Arsenic bioaccessibility in contaminated soils: coupling in vitro assays with sequential and HNO₃ extraction

Arsenic bioaccessibility varies with in vitro methods and soils. Four assays including unified BARGE method (UBM), Solubility Bioaccessibility Research Consortium method (SBRC), in vitro gastrointestinal method (IVG), and physiologically based extraction test (PBET), were used to determine As bioaccessibility in 11 contaminated soils (22–4,172 mg kg⁻¹). The objective was to understand how bioaccessible As by different methods was related to different As pools based on sequential extraction and 0.43 M HNO₃ extraction. Arsenic bioaccessibility was 7.6–25, 2.3–49, 7.3–44, and 1.3–38% in gastric phase (GP), and 5.7–53, 0.46–33, 2.3–42, and 0.86–43% in intestinal phase (IP) for UBM, SBRC, IVG, and PBET, respectively, with HNO₃-extractable As being 0.90–60%. Based on sequential extraction, As was primarily associated with amorphous (AF3; 17–79%) and crystallized Fe/Al oxides (CF4; 6.4–73%) while non-specifically sorbed (NS1), specifically sorbed (SS2), and residual fractions (RS5) were 0–10%, 3.4–20% and 3.2–25%. Significant correlation was found between As bioaccessibility by PBET and NS1 + SS2 (R² = 0.55 − 0.69), and UBM-GP and NS1 + SS2 + AF3 (R² = 0.58), indicating PBET mostly targeted As in NS1 + SS2 whereas UBM in NS1 + SS2 + AF3. HNO₃-extractable As was correlated to bioaccessible As by four methods (R² = 0.42 – 0.72) with SBRC-GP having the best correlation. The fact that different methods targeted different As fractions in soils suggested the importance of validation by animal test. Our data suggested that HNO₃ may have potential to determine bioaccessible As in soils