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

Abstract

ABSTRACT 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 pHPZC (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.