Utilization of Thermally Modified Water Treatment Plant Sludge as an Adsorbent for Fluoride Removal

ABSTRACT

The presence of fluoride (F ⁻ ) at high concentrations in drinking water is a global concern due to its adverse health effects. This study evaluated the removal of F ⁻ ions by adsorption using an adsorbent produced from thermally modified water treatment plant sludge (TWTPS), providing an efficient and low‐cost alternative for contaminated water treatment and residue valorization. Initially, the physical and chemical changes in the sludge after thermal treatment were characterized. Subsequently, batch adsorption tests were conducted at 24°C ± 1°C to investigate the effects of pH, F ⁻ concentration, and adsorbent dosage, as well as to determine the adsorption kinetics and capacity of the produced material. An equilibrium contact time of 24 h was adopted for the pH, dosage, and isotherm experiments. Characterization analyses, including surface area, pore size distribution, and functional group identification, confirmed the suitability of the thermally treated sludge as an adsorbent for F ⁻ removal. Optimal conditions were obtained at pH 4.0–6.0 and 2 g L ^{− 1} adsorbent dosage, yielding adsorption capacities of 0.600–0.634 mg g ^{− 1} . Kinetic data followed the pseudo‐second‐order and Elovich models, suggesting chemisorption through substitution of surface –OH groups by F ⁻ ions. Equilibrium data fitted both Langmuir and Freundlich isotherms, indicating adsorption on a heterogeneous surface (0 <  n ⁻¹  < 1) and a favorable process (0 <  R _L  < 1), with a maximum adsorption capacity ( q _max ) of 3.19 mg g ^{− 1} . Thermodynamic parameters confirmed that F ⁻ adsorption onto the modified sludge was spontaneous and endothermic.