Trends Sci. 2025; 22(12): 10925

Water pollution is increasingly recognized as a serious environmental issue, driven by the continuous discharge of diverse contaminants such as pharmaceutical residues (e.g., ibuprofen), pesticides, heavy metals, and organic dyes into aquatic systems [1-3]. Many sectors, such as textiles, food processing, printing, cosmetics, and medical manufacturing, heavily rely on dyes. Approximately 15% of the dyes used in textile dyeing processes are lost to wastewater. Methylene blue (MB), a widely used synthetic dye, poses significant environmental and health risks [4]. Exposure to excessive MB may cause symptoms such as nausea, vomiting, and neurotoxicity in humans [5]. Several techniques to mitigate MB contamination have been explored, including adsorption, reverse osmosis membranes [6-9], electrocoagulation [10,11], ion exchange [12], photodegradation [13,14], biodegradation [15-18], oxidation [19], and ozonation [20,21]. Adsorption stands out among these techniques due to its simplicity, low operational cost, and eco-friendly nature [22].

Due to its suitability, operational flexibility, ease of design, and affordability, especially when using low-cost and regenerable adsorbents, adsorption is often considered the most attractive method [23]. Various adsorbents such as resins [24], silica [25], biochar, zeolites, agricultural biomass, carbon nanotubes [26,27], graphene [28,29], chitosan, clays or organoclays [23], alumina [30], Mg-Al layered double hydroxides [31], and hydroxyapatite [32] have been employed for MB adsorption. However, some of these materials are not particularly effective and exhibit low to moderate adsorption capacity for MB due to their limited porosity and functionality, while others are considered high-cost adsorbents.

Among these materials, carbon-based adsorbents offer greater environmental and economic benefits. Owing to their large surface area and favorable surface functionalities, activated carbon (AC) provides high removal efficiency. AC typically contains many hydroxyl and carboxyl groups, aliphatic double bonds, and aromatic structures [26]. Recent research has shifted toward developing low-cost, readily available, renewable, eco-friendly, and effective adsorbents. Agricultural residues such as biomass, olive stones, corn stalks, rice husks, sugarcane bagasse, fruit seeds, nutshells, fruit pulp, bones, and coffee grounds have been explored as potential precursors for activated carbon production [33].

Coffee beans are widely consumed worldwide in the form of brewed coffee. According to the International Coffee Organization (ICO), 10.5 million tons of coffee beans were harvested globally in 2020 - 2021 [34]. Waste products from coffee processing include spent coffee grounds (WCGs), coffee husks, and silver skin. The annual global production of WCGs is estimated at around 6 million tons [35]. Organic compounds such as caffeine, tannins, and chlorogenic acids are present in this waste and can cause environmental pollution if not properly managed [36].

WCGs contain carboxyl, hydroxyl, carbonyl, and other functional groups that can enhance the adsorption and removal of pollutants from water through hydrogen bonding and electrostatic interactions. Additionally, π–π and hydrophobic interactions contribute to removal [37]. Activated carbon derived from WCGs (WCGs-AC) has demonstrated the ability to adsorb various contaminants, including heavy metals, such as lead [38], copper [39], chromium [40,41], nickel [42], arsenic [43], and mercury [44]; dyes, including methylene blue [45], rhodamine B [46], malachite green [47], Congo red [48], crystal violet [49], aniline yellow [50], orange G [51], and methyl orange [52]; antibiotics, such as tetracycline [53], balofloxacin [54], and sulfamethoxazole [55]; other pollutants, including the pesticides carbendazim and linuron [56], fluoride [57], and bisphenol A [58].

The main steps in preparing activated carbon from spent coffee grounds (WCGs-AC) are similar to conventional activated carbon production methods, involving activating a pore-forming agent. Activation can be performed using either chemical or physical agents. Chemical activators vary widely and may include acids, bases, or salts. Common chemical agents used in activated carbon production include acids such as H₃PO₄, H₂SO₄, and HCl; bases such as KOH and NaOH; and salts including ZnCl₂, FeCl₃, and CaCl₂. ZnCl2 is the most widely used activator for biomass-based precursors among these salts. [59-60].

