Preparation of High-Performance KOH-Activated Biochar from Agricultural Waste (Sapindus mukorossi) and Its Application in Organic Dye Removal

Abstract

The separation of organic dyes from wastewater (WW) is a major challenge in water pollution control. The present research utilised agricultural residuals from Sapindus mukorossi were utilised to prepare high-performance biochar through carbonization and KOH activation, while its efficiency was evaluated in removing methylene blue (MB). The physicochemical characteristics of the unactivated Sapindus shell biochar (SH0) and activated Sapindus shell biochar (SH2) material were characterised via EA, FTIR, BET, and SEM analyses. The findings indicated that the KOH activated Sapindus shell biochar (SH2) exhibited higher adsorption efficiency in comparison to Sapindus shell biochar (SH0). In particular, the SH2 demonstrated an 11.2-fold higher adsorption capacity for MB (502.11 mg·g⁻¹) compared to SH0, a performance enhancement driven by its remarkably porous structure, substantial total pore volume (0.56 cm³·g⁻¹), and high specific surface area (871.04 m²·g⁻¹). A high MB removal efficiency of 98.36% was achieved within 30 min under the following optimal conditions: a KOH/SH0 activation ratio of 2:1, pH 6.5, and a biochar dose of 0.2 g·L⁻¹. The MB adsorptive process was studied by applying the Langmuir isotherm and PSO kinetic models, suggesting physical and chemical interaction mechanisms between MB dyes and SH2. These findings provide a feasible strategy for the application of Sapindus shells and offer technical support for effectively removing dyes from wastewater by KOH-modified biochar.

1. Introduction

Organic dye wastewater (DW) from the production of various industries, including cosmetics, food, paper, pigments, textiles, and pharmaceuticals, has been continuously increasing [1,2]. Inadequate management of organic dye macromolecules can pose a serious threat to the photosynthesis of aquatic plants, water source quality, and human health, due to their strong colouring ability, stability, poor biodegradability, and high toxicity (teratogenicity and mutagenicity) [3]. Therefore, the development of effective dye removal strategies is crucial for the restoration and preservation of the aquatic ecological environment. Several techniques, including electrochemical processes, photolysis, membrane filtration, advanced oxidation processes, adsorption, biological treatment, and coagulation-flocculation, have been utilised for treating DW [4,5]. Among the available treatment approaches, adsorption has been extensively utilised owing to its economic feasibility, high efficiency, and environmental safety. However, the biochar produced by pyrolysis requires modification to achieve enhanced performance because of its limited specific surface area (SSA) and poor removal efficiency toward organic pollutants [6]. The chemical activation is a premier modification technology for optimising the adsorption capacity of biochar. This method enhances biochar by introducing functional groups (FGs), thus changing the pore volume, SSA, and surface charges [7].

Different modifiers (such as acid, Alkali, metal salt) exhibit distinct activation mechanisms and resultant effects. Acid treatment can introduce abundant FGs of biochar, while simultaneously removing metals (such as K, Na and Mg) and impurities from the biomass. [8]. Biochar modified with metal salts demonstrates improved porosity, electrostatic interaction, and ion exchange capabilities [9]. However, the potential environmental toxicity of some metals limits their wide use as chemical modifiers. Alkaline activators are effective for increasing SSA, total pore volume (TPV) and the number of FGs in biochar. KOH activation can efficiently etch the carbon skeleton through a series of reactions, obtaining biochar adsorbents with a relatively high SAA. The KOH activation method has been used to treat various biomass types, yielding adsorbents with outstanding dye removal capabilities [10]. In addition, surface aromaticity increased and hydrophilicity was reduced as a result of alkali modification [11]. Compared to the unactivated biochar, KOH-activated biochar derived from Pennisetum (pyrolyzed at 300–800 °C) exhibited a 65–200-fold and a 92–1532-fold increase in TPV and SSA, respectively, resulting in a highly rich microporous structure [12]. The SSA of almond shell biochar increased from only 2.3 m²·g⁻¹ to as high as 1050 m²·g⁻¹ after co-modification with KOH and EDTA-4Na [13]. After KOH activation, a decrease in oxygen content (23.64% to 12.90%), and an increase in carbon content (73.94% to 85.22%) of solid waste biochar were observed, which significantly enhanced its aromaticity [14]. The oxygen-containing FGs of Douglas Fir biochar were also significantly increased after KOH activation [15].

