Adsorption of Crystal Violet onto an Agricultural Waste Residue: Kinetics, Isotherm, Thermodynamics, and Mechanism of Adsorption

Agricultural waste can be exploited for the adsorption of dyes, due to their low cost, availability, cost-effectiveness, and efficiency. In this study, we were interested in the elimination of crystal violet dye, from aqueous solutions, by adsorption on almond shell-based material, as a low-cost and ecofriendly adsorbent. The almond shells were first analyzed by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction; then, the influence of adsorbent dose, initial dye concentration time, and pH were studied to assess adsorption capacity under optimal experimental conditions. Experimental results indicate that almond shell adsorbent removes about 83% of the dye from the solutions at room temperature and in batch mode; the kinetic study showed that the equilibrium time is about 90 min, and the model of pseudo-second order could very well describe adsorption kinetics. The modulation of adsorption isotherms showed that retention follows the Langmuir model. The thermodynamic study has shown that the adsorption is endothermic (ΔH° > 0) and spontaneous (ΔG° < 0).


Introduction
Many industries, such as the textile, plastics, and paper, use dyes to hue their products, and large volumes of water are consumed. Consequently, they generate a considerable quantity of colored wastewaters. More than 100000 dyes available in the trade with more than 7 10 5 t of dyes are produced annually in the world [1]. 1000 t of these dyes are released annually into the aquatic system [2]. e dye is the first pollutant to be detected in wastewater [3]. e presence of a very low concentration of dyes, in water, is very noticeable and undesirable. Several treatment methods were used for treating effluents containing dyes [2,4]. But these differ in their effectiveness, cost, and environmental impact [5]. Adsorption is the most effective technique widely used [6,7], in which activated carbon is the most frequently used adsorbent in the purification of water. e high cost required by adsorption using activated carbon [2] pushes researchers to find other alternatives such as waste or agricultural by-products [4]. Agricultural waste consists mainly of cellulose, hemicellulose, and lignin, which are effective adsorbents for a wide range of pollutants because of their richness in functional groups such as hydroxyl groups, carboxyl, and phenols [8]. Other advantages that make them as excellent candidates are the capacity and the rate of adsorption, the high selectivity for different pollutants, and also rapid kinetics [4]. Agricultural waste is better than other adsorbents, as they are generally used without or with minimal treatment (washing, drying, and grinding) [9]. Several recent studies have used various agricultural wastes for adsorption of dyes in effluents. Table 1 shows some examples.
Almond is the fruit of almond tree, which is the second most important fruit species cultivated in Morocco after the olive tree, with a production of 101,000 t/year in 2016, which is the equivalent of 80,000 tonnes of almond shells, according to data from the Moroccan Federation of Almond Tree Producers. ese residues are discarded as solid waste, which poses environmental problems. It is therefore necessary to find an appropriate method to solve the disposal problem. e use of this inexpensive material as an adsorbent contributes to the solution to this problem and to the application of the principle of "self-cleaning of waste." Crystal violet is widely used as a violet dye in the textile industry for dyeing cotton and silk. It is also used in the manufacture of paints and printing inks [10]. Crystal violet is carcinogenic and has been classified as a recalcitrant molecule because it is poorly metabolized by microbes, is nonbiodegradable, and can persist in various environments. It is highly toxic to the cells [11]. e purpose of this research paper is to investigate the potential use of almond shells to remove the crystal violet dye from aqueous solutions. Adsorption parameters including dye concentration, contact time, almond shell dose, and pH were studied to determine the effectiveness of the adsorbent. e characterization of the adsorbent before and after adsorption was achieved in order to identify the mechanism governing the fixation of the dye molecules on the adsorbent.

Adsorbent Preparation.
e almond shells (AS) were washed, dried for 24 hours at a temperature of 110°C in an oven, and then crushed and sieved to obtain fine and homogeneous samples (<0.2 mm).

Adsorbate Preparation.
Crystal violet (CV) (characteristics given in Table 2) dye was used as the adsorbate. A stock solution of crystal violet (1 g/L) was first prepared by dissolving a known quantity in deionized water. e stock solution was finally diluted to obtain the desired concentration.

Adsorption Study.
All adsorption experiments were realized at room temperature (≈25°C) and in the batch mode. A mass of the adsorbent was contacted with a volume V � 40 ml of the initial crystal violet solution C 0 . e assembly was agitated for a time t of adsorption, and then, the solid was separated from the solution by filtration on a microporous filter. e absorbance of the supernatant solution was measured using a UV-vis spectrophotometer at the wavelength corresponding to the maximum absorbance (λ max � 590 nm). e concentration at time t (C t ) of the dye in the mixture was calculated using a calibration curve prepared from the known concentrations of the CV. e removal percentage (R t (%)) of the CV and the quantity adsorbed to the surface of the AS (q t (mg/g)) were determined using the following equations [23]: where C 0 and C t are the CV initial and final concentration (mg/L) at time t, V is the solution volume (L), and m is the adsorbent mass (g).

