Transfer and Mobility of 137Cs and 60Co in Oxisol and Alfisol

TRANSFER AND MOBILITY OF 137CS AND 60CO IN OXISOL AND ALFISOL

Maria Angélica Wasserman*, Daniel Vidal Pérez**, Claudio Carvalho Conti*; Flavia Bartoly*, Aline Gonzalez Viana* and Júlio César Wasserman ***

* Instituto de Radioproteção e Dosimetria/CNEN Av. Salvador Allende s /n°. Recreio CEP: 22780-160, Rio de Janeiro, RJ. Brasil E-mail: [email protected]

** Centro Nacional de Pesquisa de Solos/EMBRAPA Rua Jardim Botânico, 1024. CEP: 22460-000, Rio de Janeiro, RJ. Brasil.

*** Departamento de Geoquímica/UFF Outeiro de São João Batista s/n°, Centro, Niterói, RJ, 24.020-007, Brasil.

ABSTRACT

One manner to assess potential mobility of radionuclides in soils is the use of sequential extraction procedures. These procedures intend to provide the radionuclide partitioning in geochemical phases of soil affected by changes in physico-chemical conditions. In this study a new sequential chemical extraction protocol was choose to evaluate 60Co and 137Cs mobility under a large range of physico-chemical soil properties. The results of sequential procedure was further compared with soil to plant transfer factors (TF) data for maize and radish and with soil properties, showing rather consistent results. The 137Cs distribution in soil showed that Fe oxides are the main sink for this element and after 14 years after contamination the 137Cs was still available for plants. The 60Co distribution showed that Mn oxides are the main sink for this element in Alfisol and 5 years after contamination no 60Co was detected as bioavailable neither detectable in maize. The knowledge of the bio-geochemical behavior of radionuclides in soil system can be useful for risk assessment studies, to be applied in the case of nuclear accident or contamination scenarios.

Keywords: mobility, bioavailability, sequential chemical extraction, cesium, cobalt.

