Abstract
Fluoride contamination of rural groundwater poses a significant public health risk in South Africa, where seasonal monitoring recorded concentrations of 5.8–7.1 mg/L, exceeding the WHO guideline of 1.5 mg/L. This study valorised raw sugarcane bagasse (Saccharum officinarum) as a low-cost biosorbent for defluoridation, optimised using Response Surface Methodology with a Box-Behnken Design (RSM-BBD). FTIR spectroscopy confirmed –OH and –COOH surface groups as primary fluoride binding sites, SEM-EDX revealed a porous morphology with a BET surface area of 4.1 m²/g and pore volume of 0.01 cm³ /g. The point of zero charge (pHPZC = 6.78) confirmed a net negatively charged surface at the operating pH of 7.2–7.8, indicating ligand exchange as the dominant uptake mechanism. A 17-run BBD varying initial fluoride concentration (4–8 mg/L), adsorbent dosage (5–15 g), and temperature (20–30 °C) yielded a significant quadratic model (F = 4.06, R² = 0.92, lack-of-fit p = 0.1710). Optimal conditions of 5.767 mg/L, 10.577 g, and 24.863 °C produced a predicted removal of 89.28%, confirmed experimentally at 88.61% (error = 0.75%), yielding a treated fluoride concentration of 0.76 mg/L from a worst-case inlet of 7.1 mg/L. The Langmuir isotherm provided the best equilibrium fit (R² = 0.993; qm = 6.31 mg/g; RL = 0.050–0.347) over Freundlich (R² = 0.943). Thermodynamic analysis confirmed spontaneous, endothermic adsorption across 15–35 °C. The Weber–Morris intraparticle diffusion model best described the rate data (R² = 0.979), indicating a two-stage boundary-layer and pore-diffusion process; between the reaction-order models, PFO (R² = 0.967) outperformed PSO (R² = 0.877) on R² and RMSE. Desorption screening identified 0.5 M NaOH as the optimal eluent (85% recovery), with performance confirmed over five regeneration cycles. Raw sugarcane bagasse is a viable, low-cost biosorbent for community-scale defluoridation under ambient conditions, supporting SDGs 3, 6, and 12.
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•Seasonal groundwater fluoride levels (5.8–7.1 mg/L) defined RSM-BBD optimisation range.•RSM-BBD optimised fluoride concentration, bagasse dosage, and temperature.•Optimal conditions (5.767 mg/L, 10.577 g, 24.863 °C) achieved 88.61% removal.•Fluoride concentration decreased from 7.1 to 0.76 mg/L, below WHO guidelines.•Bagasse biosorbent shows potential for low-cost, gravity-fed rural water treatment.