Characterization of stockpiled phosphogypsum waste in Santos basin, Brazil

Abstract

Phosphogypsum is a waste produced by the phosphate fertiliser industry. It' s formed by precipitation during wet sulphuric acid processing of phosphate rocks. Although phosphogypsum is mainly calcium sulphate dihydrate, it contains elevated levels of impurities, which originate from the source phosphate rock used in the phosphoric acid production. Among these impurities, radionuclides from U-238 and Th-232 decay series, particularly Ra-226, Ra-228 and Pb-210, are of most concern due to their radiotoxicity. The phosphate fertiliser complex located in Santos Basin, Southwest Brazil, is responsible for the production of approximately 2000 tons of phosphogypsum per day. This phosphogypsum waste is stockpiled and presents a potential threat to the surrounding environment. This study aims to determine the activity concentration of Ra-226, Ra-228, Pb-210 and K-40 in the stockpiled phosphogypsum of the two main local producers. As a complementary study, trace and microelements (Ba, Co, Cr, Fe, Hf, Sb, Sc, Ta, Th, U, and rare earths Ce, Eu, La, Lu, Nd, Sm, Tb and Yb) were also determined by instrumental neutron activation analysis (INAA). Mean activity concentrations of 401+108 Bqkg-', 173±65 Bqkg-1 and 389±106 Bqkg-1 were observed for Ra-226, Ra-228 and Pb-210 for one producer. The results obtained for the second producer were 840±275 Bqkg-1 , 225±32 Bqkg-' and 827±276 Bqkg-' for Ra-226, Ra-228 and Pb-210, respectively. Phosphogypsum samples are enriched in rare-earths elements, specifically Ce, Eu, La, Nd, Sm, and Tb, and the elements Ba and Ta. The radiochemical and elemental characterisation of the phosphogypsum from industry A and C show that the stacks are quite homogeneous and mainly dependent upon the origin of the phosphatic rock used as raw material. This information is important and necessary for decision making on possible uses of this waste. The radionuclides Ra and Pb, which present high radiotoxicity, although present in the phosphogypsum, are not available for the surrounding environment.