Journal of Environmental Quality 31:1516-1521 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORTS
Ecological Risk Assessment
Soil and Plant Selenium at a Reclaimed Uranium Mine
Shankar Sharmasarkar*,a and
George F. Vanceb
a Water Management Research Laboratory, USDA-ARS, 9611 South Riverbend Ave., Parlier, CA 93648
b Dep. of Renewable Resources, University of Wyoming, Laramie, WY 82071-3354
* Corresponding author (ssharmasarkar{at}fresno.ars.usda.gov)
Received for publication March 15, 2001.
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ABSTRACT
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Selenium (Se) associated with reclaimed uranium (U) mine lands may result in increased food chain transfer and water contamination. To assess post-reclamation bioavailability of Se at a U mine site in southeastern Wyoming, we studied soil Se distribution, dissolution, speciation, and sorption characteristics and plant Se accumulation. Phosphate-extractable soil Se exceeded the critical limit of 0.5 mg/kg in all the samples, whereas total soil Se ranged from a low (0.6 mg/kg) to an extremely high (26 mg/kg) value. Selenite was the dominant species in phosphate and ammonium bicarbonate-diethylenetriamine pentaacetic acid (AB-DTPA) extracts, whereas selenate was the major Se species in hot water extracts. Extractable soil Se concentrations were in the order of KH2PO4 > AB-DTPA > hot water > saturated paste. The soils were undersaturated with respect to various Se solid phases, albeit with high levels of extractable Se surpassing the critical limit. Calcium and Mg minerals were the potential primary solids controlling Se dissolution, with dissolved organic carbon in the equilibrium solutions resulting in enhanced Se availability. Adsorption was a significant (r2 = 0.76–0.99 at P < 0.05) mechanism governing Se availability and was best described by the initial mass isotherm model, which predicted a maximum reserve Se pool corresponding to 87% of the phosphate-extractable Se concentrations. Grasses, forbs, and shrubs accumulated 11 to 1800 mg Se/kg dry weight. While elevated levels of bioavailable Se may be potentially toxic, the plants accumulating high Se may be used for phytoremediation, or the palatable forage species may be used as animal feed supplements in Se-deficient areas.
Abbreviations: AB-DTPA, ammonium bicarbonate-diethylenetriamine pentaacetic acid • DOC, dissolved organic carbon
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INTRODUCTION
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SELENIUM (Se) occurs naturally in association with uranium (U) deposits, including geological formations mined in Wyoming and other parts of the western USA (Boon, 1989). Although an essential micronutrient for humans and animals, Se at low levels (0.5 mg/kg extractable soil concentrations and 5 mg/kg vegetation contents) can be toxic in nature (Rosenfeld and Beath, 1964; Maas, 1998). With grazing being the primary post-mining reclaimed land use practice in many parts of the western USA, there is cause for concern that conditions do not pose a risk to future use of these areas. Revegetation of mine backfill materials high in Se may result in plants that contain elevated levels of Se. Seleniferous mine land environments and areas affected by mine drainage waters may, therefore, be problematic to grazing animals that consume vegetation containing greater than 5 mg/kg Se for extended periods of time (National Research Council, 1976; Vance, 2000). During mining operations, excavated soils and subsurface materials are exposed to oxidative conditions, thereby elevating the potential levels of bioavailable Se species, which ultimately could enter the food chain through grazing of cattle, sheep, and other ungulates.
The availability of Se is dependent, in part, on the dissolution of different soil inorganic Se species and organic Se components. Four major inorganic species, including selenide (Se2-), elemental Se (Se0), selenite (SeO2-3), and selenate (SeO2-4), are present in soils; both SeO2-3 and SeO2-4 are the predominant Se species in oxidized and alkaline environments (Elrashidi et al., 1987; Jayaweera and Biggar, 1996). Organic Se may also be present in soil solutions (Abrams et al., 1990; Martens and Suarez, 1997), wetland aqueous samples (Zhang et al., 1999), and extracts from reclaimed coal mine soils (Sharmasarkar, 1996). Speciation of Se into SeO2-3, SeO2-4, and organic components was determined in saturated paste extracts of some Californian soils (Fio et al., 1991) and ground waters (Reddy et al., 1995), as well as in biogenic particles and sediments (Cutter, 1985). However, Se speciation in different extracts (e.g., hot water, AB-DTPA, and KH2PO4) from samples collected at reclaimed U mine sites is unavailable. These extracts have been reported to be significant in predicting soil–plant Se correlations under field conditions (Pasch and Vance, 1995; Sharmasarkar, 1996). Sorption studies have been used to understand Se mobility and availability in alluvial soils (Neal et al., 1987), coal mine soils (Sharmasarkar and Vance, 2002), oil shale (Spackman et al., 1990), goethite (Balistrieri and Chao, 1987), kaolinite (Bar-Yosef and Meek, 1987), montmorillonitic (Goldberg and Glaubig, 1988), ground waters (Vance et al., 1998), and agricultural and forest soils (Pezzarossa et al., 1999).
