Sulphuric Acid-Modified Coal Fly Ash for the Removal of Rhodamine B Dye from Water Environment: Isotherm,Kinetics,and Thermodynamic Studies

Abstract

Among the wide variety of dyes present in the environment, cationic dyes are more toxic and have complex structure. The adsorption process of rhodamine B dye was successfully carried out by sulphuric acid-treated inexpensive modified fly ash (MFA) adsorbent via batch experiments. The nature of the adsorbent was characterized by techniques, namely, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The maximum removal efficiency of RhB dye was found to be 99.78% by using 0.5 g of adsorbent dosage in 50 mg/L of dye concentration. The SEM images displayed the porous nature of the adsorbent where the EDS analysis displays the elemental compositions present in the adsorbent. XRD pattern shows the crystallinity nature of the adsorbent. Among the batch study parameters, effect of pH plays an important role in the adsorption process. The pH of 4 was found to be an ideal setting for the efficient removal of the dye RhB. The preferable elimination ability was found by keeping the dosage at 5 g/L, contact time 120 min, and dye concentration at 50 mg/L. Adsorption capacity was found to be 36.36 mg/g. This shows the ability of the MFA for the removal of wastewater contaminants. This adsorption process is well suited for the Freundlich isotherm, which displaces the process as a multilayer adsorption. Studies in kinetics and thermodynamics demonstrate that the process was well suited for its exothermic nature and pseudo-second-order. Thermal regeneration studies were carried out, and the adsorbent was effectively recycled and utilized up to four more times with minimal loses in its effectiveness. Therefore, from these obtained results, it is clear that the MFA is an effective adsorbent for the effective removal of dyes from wastewater.

1. Introduction

The primary conditions for promoting healthy living are a tidy habitat and safe drinking water. For household use as well as industrial and agricultural purposes, potable water is a necessity. Due to the increasing global population and industrial advancements, a significant amount of water was utilized by residential, industrial, and agricultural activities daily [1]. Some of the manufacturing sectors that use a lot of water include the paper, printing, plastics, leather, and fabric industries [2]. The dyeing process utilized in fabric, paint, food, and cosmetic industries requires vast amounts of water, and the resulting effluent causes serious water pollution. By nature, dyes are nonbiodegradable, carcinogenic, and hard to eliminate from the aqueous medium because of their resilience towards oxidizing chemicals [3–5]. In general, dyes are toxic molecules with benzene groups, which are particularly hazardous substances with high carcinogenicity and mutagenicity. Moreover, the contamination is disturbing public health as well as the ecosystem because it persists [6].

The most dangerous and mutagenic dyes for both humans and marine life are cationic ones. About 12% of the estimated 7 million tonnes of dyes produced each year are discharged into the environment [7, 8]. One of them is the rhodamine B (RhB) dye, which comes in both cationic and zwitterionic molecular forms [9]. These dyes have various chemical structures with substituted aromatic groups and are positively charged. They are a type of basic dye that dissolves in water and produces colored ions when dissolved [10]. RhB dye is frequently used in a variety of industries but is most prevalent in the textile, paper, and food industries. When RhB is exposed to the environment, it will affect the eyes, respiratory system, and skin of humans and animals. Medical investigations have demonstrated that drinking water contaminated with RhB produces persistent poisoning as well as neurotoxic and carcinogenic effects [11].

Previous reports have shown that RhB is resistant to natural degradation, reproductive toxicity, and animal neurotoxicity [12]. Adsorption [13], membrane filtration [14], electrocoagulation [15], flocculation [16], electrochemical oxidation [17], biological treatment [18], and coagulation are just a few of the methods in the current trend to purify the wastewater [19]. Adsorption, among the other processes, is a useful method for removing dyes because it has a cheap operational cost and good effectiveness, especially if efficient adsorbents are used [20]. Approximately 600 million tonnes of FA are produced annually by combustion power plants that use coal as fuel. Silica, alumina, calcium, unburned carbons, iron, and alkali metal oxides make up most of its composition, along with heavy metal impurities [21, 22]. Fly ashes are classified as F-type and C-type. Fly ash obtained by burning of bituminous coal is termed to be class F, while class C FA originates from burning of subbituminous coal. The major difference between these two types is that class F has no cementing property, while class C has self-cementing properties. Only 20% of the generated FA was utilized in construction purposes, soil stabilization, and zeolite preparation and mainly used as filler material in polymers. These applications cannot fully utilize the production of FA. So, safety disposal of FA is necessary, because if these FA enter into the environment, it will affect the ecosystem by causing water and soil pollution [23–25]. Currently, a significant proportion of FA is being disposed of in simple stacks, monofils, and ponds [26] which are seriously contaminating the water resources, and these disposal techniques are unfriendly to the environment. Additionally, these methods of disposal are taking up large areas of land, including productive farming [27]. So, fly ash is a serious environmental threat and still presents a substantial problem for appropriate disposal [28]. To avoid the hazardous nature of FA by disposing it, effectual utilization of FA in various fields had been emerged. Fly ash has recently been employed in wastewater treatment to remove toxic pollutants economically [29].

