Attenuation of cadmium, fluoride and uranium in phosphogypsum process water by calcareous soil

Abstract

Concerns about contaminant migration from phosphogypsum (PG) repositories prompted the present study. A sequential batch procedure, in which acidic process water (PW) from a phosphate fertilizer plant was reacted with samples of a calcareous till, was employed to determine the attenuation of environmentally sensitive elements by reaction with typical subsoils at PG waste repositories. PW leachates were monitored for total soluble fluoride (F) and pH during the experiment. Soil samples were analyzed for pH, F, cadmium (Cd), uranium (U), and calcium carbonate (CaCO3) equivalent at the end of the study. Leachate pH increased from <2 to 5.5 where it stabilized; soil pH stabilized at 6.5. Fluoride solution concentrations were reduced within regulatory limits. Soils accumulated Cd, F, and U in excess of background concentrations (up to 99 ×, 83 ×, and 12.5 ×, respectively). Soil carbonates persisted after leachate was buffered near pH 4.5, and played a major role in Cd attenuation. The results of this laboratory analog of a PW leaching gradient suggest contamination of groundwater is unlikely but accumulation of toxic elements in soil is an issue that may require consideration in the long-term decommissioning of PG waste repositories. Key words: Attenuation, cadmium, fluoride, leachate, phosphogypsum, uranium, waste management