ZnCl₂ pretreatment promotes the formation of mesopores and micropores. It acts as a dehydrating agent, facilitating the removal of moisture and volatile compounds from the biomass precursor. Additionally, ZnCl₂ catalyzes cellulose, hemicellulose, and lignin thermal degradation. Increasing the carbonization temperature leads to more mesopores and micropores [39]. ZnCl₂ enhances the specific surface area, pore volume, and methylene blue (MB) adsorption capacity of activated carbon derived from shaddock peel. ZnCl₂-activated carbon made from citrus peel has demonstrated high MB adsorption performance. The ZnCl₂ treatment increased porosity by more than 4.54% and improved MB adsorption capacity up to 336 mg/g [61].

In commercial applications of physical activation, the 2^nd step involves activation in an oxidizing gas atmosphere, such as steam, carbon dioxide, nitrogen, or a mixture with air, at elevated temperatures ranging from 400 to 900 °C. The 1^st step is carbonization (pyrolysis) conducted in an inert atmosphere. Although physical activation is a relatively costly technique for producing activated carbon, it can yield porous carbon structures with strong physical stability. However, this method has several drawbacks, including limited adsorption capacity, high energy consumption, and extended activation times [33]. To address the challenge of excessive energy consumption, activated carbon in this study was synthesized through gas-free carbonization in a relatively short duration, only 10 min. The absence of a gas flow during pore formation was compensated for by pre-washing the spent coffee grounds with hexane and hot water before activation. This washing step helps remove coffee oils [62] and other impurities [63,64].

In addition to washing and chemical activation, carbonization temperature is also a key factor in determining the properties of activated carbon. Higher carbonization temperatures - 600, 700 and 800 °C - tend to produce activated carbon from spent coffee grounds with larger specific surface areas [65]. However, care must be taken to avoid damaging the carbon microstructure, which could reduce its effectiveness in adsorbing inorganic pollutants. In highly porous materials, adsorption is typically dominated by pore-filling mechanisms, which may result in a loss of specificity and selectivity toward certain adsorbates. Furthermore, elevated carbonization temperatures can lead to the formation of more hydrophobic carbon with fewer surface heteroatoms, which is less favorable for adsorbing organic pollutants that rely on hydrophobic interactions [66].

For practical applications, an effective adsorbent must exhibit high adsorption capacity and environmental sustainability and demonstrate ease of regeneration and reusability. These requirements have driven growing interest in the recyclability and structural stability of synthesized adsorbents [67]. Regeneration and reuse studies are essential for evaluating the long-term performance and feasibility of adsorbents before their application to real environmental samples [68]. The ability of an adsorbent to maintain its performance over multiple cycles is a key factor in advancing adsorption technologies, particularly for sustainable or long-term operations [69]. Once activated carbon becomes saturated or the contaminant concentration reaches the breakthrough threshold, the spent material must be treated to remove the adsorbed pollutants and restore its adsorption functionality. The regeneration of exhausted adsorbents is a critical parameter for commercial applications, as it directly affects the cost-effectiveness and sustainability of the process [70,71].

In a previous study, waste coffee grounds (WCGs) washed with hexane were directly employed for the removal of Methylene Blue (MB), achieving a maximum adsorption capacity of 416.67 mg/g [72]. Both raw WCGs and their activated form (WCGs-AC) exhibited similar functional groups, including amine (N–H), hydroxyl (–OH), and carbonyl (C–O), which contributed to dye adsorption. However, aspects such as the influence of different washing methods and carbonization temperatures, pore characteristics, post-adsorption properties of WCGs-AC, and adsorption kinetics were not previously investigated. In the present study, 2 washing methods were applied to WCGs - using hexane and hot water - before chemical activation with ZnCl₂ and subsequent carbonization under oxygen-limited conditions to enhance their physicochemical properties. The novelty of this work lies in the improved synthesis approach and the comprehensive evaluation of pore structure (via BET analysis), functional group transformations before and after MB adsorption (via FTIR), and adsorption kinetics. The resulting WCGs-AC exhibited a high specific surface area, fast adsorption rate, and excellent reusability. The abundant surface area and active binding sites facilitate MB adsorption through diffusion and chemisorption mechanisms, enabling environmentally sustainable regeneration.

