Sustainable fluoride removal from water using banana leaf-derived adsorbents

Introduction

Fluoride is a native element commonly present in several aquifers such as groundwater, surface water and even certain bottled waters. Its presence in aquatic domains predominantly springs from the regular weathering and dissolution of fluoride-rich minerals like fluorite and apatite (Onipe et al., 2021) [21] . Fluoride burden in groundwater is fundamentally of geogenic origin, resulting from the interaction of water with fluoride-laden rocks, comprising granites rich in hornblende, biotite and muscovite near volcanic regions (Hanse et al., 2019) [8] . Groundwater carries fluoride through pore spaces within these rocks, with dissolution, ion exchange and desorption processes playing crucial roles in fluoride convemimg. Further human activities can exacerbate fluoride levels in water bodies. The chemical characteristics of aquifers, particularly the weathering of silicate minerals, nitrate oxidation and sulfate dissolution, contributing to the natural enrichment of fluoride (Yidana et al., 2012) [30] . The interplay of hydrological cycle significantly impacts fluoride concentrations, with stagnant or slow-moving water bodies often evincing elevated fluoride levels due to limited dilution. In addition to aqeous sources, fluoride can accumulate in crops, fruits and animal-derived food products through soil absorption or consumption of contaminated fodder (Han et al., 2021). In affected areas, excessive fluoride intake may lead to severe health complications, including dental and skeletal fluorosis. The World Health Organization (WHO, 1997) emphasized the narrow limit between beneficial and harmful fluoride levels in drinking water, recommending a maximum permissible limit of 1.5 mg/L to avoid adverse effects (Amereih et al., 2013) [1] . Depending on the fluoride concentration, water quality can be grouped as very good (<1 mg/L), moderate (1-1.5 mg/L), or very poor (>1.5 mg/L), corresponding with the standards set by WHO (1997). Several analytical techniques have been engaged for detection of fluoride levels in water. Colorimetric test strips, widely used For initial screening, provide a visual estimation of fluoride levels by comparing color changes with a reference scale (Barghouthi and Amereih, 2017) [3] . Ion-selective electrodes (ISEs) are picked due to higher accuracy, although they are relatively expensive (Mondal, 2021) [16] . The process of reducing superfluous fluoride content from drinking water, known as defluoridation, is essential in regions facing elevated natural fluoride burdens or human-induced pollution. In fluoride endemic regions, instant execution of defluoridation strategies is imperative (Wang et al., 2023) [27] . The Nalgonda technique, a widely adopted communitylevel solution in India developed by NEERI, involves chemical precipitation using aluminum sulfate and lime, with or without phosphate additives (Nawlakhe and Paramasivam, 1993) [20] . Advanced procedures like reverse osmosis (RO) and nanofiltration (NF) utilize semipermeable membranes to physically separate fluoride based on ionic size and charge, whereas electrodialysis (ED) uses electrical potential to achieve ionic separation (Jagtap et al., 2012) [9] . Ion exchange and adsorption processes are also employed, using specially designed resins or contact beds to selectively extract fluoride ions from water (Yang and Dluhy, 2002; Jamwal and Slathia, 2022) [11,29] . Bioadsorption has appeared as a particularly attractive approach due to the abundance, restorability and low toxicity of natural biomaterials. Hydrogel beads containing Zingiber officinale rhizome extract have achieved up to 98.5% removal efficiency (Srivastava et al., 2020) [25] , while Elettaria cardamomum and Coriandrum sativum seeds also exhibit significant adsorption potential (Santhi et al., 2005; Moradi et al., 2020) [18,23] . Powdered leaves of Ocimum sanctum and ash from Syzygium cumini have shown significant fluoride binding capacities, with optimal performance observed at pH ranges near neutrality (Bayu et al., 2022; Tirkey et al., 2020) [4,26] . The banana plant (Musa acuminata) holds particular interest as a bioadsorbent due to its global availability and complex applications. Banana peels, rich in starch, lignin and protein, serve as compelling natural adsorbents for fluoride removal (Fadaei, 2019) [6] . These biopolymers increase fluoride uptake through surface interactions and ion exchange. Banana leaves exhibit notable antioxidant activity, due to the presence of polyphenol oxidase and flavonoids, which protect the plant against oxidative stress (Karpagavalli et al., 2023) [12] . Biochar derived from agricultural waste is another sustainable alternative for fluoride adsorption. Compared to conventional activated carbon, biochar is less energyintensive and utilizes renewable feedstocks. Biochars made from coffee grounds, rice husks and banana leaves offer promising results due to their high surface area, porosity and functional group diversity, which facilitate effective fluoride binding (Santos et [13,14,24] . These findings highlight the potential of natural and modified bioadsorbents as viable solutions for fluoride remediation, aligning with global efforts toward eco-friendly and cost-efficient water purification technologies.

Materials and Methods

Place of experiment

The study was carried out in the Animal Health Laboratory of I.C.A.R.-R.C.E.R., Patna.

