Preparation and Evaluation of Monetite as a High-Capacity Adsorbent for Fluoride Removal from Drinking Water

Abstract

In an effort to further understand the fourfold higher fluoride removal capacity of chemically activated cow bone (CAB) previously reported compared to bone char, fundamental properties of CAB were evaluated in this study. The CAB media contained bassanite (CaSO4·0.5H2O) and monetite (CaHPO4) minerals that were not present in bone char. In this study, the effectiveness of these minerals for fluoride adsorption was evaluated. High-purity (99.6%) monetite was prepared in the laboratory and showed a threefold higher fluoride adsorption capacity than CAB (20.0 and 6.4 mg/g, respectively). The ethylene glycol monoethyl ether (EGME) specific surface area (SSA) of monetite was twice that of the CAB media (260 versus 134 m2/g) and thus may account for a portion of the threefold higher capacity of monetite versus CAB. The increased capacity of monetite can also be partly attributed to the high surface charge (zeta potential) on the monetite compared to CAB (27.1 and 7.5 mV, respectively, measured at pH 7). The high zeta potential suggests electrostatic adsorption as the fluoride removal mechanism. In contrast to monetite, the bassanite also present in CAB had negligible fluoride removal capacity. Therefore, monetite is found to be promising for fluoride removal to mitigate the negative health impacts of excess fluoride concentrations in drinking water.