Maggot-modified avocado peel as an innovative adsorbent for crystal violet dye wastewater treatment

2.2.1 Preparation of AP adsorbent and modification

APs (Persea americana) were cleaned thoroughly, sliced into smaller fragments, air-dried, milled, and sieved, resulting in a powder fraction of ≤ 36 µm. The AP powder was activated by immersion in 0.01 M HNO₃ solution for 3 h at a ratio of 1:3 (w/v). After activation, the material was rinsed with deionized water until a neutral pH was achieved, filtered, and air-dried to yield the AP adsorbent (AP) (Ramadhani et al., 2020).

Modification with maggot powder was performed by mixing dried maggot powder (≤36 μm) with activated AP at a 1:1 ratio (9 g each) and suspending the mixture in 75 mL of 96% ethanol. The suspension was sonicated for 15 min, followed by filtration and air-drying for 24 h, yielding the modified adsorbent (APM) (Putra et al., 2024).

2.2.2 Characterization of the adsorbent

The adsorbent was characterized before and after CV adsorption using Fourier-transform infrared spectroscopy (FTIR, Unican Mattson Mod 7000 FTIR), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX, Hitachi S-3400N), X-ray fluorescence (XRF) spectroscopy (XRF, S2 PUMA – Bruker), and Brunauer–Emmett–Teller surface area analysis (BET, Anton Paar Nova800). In addition, thermogravimetric analysis (TGA, Perkin Elmer TGA 4000) was performed on the adsorbent prior to dye adsorption.

The point of zero charge (pH_pzc) was evaluated by dispersing 0.1 g of the adsorbent in 50 mL of a 0.1 M KCl solution. The initial pH (pH₀) of the resulting suspension was adjusted to values between 1 and 8 using NaOH or HNO₃ as required. The suspensions were then shaken for 24 h, after which the final pH (pHf) of the supernatant was measured. The pH_pzc value was obtained from the intersection between the ΔpH (pH_f – pH_₀) curve and the zero line on the vertical axis (pH_pzc) (Putra et al., 2024).

2.2.3 Adsorption performance

A batch adsorption approach was applied to examine the influence of solution pH, initial dye concentration, contact duration, and thermal treatment of the adsorbent on adsorption efficiency. The experiments were carried out using raw AP and maggot-modified avocado peel (APM). For AP, the investigated pH values and initial dye concentrations ranged from 3 to 7 and 100 to 1300 mg L⁻¹, respectively, whereas for APM these ranges were extended to 5–9 and up to 1500 mg L⁻¹. Both adsorbents were evaluated under identical contact times (15–120 min) and heating temperatures (25–210 °C). The selected pH ranges were based on the pHₚzc values of each adsorbent, while variations in the initial dye concentration were performed to identify optimum adsorption conditions.

The pH values of the prepared dye solutions was regulated using HNO₃ or NaOH and stabilized with suitable buffer systems. During adsorption, the dye solution–adsorbent suspensions were continuously shaken on an orbital shaker at 100 rpm. Following the adsorption process, the solid phase was removed by filtration, and the remaining dye concentration in the filtrate was quantified using a UV–Vis spectrophotometer (PG Instrument, T70) at a wavelength of 591 nm. All experiments were conducted using a fixed dye volume of 25 mL, an adsorbent dosage of 0.1 g, and a particle size smaller than 36 µm (Hevira et al., 2025).

The values of adsorption capacity (q) and removal percentage (%R) were obtained from Eqs. (1) and (2), respectively.

in which C₀ and Cₑ (mg L⁻¹) indicate the dye concentrations at the initial stage and at equilibrium, respectively; v represents the volume of the solution (mL), while m denotes the amount of adsorbent used (g).

