1 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 × 105 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.

The techniques and procedures used for the remediation of dyes in textile industrial effluents (TIE) are broadly classified as biological [6, 7], biological-cum-chemical [8], chemical [9], electro-separation [10], electro-coagulation [11], physical methods using membranes [12, 13], use of nanomaterials and composites [14,15,16,17,18,19,20], food products [21], and activated carbon [22]. The high cost of plant establishment, increased operational cost, the additional cost of regeneration of the adsorbent, sensitivity to changes in the participation of the wastewater and its ingredients, and disposal of residual sludge [23, 24] are the major issues and serious limitations reported in the literature.

Currently, adsorption technique with the characteristics of simplicity, efficiency, minimum discharge of toxic by-products[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.

2 Experimental

2.1 Studies on the variables affecting adsorption of MG dye on NIFGS

The influence of experimental parameters (pH, initial MG dye concentration,adsorbent dose, and temperature) was analyzed using batch experiments. For preparing stock sol. of MG (1000 mg L−1), double-distilled water was used. A required solution concentration (25–200 mg L−1) was obtained from the stock solution. A series of 250-mL Erlenmeyer flask and 50-mL aqueous MG dye solution (25—200 mg L−1) were added. A required amount of NIFGS was introduced into each flask. Evaluations were conducted based on the effects of varying parameters such as pH (2, 4, 6, 7, 8, 10, and 12), MG dye initial concentration (25–200 mg L−1), and dosage of adsorbent (0.025–0.200 g 50 ml−1; 0.500–4.000 g L−1). Three temperatures (303, 313, and 323 K)were selected with an initial dye concentration of 200 mg L−1 to study the influence of temperature on the process of adsorption. The solution was subjected to stirring on a thermostatic orbital shaker at 165 rpm for 180 min. The samples were withdrawn at a predetermined equilibrium time. The unadsorbed MG dye in the solution phase was removed from NIFGS by centrifugation at 3000 rpm for 5 min. If the solution was unclear, the centrifugation was repeated for an additional 5 min. The MG dye concentration at equilibrium about the supernatant centrifuged solution was found out utilizing the Perkin-Elmer Lambda EZ-201 UV–vis spectra at λmax 590 nm. To examine the influence in a range of pH 2–12, batch experiments were performed.The pH of the solution was attuned utilizing 0.01–1.00 MHCl or NaOH solution. Adsorption kinetic studies of MG dye solution (200 mg L−1) and three temperatures (303, 313, and 323 K) were selected to carry out experiments with time as an independent variable. The data were fitted into Eqs. 1, 2, and 3—Table 1. Experiments were replicated thrice, and the mean values were considered.

Table 1 Mathematical equations used in the investigation [32]
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3 Results and discussion

3.1 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 Fig. 1b shows SEM image of MG adsorbed on NIFGS.

Fig. 1
figure 1

a SEM image of NIFGS, b SEM image of MG-NIFGS

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IR spectra of NIFGS (Fig. 2a) provides the information pertaining to the functional groups present. A stretch of C-H bond in the saturated carbon results in the band at 2926 cm−1. The phenomenon of C = C stretching results in the bands depicted at 1627 cm−1, 1638 cm−1, and 1652 cm−1. Vibrations at C-O bonds and a bend at O–H bond result in the band seen at 1374 cm−1 and 1325 cm−1, and 1246 cm−1 and 1160 cm−1, respectively. The bands at 1103 cm−1 and 1031 cm−1are attributed to C-O–H and C–O–C bond stretching, respectively.

Fig. 2
figure 2

FTIR spectra of (a) NIFGS,(b) MG dye, and (c) MG dye adsorbed onto NIFGS

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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.

3.2 Point of zero charge (pHz)

The pHz, 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 measured with a pH meter. The graphs between pHfinal and pHinitial were plotted. The point of intersection of the curves at pH 7.1 gives the pHz of NIFGS (Fig. 3).

Fig. 3
figure 3

Point of zero charges of NIFGS [5]

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3.3 The influence of variables on MG adsorption on NIFGS

3.3.1 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).

Fig. 4
figure 4

Effect of parameters on MG-NIFGS system

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3.3.2 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].

3.3.3 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.

3.4 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. The data obtained for the classical thermodynamic parameters, namely, ΔGo, ΔHo, and ΔSo indicate the nature and type of a reaction. For example, the positive ΔH° (enthalpy) values obtained from 303 to 323 K of NIFGS indicate an endothermic process. The overall negative values of ΔG° (free energy) obtained for the MG-NIFGS system confirm the adsorption process’s spontaneity and viability. The magnitude of ΔG° values shows the rapid and spontaneous adsorption at a lower temperature. The positive value of ∆S° indicates that the enhanced randomness at the solid/solution interfaces with the desired affinity of MG towards NIFGS [37].

3.5 Adsorption isotherms—modeling analysis

The main criterion of the study of adsorption isotherms was to select a model where qe (the experimental equilibrium) values were almost equal to Qm (monolayer adsorption capacity) value with a coefficient of determination (R2) value ≥ 0.90. To refine the data and distinguish between results obtained by two isotherm models, SSE and χ2 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 (Ce) as independent variable and qe 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 (nF) indicates whether the nature of adsorption is linear (nF = 1), chemisorption (nF < 1), or physisorption (nF > 1) [39]. In the present study, the value of nF about 1.096 indicates that the adsorption favored normal Langmuir isotherm and is physisorption. The fitting of Langmuir and Freundlich isotherms (R2) to the experimental is indicated by the correlation coefficient, which is 0.991 and 0.952, respectively, where R2 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.

Fig. 5
figure 5

Fitting of adsorption data to (a) Langmuir and (b) Freundlichadsorption isotherm MG-NIFGS system

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3.6 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 pseudo-second-order model has best fit for the MG-NIFGS system (Table 2).

Fig. 6
figure 6

Kinetics data fitted to pseudo-first order with initial MG concentration (a) 50 \(\upmu\) g mL−1, (b) 100 \(\upmu\) g mL−1, and (c) 200 \(\upmu\) g mL−1

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Fig. 7
figure 7

Kinetics data fitted to pseudo-second order with initial MG concentration (a) 50 \(\upmu\) g mL−1, (b) 100 \(\upmu\) g mL−1, and (c) 200 \(\upmu\) g mL−1

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Table 2 Parameters for adsorption kinetic models
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3.7 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 Ea is presented in Eqs. 9, 10, 11, and 12—Table 1. The positive values of ∆S° indicate the decrease in the randomness at the dye/NIFGS interface.

Table 3 Effect of temperature and initial dye concentration on thermodynamic parameters
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4 Conclusions

The interactions between the NIFGS as adsorbent and MG dye as adsorbate showed an experimental equilibrium (qe) 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.