Environ. Sci. Technol. 2008, 42, 140–146
A Novel Technique to Determine Cobalt Exchangeability in Soils Using Isotope Dilution L A U R A A . W E N D L I N G , * ,†,‡ JASON K. KIRBY,‡ AND M I C H A E L J . M C L A U G H L I N ‡,§ CSIRO Land and Water, Centre for Environmental Contaminant Research, Urrbrae, South Australia, and School of Earth and Environmental Science, University of Adelaide, Urrbrae, South Australia
Received June 22, 2007. Revised manuscript received September 27, 2007. Accepted October 16, 2007.
The environmental risk posed by Co contamination is largely a function of its oxidation state. Our objective was to assess the potential biological availability of Co and the reactions and fate of soluble Co(II) after addition to soils with varying physical and chemical characteristics. A potential risk in quantifying exchangeable Co in soils using isotope dilution techniques is the possible presence of two species of Co in soil solution and adsorbed on soil solid phases [Co(II) and Co(III)], coupled with the possibility that when an isotope of Co is added it may undergo a change in oxidation state during the measurement phase. In this study, we have utilized an isotope dilution technique with cation exchange and high-performance liquid chromatography-inductively coupled plasma-mass spectrometry to determine the isotopically exchangeable Co fraction in several soils with varying characteristics such as differing Al, Fe, and Mn oxide content; pH; and organic carbon content. The application of the cation exchange procedure adjusts measurements of isotopically exchangeable Co to correct for the presence of non-exchangeable 57Co not in equilibrium with the solution phase. Results indicated that oxidation of added 57Co(II) to 57Co(III) or precipitation of 57Co(II) may occur on the surfaces of some soils, particularly those with a high pH or substantial quantities of Mn oxide minerals. No detectable Co(III)(aq) was found in the aqueous extracts of the soils examined. Keywords: Cobalt; exchangeability; isotope dilution; E value
Introduction Anthropogenic activities have led to widespread Co contamination of soils (1).While Co is an essential micronutrient for mammals as the central element of vitamin B12, exposure to excess Co can lead to toxic, carcinogenic, and mutagenic effects (2). Early investigations of Co fate and behavior in soils focused on determining effective application rates of Co in micronutrient fertilizers to increase the Co content of forage crops and eliminate Co deficiencies in grazing animals. * Corresponding author e-mail:
[email protected]. † Current address of the corresponding author is CSIRO Land and Water, Centre for Environment and Life Sciences, Floreat, Western Australia. ‡ Centre for Environmental Contaminant Research. § University of Adelaide. 140
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Since that time, concerns have been raised about the migration of cobalt-60 (60Co) in mixed radioactive waste at current and former nuclear facilities (3–5). At present, there are limited scientific data available to assess the risk posed to soil environments by Co contamination. Cobalt differs from many other cationic metals such as Cd, Ni, and Zn in that Co is commonly present in two stable oxidation states in soils, Co(II) and Co(III) (6–11). Total Co concentrations in soils vary widely between about 0.05 and 300 mg kg-1, with average concentrations between 10 and 15 mg Co kg-1 (12). A number of studies have documented the strong association between Co and Mn minerals, both in soils and in marine sediments (9, 13–15). Most Co(III) salts are highly insoluble; Co bioavailability and potential toxicity are more likely related to the availability and reactions of Co(II) and its compounds in soil. Thus, evaluation of the solution and exchangeable pool of Co(II) in soil is an important determinant of the potential ecotoxicology of Co in soils. The use of isotope dilution techniques has gained increasing attention in recent years as a means of quantifying the exchangeable and potentially bioavailable pool of various elements in soils. The isotope dilution technique provides a measure of the labile pool of an element in soil, also known as an E value. The method measures the distribution of an added stable or radioactive isotope between the soil solution and the soil solid phase within a defined time and, using the total amount of isotope introduced, can determine the amount of isotopically exchangeable element in the soil (16–20). A fundamental assumption in the use of isotope dilution techniques to determine the potentially exchangeable pool of an element in soils is that the labile element in solution and the element associated with the soil solid phase must remain in equilibrium throughout the measurement period (16). The earliest studies utilizing the E value technique examined P behavior and exchangeability in soils (21, 22). Isotope dilution techniques have since been applied to other elements in soil, including Al, As, Cd, Ca, Cu, Fe, F, Pb, K, Mn, Ni, S, and Zn to evaluate the nutrient supplying capacity of soils or to examine surface-sorbed elements in relation to mineralogy, pH, and other