Low-cost and sustainable bioadsorbent from banana peel waste for crystal violet dye removal

Abstract

A direct comparison was made between raw banana peel waste (RBPW) and acid-treated banana peel waste (ABPW), under identical conditions, for adsorption of crystal violet (CV). Sorption kinetics, isotherms, and thermodynamics were considered to reveal the underlying mechanisms. The effects of contact time, pH, initial CV concentration, temperature, and adsorbent dosage were evaluated. The sorption process obeyed a pseudo-second-order kinetic model, while the Langmuir isotherm model best explained the equilibrium data with maximum adsorption capacities. The Dubinin–Radushkevich model supported the potential of ion-exchange mechanisms for the acidified sample. Adsorption was spontaneous and endothermic, as revealed by negative Gibbs free energy, positive enthalpy (+16.4 kJ/mol for RBPW and +53.5 kJ/mol for ABPW) and positive entropy (RBPW = 6.79 J/mol·K and ABPW = 14.65 J/mol·K) values. The lower ΔH for the raw peel is more consistent with physisorption, while higher ΔH of the acid-treated peel suggests stronger interactions consistent with chemisorption/ion-exchange. The FT-IR analysis confirmed that functional groups such as –OH, –COOH, C=O, C-O, and possibly aromatic moieties on banana peel waste are involved in the sorption of CV. The enhanced performance of ABPW is attributed to acid-induced surface modifications that increased porosity, making the functional groups available for sorption process.

Full Article

Low-cost and Sustainable Bioadsorbent from Banana Peel Waste for Crystal Violet Dye Removal

Salah Ud Din,^a Khairia Mohammed Al-Ahmary,^b Hamad AlMohamadi,^c Saedah R. Al-Mhyawi,^b Jawaher Saud Alrashood,^d Patrick T. Ngueagni,^e,* and Edwin A. Ofudje ^f

‏A direct comparison was made between raw banana peel waste (RBPW) and acid-treated banana peel waste (ABPW), under identical conditions, for adsorption of crystal violet (CV). Sorption kinetics, isotherms, and thermodynamics were considered to reveal the underlying mechanisms. The effects of contact time, pH, initial CV concentration, temperature, and adsorbent dosage were evaluated. The sorption process obeyed a pseudo-second-order kinetic model, while the Langmuir isotherm model best explained the equilibrium data with maximum adsorption capacities. The Dubinin–Radushkevich model supported the potential of ion-exchange mechanisms for the acidified sample. Adsorption was spontaneous and endothermic, as revealed by negative Gibbs free energy, positive enthalpy (+16.4 kJ/mol for RBPW and +53.5 kJ/mol for ABPW) and positive entropy (RBPW = 6.79 J/mol·K and ABPW = 14.65 J/mol·K) values. The lower ΔH for the raw peel is more consistent with physisorption, while higher ΔH of the acid-treated peel suggests stronger interactions consistent with chemisorption/ion-exchange. The FT-IR analysis confirmed that functional groups such as –OH, –COOH, C=O, C-O, and possibly aromatic moieties on banana peel waste are involved in the sorption of CV. The enhanced performance of ABPW is attributed to acid-induced surface modifications that increased porosity, making the functional groups available for sorption process.

DOI: 10.15376/biores.20.4.10640-10664

Keywords: Acid activation; Banana peel; Crystal violet; Isotherms; Kinetics

Contact information: a: Department of Chemistry, University of Azad Jammu and Kashmir Muzaffarabad, Pakistan; b: Department of Chemistry, College of Science, University of Jeddah, Jeddah, Saudi Arabia; c: Department of Chemical Engineering, Faculty of Engineering, Islamic University of Madinah, Madinah, Saudi Arabia; d: Department of Teaching and Learning, College of Education and Human Development, Princess Nourah bint Abdulrahman University, Saudi Arabia; e: Department of Inorganic Chemistry, Faculty of Science, University of Yaoundé, 812, Yaoundé, Cameroon; f: Department of Chemical Sciences, Mountain Top University, Ibafo, Ogun State, Nigeria;

* Corresponding author: patrickngueagn@gmail.com

INTRODUCTION

Synthetic dyes are complex organic compounds with stable structures that resist biodegradation, making them persistent environmental pollutants. It has been reported that about 12% of these dyes’ annual production is released as industrial wastewater, which greatly contributes to environmental pollution (Foroutan et al. 2020). The release of dye components into water bodies has serious environmental and health consequences (Mahmood et al. 2018). Dye contamination in aquatic environments is highly visible, and it disrupts the ecosystems by hindering photosynthesis through the blocking of sunlight penetration, thus reducing oxygen levels and harming aquatic life (Baharim et al. 2023). These dyes also pose noticeable health risks, causing toxic effects such as skin irritation, carcinogenicity, allergic reactions, teratogenic effects, and potential genetic mutations, that threaten both human health and ecological systems (Patricia et al. 2020; El-Rayyes et al. 2025).

Crystal violet (CV) dye is a highly intense cationic triphenylmethane dye that is often used in pharmaceuticals, printing inks, and textiles industries (Pashaei-fakhri et al. 2021). The CV dye is extremely toxic; it can damage aquatic ecosystems and cause severe health related issues, among which are cardiovascular effects, respiratory problems, skin and eye irritation, and increased risks of cancer and genetic mutations (Mittal et al. 2021: Pashaei-fakhri et al. 2021). The hazardous and persistent nature of crystal violet dye has led to major research interest on the effective disposal methods for eliminating this toxic dye from wastewater.

