Optimization and Kinetic Study of Treating Dye-Contaminated Wastewater Using Bio-Composite Synthesized from Natural Waste

Abstract

The main objective of the present research project was to investigate the possibility of using low cost, eco-friendly, and easily available adsorbents, such as mint biomass and marble stone waste, for the removal of dyes, DRIM blue HS-RL and DRIM black ep-B, from wastewater using an efficient procedure, which is adsorption. Nine different combinations of these adsorbents were prepared with and without modification using sodium metasilicate and potassium ferricyanide. Spectroscopic analysis was carried out to investigate the λ_max of the dyes. Adsorbent nanocomposites were characterized using Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and zeta (ζ) potential. Adsorption equilibrium studies were investigated by determining the adsorption at the following conditions: initial dye concentrations (5–50 ppm), adsorbent doses (0.005–0.5 g), contact times (15–240 min), temperatures (30–70 °C) and pH (5–10). Of all the nine adsorbents, MTPF showed the maximum adsorption capacity at 50 ppm initial dye concentration, 0.005 g dosage of adsorbent and 240 min contact time for both dyes. DRIM Blue HS-RL was adsorbed efficiently at 6 pH and temperature 60 °C and DRIM black ep-B was adsorbed at pH 5 and temperature 50 °C by MTPF (mint–tawera composite treated with potassium ferricyanide). Among the various adsorption isotherms (Langmuir, Dubinin–Radushkevich, Freundlich, Herkin–Jura, and Temkin isotherms), some adsorbent followed the Freundlich isotherm while the others followed the Langmuir isotherm. The best-fit model was decided based on their high R² value and agreement between q_e calculated from isotherms and those obtained experimentally. At equilibrium concentration, application of kinetic models (pseudo-first-order, and pseudo-second-order) revealed that the best-fit model was pseudo-second-order kinetic model for both dyes, as their R² > 0.9, and q_e calculated was close to q_e obtained experimentally.

1. Introduction

Water pollution is caused by a wide spectrum of chemicals, colorants, industrial, domestic, and radioactive wastes. Many industries, such as paper, textile, and steel, are present along riverbanks because they demand a large quantity of water in their manufacturing procedure, and finally their waste consisting of acids, salts, dyes, alkalis, and various other chemicals are discarded and dumped into the rivers as industrial effluents [1,2,3,4,5,6,7].

Ornamental stone industries are the most important subsectors of the mining industry. It produces huge quantities of stone wastes that can be used to treat dye-containing toxic water [8]. Waste produced during the manufacturing of ornamental stones may range from 30% to 50% of the total volume of all the manufactured stone blocks, depending on the way of cutting, different stone types, and different polishing techniques used. Most commonly, this waste material is discharged into the nature in an uncontrolled manner, resulting in serious environmental pollution. Interestingly, dolomite and calcite are main constituents of marble stone, that have excellent removal capacities for anionic dyes [9,10]. Therefore, the use of these stone wastes in other industrial plants will not only reduce environmental damage but will also increase its overall economic value [11].

Textile industry wastewater consists of different types of dyes as well as other inorganic and organic contaminants. According to the color index, >10,000 different varieties of dye are now being prepared around the world, resulting in the manufacture of 700,000 tons of dye. For example, dyeing 1 kg of cotton requires 70 to 150 L of water and 30 to 60 g of dye, and it has been observed that more than 50% of the color used is dumped indirectly or directly into the aquatic environment [12]. The textile industry contributes more than half of the existing dye pollutants in the global environment, that is 54%, followed by the dyeing industry (21%), the paper and pulp industry (10%), the paint and tanning industry (8%), and the dye producing industry (7%) [13]. Long-term or inadvertent exposure to colors and chemicals can pose serious health risks, especially respiratory disorders [14]. They cause severe damage to aquatic life, the food chain, and the environment’s aesthetics. Hence, it is important to take steps to reduce the severity of the harm or even achieve a risk-free level environment [15,16,17,18]. As a result, finding an appropriate solution for treating organic dyes in wastewater of printing and dyeing is an enormous challenge and a high priority.

Various techniques for the removal of dye have been investigated, including adsorption, electrochemical oxidation, chemical method, biological method, coagulation, membrane separation [19], osmosis [20], precipitation [21], and ion exchange [22]. Every technology, however, has its drawbacks too. For instance, the oxidation method produce harmful byproducts, coagulation cause colloidal pollution, while ozonation, photo Fenton and photo catalysis are expensive and difficult to handle [23]. However, adsorption has been regarded as the superior one among the other techniques in terms of its simplicity of design, flexibility, initial cost, and ease of operation [24].

Different substances have been explored in this aspect as adsorbents through various research projects, such as clay materials [25,26,27,28,29], fly ash [30], activated carbon [31], zeolite [32], the biological straw husk [33], fruit peels [34,35,36,37,38], hoshana marble waste [39], exhausted coffee ground powder [40], raw date seed [29], and fallen phoenix’s leaves [41]. Factors that influence adsorption efficiency include surface area of the adsorbent, the association between adsorbate and adsorbent, the ratio of adsorbent to adsorbate, temperature, particle size of the adsorbent, contact time and pH [24,42]. Nanocomposites are materials comprising at least one of these phases in a range of nanometer scale, having improved properties as compared to their constituent elements in uncombined form. Improved mechanical strength or high ductility, improved optical properties, and low friction are the characteristic properties of nanocomposites [43].

To the best of our knowledge, nanocomposites of mint and tawera stone waste have not been investigated to check their adsorption potential until now in the literature. Therefore, the present research work was conducted to explore the use of modified waste biomass of mint and tawera marble as a composite material. These composites were used as adsorbents to eliminate two dyes, DRIM black Ep-B and DRIM blue HS-RL. These composites were investigated for their adsorption capacity without further treatment as well as after treating them with potassium ferricyanide and sodium metasilicate. These dyes were treated with these composites at varying doses, concentrations, pH, temperatures, and times. Optimized conditions were investigated to reveal which dye removal efficiency was maximum using the kinetic and isothermal models.

2. Materials and Methods

2.1. Materials

Drimarene blue HS-RL and Drimarene black ep-B were purchased from a dyeing industry in Faisalabad, Pakistan. Tawera marble stone (500 g) was obtained from the Faisalabad Marble Industry, Pakistan. The waste mint was collected from the University of Agriculture Faisalabad, Pakistan. Chemicals/reagents, such as sodium metasilicate, potassium ferricyanide, 0.1 M sodium hydroxide, and 0.1 M sulfuric acid, were of analytical grade.