2.2.4 Adsorption isotherm studies

Adsorption isotherms are widely used to elucidate the equilibrium partitioning of adsorbates between solid surfaces and liquid media. Accordingly, the Langmuir, Freundlich, and Temkin models were employed in this study, with the Langmuir model assessed using the linearized form of Eq. (3).

where $q_m$ (mg g⁻¹) indicates the maximum monolayer adsorption capacity, ​ $K_L$(L mg⁻¹) is the Langmuir adsorption coefficient, $q_e$​ (mg g⁻¹) denotes the equilibrium adsorption amount per unit mass of adsorbent, and $C_e$​ (mg L⁻¹) is the equilibrium dye concentration in the aqueous phase. The Freundlich model was analyzed using Eq. (4).

where $K_f$​ is the Freundlich coefficient reflects the adsorption capacity, while n indicates the intensity of the adsorption process. The Temkin isotherm was analyzed using Eq. (5):

where $K_T$ (L mg^–1) is the Temkin isotherm constant, and β is related to the heat of adsorption (J mol^–1) (Azeez & Al-Zuhairi, 2022; Ume et al., 2022)

2.2.5 Adsorption kinetic studies

In this study, the adsorption kinetics were evaluated using four models: pseudo-first-order, pseudo-second-order, Elovich, and Intraparticle diffusion, as described by Eqs. (6), (7), (8), and (9).

The rate constants corresponding to the pseudo-first-order, pseudo-second-order, and intraparticle diffusion kinetic models are defined as $K_l$ (min^─1), $k_2$ (g.mg^─1 min^─1), and $k_{diff}$ (mg g^─1.min^─0.5), respectively. In the Elovich equation, β (mg g^─1 min^─1), represents the initial adsorption rate, while the desorption constant is given in (g mg^─1). The terms $q_e$ and $q_t$ indicate the adsorption capacities at equilibrium and at time t (min). The constant C (mg g^─1) reflects the intraparticle diffusion parameters in the liquid phase (Essekri et al., 2023). The kinetic data applied in these models were derived from time-dependent experiments conducted under the optimized batch adsorption conditions.

2.2.6 Thermodynamics studies

To evaluate the thermodynamic favorability of the adsorption system, 25 mL of CV solution with concentrations varying from 10 to 50 mg L⁻¹ was brought into contact with 0.1 g of adsorbent. The solution pH was set to its optimum condition, followed by agitation for the predetermined optimal contact period at temperatures of 298, 308, and 318 K under a constant shaking speed of 100 rpm. After completion of the adsorption process, the suspensions were separated by filtration, and the dye concentration remaining in the liquid phase was measured at 591 nm using a UV–Vis spectrophotometer. The change in Gibbs free energy (ΔG) was determined according to Eq. (10).

where T refers to the absolute temperature (K), K_L is the Langmuir equilibrium constant (Lmol^-1), and R denotes the universal gas constant (8,314 J mol^-1 K^-1). The relationship between Gibbs free energy (ΔG^o), enthalpy change (ΔH^o), and entropy change (ΔS^o) is expressed by Eq. (11)

The values of ΔH^o and ΔS^o were obtained from the slope and intercept of the plot of ΔG^o against T (Fabryanty et al., 2017).

2.2.7 Reusability studies

The reusability performance of the adsorbent was assessed through successive adsorption–desorption experiments. During each cycle, 25 mL of CV solution adjusted to the optimum pH was contacted with 0.1 g of adsorbent and shaken at 100 rpm for the predetermined optimal contact duration. After the adsorption step, the suspension was separated by filtration, and the remaining dye concentration in the filtrate (C_ads) was quantified using a UV–Vis spectrophotometer. The regenerated adsorbent was dried once more and reused in the next cycle. In this work, a total of four consecutive cycles were conducted using different desorbing agents. The percentage of adsorption was calculated using Eq. (12) (Handayani et al., 2024):

3.1.1 Fourier transform infra-red (FT-IR) analysis

The surface functional groups of AP and APM adsorbents were characterized using FT-IR spectroscopy both prior to and following CV adsorption. The analysis was performed over a wavenumber interval of 4000–400 cm⁻¹ to determine the functional moieties participating in the adsorption process. Variations observed in the FT-IR spectra were used to elucidate the interaction mechanisms occurring between the adsorbent surfaces and CV molecules, as illustrated in Fig. 1(a).

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Fig. 1.

(a). FTIR spectra, (b). SEM analysis of AP, (c). SEM analysis Avocado of peel-Maggot (APM).