Results and Discussion
3.1. X-Ray Diffraction Analysis. Figure 1 shows the diffractograms of the AS before and after adsorption of the dye. It can be seen on the one hand that the two diffractograms are identical which means that the material undergoes no modification after adsorption, and on the other hand, the broadband at about 22°justifies certain crystalline phases in the material. In fact, lignocellulosic materials present defects of structures offering the possibility of obtaining monocrystals called whiskers [24].

Fourier Transform Infrared (FTIR) Spectroscopy.
FTIR spectroscopy is a widely used method for determining the functional groups that serve as adsorption sites. Figure 2 shows the FTIR spectra of the AS before and after CV adsorption. e analysis of the FTIR spectrum shows the presence of many peaks in the range of wavenumbers from 4,000 to 500 cm −1 , which highlights the complex nature of the material analyzed. Before adsorption, the broad band at about 3420 cm −1 corresponds to the elongation of the O-H groups, the band at 2910 cm −1 relates to the elongation of the C-H group, and the band at 1740 corresponds to the elongation vibration of the nonconjugated C�O bonds; these vibrations are mainly due to the ester and carboxylic acid functions present in the lignin, pectin, and hemicelluloses; the 1640 cm −1 band is characteristic for the elongation of the C�C bonds of aromatic compounds, and the 1045 cm −1 band is characteristic of the deformation in the C-O plane of aromatic compounds and acetyl and carboxylic acid functions. After adsorption of the dye, the intensity of the bands decreased significantly and the band of elongation of the O-H widens, indicating the presence of interactions between the AS and CV functional groups.

Effect of Contact Time.
It is necessary to obtain the time at the end of which the adsorption equilibrium is reached. is study was conducted for concentrations of 25 mg/L and 50 mg/L. e results obtained are shown in Figure 3(a), which illustrate the evolution of the adsorbed quantity over time. From the figure, we can see that the equilibrium is reached almost at the end of 90 minutes. e results show the existence of two phases: the first rapid and the second slow. is is related to the high availability of the adsorbent-free active sites at the beginning of the experiment, which decrease as the adsorption progresses. ese curves also show that the fixed quantity q t increases with C 0 .
e curve of C 0 � 25 mg/L is lower than that of C 0 � 50 mg/L.

Effect of Initial Crystal Violet Concentration.
For this study, the initial concentration was varied in the range of 20 mg/L to 100 mg/L by maintaining the adsorbent dose at 5 g/L, the temperature at 20°C, and the pH at 6. e results are shown in Figure 3(b). It can be observed that the CV elimination rate decreases from 84% to 49% when the initial CV concentration varies from 20 to 200 mg/L. e decrease in the removal rate is probably due to the increase in the number of CV ions in the solution for the same number of sites and the same adsorbent surface area.

Effect of the Initial pH of the Solution.
pH is a critical parameter to be taken into account when removing dyes from aqueous solutions, as it can affect the charge on the surface of the adsorbent. e zero charge point pH pzc of AS was 4.7 (inset in Figure 3(c)). e percentage of CV    e Scientific World Journal 3 removal by the AS at different pH values was then studied, while keeping the other parameters at constant values. e results show that the highest removal efficiency of the CV (82%) was observed in the pH range of 6-12. is efficiency decreased to 62% at a pH of 2. Indeed, at pH > pH pzc , the surface of the AS is negatively charged, and this charge increases proportionally to the pH. erefore, removal efficiency increases when the pH is in the range of 3-6 due to attractive forces occurring between the cationic dye and the negatively charged surface. Consequently, the optimal pH value that maximizes the removal of dye from the aqueous solution is 6.

3.3.4.
Effect of Adsorbent's Dose. CV adsorption on AS was studied by varying the dose of the adsorbent from 40 to 400 mg for a concentration of 30 mg/L of CV and a pH � 6. According to Figure 3(d), it can be seen that CV removal has increased rapidly from 42% to 82% in the range of 40-200 mg SA and remains constant in the range of 200 mg-400 mg; this is due to the increase in the contact surface, thus establishing the equilibrium. e adsorbent dose was set at 200 mg for the subsequent experiments.

Adsorption Equilibrium Study.
e adsorption isotherm is the curve binding, at a fixed temperature, the quantity of product adsorbed per initial mass of adsorbent at the concentration residual in the solution after adsorption equilibrium. It was used to determine the maximum adsorption capacity and the type of interaction between the CV and AS. To exploit the data from the CV adsorption isotherm by AS, the Langmuir and Freundlich equations in their linear form were used. e linear form of the Langmuir isotherm is indicated in the following equation: e linear form of the Freundlich isotherm is indicated in the following equation:

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where C e (mg/L) is the equilibrium concentration; Q e (mg/g) is the equilibrium adsorbed quantity; Q m (mg/g) is the maximum adsorption capacity; K L (L/mg) is the Langmuir constant; and K F and n are the Freundlich constants. e representations of the Langmuir and Freundlich models are given in Figure 4, and the equilibrium parameters obtained are shown in Table 3. e value of the correlation coefficient (R 2 ) for the Langmuir isotherm is higher than that of the Freundlich isotherm; this means that the Langmuir model better represents the adsorption process of CV by the AS. is suggests that CV fixation is done in monolayer, without interaction between the adsorbed molecules, on energetically equivalent sites. In addition to that, the R L value is less than 1 which indicates that the adsorption of the dye is favorable.