The study’s unique feature is the use of plentiful, readily available, and cost-effective modified fly ash (MFA) as an adsorbent for the effective and efficient removal of RhB dye. It is an attempt for using sulphuric acid-activated fly ash as an effective adsorbent for organic pollutant removal. The MFA is an easily disposable material, and the fundamental novelty of the operation was to successfully utilize the waste material as an effective adsorbent. As a result, we decided to get rid of it by employing it as a filler material in firecrackers.

This experiment is an attempt to examine the feasibility of modified fly ash (MFA) for the effectual elimination of RhB via batch experiments. Among the various activating agents, sulphuric acid is widely used as an agent for the synthesis of activated carbon. Understanding various activation process parameters is crucial to achieving the desired porosity of carbon for a particular usage. The chemical activation by sulphuric acid enhances the pore development and increases the carbon content in the adsorbent [30]. The batch studies like influences of pH, shaking time, dye concentrations, and dose rate on RhB removal have been evaluated. Adsorption isotherm, thermodynamics, and kinetics studies were also performed and fitted with their respective models. Regeneration studies were conducted as well to determine whether the used adsorbent could be reused.

2. Experimental Studies

2.1. Adsorbent and Chemicals Utilized in This Experiment

For this experimental study, class F fly ash was obtained from the Tuticorin power plant. The main reason for using class F fly ash is that it has only a small amount of cementing properties. So, it cannot be majorly used in the construction industry. All the chemicals used in this analysis are annular class. The rhodamine B dye of C₂₈H₃₁ClN₂O₃ 99% purity with analytical reagent (AR) grade was purchased from Sigma-Aldrich Solutions Pvt. Ltd., India, and it was used. The structure of RhB dye is presented in Figure 1. For the chemical activation of fly ash, sulphuric acid (H₂SO₄, 99%, Sigma-Aldrich Solutions Pvt. Ltd., India) was used. 0.1 N of NaOH (97%, Sisco Research Lab Private Limited, India) and 0.1 N of HCl (37.0%, Merck Life Science Pvt. Ltd., India) were used for pH alteration. The stock solution was created by diluting 0.1 g of RhB in 100 mL of water to create synthetic dye wastewater with a 1000 ppm concentration. From the stock solution, the required dye concentration can be made by successive serial dilution.

Figure 1

Structure of RhB dye.

2.2. Activation and Evaluation of the Adsorbent

The obtained class F fly ash was thoroughly cleaned with double-distilled water repeatedly to get rid of any remaining organic material and then dried in a hot-air oven for 1 day. The uneven particles in the fly ash were grinded using mortar and pestle, and then, it was sieved by using the sieve analysis to get a 40 mm particle size. After acquiring the desired particle size, it was chemically activated using sulphuric acid in a ratio of 1 : 2. After that, the FA was washed several times using double-distilled water to bring back the pH level to neutral. It was then dried for six hours at 120°C in a forced-air circulating oven before being kept in an airtight vessel. After the activation, modified fly ash was characterized by several characterization techniques to examine its applicability and effectiveness. The surface morphology of the MFA was evaluated by EVO18 (Carl Zeiss) scanning electron microscope (SEM). The magnification range was varied between 1000x and 5000x with a 20 kV acceleration voltage. The elemental composition of the adsorbent was analyzed using energy-dispersive X-ray spectrophotometer (Quantax 200 with X Flash 6130). In EDX, the composition of the elements can be obtained with the concentration of the available elements. The nature of the adsorbent was evaluated by using D8 advanced X-ray diffractometer (XRD). The varying concentration of rhodamine B was measured by using Jasco-630 UV spectrophotometer. The thermal stability of the MFA was measured by using TGA-DSC with the scanning range of 10°C per minute from 40°C to 1000°C. The functional groups present in the modified FA surface were evaluated by Perkin Elmer e L1600300 Fourier transform infrared spectrometer (FTIR).

