An Eco-Friendly Modification of a Walnut Shell Biosorbent for Increased Efficiency in Wastewater Treatment

Abstract

Herein, we report the performance of some low-cost biosorbents developed by environment-friendly modification of walnut shells. Two types of biosorbents were prepared by ecological modification of walnut shell surfaces: (1) biosorbents obtained by hot water treatment (WS_H2O) and (2) biosorbents produced by mercerization (WS_NaOH). Different techniques were used to evaluate the morphological, elemental, and structural modification of the biosorbents, by comparison with raw materials. These characterization techniques involved scanning electron microscopy (SEM) coupled with energy-dispersive X-ray analysis, and Fourier-transform infrared spectroscopy (FTIR). The biosorbents were employed for the removal of methylene blue (MB) and crystal violet (CV) cationic dyes (as model organic pollutants) from aqueous solutions. The kinetic adsorption data mainly followed the pseudo-first-order model. The maximum adsorption capacities of the produced biosorbents ranged from 102 to 110 mg/g and were observed at 330 K. Equilibrium data for adsorption were fitted to Langmuir and Freundlich isotherm models. The calculated values of thermodynamic parameters suggested that the investigated adsorption processes were exergonic (ΔG < 0) and exothermic (ΔH < 0). In addition, a possible valorization of the cost-effective and eco-friendly spent biosorbents was tested by performing secondary adsorption of the anionic dyes.

1. Introduction

Creating alternative sources of freshwater and reducing pollution through effective wastewater management is becoming increasingly important for promoting a circular economy and sustainable development [1,2]. Despite the efforts made in this regard, in the last year, 52% of the total volume of wastewater produced was estimated to be treated, from which only a fraction of 11% was intended for reuse [3]. Therefore, a huge amount of polluted water (~48%) remained untreated, being discharged into the environment. Such an ecological issue posed an increased risk to human health and natural ecosystems [4].

Various hazardous compounds such as organic contaminants (e.g., dyes, pesticides, pharmaceuticals, and surfactants), inorganic pollutants (e.g., heavy metals, sulfides, ammonia, and oxides of nitrogen), pathogens, radionuclide, and others released in the watercourses represent the main cause of increased contamination. These effluents derive as sewage from household and industrial activities, animal husbandry, agriculture, or the hospital sector [5,6].

To encourage the reuse of wastewater, it is essential to find a treatment method that is ecological, reliable, and cost-effective. Over time, different physicochemical (e.g., adsorption, advanced oxidation processes, coagulation, flocculation) [7], biological (e.g., oxidation ditch process, activated sludge process, sequencing batch reactor, phytoremediation technology) [8,9] and hybrid treatment systems have been developed to mitigate the threat of pollution and reduce wastewater discharges [10,11]. The choice of a wastewater treatment method depends on the type of pollutant. For instance, synthetic dyes were designed to be resistant compounds from a chemical standpoint. Therefore, the removal of such persistent organic contaminants by biological/biotechnological methods is usually improper, and other advanced methods of treatment need to be applied. Among the treatment methods for wastewater, the United States Environmental Protection Agency (USEPA) considered adsorption at the solid/liquid interface as one of the most effective wastewater treatment techniques [11] due to its design and operational simplicity, high efficiency, universal nature, fast adsorbent/adsorbate contact time, and the possibility of sorbent recovery and reuse [5,12].

The use of biosorbents, derived from agricultural waste, represents an ideal low-cost and eco-friendly alternative for the treatment of contaminated waters [5,13]. One of the most readily available agricultural wastes is walnut shells (WS). The walnut plant is the second largest producer of nuts (after almonds), with a growing global demand and consumption due to its wide pharmacological uses and valuable health benefits [14]. Consequently, over 1.5 million tons of walnut shells are left behind every year, as the shell represents almost 67% of the fruit’s weight [15,16]. The walnut shells proved to exhibit great potential as biosorbent materials, as they possess suitable physicochemical properties (high surface area, chemical/hydrodynamic stability, and reduced ash content) and an increased affinity for different compounds (e.g., dyes, drugs, and heavy metal ions) due to their high content of hemicellulose, cellulose, and lignin [16,17].

In this context, this study aims to highlight the properties of walnut shell (WS) biomass and to promote two methods of WS modification for improved adsorption efficiency of organic compounds: (i) the eco-friendly method of modification by hot water treatment and (ii) the chemical surface treatment via mercerization (involving sodium hydroxide). Two cationic dyes with increased pharmaceutical uses (Crystal Violet and Methylene Blue) were used as organic adsorbate models. In addition, this work aims to demonstrate the ecological sustainability of walnut shell biomass, due to the possibility of valorizing the used biosorbent through the secondary adsorption of anionic dyes, thus avoiding subsequent treatments with hazardous chemicals.