There is, however, limited information on Se bioavailability in reclaimed U mine environments. Due to Se existing at higher than natural concentrations, as well as Se being potentially toxic at low levels, it is important to evaluate both soil and plant Se characteristics at U mine sites. Therefore, the objective of this study was to assess soil Se distribution, dissolution, speciation, and sorption characteristics and plant Se accumulation at a post-reclamation U mine site.
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MATERIALS AND METHODS
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Soil and plant samples were collected from a reclaimed U mine located within the southern Powder River basin of eastern Wyoming, approximately 200 km due north of the University of Wyoming at Laramie. Samples collected included both soil and subsoil materials. Four sampling sites (SA1, SA2, SA3, and SA4) were selected along a north–south transect of a reconstructed hill. Site SA1 was located at a high point of the northern end of the reclaimed U mine, with SA2, SA3, and SA4 located downslope at 150, 300, and 450 m south of SA1, respectively. At each site, soil samples were collected from four depths at 30-cm increments between 0 and 120 cm. The reclaimed soil (Typic Torriorthents) generally had alkaline pH values, moderate salt contents, low organic carbon levels, and a predominance of sand particle-sized materials. Soils were air-dried, hand-ground, and sieved (2 mm) following established processing procedures for mined materials (Spackman et al., 1994).
Selenium is known to be associated with geological materials in roll-front U mining operations (Boon, 1989). We were interested in this site because high Se indicator plants existed within the surrounding area. Our site consisted of the overburden materials collected from the U mine pit, which was deeper than the 120-cm depth of our sample cores. Vegetation at the site was primarily reseeded plant species; however, some native species were also evident and resulted from dispersion of soils that contained native plant seeds. The convex sampling transect included sites located within depression, midslope, and upland areas of a north–south facing hill at the reclaimed U mine. The native area surrounding the site was slightly undulating with a predominance of bare soil, scattered grasses, forbs, and shrubs. No trees existed in the dry, harsh landscape associated with our study site. There was more moisture in the depression site during late spring and early summer due to accumulation of snowmelt and rainfall; however, during the time of sampling, which was in July, there were no noticeable differences in the moisture status of the different sampling sites.
Using the standard Se analysis techniques of Spackman et al. (1994), we analyzed extractable soil Se in saturated paste, hot water, ammonium bicarbonate-diethylenetriamine pentaacetic acid (AB-DTPA), 1 M KH2PO4, and also total Se in HClO4 + HNO3 + HF digests. The pH, electrical conductivity, and soluble sulfate (SO2-4) were determined in saturated paste extracts. Net inorganic Se (SeO2-3 + SeO2-4) was determined with atomic absorption spectrophotometry–hydride generation (AAS–HG) after an HCl reduction step. Solution SeO2-3 concentrations were already measured prior to the reduction process, and then subtracted from the net Se values to obtain SeO2-4 data. Aqueous extracts (deionized water, soil to solution = 1:2; 4-wk shaking on an orbit rotary shaker at 200 rpm) of selected soil samples were analyzed for dissolved cations, anions, and organic carbon. Selenium species and potential solid phases were estimated with the equilibration model GEOCHEM (Sposito and Mattigod, 1980). We also studied sorption of SeO2-3, rather than SeO2-4, because of greater adsorption affinity of the former species to mine soils (Blaylock et al., 1995; Sharmasarkar and Vance, 2002). Soils were equilibrated with SeO2-3 treatments (0–2.5 mg/L; soil to solution = 1:10 w/v) for 48 h on a reciprocating shaker and solution SeO2-3 was analyzed by AAS–HG. The sorption data were evaluated with four isotherm models: linear, Langmuir, Freundlich, and initial mass (Blaylock et al., 1995; Vance et al., 1998; Sharmasarkar and Vance, 2002). Our previous studies indicated that the shaking periods were adequate for reaching equilibrium (Sharmasarkar, 1996).
