Sustainable adsorption method for the remediation of malachite green dye using nutraceutical industrial fenugreek seed spent

Nutraceutical industrial fenugreek seed spent (NIFGS), a relatively low-cost material abundantly available with nearly negligible toxicity for the bioremediation of malachite green (MG) dye from aqueous media, is reported. Studies on the various parameters affecting the adsorption capacity of NIFGS were carried out to evaluate the kinetics and the equilibrium thermodynamics. All the experiments were designed at about pH 7. The adsorption isotherm model proposed by Langmuir fits better than the Freundlich isotherm model. Kinetic study data confirms the viability of pseudo-second-order model. Calculated thermodynamic factors suggest that the adsorption phenomenon is endothermic, almost instantaneous, and physical in nature.


Introduction
A report on nutraceutical industries suggests a possible turnover of USD 722.5 billion by 2027 [1]. These industries produce unprecedented myriad tons of spent/waste. The nutraceutical industrial spent (NIS) produced as a by-product has no feed, fuel, and fertilizer value [2]. Fenugreek plant (Trigonellafoenum-graecum L.) belongs to the legume family, and fenugreek seeds are extensively used as nutraceuticals [3]. India ranks first by producing about 2.40 × 10 5 tons of fenugreek seed which is extensively used as a nutraceutical [4]. The spent left after extracting the lead compound is generally identified as nutraceutical industrial fenugreek spent (NIFGS). The use of NIFGS as an adsorbent for eliminating anionic bisazo dye from aqueous media [5] has been reported inrecent times.
Currently, adsorption technique with the characteristics of simplicity, efficiency, minimum discharge of toxic byproducts [25][26][27], and possibilities of scaling up to field-level is widely employed for remediation of toxic dye and allied materials from TIE. Pore structure, cost, abundant availability, and ready-to-use adsorbent are the qualities required for the adsorption process. Most of the proposed materials as adsorbents do not qualify for the textile industry's needs and demands. Also, scant literature available on the problem of sludge generated during the process poses an additional challenge. These limitations have created a surge amongst scientists to look for an economically feasible and environmentally acceptable alternative [28][29][30][31].
Presently, NIS produced by nutraceutical industries is disposed of as landfills and as low-calorific value fuel, which is attributed to the pore structure, which traps moisture from the atmosphere. Incineration/burning of NIS results in pollution in the form of GHG emissions and leaves a carbon footprint. Utilization of NIS as an adsorbent to amputate the toxic substances from TIE ecologically alleviates waste disposal problems and addresses sustainability needs and demands.
In the present investigation, we report the use of NIFGS for the remediation of cationic malachite green (MG) as a potential adsorbent belonging to triarylmethane class of dyesfrom water and TIE. The possibilities of the use of the resultant dye-adsorbed clay minerals commonly known as "sludge" as filler/reinforcement material in the fabrication of the composites will address the problems encountered by the polymer industry.

Surface characterization
The surface characterization was conducted through SEM. It was found that the NIFGS surface texture has a porous and fibrous structure which enhances the adsorption of the substrate (dye) on it. Figure 1a shows SEM image of NIFGS, and As seen in Fig. 2c, a clear distinction is witnessed in the spectrum of MG adsorbed onto NIFGS compared to the standalone spectrum of MG (Fig. 2b). This conclusively indicates the dye being adsorbed onto the Fenugreek spent. The bands observed between 3300 and 3600 cm −1 are due to the two phenolic groups' presence, namely hydroxyl (-OH) and carboxylic acid (-COOH), and these two groups are responsible for the interaction with the dye. Therefore, the carboxylate anion being axially deformed in an asymmetric manner results in the band between 1550 and 1650 cm −1 and asymmetrical deformation results in the weaker band at 1400 cm −1 .

