Supplementary Information for "<b>Optimized Fluoride Adsorption from Water Using Functionalized Rice Straw: A Response Surface Methodology Approach</b>"

Fluoride contamination in drinking water offers severe health risks, demanding the development of low-cost and sustainable remediation strategies. In this study, rice straw (RS), an abundant agricultural residue, was sequentially modified by alkaline pretreatment and tetraethyl orthosilicate (TEOS) functionalization to enhance fluoride adsorption. Characterization by FTIR, SEM, and EDX confirmed hemicellulose removal, silica grafting, and the existence of functional groups favorable for fluoride binding. Batch adsorption experiments were optimized using response surface methodology (RSM), which identified pH, contact time, and adsorbent dose as the most significant parameters. Fluoride removal reached its peak efficiency (~65%) under acidic conditions. The adsorption equilibrium aligned closely with the Langmuir isotherm (qₘₐₓ = 100 mg/g, R² = 0.98), signifying monolayer adsorption on a uniform surface. In contrast, the Freundlich and Temkin models provided less accurate fits. Kinetic analysis indicated that the pseudo-second-order model (R² = 0.99) most effectively represented the fluoride adsorption process, pointing to chemisorption as the primary mechanism, supported by multi-step intraparticle diffusion. The proposed mechanism involves rapid physisorption on external sites, followed by chemisorption and gradual pore diffusion. These findings highlight functionalized rice straw as a cost-effective, sustainable biosorbent with potential application in decentralized water treatment systems.