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These spectral variations suggest the participation of ether (C–O–C) and hydroxyl (O–H) functional groups during the adsorption of CV. Within the wavenumber region of 500–800 cm⁻¹, which is associated with C–Cl stretching vibrations and aromatic ring deformation, the APM sample showed characteristic bands at 818, 548, and 537 cm⁻¹. These bands remained detectable after adsorption (APM–CV), although noticeable changes in their intensities were observed. Similarly, AP showed bands at 814, 531, 519, 425, and 413 cm⁻¹, which also persisted with slight shifts. This region reflects contributions from aromatic ring vibrations and is likely associated with the triphenylmethane structure of CV. The observed shifts in band positions and intensities confirm that interactions occurred between the adsorbents (AP and APM) and CV molecules. These findings demonstrate that CV adsorption on the adsorbent surfaces involves hydrogen bonding, aromatic π–π interactions, and electrostatic interactions with polar functional groups.

In the wavenumber range of 3200–3300 cm⁻¹, which is associated with –OH stretching vibrations, the absorption band observed at 3274 cm⁻¹ for APM shifted to 3295 cm⁻¹ following CV adsorption. Similarly, for AP, the –OH stretching band moved from 3314 cm⁻¹ to 3304 cm⁻¹ after adsorption. Such spectral shifts suggest the establishment of hydrogen-bonding interactions between the hydroxyl groups on the adsorbent surfaces and CV molecules. Within the aliphatic C–H stretching region (2800–3000 cm⁻¹), the characteristic band at 2919 cm⁻¹ in APM was displaced to 2897 cm⁻¹ after CV uptake, while in AP the band at 2920 cm⁻¹ shifted to 2887 cm⁻¹. These changes indicate the participation of aliphatic C–H functional groups in the adsorption mechanism. Furthermore, in the 1600–1700 cm⁻¹ region, typically attributed to C=O or aromatic C=C stretching vibrations, a pronounced band at 1625 cm⁻¹ in APM shifted to 1965 cm⁻¹, accompanied by the emergence of a new absorption band in the APM–CV spectrum. For AP, the band at 1729 cm⁻¹ shifted slightly to 1726 cm⁻¹, with additional differences observed in the 1610–1580 cm⁻¹ range. These observations confirm the contribution of carbonyl and aromatic groups to the interaction with CV.

In the wavenumber range of 1400–1500 cm⁻¹, which is attributed to aromatic C=C and C–N stretching vibrations, absorption bands at 1416 and 1363 cm⁻¹ observed in APM remained detectable after the adsorption process, exhibiting only slight shifts. In contrast, for AP, the bands initially located at 1437 and 1354 cm⁻¹ were displaced to 1461 cm⁻¹ following CV adsorption. These spectral variations imply the participation of aromatic structures and amine functional groups in the binding of CV molecules. Additionally, within the 1000–1200 cm⁻¹ region associated with C–O–C and C–N stretching modes, the characteristic bands at 1169 and 1243 cm⁻¹ in APM showed minor positional shifts after adsorption, whereas the bands at 1248, 1168, and 1040 cm⁻¹ in AP remained present but showed variations in intensity. During the adsorption of cationic molecules, specifically CV dye, the FTIR analysis revealed that ion-exchange interactions occurred at various functional groups, including hydroxyl, amino, ester, carbonyl, and ether groups (Zein et al., 2010).

3.1.2 SEM-EDX analysis

SEM-EDX was employed to examine the surface morphology of the adsorbents at the microscopic level and to analyze their elemental composition. Figs. 1(b and c) present the surface properties of the adsorbents before and after CV adsorption.

Figs. 1(b and c)(i, ii) display the morphology and surface composition of the adsorbents prior to adsorption at magnifications of 1000X and 10,000X. At this stage, the surface appeared relatively rough, porous, and free from foreign deposits. The EDX spectra verified that the material consisted mainly of carbon and oxygen, with only small quantities of other elements present. The modification of AP into APM was successfully achieved, as evidenced by a rougher and denser surface morphology with a more heterogeneous pore distribution (Figs. 1(c)(i, ii)), features that enhance adsorption efficiency. The EDX data further indicated that the incorporation of maggot biomass enriched the adsorbent with additional mineral elements, increased the content of active carbon, and provided a more complex composition. These modifications contributed to the higher adsorption capacity observed for APM, consistent with the maximum adsorption capacity data.