Point of zero charge (pH z )
The pH z , determined by preparing 0.1 M KCl, adjusts the initial pH between 2.0 and 12.0 with HCl and NaOH, respectively. Fifty milliliters of 0.1 M KCl with pre-set was poured in a 250-mL conical flask and 50 mg of NIFGS was added. After 24 h, the change in the pH of the solutions was Table 1 Mathematical equations used in the investigation [32] Eq. no

Equation Description Parameter
General adsorption studies 1 q e = (Co−Ce)V W Adsorption capacity at equilibrium q e is adsorption capacity at equilibrium (mg L −1 ) qt is adsorption capacity at time t (mg L −1 ) C o is initial adsorbent concentation (mg L-1) Ce is adsorbent concentation at equilibrium (mg L-1) C t is adsorbent concentation at time t V is the volume of the absorbate solution (L) W is the weight of the adsorbent (g) Langmuir isotherm Q m is the monolayer adsorption capacity (mg g −1 ) Ka is Langmuir isotherm adsorption constant (L mg −1 ) R L factor implies whether the adsorption unfavourable Separation factor of Langmuir isotherm Freundlich isotherm K F is Freudlich isotherm adsorption capacity constant (mg g −1 ) n F is heterogeneity factor indicates the nature of adsorption is chemisorption (n F < 1), linear (n F = 1) or physisorption (n F = 1) Adsorption kinetic studies measured with a pH meter. The graphs between pH final and pH initial were plotted. The point of intersection of the curves at pH 7.1 gives the pH z of NIFGS (Fig. 3).

Solution pH
The adsorption capacity of NIFGS depends on pHsolution.
The pH plays two crucial roles: first, it influences the adsorbent surface's characteristics and second, the dye solution's chemistry [33]. The pH as a parameter assumes importance to substantiate the efficiency of the adsorbent under study.
The knowledge of this parameter is significant when the process is scaled to commercial levels [34]. There was a marginal increase in the adsorption capacity exhibited by the NIFGS as the solution pH increased, and when the NIFGS was subjected to acidic media, its maximum capacity of adsorption was observed. This observation is attributed to the fact that the lower pH refers to acidic media wherein the surplus of hydrogen ions compete for the adsorption at the targeted sites with the adsorbate's cations. As the solution pH increases, the surface charge per unit area lowers, wherein the repulsion of the dye's cations and the surface of NIGFS is reduced.This behavior is explained as follows: many H + ions present in the lower pH range compete with the dye's cation, which increases repulsion between the two charged species. Conversely, at higher pH, the negatively charged OH − attracts the dye's cation and negatively charged species (Fig. 4a).

Initial MG dye concentration
The profound influence of initial dye concentration on the adsorption capacity of NIFGS was studied. The dye's application onto NIFGS increased from 20 to 75 mg g −1 with the rise in the initial dye concentration ranging 25-200 mg L −1 .
The increase in the dye concentration results in an enhanced concentration gradient. The high gradient augments the driving force to the dye solution, which helps rise in the adsorptioncapacity of NIFGS (Fig. 4b). The dye uptake by NIFGS gets enhanced with the increase in contact time.
The maximum adsorption of MG from an aqueous solution was observed at 180 min of contact time. Adsorption was rapid initially due to the dye getting adsorbed onto the exterior surface. Later, the dye molecules probably enter the NIFGS pores (interior surface), which is a slow process. The adsorption of MG was more with higher concentration, and it reached almost constant after attaining equilibrium. Furthermore, it was observed that the adsorption reached almost constant after attaining equilibrium [35].

Effect of adsorbent dosage
The dosage as a parameter profoundly influences the commercialization of the process because it decides the economic viability [36]. Interestingly, the increase in adsorbent dosage from 0.500 to 4.000 g L −1 decreased MG dye quantity (Fig. 4c). This observation assumes importance in the commercialization process, which infers that the number of trials with optimum amounts of the adsorbent enhances MG dye removal efficiency from the aqueous solutions.

