Arsenic Fractionation and Bioaccessibility in Two Alkaline Texas Soils Incubated with Sodium Arsenate Rupali Datta, Konstantinos C. Makris, Dibyendu Sarkar Department of Earth and Environmental Science, University of Texas at San Antonio, 6900 N Loop 1604 W, San Antonio, TX 78249-0663, USA Received: 15 July 2006 /Accepted: 17 December 2006 Abstract. Elevated arsenic (As) concentrations in urban soils with prolonged arsenical pesticide application history have increased the risk associated with accidental hand-to-mouth soil ingestion by children. Earlier work by the authors sug- gested that the conservative statement of 100% As bioacces- sibility in soils was not valid for a set of acidic soils incubated with sodium arsenate. In this study, two alkaline Texas soils incubated with a commonly used As pesticide (sodium arse- nate) were evaluated for their potential in reducing soil As bioaccessibility. The objective of this study was to evaluate the effects of incubation time and As load on soil As fractionation and bioaccessibility. Soils were subjected to a sequential As fractionation scheme, and bioaccessible As was quantified using an in vitro stomach phase test. Results showed a reduction in the water-soluble As fraction with incubation time (after 4 months), which remained unchanged after 12 months. This reduction with time was accompanied by an increase in the NaOH- and H 2 SO 4 -extractable As fractions, suggesting As sorption by amorphous Fe/Al hydroxides and/or Ca/Mg com- pounds, respectively. Organic/sulfides-bound As increased with incubation time after 12 months but not after 4 months of incubation. The aging effect was also observed with the amount of bioaccessible As at all As loads, showing signifi- cant positive correlations with the water-extractable and exchangeable As fractions. Bioaccessible As concentrations even after 12 months of incubation were not significantly re- duced, suggesting that natural attenuation might prove inade- quate to control As bioaccessibility in these alkaline soils. Key words: Arsenic—Pesticide—Speciation—Bioacces- sibility—In-vitro—Fractionation Inorganic arsenic (As) is classified as the number one toxin in the US Environmental Protection Agency (EPA) list of prior- itized pollutants. Arsenicals can be found in surface and sub- surface water bodies, many foods, and soils, although the risk exposure is greater in drinking water than food or soil (Brown and Ross 2002). Primary anthropogenic As sources include Cu smelting, coal combustion, herbicide, pesticide and rodenticide use, as well as waste incineration, steel/glass production, and pressurized wood production (Matschullat 2000). Naturally occurring As in soils originates from the weathering of primary and secondary As-bearing minerals. Arsenic occurs in soils mostly in the V or III oxidation state. Arsenate [As(V)] is the oxidized form and occurs in well-aerated soils, whereas in chemically reduced environments, arsenite [As(III)] species prevails. Although arsenite is more toxic, arsenate is also toxic to humans, plants, and micro-organisms. The risk of human contact with soil As has greatly increased in the last two decades as a result of expanding residential areas into former agricultural land. Soil ingestion from inci- dental hand-to-mouth activity by children is the number one exposure route for As in backyards of homes or playgrounds contaminated with As (Cohen et al. 1998). Elevated As con- centrations were reported in urban soils used for residential development, which were located on former apple orchards with a history of prolonged arsenical pesticide applications (Murphy and Aucott 1998). Sodium arsenate (SA) has been extensively used as a pesticide (e.g., cotton defoliant) in agricultural fields, elevating soil As concentrations beyond background concentrations. Several studies have provided estimates of the amount of soil As ingested by children (Binder et al. 1986; Calabrese et al. 1989), thereby allowing for more accurate risk assess- ments in As-contaminated areas. Arsenic bioaccessibility is a term adopted to replace the old belief that total concentration of a chemical could predict the biological response of living organisms, including humans (NRC 2003). We utilized the term bioaccessibility to better define the ambiguous use of the term bioavailability, which has been used to define both in vivo biological and in vitro chemical extraction assays (Sem- ple et al. 2004). The bioaccessible compound is available to cross an organismÕs cellular membrane from the environment, if the organism has access to the chemical; however, the chemical might be either physically removed from the organism or only bioavailable after a period of time (Semple et al. 2004). Contaminant bioaccessibility is increasingly being used as a key indicator of contaminant risk to environmental and human health (Adriano et al. 2004). So far, lead is the only metal that Correspondence to: Rupali Datta; email: rupali.datta@utsa.edu Arch. Environ. Contam. Toxicol. 52, 475–482 (2007) DOI: 10.1007/s00244-006-0147-7