Materials and methods

Materials

The WCGs were collected from the pantry of the Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Gadjah Mada. ZnCl₂ supplied by Pudak Scientific, Indonesia, HCl supplied by Smart Lab, Indonesia, and MB (C₁₆H₁₈ClN₃S·3H₂O, MW: 373.90 g/mol) provided by Sigma Aldrich.

Fabrication of adsorbent

The fabrication of the adsorbent was conducted according to the experimental scheme shown in Figure 1. The WCGs were dried in an oven at 100 °C for 7 h and then stored in a desiccator. The dried WCGs were washed several times with hot water until the washing water became colorless. WCGs were dried in an oven at 100 °C for 7 h. Another washing method for WCGs followed the procedure reported by Nurmayasari et al. [72]. First, the WCGs were washed with hexane at a ratio of 1:10 (g/mL) and stirred using a magnetic stirrer at 750 rpm for 2 h. The WCGs were then filtered and washed with 70% ethanol. Finally, the WCGs were rinsed with distilled water until the rinsing water became colorless and then dried.

Figure 1 Experimental schematic.

A total of 20 g of the cleaned WCGs were soaked in a solution containing 9 g of ZnCl₂ in 60 mL of water for 24 h. The WCGs were then dried at 100 °C for 5 h. The activated WCGs were subsequently carbonized at 450, 500 and 550 °C for 10 min in a furnace (heating rate: 5 °C/min), cooled to room temperature in air, and neutralized with 50 mL of 5% HCl for 2 h. They were then rinsed with distilled water several times until the pH of the rinsing water was neutral. The WCGs-AC were sieved using a 200-mesh sieve to achieve uniform particle size. Then WCGs-AC was stored in a desiccator before characterization and adsorption experiments. The resulting WCGs-AC samples were named according to their washing methods and carbonization temperatures, as shown in Table 1.

Table 1 WCGs-AC adsorbents in this study.

WCGs-AC	Washing Method	Carbonization temperature (°C)
WZ450	hot water	450 ± 1
WZ500	hot water	500 ± 1
WZ550	hot water	550 ± 1
HZ450	hexane	450 ± 1
HZ500	hexane	500 ± 1
HZ550	hexane	550 ± 1

Characteristics of the adsorbent

WCGs-AC were characterized using a Particle Size Analyzer (PSA), Brunauer-Emmett-Teller (BET) analysis, Scanning Electron Microscopy (SEM), and Fourier Transform Infrared Spectroscopy (FTIR). The PSA was used to determine the particle size distribution of the material. The BET technique was applied to analyze the specific surface area, total pore volume, and size distribution. SEM imaging was conducted to investigate the microstructure and morphology of the activated coffee grounds at the micro/nano scale and to assess the elemental composition of the material quantitatively. FTIR analysis was conducted to characterize the functional groups on the adsorbent surface. FTIR measurements were performed using the KBr pellet method in the wavenumber range of 400 - 4,000 cm⁻¹ at room temperature. The FTIR spectra provide absorbance profiles indicating the functional groups of the sample. The absorption peaks observed in the spectra were compared with standard reference data to identify the types of functional groups present.