Effect of temperature
Keeping in view the focus to scale to commercial applications, evaluation of the process of adsorption of dyes onto NIFGS as a function of solution temperature was studied. The influence of temperature on MG dye adsorption onto NIFGS is presented in Fig. 4d. As seen in the temperature profile against the adsorption capacity, an exothermic process indicates a direct correlation between the temperature and adsorption capacity. This is attributed to the dye molecules' kinetic energy by which the intra-particle diffusion of the dye molecules is high as the temperature increases. This promotes a more vital interaction between the subjected dye and the nutraceutical industrial spent acting as adsorbent.

Adsorption isotherms-modeling analysis
The main criterion of the study of adsorption isotherms was to select a model where q e (the experimental equilibrium) values were almost equal to Q m (monolayer adsorption capacity) value with a coefficient of determination (R 2 ) value ≥ 0.90. To refine the data and distinguish between results obtained by two isotherm models, SSE and χ 2 as two additional error functions were incorporated in our current study. Langmuir [38] proposed a model assuming that the adsorbent will have active sites possessing almost uniform energies. The equations of the Langmuir isotherm model are shown in Eqs. 4 and 5- Table 1. This was further established on the idea that no lateral interaction takes place between adsorbed molecules. A plateau in a two-dimensional graph with equilibrium concentration (C e ) as independent variable and q e as dependent variable characterizes that the active sites are almost fully saturated on the adsorbent's surface, and the Langmuir isotherm curve is presented in Fig. 5a. This implies that further adsorption cannot occur, and the possibility of multilayer adsorption of the dye is ruled out. Compared to the Langmuir isotherm model, Freundlich proposed heterogeneity in the process of adsorption due to different energy of the surface sites, which likely results in multilayer adsorption and the Freundlich isotherm curve is presented in Fig. 5b. The heterogeneity factor (n F ) indicates whether the nature of adsorption is linear (n F = 1), chemisorption (n F < 1), or physisorption (n F > 1) [39]. In the present study, the value of n F about 1.096 indicates that the adsorption favored normal Langmuir isotherm and is physisorption. The fitting of Langmuir and Freundlich isotherms (R 2 ) to the experimental is indicated by the correlation coefficient, which is 0.991 and 0.952, respectively, where R 2 is the correlation coefficient, and the data fits well. The mathematical expression is shown in Eq. 6- Table 1. The values suggest that the dye's transfer from the bulk solution using NIFGS is physisorption and favors Langmuir isotherm.

Adsorption kinetics
Kinetic studies carried out using three concentrations 50, 100, and 200 mg L −1 and at three temperatures at 303, 313, and 323 K, respectively, provided variation in the adsorption rate. The results of the pseudo-first-order model [40] are computed using Eq. 7 (Table 1) and depicted in Fig. 6. Pseudo-second-order model [41] computed using Eq. 8 (Table 1) and depicted in Fig. 7 confirms that the pseudosecond-order model has best fit for the MG-NIFGS system ( Table 2).

Thermodynamics of the adsorption process
∆G° and ∆S° are Gibbs free energy and entropy change of MG dye-NIFGS system. They are the main features of the process design of adsorption thermodynamics. The values of the various thermodynamic parameters calculated are presented in Table 3. The decrease in the negative values of ∆G° with an increase in the temperature infers that the adsorption process is almost spontaneous at lowtemperatures [42][43][44]. The relationship of ∆H°, ∆S°, and E a is presented in Eqs. 9, 10, 11, and 12- Table 1. The positive values of

Conclusions
The interactions between the NIFGS as adsorbent and MG dye as adsorbate showed an experimental equilibrium (q e ) value of 105.00 mg g −1 at almost neutral pH, 130 mg g −1 at pH 2, and 168 mg g −1 at statistically optimized conditions. The adsorption follows the Langmuir isotherm model. The interactions between NIFGS and MG were physical in nature and followed second-order kinetics. This work has relevance to three industrial sectors, namely, textile industries, nutraceutical industries, and plastic industries. Industrial production has a linear model to create value in making the product and completes the life-cycle with the disposal after use. This model seriously affects the environment and ecology due to the unprecedented problem of resource depletion. To address the issue of resource depletion, there is a need for the industries to shift from virgin inputs to recycled and/or reuse products. This article is a step in this direction.