The Influence of the Physicochemical Parameters of An in Vitro Gastrointestinal Method On the Bioaccessibility of Arsenic and Other Trace Elements in Contaminated Soils.

Various in vitro gastrointestinal methods have been developed over the years that simulate human digestion and the potential absorption (bioavailability) of trace metals from contaminated soils. The selection of human physicochemical gastrointestinal parameters (i.e., pH, soil:solution ratio, retention time, enzymes, and ionic strength) can greatly affect the measured contaminant bioaccessibility.  However, research comparing variations of these parameters and what effects these variations can have on contaminant bioaccessibility has been limited. Many of these studies do not include rationale or data to support the selection of key parameters, investigated a limited number of soils, and/or only investigated one contaminant. The objective of this study was to compare key gastrointestinal parameters that may influence the bioaccessibility of arsenic and other toxic trace elements, including cadmium, copper, chromium, lead, and zinc, in soils.  Six contaminated soils were selected to represent commonly found sources of arsenic contamination, such as mining/smelter waste, wood treatment, and pesticide contamination, and were used to determine the range of bioaccessibility values that may result under these varying conditions.   Additionally, the primary metals that make up the soil matrix, such as aluminum and iron, were evaluated for impact.  Solution pH had the greatest influence on bioaccessibility, with differences ranging from 1 to 48% for gastric pH values ranging from 1.2 to 2.5.  Retention time was also greatly impacted by the mineral composition of the soil.  Slow-release minerals, such as arsenopyrite, exhibited continuous but significant dissolution over 4 hours, while the fast-release oxide minerals demonstrated a plateau effect for bioaccessibility between 45 and 120 minutes.  Additionally, soils with significant levels of extractable iron demonstrated the potential to complex with bioaccessible arsenic at the more neutral pH of the intestinal phase and cause arsenic to fall out of solution.