Brinjal (Solanum melongena) stalk waste as an effective scavenger for Eriochrome Black-T from water and wastewater: an approach towards waste to best

Abstract Stalks of brinjal (Solanum melongena), hereinafter SM, have been exercised as an efficient and sustainable adsorbent material for the elimination of Eriochrome Black-T (EBT) from an aqueous solution. The material was characterized by FTIR, FESEM, BET surface area, pHpzc, and proximate analysis. FTIR spectrum suggests the presence of polyphenolic moieties, responsible for successful dye binding. FESEM images show an unprecedented octopus-like texture containing micropores. The central head transforms the architecture of a flower. The evaluated BET surface area of 10.042 m2/g and pore volume 1.055 × 10−2 cm3/g suggest a porous material. The pHpzc of the material was evaluated to be 7.05, and under optimized conditions, the maximum adsorption capacity was found 52.631 mg/g at pH 7. The operational parameters were studied concerning contact time (0–90 min), pH (5–11), initial concentration (10–40 mg/L), and interfering ions (PO4 −3, AsO4 −3, Hg+2, Pb+2). Adsorption follows Langmuir isotherm best (R 2 = 0.996), and pseudo-second-order kinetics (R 2 = 0.991) indicate a monolayer and homogeneous adsorption. 83% regeneration was successful with 0.1(M) sodium hydroxide solution. The material can be reused for up to three cycles with 90% efficiency retention. Analysis of EBT containing industrial effluent indicates that 52.62% of EBT can be removed. Novelty statement Brinjal (Solanum melongena), being one of the most cultivated vegetables around the globe, generates voluminous waste as stalks which warrant proper management. With this aim, such stalks were converted to a phytosorbent and selected for removal of Eriochrome black-T (EBT), a dye that is used by industry persons and science students in their laboratory experiments. The prepared material is highly porous, water-stable, regenerable, and reusable. The protocol is economically viable, easy, and efficient for industrial effluent treatment as well. With a notable maximum adsorption capacity of 52.631 mg/g, the material could offer an ideal choice for dye decontamination.


Introduction
The world's quenchless obsession with growing industrialization has led to the generation of a disastrous range of toxins in aqueous bodies. Daily, the water resources are contaminated using effluents from industries, which affect both the quality of ecosystems and the health of all forms of life (Malhotra et al. 2018). One of the most pertinent categories of environmental pollutants includes dyes and their presence in wastewater from various industries like textile, dying, tanner, pulp and paper, paint, and pigments (Senthilkumar et al. 2019;Solangi et al. 2021). Besides their toxic and carcinogenic effects, dyes significantly affect aquatic flora and fauna (Lohani et al. 2008). In addition to this, as most dyes contain aromatic rings in the chemical structure; their decomposition into the environment is quite difficult producing secondary toxic products, to the environment (Stavrinou et al. 2018).
The extended use of EBT as an indicator in biological sample markings, paper, and textile industries, and complexometric titrations has further enhanced its subjection rate to the environment. Their discharge into aqueous bodies affects crucial variables such as the re-oxygenation ability of water bodies and the photosynthetic capacities of the phytoplanktons and causes diseases like astigmatism and skin allergies (Mohanta et al. 2021). Because of the high resistance of EBT toward light, it has been considered one of the troublesome toxins to get immediately removed from aqueous bodies (Singh et al. 2020).
Various physicochemical, as well as biological techniques, are being implemented for the elimination of dyes from the environment which include chemical precipitation, aerobic as well as anaerobic microbial degradation, ultra-filtration, coagulation, reverse-osmosis, electrochemical treatments, and adsorption (Akhouairi et al. 2019;Jahangiri et al. 2019;Tahir et al. 2020). All those techniques have limited industrial applications mainly because of high operational cost and time (Aziz et al. 2018). The adsorption technique not only removes dyes from wastewater but is also a cost-effective method Qaiyum et al. 2022). It was also seen that one of the widely used adsorbents for the removal of dyes is a biomass-based adsorbent (Kumari and Dey 2019). However, vegetable by-products and wastes, which are produced in large quantities, create a major problem since they have an impact on the environment and must be managed. The reusability of the biomass-based adsorbent is comparatively less than other adsorbents. The adsorption capacity of the material is generally low and encounters difficulty to separate the dye-loaded adsorbent from the solution.
In our present investigation, the dried stalk of brinjal (Solanum melongena) has been chosen as the adsorbent for the removal of EBT dye from wastewater. It is economical, biodegradable, and poses no threat to the environment post disposal. These substances are very convenient for being a good adsorbent due to some active functional groups like methyl, carboxyl, hydroxyl, amino, etc. (Kannaujiya et al. 2021). Herein we report the use of waste brinjal stalks to act as an efficient scavenger of EBT dye from water as well as industrial wastewater. Being an abundantly cultivated vegetable worldwide, brinjal stalks are easy to collect, converted into the desired adsorbent, and employed in large-scale detoxification economically and sustainably.