Study design

Fluoride adsorption was studied within batch experimental settings to determine the capability of Banana leaf powder and biochar for defluorination. For control purposes, distilled water (100 mL) was served as a negative control while alum was employed as a positive control at 300 ppm concentration. The fluoride containing water in the group subjected to treatment involved administering different doses of banana leaf powder and biochar at concentration of 1 × 10⁴ to 20 × 10⁴ ppm as per the procedure outlined by Mondal (2017) [17] . All treatments were conducted at an established contact period of 60 minutes to allow the adsorption of fluoride ions on the surfaces of the absorbents. The amount of fluoride ions in the treated water samples was examined both prior and after carrying out the process using a fluoride selective electrode (ISE). The effectiveness of each treatment was examined by comparing the quantities of fluoride remaining with their original concentrations, and calculation of the removal percentage of fluoride and adsorption capacity was done. The experimental groups and treatment parameters included in the study are summarized in Table 1.

Collection of sample

Approximately 1 litre of water was collected in separate clean plastic bottles from five distinct water sources located in Sheikhupura.

Measurement of fluoride

Fluoride concentration in serum and urine was quantified using an ion-selective electrode (ISE) method (Bard and Faulkner, 2001) [2] . The system employed an Orion 96-09 fluoride-selective electrode and mV meter (Thermo Fisher Scientific, USA). To ensure consistent ionic strength and pH, all standards and samples were mixed with Total Ionic Strength Adjustment Buffer (TISAB) in a 1:1 ratio. A calibration curve was generated using sodium fluoride standards (0.1-20 µg/mL), and sample readings were taken after 1-2 minutes of stabilization.

Collection of banana leaves

Raw waste banana leaves (Musa acuminata) were collected from the ICAR-Research Complex for Eastern Region (ICAR-RCER), Patna Campus.

Procurement of alum

The alum used in this study was procured from local market.

Preparation of banana leaf powder (BLP)

Raw banana leaves (Musa acuminata) were washed sequentially with tap and deionized water to remove adhering impurities. The leaves were dried in a hot-air oven at 50 °C for 12 h, followed by drying at 60 °C for 24 h to ensure complete moisture removal. The dried material was ground into a fine powder using a grinder and stored in airtight containers until further use (Mondal, 2017) [17] .

Preparation of banana leaf biochar (BLB)

Washed and oven-dried banana leaves (Musa acuminata) were subjected to controlled pyrolytic combustion at 450-600 °C on a meshed surface. The combustion process was rapidly quenched with water to prevent further oxidation, and the resulting biochar was separated from ash and airdried for 12 h. The dried biochar was ground into a fine powder and stored in airtight containers for subsequent analyses (Chun et al., 2021) [5] .

Measurement of pH

The pH of banana leaf powder and biochar was determined following the method of Malik et al. (2015) [15] . Briefly, a 1:10 (w/v) suspension of the material in distilled water was prepared and allowed to equilibrate for 30 min. The pH of the supernatant was then measured using a calibrated digital pH meter.

Determination of point of zero charge (pHₚzc)

The pHₚzc was determined using the solid addition method described by Mosoarca et al. (2022) [19] . Samples (0.1 g) were added to 50 mL of 0.1 M KNO₃ solution, and the initial pH was adjusted between 2 and 12 using HNO₃ or KOH. After 24 h of shaking, the final pH was recorded and pHₚzc was determined from the plot of ΔpH versus initial pH, where ΔpH = 0.

Measurement of adsorption efficiency

Batch adsorption experiments were conducted in 250 mL conical flasks containing fluoride solutions (1.67-3.4 μg/mL) with adsorbent dosages ranging from 10⁴ to 2 × 10⁶ ppm per 100 mL. The pH was adjusted using 0.1 N HCl or NaOH, contact time was maintained at 60 min, and experiments were performed at 300-333 K under constant shaking. Residual fluoride concentration was quantified using a fluoride ion-selective electrode (Orion 96-09, Thermo Fisher Scientific) after 1:1 dilution with TISAB, and fluoride removal efficiency and adsorption capacity were calculated according to Gul et al. (2023).

Fourier transform infrared spectroscopy (FTIR) analysis

Fourier Transform Infrared Spectroscopy (FTIR) was employed to characterize the surface functional groups of banana leaf powder and biochar. Samples were prepared as KBr pellets and scanned over a spectral range of 4000-400 cm⁻¹ using a PerkinElmer FTIR spectrometer. The analysis was conducted at the Department of Biotechnology, National Institute of Pharmaceutical Education and Research (NIPER), Hajipur, to identify functional moieties involved in fluoride adsorption.

Statistical analysis

The comparative analysis of fluoride adsorption capacities among various bioadsorbents was conducted using Microsoft Excel.

Results

Characterization of bioadsorbents

The physicochemical characteristics of Musa acuminata leaf powder and biochar were evaluated through pH, pHₚzc, and FTIR analyses to determine their suitability as fluoride adsorbents. The pH provides information on the surface acidity/alkalinity of the adsorbents, while pHₚzc indicates the pH at which the net surface charge becomes neutral, influencing fluoride adsorption behavior. FTIR analysis was performed to identify the surface functional groups involved in adsorption mechanisms. The characterization parameters of the bioadsorbents are presented in Table 2.

