Heterogeneous distribution of trace elements and fluorine in phosphogypsum by-product

Abstract

Phosphogypsum (PG), a by-product from phosphate fertilizer production, is composed mainly of gypsum (CaSO4·2H2O) but also contains minor quantities of trace elements (TE), rare earth elements (REE) and F. Some elements may be elevated in quantities to be of environmental concern. This study determined the distribution of TE, REE and F among three size fractions (<20 μm, 20–53 μm, >53 μm) in PGs derived from three different phosphate rock sources. Fine fraction PG (<20 μm) composed of <10% of total PG mass but was highly enriched in TE, REE and F compared to unfractionated PG. For PG derived from Idaho rock, Se in the fine fraction was enriched 830 times over soil and 415 times over shale while Cd was enriched in the fine fraction 70-fold over shale and soil. Fluorine was elevated 37 times in the fine fraction compared to shale. The same trends were observed for PG derived from Togo and Florida rocks. Elevated elemental concentrations in fine particles and particle sorting during PG deposition may contribute to chemical heterogeneity of PG repositories, and make elements more susceptible to mobilization processes, such as leaching and erosion. Removal of fines will improve the utilization of PG in other industries, such as for use as an amendment to agricultural soils.