Fluoride sorption and mineral stability in an Alberta soil interacting with phosphogypsum leachate

Abstract

A by-product of phosphate fertilizer production is phosphogypsum (PG). Relatively high F concentrations occur in the solid phase and in the highly acidic process water (PW) that initially fills its pores. The present research addresses the fate of F and the effects on mineral stability as phosphogypsum leachate (PGL) interacts with calcareous (Ck) soil and non-calcareous (Bt) soil. In a sequential batch equilibration, soils were exposed to fresh PGL during 30 d-long extractions. Major ions, pH and electrical conductivity (EC) were monitored. At 28 d, F concentration decreased from 31 mg L−1 in the PGL to 11 mg L−1 in the Bt-PGL mixture and 22 mg L−1 in the Ck-PGL mixture. Saturation indices (SIs) for fluorite ranged from 2.1 to −3.6. Extracts were closer to saturation with fluorite than with the other F minerals studied. Using alternative geochemical constants, fluorapatite was predicted to be stable in Bt-PGL extracts. The day 28 Bt-PGL extract was also near equilibrium with alunite, basaluminite, jurbanite, gypsum, amorphous silica, and quartz. Of the phyllosilicates in the soil-PGL mixtures at day 28, only kaolinite was predicted to be stable in only the Bt-PGL mixture. The potential for clay mineral instability and accelerated dissolution in F-rich PGL needs to be considered m the design of PG repositories. Key words: Fluoride, phosphogypsum, waste management, leachates, geochemical modeling, mineral stability