2.3. Batch Adsorption Experiments

The study of isotherm and adsorption kinetics will provide the basic knowledge required for designing and operating the instruments in the sewage treatment plant. At 554 nm wavelength, RhB dye was determined using UV spectrophotometer. The batch experiments were executed to evaluate the behaviour of rhodamine B dye on modified fly ash (MFA). The moisture content in the MFA was removed by keeping it in an oven for 3 hours at 110°C. A concentration range of 50 mg/L to 250 mg/L of RhB dye solution was mixed with the desired amount of MFA. Then, it was thoroughly shaken at 150 rpm for extended periods of time at 303 K in a shaker. The concentration of the remaining solution was then examined by UV spectrophotometer after it had been filtered using the Whatman filter paper.

The impact on batch study parameters like pH [3–8] and dose rate (0.1 g-0.8 g), the effect of initial concentration (50 mg/L–250 mg/L), the effect of contact time (15 min-180 min), and the effect of temperature (303 K-318 K) were evaluated. 0.1 N each of HCl and NaOH was used to modify the pH of the mixture.

The sorption experiment was tripled to get consistent results. Freundlich and Langmuir’s evaluations were performed to determine the type and capacity of adsorption. To assess the controlling mechanism of this process, kinetic models are used. Since the adsorption process was temperature-dependent, thermodynamic studies were conducted by changing the temperature. The thermodynamic parameters will primarily be used to assess whether this process is possible and spontaneous.

3. Results and Discussion

3.1. Surface Assessment

Using a scanning electron microscope (SEM), the morphology and nature of the modified fly ash (MFA) surface were examined. Before the activation, the surface of the adsorbent was soft and had less pores. Once it was activated with sulphuric acid, the surface becomes rough and had more pores compared with the raw FA. In Figure 2(a), in raw FA, there were no visible pores found on the adsorbent surface. Afterward, scabrous clumps and nonspecific uneven pores are found on the surface of the MFA in Figure 2(b). These active sites will be responsible for the trapping of pollutants. Figure 2(c) displays the trapped RhB molecules in the active site region, and it confirms its effectual nature. The removal of Alizarin Red dye using chemically activated avocado seed powder follows the same pattern, where the adsorbent was chemically activated by using sulphuric acid to enhance its adsorptive properties [31].

Figure 2

(a) Raw coal fly ash. (b) Modified fly ash (before adsorption). (c) Modified fly ash (after adsorption).

(b)

(c)

3.2. EDS Analysis

Figures 3(a)–3(c) and Table 1 show the EDS stretches of the raw FA and modified fly ash. From Table 1, it is clear that FA and MFA consist of C, O, Na, Mg, Al, Si, S, Ca, Fe, and Ti before and after the adsorption of rhodamine B dye. Out of these elements, carbon and oxygen present in the highest quality. Comparing the before and after activation of FA, the amount of carbon content was increased in the MFA and rhodamine B dye-loaded MFA. The increase of carbon content in MFA is mainly due to the activation by sulphuric acid. Adsorption of RhB dye was confirmed by the increase in the carbon content in rhodamine B dye-loaded MFA. Titanium, iron, sodium, magnesium, sulphur, and potassium along with calcium are present in only the trace amount. Rather than the carbon and oxygen, the amount of silica and aluminium is high because these two are the majority constituents of FA. Due to the presence of dust and dirt, the amount of silica content was increased. Additionally, the sample’s insulating properties at the time of analysis may be the cause of the increase in aluminium content. Weight percentage of Mg, Al, and Ca elements was reduced after sulphuric acid activation. This was because these elements will be present in metal oxide form in FA. While activating the FA with sulphuric acid, these metal oxides will be converted to metal sulphates and washed away by distilled water to get neutral pH for MFA.

Figure 3

(a) EDS pattern of raw coal FA. (b) EDS pattern of MFA. (c) EDS pattern of MFA (RhB loaded).

(b)

(c)

Table 1

EDS compositions.

Elements	Weight percentage
Elements	Fly ash	MFA	MFA (RhB dye loaded)
Carbon (C)	45.4	58.6	64.6
Oxygen (O)	36.1	28.3	26.1
Sodium (Na)	1	0.9	—
Magnesium (Mg)	1.2	0.4	0.3
Aluminium (Al)	4.5	3.3	2.4
Silica (Si)	5.0	5.3	4.5
Sulphur (S)	1.1	1.2	1
Potassium (K)	0.4	0.5	0.2
Calcium (Ca)	3.4	0.5	—
Iron (Fe)	1.5	0.5	0.6
Titanium (Ti)	0.3	0.4	0.3

3.3. XRD Analysis

The morphological changes on the surface of the adsorbent can be evaluated by X-ray diffraction (XRD). The results of the XRD for the FA and MFA are shown in Figure 4. The findings show that the fly ash has more peaks at different angles than the modified fly ash. Amorphous nature and more pores are present in the MFA, due to the evidence of smaller peaks [32]. Due to the sulphuric acid activation, the surface area of the MFA has been raised. The MFA sample has more intensity peaks when compared with the fly ash sample. The major phases of the FA and MFA include quartz and mullite whereas the slight peaks include hematite and magnetite [33].