Materials and methods
Eriochrome black-T (EBT) dye, sodium hydroxide (NaOH), hydrochloric acid (HCl), buffer tablets, and methanol (MeOH) were purchased from Avra Synthesis Pvt Ltd, India, and was used as received without further purification. Millipore-grade water has been used for all solution preparation, reflux, and washing.

Instrumentation
All weighing measurements were done in analytical balance (Saffron Scale SES 220 C, India Sr. NO.M2010289). Redline Binder oven was used for drying the samples at varying temperatures. FESEM images have been recorded by using a Zeiss electron microscope. All colorimetric measurements were done with a Hitachi Double Beam spectrophotometer (model U-2900, equipped with UV-solutions program NSJ). The experimental solution was centrifuged using a Remi Bench Top centrifuge (R-8M). Vanira LI 613 pH meter was used for measuring the pH of the solution. A Thermoscientific Muffle furnace (model F48020-33-80) was used for proximate analysis. Orbital Shaking incubator (model OS 100, India make) was used throughout for batch scale study. The FTIR spectra were obtained using FT-IR spectrum 2 (Perkin Elmer). BET surface area has been evaluated using Quantochrome Instruments Nova (Quantochrome Touchwin v1.22).
Collection and preparation of brinjal (Solanum melongena) stalk A large quantity of brinjal stalk was collected from the university hostels/canteens free of cost. Non-edible waste parts of brinjal were separated, washed, and cut into smaller pieces. Then they were first sun-dried for a day and then dried in an oven at 75 C for 1 h to remove excess water content followed by reflux for 12 h to remove any color. The materials were dried in an oven at 65 C for 24 h and crushed into powder using a regular mixer grinder followed bysieving to get fine particles (100-200 mesh) as the particle size of the adsorbent affects the process of adsorption.

Adsorption experiment
In a typical experiment, 0.25 g of crushed SM stalk was added to the respective dye solution (20 mg/L, 50 mL) of pH $ 7. Then the solution was agitated at 110 ± 5 rpm (Hu et al. 2019;Parvin et al. 2019). Post-agitated solutions have been centrifuged, and the concentration of residual dye solution was calculated. The equilibrium adsorption (q e ) and % percentage adsorption have been assessed using the equation: Where C 0 is the initial concentration (mg/L) and C e is the final concentration (mg/L), m is the quantity of adsorbent (g) and v is the volume of dye solution (L). The effect of temperature (308, 318, and 328 K) and pH (5-11) were analyzed. Acidic and alkaline solutions were used to assess the regeneration of EBT dye-loaded spent adsorbent material. The efficiency (%) was calculated using the equation: Scheme 1 represents the preparation and techno-economic removal of EBT from wastewater.

Results and discussion
Characterizations FTIR spectra Combined FTIR spectra are given in supplementary Figure S1. The broad peak at 3432 cm À1 (Figure 1(a)) is due to the presence of polyphenolic moieties. Upon dye adsorption ( Figure  1(b)), no significant shift of such peak has been observed at 3433 cm À1 . Such observation confirms that hydrogen bonding interaction is not the governing binding interaction between SM and EBT. This is consistent with earlier literature reports (Abu Talha et al. 2018). The peak at 2922 and 2877 cm À1 represents aromatic and aliphatic C-H stretching. A small but sharp peak was noticed at 1713 cm À1 , which suggests the presence of the NO 2 group, arising out from the EBT molecule.
Such a peak is absent in Figure 1(a), confirming that the EBT is solely responsible for the peak. A peak around 1632 cm À1 is due to aromatic C ¼ C stretching. Peaks seen at 1463, 1466, and 1475 cm À1 can be related to C-C stretching of the lignin matrix; before and after the adsorption of EBT dyes (Kumari and Dey 2019;Jawad et al. 2020;Qaiyum et al. 2022). It could be noted that there are multiple peaks around 1000-1400 cm À1 .

