Groundwater defluoridation efficacy of manganese-oxide-coated alumina prepared via two-step heating

Fluoride concentrations in groundwater can be high in some Brazilian aquifers and therefore these waters should be treated before consumption. This study assessed the properties of Mn-oxide-coated alumina (AM) prepared by two-step heating in water defluoridation. The release of secondary contaminants (e.g. Al³⁺ and Mn²⁺) from alumina was also examined, as their removal by vermiculite. The process of Mn-oxide coating changed some properties of the activated alumina (AA), decomposing the crystalline phases and reducing some parameters, e.g. specific surface area (from 295.90 to 94.51 m² g^-1) and pH_PZC (from 7.34 to 5.74). These changes increased the efficiency and kinetics of alumina in removing F^- from synthetic solutions and groundwater (from 80%/16 h to 100%/1 h). This efficiency was not affected by the presence of other anions in groundwater, such as HCO₃^- and SO₄^2-. The optimum rate of F^- removal occurred at pH 5; however, during the F^- removal, Al³⁺ and Mn²⁺ ions were released, respectively, from the AA (0.61 mg L^-1 Al³⁺) and from the AM ( 52 mg L^-1 Mn²⁺). Vermiculite used to remove these cations adsorbed about 86% Al³⁺ and 90% Mn²⁺. However, only Al³⁺ concentrations fell below the standard limit for drinking water of <0.5 mg L^-1. Therefore, AA has the advantage of not containing Mn, and after 3 h kept F^- concentrations in solutions 5 mg L^-1F^- below the standard limit of 1.5 mg L^-1. This study revealed that, depending on the groundwater characteristics, AA may be more efficient and sustainable for defluoridation than coated alumina.