Figure 4

XRD pattern of FA and MFA.

(b)

3.4. FTIR Evaluation

Fourier transform infrared spectroscopy is a predominant technique to evaluate the elements on the modified fly ash surface, which enhances the process. The FTIR stretches for the adsorption studies are displayed in Figure 5. The MFA’s peak at 1053 and 1060 indicates that silica, the primary substance in FA, is present. The presence of iron oxide components in the FA was displayed by the peaks at 580 and 585 in both evaluations. Stretching between 3200 and 3730 for both before and after adsorption indicates the existence of O-H groups in the MFA adsorbent. A narrow peak shift from 2190 to 2145 indicated the existence of (CΞC stretching) alkyne group and (N=C=N stretching) carbodiimide. A narrow peak in 1975 indicates the presence of the allene group (C=C=C stretching) [34].

Figure 5

FTIR stretches of modified fly ash (before and after removal).

3.5. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis (TGA) shows an outstanding impact of temperature over the MFA’s structure. The reaction between the sulphuric acid and the elements present in the FA will result in the destruction and removal of its amorphous nature. From Figure 6, it is observed that the weight loss occurred in three different stages. From increasing the temperature from 0 to 100^oC, only 2.5% of the weight was lost. The moisture content in the MFA is responsible for this loss. Sample weight loss of up to 14% was caused at the temperature range of 100 to 450°C. This major loss was caused by the evaporation or breakdown of physiochemically trapped water. Weight loss from 450 to 1000 is due to the burning of the remaining carbon [35].

Figure 6

TGA analysis of modified fly ash.

3.6. Impact of Shaking Time

One of the key factors used to assess the effectiveness of the adsorbent in the removal of the adsorbate is the effect of the contact time of MFA on rhodamine B dye. The rate of the rhodamine B (RhB) dye removal was evaluated by varying the agitation time from 15 min to 180 min and thereby maintaining the other parameters as same as pH 4, dye concentration 50 mg/L, dose rate 0.5 g, and temperature 30°C, and the results are graphically represented in Figure 7. For this process, the shaking time was maintained at 220 rpm to enhance the rate of the adsorption process. From the figure, it is observed that there is a noticeable increase up to 60 min, where the rate of adsorption of dyes by the active sites will be equal to the desorption of the dye molecules from the already bonded sites, and it results in an equilibrium state. Once the equilibrium state is attained, there will not be any adsorption that will take place, because the active sites and the pores are already saturated by the dye molecules. After this period, the remaining active sites will be saturated by three diffusion mechanisms, namely, film, pore, and reactions, that will take place on its surface [36]. Using chemically altered Volvariella volvacea, the same general trend of outcome was obtained for the removal of RhB dye [37].

Figure 7

Effect of contact time for the removal of RhB dye.

3.7. Impact of MFA Dose Rate

The impact of the MFA dose rate was assessed by altering the MFA dosage from 0.1 to 0.8 g at the same time keeping the other parameters as constant as pH 4, shaking time of 120 min, RhB concentration of 50 mg/L, and temperature of 30°C. With an increase in MFA dosage, rhodamine B dye removal efficiency increased, as shown graphically in Figure 8. A considerable increase from 34% to 99% of dye removal was noticed while increasing the dosage of MFA from 0.1 g to 0.5 g. Once the surface was saturated with dye molecules, further increases in the MFA dosage did not affect the effectiveness of removal. The increased surface area of the MFA and additional potential active sites may be the root cause of the increase in the removal efficiency. After attaining equilibrium, there will not be any further adsorption that will take place, because the dye molecules had already occupied the pores. The removal of RhB dye using a biosorbent from the banana peel produced results that followed a similar pattern [38].

Figure 8

Impact of adsorbent dosage for the removal of RhB dye.