Adsorption Kinetics.
In order to model the adsorption kinetics, the kinetic models of the pseudo-first order and pseudo-second order were used. e expression of the pseudo-first-order model is in the form cited by Lagergren (5) [25]: e expression of the pseudo-second-order model is in the form cited by Ho and Mckay (6) [26]: where q t (mg·g −1 ) and q e (mg·g −1 ) are the adsorbed quantity of dye at time t and at equilibrium and k 1 (min −1 ) and k 2 (g·mg −1 ·min) are the constants of pseudo-first-order and pseudo-second-order models, respectively. e curves of both models are shown in Figure 5, and the constants obtained from the different models are recapitulated in Table 4.
From the R 2 values reported in Table 4, it can be deduced that the pseudo-second-order model is the one that best describes the CV adsorption process on AS. We also observe that the adsorbed quantities calculated by this model are closer to those determined experimentally.

Effect of Temperature and ermodynamics of Adsorption.
e thermodynamic study was conducted at 25, 30, 40, and 50°C. e tests were performed on 40 ml mixtures of dye solutions at a concentration of 30 mg·L −1 , with 160 mg masses of AS in 100 mL flasks. ese mixtures were maintained at constant agitation of 200 rpm −1 for a time of 4 hours. Figure 6(a) shows the influence of temperature on the dye retention rate. From the figure, we notice that this rate increases with increasing temperature, suggesting that the process is endothermic, and that increasing temperature promotes its progress. ermodynamic parameters such as standard Gibbs free energy change (ΔG°), standard enthalpy change (ΔH°), and standard entropy change (ΔS°) were determined by using the following equations [25]: where K d � q e /C e : distribution constant; R: universal gas constant (8.314 J/mol K); and T: absolute temperature (K). Table 5 gives the values of standard Gibbs free energy change (ΔG°), standard enthalpy change (ΔH°), and standard entropy change (ΔS°), extrapolated from the plot ln (K d ) vs 1/ T (Figure 6(b)). e positive value of ΔH°shows that the adsorption process of the CV on AS is endothermic and that it is indeed a physisorption (<40 kJ·mol −1 ) [27].
e negative values of ∆G°indicate that the adsorption is spontaneous while the positive value of ∆S°indicates the increase in randomness at the solid-liquid interface during sorption. is is the normal consequence of the phenomenon of physical sorption, which occurs through electrostatic interactions. Similar results were obtained during the adsorption of malachite green by almond gum [28] as well as the adsorption of methylene blue by garlic straw [29].

Adsorption Mechanism.
To understand the adsorption mechanism, it is necessary to examine the structure of the adsorbate and the properties of the adsorbent surface. For this purpose, it should be noted that CV is a cationic dye with amine groups in its structure and in aqueous medium dissociates into CV + and Cl − [30]. On the other hand, AS is a lignocellulosic material consisting of cellulose, hemicellulose, and lignin and other minor constituents [31,32]. Cellulose and hemicellulose contain the majority of functional groupa, such as hydroxyl and carboxyl (confirmed by the FTIR spectrum), while lignin is a complex, systematically polymerized, and highly aromatic substance and acts as a cementing matrix that is maintained between and in both cellulose and hemicellulose units. In this study, the removal of CV by AS adsorption is highly pH dependent (Figure 3(c)). e CV has been adequately adsorbed for pH ≥ 5. Based on the experimental results of this study, and depending on the structure of the adsorbate and the properties of the adsorbent surface, the mechanism for removing CV by AS adsorption involves the following steps: (i) Migration of the dye from the solution to the surface of the adsorbent (ii) Dye diffusion through the boundary layer on the surface of the adsorbent (iii) Adsorption of the dye on the AS surface, which can be due to two mechanisms e first mechanism can explain the phenomenon of adsorption by the formation of hydrogen bonds between the surface hydroxyl and carboxyl groups and the nitrogen atoms of the CV as suggested in Figure 7. e second mechanism is a dye-hydrogen ion exchange mechanism because at pH ≥ 5, the surface functional groups    Table 4: Pseudo-first-order and pseudo-second-order adsorption rate constants for the different initial CV concentrations (C 0 ).

Conclusion
e almond shell, a low-cost and easily available material, has proven to be highly effective to remove crystal violet from aqueous solutions. e equilibrium data were analyzed using Langmuir and Freundlich isotherm models. e maximum monolayer adsorption capacity was equal to 12.2 mg/g. e experimental data of the adsorption isotherm follow the Langmuir model and the pseudo-second-order kinetic model. is work clearly shows that the elimination of crystal violet by the almond shell is feasible, efficient, and economical. Moreover, the almond shell is a promising candidate for wastewater treatment.

Data Availability
No data were used to support this study.