To address this pollution challenge caused by CV and other contaminants efficiently, various techniques have engaged, and among them are chemical oxidation, coagulation, electrochemical, ozonation, nanofiltration, biological degradation, advanced oxidation processes, reverse osmosis, adsorption, and photocatalytic degradation (Ofudje et al. 2020; Wang et al. 2020; Muhammad et al. 2021; Yuan and Jiang 2021; Ngueagni et al. 2025). However, most of these conventional methods can produce secondary waste, and expensive, highlighting the need for more sustainable, and affordable solutions like adsorption (Ofudje et al. 2020). Adsorption is a very effective dye removal method, which is known for its potential for reusability, simple to operate and cost-efficiency, making it an excellent choice (Adegoke et al. 2023; Ngueagni et al. 2025). The choice of using activated carbon, which is the most common adsorbent in developing nations like Cameroon and Nigeria, is often threatened by its high cost of production (Ofudje et al. 2020). Agricultural waste-derived adsorbents are increasingly popular for wastewater treatment because of their numerous advantages, such as being efficient, cost-effective, renewable, and biodegradable (Jia et al. 2017).

Recent research has explored eco-friendly bio-adsorbents from agricultural waste, including banana trees (Abdelkhalek et al. 2022), olive stones (Abed et al. 2019), apple peels (Enniya et al. 2017), peach and apricot kernels (Rehali et al. 2021), tea waste (Liu et al. 2018), coffee grounds (Pagalan Jr et al. 2020), Kepok banana (Silviyanti et al. 2025), and corn cobs (Aljeboree et al. 2021) for effective dye and colour removal. Bananas are widely consumed worldwide, and their peels, which make up about 40% of the fruit’s weight, are often disposed of as waste, causing a lot of waste management issues. However, harnessing their abundance could also make them an attractive biosorbent material for dye removal from wastewater, thus adding value to them (Akpomie and Conradie 2020). Banana peels are rich in cellulose, hemicellulose, pectin, and lignin with functional groups such as carboxyl, hydroxyl, and others which could enhance their interactions with several adsorbate, thus facilitating an effective adsorption process (Yang et al. 2019; Thomas et al. 2021). The adsorption capacity of banana peel-derived adsorbents can be improved by chemical treatments such as the use of acids, bases, and salts that remove impurities and enhance properties including high porosity, and surface area (Akter et al. 2021; Channei et al. 2025). This evidence supports banana peels as promising candidates for cationic dye removal due to their natural abundance of active functional groups and modifiable structure.

Channei et al. (2025) reported that banana peel treated with sulfuric-acid solution resulted in a huge increase in BET surface area from just 3.2 to 339 m²/g, coupled with increased in pore volume were observed. It was reported in their work that the treatment by an acid opened a porous network in the adsorbent by breaking the lignocellulosic structures. In a study conducted by Huang et al. (2024) on Cr(VI) uptake, the treatment of banana peel with 50 % H₂SO₄ at 50 °C for 24 h caused an increased its surface area from 0.036 to 0.0507 m²/g and that this improved the adsorption capacity of the adsorbent.

Many earlier works primarily focused on examining the adsorption capacity of banana peel without providing detailed mechanistic insights or comprehensive physicochemical characterizations. The present work builds upon these findings by systematically comparing raw banana peel waste (RBPW) with acidified banana peel waste (ABPW) under identical conditions. Another unique feature of this work is the integration of desorption and regeneration studies, which assessed the performance of banana peel waste across multiple adsorption–desorption cycles using acetic acid as an eluent. This sustainability aspect is often overlooked in previous banana peel studies, and it plays a critical role for practical wastewater treatment applications. The selection of acetic acid as the desorbing agent for regeneration studies was because under acidic conditions, the carboxyl groups present in lignocellulosic components of banana peel become protonated. This protonation reduces the negative surface charge that normally promotes electrostatic attraction with cationic dyes such as crystal violet. As a result, the electrostatic interactions weaken, making it easier for the bound dye molecules to be released back into solution.

This study investigated the ability of raw and acidified banana peels in adsorbing CV from wastewater. It critically examines the effects of various functions including pH, contact time, CV initial concentration, and temperature on the removal efficiency of the adsorbent. Furthermore, the research explored the thermodynamics, adsorption isotherms and kinetics involved in the process, offering a comprehensive understanding of the mechanisms behind dye removal.

MATERIALS AND METHODS

Preparation and Characterization of Acidified Adsorbent

Banana peel waste (BPW) was collected, thoroughly cleansed with deionized water, and sun-dried for seven days. The dried BPW (40 g) were subjected to acidification with 0.1 M sulfuric acid for 24 h and the content was filtered, washed using deionized water several times until neutral pH was attained, dried, and ground to powder and sieved to obtain desired particles with sizes less than 250 µm. The acidified banana peel waste (ABPW) and raw banana peel waste (RBPW), which is the untreated sample, were stored in airtight containers for further analysis.