3.8. Impact of pH

To determine the impact of pH, the pH of the solution was varied between 3 and 8. From Figure 9, the adsorption of the dye was reduced while increasing the pH of the solution from 3 to 8. The maximum removal efficiency has occurred at pH 4. The interaction between the rhodamine B dye and the MFA surface was influenced by the ionization states of the different elements held in the surface of the dye molecule and adsorbent. Throughout the process, the quantity of rhodamine B dye adsorbed decreases with increases in pH, and this may be due to the charges on the surface of MFA. At lower pH, it tends to increase the hydronation effect, and it will enhance the adsorption capacity. When the pH of the solution is increased, desorption will take place due to the DE hydronation effect and the adsorption efficiency of rhodamine B will tend to decline. The pH at zero point charge for the MFA was estimated by the drift method, and it is shown in Figure 10. When the pH is higher than the pH_ZPC, electrostatic repulsion will happen between the adsorbent and the adsorbate and the removal efficiency will also decrease [35]. Using biomass of cypress/false cypress, the removal of RhB dye produced results that followed an identical trend [39].

Figure 9

Effect of pH for the removal of RhB dye.

Figure 10

Zero point charge of MFA.

3.9. Impact of Rhodamine B Concentration

The impact of rhodamine B concentration was evaluated by altering the dye concentration from 50 to 250 mg/L. According to the results in Figure 11, the removal efficiency decreases as the initial dye concentration of rhodamine B dye rises. But when raising the concentration of the rhodamine B, the adsorption capacity of the dye keeps on increasing. It is because when the concentration of the adsorbate rises, the driving force also rises [40]. The main reason for more removal at lower concentrations is because of the availableness of binding sites and pores on the modified fly ash (MFA). The decrease in the removal efficiency is due to the unavailability of active sites for the dye molecules to attach to the adsorbent surface. The same tendency of result was observed in the effectual removal of rhodamine B dye by using MgO-loaded Fe-Co-Mn nanoparticles [11].

Figure 11

Impact of initial dye concentration for the removal of RhB dye.

3.10. Impact of Temperature and Thermodynamics

To examine the suitable temperature, the investigations were by altering the temperature ranging from 303 K to 318 K and thereby keeping the other parameters like pH [4], RhB dye concentration (50 mg/L), shaking time (120 min), and dose rate (5 g/L) as constant. From Figure 12, it is noticeable that the increasing temperature will decrease the adsorption efficiency. The weakening of the electrostatic field of attraction may be the main cause of the decreased efficacy of RhB uptake by the MFA at higher temperatures. At higher temperatures, the dye molecules will desorb from the adsorbed active sites. Hence, it is evidenced that the lower temperature will be evident for an enhancement in the adsorption effectiveness of RhB dye by using MFA adsorbent. Using magnetic silica alginite beads (MSAB) as an adsorbent, a similar sequence of outcomes was obtained for the adsorption of RhB dye [41].

Figure 12

Effect of temperature for the removal of RhB dye.

The main reason for performing the thermodynamic evaluation is to determine the suitable nature and spontaneity of this process. By utilizing Van ‘t Hoff’s equations, the thermodynamic parameters for adsorption studies can be calculated by the following equations: $\begin{matrix} (1) & Kc = \frac{C_{Ae}}{C_{e}}, \\ Δ G^{o} = - RTlnKc, \\ Δ G^{o} = Δ H^{o} - T Δ S^{o}, \\ lnKc = - \frac{Δ H^{o}}{RT} + \frac{Δ S^{o}}{R}, \\ \log Kc = - \frac{Δ H^{o}}{2.303 RT} + \frac{Δ S^{o}}{2.303 R} . \end{matrix}$

By plotting a graph for $1 / T$ vs. log Kc, $Δ H^{o}$ and $Δ S^{o}$ values can be determined by using slope and intercept readings from Figure 13. The determined values are listed in Table 2. From the table, it is evident that $Δ G^{o}$ , $Δ H^{o}$ , and $Δ S^{o}$ values belong to negative. $Δ G$ negative value shows the feasibility of the process. The negative value of $Δ H^{o}$ describes the exothermic nature. The negative value of $Δ S^{o}$ demonstrates that the adsorption process was enthalpy-driven, with reduced randomness [42].

Figure 13

Thermodynamic evaluation.

Table 2

Parameters of thermodynamics.