The particle size distribution was analyzed using a Microtrac Flex 11.106. The BET surface characteristics and porosity of the WCGs-AC were measured using a QuantaTec 11.06 instrument. Surface morphology and elemental composition of WCGs-AC before and after adsorption were examined using a scanning electron microscope equipped with energy-dispersive X-ray spectroscopy (SEM/EDX, JEOL JSM-6510). Diffraction patterns were collected using a Bruker D8 Advance Eco X-ray diffraction (XRD) system. FTIR spectra (Thermo Nicolet iS10) of WCGs-AC before and after adsorption were also analyzed to investigate the adsorption mechanisms.

Batch experiments

A 10 ppm MB solution was treated with WCGs-AC adsorbents listed in Table 1, each applied at 1 g/L. The WCGs-AC was stirred in the MB solution at 500 rpm for 10 to 60 min at room temperature. At 10-min intervals, the absorbance of the aliquots was recorded at 664 nm using a UV-Vis spectrophotometer. The adsorption capacity (q_e) and MB removal efficiency (R) of the WCGs-AC were calculated using Eqs. (1) and (2).

(mg/g) is the adsorption capacity, (mg/L) is the initial MB concentration, and C_e is the MB concentration at equilibrium, (L) is the MB volume, (g) is the WCGs-AC mass, and R (%) is the removal efficiency.

The pseudo-1^st-order, pseudo-2^nd-order, Elovich, and Weber-Morris models fit the time series experimental data [73]. The models are described in Eqs. (3) - (9).

q_t (mg/g) denotes the quantity of adsorbate adsorbed at time t (m), k₁ (/m) is the rate constant of the pseudo-1^st-order equation, k₂ (L/mg m) is the rate constant of the pseudo-2^nd-order equation, is the initial adsorption rate constant of the Elovich model (mg/g h), is the desorption rate constant of the Elovich model (g/mg), k_WM (mg/g m^0,5) is the rate constant of the intraparticle equation, and C is the linear intercept.

WCGs-AC 1.00 g/L had been tested for MB adsorption for 60 min and showed a MB degradation percentage of 100%. Repeated adsorbent testing was carried out up to 10 cycles. Table 2 summarizes the preparation methods, pore characteristics, and kinetic performance of various activated carbon-based adsorbents for methylene blue removal, providing a comparison for this study.

Table 2 Preparation, pore properties, and kinetics performance of different AC-based adsorbents used for MB removal.

Adsorbent types	Washing method and carbonization temperature	Pore properties			Kinetic performance			Removal (%)	No. of cycle	Ref.
		SBET	Vt	dp	qe	k2	R2
		m2/g	cm3/g	nm	mg/g	g/mg min
WCGs/KOH	800 °C	1,199	0.42	-	106.38	2231.65	0.9995	-	-	[45]
Commercial	-	833	2.5	-	100.38	2179.47	0.9873	-	-	[45]
WCGs	500 °C	-	-	-	5.8959	0.0547	0.9827	-	-	[74]
WCGs/ H3PO4	WCGs boiled in DW* 3 times, 600 °C	-	-	-	25.19	0.003	0.9958	77.8	3	[75]
WCGs/ZnCl2	WCGs boiled in DW* 3 times, 600 °C	-	-	-	42.92	0.001	0.9921	78.9	3	[75]
WCGs/NaOH	WCGs are washed with NaOH, 300 °C	-	-	-	142.8	3.7×10–4	0.932	-	-	[76]
WCGs/H3PO4+P4O10	DW* washing	662.4	0.377	< 2	101.17	9.4×10–7	0.9905	52.4	6	[77]

*rT: room temperature

Results and discussion

Grain size of adsorbents

The dominant particle size distribution of WCGs-AC, as measured using a Particle Size Analyzer (PSA), is shown in Table 3. A comparison between WCGs-AC washed with hot water (WZ) and those washed with hexane (HZ) reveals that WZ exhibits a larger particle size than HZ. These findings imply that the hot water washing process leads to greater particle agglomeration than hexane washing. The increase in carbonization temperature resulted in a corresponding particle size enlargement, presumably due to intensified structural reorganization and material densification during thermal processing. Furthermore, the WCGs-AC samples washed with hot water exhibited larger particle sizes than those treated with hexane, indicating that the washing medium exerts a distinct influence on particle aggregation behavior. Although larger particle sizes tend to exhibit higher adsorption kinetic rates, particle size has a negligible effect on the adsorption capacity at equilibrium. In contrast, previous studies have reported that smaller particle sizes enhance adsorption capacity due to the increased surface area and shorter diffusion pathways [78].