factors (16). When coupled with modern analytical technologies such as high-performance liquid chromatography-inductively coupled plasma-mass spectrometry (HPLC-ICP-MS), isotope dilution techniques can successfully be adapted to measure the labile pools of multiple species for elements that are redox-sensitive (17, 19). A recent study (17) demonstrated the importance of understanding the solution-solid-phase partitioning of individual As species for the accurate determination of As exchangeability in soils. Hamon et al. (17) determined that As(III) and As(V) were rapidly isotopically self-exchangeable and were able to use isotope dilution and speciation techniques to identify the mechanisms responsible for As mobilization by determining a pseudoequilibrium for both As(V) and As(III). In contrast to As, no rapid isotope self-exchange was observed for Se during a 120 h equilibration period [76Se(IV) and 78Se(VI)] (19). This complication in E value determinations due to changes in speciation and differences in solution-solid-phase partitioning has the potential to occur for any element that can exist in multiple oxidation states in the environment. Previous studies of exchangeable, extractable, or potentially bioavailable Co have only quantified total Co and have not taken into account possible changes in Co speciation. An examination of isotopically exchangeable Co in soils showed significant correlation between isotopically ex10.1021/es071526n CCC: $40.75
Published 2008 by the American Chemical Society
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changeable Co and ammonium acetate- (CH3CO2NH4) extractable Co (23). The CH3CO2NH4-extractable fraction of Co was determined using a technique similar to the standard method for determining easily reducible Mn in soils (24), which suggests that the isotopically exchangeable fraction of Co in soils may be associated with easily reducible Mn. In contrast to these findings, however, a later investigation indicated that the bulk of soil Co was associated with highly crystalline Fe/Mn oxide minerals or was present in the structures of primary and secondary soil minerals (25), and neither ethylenediaminetetraacetic acid (EDTA)- nor acetic acid (CH3CO2H)-extractable Co were significantly correlated with isotopically exchangeable Co. The poor relationships observed between different measures of Co lability may be due to the oxidation and fixation of introduced Co(II) and differential extraction and measurement of Co(III) species by these techniques. Cobalt sorption to soils has been adequately described by a simple Langmuir adsorption equation, and the specific sorption capacity for Co closely related to both total soil Co and Mn concentrations and to specific surface area of the soils (23). Tiller et al. (23) observed Co to sorb to soil surfaces more strongly at low saturation, indicating that the selectivity of soils for Co is dependent upon the proportion of highly Co-selective sorption sites available. The greater specific sorption of Co at low saturation indicates that soils with low concentrations of Co addition and appreciable quantities of soil Mn oxide/oxyhydroxide minerals are more likely to exhibit a significant oxidation of Co(II) to Co(III) or a fixation of Co in non-exchangeable pools. There are a number of potential problems in quantifying labile Co in soils using isotope dilution techniques that have not been considered in previous studies, including (i) Co speciation: the possible presence of nonrapidly selfexchangeable Co(II) and Co(III) in soil solution and adsorbed on soil solid phases (ii) colloidal interferences: the possible presence of nonisotopically exchangeable Co in filtered soil extracts (18) (iii) isotope fixation: the possible conversion of added Co isotopes into non-exchangeable solid-phase forms. Isotope fixation could introduce serious errors into determinations of labile Co pools in soils. The strong association between Co and Mn oxyhydroxide minerals has the potential to induce overestimations in labile Co determinations through the removal of the added Co(II) isotope tracer via surface oxidation processes to non-exchangeable Co(III). The objective of this study was to develop an accurate isotope dilution procedure that accounts and corrects for the above potential errors in the determination of labile Co in soils.
Materials and Methods Description of Soils. This investigation is part of a risk assessment study to examine the fate, behavior, and toxicity of Co in terrestrial environments. Soils with varying physical and chemical characteristics were selected from locations in Europe and Australia (Table S1, Supporting Information). To determine the risk associated with Co in terrestrial environments, experiments were undertaken on soils spiked with cobalt chloride (CoCl2 · 6H2O, Sigma) at various concentrations from nontoxic to lethal to soil microorganisms, invertebrates, and plants. Soil Co chemistry was investigated in soils with sufficient Co added to inhibit microbial nitrification by 10% and 90% (EC10 and EC90, respectively; Table S1, Supporting Information). After spiking, soils were leached with artificial rainwater, air-dried, crushed, sieved to
