Zirconium-functionalized microporous hybrid hydrogel beads for fluoride sequestration from aqueous media

Performance of microporous zirconium-loaded chitosan hybrid hydrogel beads (H-HGB) is investigated for its de-fluoridating capacity from drinking water. The adsorbent was prepared by ionotropic gelation method and characterized using SEM, FTIR, XRD, BET, TG/DTA and EDX. The H-HGB worked efficiently at pH range 2–4 with a maximum removal percentage of 70% at pH 2 and a contact time of 120 min. The point of zero charge was determined to be 7.4. Anions like bicarbonate (HCO3−) and phosphate (PO43−) at concentrations above 100 mg/L showed a significant decline in fluoride adsorption by the hydrogel. Among cations, the presence of magnesium (Mg2+) lowered the adsorption capacity of H-HGB even at lower concentrations (30–100 mg/L). The experimental data fitted well with the Freundlich adsorption model, revealing a multilayer adsorption pattern and the maximum adsorption capacity was calculated as 24.2 mg/g. The swelling ratio demonstrated that the hydrogel beads exhibited rapid water absorption during the initial 20 min, achieving a swelling of 53%. The experimental data correlated well with pseudo-second order kinetics, suggesting that fluoride adsorption occurred by chemisorption. Using H-HGB for de-fluoridation is an economical and sustainable option as it can be reutilized up to 4 cycles after regeneration with 5M NaOH.