Sapindus, an important tropical hardwood agroforestry tree, has been extensively planted in southern China. Sapindus is primarily used for the manufacture of detergent products [16], soap [17], biomedicines [18], and biodiesel [19], and as a wood supply. The carbon content of Sapindus residue after the extraction of biodiesel and soap can be as high as 50%, which suggests that Sapindus biomass may be an suitable raw precursor for the activated biochar (AC). The application potential of Sapindus-derived carbon materials has been investigated by various works in the literature. For example, the thermally treated S. mukurossi pericarp biomass removed 95% of the methyl violet dye (28 mg·L⁻¹) from WW at pH 4 with a dose equal to 0.1 g·L⁻¹ [20]. The AC sample obtained from the Sapindus mukorossi nut efficiently removed bisphenol A (BPA) from water [21]. The optimal dosage of clay (2.5 g) and AC (0.5 g) prepared from Sapindus seed achieved a removal efficiency (R) equal to 86% for yellow dye WW (10 mg·L⁻¹) [22]. A porous adsorbent prepared from S. mukorossi and nano-clay attapulgite demonstrated high adsorption efficiency for antibiotics in a soil environment [23]. While Sapindus represents a promising carbon source for pollutant adsorption, these studies have obvious limitations. On the one hand, the existing research mainly focuses on antibiotics or low-concentration dyes as pollutants, with limited investigation into the removal efficiency of typical high-concentration cationic dyes (such as Methylene Blue (MB), Crystal Violet (CV), and Rhodamine B (RhB)). On the other hand, the existing research mainly focuses on the preparation methods of Sapindus mukorossi by pyrolysis or physical mixing with other materials. The use of the KOH chemical activation method to treat Sapindus mukorossi shells has not been yet reported in the literature. Furthermore, a comprehensive evaluation of its performance and underlying mechanism as an efficient dye adsorbent remains lacking.

In this study, modified biochar was synthesised by carbonization combined with KOH chemical activation using Sapindus mukorossi shells as raw materials, and its adsorption performance and mechanism for MB were systematically investigated. The goals of this study were to: (I) characterise the key morphological and structural features of the activated Sapindus mukorossi shells biochar, (II) evaluate its adsorption performance for MB through batch adsorption experiments, and (III) analyse the mechanism of MB adsorption onto activated Sapindus mukorossi shell biochar via adsorption isotherms, thermodynamics, and kinetics models. These findings inform the utilisation of S. mukorossi as a cost-friendly biochar source for the separation of organic dyes from WW.

2. Materials and Methods

2.1. Reagents and Biochar Biomass

Analytical grade hydrogen chloride (HCl), KOH, and MB (C₁₈H₁₈ClN₃·3H₂O) were procured from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The experimental water was treated by a Milli-Q deionization system (EQ700, Sigma-Aldrich (Wuxi, China) Biochemical Technology Co., Ltd.). The Sapindus mukorossi biomass (S. mukorossi Gaertn.) was obtained in Hangzhou (Zhejiang).

2.2. Synthesis of Adsorbents

The synthesis method of Sapindus mukorossi shell biochar as described by Mi et al. [24]. The Sapindus mukorossi shells powder (SH) was cleaned many times with H₂O, dried off at 60 °C till reaching a steady weight reading, and then sieved through a 30-mesh sieve. After that, 10 g of dried SH powder was loaded into a quartz boat and carbonised inside a horizontal tube furnace (OTF-1200X, Hefei Kejing Materials Technology Co., Ltd., Hefei, China) under flowing N₂ for 1 h at 400 °C. The product was then cooled to the ambient temperature and denoted Sapindus shell biochar (SH0). SH0 was added to the aqueous KOH solution (50 mL) in different mass ratios of KOH to SH0 (1:1, 2:1, 3:1). The resultant mixtures were continuously stirred for 24 h, then dried off for 4 h at 105 °C until all moisture was removed. The resulting powder was pyrolyzed for a duration equal to 2 h at 800 °C within an N₂ environment. The heating rate and N₂ flow rate during the two pyrolysis processes were 20 °C·min⁻¹ and 200 mL·min⁻¹, respectively. After activation, the samples were stored under N₂ flow till they reached ambient temperature. Finally, the obtained specimens were cleaned with 1M HCl and ultrapure H₂O till the filtrate pH was 7 and dried off until the weight remained unchanged at 100 °C. The prepared specimens were denoted SH1, SH2, and SH3 for KOH/SH0 ratios of 1:1, 2:1, and 3:1, respectively.