RhB concentration (mg/L)	$Δ H^{o}$ (kJ/mol)	$Δ S^{o}$ (J/mol·K)	$Δ G^{o}$ (kJ/mol)
RhB concentration (mg/L)	$Δ H^{o}$ (kJ/mol)	$Δ S^{o}$ (J/mol·K)	303 K	308 K	313 K	318 K
50	-80.822	-568.166	-15.409	-9.188	-7.756	-6.522
100	-22.661	-146.954	-7.643	-6.904	-6.278	-5.400
150	-16.714	-108.219	-5.713	-5.136	-4.594	-4.092
200	-11.175	-72.031	-3.867	-3.604	-3.187	-2.803
250	-9.112	-61.921	-2.176	-1.954	-1.667	-1.236

3.11. Adsorption Isotherms

The effectiveness and the controlling mechanism of the adsorption process can be approximately determined by using isotherm studies. These studies will explain about how the adsorbate molecules at equilibrium conditions interacted with the adsorbent surface. The Langmuir and Freundlich adsorption isotherms were implemented to obtain a proper interactivity between the adsorption and desorption rate of rhodamine B dye onto modified fly ash (MFA). The isotherm model and isotherm data were obtained by plotting a graph between $C_{e}$ vs. $q_{e}$ .

The Langmuir adsorption isotherm model is a mathematical model in which the surface of the adsorbent is assumed to be homogeneous, and the adsorbate is assumed to adsorb in a monolayer [43]. The mathematical expression was given as $\begin{matrix} (2) & q_{e} = \frac{q_{\max} K_{L} C_{e}}{1 + K_{L} C_{e}}, \end{matrix}$ where $q_{e}$ corresponds to the quantity of RhB adsorbed per gram of the MFA at equilibrium (mg^-1), $K_{L}$ is termed to be Langmuir constant (L/mg), $C_{e}$ is the equilibrium concentration of the RhB (mg/L), and $q_{\max}$ is the monolayer adsorption capacity (mg/g).

The Freundlich isotherm is a nonideal, reversible, and multilayer adsorption isotherm model. The Freundlich model explains the heterogeneous surface with varying active sites’ energy [44]. The mathematical expression of the Freundlich isotherm was given as $\begin{matrix} (3) & q_{e} = K_{F} C_{e}^{1 / n}, \end{matrix}$ where $K_{F}$ ((mg/g) (L/mg)^1/n) is termed to be Freundlich constant, it will be used to examine the adsorption degree, and $1 / n$ represents the heterogeneity factor of adsorption.

Figure 14 and Table 3 show the equilibrium evaluation and the parameters of the isotherm analysis. Based on the $R^{2}$ value and the error readings, the best model that suit for this adsorption process was chosen. From Table 3, it is clear that the system follows the Freundlich isotherm because it has a greater $R^{2}$ value (0.9812) and lower error value of SSE (8.1) and RMSE (1.643).

Figure 14

Isotherm evaluation for the removal of RhB by MFA.

Table 3

Adsorption isotherm parameters.

Models	Parameters	Values
Langmuir	$K_{L}$ (L/mg)	0.2507
	$q_{\max}$ (mg/g)	36.36
	$R^{2}$	0.8077
	SSE	82.99
	RMSE	5.26

Freundlich	$K_{F}$ ((mg/g) (L/mg) $^{1 / n}$ )	15.27
	$n$	4.956
	$R^{2}$	0.9812
	SSE	8.1
	RMSE	1.643

The mathematical expression of SSE and RMSE [45] is given below. $\begin{matrix} (4) & SSE = \sum_{i = 1}^{n} {(A_{i} - P_{i})}^{2}, \\ RMSE = \sqrt{\sum_{i = 1}^{n} \frac{{(A_{i} - P)}^{2}}{n}} . \end{matrix}$

So, in this process, the removal of RhB dye was carried out in a multilayer with a heterogeneous structure. By calculating the number of adsorbates adsorbed on the adsorbents using adsorption isotherms, the saturated adsorption capacity for adsorbates for various adsorbents can be compared. Various adsorbents synthesized from the waste materials and CFA are listed in Table 4. And this table comprises the bibliographic review of adsorbents with their adsorption capacity and the nature of adsorption. While being compared with the adsorption capacities of biomass wastes like paper industry waste [46], saw dust [47], and coffee waste power [48], MFA (this work) displays higher adsorption capacity of 36.36 mg/g. The prepared absorbent was cost-effective and had multilayers for better removal efficiency. Compared with the zeolites prepared from the CFA, this adsorbent was easy to synthesize and it can be regenerated up 4 cycles with minimal loss in its efficiency.

Table 4

Adsorption capacities of already available adsorbents.