2. Materials and Methods

2.1. Materials

Sodium hydroxide ≥97.0% in pellet form, Methylene Blue (MB), Orange II, and Congo Red dyes were purchased from Merck Chemical (Saint Louis, MO, USA). The solution of 1% Crystal Violet (CV) in water/ethanol (1:10 v/v) was acquired from TIS Farmaceutic S.A. (Bucharest Romania). Stock experimental solutions of both cationic dyes were prepared by using bi-distilled water (concentrations of 1 g/L). The walnut shell biomass was collected from a village in Neamt County, Romania. After eliminating the solid impurities by multiple washing with distilled water and drying at 378 K in a laboratory oven, the walnut shells were subjected to mechanical grinding using a Knife Mill Pulverisette 11 (Fritsch, Idar-Oberstein, Germany). Grains with dimensions ranging between 1 and 2 mm were further stored and used for the experiments.

2.2. Eco-Friendly and Chemical Modification of Walnut Shell Biosorbents (WS_H2O and WS_NaOH)

Two types of walnut shell biosorbents were obtained via WS grain surface modification (1) by adopting an eco-friendly method described in the literature [18] and (2) by a commonly used alkaline treatment, namely mercerization, respectively. For the environmentally friendly treatment of the walnut shells, the 2 g of WS grains were placed in 100 mL of hot water, previously brought to a temperature of 353 K, and kept under continuous stirring (150 rpm) for 1 h. An eco-friendly sorbent was obtained (WS_H2O) after drying the immersed WS grains at 378 K for 24 h. For the mercerization treatment, 2 g of WS were immersed in 20 mL of 1.25 M NaOH aqueous solution and gently stirred (150 rpm) for 48 h at room temperature. The treated WS grains were filtered, washed abundantly with bi-distilled water (to remove the excess sodium hydroxide), and dried at 338 K in an oven for 24 h, obtaining the final mercerized biosorbent (WS_NaOH).

2.3. Biosorbents Characterization

The surface morphological features and elemental composition of raw and modified walnut shell biosorbents were examined by using a Quanta 200 scanning electron microscope (SEM), equipped with an Energy Dispersive X-ray (EDX) detector (Brno, Czech Republic). Dye adsorption onto the biosorbents was observed on a Polarized Optical Microscope (Leica Microsystems, Wetzlar, Germany).

Structural characterization of the walnut shells (before and after surface modification) was performed by using a Bruker Vertex 70 Fourier-transform infrared spectrophotometer (FTIR) (Ettlingen, Germany), using the KBr pellets technique (4000–400 cm⁻¹ wavelength range, 2 cm⁻¹ resolution, and 64 scans). In addition, ash content was gravimetrically determined in accordance with ISO 1171 standard (as described in Supplementary Materials, Equation (S1)).

2.4. Adsorption Assays for Cationic Dyes Removal

Methylene Blue (MB) and Crystal Violet (CV) adsorption onto mercerized (WS_NaOH) and hot water treated (WS_H2O) walnut shell-based biosorbents were assessed by using an Orbital Shaker-Incubator Biosan ES-20/60 (Riga, Latvia). The concentration of the cationic dyes in the aqueous solutions was determined with a double-beam UV–Vis spectrophotometer Hitachi U-2910 (Hitachi High-Tech Corporation, Tokio, Japan), based on previously performed calibration curves and absorption bands identification (664 nm for MB and 583 nm for CV), as shown in Figure S1 (from Supplementary Materials).

The kinetic studies were performed at 300 K in batch mode, for each system, in order to determine the rate of the adsorption process. To this end, the sorbent (0.2 g of WS_NaOH or WS_H2O) was added to 50 mL of MB or CV solution (initial concentration of 50 mg/L, 180 rpm, 4 h). Aliquots were taken from the dye solutions at different predetermined contact times and the final dye concentration in the solution was determined. The adsorption isotherms were performed at two different temperatures, 300 K and 330 K, respectively. Herein, for the same dose of sorbent (0.2 g) the initial concentration of the dye solutions varied between 10 and 500 mg/L, and contact time was set for 4 h (180 rpm).

The adsorption capacity as a function of time was calculated according to Equation (1):

$$q_t\left(\frac{\mathrm{mg}}{\mathrm{g}}\right)=\frac{\left(C_0-C_t\right)V}{m·1000},$$

(1)

where q_t (mg/g) represents the amount of dye per weight of sorbent that is absorbed in time t and takes the higher value (q_e) when the adsorption process reaches equilibrium (adsorption capacity of biosorbents); C₀ and C_t (mg/L) indicate the dye concentrations in the initial and final solution (after contact time t), respectively; V (mL) refers to the volume of the working solution; and m (g) is the weight of the walnut shell-based biosorbents (biosorbent dose).