Adsorbent Characterizations

Fourier transform infrared spectroscopy (FT-IR) (CARY630 NBY, Thermo Fisher Scientific, Waltham, MA, USA) was used for the identification of the functional groups involved in dye adsorption. Scanning electron microscopy (SEM) (Phenom ProX, USA) was used in the examination of surface morphology and porosity. Brunauer-Emmett-Teller (BET) analysis using a Quantachrome NOVA 2200C (USA) was used to determine the pore size distribution and specific surface area. The point of zero charge (pH_PZC) was investigated using a Nano ZS analyzer (Malvern, UK) that assessed the surface charge behavior across pH ranges.

Crystal Violet Solution Preparation and Adsorption Study

1000 mg/L stock solution of crystal violet (CV) was made following the dissolution of analytical-grade CV in deionized water. Different working concentrations were obtained through serial dilution. The CV solution pH was adjusted by adding either 0.1 M HCl or NaOH. For the adsorption study, 0.3 g of either ABPW or RBPW was added into 25 mL of CV solution in conical flasks and shaken at a constant temperature of 45 °C for a predetermined time. After each adsorption process, the mixtures were filtered, and the residual CV concentrations were measured using a UV-Vis spectrophotometer (Shimadzu UV-3600 UV-Vis-NIR). The CV amount sorbed at equilibrium (Qₑ, mg/g) and the percentage removal (%) were estimated using Eqs. 1 and 2:

In these equations, C₀ is the initial CV concentration (mg/L), C_e denotes the equilibrium CV concentration (mg/L), V stands for the volume of solution (L), and m represents the mass of adsorbent used (g).

Desorption Study

To investigate the reusability of the adsorbent, desorption experiments were done using O.5M of acetic acid as the desorbing agent. The used adsorbent biomass was subjected to multiple adsorption–desorption cycles of experiments, and the percentage of CV recovered was estimated to compute the regeneration potential of the adsorbent using the equation,

where AA represent the amount of CV adsorbed, DA is the amount of CV desorbed.

All experiments were performed in triplicate, and the results are represented as mean values with associated standard deviations. Error bars were included in the relevant figures.

RESULTS AND DISCUSSION

Effects of Reaction Time and CV Concentration

Crystal violet sorption onto raw and acidified banana peel waste was studied as a function of CV concentration and contact time, as presented in Figs. 1 and 2, respectively. The data indicated that the sorption process was both time- and concentration-dependent, exhibiting a rapid uptake initially, followed by a gradual approach to equilibrium. For the raw sample, at a CV 300 mg/L, the uptake capacity rose from 25.5 mg/g just after 10 min to 73.4 mg/g at 100 min. But as the contact time was extended beyond 120 min, the sorption rate slowed, and equilibrium was attained at 140 min with maximum sorption capacity of 88.5 mg/g, and beyond which no meaningful increase in the dye uptake was observed. This suggests that the available binding sites became filled with CV molecules and the system approached dynamic equilibrium (Ogundiran et al. 2025). The rapid uptake at the early stages could be due to the abundance of available binding sites on the banana peel surface (El-Rayyes et al. 2025). For the acidified sample (Fig. 2), at 300 mg/L, the sorption capacity rose from 33.5 mg/g at 10 min to 115.5 mg/g at 120 min, and beyond which no meaningful increase in the dye uptake was observed. It was observed that the acidified banana peel consistently outperformed the raw variant at all conditions tested. For instance, the acidified peel adsorbed 96.4 mg/g of CV by 100 min, compared to 73.4 mg/g for the raw peel. The enhanced adsorption of the acidified adsorbent can be attributed to acid-induced surface modification by the removal of impurities, which increased porosity, exposed more active sites, and improved functional group affinity for CV molecules.

On the role of CV concentration, as can be seen from Figs. 1 and 2, when CV concentration was increased from 50 to 300 mg/L, the adsorption capacity also increased, reaching 19.5 mg/g at 50 mg/L and 88.5 mg/g at 300 mg/L for the raw peel and 23.4 mg/g and 115.2 mg/g in the case of the acidified peel. This increase could be a result of higher driving force for mass transfer at higher concentrations, which enables the interaction between dye molecules and the adsorbent surface to become faster. The availability of a greater number of CV molecules could promote more frequent collisions with the adsorbent, thus improving the uptake efficiency until the surface sites are fully occupied (Ogundiran et al. 2025). This consistent increase across all concentrations suggests that acid treatment enhanced the surface chemistry of the banana peel, making it more effective at capturing dye molecules. Comparable results were found by Sen et al. (2024) in the study where palm leaf sheath powder was used, showed that an increased in initial CV concentrations led to higher adsorption capacities, thereby underscoring the contribution of concentration gradient in mass transfer. Research conducted by Azhar-ul-Haq et al. (2022) employed RBPW obtained 93% CV removal and 91% desorption efficiency, with adsorption equilibrium reached within 10 min, highlighting rapid kinetics even with untreated biomass.

Fig. 1. Graphs of contact time and CV concentrations for the sorption by RBPW

Fig. 2. Graphs of contact time and CV concentrations for the sorption by ABPW

Effect of Banana Peel Waste Dosage

The influence of banana peel dosage on the removal of CV dye was evaluated using both raw banana peel waste (RBPW) and acidified banana peel waste (ABPW), with dosages ranging from 0.1 g to 0.4 g as depicted in Fig. 3.

