Mg-doped nano ferrihydrite—A new adsorbent for fluoride removal from aqueous solutions

The present study evaluates synthesized Mg-doped nano ferrihydrite powder as an adsorbent for F− removal from aqueous solutions. High surface area Mg-doped ferrihydrite was prepared by co-precipitation method under controlled conditions. Samples were prepared by varying Mg content in the range of 0.39–1.12%. Preliminary test work revealed that under similar conditions, with the increase in Mg content from 0.39 to 0.98% in doped ferrihydrite, % F− adsorption increased from 66 to 91%. Hence this sample was characterized by XRD, TEM, SAED and TG–DTA. Batch adsorption experiments were carried out by varying contact time (30–480 min), initial pH (1.0–10), initial fluoride concentration (10–150 mg/L), adsorbent dose (0.5–4 g/L), temperature (20–45 °C) and Cl− or SO42− concentrations (nil to 50 mg/L). The results showed 0.98% Mg-doped ferrihydrite to be an excellent fluoride adsorbent giving maximum adsorption capacity of 64 mg/g. The time data fitted well to pseudo second order kinetic model. The isothermal data followed Langmuir model. Thermodynamic parameters confirmed the adsorption process to be spontaneous and endothermic. 89% of fluoride could be desorbed from loaded sample using 1 M NaOH.