Environ. Sci. Technol. 2005, 39, 7102-7110
Adsorption, Oxidation, and Bioaccessibility of As(III) in Soils
properties, the resulting estimates are valid as long as the major soil properties do not change, thus providing some confidence in the long-term applicability of the estimates.
J A E - K Y U Y A N G , † M A R K O . B A R N E T T , * ,† JINLING ZHUANG,† SCOTT E. FENDORF,‡ AND PHILIP M. JARDINE§ Department of Civil Engineering, 238 Harbert Engineering Center, Auburn University, Auburn, Alabama 36849, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, and Environmental Sciences Division, Oak Ridge National Laboratory, Post Office Box 2008, Oak Ridge, Tennessee 37831
Introduction
At As-contaminated sites, where the ingestion of soil by children is typically the critical human-health exposure pathway, information on the bioavailability of soil-bound As is often limited. The influence of various soil physical and chemical properties (iron and manganese oxides, pH, cation exchange capacity, total inorganic and organic carbon, and particle size) on As(III) adsorption, sequestration, bioaccessibility (as a surrogate for oral bioavailability), and oxidation was investigated in 36 well-characterized soils by use of a physiologically based extraction test (PBET). These results were compared to an earlier published study with As(V) on the same set of soils. The properties of the soils were able to explain >80% of the variability in the adsorption and sequestration (as measured by the reduction in bioaccessibility over time) of As(III) in these soils. The initial bioaccessibility of As(III) was significantly higher than the initial bioaccessibility of As(V) on the same set of soils. However, over a 6-month period of aerobic aging, a significant portion of the solid-phase As(III) on these soils was oxidized to As(V), decreasing its bioaccessibility markedly. A multivariable linear regression model previously developed to predict the steady-state bioaccessibility of As(V) in soils was able to predict the bioaccessibility in As(III)-spiked soils within a root-meansquare error (RMSE) of 16.8%. Generally, soils having a higher iron oxide content and lower soil pH exhibited lower bioaccessibility. This model was also able to predict the in vivo bioavailability of As in contaminated soils previously used in an independent juvenile swine dosing trial within an RMSE of 15.5%, providing a greatly improved yet conservative estimate of bioavailability relative to the typical default assumption of 100%. However, the model was not able to accurately predict the bioavailability of As in a different set of contaminated soils previously used in an independent Cebus monkey dosing trial, consistently overpredicting the bioavailability, resulting in an RMSE of 42.7%. This model can be used to provide an initial estimate of As bioavailability in soil to aid in screening sites and justifying expensive site-specific animal feeding studies. Further, as the model is based on major soil * Corresponding author phone: +1 (334) 844-6291; fax: +1 (334) 844-6290; e-mail:
[email protected]. † Auburn University. ‡ Stanford University. § Oak Ridge National Laboratory. 7102
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 18, 2005
Arsenic contamination in soils results from various anthropogenic sources such as mining activities, the discharges of industrial wastes, and agricultural application as well as from geochemical reactions (1-4). Although As has multiple oxidation states (+5, +3, 0, and -3), arsenite As(III) and arsenate As(V) are the most common in natural environments (5, 6). Generally, As(III) is favored in sediments or in the aqueous phase under anaerobic conditions, while the reverse order of species occurs under aerobic conditions. The speciation and solubility of inorganic As is sensitive to redox conditions and the pH of the environment, which affects the bioavailability, toxicity, and mobility of As in soils (7, 8). Since As(III) is much more toxic and mobile than As(V) (9), the oxidation of As(III) to As(V) is important in reducing the risk of As-contaminated soils. Children are often exposed to soils contaminated with toxic metals such as As through hand-to-mouth activity. Therefore, the ingestion of soils contaminated with toxic metals is of great concern because of their toxicity and threat to human health. The ingestion of As-contaminated soils by children is typically the risk driver at As-contaminated sites (10). This risk has often been estimated by assuming that soil-bound metals were completely absorbed (100% bioavailable) through the human gastrointestinal tract upon ingestion, although
