Optimised fluoride adsorption from water using functionalised rice straw: a response surface methodology approach

Rice straw’s biosorbent potential for fluoride remains underexplored. Few studies explored alkali pretreatment with TEOS functionalization, leaving clear gap, how dual modification boosts reactivity. In this work, rice straw is alkali-pretreated and TEOS-functionalised, producing silica-rich surface enhancing fluoride affinity. FTIR, SEM, and EDX confirmed hemicellulose removal, silica grafting, and adsorption‑enhancing structural changes. A Central Composite Design-based RSM framework comprising 30 batch experiments (50 mL per run) identified pH, dosage, and contact time as the most influential factors. Under optimised acidic conditions, the modified adsorbent achieved ~65% fluoride removal, with equilibrium behaviour following the Langmuir isotherm (qₐₓ = 100 mg g−1, R2 = 0.98) and kinetics governed by the pseudo-second-order model (R2 = 0.99), indicating a chemisorption-dominated, multi-step mechanism. This performance surpasses that of many biomass-derived adsorbents such as wheat straw (40.2%), sawdust (49.8%), activated bagasse carbon (56.4%), carbonised activated carbon (57.6%), and coffee-ground biochar (20.62%) which typically achieve only 20–60% fluoride removal under comparable conditions. FTIR, EDX, and storage observations confirmed TEOS‑modified rice straw remained structurally, functionally stable before and after adsorption. Overall, this study demonstrates a chemically enhanced, statistically optimised biosorbent from agricultural waste, mechanistically validated for integration into decentralised defluoridation units and deployment in fluoride‑affected regions.