Characterization of stockpiled phosphogypsum waste in Santos basin, Brazil

Abstract

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 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±32Bqkg-' and 827±276 Bqkg-' for Ra-226, Ra-228 and Pb-210, respectively. Phosphogypsum samples are enriched in rareearths 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.

Distribution of natural radionuclides in the production and use of phosphate fertilizers in Brazil

“…Although several publications were found related with the characterization of Brazilian phosphate rock and phosphogypsum (Mazzilli et al, 2000;Silva, 2001;Silva et al, 2001;Santos, 2002;Santos et al, 2002), no specific reference was found concerning the radiological characterization of the Brazilian phosphate fertilizers and the long term exposure due to its application along the years.…”

Partitioning of radionuclides and trace elements in phosphogypsum and its source materials based on sequential extraction methods

“…This waste is then moved to nearby storage areas. Santos et al (2001) performed the characterization of natural radionuclides of the U and Th series in the stockpiled phosphogypsum waste in Cubatão, where are located two main producers, named A and C. 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 in the samples (Santos et al, in press). The aquatic environment near the disposal area was assessed by measuring natural radionuclide's activity in monitor wells, river water and sediment samples.…”

Distribution and mode of occurrence of radionuclides in phosphogypsum derived from Aqaba and Eshidiya Fertilizer Industry, South Jordan

“…During the production of phosphoric acid, the radioactive equilibrium originating in the phosphate rock is disrupted and a redistribution of the radionuclides occurs. Some work has been performed on the re-distribution of 238 U decay-series radionuclides during ore processing and during storage on stacks (Luther et al, 1993;Burnett et al, 1995;Rutherford et al, 1995a;Hull and Burnett, 1996;Al-Masri et al, 1999;Santos et al, 2001;Santos et al, 2006). The results showed that most of the 238 U, 234 Th, 210 Pb tend to be incorporated in the phosphoric acid and then in the phosphate fertilizer, while 226 Ra and 210 Po are being deposited into the PG (Miller and Sutcliffe, 1984;Burnett et al, 1988;Hull and Burnett, 1996;Al-Masri and Al-Bich, 2001).…”