Table 3 The particle size distribution of WCGs-AC

WCGs-AC	Diameter (nm)
WZ450	368.00
WZ500	471.00
WZ550	1376.00
HZ450	1.34
HZ500	432.00
HZ550	643.00

Surface area, pore volume, and pore size of adsorbents

The surface area, pore volume, and pore size of WCGs-AC are summarized in Table 4. The material exhibits relatively high surface area and pore volume, with surface area values ranging from 1,304 to 3,450 m²/g, which are considered exceptionally high. According to Hou et al. [79], such values fall into the “super high” classification. Compared with commercial adsorbents and adsorbents produced by other researchers in Table 2, the commercial adsorbents and those in this study have the highest surface area and pore volume. Based on IUPAC pore size classification, WCGs-AC in this study is categorized as mesoporous, with pore diameters between 2 and 50 nm [80]. Mesopores provide a large surface area, significantly enhancing the adsorption capacity of WCGs-AC [39]. A comparison of different WCG washing methods revealed that WCGs-AC washed with hot water had a higher surface area than those washed with hexane. However, Zhao (2022) reported that WCGs washed with hot water showed a much lower surface area (0.6744 m²/g), total pore volume (6.1×10⁻⁵ cm³/g), and average pore diameter (2.126 nm), which is still within the mesoporous range [81]. In contrast, the current study indicates that hot water washing did not significantly influence the pore characteristics of WCGs-AC. Therefore, it can be reasonably concluded that ZnCl₂ activation and carbonization temperature are the dominant factors affecting the pore structure of WCGs-AC.

The specific surface area (S_BET), total pore volume (V_T), and average pore diameter (D) of WCGs-AC tend to decrease with increasing carbonization temperature, regardless of whether the samples were washed with hot water or hexane. Notably, WCGs-AC washed with hexane exhibited a more pronounced decline. The reductions in SBET and VT suggest the occurrence of pore degradation. Using WZ450 as a reference, the pore degradation observed in WZ500 and WZ550 was 19.59% and 12.53%, respectively. For the hexane-washed samples, pore degradation increased progressively with temperature. Compared to HZ450, HZ500 showed a 7.62% decrease, while a substantial degradation of 56.47% was observed in HZ550.

During the impregnation and carbonization processes, WCGs-AC undergoes pore formation, pore enlargement, and pore degradation [82]. At 450 °C, the rate of pore formation exceeds that of enlargement and degradation, resulting in higher surface area and pore volume. However, at 500 and 550 °C, the pore enlargement and degradation rates become dominant, leading to a net loss in pore structure. Consequently, this degradation contributes to decreased pore volume and surface area.

Table 4 WCGs-AC surface area, pore volume, and pore size.

WCGs-AC	Surface area (m2/g)	Pore volume (cm3/g)	Average pore radius (nm)
WZ450	3405	4.079	2.396
WZ500	2792	3.280	2.350
WZ550	2977	3.568	2.397
HZ450	2382	2.899	2.434
HZ500	2302	2.678	2.327
HZ550	1304	1.262	1.936

The consistency between BET and PSA analyses reinforces the interpretation of the mesoporous structure’s role in enhancing adsorption. Despite the increase in particle size at higher carbonization temperatures, surface area may decline due to structural densification and pore degradation. A trade-off exists between particle size and surface area, which are crucial for maximizing adsorption efficiency. The high specific surface area of the activated carbon likely enhances its adsorption capacity by providing more active sites for dye molecules to interact with the adsorbent surface [50].