2.2. Preparation of Adsorbents

The collected mint waste was washed several times and boiled in the distillation tank for about 45 min. After boiling, the material was washed carefully with distilled water to wash out any adherent impurity. The washed material was shade dried for several days until complete drying. The dried biomass was then grounded and sieved to obtain the particle size in the range 250–500 µm. After drying at 60 °C, this powdered waste was stored in an airtight container. The other adsorbent, tawera stone, was also washed with distilled water to remove adherent dirt particles and then grinded [44]. After thorough washing, residue was collected and dried in oven at 60 °C. Equal amounts of mint powder (7 g) and tawera stone powder (7 g) were mixed and grinded in a pestle mortar. For the modification, mint powder (7 g), tawera stone powder (7 g), and mint tawera composite (7 g) were separately treated with 10 mL of potassium ferricyanide solution in pestle and mortar to make fine paste. These pastes were kept at room temperature for about 24 h and then washed and filtered until a clear filtrate was obtained. Then, after drying at 60 °C, these dried powders were stored in airtight jars to be used as adsorbents and labelled as MPF (mint treated with potassium ferricyanide), TPF (tawera stone waste treated with potassium ferricyanide), and MTPF (mint–tawera composite treated with potassium ferricyanide). Same method was used to treat mint and stone waste with sodium metasilicate, and the prepared modified materials were stored to be used as adsorbents and they were labelled as follows: MSM (mint treated with sodium metasilicate), TSM (tawera stone waste treated with sodium metasilicate), MTSM (mint–tawera composite treated with sodium metasilicate).

2.3. Adsorbents

Nine combinations of adsorbents were prepared as MB (mint biomass), TSW (tawera stone waste), MTC (mint–tawera composite), MPF (mint treated with potassium ferricyanide), MSM (mint treated with sodium metasilicate), TPF (tawera stone waste treated with potassium ferricyanide), TSM (tawera stone waste treated with sodium metasilicate), MTPF (mint–tawera composite treated with potassium ferricyanide), MTSM (mint–tawera composite treated with sodium metasilicate).

2.4. Spectrophotometric Analysis

The wavelength of maximum absorbance for both dyes (DRIM blue HS-RL and DRIM black ep-B) was determined using UV-visible spectrophotometer (Wincom Company Ltd., Changsha, China). The solutions of both dyes (DRIM blue HS-RL and DRIM black ep-B) for this purpose were prepared according to standard calculation. Exactly, 0.005 g of each dye was dissolved in 50 mL of distilled water to prepare a 25-ppm dye solution. Wavelength range of 340–1000 nm was studied to determine the wavelength at which these dyes show maximum absorbance [45].

2.5. Adsorption Study

To investigate the adsorption process, 300 mL of both dye solutions with 25 ppm concentration was prepared. The weighed amount (0.01 g) of each of nine adsorbents was treated with 10 mL of DRIM blue dye solution in 15 mL test tube. All experiments were triplicated. All these 90 test tubes along with a test tube containing 10 mL of control solution were agitated on an orbital shaker with 300 rpm for 2 h on a test tube rack. After that, all these 90 test solutions were filtered using filer papers, micro syringes and micro filters, and their maximum absorbance was studied at 610 nm and then compared with the control solution. Same procedure was applied separately to investigate the maximum adsorption of DRIM black dye at the wavelength of its maximum absorbance (590 nm).

2.6. Characterization of Adsorbent

Characterization of adsorbent samples was carried out using Fourier transform infrared spectroscopy and scanning electron microscopy. FTIR of Agilent technology Cary 360 FTIR spectrophotometer was used to investigate the presence of functional groups. The scanning electron microscope with TLD and ETD detectors (FEI Nova 450 NanoSEM, Hillsboro, OR, USA) was used to scan the morphology of the adsorbent samples. Zeta sizer and zeta potential analysis was used to measure the particle size (hydrodynamic diameter) and size distribution (Zetasizer Nano ZSP, Malvern, Worcestershire, UK).

2.7. Optimization Study

The effect of varying concentrations of dyes on adsorption capacity of different adsorbents was investigated by treating 10 mL of both dye solutions having concentrations of 5, 10, 15, 25, and 50 ppm with 0.01 g of each adsorbent on an orbital shaker for two hours at 300 rpm. The effect of the dose of the adsorbent on adsorption of dye was investigated by treating 10 mL of 50 ppm dye solution with different doses of the adsorbent, which were 0.005 g, 0.01 g, 0.02 g, 0.03 g and 0.04 g in several 15 mL closed test tubes that were agitated for 2 h at 300 rpm at room temperature and then their maximum absorbance was analyzed using spectrophotometer. The effect of time on the removal of adsorbate was analyzed over the various time intervals, i.e., 15, 30, 60, 120, and 240 min. In this study, 10 mL of 50 ppm dye solution was treated with nine adsorbents with the dose of 0.005 g on an orbital shaker with 300 rpm for 15, 30, 60, 120 and 240 min, at room temperature. The maximum absorbance for each dye test solution were measured after agitation using a spectrophotometer. The effect of varying temperatures on the removal of dye was analyzed over the various temperatures, i.e., 30, 40, 50, 60, and 70 °C. In this study, 10 mL of 50 ppm dye solution and all the adsorbents with dose 0.005 g were taken in closed test tubes that were agitated for 2 h with 300 rpm at all the temperatures and then maximum absorbance of all the test solutions were analyzed using a spectrophotometer.

The effect of various pH on the removal of adsorbate by adsorbent was analyzed for the pH range 5–10. The pH was adjusted using 0.01 M sodium hydroxide and 0.01 M hydrochloric acid solution. In this study, 10 mL of dye solution with fixed initial concentration of 50 ppm was taken in a closed test tube and was agitated with 0.005 g of each adsorbent for two hours at room temperature for all the pH (5, 6, 7, 8, 9, and 10) and then analyzed under a spectrophotometer. In all these experiments, three replicates were taken in each case [44].

2.8. Equilibrium Studies and Modelling

Equilibrium studies were carried out by treating 0.005 g of adsorbent with 10 mL of different concentrations (5, 10, 15, 25, and 50 ppm) of dye solution in 15 mL falcon tube, using an orbital shaker for two hours at room temperature. After completion of the experiment, sample solutions were filtered using micro filters (disposable). The equilibrium and initial concentrations of both dyes were analyzed using a UV-vis spectrophotometer at wavelength of maximum absorbance. At equilibrium, the amount of adsorption, q_e, was calculated using the following formula,

$$\mathrm{q}=\frac{{\mathrm{C}}_{\mathrm{e}}-{\mathrm{C}}_0}{\mathrm{W}}$$

In this equation, C_e (mg/L) is equilibrium concentration, and C₀ (mg/L) is the initial concentrations of dye while W is the amount of adsorbent used in the volume of the solution. Percentage removal of each dye was calculated using the following formula:

$$\mathrm{\%}\mathrm{r}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{a}\mathrm{l},{\mathrm{q}}_{\mathrm{e}}=\frac{{\mathrm{C}}_{\mathrm{e}}-{\mathrm{C}}_0}{{\mathrm{C}}_0}\times 100$$

To study the interaction of the adsorbate with the adsorbent, different isothermal models were applied. These models infer the association of the adsorption capacity (q_e) and the adsorbate concentration of C_e in the liquid state.

