Fractionation And Extraction Of Persistent Organic Pollutants In Soil

Oral intake is one of the most important pathways of human exposure to toxic substances. Since not all contaminants in soil can be absorbed by human digestive system, inaccessibility is critical in terms of risk assessment for oral exposure. In-vitro gastrointestinal model is a commonly used screening method for evaluating inaccessibility of various toxic substances. A previous study on oral bioaccessibility of contaminated soil revealed that the total amount of organic chlorine pesticides in digestive fluid and chyme extracted by a gastrointestinal model seems to be higher than the quantity directly extracted by harsh solvent extraction, providing a clue that bound residue of these contaminants could be released. The result not only provided mobilization potential of sequestrated organic contaminants, but also suggested a hypothesis to be tested in this study, e.g. the conventional procedure of harsh solvent extraction may underestimate risk of oral intake of contaminated soils. This hypothesis was tested in this study for both organic chlorine pesticides (OCPs) and polycyclic aromatic hydrocarbons (PAHs). It was found that.Three soils with different contents of soil organic carbon (SOC) have been collected in this study. The mobilization of the sequestrated OCPs and PAHs has been observed. For the three soils with different organic carbon (SOC) content tested the bound residue of individual compounds of OCPs and PAHs were mobilized during in vitro gastrointestinal digestion. The qualities of bound residues mobilized varied from 80% to580% of amount extracted by conventional extraction procedure without digestion. It was found that the ratio of the extracted bound residue was positively correlated with SOC and negatively correlated with total concentration of the contaminants in the soil. Besides,among five constitutes in the digestive juice, only bile salt served to mobilized the bound residue and the extractability of bile salt was constant over a concentration range from 2 to 20 mg/mL. The result of kinetic experiments indicated that the process of the mobilization followed a typical first-order kinetics which can be quantified by an exponential function. By fitting the kinetics model, the total amounts of extractable residue and bound residue could be quantified. The calculated rate constant suggested a fast mobilization processes. Ninety percent of bound OCP and PAH residues could be mobilized within 2.4 h and 4.8 h, respectively.These findings provided strong evidence indicating that the oral inaccessibility of OCPs and PAHs in contaminated soil can be underestimate significantly even using harsh organic solvent extraction.