2.3. Adsorption Experiment

The activated biochar powder and MB aqueous solution (100 mL) were transferred to a 150 mL conical bottle. The mixture was shaken (200 rpm) at 25 ± 1 °C in an incubator shaker (YCT-80B, Shanghai Jie Cheng Experimental Instruments Co., Ltd., Shanghai, China) prior to the adsorption experiments. At specific time intervals, including 2, 5, 10, 20, 60, 120, and 180 min, the mixed liquid was passed via a 0.45 μm filter membrane. A Spectrophotometer was used to quantify the concentration value of MB at 664 nm (UV5, Mettler Toledo Technology (China) Co., Ltd., Shanghai, China). The impact of adsorption duration (0–180 min), initial MB concentration (C₀) (20–200 mg·L⁻¹), absorbent dose (10–400 mg biochar·L⁻¹ MB solution), and initial pH (0–12) on the adsorption efficiency for MB was thoroughly investigated. The calculation formulas for the values of removal efficiency (R) and adsorption capacity (Q_t) can be found in the Supplementary Materials. The method for determining the point of zero charge (pH_pzc) of SH2 was adapted as Dalvand et al. [25] and the pH values were performed using a multi-parameter tester (HQ40d, Hach Water Quality Analytical Instruments Co., Ltd., Shanghai, China). Each adsorption test was conducted 3 times, and the mean value of the obtained data was found.

2.4. Characterisations

The experimental results on MB removal performance identified the activated biochar with a KOH/SH0 ratio of 2:1 (SH2) as the optimal material. The raw powder (SH), unactivated biochar (SH0) and activated biochar (SH2) were adopted to explore the differences in their morphology and composition through several characterisation methods. The elemental N, H, and C content was identified by carrying out elemental analysis (EA; Vario EL cube, Elementar Trading Co., Ltd., Shanghai, China), and the FGs were determined via a Fourier transform infrared spectroscopy (FTIR; Nicolet Nexus 670, Thermo Nicolet Corporation, Waltham, MA, USA). FTIR analysis was conducted at a resolution of 4 cm⁻¹ with 32 scans min⁻¹ over the 4000–400 cm⁻¹ range. A scanning electron microscopy (SEM; Sigma 300, Carl Zeiss AG, Oberkochen, Germany) system was used to determine the microscopic structure. The N₂ desorption/adsorption isotherms were characterised by surface area and porosity analysis (ASAP2460, Micromeritics Instruments Co., Ltd., Shanghai, China). More details regarding the characterisation methods of EA, SEM, and BET can be found in the work of Herath et al. [15].

2.5. Adsorption Isotherms and Kinetics

Adsorption isotherms along with kinetic and thermodynamic models were analysed to determine the mechanism of MB adsorption onto SH2. The kinetics of MB transferred from the solution to SH2 was explored using the pseudo-first-order (PFO), Weber-Morris intra-particle diffusion (IPD), and pseudo-second-order (PSO) models [26]. The Freundlich and Langmuir models were utilised to analyse the isotherms [27]. The Gibbs-Helmholtz relationship was utilised to explore the thermodynamics of MB adsorption onto SH2. The equations of these models can be found in the Supplementary Material.

2.6. Data Analysis

Each test was conducted three times. The experimental data were described as the mean ± SE. All figures were plotted using GraphPad Prism (version 10.4.2). To compare the different treatments, One-way ANOVA (Tukey’s test) was implemented in IBM SPSS Statistics V25.