Grass, forb, and shrub samples were also collected within a 10-m radius of each soil sampling site. Plant samples were dried, finely ground, and sieved through a 0.25-mm screen. The plant materials were digested with a mixture of nitric and perchloric acids (HNO3 + HClO4) and analyzed for total Se following the protocol described in Steward et al. (1994). All experiments were conducted with three replicates, and statistical correlations between adsorbed and solution Se concentrations as well as with soil properties were tested at the 0.05 level of significance.
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RESULTS AND DISCUSSION
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Soil Characterization
The U mine soils were alkaline (pH 7.7–8.1) with low to moderate electrical conductivity (EC) and SO2-4 levels and low to high Se concentrations (Table 1). A level as high as 26 mg/kg total Se (Site SA1) was detected in the 90- to 120-cm depth sample. Total Se levels within the 0- to 30-cm depth samples ranged from 1.39 to 7.06 mg/kg, indicating that the mining area was highly seleniferous. Site SA1 had higher EC, SO2-4, and Se levels as compared with other sites. While we refrained from inferring a definite S–Se relationship with the limited data points, a pattern of increasing Se and decreasing SO2-4 concentrations with depth was noted within SA1 samples. This observation conformed to the reports of S–Se antagonism (Sposito, 1989; Vance et al., 1998). The EC, which is a function of the mobility of various ions, decreased with depth at SA1.
Selenium Extraction and Speciation
Phosphate-extractable Se (SeO2-3 and SeO2-4) at all four sites exceeded the Wyoming Department of Environmental Quality critical limit of 0.3 to 0.8 mg/kg for mine soils (Table 2). Extractable Se concentrations followed an order of KH2PO4 > AB-DTPA > hot water > saturated paste; saturated paste–extractable Se levels were very low (<0.05 mg/kg). Pasch and Vance (1995) also found a similar trend in the extractable Se concentrations of numerous soil and overburden samples collected from coal mines and native environments within the Powder River basin, Wyoming.
In addition to extracting the greatest concentrations of Se, the phosphate extract also can be used to determine the relationship between total extractable Se and Se species. Compared with the other sites, SA1 had the highest concentrations of phosphate-extractable SeO2-3 and SeO2-4. The data indicated that SeO2-3 was dominant in KH2PO4 and AB-DTPA extracts, whereas SeO2-4 was the major Se species in hot water extracts. The low level of Se in the saturated paste extracts precluded our ability to determine Se species in these solutions.
From studies conducted on the fractionation of solid-phase Se species, Sharmasarkar (1996) and Martens and Suarez (1998) reported that SeO2-3 and SeO2-4 were the dominant species in phosphorus and aqueous extracts, respectively. Conceptually, the SeO2-4 extracted by distilled-deionized and/or hot water are loosely bound to the soil particles and hence brought into solution by solubilization or low thermal energy processes. In contrast to the inner-sphere complexation of SeO2-3, bonding of SeO2-4 is considered to occur mainly through diffuse-ion swarm association and outer-sphere complexation (Sposito, 1989). The relatively lower strength of SeO2-4 stems from its electrostatic nature of bonding as compared with the more stable nature of combined ionic and covalent bonds involving SeO2-3. Subsequently, the stabilization energy (heat of bond formation) for SeO2-3 bonding is larger than that for SeO2-4. The hydroxyl ions (OH-) in the hot water extract can substitute for nonspecifically adsorbed SeO2-4 that is weakly bound in outer sphere complexes. This is also consistent with thermodynamic predictions regarding SeO2-4 solubility (Elrashidi et al., 1987, 1989). Phosphate can solubilize specifically adsorbed SeO2-3 by means of anionic competition and/or ligand exchange mechanisms (Parfitt, 1978). The SeO2-3 is generally present as an inner sphere complex associated with clay minerals such as kaolinite and montmorillonite (Bar-Yosef and Meek, 1987). Extraction with AB-DTPA, a strong chelating ligand, can result in metal cation complexes that release Se compounds, thereby solubilizing SeO2-3. However, the predominance of a particular species does not exclude the occurrence of other species in solution in relatively lower concentrations. Thus, SeO2-4 was detected in KH2PO4 and AB-DTPA extracts, and SeO2-3 was also present in the other extracts.
