Calcined and uncalcined carbonate layered double hydroxides for possible water defluoridation in rural communities of the East African Rift Valley

Health risks linked to the regular consumption of water with high fluoride (F-) content seriously affect rural areas of the East African Rift Valley. The F- removal capacity has been tested on uncalcined and calcined carbonate layered double hydroxides (LDHs), with different cationic compositions (M2+ = Mg2+, Zn2+; M3+ = Al3+, Fe3+) and M2+/M3+ molar ratio (2, 3, 4), taking into account the necessity of a simple defluoridation method. The 3MgAlFe-cal phase derived from calcination of a hydrotalcite-like compound with composition Mg/(Al + Fe) = 3/(0.5 + 0.5) results to be the best F- remover (43 mg/g) and can uptake up to 92.3 mg/g when the interlayer is fully saturated with F-. At the high pH reached during the experiments (up to pH 12 for calcined LDHs), the presence of carbonate species in solution significantly affects the F- removal capacity. The recyclability of the sorbent was tested by regeneration through calcination of the 3MgAlFe-cal phase: under F- unsaturated interlayer conditions, up to 80% of the starting F- removal capacity is still preserved after four regeneration cycles, indicating that LDHs can be reused after regeneration and have a potential use in water defluoridation.