WCGs-AC morphology

SEM images indicate that pore structures begin to form following hot water and hexane washing, as shown in Figure 2. Hot water-washed WCGs show a higher density of open pores, highlighting the effectiveness of this pretreatment method. Pretreatment with hot water appears more effective in promoting porosity compared to hexane.

Figure 2 WCGs are (a) before washing, (b) washed with hot water, and (c) with hexane.

WCGs-AC exhibits an irregular and heterogeneous surface morphology characterized by pores of various shapes and sizes, as shown in Figure 3. A comparison of WCGs-AC samples reveals that WZ exhibits larger and more numerous pores than HZ. This observation is consistent with the BET analysis, confirming that WZ-derived WCGs-AC has a greater total surface area and pore volume than HZ-derived WCGs-AC. The effect of carbonization temperature on surface morphology is evident when comparing the 2 columns in Figure 3. As the carbonization temperature increases, both the size and number of pores tend to decrease, resulting in a reduced total surface area, as indicated by the BET results. This suggests that although higher temperatures promote carbon structure development, excessive carbonization may lead to pore shrinkage or collapse, reducing the number of available adsorption sites.

SEM images confirm the porous nature of the WCGs-AC, while BET analysis indicates an average pore radius ranging between 1.936 and 2.434 nm. Considering the molecular size of MB (1.382 - 1.447 nm in length and ~0.95 nm in width [74], it is likely that the pores of WCGs-AC provide sufficient space for adsorption.

Figure 3 SEM image of WCGs-AC: (a) WZ450, (b) WZ500, (c) WZ550, (d) HZ450, (e) HZ500, (f) HZ550.

WCGs-AC functional groups before and after MB adsorption

Figure 4 presents the FTIR spectra of WCGs-AC washed with hexane (HZ550) and hot water (WZ550). Functional groups identified include hydroxyl (–OH), amine (N–H), methylene (C–H), and carbonyl (C–O), which contain electronegative atoms (O, H, and N) capable of interacting with corresponding atoms in methylene blue (MB) molecules [74]. The characteristic bands of –OH and N–H stretching (3,100 - 3,600 cm⁻¹), C–H (2,924, 2,854, 1,465 and 1,381 cm⁻¹), C≡C (2,368 and 2,337 cm⁻¹), C=C (1,566 cm⁻¹), and C–O (1126 cm⁻¹) exhibited similar intensities in both samples, except for the C≡C band, which was more pronounced in HZ550, suggesting a higher concentration of this group. These findings indicate that the washing method had minimal influence on the surface functional groups of WCGs-AC. Figure 5 illustrates the impact of carbonization temperature on the functional groups of WCGs-AC. All samples displayed the presence of –OH, N–H, C–H, C≡C, C=C, C–O, and aromatic structures, with varying intensities. Among them, WZ550 exhibited the highest overall band intensities, indicating a richer surface chemistry. Accordingly, 550 °C was identified as the optimal carbonization temperature in this study.

Following MB adsorption, a marked increase in the –OH band intensity was observed, indicating the formation of strong covalent or ionic interactions between –OH groups on WCGs-AC and MB molecules (Figure 6). The C=C band shifted from 1,566 to 1,627 cm⁻¹, while the C–H bands at 1,465 and 1,381 cm⁻¹ intensified. New absorption bands also emerged corresponding to N–O (1,319 cm⁻¹) and C–N (1242 cm⁻¹). The shift and new groups may occur due to hydrogen bonding in WCGs-AC with nitrogen or other atoms in MB. Additionally, aromatic C–H (825, 810, and 756 cm⁻¹) and C=C (694 and 678 cm⁻¹) groups present in WCGs-AC are associated with π-electron systems that can engage in π–π stacking interactions with the aromatic rings of MB, thereby enhancing the adsorption capacity [74]. The appearance of additional aromatic peaks at 871, 794, and 678 cm⁻¹ after MB adsorption further supports this mechanism.