Fig. 3. Graph of dosage effects on the uptake of CV by RBPW and ABPW

For RBPW, adsorption percentage rose from 53.8% at 0.1 g to a peak of 74.9% at 0.3 g and above this point, a marginal decrease was noted, with values of about 71.5% recorded at 0.4 g. Similarly, ABPW demonstrated improved performance across all dosages, achieving a maximum adsorption of 78.7% at 0.3 g, then slightly decreasing to 74.9% at 0.4 g. In both cases, the sorption percentage increased with dosage up to 0.3 g, after which a slight decline was observed. While this trend is often attributed to overlapping and aggregation of active sites at higher biomass concentrations (Shridhar and Sanjaykumar 2022; Ogundiran et al. 2025), another important factor is the physical nature of banana peel material. Being pulpy and crumbly, higher dosages increase the likelihood of generating very fine particles that disperse in solution. These fines can adsorb dye molecules but may not be fully retained during filtration, causing an underestimation of adsorption performance.

In a study by Baharim et al. (2023), acidic post-treatment of banana pseudo–stem biochar showed enhanced CV uptake up to an optimal dose after which additional dose yielded negligible benefits due to saturation effects. Furthermore, Shridhar and Sanjaykumar (2022) observed that adsorbent obtained from banana stems demonstrated exceptionally high CV adsorption capacities (up to 208 mg/g), indicating that the chemical treatment of banana-based biomass enhanced dye uptake beyond quantities observed here and in RBPW.

Effect of pH on the Adsorption of Crystal Violet

Solution pH plays a vital role in the uptake of CV dye onto banana peel-based adsorbents, as it influences both the adsorbent’s surface charge and the dye molecules’ ionization state. In this study, the adsorption capacities of RBPW and ABPW were evaluated over a pH range of 2.0 to 6.0, as shown in Fig. 4. For both adsorbents, the percentage removal rose with rising pH from 2.0 to 4.5, reaching a maximum at pH 4.5 for ABPW (80.5%) and pH 5.0 for RBPW (74.83%). At lower pH values, protonation of functional groups on the banana peel surface could result in an overall positive surface charge, which creates electrostatic repulsion between the cationic CV molecules and the surface of the adsorbent. This could lead to lower adsorption capacities at acidic pH conditions, as seen in this current study with values of 49.6% for RBPW and 53.7% for ABPW at pH 2.0.

As the pH rises, adsorbents surface becomes less protonated and more negatively charged, enhancing electrostatic attraction between the CV cations and the adsorbent surface. This leads to increased dye uptake. The higher performance of ABPW across all pH values can be attributed to acid treatment, which enhances surface area, introduces more acidic functional groups, and increases the number of negative sites available for cationic dye binding. The point of zero charge (pH_ZPC) were found to be 4.7 in the raw material and 4.1 in the acidified form. These values indicate the pH at which the surface of the adsorbent carries no net charge. At pH values less than the pH_ZPC, the surface will be positively charged, but at pH values above it, the surface is negatively charged. Since CV is a cationic dye, it’s uptake is generally favored on negatively charged surfaces due to electrostatic attraction. For the raw adsorbent, maximum adsorption was observed at pH 4.5, which is slightly below its pH_ZPC of 4.75. At this pH, the surface of the adsorbent is expected to be mildly positively charged, which would typically repel the CV molecules which are positively charged. However, the high adsorption observed suggests that mechanisms other than electrostatic attraction, such as Van der Waals forces, π–π interactions or hydrogen bonding between the aromatic rings of the dye and the lignocellulosic components of the bark, likely played a significant role in dye uptake. This implies that despite mild electrostatic repulsion, other physicochemical interactions were strong enough to drive substantial adsorption at this pH.

Fig. 4. Graphs of effect of pH on the uptake of CV by RBPW and RBPW

In contrast, the acidified sample showed maximum adsorption at pH 5.0, which is above its pH_ZPC of 4.14. This implies that the surface was negatively charged at the optimum pH, favoring electrostatic attraction with the cationic dye. The acid treatment likely enhanced the surface acidity and shifted the pH_ZPC to a lower value. This modification improved the surface’s affinity for positively charged species at slightly acidic to neutral pH levels, thereby increasing the adsorption efficiency. However, beyond the optimum pH, a gradual decline in adsorption percentage was observed. For ABPW, the percentage dropped from 80.5% at pH 4.5 to 73.1% at pH 6.0, and for RBPW, from 74.8% at pH 5.0 to 70.3% at pH 6.0. The decline may result from saturation of binding sites and competition with OH⁻ ions at higher pH, limiting dye uptake. The observed trends are consistent with prior studies. For example, Azhar‑ul‑Haq et al. (2024), reported optimal CV binding by banana peel powder at pH near neutral, achieving ~93% removal in a rapid 10 min contact time confirming that CV adsorption is favored in mild-to-moderately acidic conditions.

