Journal of Radioanalytical and Nuclear Chemistry, Vol. 269, No.3 (2006) 739–745
Environmental contamination by technologically enhanced naturally occurring radioactive material – TENORM: A case study of phosphogypsum P. S. C. Silva,1 B. P. Mazzilli,1* D. I. T. Fávaro2 Instituto de Pesquisas Energéticas e Nucleares 1 Divisão de Radiometria Ambiental 2 Divisão de Análise por Ativação Caixa postal 11049, São Paulo, Brazil (Received April 6, 2006)
In the last decades considerable attention has been given to technologically enhanced natural occurring radioactive material (TENORM). Within this frame, of particular concern is the phosphate fertilizer industry, located in Cubatão, São Paulo State, Southwest Brazil. This industry is responsible for the production of 69 million tons of phosphogypsum waste, which is stockpiled in the surrounding environment. This waste concentrates radionuclides of the natural series originally present in the phosphate rocks used as raw material. This study aims to evaluate the environmental impact of such activities in the sediments of the estuarine system. Contents of natural radionuclides from thorium and uranium series were measured in sediments from Cubatão estuarine system, using high-resolution gamma-spectrometry. U and Th were determined by instrumental neutron activation analysis (INAA). It was observed that U and Th concentration is higher in the rivers close to the phosphogypsum piles, at least five points were identified as being affected by anthropogenic factors.
Introduction The mining and processing of phosphate rock that is processed to phosphoric acid production generate TENORM containing residues. This intermediate is then further processed into phosphate fertilizers, detergents, animal feeds, food additives, pesticides and other phosphorous chemicals. Phosphate fertilizers are primarily derived from phosphate rock mined as naturally occurring ores. The principle constituent of the Brazilian phosphate rock is the mineral apatite (carbonatite). The fresh rock is a phoscorite cut by abundant carbonatitic veins. The typical phosphate (P2O5) concentration of the rock is of the order of 15– 30%, with clay, sand, carbonate and other impurities present in varying quantities. Processing of phosphate ore in order to produce intermediate products can be done by acid leaching of the ore resulting in phosphoric acid. Most acidulation is done with sulphuric acid, which leads to the formation of phosphogypsum (CaSO4.xH2O), which is not very soluble in the resulting reaction mixture. Solid gypsum crystals precipitate and can be easily separated from the raw phosphoric acid by filtration following a washing step. This waste is then moved to nearby storage areas, the so-called gypsum stacks. In Brazil, two main industries (named here A and C) are responsible for the production and storage of about 4,000 ton per day on several stacks in Cubatão City, located in Santos Basin. The amounts of radioactivity that are fractionated to phosphogypsum vary significantly. MAZZILLI et al.1 found percentages (phosphogypsum to ore rocks) of 90% for Ra isotopes, 100% for 210Pb and 80% for Th isotopes, in the
phosphogypsum stored in Cubatão. Several papers have already been published concerning the characterization of the Brazilian phosphogypsum.1–3 Mean activity concentrations of the same order of magnitude was observed for 226Ra and 210Pb (849 Bq.kg–1 and 837 Bq.kg–1 for industry A and 357 Bq.kg–1 and 342 Bq.kg–1 for industry C) and for 232 Th and 228Ra (222 Bq.kg–1 and 229 Bq.kg–1 for industry A and 172 Bq.kg–1 and 163 Bq.kg–1 for industry C).3 SANTOS et al.3 evaluated the environmental impact of the storage of phosphogypsum stacks in Cubatão. The aquatic environment near the disposal area was assessed by measuring natural radionuclide activities in groundwater, river water and sediment samples in the vicinity of gypsum stack from industry C. They found that the most critical pathway between phosphogypsum and the environment was through water contamination. Sediments from rivers, in the area of influence of the stacks, presented higher concentrations of U and Th, when compared with reference values. In order to verify the extent of such influence, this paper is concerned with the measurement of natural radionuclides of the U and Th series in the sediments of the region. In Fig. 1 are depicted the location of the two industries, the surrounding aquatic system and the location where sediments were collected.
The study area Santos Basin, located in Southwest Brazil, São Paulo State, comprising the counties of Santos (725 km2), São Vicente (131 km2) and Cubatão (160 km2), is considered the most important industrial region of the country.
