Phosphogypsum Waste Anion Removal by Soil Minerals

The removal of F-, PO4-3, and SO4-2 from phosphogypsum leachate (pH 1.7) by soil minerals was investigated using a serial batch procedure. PH was the controlling factor in removal of orthophosphate and fluoride, but had less effect on sulfate. Calcium carbonate precipitated major fractions of fluoride and orthophosphate, and smaller amounts of sulfate, during the early stages of leaching; with continued leaching, redissolution of fluoride and orthophosphate occurred, resulting in release of these anions at concentrations exceeding those in the original phosphogypsum leachate. Removal of fluoride and orthophosphate by kaolinite and montmorillonite was strongly affected by pH, due to complex formation, degree of ionization, and charge on the clays. Iron (III) hydroxide had little effect on fluoride or orthophosphate removal due to formation of Ca‐, Al‐, and Fe‐complexes which possibly interfered with adsorption. The iron hydroxide was important to sulfate removal at all pH values; the removal was markedly enhanced by the presence of calcium, presumably due to formation of calcium phosphate complexes that prevented orthophosphate competition with sulfate for adsorption sites.