I. INTRODUCTION One manner to assess potential mobility of radionuclides in soils is the use of sequential extraction The study of 137Cs in agricultural areas is of large procedures. These procedures intend to provide the concern because, once in the soil, it can recycle within the radionuclide partitioning in geochemical phases of soil biota, similarly to potassium and as long as its radioactive affected by changes in the physico-chemical conditions half-life permits. Interactions between radionuclides and [10-12]. soil depends on the chemical form of element and some The results of sequential procedure for 137Cs are soil properties as pH, mineralogical composition, organic going to be discussed considering soil properties and soil matter content and nutrient status [1-6]. The knowledge of to plant transfer factors (TF) for maize and radish obtained the mobility of radionuclides in soils is of special interest to in an Oxisol soil accidentally contaminated in Goiânia, and assess potential hazards from the radionuclide inputs into in an Alfisol and Oxisol, both artificially contaminated. food chains. The plant uptake of radionuclides will be The bio-geochemical behavior of 60Co in Alfisol soils is dependent on interactions occurring in soils as well going to be discussed considering soil properties and soil metabolic and physiological characteristics of the species to plant transfer factors (TF). [1].The differences found between types of soils and vegetation engender a variety of measured soil-to-plant transfer factors (TF) [7]. This figure is still complicated by II. MATERIALS AND METHODS the use of fertilizers and other agricultural practices that modifies plant uptake by direct changes on some physico- Lysimeters measuring 1m2 of area and with a depth chemical properties of soils [1,2,8,9]. of 1 m were installed in a restricted area of the Institute for Radioprotection and Dosimetry (CNEN/Brazil). Six lysimeters were filled with the Oxisol soil [13], Where, Ap = Activity in the edible part of the plant contaminated with 137Cs in 1992. One lysimeter was filled (Bq.kg-1 dry weight) and As = Activity in the soil (Bq.kg-1 with the Alfisol soil [13], contaminated with 60Co and dry weight). 137Cs in 1996. Each lysimeter received a liter of solution Edible parts of plants were collected and washed. containing approximately 60µCi/L of 137Cs or 60Cs. The Plants and soils were dried, ground and sieved through a contamination was done directly in the lysimeters. The 2mm screen before direct measurements of the of 137Cs solution was sprayed in every 2 cm of soil, layer by layer, and 60Co activity by gamma ray spectrometry using a Ge up to 40 cm. Another Oxisol soil was collected in the city detector ORTEC with a relative efficiency of 30%. of Goiânia, in a site where the radiological accident Individual counting errors for the used geometry (pot of occurred in 1987 [14]. Three lysimeters were filled with 250 g) were less than 3%. Goiânia soil. Samples of soil were collected at lysimeters filled with Oxisol, Goiânia soil and Alfisol to perform III. RESULTS AND DISCUSSIONS sequential extraction analyses. Sequential extractions were done as described in Table 1. Details of protocol can be The main chemical and physical properties of found in Wasserman et al. [10]. All the extracts were studied soils are presented in Table 2. The Oxisol soil has analyzed by gamma ray spectrometry using an Ge detector sandy clay loam texture, low cation exchange capacity ORTEC with a relative efficiency of 30%, for the (CEC) and low content of nutrient elements. The determination of 137Cs and 60Co. Individual counting errors mineralogical analyses indicate the presence of gibbsite for the used geometry (vial of 250 mL) were less than 3%. and kaolinite as main constituents of the clay mineral The Soils Office of the Brazilian Agricultural fraction. Fertilizers were applied in this soil, in Research Corporation (EMBRAPA-Soils) performed soil formulations and sometimes alone, depending on the crops analyses and clay mineralogy determinations in order to cultivated. The Goiânia soil is also an Oxisol, but it is an characterize samples according to their standard manual urban soil with contributions from urban residues, like [15]. lime and cement (from building construction). No fertilization procedure was applied during these TABLE 1. Protocol of sequential extraction method experiments. The mineralogical analyses of the clay considering geochemical meaning by phase. fraction indicate the presence of gibbsite and kaolinite Slightly acidic phase: (Table 2). The 137Cs activity in this soil is lower than that CH3COOH + CH3COONa 1:1; measured in Oxisol. pH 4.7; At room temperature. The Alfisol is an acid silty clay loam soil, with low geochemical meanings: elements readily bioavailable. nutrient content and low CEC. The mineralogical analyses Easily reducible phase: indicate the presence of hematite, goethite and traces of 137 NH2OH.HCl (0.1 M); vermiculite. The Cs activity in this soil is comparable to pH 2; At room temperature. that of Oxisol. geochemical meanings: elements mainly bound to Mn 137 60 oxides Geochemical Partitioning of Cs and Co. The Oxidizable phase: geochemical distribution of the radionuclides furnishes a first approach to the investigation of their potential H2O2 (30%) + CH3COONH4 (1M); pH 2; At room temperature. mobility in the studied soils. Figure 1 shows the geochemical partition of 137Cs. geochemical meanings: elements bound to labile organic 137 matter The distribution of Cs in Oxisol and in the Goiânia soil in all considered phases were very similar Alkaline phase: (Figure 1). No significant differences were observed in the NaOH (0.1 M) ; soil properties considering texture, pH, organic matter pH 12; At room temperature. content and secondary minerals (Table 2). geochemical meanings: elements bound to Fe The 137Cs transfer factor for maize and radish was compounds comparable in Oxisol and Goiânia as showed in the Figure Resistant phase: 2. These results agree with the similar results for 137Cs Aqua regia. obtained in the slightly acidic phase to Oxisol (8% readily Heat to 50°C/ 30mim. bioavailable) and Goiânia soil (9% readily bioavailable). geochemical meanings: elements not available for In the Alfisol, very small activity of 137Cs was transfer processes detected in the slightly acidic phase as well TF in this soil was generally lower than in the other 2 soil, despite of time The transfer factor (TF) was determined for maize after contamination be lower than the other two soils and (Zea mays, L.) and radish (Raphanus sativus, L.) following level of contamination be comparable to the Oxisol (Figure the IUR protocols to determine transfer factors [16]: 2). The conditions for extraction used in the slightly acidic phase seem to be very consistent with the transfer factor TF =Ap/As results, corroborating that this phase represents the bioavailable fraction.

TABLE 2. Pedological analyses of artificially contaminated soils and Goiânia soil. Parameters Oxisol Goiânia soil Alfisol N=6 N=3 N=1 pH (water) 6.3 7.8 4.9 pH (KCl 1M) 6.7 8.0 4.2 Sand (%) 71 68 10 Clay (%) 18 16 49 CEC (cmolc kg-1) 3.2 5.2 8.4 C (dag kg-1) 1.3 1.14 1.24 N (dag kg-1) 0.12 0.12 0.13 P (mg kg-1) 5 53 8 K+ (cmol kg-1) 0.13 0.25 0.75 SiO2 (dag kg-1) 3.3 5.8 15.7 Al2O3 (dag kg-1) 10.5 10.9 10.2 Fe2O3 (dag kg-1) 3.4 5.4 13.3 Ca++ (cmolc kg-1) 1.6 4.2 not determined Mg++ (cmolc kg-1) 0.9 0.4 2 Al+++ (cmolc kg-1) 0.0 0.0 0.05 H+ (cmolc kg-1) 0.5 0.4 5.5 137Cs (Bq/Kg dry wet) ± 1s 5969 ± 47 2397 ± 37 6227 ±32 60Co (Bq/Kg dry wet) ± 1s - - 3087 ± 25 Time after contamination 9 years 14 years 5 years Main clay mineral type gibbsite, kaolinite gibbsite, hematite, goethite, (in order of predominance) kaolinite vermiculite