The Langmuir isotherm model suggests that a maximum capacity of adsorption is demonstrated for single-layer adsorption. The linear equation of this isotherm is described as follows:

$$\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{Q}}_{\mathrm{e}}}=\frac{1}{{\mathrm{Q}}^0}+\frac{1}{{\mathrm{Q}}^0}{\mathrm{C}}_{\mathrm{e}}$$

The Freundlich isotherm model studies the reversible and non-ideal adsorption. According to an assumption regarding the energetic surface heterogeneity, the Freundlich isotherm can be used for multilayer adsorption. The linearized form of the Freundlich isotherm is given as follows:

$$\mathrm{l}\mathrm{n}{\mathrm{q}}_{\mathrm{e}}=\mathrm{l}\mathrm{n}{\mathrm{k}}_{\mathrm{f}}+\frac{1}{\mathrm{n}}\mathrm{l}\mathrm{n}{\mathrm{C}}_{\mathrm{e}}$$

The Dubinin–Radushkevich isotherm model is used to exhibit the adsorption mechanism using a Gaussian energy distribution on a heterogeneous surface. Linear form of the (D–R) model is expressed as:

$$\mathrm{l}\mathrm{n}{\mathrm{q}}_{\mathrm{e}}=\mathrm{l}\mathrm{n}{\mathrm{q}}_0-\mathsf{\beta }{\mathsf{\epsilon }}^2$$

The Temkin adsorption isotherm studies the non-distinguishable distribution of the binding energies present over many exchanging sites onto the surface of the adsorbent. The linear form is expressed as:

$${\mathrm{q}}_{\mathrm{e}}=\mathrm{B}\mathrm{l}\mathrm{n}{\mathrm{A}}_{\mathrm{t}}+\mathrm{B}\mathrm{l}\mathrm{n}{\mathrm{C}}_{\mathrm{e}}$$

Harkin–Jura isotherm illustrates heterogeneous distribution of pores and is used to describe multilayered adsorption phenomena. The linearized equation of this isotherm is expressed as follows:

$$\frac{1}{{\mathrm{q}}_{{\mathrm{e}}^2}}=\frac{\mathrm{B}}{\mathrm{A}}-\left(\frac{1}{\mathrm{A}}\right)\mathrm{l}\mathrm{o}\mathrm{g}{\mathrm{C}}_{\mathrm{e}}$$

Kinetic modeling was carried out to optimize the contact time at various temperatures for both dyes. Pseudo-first- and pseudo-second-order kinetic model was applied during the present study.

Lagergren’s kinetics equation or pseudo-first-order equation explains liquid-based adsorption on solid capacity. Kinetic describes that the rate of change of the solute uptake with time is directly proportional to the difference in saturation concentration and solid uptake with time.

$$\mathrm{Log} ({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}})=\log {\mathrm{q}}_{\mathrm{e}}-{(\mathrm{K}}_{1,\mathrm{a}\mathrm{d}\mathrm{s}}\mathrm{t})/203.3$$

Here, t is time (min), q_e (mg/g) represents the equilibrium capacity of the adsorption, qt for concentration (mg/g) at any time t, and K_1,ads is the pseudo-first-order rate constant.

Pseudo-second-order kinetic model was applied to simulate adsorption kinetics. Linear form of the pseudo-second-order equation is as follows:

$$\frac{\mathrm{t}}{\mathrm{q}}=\frac{1}{{\mathrm{K}}_{2,\mathrm{a}\mathrm{d}\mathrm{s}}·{\mathrm{q}}_{\mathrm{e}}^2}+\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}$$

where, q_e (mg/g) represents the mass of dye absorbed at equilibrium, q_t (mg/g) is the mass of dye adsorbed at time t (min), and K_2,ads (g/mg·min) pseudo-second-order rate constant of adsorption. Parameters of q_e and K_2,ads were computed from the y-axis intercept and the slope of the graph [46].

3. Results and Discussion

3.1. Determination of Wavelength of Maximum Absorbance of Dye Solutions

Spectrophotometric analysis was performed to calculate the wavelength of maximum absorbance of both the dye solutions (DRIM blue HS-RL and DRIM black ep-B) using 25 ppm concentration of dye solution. Scanning the DRIM blue HS-RL and DRIM black ep-B dyes solution through 340 nm to 1000 nm wavelength via the UV visible spectrophotometer revealed that they showed maximum absorbance at wavelengths of 610 nm and 590 nm, respectively [47], as presented in Figure 1.

3.2. Characterization

Mint–tawera composite (MTC) and mint–tawera composite treated with potassium ferricyanide (MTPF) were subjected to characterization via Fourier transform electron microscopy and scanning electron microscopy to investigate the presence of different functional groups of these adsorbents and their surface morphology, respectively.

3.2.1. FTIR Analysis

Fourier transforms infrared spectroscopy is a fundamental technique used for the functional group identification of different species [48]. Absorption peaks were observed at 711, 1390, 1795 and 2513 cm⁻¹ which show the presence of calcite (CaCO₃) in the composite [49]. The absorption peak observed at 874 cm⁻¹ revealed the presence of silicate in the material. The absorption peak at 710 cm⁻¹ showed the symmetric vibration of Ca-O bond of CaCO₃ and the absorption peak observed at 1410 cm⁻¹ is assigned to the asymmetric stretching vibrational frequency of the carbonate ion [50]. The absorption peak observed at 3309 cm⁻¹ shows the OH while the absorption peak observed at 2918 cm⁻¹ shows the C–H stretching band. There was no substantial difference between the two spectra. The spectra of the potassium ferricyanide-treated composition have shown a shift of the OH band to a higher vibration frequency (Figure 2).

3.2.2. SEM Analysis

The surface morphology of the adsorbent samples was investigated using a scanning electron microscope. SEM images of mint–tawera composite is shown in Figure 3. It illustrated the porous morphology of the sample with different pore shapes and different pore sizes. Various cavities were also observed in the images that may cause good accessibility of the adsorbate to the adsorbent surface for adsorption. The SEM images of the mint–tawera compound treated with potassium ferricyanide also illustrated porosity in the structure, which caused the absorptivity of the adsorbate. When the MTPF image was compared with that of the untreated MTC composite, the agglomeration of the particles was reduced, leading to its increased absorptivity compared to the untreated mint–tawera composite (Figure 3).

