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.

(a) TEM images Mg-doped ferrihydrite, (b) TEM image fluoride loaded Mg-doped ferrihydrite (c) EDAX pattern of fluoride loaded Mg-doped ferrihydrite showing presence of fluoride.Highlights► High surface area nano powder of Mg-doped ferrihydrite was synthesized. ► Batch adsorption data for fluoride adsorption has been generated. ► Kinetic and isotherm models were applied to adsorption data. ► Thermodynamic parameters were evaluated. ► Regeneration studies were carried out and fluoride adsorption capacity of 64 mg/g was achieved.