2.5. Dessorption Assay of MB and CV Cationic Dyes

Both spent WS_H2O and WS_NaOH biosorbents (loaded with MB and CV cationic dyes after adsorption) were subjected to desorption experiments (in batch mode) in various eluents, including distilled water, 0.01 M HCl, 0.01 M NaCl, 0.5 M NaOH, and ethanol. In this respect, the spent sorbents (0.1 g) were immersed in 10 mL of each eluent (stirring at 180 rpm for 1 h).

2.6. Secondary Adsorption of Anionic Dyes

Both spent WS_H2O and WS_NaOH biosorbents (after MB and CV cationic dyes adsorption) were tested in terms of the secondary adsorption efficiency of anionic dyes. Therefore, after the adsorption of MB and CV, the biosorbents were removed from the aqueous solution and were dried at 338 K in an oven for 24 h. Then, each used biosorbent (0.2 g) was further immersed in a 20 mL solution of Orange II or Congo Red anionic dyes (a concentration of 10 mg/L, 300 K, 180 rpm, for 1 h). The removal efficiency of anionic dyes was calculated in accordance with Equation (2):

$$Y\left(\%\right)=\frac{C_0-C_e}{C_0}·100,$$

(2)

where C₀ is the dye concentrations in the initial and final solution (after contact time t), respectively; V (mL) refers to the volume of the working solution; m (g) is the weight of the walnut shell-based biosorbents (biosorbent dose); and C_e represents the dye concentration at equilibrium.

3. Results and Discussion

3.1. Modification of Walnut Shells

In order to provide a sustainable solution for wastewater treatment, the use of agro-based natural sorbents, which are usually discharged as waste, was taken into consideration. Walnut shells (WS) are characterized by the abundance of cellulosic fibers and lignin structures, so they possess numerous functional groups (hydroxyl, carboxyl, etc.), as highlighted in Figure S2a, which are proper for the direct binding of various pollutants in aqueous solutions [19,20] or subsequent surface modifications [21]. Thus, to improve the physicochemical properties of the biosorbents, different physical (e.g., low temperature or pyrolysis treatment), chemical (e.g., alkali, acylation, acrylation, grafting), and eco-friendly (with hot, plasma, bacteria, plant triglycerides) treatment methods were approached in the literature [18].

To avoid the drawbacks imposed by the physical and chemical methods (high costs, time-consuming, hazardous chemicals, and others), this study focused on using the hot water treatment of walnut shells (immersion for 1 h in water with a temperature of 353 K), as it represents an excellent environmentally friendly option, being a simple, time-saving and cost-effective modification method. Moreover, the immersion of biosorbents in hot water leads to the removal of impurities, such as waxes, inorganic ashes, volatile compounds, and others, facilitating the adsorption processes [18,20]. This work is the first report on the use of this method for the modification of walnut shell agro-based sorbents in order to evaluate the adsorption affinity for Methylene Blue (MB) and Crystal Violet (CV) cationic dyes.

However, the mercerization of the walnut shells was also considered, being a commonly used technique for cellulose activation, when using NaOH solution with concentrations between 1.25 M and 1.75 M [18,22]. To reduce the impact on the environment, the lowest concentrations (of 1.25 M NaOH) were used. In addition, the produced mercerized walnut shells (WS_NaOH) were abundantly washed with water to remove the sodium hydroxide and/or sodium residues. Two types of walnut shell-based biosorbents were finally obtained (WS_H2O and WS_NaOH), each of them being employed in the adsorption of both MB and CV dyes (Figure S2b).

3.2. Morphological and Elemental Analysis of the Biosorbents

The surface morphology of the WS_H2O and WS_NaOH biosorbents was investigated by comparison with the raw walnut shells (WS) sample, and the SEM images are given in Figure 1. The unmodified WS sample exhibits a rough heterogeneous structure, characteristic of lingo-cellulosic biomass, covered by numerous irregular-shaped small particles as a result of the grinding process and the presence of impurities, such as inorganic ashes (Figure 1a). On the other hand, the hot water and chemical treatment of the WS (Figure 1b,c, respectively) lead to significant surface modifications of the biosorbents. Both WS_H2O and WS_NaOH samples are characterized by a cleansed surface, by removal of soluble impurities, and the appearance of a porous texture with many cavities. These morphological features are considered to be ideal for a sorbent material, as they facilitate the adsorption of pollutants from aqueous solutions, especially the adsorption of organic dyes [23]. Similar surface morphologies were obtained for raw and NaOH-modified cashew nut shells, designed to remove Pb(II) ions from an aqueous solution [24].