3.2.3. Zeta (ζ) Potential and Particle Size of Nanocomposite

MTPF was selected for zeta (ζ) potential and sizer analysis due to its highest adsorption capacity among all the nine adsorbent combinations. Zeta (ζ) analysis was performed to check the nanocomposite’s (MTPF) stability. Zeta sizer analysis of MTPF is presented in

Table 1. Zeta potential (ζ) and the particle size of composites.

Nanocomposite	Zeta Potential (mV)	Particle Size (nm)
MTPF	−18.5	1055

. The graph of size distribution versus intensity of MTPF resulted in two prominent peaks. The first peak showed 746.7 nm (size), 91.7% (intensity) and 194.3 (St Dev). The second peak depicted 187 nm (size), 8.3% (intensity) and 32.70 (St Dev). The zeta (ζ) average diameter was 1055 nm. These peaks revealed the presence of variable-sized particles along with some extent of agglomeration in the MTPF adsorbent. This heterogeneity in particle size is also supported through the SEM image of MTPF as presented above. The nanocomposite adsorbent MTPF showed a zeta (ζ) potential measurement of −18.5 mV (Table 1), with a standard deviation of 5.98 mV and conductivity of 0.0654 mS/cm. The negative value of zeta (ζ) potential indicated the presence of highly electronegative groups on the surface of the nanocomposites. While zeta potential <−30 mV investigates the anionic character, >+30 mV revealed the cationic nature of the particles. In the case of MTPF, zeta potential of −18.5 mV indicates the anionic nature of the adsorbent particles. Moreover, the negative value of the zeta potential showed promising results for the adsorption process.

3.3. Optimization of Different Parameters

3.3.1. Effect of Initial Dye Concentration

Dye removal is largely concentration-dependent. As the concentration of dye is increased, the time taken to reach equilibrium also increased (Figure 4). The dye adsorbed per unit mass of the adsorbent increased with the increasing initial dye concentration due to the high driving force for mass transfer. As in the case of both dyes, maximum removal capacity for all the adsorbents was found at 50 ppm when treated with 5, 10, 15, 25, and 50 ppm dye solutions. Hence, the higher the concentration, the better the adsorption [51].

Among all the nine adsorbents, MTC has shown the highest adsorption capacity (176.93 mg/g) to remove DRIM blue, which was further improved up to 209.20 mg/g after its treatment with potassium ferricyanide to make the MTPF composite. Other adsorbents, such as MB, TSW, MPF, MSM, TPF, TSM, and MTSM showed adsorption capacity values of 54.68, 53.15, 141.99, 102.38, 94.84,94.94 and 159 mg/g, respectively. MTC also efficiently adsorbed DRIM black up to 107.57 mg/g, which was further improved to 123.95 mg/g after it was treated with potassium ferricyanide. Other adsorbents including MB, TSW, MPF, MSM, TPF, TSM, and MTSM showed adsorption capacity values of 56.93, 56.56, 62.90, 83.0, 90.50, 90.08, and 93.79 mg/g, respectively.

3.3.2. Effect of Adsorbent Dose

The effect of adsorbent dose was investigated by treating the adsorbent–adsorbate solution with 50 ppm initial dye concentration and varying adsorbent doses of 0.005, 0.005, 0.01, 0.02, 0.03, and 0.04 g (Figure 5). These solutions were shaken on an orbital shaker at 300 rpm for 2 h. Absorbance of each solution was measured after treatment and compared with the absorbance of the dye solution. The removal percentages versus adsorbent dose were plotted [52].

Figure 5 shows that as the adsorbent dose increased, the proportion of dye removed decreased. The percentage removal of dye was rapid at first, but it slowed as the dosage was increased. This phenomenon can be attributed to the fact that the adsorbate or dye was more easily accessible at smaller doses of adsorbent, resulting in an increased removal of the adsorbent or dye per unit of mass. As the adsorbent dosage was increased, there was less equivalent adsorption, because several sites remained unsaturated during this adsorption [53]. Consequently, with increasing adsorbent dosage, the percentage adsorption of adsorbate adsorbed per unit weight of adsorbent was reduced and caused a decline in q_e value. Maximum percentage removal per unit mass of the adsorbent was investigated at 0.005 g adsorbent for both dyes [41].

In the case of DRIM blue, as the dosage of the adsorbent was increased from 0.005 g to 0.04 g, adsorption capacity of different adsorbents decreased: MB (48 mg/g to 11 mg/g), TSW (28 mg/g to 12 mg/g), MTC (22 mg/g to 4 mg/g), MPF (44 mg/g to 7 mg/g), MSM (38 mg/g to 11 mg/g), TPF (60 mg/g to 3 mg/g), TSM (99 mg/g to 2 mg/g), MTPF (35 mg/g to 7 mg/g), and MTSM (53 mg/g to 4 mg/g). For DRIM black, as the dosage of the adsorbent was increased from 0.005 g to 0.04 g, adsorption capacity of different adsorbents decreased: MB (34 mg/g to 5 mg/g), TSW (42 mg/g to 2 mg/g), MTC (17 mg/g to 1 mg/g), MPF (24 mg/g to 3 mg/g), MSM (22 mg/g to 6 mg/g), TPF (18 mg/g to 3 mg/g), TSM (17 mg/g to 2 mg/g), MTPF (28 mg/g to 1 mg/g), and MTSM (17 mg/g to 3 mg/g).

3.3.3. Effect of Time at Different Temperature

The effect of contact time was investigated at an initial concentration of 50 ppm, adsorbent dose of 0.005 g and contact time of 15, 30, 60, 120, and 240 min at various temperatures (30–70 °C). The removal efficiency of all dyes increased with contact time at all temperatures [54], as presented in Figure 6 and Figure 7. The time taken to attain the equilibrium state is called equilibrium time, and the amount of adsorbate deposited at this equilibrium time represents the maximum adsorption (deposition) capacity of the adsorbent under specific operating conditions for both dyes [55].

The graphical results (Figure 6 and Figure 7) revealed that at all the temperatures, adsorption capacity per unit mass of adsorbent increased with increasing contact time. Further increase in contact time at 70 °C did not increase the adsorption. It might be due to the deposition of adsorbate on the available active sites for adsorption on the adsorbent material. At this point, the rate of desorption of the adsorbate from the adsorbent is in dynamic equilibrium with the rate of the adsorbate being adsorbed onto the adsorbent. The time taken for reaching the equilibrium state is called equilibrium time, and the amount of adsorbate deposited at this equilibrium time represents the maximum adsorption (deposition) capacity of the adsorbent under specific operating conditions [55].

