Fluoride retention kinetic and equilibrium studies on layered double hydroxides under ambient conditions: Implications on pond-stream-hot spring-well water remediation.

Abstract

Fluoride deficiency and toxicity severely affect a large population globally. Hence, a low-cost geosorbent is in demand to overcome fluorosis hazards where in situ retardation is prioritized over pilot-scale waste water treatment. This study reports the fluoride removal potential of MgFe-type layered double hydroxide (HT-LDH) and its calcined form at 500 and 800°C for their usability for treatment of polluted streams, ponds, wells, and hot spring water. Rapid uptake with >33% removal in 24 h was found by the adsorptive method, whereas the co-precipitation process removed >16 mg/L (>83%) in 1 h. The efficiency was further enhanced upon calcination at 500°C with >95% removal up to five times regeneration, unlike that at 800°C. It was demonstrated that multilayer sorption onto heterogeneous surface sites is majorly controlled by surface adsorptive and ion exchange mechanisms. Acidic pH, low aqueous , and temperature >25°C favored greater uptake, whereas competitive anions slightly enhanced its potential in the order in adsorptive removal. In the co-precipitation process, competing anions slightly hindered retention, whereas high temperature and low aqueous led to greater retention. No F-bearing solid phases were evidenced upon its retention, except lowering the HT-LDH crystallinity and rearrangement of surface functional groups. The fluoride contents of natural water were lowered drastically up to 77% in 60 min upon precipitation of HT-LDH by maintaining alkalinity and a di-/trivalent cationic ratio of 2.0. PRACTITIONER POINTS: F^- uptake is rapid on heterogeneous surface sites with multilayer sorption mechanism Greater F^- removed by coprecipitation technique compared with adsorptive pathway Acidic medium and temperature >25°C favor greater F^- retention NO_3^-+PO_l4^(3-) slightly enhanced F^- uptake by adsorption but lowered in coprecipitation Formation of LDH from natural waters lowered F^- content up to <77.