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Influence of In Vitro Assay pH and Extractant Composition on As Bioaccessibility in Contaminated Soils
Citation:
Smith, E., K. Scheckel, B. Miller, J. Weber, AND A. Juhasz. Influence of In Vitro Assay pH and Extractant Composition on As Bioaccessibility in Contaminated Soils. B. Culleres and J.P. Bennett (ed.), SCIENCE OF THE TOTAL ENVIRONMENT. Elsevier B.V., Amsterdam, Netherlands, 473-474:171-177, (2014).
Impact/Purpose:
Arsenic (As) is a commonly encountered contaminant identified at many sites associated with former anthropogenic activities. Arsenic contamination occurs in more than 30 % of US superfund sites (deLemos et al., 2006) and has been extensively identified as a contaminant throughout Europe (Keegan et al., 2006; Navarro et al., 2006; Schulin et al., 2007), Asia (Kocar and Fendorf, 2012; McCarty et al., 2011), South America (Castro de Esparza, 2009; Concha et al, 2010) and Australia (Smith et al., 2006). Surface soil contamination may be a result of natural processes, such as weathering of the regolith, but the major source of elevated As contamination of soils is through anthropogenic activities using As-based herbicides, pesticides, tanning solutions, timber preservatives, as well as through mining and smelting processes. Anthropogenic activities have resulted in As contamination of surface soils ranging from <50 to >15000 mg As kg-1 (Ellice et al., 2001), and although higher reports of As surface contamination has been documented, the occurrence of As concentrations >15000 mg As kg-1 is not common in the soil environment. Exposure to As contaminated soils is of concern to regulatory agencies and the community at large due to As related health disorders which have been linked to As exposure (National Research Council, 1999; Mandal and Suzuki, 2002). Therefore assessing human exposure to As-contaminated soil is a major consideration when undertaking a contaminated site assessment. The objective of this study was to investigate the in vitro assay factors that influence As bioaccessibility in As contaminated soils utilised by Juhasz et al. (2009) using solution, and both surface and bulk soil analysis to determine the major in vitro assay variables that influence As bioaccessibility in contaminated soils.
Description:
In vitro bioaccessibility assays are often utilised to determine the potential human exposure to soil contaminants through soil ingestion. Comparative studies have identified inconsistencies in the results obtained with different in vitro assays. In this study we investigated the potential causes for the variability between in vitro assay results using the PBET and SBRC assays to assess As bioaccessibility in 5 brownfield contaminated soils. Total As concentration in the 5 soils ranged from 227 to 807 mg As kg-1 in the <250 µm particle fraction while XANES analysis identified that As was predominately present as AsV (>88% was sorbed to Fe mineral phases) with the remaining As present as beudantite or orpiment mineral phases. Arsenic bioaccessibility varied depending on the in vitro methodology; markedly higher values were obtained using the SBRC gastric phase compared to the PBET gastric phase, however, similar As bioaccessibility values were obtained in both the SBRC and PBET intestinal phases. The difference in As bioaccessibility following SBRC and PBET gastric phase extraction appeared to be due to the difference in gastric phase pH (i.e. 1.5 versus 2.5 respectively), however, modifying the PBET gastric phase to pH 1.5 (that of the SBRC gastric phase) enhanced As bioaccessibility up to 1.6 fold, but was still markedly lower than SBRC values. Although As bioaccessibility was enhanced, the increase did not occur as a result of the solubilisation of As associated Fe mineral phases suggesting As bioaccessibility may also be strongly influenced by the in vitro assay extractant composition. The extractant composition of the PBET assay incorporates a number of organic acids in addition to pepsin which may inhibit the sorption of As onto iron oxide surfaces, therefore increasing As solubility at the modified (pH 1.5) gastric phase pH.