3.3.4. Effect of Temperature

The effect of temperature was investigated at an initial dye concentration of 50 ppm, adsorbent dose of 0.005 g, contact time of 240 min and temperatures of 30, 40, 50, 60 and 70 °C (Figure 8a,b). In the case of DRIM blue (Figure 8a), MPF and TSM showed maximum removal efficiency at 30 °C, MTC and TPF at 40 °C, MB and MTSM at 50 °C, and MSM and MTPF showed maximum efficiency at 60 °C. The above results revealed that adsorption may be endothermic as well as exothermic that vary from adsorbent to adsorbent. In the case of DRIM black (Figure 8b), MB, TSW, MTC, and MSM, showed maximum adsorption capacity at 60 °C and MTSM showed maximum capacity at 70 °C, which gives us the evidence of an endothermic adsorption between these adsorbents and dye. Increased adsorption at high temperature is due to the increased number of available active sites but to a certain limit, it is also because further increase in temperature can cause hindrance due to high mobility of the dye molecules [56].

3.3.5. Effect of pH

The adsorption capacity is significantly influenced by pH. To investigate the effect of pH on the adsorption capacity of different adsorbents, pH range from 5 to 10 was studied. Solutions of different pH were prepared using 0.1 M NaOH and 0.1 M dilute HCl solution. These experimental solutions were shaken for 2 h at 300 rpm on an orbital shaker. Figure 9a,b show that the most favorable adsorption occurred at pH 6 and 5 for DRIM blue HS-RL and DRIM black ep-B, respectively. With increasing pH of the solutions, adsorption capacity was reduced because they are reactive anionic dyes. Higher adsorption at acidic pH was due to the presence of excessive H⁺ ions on the surface of the adsorbent, so there is more attraction for the neutral charged anionic dye. Consequently, the adsorption of the anionic dye increased [57].

3.4. Plausible Nanocomposite Adsorption Process and Mechanism

Figure 10 illustrates the adsorption mechanisms along with the adsorption process. In the adsorption process, the adsorbate molecules adsorb to the adsorbent surface in the form of monolayer or they may adsorb in the form of multilayers from the bulk solution, as evident from the adsorption isotherm equations. The difference in concentration between the adsorbent and the bulk solution is basically the driving force behind this adsorption [58]. The efficient adsorption mechanism may involve π–π interactions between double bonds present in the adsorbate and adsorbent molecules: hydrogen bonding between H atom of adsorbent and electronegative elements of adsorbate (-F, -O, an -N), and electrostatic interactions of opposite charges present in the anionic adsorbate and adsorbent [59]. Hence, this is a chemisorption type of adsorption in which the adsorbate–adsorbent molecules interact chemically following the pseudo-second-order kinetic equation as evident from the kinetic modeling below.

3.5. Adsorption Isotherms

Adsorption of dyes onto the nine different adsorbents was examined using the Langmuir, Dubinin–Radushkevich, Freundlich, Temkin and Harkin Jura isotherm models (

Table 2. A comparison of various isotherm models for DRIM blue HS-RL adsorption.

Isotherm Models	Parameters	MB	TSW	MTC	MPF	MSM	TPF	TSM	MTPF	MTSM
Maximum adsorption capacity (experiment)	qmax (exp)	54.68	53.15	176.92	142	102.4	94.85	94.95	209.2	159.1
Langmuir isotherm	qmax (cal) (mg/g)	82.64	71.43	179.29	153.85	161.29	109.89	156.25	263.16	243.91
	KL (L/mg)	0.04	0.06	0.18	0.29	0.05	0.15	27.48	0.11	0.05
	R2	0.83	0.98	0.94	0.99	0.99	0.96	0.97	0.97	0.82
Freundlich	qmax (cal) (mg/g)	50.71	59.78	268.71	153.36	139.93	94.33	97.46	218.95	145.57
	R2	0.97	0.96	0.99	0.96	0.99	0.90	0.99	0.97	0.98
	Kf (L/mg)	5.56	6.85	47.82	47.41	8.37	28.19	8.73	35.98	17.86
	N	1.72	1.76	1.82	30.5	1.31	3.07	1.54	1.87	1.69
Dubinin–Radushkevich	qo cal (mg/g)	34.11	39.19	160.90	108.42	73.09	67.38	61.34	136.66	89.36
	β (mol2/J2)	3 × 10−6	3 × 10−6	3 × 10−6	4 × 10−7	3 × 10−6	3 × 10−6	4 × 10−7	3 × 10−6	7 × 10−7
	R2	0.71	0.84	0.76	0.82	0.83	0.49	0.75	0.81	0.64
	E (KJ/mol)	408.3	408.3	597.6	1290.9	408.3	1118	408.3	845.2	707.1
Temkin	B	16.19	15.43	66.54	27.11	33.17	19.43	30.16	53.68	44.12
	R2	0.88	0.98	0.93	0.99	0.98	0.85	0.95	0.97	0.87
	At (L/g)	2.29	1.65	133.37	5.09	1206.1	2.69	2.19	1.29	1.55
Herkin–Jura	R2	0.81	0.75	0.73	0.78	0.72	0.95	0.79	0.71	0.86
	A	172.41	227.27	2500	3333.4	454.54	2000	400	2000	1111.1
	B	1.55	1.52	1	1.34	1.41	1.8	1.44	1.2	1.43

and

Table 3. A comparison of various isotherm models for DRIM black ep-B HS-RL adsorption.

Isotherm Models	Parameters	MB	TSW	MTC	MPF	MSM	TPF	TSM	MTPF	MTSM
Maximum adsorption capacity (experimental)	qmax (exp)	56.93	56.56	107.57	62.91	83.01	90.51	90.07	123.95	93.7
Langmuir isotherm	qmax (mg/g) cal	92.59	98.04	92.59	73.53	108.6	125	47.62	46.29	55.86
	KL (L/mg)	0.112	0.074	0.108	0.11	0.07	0.06	0.24	0.22	0.15
	R2	0.96	0.96	0.99	0.96	0.96	0.98	0.98	0.99	0.99
Freundlich isotherm	qmax (cal) (mg/g)	77.70	640.7	84.26	62.67	83.16	97.99	47.08	45.58	48.99
	R2	0.99	0.93	0.94	0.95	0.99	0.98	0.66	0.91	0.91
	−Kf (L/mg)	19.21	17.29	15.35	14.12	13.95	0.93	17.24	15.34	16.97
	N	2.676	0.356	2.201	2.534	2.636	1.803	3.799	3.511	3.591
Dubinin–Radushkevich	qo (mg/g)cal	58.66	1.382	56.55	49.25	58.38	66.74	42.10	37.77	38.95
	β (mol2/J2)	7 × 10−5	9 × 10−5	1 × 10−4	1 × 10−4	1 × 10−4	1 × 10−4	1 × 10−4	8 × 10−5	5 × 10−5
	R2	0.75	0.66	0.87	0.85	0.69	0.76	0.90	0.78	0.519
	E (KJ/mol)	84.51	74.53	70.71	70.711	70.71	70.711	70.711	79.056	100
Temkin	B	17.74	17.74	20.35	14.756	22.22	26.914	7.816	8.559	9.619
	R2	0.74	0.94	0.96	0.95	0.95	0.97	0.71	0.92	0.90
	At (L/g)	1.543	1.195	1.046	1.268	1.241	1.517	6.859	3.623	3.253
Herkin–Jura	R2	0.88	0.93	0.77	0.76	0.91	0.83	0.58	0.85	0.88
	A	1000	666.6	666.6	555.56	714.2	588.23	588.24	625	833.34
	B	1.6	1.6	1.467	1.556	1.643	1.471	1.706	1.875	1.916