FESEM
FESEM images have been recorded to ascertain the surface morphology and texture of SM. Figure 1(a) represents the microphotograph of free SM which reveals a plate-like flaky architecture with numerous pores, evenly distributed over the entire surface. Non-even pits, fractures, and ridges could be noticed. Measured diameters of the macropores are in the range of mm, while smaller pores <500 nm could also be seen. After the adsorption of EBT, the surface significantly became smooth, packed, and somewhat evenly saturated with a remarkable reduction of pores (Figure 1(b)). An astonishing texture was seen when the FESEM image was recorded at a resolution of 40 mm. An unprecedented septopus (like an octopus, an eight-legged sea animal) like 3 D architecture has been noticed (Figure 1(c)). There is a bridgehead junction of all seven strands which looks like the center of a lotus. Upon dye adsorption, such 3 D architecture was found to be changed. Figure 2 represents the N 2 adsorption-desorption curve that reveals a surface area of 10.042 m 2 /g which favors good adsorption of dye molecules from the wastewater. The pore volume was estimated to be 1.055 Â 10 À2 cm 3 /g. The BJH adsorption-desorption method gave the pore diameter of SM to be in the nano-range which is 3.399 nm which is suitable to trap the dye molecules in its pores. The curve signifies multi-anchorage adsorption with negative concavity.

Proximate analysis
To have an idea of the tentative chemical composition, a proximate analysis was done (Aichour et al. 2022). It was found that the moisture content: was 2.19, ash content: was 68.94, volatile matter: 8.77, and fixed carbon content: 20.1 Figure 3(a), depicts that the dye uptake is consistently increasing with time, and becomes maximum at 75 minutes (75%), after that no more uptake is seen. Initially, the higher availability of pores leads to faster adsorption, which gets reduced with time (Karoui et al. 2018;Akhouairi et al. 2019;Jawad et al. 2021). This indicates the saturation of available vacant pore sites on the adsorbent surface.

Effect of pH
The binding of dye solution to the adsorbent surface is highly influenced by pH. Adsorption of EBT was investigated in the pH range of 5-11 (Figure 3(c)). Adsorption is moderate at lower pH, but it gradually increased with an increase in pH, and its maximum adsorption was found to be 73% at pH ¼ 7. This can be well explained by evaluating the point of zero charge (pH pzc ¼7.05) (Figure 3(d)). Below pH pzc , the surface is cationic, which favors the binding of anionic dye EBT through electrostatic attraction. At higher pH (pH > pH pzc ), the surface of the dried SM becomes negatively charged. It disfavors the binding of an anionic dye onto the surface ). Moreover, a competitive inhibition between the dye molecules leads to lesser uptake. pH beyond 11 was not studied as it doesn't have practical utility.

Kinetic study
Different models such as pseudo-first-order, pseudo-secondorder, second-order and intra-particle diffusion were analyzed and those were presented in Figure 4(a-d). According to the pseudo-first-order model, the rate of adsorption is determined by the number of active vacant sites on the surface of the adsorbent (Alp et al. 2020). The corresponding rate equation is given below: log q e À q t ð Þ ¼ logq e À k 1 2:303 Where q t and q e indicate the quantity of dye adsorbed after time t (min) and the same at equilibrium respectively. The k 1 (min À1 ) is the rate constant.  According to the pseudo-second-order model, the site occupancy is proportional to the square of sites unoccupied (Jawad et al. 2018a,b). The corresponding equation is given below: k 2 (g/mg.min À1 ) is the pseudo-second-order rate constant. Second order model follows the given equation: k 3 (g.mg À1 .min À1 ) is the second order rate constant. Mass transfer from solution to the solid phase is aided by intra-particle diffusion. The corresponding Equation for intra-particle diffusion is given below: k id (mg/g.min 1/2 ) is the rate constant for intra-particle diffusion. All kinetic data are graphically presented in Figure 4 and Table 1. From experimental data, it is evident that the pseudo-second-order model is followed in the adsorption of EBT by SM with R 2 ¼ 0.991 (Benkartoussa et al. 2021). It is further supported by the modest regeneration of the spent material. Besides, a pore diffusive transfer may also contribute to the uptake. This is consistent with our earlier results.