Figure 4 WCGs-AC washing comparison.

Figure 5 WCGs-AC carbonization temperature comparison.

Figure 6 WZ550 before and after MB adsorption.

Characteristic and kinetic comparison study

The kinetic data reveal that the adsorption of MB onto WCGs-AC is best fitted by the PSO model, indicating that chemisorption is the dominant mechanism [78]. This is further supported by FTIR analysis, which confirms the involvement of chemical interactions. The coefficients of determination (R²) for the kinetic models are summarized in Table 5. In contrast, HZ450 and HZ500 exhibited good fits with the PFO and PSO models, suggesting a mixed mechanism involving physisorption and chemisorption [83]. The experimentally determined adsorption capacity (qₑ,_exp) for WCGs-AC is 10.20 mg/g, while the theoretical kinetic parameters are listed in Table 6. Pretreatment with hot water and higher carbonization temperatures enhances the adsorption rate, as evidenced by increased k₁ and k₂ values. WZ550 exhibited the highest rate constant among all samples, indicating superior adsorption kinetics. These findings suggest a synergistic effect between the pretreatment method and thermal processing in improving adsorption performance.

Table 5 Regression of determination (R²) of WCGs-AC kinetic models.

WCGs-AC	R2
	PFO	PSO	Elovich	Weber-Morris
WZ450	-	0.9999	0.6572	0.6906
WZ500	-	0.9999	0.6572	0.6602
WZ550	-	1.0000	-	0.6590
HZ450	0.9785	0.9979	0.7557	0.8079
HZ500	0.9830	0.9999	0.7415	0.6839
HZ550	-	0.9999	0.6572	0.6833

Table 6 Kinetic parameters of triplication MB adsorption on WCGs-AC.

WCGs-AC	k1 (/min)	qe-kinetic theory (mg/g)	k2 (g/mg min)	qe-kinetic theory (mg/g)
WZ450	-	-	0.29 ± 0.033	10.27 ± 8.537×10-3
WZ500	-	-	8.6 ± 1.4	10.20 ± 4.408×10-4
WZ550	-	-	infinite	10.20 ± 0
HZ450	0.206 ± 0.00752	14.1 ± 0.860	0.0364 ± 0.00132	10.74 ± 1.958×10-2
HZ500	0.274 ± 0.00629	8.30 ± 0.259	0.347 ± 0.00419	10.26 ± 8.652×10-4
HZ550	-	-	0.390 ± 0.00592	10.25 ± 8.270×10-4

The kinetic parameters obtained in this study are compared with those reported in previous studies (Table 2), with WZ550 representing the current findings due to its superior performance. Although the q_e of WZ550 is relatively lower than that reported in some studies, its k₂ is extremely high. Furthermore, the PSO model provides an excellent fit to the experimental data for WZ550, with an R² of 1.0.

Adsorbents reusability

WZ500 retained its adsorption capacity over 6 cycles, indicating superior structural resilience. Compared to HZ500, WZ500 shows extended usability with minimal decline in performance over time (Figure 7(a)). Thermal activation appears to strengthen the adsorbent matrix, contributing to enhanced reusability. WZ450 maintained full adsorption efficiency for 4 cycles, WZ500 for 6 cycles, and WZ550 for 7 cycles (Figure 7(b)). This study demonstrates the importance of thermal treatment in developing regenerable adsorbents for wastewater treatment.

Figure 7 The reusability of WCGs-AC is influenced by (a) washing methods and (b) carbonization temperatures.

Table 2 compares the reusability study conditions and results of various AC-based adsorbents used for MB removal. Due to differences in adsorption, desorption, and regeneration parameters, the reusability performance varied significantly among the adsorbents. The WCGs-AC developed in this study sustained a high removal efficiency over 10 regeneration cycles. Particularly, WZ550 maintained a removal efficiency of 96.39 ± 0.01 % in the tenth cycle, indicating excellent reusability.