). Adsorption equilibrium studies demonstrated the adsorbent surface properties and affinity of the adsorbents. Various isotherms were compared in terms of their q_max (mg/g) and R². According to Table 1 and Table 2, for adsorption of DRIM blue, using adsorbents MB, TSW, MSM, TPF, TSM, MPTF, and MTSM, and DRIM black uptake using adsorbents, such as MB, MPF, MSM, and TPF, followed the Freundlich isotherm. Freundlich isotherm investigates the physiochemical type of adsorption on heterogeneous sites of adsorbent with different energies as well as non-equal availability of adsorbent sites for adsorption. For the adsorption of DRIM blue, two adsorbents, MTC and MPF, and for the adsorption of DRIM black, TSW, MTC, TSM, MPTF, and MTSM followed the Langmuir isotherm [60]. This isotherm considers the monolayer adsorption of the adsorbate on the adsorbent surface having uniform or homogeneous sites for adsorption that do not provide the transmigration phenomenon of dye in the plane of the adsorbent surface [61].

3.6. Kinetic Studies

Kinetic modeling was carried out to optimize the contact time at various temperatures for both dyes. Lagergren’s kinetics equation or pseudo-first-order equation explains liquid-based adsorption on solid capacity. It states that the rate for the change of the solute uptake with increasing time is proportional to the difference in solid uptake and saturation concentration. Pseudo-second-order model was also applied to simulate adsorption kinetics (

Table 4. A comparison of pseudo-first-order and pseudo-second-order kinetics at 30 °C.

Adsorbent	Pseudo-First-Order Kinetics			qe exp (mg/g)	Pseudo-Second-Order Kinetics
	qe Cal (mg/g)	K1,ads	R2		qe (mg/g)	K2,ads	R2
DRIM blue HS-RL
MB	439.54	1 × 10−2	0.94	282.09	370.37	6.2 × 10−5	0.99
TSW	345.06	1 × 10−2	0.97	308.86	357.14	9.5 × 10−5	0.99
MTC	118.35	1.7 × 10−2	0.98	311.46	312.5	3.36 × 10−4	0.99
MPF	285.82	1.8 × 10−2	0.97	406.51	737.78	1.5 × 10−4	0.99
MSM	156.89	1.7 × 10-	0.94	333.18	344.83	3.0 × 10−4	0.99
TPF	340.48	1 × 10−2	0.97	239.43	322.58	6.9 × 10−5	0.98
TSM	271.45	1.8 × 10−2	0.89	358.04	416.66	8.6 × 10−5	0.99
MTPF	231.68	1.8 × 10−2	0.96	290.59	312.50	1.9 × 10−4	0.99
MTSM	280.73	1.8 × 10−2	0.93	352.02	370.37	1.6 × 10−4	0.99
DRIM black ep-B
MB	512.86	1 × 10−2	0.94	438.01	500.0	8.6 × 10−5	0.99
TSW	371.53	1 × 10−2	0.98	375.00	500.0	1.18 × 10−4	0.99
MTC	575.40	1 × 10−2	0.93	412.94	500.0	6.5 × 10−5	0.96
MPF	446.68	1.1 × 10−2	0.98	437.08	500.0	8.6 × 10−5	0.98
MSM	316.22	9 × 10−3	0.96	406.30	500.0	1.5 × 10−4	0.99
TPF	371.53	9 × 10−3	0.90	408.90	500.0	1.2 × 10−4	0.98
TSM	154.88	8 × 10−3	0.87	283.79	333.3	2.49 × 10−4	0.99
MTPF	380.19	9 × 10−3	0.91	351.68	500.0	1.00 × 10−4	0.98
MTSM	173.78	9 × 10−3	0.98	290.29	333.3	2.3 × 10−4	0.98

,

Table 5. A comparison of pseudo-first-order and pseudo-second-order kinetics at 40 °C.

Adsorbent	Pseudo-First-Order Kinetics			qe (mg/g) exp	Pseudo-Second-Order Kinetics
	qe (mg/g) Cal	K1,ads	R2		qe (mg/g)	K2,ads	R2
DRIM blue HS-RL
MB	431.51	1 × 10−2	0.91	333.91	370.37	8.9 × 10−5	0.97
TSW	265.82	9 × 10−3	0.97	361.63	384.62	1.9 × 10−4	0.99
MTC	491.69	1 × 10−2	0.93	327.21	400.00	6.67 × 10−5	0.99
MPF	369.14	1 × 10−2	0.95	270.23	333.33	8.5 × 10−5	0.99
MSM	120.69	8 × 10−3	0.99	387.25	285.71	2.2 × 10−4	0.99
TPF	615.88	1 × 10−2	0.93	358.63	500.00	5.9 × 10−5	0.99
TSM	363.83	1 × 10−2	0.96	284.34	400.00	2.0 × 10−4	0.99
MTPF	216.97	9 × 10−3	0.97	352.20	333.30	1.3 × 10−4	0.99
MTSM	372.30	9 × 10−3	0.92	362.12	384.62	1.0 × 10−4	0.99
DRIM black ep-B
MB	131.83	9 × 10−3	0.93	245.22	333.3	3.3 × 10−4	0.99
TSW	204.17	9 × 10−3	0.93	258.17	333.3	2.1 × 10−4	0.99
MTC	169.82	9 × 10−3	0.98	242.28	333.3	2.4 × 10−4	0.99
MPF	177.82	8 × 10−3	0.97	234.95	250.0	2.2 × 10−4	0.99
MSM	194.98	9 × 10−3	0.98	247.87	333.3	2.0 × 10−4	0.99
TPF	218.77	9 × 10−3	0.94	247.64	333.3	1.8 × 10−4	0.99
TSM	281.94	9 × 10−3	0.99	253.54	333.3	1.2 × 10−4	0.99
MTPF	194.98	9 × 10−3	0.97	247.10	333.3	2.0 × 10−4	0.99
MTSM	208.92	8 × 10−3	0.91	255.93	333.3	1.9 × 10−4	0.98

,

Table 6. A comparison of pseudo-first-order and pseudo-second-order kinetics at 50 °C.