Selenium Dissolution
Both [>SeO3]0 and [>SeO4]0 complexes comprising H, Ca, Mg, Mn, and Zn were predicted to be present in the mine soil solutions (Table 3). The predominant species were CaSeO04, MgSeO04, and MgSeO03 as compared with the transition metal complexes. Site SA1 had higher estimated activities of these species than the other sites. In spite of elevated SeO2-3 and SeO2-4 concentrations in different extracts, as discussed earlier in Table 2, the saturation index values for these mine soils indicated that the solutions were undersaturated with respect to various Se solid phases (Table 3). This observation suggests that Se precipitation is not likely to be a primary process. Instead, a dissolution mechanism would contribute to Se availability. A closer inspection of the data indicates there was an estimated predominance of Mg and Ca complexes in soil solutions, with MgSeO3, CaSeO3·H2O, and CaSeO3 being closest to the equilibrium saturation index. Thus, salts of Ca and Mg were likely to be primary solids controlling Se dissolution. There remains a possibility that these solid phases may be able to partially control solution chemistry if saturation levels are reached through long-term pedochemical changes. Definite nature of such weathering process involving Se is yet to be known. Site SA1 had a greater saturation condition than the other sites, as noted with the saturation indices of Ca and Mg minerals.
Influence of Dissolved Organic Carbon
When dissolved organic carbon (DOC) was considered, GEOCHEM modeling predicted that there would be a decrease in solution metal activities and an increase in SeO2-3 and SeO2-4 concentrations (Table 4). The DOC concentrations in SA1, SA2, SA3, and SA4 were 4.93, 3.46, 19.0, and 14.5 mg/L, respectively. Apparently, greater metal–DOC complexation resulted in higher SeO2-3 and SeO2-4 levels in solutions. Formation of metal–DOC chelates is very likely to cause dissociation of metal–Se complexes, and subsequent release of SeO2-3 and SeO2-4 into solution. Also, DOC would compete with Se species for sorption sites, which could lead to desorption of SeO2-3 and SeO2-4. The effect of DOC was most noticeable for SeO2-4 in the SA3 soil, the sample with the maximum DOC level. Percentages of both Se ions as "free" components were higher when DOC was included in GEOCHEM modeling, whereas calculations without considering DOC resulted in greater ionic distributions in "bound" components. Vance et al. (1998) reported that the DOC fractions, hydrophobic and hydrophilic acids, can outcompete SeO2-3 for adsorption sites, thus resulting in greater mobility of SeO2-3 in mining environments.
Selenium Sorption
Due to the undersaturated soil conditions with respect to various Se solid phases, albeit with high levels of phosphate-extractable Se surpassing the Wyoming Department of Environmental Quality critical limits, sorption–desorption processes appeared to be a more plausible mechanism governing U mine Se immobilization chemistry (Table 5). We studied the sorption characteristics of SeO2-3, rather than SeO2-4, because of the greater sorptivity of the former species in mine environments (Blaylock et al., 1995; Sharmasarkar, 1996; Vance et al., 1998). As compared with the other sites, SA1 was characterized by the highest SeO2-3 as well as total Se concentrations, and therefore, selected for the sorption study.