Materials	Dyes	Favourable pH	Time (min)	Isotherm model	Maximum adsorption capacity (mg/g)	Ref.
Waste of seeds of Aleurites moluccanus	RhB	6	120	Sips	117	[49]
Paper industry waste	RhB	2.4	60	Langmuir	6.711	[46]
Okra waste	RhB	3.5	600	Langmuir	321.50	[50]
Saw dust	RhB	1-11	10	Langmuir	35.7	[47]
Sulphur-doped biochar from tapioca peel waste	RhB	8	120	Langmuir	32.810	[51]
Coffee waste powder	RhB	2	180	Langmuir	5.26	[48]
Artocarpus odoratissimus leaves	RhB	3	30	Langmuir	104.96	[52]
Kaolinite	RhB	7	80	Langmuir	46.08	[53]
Linde-type A zeolite from CFA	Acid red 66	4	10	Freundlich	416.7	[54]
CFA	Methylene blue (MB)	—	—	Langmuir	2.88	[55]
Activated CFA	MB	—	—	Langmuir	37.08	[55]
Bentonite carbon nanotubes	RhB	9	30	Freundlich	142.8	[56]
Sodium dodecyl sulfate-modified Fe₂O₃ (zeolite)	RhB	3	60	Freundlich	89.3	[57]
A coral reef-shaped mesoporous silica material obtained from CFA	MB	9	60	Langmuir	10.86	[58]
Modified fly ash (MFA)	RhB	4	120	Freundlich	36.36	This study

3.12. Adsorption Kinetics

The kinetic models will provide the information about the equilibrium time required to carry out the adsorption process. It is an essential tool for developing an adsorption system. It delivers details regarding the rate of the process and the effectiveness of the material used. Kinetic models, namely, pseudo-first-order (PFO) [59] and pseudo-second-order (PSO) [60] and intraparticle diffusion (IPD) [61], are used in this study to evaluate the adsorption kinetics and mechanism of MFA adsorbent. The nonlinear equations of the kinetic models are listed below. $\begin{matrix} (5) & q_{t} = q_{e} (1 - \exp (- k_{1} t)) (PFO), \\ q_{t} = \frac{q_{e}^{2} k_{2} t}{1 + q_{e} k_{2} t} (PSO), \\ q_{t} = k_{p} t^{1 / 2} + C (IPD), \end{matrix}$ where $q_{t}$ is the concentration of the pollutant adsorbed at equilibrium time ( $t$ ), $q_{e}$ is the concentration of the pollutant adsorbed at equilibrium, $k_{1}$ is the PFO rate constant, $k_{2}$ is the PSO rate constant, and $k_{p}$ is the IPD rate constant.

By using the kinetic model equations listed above, the kinetic models (Figure 15) and the obtained data are tabulated in Table 5. From the table, it is clear that the correlation coefficient ( $R^{2}$ ) of PFO is considerably higher than the PSO model. There was no major difference in the $q_{e}$ cal and $q_{e}$ exp values in the PFO model as compared with PSO model. So, it was decided that the adsorption of RhB dye by MFA adsorbent fitted in the PFO model and the nature of adsorption was physisorption. The gradual decrease in the value of rate constant ( $k$ ) while increasing the concentration of the dye indicates the lower availability of the active sites for the adsorbates to adhere on the adsorbent surface. At higher concentrations, there will be more competition for the RhB dye to attach on the surface of the adsorbent. Diffusion mechanism may be the controlling mechanism of the adsorption process, where intraparticle diffusion (IPD) is a mass transfer phenomenon that occurs at three different stages (Figure 16) that include external diffusion which is the first stage in which the pollutant, namely, the adsorbate, will diffuse through the aqueous film nearer to the adsorbent. Internal diffusion is the second stage, in which the pollutant will be dispersed into the inner region of the pores on the surface of the material.

Figure 15

Adsorption kinetics for the elimination of RhB by MFA.

Table 5

Kinetic parameters of MFA for the removal of RhB dye.

Models	Parameters	Values
Models	Parameters	50 mg/L	100 mg/L	150 mg/L	200 mg/L	250 mg/L
PFO	$q_{e} \exp$ (mg/g)	10.12	19.25	27.35	33.11	35.32
	$k_{1}$ (min^-1)	0.02192	0.018	0.01571	0.01401	0.01285
	$q_{e} cal$ (mg/g)	10.36	20.55	30.19	37.71	41.04
	$R^{2}$	0.9742	0.9815	0.9831	0.976	0.984
	SSE	1.368	4.52	9.671	21.68	16.9
	RMSE	0.3699	0.6723	0.9834	1.472	1.3

PSO	$k_{2}$ (g/mg·min)	0.00176	0.00061	0.00032	0.00021	0.00017
	$q_{e} cal$ (mg/g)	12.95	26.94	41.06	52.63	58.28
	$R^{2}$	0.9754	0.9736	0.9732	0.9673	0.9781
	SSE	1.301	6.447	15.36	29.52	23.11
	RMSE	0.3606	0.8029	1.239	1.718	1.52

IPD	$k_{p}$ (mg/(g·min^(1/2))	0.6997	1.2039	1.8315	2.3418	2.5686
	$C$	1.615	6.2437	7.569	7.7712	7.2541
	$R^{2}$	0.9298	0.8485	0.857	0.8684	0.8924

Figure 16

Intraparticle diffusion.