Adsorbent	Pseudo-First-Order Kinetics			qe (mg/g) exp	Pseudo-Second-Order Kinetics
	qe (mg/g) Cal	K1,ads	R2		qe (mg/g)	K2,ads	R2
DRIM blue HS-RL
MB	318.19	1 × 10−2	0.99	341.17	500.00	1.2 × 10−4	0.99
TSW	371.53	1 × 10−2	0.93	299.19	384.61	9.3 × 10−5	0.99
MTC	257.04	9 × 10−3	0.96	302.59	344.83	1.8 × 10−4	0.99
MPF	354.81	9 × 10−3	0.91	347.31	333.33	1.2 × 10−4	0.99
MSM	416.87	1 × 10−2	0.98	318.17	370.37	7.6 × 10−5	0.96
TPF	407.38	9 × 10−3	0.90	322.07	416.66	9.6 × 10−5	0.97
TSM	446.68	0.010	0.94	364.73	357.14	8.3 × 10−5	0.99
MTPF	301.99	9 × 10−3	0.97	331.38	416.66	1.1 × 10−4	0.99
MTSM	275.42	9 × 10−3	0.93	373.32	357.14	1.7 × 10−4	0.99
DRIM black ep-B
MB	213.76	9 × 10−3	0.93	503.19	1000	3.9 × 10−4	0.99
TSW	177.82	9 × 10−3	0.99	578.86	500.0	2.9 × 10−4	0.99
MTC	72.44	7 × 10−3	0.95	430.86	500.0	6.7 × 10−4	0.99
MPF	154.88	9 × 10−3	0.99	447.35	500.0	5.0 × 10−4	0.99
MSM	229.08	9 × 10−3	0.97	437.41	500.0	2.6 × 10−4	0.99
TPF	794.32	1 × 10−2	0.93	571.35	1000	5.1 × 10−5	0.99
TSM	676.08	1.1 × 10−2	0.93	505.60	1000	6.5 × 10−5	0.99
MTPF	190.57	9 × 10−3	0.99	420.21	500.0	2.8 × 10−4	0.99
MTSM	169.82	9 × 10−3	0.98	540.09	1000	3.4 × 10−4	0.99

,

Table 7. A comparison of pseudo-first-order and pseudo-second-order kinetics at 60 °C.

Adsorbent	Pseudo-First-Order Kinetics			qe (mg/g) exp	Pseudo-Second-Order Kinetics
	qe (mg/g) Cal	K1,ads	R2		qe (mg/g)	K2,ads	R2
DRIM blue HS-RL
MB	316.22	9 × 10−3	0.97	334.14	500.0	1.2 × 10−4	0.99
TSW	275.42	9 × 10−3	0.96	412.95	500.0	1.9 × 10−4	0.99
MTC	398.11	1 × 10−2	0.91	298.54	500.0	8.6 × 10−5	0.98
MPF	234.42	9 × 10−3	0.96	277.80	333.3	1.8 × 10−4	0.99
MSM	223.87	9 × 10−3	0.96	324.31	500.0	2.3 × 10−4	0.99
TPF	223.87	9 × 10−3	0.97	308.88	333.3	3.5 × 10−4	0.99
TSM	275.42	9 × 10−3	0.92	287.93	333.3	4 × 10−4	0.99
MTPF	87.09	8 × 10−3	0.99	326.08	333.3	9.4 × 10−4	0.99
MTSM	98.49	8 × 10−3	0.99	198.04	333.3	4.2 × 10−4	0.99
DRIM black ep-B
MB	562.34	1 × 10−2	0.95	517.39	1000	9.3 × 10−5	0.99
TSW	851.13	1 × 10−2	0.88	541.21	1000	8.8 × 10−5	0.96
MTC	1202.1	1 × 10−2	0.88	502.39	1000	6.6 × 10−5	0.95
MPF	109.6	8 × 10−3	0.99	251.29	333.3	3.9 × 10−4	0.99
MSM	0.47	1.1 × 10−2	0.91	616.09	1000	4.3 × 10−5	0.99
TPF	123.02	8 × 10−3	0.97	224.79	250.0	3.3 × 10−4	0.99
TSM	213.79	9 × 10−3	0.98	321.24	500.0	2.4 × 10−4	0.99
MTPF	208.92	8 × 10−3	0.89	264.23	333.3	2.0 × 10−4	0.99
MTSM	426.57	1 × 10−2	0.92	409.23	500.0	3.0 × 10−4	0.98

and

Table 8. A comparison of pseudo-first-order and pseudo-second-order kinetics at 70 °C.

Adsorbent	Pseudo-First-Order Kinetics			qe (mg/g) exp	Pseudo-Second-Order Kinetics
	qe (mg/g) Cal	K1,ads	R2		qe (mg/g)	K2,ads	R2
DRIM blue HS-RL
MB	87.09	8 × 10−3	0.99	253.48	333.3	5.2 × 10−4	0.99
TSW	275.4	9 × 10−3	0.94	282.06	333.3	1.5 × 10−4	0.99
MTC	467.7	1 × 10−2	0.93	264.27	500.0	4.4 × 10−5	0.96
MPF	269.2	9 × 10−3	0.93	300.08	333.3	1.5 × 10−4	0.99
MSM	91.2	9 × 10−3	0.99	215.66	250.0	5.4 × 10−4	0.99
TPF	239.88	8 × 10−3	0.95	242.48	333.3	1.5 × 10−4	0.99
TSM	398.1	9 × 10−3	0.92	250.28	333.3	6.6 × 10−5	0.98
MTPF	295.1	1 × 10−3	0.97	293.11	333.3	1.2 × 10−4	0.99
MTSM	239.8	9 × 10−3	0.96	277.19	333.3	1.8 × 10−4	0.99
DRIM black ep-B
MB	537.03	1 × 10−2	0.91	416.42	500.0	8.2 × 10−5	0.98
TSW	190.54	9 × 10−3	0.95	293.43	333.3	2.3 × 10−4	0.99
MTC	151.2	7 × 10−3	0.99	130.97	142.85	3.2 × 10−4	0.99
MPF	138.3	3.5 × 10−3	0.99	245.10	333.3	1.1 × 10−4	0.99
MSM	114.4	3 × 10−3	0.99	297.88	333.3	1.8 × 10−4	0.99
TPF	104.6	2 × 10−3	0.99	345.04	500.0	4.2 × 10−4	0.99
TSM	107.8	3 × 10−3	0.99	276.17	333.3	4.3 × 10−4	0.99
MTPF	112.3	3 × 10−3	0.99	260.57	333.3	2.9 × 10−4	0.99
MTSM	223.8	9 × 10−3	0.94	171.78	250	1.3 × 10−4	0.98

). A comparison between the pseudo-first- and second-order kinetic models at different temperatures revealed the best effectiveness of the pseudo-second-order kinetic model for all the adsorbents as their R² are higher when compared with the pseudo first-order R². Moreover, the calculated values of q_e from the pseudo-second-order model agreed best with the q_e obtained experimentally at all the temperatures. Hence, this best-fit model investigates the dependence of adsorption rate on the adsorption capacity of the adsorbent as well as the chemisorption type of adsorption. Lagergren’s first-order model that describe the adsorption through interface by diffusion process, failed to be a fit on our experimental data as it does not describe the rate constant, K_1,ads, and the calculated q_e also not agreed with experimental q_e. These kinetic results are very helpful technologically, as they save the time and usage of experimental material to investigate the equilibrium adsorption capacity experimentally [62]. Existing literature clearly shows that the adsorbents used in the present study have better adsorption capacities than most of the previous adsorbents (

Table 9. A comparison of various adsorbents for their dye adsorption capacities.

