Phosphogypsum applications in the cerrado agriculture and his radiological implications

The rate of generation of phosphogypsum is approximately 4.8 tons for each ton of phosphoric acid produced. The annual world production can be estimated at 150 million of tons. It is classified as NORM (Naturally Occurring Radioactive Material), in other words, it is a solid waste that contains radioactive elements of natural occurrence from the source rock. The large amount of phosphogypsum produced has been attracting attention of radiological protection institutions and environmental protection agencies in the world, given its high potential for contamination of the environment. In Brazil, this material has been used for several decades, especially for agricultural purposes. In this case, the phosphogypsum is used as a source of calcium and sulfur, as conditioner, and for correction of subsurface soil saturated with sodium, potassium and aluminum. Due to the presence of radionuclides in phosphogypsum, it is necessary to understand the mechanisms for transferring of natural radionuclides in the system soil/plant and to evaluate if the use of phosphogypsum in soil contributes to increased exposition of humans to the natural radioactivity. Experiments were accomplished in a greenhouse with lettuce cultivation in two types of soil (sandy and loamy) fertilized with four different amounts of of phosphogypsum. Samples of phosphogypsum, soil, lettuce and drainage water were analyzed being determined the radionuclides of interest (238U, 232Th, 226Ra, 228Ra, 2'1'0Pb and 210Po). 238U and 232Th have been carried out by neutron activation analysis, Ra, Ra and Pb by gamma spectrometry and Po by alpha spectrometry techniques. Finally, Transfer Factor of soil-plant was calculated and the annual contribution to the effective dose committed due to the ingestion of lettuces of the experiment. Analyses of chemical, physical and mineralogical characterization of soil and phosphogypsum samples were also accomplished. The phosphogypsum was classified as Class II A - Not Dangerous, Not Inert, Not Corrosive and Not Reactive. The soil samples analyzed as were acids, with low content of organic matter and high potential acidity. The average of specific activity for 226Ra in phosphogypsum samples (252 Bq.kg-1) was below of the maximum level recommended by USEPA, which is 370 Bq.kg-1 for agricultural use. Although the most of the results of mean specific activity of radionuclides present in samples of lettuce present values below the Minimum Detectable Activity (MDA), the Transfer Factors were estimated for those conditions in which the mean specific activity proved to be superior to MDA. The values ranges from 1.8 10-3 to 2.3 10-2 for 232Th; 3.5 10-2 to 4.1 10-2 for 226Ra, 2.4 10-1 to 3.2 10-1 for 228Ra and 3.5 10-2 to 8.5 10-2 for 210Po, depending on the type of soil used in the planting of the vegetables. In general, the results obtained in the present study indicated that the mobility of radionuclides in both studied soils was low. The effective doses committed calculated well below the limit of 1 mSv.year-1 established by ICRP, for the public in general (4,3 10-3 mSv for the experiments in loamy soil and 7,5 10-3 mSv for the experiments in sandy soil). It is possible to conclude that from the point of view of the radiological protection, the data obtained in this work demonstrated the viability of the use of phosphogypsum in agriculture of the Cerrado, Brazil. (author)