–30 mg) of adsorbents (A-BFA and BFA–APTES) to the dye solutions at the pH optimal for adsorption. To study the effect of contact time, the dye content remained in the solution treated with the adsorbents at predetermined time intervals (5, 10, 15, 20, 25, and 30 min) was determined spectrophotometrically. The effect of temperature on the adsorption of dyes was studied by performing adsorption at different temperatures (298, 308, 318, 328, and 338 K). In each adsorption experiment, 10 mL of dye solutions were used and the suspensions were shaken at 250 rpm. After shaking at the optimized conditions, the samples were centrifuged and the concentrations of dyes in the supernatant solutions were determined using a spectrophotometer. The calibration curves were obtained by recording absorbance of dye solutions of known concentrations at λ_max (422 nm for ARS and 432 nm for BTB). Different reagents (1 M NaOH, 1 M HCl, methanol, 1 M acetic acid, and a 1:1 by volume mixture of methanol and acetic acid) were tested for regeneration of the used adsorbents. To study the reusability of both the adsorbents, the dye-loaded A-BFA and BFA–APTES were separately dispersed in 10 mL of 1 M NaOH solution. After desorption, adsorbents were washed with deionized water, dried, and reused for the next adsorption cycle. The concentrations of the desorbed dyes in the supernatants were determined spectrophotometrically.

The column adsorption studies were conducted in a glass column with a diameter of 6 mm and a length of 22 mm. Known quantities (500 mg) of both adsorbents (A-BFA and BFA–APTES) were packed in separate columns to yield the desired bed height of the adsorbent; 200 ppm solution of ARS dye was channeled into the column. Samples of the water coming out of the column were collected and analyzed spectrophotometrically to estimate the concentration of dye.

The amount of dye adsorbed at equilibrium q_e (mg/g) was calculated from the following equation

6

where, q_e is the adsorption capacity (mg/g) of the adsorbent at equilibrium, V is the volume of dye aqueous solution in litres, C_o and C_e (mg/g) are the initial and equilibrium concentrations of dye, and W is the mass of the adsorbent in grams.

Please note that the dyes can photobleach overtime, and this aspect should be considered by performing the dye removal experiments that are based on UV/vis absorption of the dye solutions. In our case, we monitored the UV/vis absorption of 15 ppm solution of ARS and 10 ppm solution of BTB for three consecutive days (stored in the dark) and observed that upon careful storage, there was no change in the intensity of absorption for both the dyes during first two days. A small decrease in the intensity of absorption of both the dyes was observed on the third day; however, this timeframe is beyond the timeframe used in the remediation experiments reported in this study. Therefore, photobleaching does not contribute to the data presented in this study.

4.6. Adsorption Isotherms

Adsorption of ARS and BTB onto A-BFA and BFA–APTES was investigated by two-parameter (Langmuir and Freundlich) and three-parameter (MLF) nonlinear models. The langmuir model describes that the adsorbent sites have identical energy, and each adsorbate molecule is located on a single site. This model depicts the formation of the monolayer of the adsorbate on the homogeneous adsorbent surface. The nonlinear form of Langmuir isotherm is given as

7

while the linearized Langmuir isotherm is

8

where C_e is the equilibrium concentration of the adsorbate (mg L^–1), q_e is the amount of the adsorbate adsorbed per unit amount of the adsorbent at equilibrium (mg g^–1), and q_max (mg g^–1) and K_L (L mg^–1) are the Langmuir constants related to maximum monolayer adsorption capacity and energy change during adsorption.⁸⁴

The dimensionless separation factor (R_L) is generally used to express the feasibility of adsorption and affinity between the adsorbent and adsorbate. The value of R_L indicates the shape of the isotherm to be either unfavorable (R_L > 1), linear (R_L = 1), favorable, (0 < R_L < 1) or irreversible (R_L = 0)⁸⁷ and can be calculated by the following equation

9

where C_o (mg L^–1) is the initial concentration of the adsorbate. In the present study, the calculated R_L values were in the range of 0 < R_L< 1, indicating favorable adsorption of dye molecules on both the adsorbents.

The Freundlich isotherm is not restricted to the formation of the monolayer and describes the nonideal adsorption that involves the heterogeneous surfaces. The nonlinear Freundlich isotherm is expressed as

10

whereas the linearized Freundlich isotherm equation is

11

where q_e represents dye concentration adsorbed on an adsorbent (mg g^–1) at equilibrium, K_f is the Freundlich constant which represents the adsorption capacity (mg g^–1), C_e represents equilibrium dye concentration in solutions (mg L^–1), and the slope, 1/n with favorable range between 0 and 1, is a measure of the adsorption intensity or surface heterogeneity.⁴³ The calculated values of 1/n are 0.58 ± 0.12 and 0.11 ± 0.07 for the adsorption of ARS and BTB on BFA–APTES, respectively. The smaller value of 1/n for BTB indicated better interaction with the adsorbent and hence more favorable adsorption of BTB on BFA–APTES than ARS.

The MLF is a three-parameter empirical model, and there is linear dependency on the concentration in the numerator and exponentially increases in the denominator to enhance the wide range of concentration of adsorption equilibrium.

12

and the linearized MLF model can be written as

13

where q_mon is the adsorption capacity (mg g^–1), and K_MLF (L mg^–1) and n are the MLF constants. The value of 1/n lies between zero and unity.

The linear regression coefficient (R²) and least-square regression based on the sum of the squares of residues (SSE) are applied as error functions in order to identify the best adsorption isotherms and kinetic models for linear and nonlinear forms of corresponding models.

14

4.7. Adsorption Kinetics

Kinetics of the adsorption process provides essential information about the reaction pathways and the solute uptake rate. The linear forms of pseudo-first-order and pseudo-second-order kinetics were applied to study the adsorption kinetics.⁸⁸ The nonlinear pseudo-first-order model was described as

15

While linear version of the pseudo-first-order model was described by Lagergren

16

where q_e and q_t are the adsorption capacities (mg g^–1) at equilibrium and at time t, respectively, and k₁ is the rate constant of pseudo-first-order adsorption (L min^–1).

The nonlinear pseudo-second-order rate equation is

17

and linear from of pseudo-second-order rate equation of McKay and Ho can be expressed as

18

where the equilibrium adsorption capacity q_e and the pseudo-second-order constants k₂ (g/mg min) can be determined experimentally.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c00889.

• Demonstration of remediation on the column packed with the developed adsorbent (MP4)

Author Present Address

^∥ Department of Chemistry, Iowa State University, 2415 Osborn Drive, 1605 Gilman Hall, Ames, IA 50011-1021, USA.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.