3. Results and Discussion

3.1. Characterisation of Adsorbents

3.1.1. Elemental Analysis and Surface Morphology

The surface morphology images of SH, SH0, and SH2 are shown in Figure 1. The surface of SH had few pores and cavities (Figure 1a), the surface of SH0 was rough and partially porous after the carbonisation of SH at 400 °C (Figure 1b). Furthermore, the addition of the KOH activator significantly increased the pore density of SH2, as seen in Figure 1c. Yang et al. found that activation by KOH is a crucial factor in generating numerous disordered pores at high temperature values [28]. The irregular pores and cavities on the surface of SH2 indicated that KOH had broken ether bonds within lignin and hydrolysed glycosidic bonds within hemicellulose and cellulose [29]. The MB dye macromolecules can migrate through these pores and cavities to the active regions within the inner pores of SH2.

As shown in

Table 1. Elemental analysis results (mass, %).

Sample	N (%)	C (%)	H (%)	H/C
SH	0.54	45.04	5.402	0.1199
SH0	0.35	73.60	3.519	0.0478
SH2	0.13	84.73	0.632	0.0075

, KOH activation resulted in a decrease of 83% in hydrogen (H) content (from 3.5% to 0.6%) and a decrease of 63% in nitrogen (N) content (from 0.35% to 0.13%). Concurrently, the carbon (C) content increased from 73.4% to 84.73%. Both the evolution of volatile matter and the removal of ash and heteroatom compounds during the pyrolysis process and KOH modification contributed to the consumption of non-carbonaceous components (H and N) and the enrichment of C [30]. Additionally, the aromaticity of biochar is inversely proportional to the hydrogen/carbon (H/C) ratio [11]. As a result of KOH modification, the H/C content ratio of SH2 reduced by 0.0403. This outcome indicated that KOH activation coupled with the elevated pyrolysis temperature promoted the cracking of the surface FGs and the complete destruction of aliphatic carbon structures, resulting in the activated biochar (SH2) with highly aromatic structures. The specific chemical reaction process involving KOH can be described as follows [31].

$$6\mathrm{K}\mathrm{O}\mathrm{H}+2\mathrm{C}\to 2\mathrm{K}+2{\mathrm{K}}_2{\mathrm{C}\mathrm{O}}_3+3{\mathrm{H}}_2$$

(1)

$${\mathrm{K}}_2{\mathrm{C}\mathrm{O}}_3\to {\mathrm{K}}_2\mathrm{O}+{\mathrm{C}\mathrm{O}}_2$$

(2)

$${\mathrm{K}}_2{\mathrm{C}\mathrm{O}}_3+2\mathrm{C}\to 2\mathrm{K}+3\mathrm{C}\mathrm{O}$$

(3)

$${\mathrm{K}}_2\mathrm{O}+\mathrm{C}\to 2\mathrm{K}+\mathrm{C}\mathrm{O}$$

(4)

3.1.2. Textural Analysis

The structural characteristics of SH0 and SH2 are presented in

Table 2. Textural characteristics of SH, SH0, and SH2.

Sample	BET Surface Area (m2·g−1)	TPV (cm3·g−1)	Mean Pore Diameter (nm)
SH	0.50	0.000701	5.5575
SH0	2.04	0.002387	4.6837
SH2	871.04	0.556474	2.5555

. Compared with SH0, KOH activation reduced the average pore size and increased the TPV and the SSA of SH2. As shown in Figure 2., the N₂ isotherms of SH and SH0 were type II. This result verified the non-porous nature of these materials. In contrast, SH2 exhibited a Type IV adsorption–desorption isotherm (Figure 2), characteristic of mesoporous materials [32]. The pore diameter (2.5555 nm) of SH2 also indicated its mesoporous nature. Additionally, SH2 demonstrated a surface area equal to 871.04 m²·g⁻¹, which was 427-fold larger compared to that of SH0 (2.0385 m²·g⁻¹). The increased SSA and the development of porous structures in SH2 were attributed to a series of chemical reactions involving KOH. The release of the generated gas from the solid, and the embedding of metal (K) in the carbon structure [33]. In general, a developed mesoporous structure along with a large SSA facilitates the absorption of organic dyes, resulting in higher adsorption rates [34].