Meanwhile, Figs. 1(b and c)(iii, iv) illustrate the surface morphology and composition after the adsorption process, also at 1000X and 10,000X magnifications. The surface pores were partially covered by layers or particles resulting from interactions with adsorbed dye molecules/ions. The EDX spectra revealed significant changes in elemental composition, including the emergence of new peaks attributed to the adsorbed species, along with changes in the relative atomic composition on the adsorbent surface. These findings confirm that adsorption occurred effectively, as reflected by the clear differences in surface morphology and elemental composition.

Overall, the modification of AP into APM resulted in a more compact and heterogeneous porous surface structure, while EDX analysis demonstrated enrichment with additional minerals and an increase in active carbon content. These characteristics explain the enhanced adsorption capacity of APM, which is consistent with the comparative maximum adsorption capacities obtained for each adsorbent.

3.1.3 XRF

XRF analysis was performed to examine the elemental makeup of the adsorbents both prior to and following the adsorption process. This method is particularly effective for confirming changes in composition or detecting metal ions associated with the adsorbent surface, thereby contributing to a better understanding of the adsorption mechanism. The elemental composition of the adsorbents obtained from XRF measurements is summarized in Table 1.

As shown in Table 1, noticeable changes in elemental composition occurred after adsorption. For the unmodified adsorbent (AP), the dominant components were CaO, Ca, and P₂O₅. Following adsorption (AP-CV), these components decreased, accompanied by an increase in Fe₂O₃ and Fe content. These differences can be explained by the cationic character of CV⁺, a basic dye containing three positively charged dimethyl amino groups. These positive charges interact with negatively charged metal oxides on the adsorbent surface, promoting electrostatic attraction and possible ion exchange, which is reflected in the redistribution of elements in the XRF spectra.

For the modified adsorbent APM, the CaO content was initially higher than that of AP-CV. However, after adsorption (APM-CV), a reduction in CaO was observed along with a corresponding increase in Fe₂O₃ and Fe. This suggests the active involvement of metal oxides in binding CV molecules. Overall, the relationship between the positively charged nature of CV and the changes in mineral composition detected by XRF suggests that the adsorption mechanism involves both physical and chemical interactions, with metal oxide components acting as the main active sites for dye attachment.

3.1.4 Brunauer–emmett–teller (BET)

BET analysis was performed to assess the textural properties of the adsorbents, such as surface area and pore size, which are closely associated with adsorption performance and mechanism. A comparative evaluation of the adsorbents before and after adsorption offers insight into changes in pore structure resulting from CV binding.

The BET analysis results are as follows: the pore width of AP slightly decreased from 2.5830 to 2.3450 nm after CV adsorption, suggesting partial occupation of the pore structure by dye species. Meanwhile, its specific surface area rose from 0.3926 to 1.5561 m² g⁻¹, suggesting surface restructuring during the adsorption process. For avocado peel modified with maggot (APM), the initial pore width and surface area had corresponding values of 4.8864 nm and 2.1278 m² g⁻¹, respectively, but both values decreased to 3.3530 nm and 0.9012 m² g⁻¹, indicating pore blockage by CV dye molecules. These structural transformations confirm that maggot modification enhances surface reactivity and adsorption capacity, consistent with the higher maximum adsorption of APM compared with AP. Overall, the results suggest that adsorption mainly occurs through physical interactions within the pore structure, supporting a pore-filling mechanism (Zein et al., 2023).

3.1.5 pHpoint of zero charge (pH_pzc)

The determination of pH_pzc is a key step in interpreting the surface characteristics of an adsorbent, as it defines the pH condition at which the surface exhibits an overall neutral charge. The pH_pzc value plays an important role in predicting electrostatic interactions occurring between the adsorbent surface and dissolved ionic or molecular species, thereby offering insight into the adsorption mechanism. The pH_pzc values obtained in this work are shown in Fig. 2(a).

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Fig. 2.

