Natural radioactivity in phosphate rock, phosphogypsum and phosphate fertilizers in Brazil

Phosphate deposits are generally characterized by enhanced radionuclide concentrations compared to natural levels. The mining and processing of this phosphate ore redistribute radionuclides throughout the environment and introduce them into phosphoric acid and phosphogypsum. Phosphoric acid is the starting material for triple superphosphate (TSP), single superphosphate (SSP), monoammonium phosphate (MAP), diammonium phosphate (DAP), NPK fertilizers and di-calcium phosphate (DCP). Contents of natural radionuclides from thorium and uranium series, (226)Ra, (210)Pb and (228)Ra, were measured in Brazilian igneous phosphate rock, phosphoric acid, phosphogypsum and phosphate fertilizer samples, using high-resolution gamma-spectrometry. Neutron activation analysis was used for the determination of U and Th in the same samples. The fertilizers samples which are derived directly from phosphoric acid, MAP and DAP, presented activity concentrations around the detection limits of the counting system for (226)Ra (<5.0 Bq center dot kg(-1)), for (228)Ra (<3.0 Bq center dot kg(-1)) and for (210)Pb (<19 Bq center dot kg(-1)). As for U and Th, the concentrations found in MAP and DAP are more significant, up to 374 and 250 Bq center dot kg(-1), respectively. SSP, TSP and NPK, which are obtained by mixing phosphoric acid with different amounts of phosphate rock and NH(3), presented higher concentrations of radionuclides, up to 871 Bq center dot kg(-1) for (226)Ra, 283 Bq center dot kg(-1) for (228)Ra, 1255 Bq center dot kg(-1) for (210)Pb, 413 Bq center dot kg(-1) for U and 538 Bq center dot kg(-1) for Th.