The FT-IR analysis indicated shifts in –OH and C=O stretching bands after the sorption, suggesting the likely involvement of hydrogen bonding between CV molecules and surface hydroxyl/carbonyl groups (Ahmed et al. 2024; Cao et al. 2024). In addition, the shifts noticed in aromatic C=C bands support the occurrence of π–π stacking interactions between the lignin/phenolic structures of banana peel and the aromatic rings of CV (Guo et al. 2023; Ahmed et al. 2024). These extra interactions complement electrostatic attraction, explaining the relatively high adsorption even at pH values close to the pHpzc where electrostatic effects alone would be unfavorable. For instance, Guo et al. (2023) noted that the combining effect of π–π and hydrogen bonding interactions can double adsorption energy for dyes on biochar, when compared to electrostatic-only mechanisms. A study by Cao et al. (2024) using DES-functionalized ZIF-8/carbon adsorbents reported enhanced adsorption of mephedrone from pH 5 to 11, which was concluded to have been driven by the synergy of π–π stacking and hydrogen bonding.

Another feature that can influence the pH is the charge characteristics of the dye, as crystal violet contains one quaternary amine group and two tertiary amine groups. The quaternary amine can remain positively charged at all pH values, whereas the tertiary amines are protonated at very low pH in the range of 1 to 2 but progressively deprotonate as pH increases (Huang et al. 2023). Thus, at highly acidic pH, the molecules of CV carry multiple positive charges, which, together with the protonated adsorbent surface, creates strong electrostatic repulsion and results in low dye uptake. But as the pH rose, the tertiary amines lose protons, reducing the dye’s net positive charge and decreasing repulsion (Huang et al. 2023). Within this range, electrostatic attraction between the negatively charged adsorbent sites and remaining positively charged quaternary group, along with hydrogen bonding and π–π interactions, become more dominant, leading to enhanced sorption.

Effect of Temperature on CV Adsorption by RBPW and ABPW

In this study, the sorption percentage of CV onto RBPW and ABPW was evaluated from 25 to 55 °C, as shown in Fig. 5. For both adsorbents, an elevated temperature caused an improved adsorption performance. RBPW’s adsorption percentage rose from 51.1% at 25 °C to 76.1% at 45 °C, maintaining this peak value at 50 and 55 °C. Similarly, ABPW showed a higher increase, from 58.7% at 25 °C to a maximum of 84.6% at 50 and 55 °C. The adsorption efficiency increasing with rising temperature is an indication that the overall process of CV uptake by banana peel waste is endothermic in nature. This enhancement can be attributed to stronger interactions with surface functional groups and mobility of dye molecules at elevated temperatures. This trend aligns with findings from Gemici (2023), who investigated CV adsorption onto sulfuric-acid-treated chestnut shell and observed an increased in adsorption capacities with rising temperatures (25 to 50 °C).

Fig. 5. Graphs of effect of temperature on the uptake of CV by RBPW and ABPW

Kinetic Modeling of CV Adsorption onto RBPW and ABPW

The pseudo-first-order model (Eq. 4), pseudo-second-order model (Eq. 5), and intraparticle diffusion model (Eq. 6) were used to describe the kinetic behavior of the sorption of CV by RBPW and ABPW (Parlayıcı and Baran 2025):

Here k₁ stands for pseudo-first-order rate constant (min^-1), k₂ denotes pseudo-second-order rate constant (g/mg·min), k_p stands for intra-particle diffusion rate constant (mg/g/min^1/2), C_i denotes the boundary layer thickness, Q_t and Q_e have been defined previously, and t is time (min). The suitability of the most fit was determined by root mean square error as listed in Eq. 7 below (Sahmoune et al. 2024),

where N expresses the data points number, while the remaining parameters have been defined previously.

Fig. 6. Graphs of pseudo-first-order for the sorption of CV by RBPW and ABPW

Fig. 7. Graphs of pseudo-second-order for the sorption of CV by RBPW and ABPW

Fig. 8. Intra-particle diffusion plots for the adsorption of CV by RBPW and ABPW

The data obtained from Figs. 6, 7, and 8 for the kinetics study are listed in Table 1. The pseudo-first-order model, reflecting diffusion-limited adsorption, fitted the RBPW and ABPW data well in terms of R² (0.994–0.998), but it consistently underestimated equilibrium adsorption capacity (Qₑ cal). In contrast, the pseudo-second-order model, provided calculated Qₑ values closely matching with experimental ones, with high coefficients of determination (R² ≈ 0.966–0.998) and very low SSE percentages (< 0.05). These results clearly indicate that the adsorption process was well fitted by both the pseudo-first-order and pseudo-first-order models. Research on palm leaf sheath powder by Sen et al. (2024) confirmed pseudo-second-order as the most appropriate kinetic model for CV adsorption.

The intraparticle diffusion model revealed high R² values (0.975 to 0.996) for both raw and acidified adsorbents, but the graph did not pass through the origin, indicating that intraparticle diffusion was substantial but not the sole rate-limiting process. Constant C_i values ranging from ~0.099 to ~2.19 further confirm that boundary-layer diffusion influences adsorption. Although both the pseudo-first-order and pseudo-second-order models showed a good fit for CV adsorption, this alone does not conclusively indicate a specific mechanism. However, insights from the intraparticle diffusion model suggest that the overall adsorption is likely diffusion-controlled, particularly within a network of fine pores in an adsorption mechanism that aligns well with the porous structure of adsorbents derived from biomass such as banana peel waste (Hubbe et al. 2019).