(a). Phpzc of AP and Avocado peel maggot (APM), (b). Effect of pH on CV onto AP and APM. Experimental condition: AP and APM adsorbent: initial concentration = 100 mg L^-1; contact time = 60 min, adsorbent mass = 0.1 g.

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The findings indicated that AP displayed a pH_pzc value of 3.7, whereas APM showed a higher value of 7.1, indicating differences in surface characteristics and their potential interactions with cations and anions across different pH ranges. Accordingly, both AP and APM are expected to adsorb cations more effectively at pH values above 3.7 and 7.1, respectively, while demonstrating a greater affinity toward anions at pH levels below 7.1, in line with the charge distribution of each adsorbent. Since CV is a cationic dye, its adsorption is more favorable at solution pH values above the pH_pzc (Bazzo et al., 2016).

3.2.1 Effect of pH on CV adsorption

Changes in solution pH can alter the surface charge properties of an adsorbent, which in turn influence the interaction strength between the adsorbent and dye molecules. Therefore, examining the effect of pH is essential for identifying the most favorable conditions for the adsorption of crystal violet. Adsorption experiments using AP were conducted over a pH range of 3–7, whereas APM was evaluated at pH values between 5 and 9. The optimum pH for CV adsorption onto AP and APM differed, as the solution pH was adjusted based on the pH_pzc of each material. The pH_pzc values were found to be 3.7 for AP and 7.1 for APM. The outcomes of this analysis are illustrated in Fig. 2(b).

As shown in Fig. 2(b), the AP adsorbent exhibited its optimum performance for CV adsorption at pH 4, achieving an adsorption capacity of 22.24 mg g⁻¹ and a removal efficiency (%R) of 99.73%. At this pH, the initial adsorption capacity was 20.97 mg g⁻¹; however, the adsorption capacity declined at pH 5 and remained nearly unchanged up to pH 7, with a value of approximately 19.9 mg g⁻¹. In contrast, the APM adsorbent reached its maximum adsorption of CV at pH 8, with an adsorption capacity of 27.15 mg g⁻¹ and a removal efficiency of 98.85%. The initial adsorption capacity was recorded as 24.65 mg g⁻¹, increased with rising pH until reaching the optimum value, and subsequently decreased to 26.07 mg g⁻¹. Notably, the optimal pH for CV adsorption on both adsorbents was higher than their respective pH_pzc values. At pH above the pH_pzc, there is an increase in OH⁻ concentration, which attracts the cationic CV dye by electrostatic attraction, thereby increasing adsorption capacity. Electrostatic interaction mechanisms also occur for other cationic dyes, such as methylene blue, as reported in studies on its removal from water using acid-modified grape leaves (Al-Qaim et al., 2024)

In the study of the pH effect, the optimum adsorption capacity was achieved at pH 4 for the AP adsorbent with a pH_pzc of 3.7, while at pH 8 for the APM adsorbent with a pH_pzc of 7.1. Under conditions of pH > pH_pzc, the surface of AP and its modified adsorbent contains more OH⁻ ions or negative charges, which causes a stronger attraction between the adsorbent surface and the cationic dye CV (Aranda-García & Cristiani-Urbina, 2019; Essekri et al., 2023). A similar result was also found in the adsorption of methylene blue using Opuntia ficus-indica, where the optimum pH was above the pH_pzc (Boumezough et al., 2025).

3.2.2 Effect of initial concentration of CV dye on adsorption capacity

The influence of the initial adsorbate concentration is closely associated with the mass transfer of dye molecules from the liquid phase to the adsorbent surface. In this study, the initial CV concentrations ranged from 100 to 1300 mg L⁻¹ for the AP adsorbent and from 100 to 1500 mg L⁻¹ for the APM adsorbent. The outcomes of this analysis are illustrated in Fig. 3.

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Fig. 3.

(a). Effect of initial concentration on the adsorption capacity of CV dye onto AP and APM. Experimental condition: AP adsorbent (pH = 4; contact time = 60 min), APM adsorbent (pH = 8; contact time = 60 min), adsorbent mass = 0.1 g. b). Effect of contact time on the adsorption capacity of CV dye onto AP and APM. Experimental condition: AP adsorbent (pH = 4; initial concentration = 1000 mg L^-1), APM adsorbent (pH = 8; initial concentration = 1300 mg L^-1), adsorbent mass = 0.1 g.

