Soil Controls On Arsenic Bioaccessibility: Arsenic Fractions and Soil Properties.

     In vitro gastrointestinal methods provide a rapid and inexpensive measure of bioaccessible arsenic which can be used to conduct more accurate and site-specific human health risk assessments of contaminated soil. However, in order for in vitro methods to become widely accepted as tools that accurately assess soil As exposure through the oral ingestion pathway, a better understanding of the fractions of soil arsenic that are measured by the in vitro extraction and the underlying soil properties and components associated with As bioaccessiblility (BA) are needed.  Bioaccessible As in 19 soils was determined using The Ohio State University in vitro gastrointestinal method (OSU-IVG) and soil As was fractionated using a four step sequential extraction.  Soil type had a large effect on As BA, ranging from 11 to 82% with a mean of 38% in the gastric phase and from 12 to 84% in the intestinal phase with a mean of 39%.  Highly significant (P < 0.0001) relationships exist between bioaccessible As and the combination of soil pH and Fe extracted by citrate bicarbonate dithionite (Fecbd) or soil pH and Fe extracted by acid ammonium oxalate (Feox).  Soil pH explains more of the variation in bioaccessible As (r² = 0.67) than Fecbd (r² = 0.45) or Feox (r² = 0.38). The sequential extraction results indicate that As extracted from the non-specifically sorbed (F1) and specifically sorbed (F2) fractions provide a good measure of soil property controlled As BA (GE, r² = 0.92, m = 1.06; IE, r² = 0.94, m = 1.07).  The largest contribution to bioaccessible As is released from the specifically sorbed (F2) fraction of soil As.  Because F2 As is the largest contribution to bioaccessible As, it is the strong linear relationship between F2 and soil pH that controls the relationship between bioaccessible As and soil pH.