Table 1. Kinetic Parameters of CV Adsorption by RBPW and ABPW

Isotherms of CV Adsorption onto RBPW and ABPW

Three isotherm models of Langmuir, Freundlich, and Dubinin–Radushkevich (D–R) were used to explain the relationship between the adsorbent and the adsorbate at equilibrium, as represented in Eqs. 8, 9, and 10 below (Dada et al. 2012; Ofudje et al. 2024; Sen et al. 2024; Channei et al. 2025; Ogundiran et al. 2025):

The maximum adsorption capacity is denoted as Q_max (mg/g), b depicts the Langmuir constant in L/mg, Freundlich constant is given as K_F (mg/g), and n represents the adsorption intensity, which determines the favorability of adsorption process or otherwise. From D-R, q_e and q_m have been defined previously, ϵ is the Polanyi potential. The separation factor (R_L) from the Langmuir isotherm that determines the favorability of adsorption can represented as (Ofudje et al. 2024):

When 0 < R_L < 1, the adsorption is said to be favorable, but when R_L > 1, it’s unfavorable adsorption, with R_L = 1, the process is linear adsorption, and irreversible adsorption if R_L = 0. The data obtained from Figs. 9, 10, and 11 for the isotherms study are listed in Table 2. The sorption equilibrium data as obtained for both RBPW and ABPW fit well to the Langmuir isotherm model. RBPW had a maximum adsorption capacity (Qₘₐₓ) of 93.15 mg/g (R² = 0.975), while ABPW shows a notably higher Qₘₐₓ of 122.53 mg/g (R² = 0.988). Additionally, the Langmuir separation factor (Rₗ) values of 0.524 for RBPW and a significantly lower 0.041 for ABPW confirm favorable adsorption, with ABPW presenting a highly favorable process. This observation is consistent with recent studies where acid or phosphoric acid treatment of banana peel and similar biomass adsorbents increased Qₘₐₓ and reduced Rₗ, indicative of improved monolayer dye uptake (Channei et al. 2025). When the Freundlich isotherm was assessed, it equally presented a good fit, especially for ABPW (R² = 0.995). The Freundlich constant K_F for ABPW (76.41) exceeded that of RBPW (64.61), signifying higher adsorption intensity, while the 1/n values (0.630 for RBPW; 0.231 for ABPW) imply both materials had favorable adsorption affinity, though ABPW displayed a much stronger adsorption intensity towards the adsorbent (Palle et al. 2022). Such dual fitting to Langmuir and Freundlich models is commonly observed in chemically modified bioadsorbents, especially where surface treatment enhances dye–adsorbent interactions.

The Langmuir model fit suggests that monolayer adsorption of crystal violet occurs on a relatively homogeneous surface of banana peel-based adsorbents, particularly after acid modification, which increases uniformity of active sites. On the other hand, the good fit of the Freundlich model indicates surface heterogeneity and the possibility of multilayer adsorption, reflecting the complex structure of lignocellulosic biomass that contains different types of functional groups (–OH, –COOH, C=O, aromatic rings). The dual fitting therefore implies that while adsorption is predominantly monolayer (supported by high Q_max values), additional multilayer interactions may also occur, especially at higher dye concentrations, likely due to hydrogen bonding and π–π stacking between dye molecules and the biomass surface. This complementary behavior of both models has been observed in other dye–biomass systems and highlights that the adsorption process is controlled by a combination of homogeneous monolayer binding sites and heterogeneous multilayer sorption sites . Jahin et al. (2024) reported dual fitting as evidence of both homogeneous binding sites and multilayer or heterogeneous adsorption phenomena.

The Dubinin–Radushkevich (D–R) isotherm further elucidates the adsorption mechanism by estimating the mean free energy E and Polanyi potential constant, ϵ as presented in Eqs. 12 and 13. respectively (Dada et al. 2012; Ofudje et al. 2024),

where Σ is defined as the Polanyi potential constant, the mean free energy of adsorption is presented as E (kJ/mol). An insight from the value of E could provide information about the adsorption mechanism; with E found between 8 and 16 kJ/mol, to be chemical or ion exchange, but E < 8 kJ/mol, is said to be physical adsorption process (Dada et al. 2012; Ofudje et al. 2024). For RBPW, E is 4.34 kJ/mol, indicating physical adsorption via Van der Waals forces, but in contrast, ABPW’s E value of 10.8 kJ/mol bridges the range into ion-exchange or chemical adsorption territory. Literature report by Ofudje et al. (2024) corroborates this, with study on unmodified and modified sugarcane peel biomass (including banana peel) showing similar E values of 4.642 kJ/mol for unmodified and 31.0 kJ/mol for modified sample, suggesting physisorption and chemisorption mechanism in the uptake of methylene blue.