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As shown in Fig. 3(a), the optimal adsorption conditions for CV were achieved at initial concentrations of 1000 mg L⁻¹ for AP and 1300 mg L⁻¹ for APM. Under these conditions, the maximum adsorption capacity of the AP adsorbent reached 151.91 mg g⁻¹, whereas CV adsorption by the APM adsorbent attained a higher maximum capacity of 216.99 mg g⁻¹. The adsorption capacity increased progressively with increasing initial dye concentration until the optimum level was reached, after which a decline was observed for both adsorbents. This decrease is associated with the active sites on the adsorbent surface becoming saturated by dye molecules, thereby preventing further adsorption (Ramadhani et al., 2020). The same result has been reported by (Nguyen et al., 2023), who observed that water hyacinth (Eichhornia crassipes) powder exhibited a relatively high optimum initial concentration (615,865 mg/L) with good capacity for the adsorption of CV dye (180,336 mg/g). These findings confirm the consistency of our results with those of other bioadsorbents, emphasizing that the maximum adsorption capacity is largely governed by the interplay between the initial dye concentration and the number of available active sites on the adsorbent surface.

3.2.3 Effect of contact time of CV dye on adsorption capacity

The influence of contact time reflects the period necessary for the adsorbent to effectively capture the adsorbate. In this work, the impact of contact time on adsorption capacity was evaluated over a duration ranging from 15 to 120 min. As illustrated in Fig. 3(b), the maximum adsorption of CV using AP and APM was attained at contact times of 45 and 60 min, respectively. Under these conditions, the adsorption capacities of AP and APM reached 153.48 and 218.77 mg g⁻¹, respectively. Although APM required a longer contact time to achieve equilibrium, it exhibited a higher adsorption capacity compared with the AP adsorbent. This behavior can be explained by the presence of additional functional groups introduced through maggot modification.

The increase in adsorption capacity occurred because sufficient time was available for the interaction between AP and APM adsorbents and the dye molecules (Jabar et al., 2022). The dye species occupied the available adsorption sites on the adsorbent surface until saturation was reached. Subsequently, the adsorption capacity decreased after the optimum time, which can be explained by the fact that prolonged contact may lead to desorption of dye molecules from the active sites or resistance effects (Arenas et al., 2017).

3.2.4 Effect of heating the adsorbent of CV dye on adsorption capacity

Adsorption performance was examined as a function of adsorbent heating within the temperature range of 25 – 210°C. As shown in Fig. 4(a), the optimal heating temperature for AP was identified as 60 °C, whereas APM exhibited an optimum temperature of 90°C.

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Fig. 4.

(a). Effect of heating adsorbent on the adsorption capacity of CV dye onto AP and APM. Experimental condition: AP adsorbent (pH = 4; initial concentration = 1000 mg L^-1; contact time = 45 min), APM adsorbent (pH = 8; initial concentration = 1300 mg L^-1, contact time = 60 min), adsorbent mass = 0.1 g. b). TGA curves of AP and APM adsorbents.

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The optimum adsorption capacities under heating influence were 165.16 mg g⁻¹ for AP and 223.49 mg g⁻¹ for APM. At these optimum temperatures, the heating was sufficient to reduce the moisture content of the adsorbents, thereby opening the pores. Beyond these optimum conditions, a gradual decline in adsorption capacity was observed, which may be ascribed to structural damage of the adsorbents at higher temperatures. Several functional groups serving as active sites for dye adsorption were degraded, leading to reduced effectiveness in adsorbing the dye. This result is consistent with the characterization obtained from TGA analysis for both adsorbents prior to CV dye adsorption.

