Influence of Mg components in hydroxylated calcined dolomite to (co-)precipitation of fluoride with apatites

Abstract

Hydroxylated calcined dolomite (HCD) is practically used as a mineralizer to immobilize fluoride by co-precipitation with hydroxyapatite (HAp) and precipitation as fluoroapatite (FAp). It has an advantage to remove fluoride without stagnation, which is always observed with Ca(OH)2 alone. HCD includes a certain content of MgO and Mg(OH)2 as well as Ca(OH)2. In the present work, the mechanism to explain the enhancing role of Mg components on removal kinetics of fluoride was aimed to elucidate. Nano-domain observation approaches were conducted by TEM when Ca(OH)2 is added as a Ca source in the presence of different Mg2+ concentrations, instead of HCD. With increase in Mg2+ concentrations in the presence of Ca(OH)2, removal rate of fluoride was enhanced. The enhancement was also observed with HCD. Mg2+ and [Mg(OH)]+ species, which can be derived from HCD, are adhered to the negatively charged c-plane of freshly formed amorphous HAp/FAp and CaCO3. As a result, the surface of Ca(OH)2 particles are gotten rid of hetero-coagulation with HAp/FAp and CaCO3 in the presence of Mg2+ and [Mg(OH)]+ species, and maintained to release Ca2+ and OH− ions, facilitating to precipitate HAp and FAp for immobilization of fluoride. Without Mg2+ additives the Ca(OH)2 surfaces are blocked by insoluble HAp/FAp through hetero-coagulation. This interpretation is also supported by TEM observation, which is well consistent with XRD results. The findings indicate that MgO and Mg(OH)2 contents in HCD influence the removal kinetics of fluoride by (co-)precipitation as apatites in industrial water treatment. Understanding the surface science in nano-domain is important in parallel reactions of dissolution and precipitation.