Adsorbents	Dyes	Adsorption Capacity (mg/g)	References
The yellow passion fruit	Methylene Blue	44.67	[13]
Banana peel	Basic violet 10	20.6	[13]
Dragon Fruit Peels	Alcian blue	71.85	[34]
Dragon fruit leaves	Methylene blue	62.58	[34]
Wheat shell	Methylene blue	21.5	[51]
Wood apple shell	Malachite green	80.64	[54]
Fallen phoenix tree’s leaf	Methylene blue	89.7	[41]
Bentonite clay	Reactive black 5	29.38	[46]
Hoshanar marble waste	Direct violet 51	105.31	[39]
Raw date seed	Methyl violet	59.5	[29]
Banana peel	Reactive dye	28.8	[38]
MB	DRIM blue	54.68	Present study
TSW		53.15
MTC		176.93
MPF		141.99
MSM		102.38
TPF		94.84
TSM		94.94
MTPF		209.22
MTSM		159.11
MB	DRIM black	56.93
TSW		56.56
MTC		107.57
MPF		62.90
MSM		83.00
TPF		90.50
TSM		90.08
MTPF		123.95
MTSM		93.79

).

3.7. Reusability of Adsorbent

The reusability of MTPF having the maximum adsorption capacity was investigated using CH₃OH as an eluent for up to five consecutive cycles (Figure 11). The amount of dye present in the solution was checked to calculate the removal efficiency after each cycle. It can be concluded from the results that dye uptake capacity of the adsorbent was very little affected after each cycle. Thus, this adsorbent can be effectively used for dye removal for several cycles.

3.8. Comparison of Published Studies on Adsorbents for Their Dye Adsorption Capacities

Table 8 compares the produced adsorbent with various other adsorbents for their dye adsorption capacities. As seen in Table 9, the produced adsorbent shows higher dye adsorption capacity as compared to the ones published in the literature. Methylene blue removal over yellow passion fruit, dragon fruit leaves, wheat shell and fallen phoenix tree’s leaf depicted adsorption capacities of 44.67, 62.58, 21.5 and 89.7 mg/g, respectively. Gisi et al. [13] studied the adsorption of basic violet 10 by banana peel and the dye removal was 20.6 mg/g. In another study, Mallampati et al. [34] studied the removal of dissolved heavy metals and dyes from water using dragon fruit peels and it was noted that the adsorption capacity was 71.87 mg/g. In Table 8, a brief comparison is reported for the adsorption capacities of dye removal using different adsorbents.

4. Conclusions

The use of natural waste products not only makes the adsorption process economical, but it also gives us an opportunity to reduce the risk of pollution caused by such waste products. This study involved the use of nine different combinations of waste mint and tawera marble in modified and unmodified forms to treat dye-contaminated wastewater: MB, TSW, MTC, MPF, MSM, TPF, TSM, MTPF, and MTSM. FTIR analysis of MTC and MTPF showed the presence of calcite and silicates in the composites. SEM revealed the porous structure of MTC with some extent of agglomeration which was reduced by treating the adsorbents with potassium ferricyanide (MTPF). This reduced the agglomeration and significantly enhanced the adsorption process. The MTPF nanocomposite particles have a negative net surface charge (zeta potential = −18.5 mV). Spectrophotometric analysis showed that DRIM blue provides the maximum absorbance at wavelength 610 nm, while for DRIM black it is at wavelength 590 nm. Optimization of different parameters, such as initial concentration of dye (5–50 ppm), dose of adsorbent (0.005–0.5 g), time (60–240 min), temperature (30–70 °C), and pH (5–10), revealed the optimum conditions for the effective removal of DRIM blue: 50 ppm initial concentration, 0.005 g dose of adsorbent, 5 pH, and 240 min contact time. Maximum adsorption of DRIM black was also investigated at 50 ppm initial concentration, 0.005 g adsorbent dosage, 6 pH, and 240 min of contact time. The optimum temperature varied from adsorbent to adsorbent and dye to dye. To adsorb DRIM blue, MB showed an adsorption capacity of 54.68 mg/g, followed by TSW (53.15 mg/g), MTC (176.93 mg/g), MPF (141.99 mg/g), MSM (102.38 mg/g), TPF (94.84 mg/g), TSM (94.94 mg/g), MTPF (209.22 mg/g), and MTSM (159.11 mg/g). The results revealed that the highest adsorption capacity was shown by MTC, which was 176.93 mg/g at 40 °C, which was further improved up to 209.22 mg/g when it was treated with potassium ferricyanide at 60 °C. To adsorb DRIM black, MB showed the maximum adsorption capacity of 56.93 mg/g, followed by TSW (56.56 mg/g), MTC (107.57 mg/g), MPF (62.90 mg/g), MSM (83.00 mg/g), TPF (90.50 mg/g), TSM (90.08 mg/g), MTPF (123.95 mg/g), and MTSM (93.79 mg/g). The results revealed that MTC showed the highest adsorption capacity of 107.57 mg/g, which was further improved up to 123.95 mg/g when it was treated with potassium ferricyanide. Different adsorption isotherm models (such as Langmuir, Freundlich, Temkin, and Herkin–Jura) were applied to investigate the behavior of adsorption for both dyes. MB, TSW, MSM, TPF, TSM, MTPF, and MTSM followed the Freundlich isotherm revealing multilayer adsorption, while MTC, and MPF followed the Langmuir isotherm revealing monolayer adsorption for DRIM blue. MB, MPF, MSM, and TPF followed the Freundlich isotherm revealing multilayer adsorption and TSW, MTC, TSM, MTPF, and MTSM followed the Langmuir isotherm revealing monolayer adsorption for DRIM blue. Application of the pseudo-first- and pseudo-second-order kinetic models revealed the phenomenon of chemisorption and established the pseudo-second-order kinetics model as the best fit for both the dyes.