3.1.3. FTIR Analysis

Variations FGs of the biochars (SH, SH0 and SH2) were investigated by performing FTIR characterisation. The consistent absorption peaks of SH0 and SH2 in Figure 3 indicated that the Sapindus mukorossi shell biochar has a relatively stable chemical structure. The broad signal forming between 3300 and 3500 cm⁻¹ was characteristic of N–H and O–H stretching vibrations [35]. The absorption bands between 2800 and 2900 cm⁻¹ corresponded to C-H and -CH₂- stretching vibrations [36]. The characteristic bands formed were ascribed to the carboxylic C=C (between 1599 and 1623 cm⁻¹) and C-O groups (and 1030–1250 cm⁻¹), respectively [37]. These oxygen-containing FGs can enhance the removal of dyes from WW. The peak intensity of O-H was significantly decreased after the pyrolysis of SH. This result indicated the dehydration and decomposition of -OH and an improvement in hydrophobicity [38,39]. Additionally, after activation, the characteristic peak intensities for SH0 and SH2 weakened, mainly resulting from the volatilisation of oxygenate (1045 cm⁻¹), decomposition of aliphatic substances (1400 cm⁻¹) and dehydration of biochar under high temperatures (3449 cm⁻¹) [40].

Compared with the FTIR spectrum without adsorbed MB, some characteristic peaks were shifted in the FTIR spectrum after SH2 adsorbed MB. After adsorbing MB, the characteristic aromatic structure peak (C=C) at 1623 cm⁻¹ was shifted to 1619 cm⁻¹, suggesting π-π stacking between MB and the aromatic structures of SH2 [41,42]. The absorption peak corresponding to the C-O vibration at 1083 cm⁻¹ significantly shifted by 11 cm⁻¹ (1072 cm⁻¹), indicating the involvement of oxygen-containing FGs in the adsorption of MB by SH2 [43]. The aromatic C-H out-of-plane bending vibration of adsorbed the dye might be the reason of the characteristic peak shifting from 613 cm⁻¹ to 603 cm⁻¹ [44]. Furthermore, the slight displacement and weakened intensity of the O-H/N-H stretching vibration peak at 3446 cm⁻¹ suggest that the involvement of hydrogen bonding as an auxiliary mechanism in the adsorption process [2].

3.2. Batch Experiments

3.2.1. Impact of KOH Activation Ratio

The influence of different mass ratios of KOH to SH0 (1:1, 2:1, 3:1) on the adsorption capacity of MB under the fixed conditions of 0.2 g·L⁻¹ adsorbent dose, 100 mg·L⁻¹ MB concentration, and pH = 6.5, as shown in Figure 4. The Q_t values of SH1, SH2, and SH3 for MB exceeded that of SH0, as KOH activation increased their SSA values and developed richer porous structures, thereby providing more active sites for dye [45]. Additionally, the interactions between biochar and MB, including electrostatic attractions, the π-π stacking, and the H-bonds may have been strengthened after biochar activation with KOH [46]. Among the three materials (SH1, SH2, and SH3), the adsorption quantity of SH3 for MB was the highest, reaching 507.06 mg·g⁻¹. This was followed by SH2 and SH1 with values equal to 502.11 and 259.48 mg·g⁻¹ at 180 min, respectively. However, no significant difference in the adsorption capacity between SH2 and SH3 was observed by one-way ANOVA (Figure 4). Considering its lower cost and excellent MB removal performance, subsequent experiments were conducted using SH2.

