RADIUM RESIDENCE IN PHOSPHOGYPSUM OF JORDAN: IMPLICATIONS FOR INHALATION EXPOSURE

Phosphogypsum (PG) is an industrial by-product of the reaction of phosphate rock with sulfuric acid to produce phosphoric acid (wet process). Voluminous stockpiles of PG are stored at many sites around the world but beneficial use of this gypsum is limited by impurities inherited from the mined phosphate rock. In particular, most of the 226Ra (half-life=1600 y) dissolved from phosphate rock is transferred to the PG and poses a radiation hazard and a source of radon gas. The distribution of radium between gypsum (major) and coexisting minor and trace mineral constituents of PG influences Ra leachability, radon emanation, and radiation exposure to workers and the public. In the arid setting of Jordan, unlined and uncovered stacks of sand-to-silt-size PG are highly susceptible to wind transport. Activity-concentrations of 226Ra in ten samples of Jordan PG of varying age averaged 601±98 Bq/kg. In comparison, the U.S. Environmental Protection Agency has established an upper limit of 370 Bq/kg for PG used as a soil amendment. Shale or granite containing 10 ppm U contains 122 Bq/kg of 226Ra, assuming radioactive equilibrium. Jordan PG shows no analytically significant enrichment (<10%) of 226Ra in the finer (<53 micrometer) grain size fraction compared to coarser fractions. Water-insoluble residues of Jordan PG contain <10% of PG mass, but contain 30-65% of the 226Ra. Radium correlates positively with barium (R=0.90) in the residues. Uniformly tiny (~10 micrometer), delicate grains of barite were identified in the residues using a scanning electron microscope. Barite is a well documented scavenger of dissolved radium and likely precipitates during dissolution of the phosphate rock (Ba, Ra source) by sulfuric acid (sulfate source). Barite formation in PG is limited by the amount of Ba provided by mined phosphate rock. Inhaled ~10 micrometer particles of “radiobarite” are slowly expelled from the lungs, are insoluble in lung fluids, and provide an intense source of alpha particles to surrounding lung tissue. Fortunately, Ra-bearing barite particles are trace mineral components of PG and may be included within or attached to larger grains of sparingly-soluble gypsum that are expelled more efficiently from the lungs.