Trace Elements and Fluoride in Phosphogypsum Leachates

Phosphogypsum (PG), produced in large quantities world-wide, is an acidic by-product of the phosphate fertilizer industry. Phosphogypsum is composed mainly of gypsum (CaSO4·2H2O), but contains impurities of environmental concern such as F−, acids, certain trace elements and naturally-occurring radionuclides which originate from sedimentary phosphate rock used in processing. Most PG produced is sluiced out to repositories, forming large stockpiles. Downward movement of impurities into groundwater is a potential problem associated with the stockpiling of PG. The objective of this laboratory study was to investigate the leachate composition of PG produced from Togo phosphate rock for F− and selected trace elements. Phosphogypsum (initial mass 400 g) was extracted 30 times with d.d. H2O (2000 mL) over 30 days. Extractable F− concentration was maximal (64 mg L−1) on Day-1 and decreased to 4.6 mg L−1 by the last extraction. Uranium concentration was maximal on Day-1 (0.37 mg L−1), then fell below the detection limit (0.03 mg L−1) for the remainder of the study. Cadmium concentration was maximal on Day-1 (0.22 mg L−1) and decreased to below the detection limit (0.004 mg L−1) between the fourth and seventh extractions. Concentrations of F− consistently exceeded certain drinking water regulations (DWR); however, concentrations of U and Cd only exceeded certain DWR for initial extractions. High initial concentrations of F−, U and Cd were attributed to residual process water within PG porosity. Leachates of PG washed free of process water may be environmentally benign and pose little hazard except for slightly elevated levels of F−.