The modification of WS surfaces by eco-friendly and chemical modification is confirmed also by EDX analysis (Figure 1d). As compared to raw WS sample that mainly consists of C, O, P, K, and Ca elements, the WS_H2O and WS_NaOH modified samples revealed a considerable decrease in the inorganic elements (usually found in WS ash) [21], most pronounced for the samples treated in hot water. Moreover, the treatment of walnut shells with an alkali solution led to the appearance of sodium (Na) element on the biosorbent surface. In order to verify WS modification, the ash content of WS before and after surface modification was calculated. As shown in Figure S1, the decrease in ash content in WS_H2O samples (from 0.405% in raw WS to 0.078%) confirms the efficiency in the removal of the inorganic ash by hot water treatment. The mercerized samples, however, showed a higher ash content, explained by the presence of Na alkali metal and the mercerization process, which induce a reduction in crystallinity and an increase in the ash agglomeration tendency [25,26]. In addition, Figure 1d reveals the increase in carbon (from 16 % (WS) to 53 % (WS_H2O) and 49 % (WS_NaOH)), respectively) and oxygen (from 20 % (WS) to 44 % (WS_H2O) and 37 % (WS_NaOH)). This can also be explained by the slow decomposition of hemicellulose due to its reduced instability and by the increase in the lignocellulosic fraction [27,28].

3.3. Structural Characterization

In addition to the porous morphology of the sorbent, the surface’s chemistry also plays an important role in adsorption processes. Therefore, the presence of the functional groups (existing at WS-based materials surface and the structural modifications of the raw WS) was examined by FTIR analysis (Figure 2).

The unmodified WS spectrum typically shows absorption bands, as follows: a broad band centered at 3431 cm⁻¹ is due the stretching vibrations for O-H functional groups in cellulose, lignin, hemicellulose and from the adsorbed water; the band at 2924 cm⁻¹ (with a shoulder at 2856 cm⁻¹) appears as a result of C-H stretching vibrations in methyl and methylene groups; the absorption at 1741 cm⁻¹ corresponds to the C=O group of acetyl and esters in hemicellulose and to the ester linkages of the hydroxycinnamic acids’ carboxylic groups that bind lignin and hemicellulose [27]; the band at 1619 cm⁻¹ is attributed to the C=O stretching vibrations from the aromatics skeletal of hemicellulose and lignin, superimposed on C=C stretching vibrations; the C=C from the aromatic rings in lignin is also found at 1514 cm⁻¹ and 1446 cm⁻¹ [29]; in-plane C-H bending vibration from lignin metoxy groups gives the 1377 cm⁻¹ absorption band, while C-C aromatic bonds are found at 1323 cm⁻¹; the C-O, C-O-C, and O-H groups from anhydrides, ethers, esters, phenols, carboxylic acids and derivatives are found at 1259 cm⁻¹ and 1158 cm⁻¹; the broad band centered at 1052 cm⁻¹ is attributed to the secondary and primary C-O, C-O-C, R-O-R and R-O-H bonds (lignin, hemicellulose, polysaccharides); stretching vibrations of C-O-C glycosidic linkage is found at 896 cm⁻¹ and the absorption band corresponding to aromatic sp² bending of (C-H) from lignin is found at 803 cm⁻¹ [30]; and the presence of α-branched aliphatic monocarboxylic acids is evidenced by the three bands characteristic for the O-C-O in-plane deformation are found at 667, 636 (shoulder), and 614 cm⁻¹, and C-C-O in plane deformation vibrations (around 520 cm⁻¹) [31].

Walnut shell treatment with hot water and an alkali solution led to the expected structural modification on the biosorbent surface. Both WS_H2O and WS_NaOH spectra confirm the washing out and removal of impurities (e.g., ash, oils), the alteration of hemicellulose structure and thus, the increase in the lignocellulose ratio, mainly by the following spectral changes, including the presence of O-H cellulosic and lignin functional groups (3433 cm⁻¹ for WS_H2O and 3432 cm⁻¹ for WS_NaOH), the clear appearance of C-H asymmetric and symmetric stretching vibration bands in the region 2995–2810 cm⁻¹ (due to higher content of lignin); the band at 1741 cm⁻¹ (C=O groups) is reduced by the hot water treatment (1740 cm⁻¹) and almost vanished because the alkali treatment; the band attributed to the aromatic C=O of hemicellulose and lignin shifts from 1619 cm⁻¹ to 1632 cm⁻¹ (WS_H2O) and 1633 cm⁻¹ (WS_NaOH), respectively; modification of the bands located in the range 1400–1300 cm⁻¹ particularly due to lignin C-H and C-C deformation vibration; the broad band centered at 1052 cm⁻¹ in WS split in two bands and increases in intensity due to the higher content of cellulose (O-H and C-O stretching vibration of polysaccharide) [27,32]; and the increase in intensity of the band corresponding to C-O stretching (1262 cm⁻¹ and 1261 cm⁻¹, respectively) and C-H bending (803 cm⁻¹ and 802 cm⁻¹, respectively) from lignin and cellulose.