Selenite sorption isotherms for the different depth samples collected from Site SA1 had high correlations (r2 = 0.761 to 0.998) according to three of the four models evaluated (initial mass = linear > Freundlich); Langmuir isotherms resulted in poor correlations (r2 = 0.008 to 0.564). With the Langmuir model, the energy of adsorbed molecules is assumed to be uniform, which is possible only for an intrinsically homogeneous surface. Based on this assumption, intra- and inter-molecular forces of attraction or repulsion should be zero, which is very unlikely in a heterogeneous system like soil. Hence, it is not unexpected that our data do not conform to the Langmuir model. The sorption–desorption coefficient (K) for all models was highest for the surface (0–30 cm) sample, as compared with other sample depths. Freundlich n values ranged from 1.0 to 1.7 with the lowest value (90–120 cm depth sample) suggesting a linear relationship. Freundlich fitting of Se sorption was also reported by Fio et al. (1991), who observed that SeO2-4 was not retained by alluvial soils, whereas SeO2-3 was rapidly adsorbed and conformed to the Freundlich model under alkaline soil conditions. In a study with oil shales, Spackman et al. (1990) concluded that SeO2-3 adsorption was described by both Langmuir and Freundlich isotherms, with greater fitting to the Freundlich model. For the present study, the best fitting was found with the initial mass isotherm. In a study with SeO2-3, Blaylock et al. (1995) concluded that the initial mass isotherm best described sorption behavior, and observed that "the Freundlich and Langmuir isotherms did not effectively predict Se sorption." The linearity in the initial mass isotherm results suggested that at low Se concentrations the sorption sites at surface mine environments were probably never saturated. This was consistent with our observation of undersaturated soil conditions as described in previous sections. A close review of the data indicated that the net initial mass B value in the whole profile (0.74 mg/kg for 120 cm) was nearly 75% of the phosphate-extractable SeO2-3 concentration at SA1.
Plant Selenium Accumulation
Within five years of soil reclamation and seeding, various grass, forb, and shrub vegetation were determined to accumulate very high levels of Se (Table 6). Similar findings of high plant Se accumulations have been reported by Sharmasarkar (1996), Schladweiler et al. (1999), and Vance (2000) for coal mines, wetland and riparian ecosystems, and a phosphate mine, respectively. At the reclaimed U mine site, forbs tended to be more prolific than grass or shrub vegetation. Forbs also accumulated the maximum Se concentrations (>1000 mg/kg). All the vegetation sampled at the U mine site had accumulated Se at concentrations in excess of the NRC's critical limit of 5 mg/kg for prolonged animal consumption, and thus would not be suitable for confined grazing. While elevated levels of bioavailable Se in this reclaimed area may be potentially toxic, some of the revegetated plants may have possible applications for phytoremediation as well as animal feed supplements in Se-deficient areas. For example, due to the high Se accumulation potential (>1000 mg/kg) of Astragalus and Grindelia species, these may be useful for Se phytoextraction. The grass species with moderate Se accumulation levels (<20 mg/kg), including Agropyron, Bromus, Elymus, and Stipa, could be used in a mixture with nonseleniferous vegetation as animal feed for nutritional purposes. Although it is beyond the scope of this paper, we recommend a Se mass balance and plant biomass analyses prior to commercial decision-making by the potential users interested in using plant species for either phytoremediation or animal nutrition purposes.
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CONCLUSIONS
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The results of this study provide information regarding the soil–plant Se characterization, speciation, and distribution behaviors of Se in a reclaimed U mining environment within eastern Wyoming. Anionic Se species can participate in biogeochemical processes; complex Se species, though neutral in charge, may also be transformed into anions over time and become part of the Se cycle. Selenite was the dominant Se species in KH2PO4 and AB-DTPA extracts, whereas SeO2-4 was the major Se species in hot water extracts. From a thermodynamic standpoint, SeO2-3 remains stable over a redox intensity (pe) + pH range of 7.5 to 14.5, and above this value SeO2-4 predominates in soil solutions (Elrashidi et al., 1989). Thus, occurrence of these two Se species was expected in our U mine site where mining activities have exposed soils to oxidative (atmospheric) conditions. Presence of DOC in the soil system would be predicted to enhance Se bioavailability. The soils were undersaturated with respect to various Se solid phases, albeit with high levels of extractable Se surpassing the Wyoming Department of Environmental Quality critical limit of 0.3 to 0.8 mg/kg Se. Salts of Ca and Mg were likely to be the primary solids controlling Se dissolution. Sorption–desorption processes appeared to be a more plausible mechanism governing Se immobilization chemistry. Different plant species collected from the reclaimed U mine included various grass, forb, and shrub samples that contained high Se concentrations. There may be a potential for on-site phytoremediation or possibly harvesting the vegetation for use as a feed supplement. Overall, it can be inferred that solution-phase Se extraction and subsequent speciation provide a means of evaluating the distribution of different Se components in various extracts. In addition, this information is important for examining the mutual relationships among Se forms and species, which can be useful in understanding the solubility and toxicity behavior of Se and developing reclamation strategies in U mine environments.
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ABSTRACT
INTRODUCTION