3.2.2. Impact of Absorbent Dose

The influence of varying SH2 amounts on the MB adsorption capacities was explored (Figure 5a,b). The removal rate of MB was enhanced from 19.79% to 99.52% as the dosage of SH2 was raised from 0.01 to 0.4 g·L⁻¹ at a fixed pH (6.5) and dye concentration (100 mg·L⁻¹). The MB dye was completely removed as the SH2 dose was raised to 0.6 g·L⁻¹. Similarly, the number of available active regions and pores increased as the adsorbent dosage was raised [7], which facilitated the binding of more dye molecules. However, the Q_t value of MB decreased as the dosage of SH2 increased (Figure 5b). This effect was linked to the reduced availability of MB dye per unit mass of the SH2, as well as to potential particle aggregation that renders active sites inaccessible [47]. Taking into account the lower cost and the high removal rate, a dose of 2 mg SH2 per 100 mL MB solution was chosen for the subsequent tests.

3.2.3. Impact of Dye Concentration

The changes in the adsorption efficiency of SH2 at different MB concentrations (20–200 mg·L⁻¹) are shown in Figure 5c,d. As the concentration of MB increases, the quantity of MB adsorbed on SH2 enhanced from 105.08 to 598.94 mg·g⁻¹, while the removal efficiency of MB reduced gradually from 100% to 54%. The enhanced Q_t value of SH2 originated from the enhancement of the MB concentration gradient. Conversely, the insufficient number of active regions on SH2 to adsorb the high MB concentrations, leading to a decrease in removal efficiency.

Within the initial 10 min, SH2 removed between 50% and 80% of the MB. This was linked to the high SSA value and the porous structure of SH2, which facilitated the rapid initial dye adsorption [48]. The adsorption efficiency of MB dye increased within 60 min, before stabilising as the contact duration between the dye and the SH2 increased. As adsorption progressed, the aggregation of MB dye on the SH2 surface impeded the pore diffusion, thereby reducing the adsorption rate [49].

3.2.4. Impact of pH Value

The change in aqueous solution pH significantly influences the Q_t value of an adsorbent. The Q_t value of SH2 for the MB dye at a starting pH lying within the range extending from 2.0 to 12.0 was explored (Figure 5e,f). The results demonstrated the potential application of SH2 in aqueous environments, due to its consistently high Q_t value for MB (466.09–522.68 mg·g⁻¹) from pH 2 to 12. After adsorption for 30 min, the removal efficiency of MB significantly increased from 64.75% (pH = 2) to 88.47% (pH = 12) as the initial pH values were increased. The pH_pzc of SH2 has been determined to be 6.35. This outcome indicated that when the pH values were below 6.35, the positive charge on SH2 caused electrostatic repulsion with the cationic dye, which impeded the adsorption of the MB. Under alkaline conditions (pH > 6.35), the SH2 surface became negatively charged due to deprotonation. This promoted the adsorption efficiency between SH2 and MB via electrostatic attraction [31,45]. Similar adsorption behaviours were identified by Zhang et al. [50].

3.3. Adsorption Kinetics

At the dye concentration of 100–400 mg·L⁻¹, the adsorption kinetics of SH2 for MB was explored via PSO and PFO models. The fitted curves for MB absorption on SH2 and the adsorption kinetics parameter values are presented in Figure 6a,b and Table S1, respectively. The PSO model had higher regression coefficients (R²) compared to the PFO model. Furthermore, the Q_t value obtained for MB at equilibrium was determined via the PSO model combined with experimental data. These findings demonstrated that the PSO model had great applicability MB adsorption by SH2, confirming that the chemisorption likely playing a crucial role in the process of SH2 adsorbing MB [51].

Furthermore, to identify the MB adsorption process on SH2, the adsorption kinetics of SH2 was explored via the IPD model. The relationship between qt and t^1/2 displayed multilinear characteristics as seen in Figure 6c and

Table 3. The parameter values of the IPD model for MB adsorption on SH2 at various C₀ values.