The TGA curves in Fig. 4(b) show the thermal degradation profiles of AP and APM adsorbents over a temperature interval of 30–500 °C. In general, both samples exhibited similar weight loss patterns but with different thermal stabilities. A relatively small weight loss occurred in both samples, which may be ascribed to the elimination of physically bound moisture or volatile compounds. The AP sample lost approximately 9.061%, whereas APM exhibited a higher mass loss of 11.474%. This indicates that APM contained a greater amount of moisture or volatile groups compared with AP, and that both adsorbents remained structurally intact at heating temperatures below 120 °C (Escalante et al., 2022). This finding is consistent with the finding that the adsorption capacity did not show a notable reduction up to 120 °C; however, at higher temperatures, the adsorption capacity decreased markedly. The curve stays almost constant until about 250–300 °C. After that, it decreases sharply starting from around 300 °C, showing the main thermal decomposition of the material. The sharp drop ends near 400 °C, then the curve becomes gradual up to 500 °C. Thus, the major mass loss takes place in the 300–400 °C range, indicating the primary phase of thermal degradation.

The good thermal stability of AP and APM below 100 °C is indicated by the minor weight loss observed at low temperatures up to approximately 100–150 °C, which is mainly ascribed to the release of physically adsorbed moisture. A significant and pronounced mass loss occurred between 200 and 400 °C.

3.3.1 Adsorption kinetic studies

To elucidate the adsorption mechanism and rate, a kinetic evaluation was carried out employing several mathematical models, including the pseudo-first-order (PFO), pseudo-second-order (PSO), intraparticle diffusion, and Elovich models. Experimental data obtained from contact time studies were applied to determine the adsorption kinetics based on the corresponding equations of each model.

The kinetic analysis results in Table 2 show that CV adsorption on AP and APM follows the PSO model most accurately, as indicated by the high coefficients of determination (R² = 0.9939 for AP and 0.9829 for APM). The equilibrium adsorption capacities (qe) estimated using the PSO model were also close to the experimental values, confirming that this model accurately represents the adsorption process. The agreement with the PSO model further indicates that the adsorption rate behavior is dominated by a chemisorption mechanism, involving chemical interactions between CV species and the available active sites on the surface of the adsorbent. This mechanism typically includes the formation of chemical bonding or electron transfer, which explains the strong binding between CV and both adsorbent materials. Therefore, the PSO model can be considered the most relevant representation of the adsorption kinetics for this system, and similar findings have also been reported for CV adsorption using Platanus orientalis leaf (Ahmad Khan et al., 2023).

3.3.2 Thermodynamics studies

Thermodynamic analysis is crucial for providing essential information regarding the mechanism and energy characteristics underlying the adsorption process based on the parameters ΔG°, ΔH°, and ΔS°. In this study, the thermodynamic parameters presented in Table 3. were obtained through mathematical calculations derived from the regression equation of the lnK_L versus 1/T curve, which was used to determine the values of ΔH° and ΔS° according to Eq. (10) (Fabryanty et al., 2017).

The thermodynamic evaluation summarized in Table 3 shows that the adsorption of CV onto AP and APM occurred spontaneously, as demonstrated by the negative values of ΔG° across the investigated temperature range (Prabakaran et al., 2022). The negative ΔH° value further suggests that the adsorption mechanism is exothermic (Sadoq et al., 2024), while the negative ΔS° value reflects a decrease in system randomness due to the reduced mobility of CV molecules after binding to the adsorbent surface (De Castro et al., 2018). Accordingly, it can be inferred that the adsorption of CV onto both adsorbent materials occurs spontaneously, releases heat, and is accompanied by a decrease in system entropy.

3.3.3 Reusability studies

Reusability testing evaluates the ability of AP and APM to be applied over repeated adsorption–desorption cycles for CV without a noticeable decline in adsorption capacity, thereby reflecting stability, long-term effectiveness, and potential as environmentally friendly adsorbents. In this case, the same adsorbent was tested repeatedly for up to four cycles using four types of eluents, namely acetic acid, hydrochloric acid, and citric acid. The eluents were chosen to be acidic since CV is cationic, making desorption more favorable in acidic conditions (Mussa et al., 2025). Fig. 5 shows that the adsorbents were still able to adsorb CV effectively even after repeated use of the same material from the first cycle.

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Fig. 5.

Reusability studies of (a) AP and (b) APM adsorbents with different eluents in four cycles.

