Potential groundwater contamination by fluoride from two South African phosphogypsums

Abstract

A study to investigate the fluoride content in two South African phosphogypsums, Kynoch and Omnia, and the potential threat to water sources was undertaken. Kynoch and Omnia phosphogypsums were found to consist of fluoride in the region of 0.12% and 0.03% by mass, respectively. The phosphogypsum samples were mixed with ZnO and anhydrous Na₂CO₃ and fluxes thereof prepared at 900^oC. The cooled melt was dissolved in deionised water and the fluoride content of each sample determined using the ISE method. These unwashed samples were compared with purified phosphogypsum samples, prepared through a leaching process using deionised water, lime or diluted sulphuric acid solutions. The fluoride content for melts of the purified samples was also determined by the ISE method. The amount of fluoride leached from Kynoch and Omnia phosphogypsums when treated with water was 9% and 22% respectively. Lime pretreatment of Kynoch and Omnia phosphogypsums leached out 37% and 6% of fluoride respectively. On the other hand, the diluted sulphuric acid solution leached out in excess of 70% of the original fluoride from both Kynoch and Omnia phosphogypsums. The presence of fluoride, a species of environmental concern, in South African phosphogypsum as indicated in this study, indicates the need to have waste disposal facilities constructed in such a manner that safeguards the environment. The study also indicates that regular monitoring of fluoride in the environment in the vicinity of phosphogypsum waste disposal sites has to be undertaken.


Key Words: Fluoride, Chemical gypsum, Phosphogypsum, Impurities


WaterSA Vol.30(4) 2004:465-468