C0 (mg·L−1)	K1d (mg·g−1·min−1/2)	C1 (mg·g−1)	${\mathbf{R}}_1^2$	K2d (mg·g−1·min−1/2)	C2 (mg·g−1)	${\mathbf{R}}_2^2$
100	62.80	98.06	0.9953	13.791	316.51	0.9261
200	62.36	140.0	0.9981	16.485	363.2	0.9765
300	57.7	277.0	0.8642	21.263	355.43	0.9295
400	61.65	426.9	0.9898	5.0917	655.59	0.8386

. The non-zero interceptions at different stages indicated that both film diffusion and intraparticle diffusion affect the adsorption of MB by SH2 [52]. The two-stage of the MB adsorption process onto SH2 was evidenced by the IPD model. The initial adsorption process was the rapid binding of MB molecules to the surface active sites of SH2. The second step was the gradual adsorption stage, where the IPD process controlled the rate of dye molecules entering SH2 pores and binding to the effective active sites present therein. The slopes of the two stages (K_1d and K_2d) represent the rate of the adsorption process. A smaller slope (K_id) indicates a slower adsorption rate [53]. The K_1d value was greater compared to the K_2d, revealing that the Q_t value obtained for SH2 was higher upon the adsorption of MB to its outer surface. The rate-limiting step may have been internal diffusion, wherein MB adsorbed on the biochar surface gradually diffused into the pore interiors [5]. The intercept value also gradually increased from the first stage (C₁) to the second stage (C₂). A larger C_i values indicates a greater boundary layer effect [54]. Additionally, the C_i values were greater at higher MB concentrations than at lower values. This may have increased the diffusion resistance of MB, as commonly observed during the mass transfer process of high-concentration dye solutions [55].

3.4. Adsorption Isotherm

At the adsorption temperature of 298–318 K, the adsorption isotherms of SH2 for MB are presented in Figure 7, and adsorption isotherm parameter values are shown in Table S2. At all temperature, the R² values of the Langmuir model were higher compared to those of the Freundlich model. Hence, the Langmuir model provided a better visualisation of the adsorption behaviour of SH2, which suggested that MB adsorption by SH2 was monolayer adsorption with greater surface uniformity [56]. Additionally, the maximum theoretical Q_t value of SH2 increased with rising pyrolysis temperatures, suggesting that higher temperatures can enhance promote the diffusion of MB dye both within the pores and upon the surface of SH2 through an accelerated rate of molecular migration.

3.5. Adsorption Thermodynamics Study

Based on the adsorption isotherm results (Table S2), a positive relationship between temperature and Q_t can be extracted. The thermodynamic parameters at various temperatures were calculated using the Gibbs-Helmholtz equation (

Table 4. Thermodynamic parameters at various temperatures of MB adsorption using SH2.

T (K)	Kc (L·g−1)	$∆\mathit{G}$ (kJ·mol−1)	$∆\mathit{H}$ (kJ·mol−1)	$∆\mathit{S}$ (J·mol−1·K−1)
298	10.019	−5.71	16.77	75.34
308	12.029	−6.34
318	15.349	−7.22

). The ΔH value was 16.77 kJ·mol⁻¹ (>0), suggesting an endothermic adsorption process. Hence, higher temperatures are favourable for enhancing MB adsorption on SH2. Furthermore, the adsorption of MB on SH2 was identified as a spontaneous physisorption process, based on the ΔH value was lower than 40 kJ·mol⁻¹ [44], and the ΔG < 0. The ΔG values reduced as the adsorption temperature was raised, which confirmed that higher temperatures enhanced the MB adsorption on SH2. Additionally, the ΔS value (>0) indicated an enhancement in the disorder of the solid-solution interface, conducive to MB dye adsorption on SH2.