However, all spectra presented the characteristic doublet (2359 and 2337 cm⁻¹ for WS, 2372 and 2340 cm⁻¹ for WS_H2O, 2372 and 2341 cm⁻¹ for WS_NaOH) for R-branch and P-branch of the ¹²CO₂ absorption from atmospheric air. Moreover, a small absorption band located around 2279 cm⁻¹ (most visible in WS_H2O spectra) appears due to the P-branch of the ¹³CO₂ absorption, while the corresponding R-branch is superimposed with the P-branch of the ¹²CO₂ absorption band [33]. It can be mentioned that the increased intensity of these adsorption bands for the modified walnut shells, as compared with raw WS, can be correlated with their morphological porous structure.

3.4. Kinetic Adsorption Studies

The biosorbents obtained by modification of walnut shells (WS_H2O and WS_NaOH) were tested for removal of hazardous pollutants, Methylene Blue (MB) and Crystal Violet (CV), from aqueous solutions. Thus, adsorption kinetics was performed for each system, as depicted in Figure 3, by determining the adsorption capacity q_t (mg/g) variation as a function of contact time t (min). As expected, the increase in contact time led to an increase in the adsorption capacity of MB (Figure 3a) and CV (Figure 3b) onto both types of biosorbents. The adsorption kinetics of the MB and CV cationic dyes onto the WS_H2O and WS_NaOH biosorbents exhibited a two-stage behavior. An accelerated increase was observed within the first 30 min (first stage), followed by a slower and gradual adsorption increase (second stage), which maintained during the 240 min of experiments.

The surface activation by alkali treatment is expected to induce a higher rate of cationic dye adsorption due to the penetration of Na⁺ cations in the intracrystalline lignocellulosic spaces, thus increasing the material swelling capacity (increasing the number of available –OH groups) [22]. As a consequence, the adsorption capacities of MB and CV onto the WS_NaOH biosorbent are higher, with about 3.4 mg/g more than those corresponding to the WS_H2O sorbent (Figure 3a,b). It also can be noticed both types of biosorbents show a slightly increased affinity for MB adsorption (Figure 3a), as compared to CV adsorption (Figure 3b). Considering that both dyes have the same ionic charge, this difference in their adsorption capacity can be attributed to their difference in chemical geometries. For instance, MB has a molecular surface area of 273 Ų, while CV has a greater molecular surface area (374 Ų) with additional hydrophobic methyl groups (Figure S2b) [34].

The experimental data were interpolated by applying pseudo-first-order (PFO), pseudo-second-order (PSO), and intra-particle diffusion kinetic models [35,36]. The predictions by different models are represented in Figure 3 by solid, dashed, and dash-dot lines. The corresponding non-linear regression equations and the kinetic parameters are given in given in Table S1. The chi-squared (χ²) statistical test was performed to evaluate the goodness-of-fit (smaller values indicate a better agreement of the predicted data with the experiment), according to Equation (S2) in Supplementary Materials [36,37]. As observed in Figure 3 and Table S1 (χ²-values), the best-fitting model for MB adsorption kinetics was the PFO model, while for CV adsorption, the PSO model provided the best-fitting results.

3.5. Adsorption Isotherms

The adsorption isotherms were determined at two temperatures (300 K and 330 K, respectively), in order to evaluate the relationship between the equilibrium concentrations of MB and CV dyes in aqueous media and the biosorbents’ adsorption capacities. Figure 4 shows the experimental results, which indicate the relationship between equilibrium concentration C_e (mg/L) and adsorption capacity q_e (mg/g). Additionally, it is observed that both dyes are more readily absorbed by the investigated materials at higher temperatures. Therefore, the highest adsorption capacities of MB (Figure 4c) and CV (Figure 4d) were registered at 330 K by the WS_NaOH biosorbent (110.5 mg/g and 105.5 mg/g, respectively). However, it should be noticed that the WS_H2O biosorbent, obtained by eco-friendly treatment, recorded maximum adsorption capacities at 330 K very close to those of the mercerized sorbent (105.9 mg/g for MB and 102.5 mg/g for CV). By contrast, at a lower temperature (300 K), there is a significant difference between the adsorption efficiency of the WS_H2O (13.1 mg/g for MB and 11.1 mg/g for CV) and WS_NaOH (66.9 mg/g for MB and 61.8 mg/g for CV) biosorbent, as shown in Figure 4a,b.

To investigate the adsorption process, the experimental data were interpolated by Langmuir and Freundlich isotherm models [36,37], and the calculated predictions are shown in Figure 4 (solid and dashed lines). Likewise, the chi-square test (χ²) was used to assess the agreement between the adsorption isotherms models and the experimental data. The corresponding values are given in Table S2, together with the isotherms equations and calculated parameters. According to the data reported in Table S2, in most of the cases, the Langmuir equation is the predominant best-fitting model for the investigated systems, indicating monolayer-type adsorption of the dyes [38].

