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Radium in Phosphogypsum Leachates

Dep. of Renewable Resources, 442 Earth Sciences Bldg., Univ. of Alberta, Edmonton, AB, Canada, T6G 2E3;

J. M. Arocena

Faculty of Natural Resources and Environ. Studies, Univ. of Northern British Columbia, Prince George, BC, Canada, V2N 4Z9.

^* Corresponding author.

ABSTRACT

Phosphogypsum (PG) is an acidic by-product of the phosphate fertilizer industry, and is produced in large quantities by the wet phosphoric acid process. Most PG is sluiced out to repositories, forming large stockpiles. Phosphogypsum is composed mainly of gypsum (Ca-SO₄·2H₂O), but contains impurities of environmental concern such as F^–, acids, trace elements, and naturally occurring radionuclides, which originate from the phosphate rock used in processing. Possible movement of these impurities into groundwater is an issue. ²²⁶Radium is the major source of radioactivity in PG produced from sedimentary phosphate rock. Few studies have addressed the leachability of ²²⁶Ra because solid solutions of Ra, Ba, and Sr are very insoluble. The objective of this study was to investigate the concentrations of ²²⁶Ra, Ba, and Sr in leachate generated from PG produced from Togo phosphate rock. Phosphogypsum was extracted 30 times with deionized distilled (d.d.) H₂O over 30 d. Extractable ²²⁶Ra was maximal (0.55 Bq L^–1) for Day 1 but remained relatively constant between Day 2 and Day 30. Minimum extractable ²²⁶Ra (0.23 Bq L^–1) occurred on the Day 30 extraction but still exceeded the current U.S. drinking water standard. Solid phase ²²⁶Ra increased between Day 0 (850 Bq kg^–1) and Day 30 (1120 Bq kg^–1). The ²²⁶Ra/Ba ratios in the solid phase and in the extractable liquid phase were nearly equal over the last half of the extraction period. If this relationship holds for other PGs, then solution ²²⁶Ra activities can be estimated if solid-phase ²²⁶Ra/Ba ratios are known and Ba solution concentrations are known or estimated.