Limitations and future work

Despite the promising performance of WCGs-AC, several limitations should be acknowledged. First, there is a potential risk of leaching of residual ZnCl₂ or its transformation products (e.g., ZnO) into aqueous environments [84], even after post-treatment washing. Trace amounts of Zn may still be released under varying pH and ionic strength conditions, potentially contributing to secondary contamination [85]. Second, the thermal treatment employed in this study involved a short carbonization time (10 min) under ambient atmospheric conditions. These parameters may lead to incomplete carbonization, limited pore development, and partial oxidation due to the presence of atmospheric oxygen [86], thereby compromising the structural quality of the resulting material [87]. Third, the adsorbent’s performance was evaluated only under controlled laboratory conditions using synthetic methylene blue solutions. However, real wastewater typically contains a complex matrix of competing ions and organic substances, which may interfere with the adsorption process and reduce the overall efficiency [88].

To address these limitations and enhance the environmental sustainability of the material, future studies should include systematic Zn leaching assessments under diverse water chemistries and explore optimized washing or recycling protocols to minimize metal release [85]. The carbonization process should be refined by employing inert gas atmospheres (e.g., nitrogen or argon) and extending residence time to improve the physicochemical properties and reproducibility of the adsorbent [87]. Furthermore, adsorption experiments should be conducted using actual wastewater samples to evaluate the material’s practical applicability and robustness under realistic treatment conditions [88]. Alternative green activation strategies, such as phosphoric acid (H₃PO₄), offer a promising substitute for ZnCl₂, as they eliminate the need for inert atmospheres and reduce environmental impacts while maintaining high surface area and adsorption performance [89]. Additionally, the integration of environmentally friendly technologies, such as the Continuous Submerged Solid Small-Scale Laboratory Photoreactor (CS4PR) employing TiO₂/NO₃⁻ catalysts, may further support sustainable wastewater treatment approaches [90]. Finally, the multipollutant adsorption capacity of WCGs-AC - including for heavy metals, dyes, and pharmaceutical compounds - should be further explored to validate its potential as a versatile and cost-effective adsorbent for diverse water treatment applications [91].

Conclusions

WCGs-AC synthesized using the method proposed in this study exhibited a micro- to nano-scale grain size, non-uniform surface morphology, a large specific surface area, and the presence of amine, hydroxyl, and C–O functional groups, which serve as active sites for MB adsorption. The adsorption mechanism was predominantly governed by covalent interactions and π–π bonding. The PSO model best described the adsorption kinetics, with kinetic capacities ranging from 10.20 ± 0.00 to 10.74 ± 0.02 mg/g. The WZ550 adsorbent demonstrated an infinite adsorption rate and maintained 100% removal efficiency over 6 consecutive reuse cycles. Its consistent performance across multiple cycles highlights its potential for practical application in industrial-scale wastewater treatment.

Acknowledgements

Indonesian Education Scholarship (BPI), Center for Higher Education Funding and Assessment (PPAPT), Ministry of Higher Education, Science, and Technology of the Republic of Indonesia, Indonesia Endowment Fund for Education (LPDP) have funded/sponsored the implementation of this work.

Declaration of Generative AI in Scientific Writing

The authors acknowledge the use of generative AI tools (e.g., Grammarly and ChatGPT by OpenAI) during the preparation for language editing and grammar correction. No content generation or data interpretation was carried out by AI. The authors take full responsibility for the content and conclusions presented in this work.

CRediT Author Statement

Wa Ode Sitti Ilmawati: Methodology, Data curation, Data analysis, Investigation, Funding acquisition, and Writing –original draft.

Nurmayasari: Data curation, Data analysis, and Investigation.

Diki Purnawati: Resources, Data analysis, and Investigation.

Sholihun: Conceptualization, Software, and Writing –review & editing.

Ari Dwi Nugraheni: Conceptualization, Methodology, Resources, and Supervision.

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