Based on the Langmuir model equation, the dimensionless separation factor R_L (equilibrium parameter) was further determined, according to Equation (3):

$$R_L=\frac{1}{1+K_L{C}_0},$$

(3)

where K_L is the Langmuir constant and C₀ is the initial dye concentration. The R_L parameter value offers information about the nature of the adsorption process (R_L = 0 describes irreversible adsorption; R_L ranging between 0 and 1 indicates favorable adsorption, and R_L = 1 suggests a linear unfavorable adsorption) [39]. As detailed in Table S2, the values obtained for the R_L factor were between 0 to 1 (for initial dye concentrations ranging from 10 to 500 mg/L), denoting favorable adsorption of both MB and CV dyes onto the investigated biosorbents.

In addition, the Dubinin–Radushkevich (D–R) isotherm model was employed to elucidate the type of the adsorption process by determining the free energy of sorption E_S (kJ/mol). Three situations can be distinguished based on the values of this main parameter. (1) E_S < 8 kJ/mol indicates that the adsorption process is based on physical forces (such as van der Waals); (2) E_S in the range of 8–16 kJ/mol suggests an ion exchange mechanism; and (3) E_S values higher than 16 kJ/mol implies chemisorption processes [36,40]. As shown in Table S2, the mean free energy (E_S) values for many of our investigated systems ranged between 9.91–16.48 kJ/mol, suggesting that the adsorption process of MB and CV could rely on ion exchange mechanisms (electrostatic interactions between the negatively charged carboxylic (−COO⁻) groups from biosorbents and positively charged amino groups from dyes).

Both cationic dyes (MB and CV) are known to be exogenous fluorophores [41,42]. Therefore, their adsorption onto the surface of the biosorbents (WS_H2O and WS_NaOH, respectively) was analyzed by polarized light microscopy, as shown in Figure 5b–f. As compared to raw walnut shell grains (Figure 5a,b), both types of biosorbents revealed homogenous adsorption of cationic dyes on the entire surface (Figure 5c–f).

Additionally, we performed a comparative literature study regarding the performance of our eco-friendly modified biosorbents by reporting to other sorbent materials based on walnut shells, untreated or treated by different chemical methods (

Table 1. Comparison of the maximum adsorption capacities of walnut shell-based biosorbent materials for retention of MB and CV cationic dyes.

Walnut Shell (WS)-Based Adsorbent Material	Treatment	Dye	Maximum Adsorption Capacity (mg/g)	Ref.
WS	washed, immersed in distilled water for 24 h, and dried at 353 K	MB	80.4 mg/g	[19]
WS	crushed, washed with distilled water, and dried at 378 K	MB	7.81 mg/g	[45]
WS	rinsed with distilled water, dried at 378 K, ground, and sieved	MB	51.55 mg/g	[46]
WS	washed, ground, and dried at 343 K	MB	36.63 mg/g	[47]
WS	crushed and dried at room temperature	MB CV	70.41 mg/g 66.00 mg/g	[48]
WS	washed with distilled water, dried at room temperature, powdered, and sieved	CV	22.67 mg/g	[49]
WS acetic acid WS acetic acid/hydrogen peroxide	chemical modification	MB	65.7 mg/g 55.7 mg/g	[50]
WS malonic acid cyclic isopropylidene ester	chemical modification	MB	98.89 mg/g	[21]
WS levulinic acid	chemical modification	MB	294.1 mg/g	[43]
WS hydrochloric acid WS sulfuric acid WS phosphoric acid	chemical modification	MB	20.04 mg/g 19.61 mg/g 38.46 mg/g	[51]
WS sodium hydroxide	chemical modification	MB	5.78 mg/g	[52]
WS sodium hydroxide	chemical modification	CV	123.2 mg/g	[44]
WSH2O	hot water treatment	MB CV	103.9 mg/g 102.5 mg/g	This work
WSNaOH	chemical modification	MB CV	110.5 mg/g 105.5 mg/g	This work

). Thus, it can be noticed that the maximum adsorption capacity values of MB and CV onto WS_H2O and WS_NaOH biosorbents (obtained in this study) recommend these materials for the removal of organic dyes from aqueous solutions. However, higher adsorption capacities of the dyes revealed by other studies [43,44] are strongly dependent on several factors, such as type of sorbent modification, sorbent dose, temperature, initial concentration, and others.