FTIR analysis

The FTIR spectrum of banana leaf powder revealed prominent absorption bands at 3401.11, 2995.97, and 2919.52 cm⁻¹, corresponding to O-H stretching vibrations of hydroxyl groups, indicating the presence of polar functional moieties capable of participating in hydrogen bonding and fluoride adsorption. Additional peaks observed at 1647.10 and 1484.99 cm⁻¹ were attributed to O-H bending vibrations, suggesting the abundance of oxygen-containing functional groups on the adsorbent surface that may contribute to fluoride binding. Similarly, the FTIR spectrum of banana leaf biochar exhibited characteristic peaks at 3430.08, 2977.48, and 2921.44 cm⁻¹, corresponding to O-H stretching vibrations associated with hydroxyl-containing surface functionalities. Bands observed at 1615.00 and 1429.08 cm⁻¹ were assigned to O-H bending vibrations, indicating the retention of oxygenated functional groups following pyrolysis. The presence of these hydroxyl-rich surface groups suggests their potential involvement in fluoride adsorption through electrostatic interactions, hydrogen bonding, and surface complexation mechanisms.

Adsorption of fluoride

The Adsorption Efficiency (AE) of banana leaf powder, banana leaf biochar, and alum is presented in Table 2, and their fluoride removal performance is illustrated in Figure 3. Banana leaf biochar consistently exhibited higher fluoride adsorption than banana leaf powder across all tested dosages. At 1 × 10⁴ ppm, biochar achieved 11.38% fluoride removal compared to 0.00% for leaf powder, while at 5 × 10⁴ ppm it was approximately ten-fold more effective (8.00% vs. 0.80%). At higher dosages (10⁵-2 × 10⁵ ppm), biochar maintained 1.3-1.7 times greater removal efficiency than leaf powder. Overall, banana leaf biochar demonstrated superior defluoridation potential, although the increase in adsorption capacity diminished with increasing adsorbent dose. The adsorption efficiency of different adsorbents used in the study are listed in Table 3.

Discussion

Defluoridation of drinking water is essential in fluorosisendemic regions to reduce fluoride exposure and associated health risks. Although conventional methods such as the Nalgonda technique, which employs alum and lime for fluoride precipitation, are widely used in India, they often involve chemical inputs and sludge management issues (Nawlakhe and Paramasivam, 1993) [20] . Consequently, bioadsorbents have gained attention as economical, ecofriendly, and readily available alternatives for fluoride removal (Yihunu et al., 2019) [31] . In the present study, banana (Musa acuminata) leaf powder and biochar were evaluated as bioadsorbents for fluoride removal. Banana leaf biochar consistently exhibited higher adsorption efficiency than banana leaf powder across all tested dosages, indicating that pyrolysis substantially enhanced the fluoride adsorption potential of the biomass. Similar applications of banana-derived adsorbents have been reported previously by Mondal (2017) and Kumar et al.

(2021) [14,16] . The superior performance of banana leaf biochar can be attributed to its favorable physicochemical properties. Biochar exhibited a higher pH (9.08) than banana leaf powder (6.36), creating conditions that favor fluoride adsorption (Jain and Singh, 2014) [10] . In addition, the higher adsorption efficiency and adsorption capacity observed for biochar indicate a greater affinity for fluoride ions over the entire dosage range tested. FTIR analysis further revealed the presence of abundant hydroxyl and other oxygen-containing functional groups on both adsorbents, with biochar exhibiting more prominent absorption bands associated with surface hydroxyl functionalities. These groups provide active adsorption sites and facilitate fluoride binding through hydrogen bonding, electrostatic interactions, and surface complexation mechanisms (Mondal, 2021) [16] . Furthermore, the pyrolysis process enhances the porosity and surface area of biochar, increasing the availability of adsorption sites and improving fluoride uptake (Santos et al., 2023) [24] . Overall, the results demonstrate that banana leaf biochar is a more effective fluoride adsorbent than untreated banana leaf powder and has considerable potential as a sustainable, lowcost material for water defluoridation.

Conclusion

The present study demonstrates that Musa acuminata leafderived bioadsorbents can effectively remove fluoride from contaminated water, with banana leaf biochar exhibiting substantially higher adsorption efficiency than banana leaf powder. The enhanced performance of biochar is attributed to its favorable surface chemistry, greater abundance of oxygen-containing functional groups, and improved porosity generated during pyrolysis. These findings highlight the potential of converting agricultural waste into value-added bioadsorbents for sustainable water treatment applications. Given its low cost, environmental compatibility, and superior fluoride removal capacity, banana leaf biochar may serve as a promising alternative to conventional defluoridation methods, particularly in rural and resourcelimited settings. Further studies should focus on optimizing operational parameters, regeneration potential, adsorption kinetics, and field-scale applicability to facilitate its practical implementation in community-based defluoridation systems.

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