Isotherm study
The dispersal of adsorbate in the given solution and the quantity of adsorbate adsorbed on an adsorbent at a certain temperature can be described from the isotherm study (Al-Ghamdi et al. 2022). Isotherm models namely Freundlich and Langmuir models were studied (Munagapati et al. 2019). Freundlich isotherm model depicts multilayer dyes adsorption onto the corresponding heterogeneous surface. The corresponding equation is described below: Where, C e (mg/L) indicates concentration at equilibrium, q e (mg/g) indicates the quantity of dye being adsorbed, and n  (pH variation of NaCl Solution: 3,5,7,9,11). and K f are Freundlich constants. For a favorable adsorption process, the n value usually lies within 1-10.
Langmuir adsorption isotherm depicts monolayer adsorption of dye onto a homogeneous surface (Kannaujiya et al. 2021). It can be described by the given equation: Where, C e (mg/L) indicates equilibrium concentration, q e (mg/g) indicates the equilibrium uptake, q m (mg/g) is the maximum capacity of adsorption and b is Langmuir constant (L/mg). R L is a dimensionless parameter, which is the Langmuir constant (Inthapanya et al. 2019). It is being calculated using the equation given below: Figure 5 and Table 2 represents the isotherm study. According to the experimental results, EBT adsorption preferably follows the Langmuir isotherm model with an R 2 ¼ 0.996. Maximum adsorption capacity at 308 K was evaluated to be 52.631 mg/g. However, with increasing temperature, such capacity was found to decrease rapidly, suggesting that higher temperature disfavors the adsorption. Possibly the weak interactions are not sufficient enough to hold the dye molecules. Freundlich's model was also found to be in good agreement. The n value ranges from 1.095 to 1.246, which advocates that the adsorption is a favorable one for a heterogeneous surface (Shakoor and Nasar 2018; Akpomie and Conradie 2021). However, a high q m value suggests that it could be a combination of both Langmuir and Freundlich processes. A high BET surface area justifies the above observations.

Regeneration study
Regeneration is considered very crucial to assess the recovery extents of EBT from the spent adsorbent and prediction of the reusability of the material (Bansal et al. 2020). Four   1 M), and MeOH. The maximum regeneration of 83% was observed in 0.1 (M) NaOH solutions. Figure 6(b) represents the regeneration of spent SM. With methanol, about 20% regeneration was seen. However, the rate of regeneration was found to decrease by $10% with consecutive cycles . The cyclic reusability test suggests a similar pattern.

Interference by coexistent ions
It is well-known that wastewater invariably contains various co-existent ions. The percentage elimination of a dye is influenced by multi-entity presence. These ions may occupy adsorption sites, reducing the amount of adsorption i.e., percentage removal. Figure 6(a) represents the effect of coexistent ions (Grabi et al). Because EBT is an anionic dye, other ions greatly interfere with adsorption by occupying adsorption sites, but anions do not do so due to the absence of electrostatic interactions. The effect of interfering ions is given below (Figure 6(a)). In presence of Hg þ2 ions, there was a 12% decrease in adsorption.

Proposed mechanism
It is noteworthy that the surface charge may tune the driving force of dye adsorption. Therefore, the present adsorption is suggested to be an amalgamation of electrostatic attraction, weak van der Waals force, and p -p stacking (Dey et al. 2007;Kumari and Dey 2019). The lower rate of adsorption above pH pzc could be attributed to the steric repulsion between anionic dye and the negative surface charge.

Industrial effluent analysis
An adsorbent must show its efficiency in real wastewater treatment so that it can be assorted as a utility material. A batch experiment was carried out using an industrial sample to examine the efficiency of the brinjal stalk adsorbent. From a nearby unit, the industrial effluent was collected; it was filtered and centrifuged followed by neutralization. Neutralization was done as the sample was found to be alkaline (pH > 8.5). Then 50 mL of the effluent was taken and 0.25 g of SM was added to it and was subjected to shaking in an orbital shaker for 75 mins. The final concentration of the effluent was measured. 52.62% adsorption was found to take place, which indicates the realistic application of the material. The first round of treated material was subjected to a second run, which was sufficient enough to reduce up to 98% of initial strength.

Economic viability and sustainability
Since the brinjal stalks were collected from our university hostels/canteens free of cost, no additional material preparation cost had been incurred because no activation, grafting, fabrication, or chemical treatments have been done. Moreover, the present methodology ensures the disposal problem to some extent. Considering the above aspects, it can be concluded that the method is highly economical. The water stability was tested by suspending fixed weighed material in water for one week. The resultant solution was scanned in a UV-Vis spectrophotometer. No peak/band was found in the entire range (200-1100 nm), eliminating any possibility of leaching. However, safe disposal/recovery of dye-saturated SM remains a concern.
Conclusion Brinjal stalk, a voluminous bio-waste has been exercised as a promising adsorbent for the elimination of EBT dye from water as well as wastewater. Structural characterization using FESEM indicates an unprecedented septopus-like architecture with a highly porous surface, favorable for adsorption.  With a BET surface area of 10.042 m 2 /g and pore volume of 1.055 Â 10 À2 cm 3 /g, the material is efficient for dye removal.The maximum capacity of adsorption was found to be 52.631 mg/g at pH 7. The process followed the pseudosecond-order model and Langmuir isotherm. Industrial effluent analysis suggests a good possibility of field application, albeit on a modest scaled up to 50% detoxification.

Acknowledgment
MAQ, PPS, and SD thank the Central University of Jharkhand for providing a fellowship

Disclosure statement
No potential conflict of interest was reported by the author(s).