3.13. Effect of Coions

Common dye wastewater usually contains some specific amount of other existing materials, namely, inorganic salts. For the industrial implementation of modified fly ash (MFA) for adsorption application, the impact of the other coions, namely, organic salts, needs to be evaluated. This study includes some of the inorganic salts, namely, KCl, NaCl, K₂SO₄, and KNO₃, that were used for evaluation. Figure 17 shows that there were negligible impacts on the adsorption effectiveness of RhB dye in the existence of these organic salts. Therefore, modified fly ash has a higher ability, and it can be implemented for real-time wastewater treatment.

Figure 17

Effect of coions for the removal of RhB dye.

3.14. Regeneration Studies

The process of regeneration is another crucial criterion to examine the efficiency of the material. Before starting the regeneration process, the adsorbate molecules need to be desorbed from the adsorbent with some methods. And the method we used needs not to disintegrate or disturb the structure of the adsorbent. A wide variety of methods are available for desorption and regeneration studies. Modified fly ash is an inorganic material, so it has high thermal stability when compared with other organic adsorbents. The MFA was regenerated using the technique of heating because of its high thermal stability. For the regeneration studies, the adsorbent was saturated by agitating 0.5 g of MFA with 100 mL of a 100 mg/L RhB dye solution for 24 hrs, and then, it was filtered. The filtrate was oven dried at 120°C. The temperature from 250°C, 350°C, 450°C, 550°C, and 650°C was utilized to investigate the adsorption efficiency of the regenerated MFA. Figure 18 displays that 350°C will be the suitable temperature. MFA can also be recycled and reused up to four times with little efficiency loss.

Figure 18

Regeneration studies of MFA.

3.15. Adsorption Mechanism

There exist $π ‐ π$ interaction, hydrogen bonding, and electrostatic attraction between the adsorbate RhB and the adsorbent MFA. From the obtained FTIR spectrum results of before and after adsorption, the date revealed the formation of carbodiimide bond between MFA and RhB. The $π ‐ π$ interaction occurred between the unsaturated aromatic ring in the RhB dye and the alkyne system in the MFA. Then, from the observed FTIR, data above 3100 cm^-1 express the -OH stretching frequency, and this reveals the existence of hydrogen bonding between the adsorbent and the adsorbate [9, 62].

4. Conclusion

The present experiment displays the elimination of rhodamine B (RhB) dye as a synthetic effluent using the modified fly ash (MFA). The characterization experiments like FTIR, TGA, XRD, SEM, and EDS studies were carried out. The XRD results clearly show that once the adsorbent is activated by using the sulphuric acid, its crystallinity nature was reduced and it becomes more amorphous in nature. Among the batch study parameters, effect of pH plays a major impact. The pH of 4 was determined as a suitable state for the effectual removal of RhB. The preferable elimination ability was found by keeping the dosage at 5 g/L, contact time 120 min, and dye concentration at 50 mg/L. This adsorption process is well suited for the Freundlich isotherm, which displaces the process as a multilayer. Studies in kinetics and thermodynamics demonstrate that the process was well suited for its exothermic nature and pseudo-second-order (PSO). According to studies on reusability, the adsorbent can be reused up to four times without significantly losing effectiveness. Based on the results, it is concluded that the activation of fly ash using sulphuric acid enhances the surface area and porous nature of fly ash and thereby promotes the adsorption effects on RhB. For the future prospective, MFA can be applied in a column study for treating a continuous flow of water rather than a batch study. Moreover, the usability of the MFA adsorbent for the treatment of real wastewater can be evaluated. Additionally, the reduction in the chemical oxygen demand (COD) using MFA can be evaluated.

Footnotes

Data Availability

All data generated or analyzed during this study are included in this published article.

Conflicts of Interest

The authors declare that there are no conflicts of interest for this article. No copyediting or translation services were used for the preparation of this manuscript.

Authors’ Contributions

G. Bharath Balji was responsible for the investigation and methodology and wrote the original draft. P. Senthil Kumar was responsible for the conceptualization, validation, and supervision.

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