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P. S. C. SILVA et al.: ENVIRONMENTAL CONTAMINATION
Fig. 1. Study area showing the sampling location. Axes x and y mean geographic coordinates
These man-made activities, on the other hand, represent a potential threat to the surrounding environment. Its estuarine system is responsible for the catchments and deposition of a considerable amount of material loaded by rivers, such as major and traces metals and other pollutants discharged by the local industrial activity. For a better understanding of the results, in Fig. 1 the study region was divided in rivers (identified by R), Santos Channel (CS), São Vicente Channel (SV) and Santos Bay (B). The rivers samples were divided into RC-samples, which represent sediments from Cubatão River; RM-samples, which represent sediments from Mogi River and R-samples, which represent sediments from others estuarine rivers that are subjected to tidal effects, and, therefore, possess salinity values higher than the former rivers. In Santos and São Vicente Channel the salinity ranges from 0.5 to 35‰ and Santos Bay possesses salinity of 35‰. In the surrounding of Cubatão and Mogi rivers a large number of industries are located including two phosphate fertilizer plants, which are the object of this study. In Santos Channel the most important Brazilian harbor is located. The sampling locations were chosen according to the monitoring program established by “Companhia de Tecnologia de Saneamento Ambiental” CETESB, which is the state company responsible for formulation and application of public health measures; and the location of the phosphate fertilizer plants (Fig. 2).
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Experimental Samples were collected in 1999 in Santos and São Vicente estuary with a Van Veen grab and in 2002 in Cubatão and Mogi Rivers using PVC core with 20 cm of length and 4 cm diameter. All samples were maintained under 4 °C before laboratory treatment. Samples were dried at temperature of 65 °C till constant weigh and sieved to a grain size of 150 mesh. Part of the sample was packed in plastic vessels for gamma-spectrometry, for the determination of radium isotopes and 210Pb. Approximately 150 mg were used for Th and U measurement by neutron activation analysis. Activity concentrations of 226Ra, 228Ra and 210Pb were measured by gamma-spectrometry with a hyperpure germanium detector, EGNC 150-190 R, from Eurisys Measures, with resolution of 1.8 keV for the 1332 keV 60Co photopeak, for 4096-channels. The detector was calibrated using natural soil spiked with radionuclides certified by Amersham. Samples were packed in 100 cm3 cans and sealed for about four weeks prior to the measurement in order to ensure that equilibrium has been reached between 226Ra and its decay products of short half-life. The 226Ra activities were determined by taking the mean activity of three separate photopeaks of its daughter nuclides: 214Pb at 295 keV and 352 keV, and 214Bi at 609 keV. The 228Ra content of the samples was determined by measuring the
P. S. C. SILVA et al.: ENVIRONMENTAL CONTAMINATION
intensities of the 911 keV and 968 keV gamma-ray peaks from 228Ac. The concentration of 210Pb was determined by measuring the activity of its low energy peak (47 keV). Self-absorption correction was applied since the attenuation for low energy gamma-rays is highly dependent upon the composition of the sample. The approach used was that suggested by CUTSHALL et al.4 Typical lower limits of detection for gammaspectrometry were 4.5 Bq.kg–1 for 226Ra, 3.1 Bq.kg–1 for 228Ra and 25 Bq.kg–1 for 210Pb, for a counting time of 50,000 seconds. U and Th were determined by instrumental neutron activation analysis (INAA) by irradiation of approximately 150 mg of each sample, during 8 hours in a neutron flux of 1012 n.cm–2.s–1, at the Instituto de Pesquisas Energéticas e Nucleares (IPEN) research reactor IEA-R1. The induced radioactivity was measured with a Ge-hyperpure detector (Intertechnique), with 2.1 keV resolution for the 1332 keV 60Co photopeak. The concentration of the elements was determined by comparing activities obtained in the samples with standard materials Buffalo River Sediment (NIST-2704) and Soil-7 (IAEA). All uncertainties presented correspond to propagation of errors. Results and discussion Table 1 shows the activity concentrations of U, Th and 228Ra obtained in the sediment samples for the six compartments studied (Santos Channel-CS, São Vicente Channel-SV, Santos Bay-B, Cubatão River-RC, Mogi River-RM and river samplesR). In the same table are also presented reference values obtained for a deep core profile collected in the region.5 These values were considered as the background of the region (BG), since no data is available in the literature so far. In Fig. 3 are depicted the results obtained for the sediment concentration normalized by the reference values. A normalized value higher than one indicates enrichment in the sediment compared to the background values and is generally used as an indicator of anthropogenic contribution. Despite the geochemical differences, such as salinity and tide, it can be observed identical behavior for 226Ra and 228Ra, with the following distribution among the studied compartments: 226Ra, 210Pb,
[RM]
[RC] > [R]
[CS]
[SV]
[B]
for U and Th the distribution: [RM] > [RC] > [R] > [CS] > [SV] and for
210Pb
[RM]
[B]
the distribution: [RC]
[R]
[CS]
[SV]
[B]
Statistical analysis was used for the comparison of the compartments studied.5 Considering all the analyzed samples, the activity concentration of U varied from 14±3 to 348±27 Bq.kg–1 (points B4 and R4, respectively); the activity concentration of 226Ra varied from 6±2 to 60±3 Bq.kg–1 (points B2 e RM2, respectively); 210Pb varied from
