Influence of Blending Phosphogypsum to Mitigate Radium Leaching and Enhance Reuse As Road Base Material

Abstract

Abstract The processing of phosphate rock to produce phosphoric acid results in a calcium sulfate byproduct known as phosphogypsum (PG). This byproduct is regulated in the United States as a Technologically Enhanced Naturally Occurring Radioactive Material (TENORM) and is managed indefinitely in monofills or gypstacks. Reuse of this material is inhibited by regulations surrounding radon release and total and leachable 226Ra concentrations, however increased interest in using PG as a road base material has developed. Many sources of PG cannot be used as a road base material alone largely due to insufficient strength and potential for 226Ra leaching. This paper investigates the influence of blending PG with common alkaline road base aggregates such as limerock and recycled concrete aggregate (RCA) to reduce 226Ra leaching from PG amended road base and provide an avenue for widespread reuse. Two sources of PG were investigated for 226Ra leaching at various pH conditions and blended with each of the two aggregate sources at up to 0,25,50, and 75% (by mass) PG to understand the behavior of 226Ra leaching. The results reveal an amphoteric leaching behavior from the PG sources alone. The blending experiment indicates an increase in leaching from PG:limerock blends initially, followed by a reduction with increasing limerock addition, and a decrease in leaching from PG:RCA blends. The amphoteric behavior of the PG paired with the leaching curves for these material blends indicates a mechanism of control of 226Ra mobility aside from change in pH, perhaps due to binding of radium by elevated concentrations of Ba, Sr, or Ca in the RCA blends. This data provides novel insight into controlling a limiting factor to PG reuse as a road base material and suggests that blending with RCA could allow for lower 226Ra mobility from PG blends and allow a larger application of PG reuse and an alternative scenario to end-of-life stacking.