Immobilization of fluoride in artificially contaminated kaolinite by the addition of commercial-grade magnesium oxide

Soil contamination and accompanied groundwater contamination by fluoride is becoming a serious environmental problem worldwide. In this study, a series of experiments were performed to understand the behavior of fluoride in the water-kaolinite-MgO system better with the final objective of evaluating the use of commercial-grade MgO as a fluoride immobilization agent. Leaching tests performed with leaching solutions of various pHs and ionic concentrations at a solid-liquid ratio of 1:10 (w/v) indicated that the addition of a sufficient amount of MgO effectively immobilized fluoride below 0.8 mg/L (the permissible level for leaching tests in Japan) against exposure to pH 4.0 acid rain for 100 years. Furthermore, experiments using diffusion cells demonstrated that the fluoride that desorbed from kaolinite by the addition of MgO (or rather, pH increase) was quickly re-sorbed by MgO. These experimental results demonstrated that the addition of commercial-grade MgO is quite an effective soil treatment for the quick immobilization of fluoride, and the effectiveness of the immobilization against exposure to acid rain is long-lasting, although the MgO dosage needs to be carefully determined, considering the diminished capability of kaolinite as a fluoride adsorbent at high pH conditions.