3.6. Comparison of Different Adsorbents

The differences in the maximum adsorption capacity (Q_m) of MB between SH2 and other biochar adsorbents are shown in Table S3. The adsorption performance of SH2 (724.68 mg·g⁻¹) is relatively high among these reported adsorbents, exceeding that of most common biochar adsorbents from agricultural and forestry wastes, including rice straw, wheat straw, pine tree and bamboo, etc. The physicochemical properties of biomass raw materials are crucial for the porosity and adsorption capacity of biochar, which further indicates that S. mokorossi can serve as an appropriate raw precursor. It is worth noting that, according to several works in the literature, the Q_m of SH2 exceeds that of many adsorbents co-activated by KOH and metals such as Fe, Ni, and Cu. SH2 may be able to provide sufficient pore bases and active sites even without metal doping. However, in several other studies [2,24], phenol-formaldehyde resin modified wood and Taihu blue algae biochar exhibited impressive SSA, pore structure and oxygen-containing function after KOH activation. It exhibited better adsorption capacity for MB dyes (1112.35 and 1143 mg·g⁻¹, respectively). The modification of PF resin increased the disorder of biochar, further enhancing its adsorption capacity. Meanwhile, in the application of Taihu blue alga biochar to adsorb cationic dyes, the alkaline pH environment enhanced the electrostatic attraction between the Taihu blue alga biochar and MB, thus increasing its adsorption capacity. Overall, S. mukorossi is a promising precursor for efficient and economical biochar production, with enormous potential in removing cationic dyes from WW, while also achieving value-added utilisation of S. mukorossi waste. In future studies, we plan to further enhance the adsorption performance of SH2 by tuning its physicochemical properties through adjustments in pyrolysis temperature, duration, and the use of various modifiers.

3.7. Adsorption Mechanism

The mechanism of MB adsorption by SH2 is shown in Figure 8. The BET analysis indicated that the ultra-high SSA and mesoporous structure of SH2 provide abundant active sites and promote the pore filling effect. The pH experiment results and the pHpzc value (6.35) also indicated that under alkaline conditions, the electrostatic attraction between the MB dye (positive charge) and the SH2 surface (negative charge) is the dominant mechanism. Insights from FTIR analysis further confirmed the role of the FGs on the surface of SH2 in the removal of MB. Furthermore, the entire adsorption behaviour can be described by the Langmuir isotherm and the PSO model, confirming that the process is mainly characterised by monolayer chemical adsorption. In conclusion, the efficient adsorption of MB onto SH2 is driven by the synergy of multiple mechanisms, including pore filling, electrostatic attraction, π-π interaction, and hydrogen bonding.

3.8. Application and Prospect

The KOH-activated Sapindus mukorossi shell biochar (SH2) demonstrated outstanding adsorption performance for MB (Q_max = 869.6 mg·g⁻¹). Based on a model from the literature [57], a preliminary cost–benefit analysis was conducted. Using a lab-scale tube furnace as an example, the production cost of one kg of the SH2 was estimated at 2.89 USD·kg⁻¹ was higher than that of commercial activated carbon (1.2–2.0 USD·kg⁻¹). However, due to its outstanding adsorption performance, the cost-effectiveness ratio (3.54 USD·kg⁻¹) of SH2 was significantly better than most commercial activated carbon (about 3–10 USD·kg⁻¹). This indicates that SH2 is not only a high-performance laboratory material but also a potential industrial adsorbent with the potential for large-scale application and cost competitiveness.

The potential environmental benefit of this work is the establishment of a “waste treatment with waste” process technology. On the one hand, it has achieved the value-added utilisation of agricultural waste, avoiding greenhouse gas emissions and soil pollution caused by its natural degradation or incineration. On the other hand, it has replaced some activated carbon from non-renewable resources by producing highly efficient adsorbents. In future work, the environmental impact of the production of SH2 will be evaluated through the Life Cycle Assessment. In addition, the adsorption performance and mechanism of SH2 for various typical pollutants (such as anionic dyes, emerging pollutants, and heavy metals) will be explored. The focus is on evaluating the performance and stability of SH2 in complex systems of real industrial wastewater (such as the coexistence of multiple pollutants, high salinity, and extreme pH), and clarifying the feasibility of its practical application. This aims to advance the recovery and recycling of biochar, supporting its environmental sustainability.

4. Conclusions

The process of preparing bioadsorbents using S. mukorossi shells as the raw material through KOH activation, and their removal effect for MB dye were studied. After activation, SH2 developed a rich micropore-mesoporous structure, a higher SSA and an increased number of FGs and hydrophobicity. The adsorption process of MB by SH2 is a synergistic effect of electrostatic attractions, π-π interactions, and pore filling. Additionally, the adsorption rate of MB on SH2 was initially influenced by film diffusion and later controlled by intraparticle diffusion. This study develops a promising technology for efficient organic dye removal from WW by repurposing S. mukorossi shell residues.