Table 3 presents a comparative analysis of different adsorbents based on their maximum adsorption capacities for CV dye removal, highlighting the performance of banana peel-based adsorbents. Among all materials studied, acidified banana peel waste from this study exhibited the high adsorption capacity at 122.5 mg/g, surpassing both natural and chemically modified adsorbents reported in the literature. Raw banana peel waste also demonstrated a strong performance with an adsorption capacity of 93.2 mg/g, indicating the intrinsic potential of banana peel as an effective biosorbent. Comparatively, other natural biomass adsorbents such as banana pseudo stem (48.2 mg/g), Prunus spinosa (53.2 mg/g), and Raphanus caudatus biomass (48.7 mg/g) showed moderate adsorption capacities. While chemical modifications such as polyacrylamide grafting enhanced the performance of kiwi fruit peel powder (69.9 mg/g) and Actinidia deliciosa peels (75.2 mg/g), they still fell short of the performance exhibited by ABPW. Notably, even advanced materials like Fe₃O₄-decorated bacteria (114.6 mg/g) were outperformed by ABPW. These findings underscore the effectiveness of banana peel waste, particularly when acidified, as a low-cost, high-efficient adsorbent for dye removal, reinforcing its potential for use in wastewater treatment applications.

Fig. 9. Langmuir graphs of CV uptake by RBPW and ABPW

Fig. 10. Freundlich graphs of CV uptake by RBPW and ABPW

Fig. 11. Dubinin–Radushkevich graphs of CV uptake by RBPW and ABPW

Table 2. Isotherms Parameters of CV Adsorption onto RBPW and ABPW

Table 3. Comparative of Different Adsorbents Capacities with Banana Peel Waste for CV Adsorption

Thermodynamics of Crystal Violet Adsorption

Thermodynamic information was obtained from free energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°) using equations 14, 15, and 16 (Edet and Ifelebuegu 2020; Ofudje et al. 2024),

where T is temperature in Kelvin unit (K) and the universal gas constant in 8.304 J mol⁻¹K⁻¹ is denoted by R. Figure 12 was used to compute the values of parameters listed in Table 4. The Gibbs free energy gave negative values (ΔG°) across all temperatures for both RBPW (–0.25 to –1.25 kJ/mol), and ABPW (–0.43 to –2.03 kJ/mol) is an indication that CV adsorption is spontaneous (Azhar‑ul‑Haq et al. 2022). The trend toward more negative ΔG° as temperature increases underscores the favorability of adsorption at higher temperatures (Edet and Ifelebuegu 2020). The positive enthalpy changes (+16.35 kJ/mol for RBPW and +53.46 kJ/mol for ABPW) indicate the endothermic nature of the adsorption (Dada et al. 2012). These findings are consistent with the outcome of the findings reported by Azhar‑ul‑Haq et al. (2022) where the uptake of CV by banana peel–based adsorbent gave ΔG° to be negative, and ΔH° as positive, indicating spontaneous, endothermic adsorption. Adsorption enthalpies smaller than 40 kJ·mol⁻¹ are classified as physisorption, whereas those with values greater than this are said to be an indicative of chemisorption (Girish 2025). In this study, the ΔH values from RBPW and ABPW suggest physisorption and chemisorption/ion-exchange respectively.

Fig. 12. Thermodynamic graphs of CV uptake by RBPW and ABPW

Literature supports this with Gemici et al. (2023) reporting a similar trend of endothermic adsorption when studying chemically modified lignocellulosic wastes for CV removal. The entropy change analysis showed that adsorption of CV onto both raw and acidified banana peel waste was accompanied by an increase in randomness, which becomes more pronounced with acid treatment (RBPW = 6.79 J/mol·K and ABPW = 14.65 J/mol·K). Acidification possibly removed surface impurities or modified surface charges, leading to better dye dispersion and less structured adsorption layers. This enhanced entropy effect contributes to the greater spontaneity and thermodynamic favorability of dye removal using ABPW compared to RBPW.

Table 4. Values of Thermodynamics Constants for the Sorption of CV by Raw and Acidified Banana Peel Waste

Fourier Transform Infra-Red Analysis

The Fourier transform infrared (FTIR) spectra of banana peel waste before and after sorption of CV is provided in Fig. 13, which gave valuable insights into the functional groups involved in the sorption process and confirm the interaction between the dye molecules and the adsorbent surface. Before sorption, a broad, strong peak indicating hydroxyl groups (–OH), typical of cellulose, hemicellulose, and lignin in banana peels was seen at 3350 to 3565 cm⁻¹ (Popoola et al. 2024). After sorption, this peak shifted to 3325 to 3478 cm⁻¹ and became reduced in intensity, suggesting that –OH groups are likely involved in hydrogen bonding or electrostatic interactions with CV dye molecules. The peak found around 2965 cm⁻¹, which is attributed to C–H, is present in both spectra but showed a minor shift and intensity change, indicating that aliphatic chains may experience minor interactions during dye binding. The band near 1730 cm⁻¹, which is assigned to C=O stretching of carboxylic acids or esters in the sample before adsorption, was shifted to 1745 cm^-1 in the sample after adsorption of CV (Popoola et al. 2024). The prominent bands at 1685 cm⁻¹ and 1466 cm⁻¹ indicating the presence of aromatic C=C stretch before adsorption also were shifted to 1674 cm⁻¹ and 1457 cm⁻¹ after the adsorption process. These clear shifts and reduced intensity after sorption are likely indicative of the involvement of aromatic rings group, supporting π–π interactions and/or electrostatic attraction between CV and the banana peel surface. The peak seen at 1150 cm^-1 in the banana peel before CV adsorption was reduced to 1143 cm^-1, indicating the involvement of C-O stretch.