The results obtained in the oxidizable phase (organic present in the structure of primary minerals, or to a lesser bound) for Oxisol, Goiânia and Alfisol soil were extension, those trapped in the structure of secondary associated with the similar organic C content in these soils minerals. Release of elements bound to the lattice of (Table 2). minerals is not a natural process observable on a human The results of this operational method mobilize time scale [19] and refers to the background levels for the 137Cs associated with iron oxides in the alkaline phase. It region. Elements originated from human activities are can be observed that iron oxides have an important role as generally distributed among other physico-chemical a sink for 137Cs in these soils. In the Alfisol that is an iron phases, according to their chemical form and behavior. rich soil, the iron oxides can be responsible for the Since 137Cs has no natural origin, some radiocesium found retention of more than 60% of 137Cs. Such behavior in the residual phase of classical methods suggest that this corroborates the findings of Sheppard et al. [17]. These radionuclide could be progressively fixed in some clay authors, using the sequential extraction method of Tessier structures (mainly 2:1 clay types). In the presence of 2:1 137 [18], obtained in four Canadian soil more than 40% of the clay mineral type, the fixation of Cs in the internal faces total Cs associated with Fe and Mn oxides. of these clays can occur after contamination (less than 3 years), reducing transfer to plants [12,20-22]. Wasserman et al. [23] using the classical method of 137Cs Tessier, noticed that no significant radiocesium was 100% observed in the residual phase of the Oxisol and the Resistant Goiânia soil, where kaolinite and gibbsite are predominant 80% in the clay fraction and there’s no 2:1 clay mineral types in Alkaline these soil to fixation processes. The Alfisol presents traces 60% of vermiculite (2:1 clay mineral type) and it was Oxidizable contaminated with 137Cs in 1996, that represents time 40% enough for Cs fixation. This can partly explain why, Easily 137 20% reducible despite of the similar concentration of Cs in Alfisol compared with Oxisol (Table2), the concentration in the Slightly 0% acidic bioavailable phase in the Alfisol was lower (Figure1). The Oxisol Goiânia Alfisol absence of 2:1 clay mineral in Oxisol and Goiânia soil explain why many years after contamination the soil to 137 Figure 1. Geochemical partitioning of 137Cs in tropical plant transfer factor for Cs remains the same in the soils. Goiânia soil (Figure 3). This also partially explain why transfer factor is higher in some tropical soil than IUR Elements present in the residual phase of the values determined to temperate soils. classical sequential extraction of Tessier [18], are those In the resistant phase, the fluorhydric acid or other strong condition able to break down mineral structures was not used, so it does not account for elements present in the The 60Co distribution in the Alfisol is presented in the mineral structures. The resistant phase correspond mainly Figure 4. In this Figure it is clear that 60Co is mainly to elements associated to compounds not destroyed in the associated with Mn oxides, followed by organic compounds previous phases and possibly not available for transfer and Iron oxides. An important thing to note with this result is processes in the water cycle. The results obtained using that no 60Co was observed in the bioavailable phase. This this sequential extraction protocol showed that Oxisol and result was very consistent with plant results, since no 60Co Goiânia soil present an important percent of 137Cs (about was detected in the maize plants (grain or leaves) and very 30%) resistant to be mobilized under normal and cultural few was transferred to radish root: 3% of soil total environmental conditions. In the Alfisol the percent is concentration. All these findings were similar to those lower (11%) than the Oxisol (Figure 1). It is possible that reported by Colle et al. [25] reviewing the Co behavior in in the old contamination, the association of radionuclides soils and plants, supporting that no specific behavior is with soil compounds be more refractory. expected for Co in tropical soil.

IV. CONCLUSIONS 10,00 Changes in bioavailability of 137Cs were mainly due 1,00 to differences in some soil properties. The results for sequential extraction method used in this study shows that TF 137Cs is mainly associated with iron oxides in Al rich soil 0,10 or Fe rich soil. Basic conditions can be more efficient to mobilize 137Cs present in soil. These results showed that 0,01 part of 137Cs can be strongly retained in the soil particles Alfisol Oxisol Goiânia soil independent of the presence of 2:1 clay mineral. In the Alfisol, part of 137Cs can be chemically fixed in the Radish Maize internal faces of 2:1 clay mineral type. The 60Co distribution showed that Mn oxides are Figure 2. 137Cs Transfer factor for radish and maize the main sinks for this element. Four years after the obtained in Alfisol, Oxisol and Goiânia soil. contamination no 60Co was detected as bioavailable in soil and in the same way it was not detectable in plants. A slightly reducible condition seems able to mobilize important fraction of 60Co in Alfisol. A specific behavior 10,00 for Co in tropical soil is not expected, since these findings have been reported in the specialized literature for temperate

1,00 climate. Data obtained with the new method are coherent TF with transfer factor results and soil properties and seems 0,10 be adequate for radiological risk assessment purposes or to remediate or to manage radiological contaminated sites.

0,01 Goiânia Goiânia Goiânia IUR 1989* 1996 2000 ACKNOWLEDGMENTS

Figure 3. 137Cs Transfer factor for radish obtained in This work received a financial support from the Goiânia soil in 1989 data from [24], 1996, 2000 and IUR International Atomic Energy Agency (contract data [16]. BRA10456/RO), and received student’s fellowship from the Ministério de Ciência e Tecnologia.

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