Radon emanation coefficients for phosphogypsum

Phosphogypsum is a by-product of the phosphate fertilizer industry which is stockpiled in large quantities world-wide. Phosphogypsum consists mainly of dihydrate gypsum (CaSO42H2O) but also contains elevated concentrations of 226Ra and other inorganic species which originate from the processing of phosphate rock. 222Rn gas is the first decay product of 226Ra and has been identified as one of the major environmental concerns associated with phosphogypsum. This study was conducted to determine effects of particle size, weathering, and moisture content on the 222Rn emanation coefficient (epsilon) for phosphogypsum. Average epsilon for air-dry, unfractionated phosphogypsums derived from Togo, Florida, or Idaho rock was approximately 12%. Average epsilon for fine fraction phosphogypsum (< 20 microns diameter) was greater than for unfractionated phosphogypsum by a factor of 4.6, 1.4, and 4.4 for samples derived from Idaho rock, Togo rock, and Florida rock, respectively. Phosphogypsum samples subjected to an artificial weathering procedure lost 40% mass, with no change in epsilon. Increasing water content was found to first slightly decrease, then to increase epsilon compared to air-dry samples; epsilon for 100% saturated phosphogypsum was 1.9-fold greater than in air-dry phosphogypsum. Particle size sorting could account for variability of 222Rn exhalation at repositories. Very high moisture contents could slightly increase 222Rn emanation, but exhalation would likely be reduced due to slow diffusion through porosity of saturated phosphogypsum.