3.6. Thermodynamic Parameters of the Adsorption Process

The main thermodynamic parameters (such as Gibbs free energy, enthalpy, and entropy) give in-depth information about inherent energetic changes associated with the adsorption phenomena [53]. To assess these thermodynamic parameters for the adsorption, one should take into account the equilibrium constants at least for two distinct temperature levels. In this study, the apparent equilibrium constant of adsorption (K_d) was approximated on the basis of the Langmuir equilibrium parameter K_L; the latter was firstly converted from the (L/mg) unit to (L/mol) unit, according to the following scheme:

$$K_L\left(\mathrm{L}/\mathrm{mol}\right)=K_L\left(\mathrm{L}/\mathrm{mg}\right)·1000 \left(\mathrm{mg}/\mathrm{g}\right)·M_w\left(\mathrm{g}/\mathrm{mol}\right),$$

(4)

where M_w (g/mol) is the molecular weight of the cationic dye (adsorbate). Further, the apparent equilibrium constant of adsorption (K_d) was estimated as follows [54]:

$$K_d=K_L\left(\mathrm{L}/\mathrm{mol}\right)·C_{ref}\left(\mathrm{mol}/\mathrm{L}\right)·\frac{1}{\gamma },$$

(5)

where γ denotes the activity coefficient calculated on the basis of the ionic strength [55,56,57] and C_ref is the molar concentration of the reference state. Usually, in the conventional approach, the concentration of solute (in the reference state) is considered equal to 1M, i.e., C_ref = 1 mol/L [54]. For this case, the adsorption equilibrium constant K_d (dimensionless) is numerically identical to K_L. However, there are situations when the reference concentration of 1 mol/L cannot be applied, since the water solubility of the considered solute (pollutant) is lower than 1 mol/L. In this case, one might consider the saturated solution (C_S) as the reference concentration [55]. For instance, the water solubility of the CV dye is equal to 4 g/L (0.0098 mol/L), whereas the solubility of MB dye is equal to 43.6 g/L (0.1363 mol/L). Therefore, in our study, we calculated the apparent equilibrium constant of adsorption (K_d) by using Equation (5), and as the reference concentration, we considered the saturated solution (i.e., C_ref = C_S, mol/L). The activity coefficient (γ) was calculated by the Debye–Huckle and Davis relationships, depending on the ionic strength [54,56]. The variation of the Gibbs free energy (ΔG) was calculated as follows [52,53,54,55,56]:

$$\mathsf{\Delta }G=-R_{g}T ln\left(K_d\right),$$

(6)

where T is the absolute temperature (K), R_g is the universal gas constant (R_g = 8.314 J/(K∙mol)), and K_d is the equilibrium constant of adsorption (without units). The enthalpy changes (ΔH) of the adsorption process were assessed from the well-known isochore Van ’t Hoff equation, which can be written as [57]:

$$\mathsf{\Delta }H=\frac{-R_g\left(\frac{K_{d,T_2}}{K_{d,T_1}}\right)}{\frac{1}{T_2}-\frac{1}{T_1}},$$

(7)

where T₁ and T₂ are two distinct levels of temperature, and $K_{d,T_1}$ and $K_{d,T_2}$ are equilibrium constants determined at these levels of temperature. The change in entropy (ΔS) was calculated according to the following thermodynamic relationship [54,55,56]:

$$\mathsf{\Delta }S=\frac{\mathsf{\Delta }H-\mathsf{\Delta }G}{T},$$

(8)

The calculated values of thermodynamic parameters are summarized in

Table 2. Values of various thermodynamic parameters for the adsorption of cationic dyes (MB and CV) onto low-cost biosorbents (WS_H2O and WS_NaOH).

Biosorbent	Dye Adsorbed	T(K)	ΔG (kJ/mol)	ΔH (kJ/mol)	ΔS (J/K·mol)
WSH2O	MB	300 330	−20.99 −23.15	+0.65	+72.15
	CV	300 330	−11.39 −14.91	+23.80	+117.32
WSNaOH	MB	300 330	−22.91 −24.88	−3.15	+65.86
	CV	300 330	−14.21 −15.27	−3.60	+35.37

. The negative values of Gibbs free energy (ΔG < 0) indicated that the investigated adsorption processes were spontaneous (exergonic). Likewise, the values of the ΔG parameter (ranging from −23.15 to −11.39 kJ/mol) suggested that the adsorption mechanism was based on both physical adsorption and ion exchange. The more negative values of ΔG for the adsorption of MB disclosed a better affinity of sorbents to MB compared to CV. This outcome corroborates the previous kinetic study (Figure 3), where a slightly increased affinity was observed for MB as compared to CV.

As outlined in Table 2, endothermic effects (ΔH > 0) were noticed for the WS_H2O adsorbent. These outcomes corroborate the increment of adsorption capacity as the temperature increases for the systems based on the WS_H2O adsorbent. In turn, for the WS_NaOH adsorbent, exothermic effects (ΔH < 0) were determined. For this case, the increase in the adsorption capacity with temperature might be explained by the intensification of the intraparticle diffusion of dye molecules into the porous structure of WS_NaOH material as the temperature is increased. These results might suggest that the treatment of cellulose-based materials with NaOH (mercerization) is obviously different than the treatment with hot water, thereby leading to more intense textural modification. According to Reference [26], in the mercerization process of cellulose-based material, sodium hydroxide (NaOH), is able to penetrate the structure of cellulose, thus reducing its crystallinity and increasing porosity. Hence, the enhancement of the intraparticle diffusion as the temperature gets higher might be the plausible mechanism for the mercerized sample (WS_NaOH).