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The percentage of adsorption, calculated using Eq. (12), remained above 95% for the AP adsorbent, while it was above 82% for APM. The use of acetic acid as an eluent for CV desorption was more effective compared with the other two eluents. The reusability results revealed that APM exhibited a faster decrease in removal percentage compared with AP in subsequent cycles, despite modification. This can be ascribed to the pore architecture and surface functional groups generated during the APM modification process being less stable under acidic eluents, leading to partial degradation or blockage of active sites, as well as incomplete desorption due to dye molecules being strongly retained within the pores. Moreover, APM appeared to be more susceptible to damage or less fully regenerable because repeated interactions with acidic eluents could degrade its functional groups. Conversely, AP, with its more physically dominant adsorption mechanism, tended to be more stable, thereby maintaining its performance over several regeneration cycles.

3.3.4 Application to laboratory wastewater

The application study for wastewater treatment was conducted using effluent samples from the Environmental Chemistry Laboratory, Andalas University, in July 2025. This wastewater can represent industrial effluent as it contains various types of dyes, metals, acids, and bases. The optimal adsorption conditions applied to this liquid waste included an adsorbent dosage of 0.1 g with a particle size not exceeding 36 µm, a contact time of 60 mins., and agitation at 100 rpm.

Both AP and APM exhibited excellent pollutant removal performance in real wastewater samples. Even under strongly acidic conditions (pH = 1.28), AP achieved a removal efficiency of 96.35%, while APM maintained a high efficiency of 86.26%, indicating that both adsorbents remained active under extreme environments. However, under optimum pH conditions, the adsorption performance improved further, with AP reaching its maximum removal at pH 4 at 97.98%, while APM achieved its highest performance at pH 8 at 92.80%. These results emphasize the critical role of pH optimization in enhancing adsorption capacity and demonstrate that the modification produced distinct surface characteristics, thereby extending the range of optimal pH values. Accordingly, AP is more suitable for acidic conditions, while APM provides advantages under near-neutral to alkaline conditions, suggesting that both adsorbents complement each other for application across different types of wastewaters.

3.3.5 Mechanism of adsorption

The adsorption mechanism of the cationic dye CV onto AP and APM adsorbents is illustrated in Fig. 6. The adsorption process occurs through several major interactions, including hydrogen bonding, π–π interactions, cation exchange, electrostatic attraction, and pore filling. Hydrogen bonding takes place when –OH and –O– functional groups located on the adsorbent surface interact with hydrogen atoms associated with CV species, thereby strengthening the binding. The π–π interaction arises from attraction between the aromatic rings of CV and the aromatic groups of organic compounds in AP or APM, resulting in π–π stacking forces. Cation Exchange occurs when the positively charged ions of CV (such as N⁺) replace other ions (such as H⁺ or light metals) on the adsorbent surface. Electrostatic Attraction happens involving negatively charged functional moieties on the adsorbent (–OH or –COO⁻ groups) and the positively charged CV molecules. In addition, some CV molecules may penetrate and become trapped within the pores of the adsorbent, confirming the occurrence of pore-filling phenomena.

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Fig. 6.

Prediction mechanism of adsorption CV.

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Overall, the modification of AP into APM resulted in an increase in both the number of available active sites and the surface area, as supported by characterization analyses showing clear differences before and after modification. Consequently, the conversion of AP to APM provides multiple adsorption pathways and significantly enhances the adsorption capacity toward crystal violet.

3.3.6 Comparison with previously reported adsorbent materials

The adsorption efficiency of CV on this adsorbent was compared with that of various adsorbents previously reported in the literature. The comparison results are presented in Table 4 to provide a clearer overview of the adsorbent performance in removing crystal violet.

Based on Table 4, different types of adsorbents demonstrate strong potential as a sustainable adsorbent for the removal of crystal violet, which was influenced by the nature of their raw materials. Mineral- and carbon-based adsorbents primarily rely on physical properties, whereas natural biomass offers active functional groups that enhance interactions with dye molecules. Overall, these findings demonstrate that AP, particularly after modification with maggot, demonstrates strong potential as a sustainable adsorbent for the removal of crystal violet. The high adsorption capacity confirms that this biomass waste is not only competitive with commercial adsorbents but also more sustainable in terms of raw material availability.