Radiochemistry of Florida phosphogypsum

Phosphogypsum is the by-product of producing phosphoric acid by reacting phosphate rock with sulfuric acid. The phosphoric acid industry in Florida is the largest in the world and it is estimated that 1 × 109 tonnes of phosphogypsum will soon be stored in 18 subaerial sites, the so-called Florida “gyp-stacks”. Florida phosphate rock samples analyzed in this study have 238U activities which range from approximately 250 to 2000 Bq kg−1 with 234U, 230Th, 226Ra, 210Pb and 210Po in approximate radioactive equilibrium. During production of phosphoric acid, about 90–100% of the 226Ra, 210Pb and 210Po in the phosphate rock is fractionated to the phosphogypsum. In most cases the activities of 226Ra, 210Pb and 210Po in the phosphogypsum are directly and predictably proportional to the activities in the input phosphate rock. Studies carried out on the disequilibrium relationships in 74 samples of Florida phosphogypsum show that 226Ra is relatively immobile on the unsaturated, aerobic portions of these gyp-stacks. The 210Pb seems to be remobilized preferentially to 210Po or 226Ra and solutions percolating through Florida gyp-stacks have added 210Pb to some phosphogypsum samples.