The –OH stretching band, which shifted from 3350–3565 cm⁻¹ to 3325–3478 cm⁻¹, the C=O band that moved from 1730 cm⁻¹ to 1745 cm⁻¹, and the aromatic C=C bands that shifted from 1685 and 1466 cm⁻¹ to 1674 and 1457 cm⁻¹ suggest possible involvement of hydroxyl, carbonyl, and aromatic groups in the adsorption of CV molecules.

Fig. 13. FTIR graphs of RBPW before and after adsorption of CV

Scanning Electron Microscopic Evaluation of Banana Peel Waste

The scanning electron microscopic (SEM) micrographs labeled as a and b in Fig. 14, represent the surface morphologies of banana peel waste before and after adsorption of CV dye, respectively.

Fig. 14. SEM graphs of RBPW before and after adsorption of CV

Before adsorption of CV, the surface of the banana peel waste appeared porous, rough, and irregular. Some cavities and channels were clearly visible, suggesting a highly porous structure with a high surface area suitable for capturing dye molecules from aqueous solutions. After adsorption, the surface had undergone a noticeable transformation with the texture becoming more compact and smoother. The porous and irregular morphology observed in SEM images is consistent with the BET results, which indicated that acid treatment increased the surface area and also enhanced pore volume. Similarly, the average pore size increased after acid-treatment, indicating the development of a more mesoporous structure. These quantitative values corroborate the visual evidence from SEM, confirming that acid treatment improved porosity and pore connectivity, which contributed to the higher adsorption capacity of ABPW.

Physicochemical Properties of Raw and Acidified Banana Peel Waste

The physicochemical characteristics of RBPW and ABPW revealed significant improvements following acid treatment, as shown in Table 5. One of the most notable changes was the increase in surface area, from 24.7 m²/g for RBPW to 74.2 m²/g for ABPW. This suggests that acidification effectively removed surface impurities, exposing more active sites for adsorption. As a result, the acidified material offered a much larger surface for pollutant interaction, directly contributing to improved adsorption capacity. Similarly, the pore volume increased from 0.32 cm³/g in RBPW to 0.87 cm³/g in ABPW, further demonstrating the structural benefits of acid treatment, thus providing more or larger pores, allowing better accessibility and retention of dye molecules.

The average pore size also increased slightly from 2.15 nm to 2.86 nm, indicating a shift toward mesoporous structure which could enhance the material’s ability to adsorb larger molecules, as mesopores are more favorable for liquid-phase adsorption compared to micropores. A study by Huang et al. (2024) noted that treated banana peel powder with 50% H₂SO₄ at 50 °C for 24 h, resulted in an increase in surface area from 0.036 to 0.0507 m²/g, which resulted in impressive Cr(VI) adsorption capacity of 161 mg/g. They further reported that the acid treatment improved porosity, which facilitated pollutant uptake. This is consistent with the changes in surface area and pore volume seen in the current study. A similar study by Channei et al. (2025) compared H₂SO₄ and CH₃COOH treatments on banana peel and found that strong acid (H₂SO₄) produced the highest BET surface area (~339 m²/g) and pore volume (~0.059 cm³/g), while acetic acid yielded moderate increases (~202 m²/g and 0.041 cm³/g), which is closely related to the increase in surface area and pore volume seen in our study after acidification.

Table 5. Physicochemical Properties of Raw and Acidified Banana Peel Waste

The observed increase in surface properties after acid treatment can be partly attributed to structural changes caused by the removal of mineral components. In particular, calcium ions bound to carboxyl groups in the lignocellulosic matrix of raw banana peel may act to hold the structure together more tightly. The acid treatment could displace these ions, loosening the structure and exposing more pores, which in turn improves the surface chemistry and adsorption capacity.

CONCLUSIONS

From this study on the use of raw and acidified banana waste peel as adsorbent for crystal violet removal, the following conclusions were made:

1. The acidified banana peel achieved higher adsorption capacities in a shorter time frame of 120 min, whereas the raw peel required more time to approach similar capacities and reached equilibrium at 140 min.
2. The positive enthalpy changes (+16.4 kJ/mol for raw banana peel waste (RBPW) and +53.5 kJ/mol for acid-treated banana peel waste (ABPW)) indicate the endothermic nature of the adsorption, while positive entropy change (ΔS°) values of +6.79 J/mol·K for RBPW and +14.65 J/mol·K for ABPW suggests increased randomness at the adsorbent–solution interface.
3. Fourier transform infrared (FTIR) analysis indicates the presence of –OH, –C=O, –C–O, and aromatic moieties and shows some spectral shifts after sorption process,
4. The acid treatment enhanced the surface area, porosity, and surface chemistry of banana peel waste, making ABPW a far more effective adsorbent than its raw counterpart. These improvements make it suitable for a wide range of applications in water and wastewater treatment, particularly for removing cationic contaminants.

ACKNOWLEDGMENTS

The authors express their gratitude to Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2025R581), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

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Article submitted: July 21, 2025; Peer review completed: August 24, 2025; Revised version received: August 26, 2024; Accepted: October 13, 2025; Published: October 23, 2025.

DOI: 10.15376/biores.20.4.10640-10664