The entropy (ΔS) was found to be positive for all investigated systems (Table 2), suggesting an increment of the disorderliness at the solid/solution interface.

3.7. Desorption Assays

The desorption assays for spent WS_H2O and WS_NaOH biosorbents (loaded with MB and CV cationic dyes) were carried out in five types of eluents (distilled water, 0.01 M HCl, 0.01 M NaCl, 0.5 M NaOH, and ethanol), as shown in Figure S3. The results showed maximum release efficiencies for MB (23.6%) and CV (16.3%) from the WS_H2O biosorbent in HCl solution and ethanol, respectively. As a result of the low desorption values in the water medium, and to avoid the use of additional chemicals, the spent materials were further tested for direct adsorption of other dyes (anionic species) from the aqueous solution. This option is supported by the fact that the desorption of cationic dyes in water is negligible (<0.9%).

3.8. Valorization of Spent Biosorbents

After the primary adsorption of cationic dyes, the surface of the walnut shell-based biosorbents can be seen as a chemically modified surface, which might have an affinity for the retention of anionic dyes (secondary adsorption). According to the literature, the double adsorption approach is suitable as an effective alternative for sorbent reusability by treating wastewater derived from different industries such as textile, dyeing, printing, leather, and coating industries [58,59,60]. In these industrial applications, large amounts of water are used in coloring and finishing processes. Consequently, the effluents derived from these industries contain biodegradable and non-biodegradable chemical pollutants such as dyes (both cationic and anionic), surfactants, various cations, anions, etc. In this respect, after the MB and CV cationic dye adsorption, the WS_H2O and WS_NaOH spent biosorbents were tested for a secondary direct adsorption of the Orange II (OII) and Congo Red (CR) anionic dyes. The removal efficiency (%) of the anionic dyes is represented in Figure 6.

It can be noticed that the spent biosorbent WS_H2O disclosed higher removal efficiency of OII and CR in the secondary adsorption test as compared with spent biosorbent WS_NaOH. This might be attributed to the less loaded surface of the spent sorbent WS_H2O compared to the spent sorbent WS_NaOH. Moreover, in the secondary adsorption test, the removal of the CR dye was favored compared to the OII dye, mainly due to the higher reactivity of CR given by the presence of two anionic (–SO₃^–) groups. Thus, the highest CR anionic dye removal efficiencies were about 53–57 % (onto the spent WS_H2O biosorbent). Similar tests were reported by other authors [60] by using spent sorbents (loaded with anionic dyes) in a secondary adsorption test to eliminate MB cationic dye.

4. Conclusions

In summary, two types of low-cost walnut shell-based biosorbents were produced by surface modification: (1) WS_H2O (by hot water treatment) and (2) WS_NaOH (by low-concentration alkali treatment). The morphological, structural, and elemental modifications of the investigated materials were confirmed by SEM, EDX, and FTIR techniques of characterization. Both types of biosorbents were subjected to the removal of MB and CV cationic dyes from aqueous solutions, disclosing an increased affinity for MB adsorption, as compared to CV. Adsorption kinetics analysis revealed the best-fitting model for MB dye adsorption was the PFO model, whereas for CV dye adsorption, the kinetic model PSO provided the best predictions. The maximum adsorption capacities of MB and CV determined at 330 K for the WS_NaOH biosorbent were 110.5 mg/g and 105.5 mg/g, respectively). Likewise, the WS_H2O biosorbent presented close values to those of the mercerized sorbent (105.92 mg/g for MB and 102.5 mg/g for CV at 330K).

Negative Gibbs free energy (ΔG < 0) disclosed the spontaneous nature of the investigated adsorption processes. In addition, the values of this thermodynamic parameter (ΔG = −23.15 to −11.39 kJ/mol) suggested that the adsorption mechanism might rely on both physical adsorption and ion exchange. Regarding the enthalpy (ΔH), endothermic and exothermic effects were determined for WS_H2O and WS_NaOH biosorbents, respectively. For all investigated adsorption systems, the entropy (ΔS) was found to be positive, suggesting an increase in the disorderliness at the solid/solution interface and corroborating the spontaneous nature of the adsorption processes.

Desorption, from spent biosorbents of cationic dyes in water, was negligible (<0.9%). Therefore, the spent biosorbents were successfully valorized in a secondary adsorption application for additional retention of the anionic dyes.

However, the eco-friendly modification of walnut shell agro-waste remains challenging and there is huge interest in the adsorption of toxic organic compounds from wastewater. For this reason, we propose future research directions regarding the optimization of this system (hot water treatment) in order to increase the adsorption efficiency of cationic dyes and the desorption efficiency for increasing the possibility of a multiple cycle reuse of the spent biosorbents.