Bioavailability of freshly added and aged naphthalene in soils under gastric pH conditions

The bioavailability of hydrophobic organic chemicals decreases with aging i n soil because of sequestration. However, assessments of the risk of exposu re to contaminated soils are usually dependent on either chemical concentra tions, which are measured using vigorous extraction methods, or models that assume an equilibrium without considering the actual conditions. The objec tive of this research was to determine the availability and desorption kine tics of freshly added and aged naphthalene from a peat and a mineral soil; naphthalene was desorbed into solutions with pH levels that approximate tho se found in different gastric regions. Soil and peat samples were spiked wi th radiolabeled and unlabeled naphthalene at 2 and 20 mu g/g and were aged from 0 to 135 d. Desorption kinetics were determined using a simulated stom ach solution (0.1 M NaCl, 0.1 M HCl, 0.01 M NH4Ac, pH = 1.0) and a neutral solution (0.2 M NaCl, pH = 6.7) that represented the pH of intestinal condi tions and most soils. Feat sorbed much more naphthalene than did soil, and it allowed little desorption. Though both acidic and neutral extracting sol utions could desorb naphthalene, little apparent effect of aging was observ ed in peat, whereas desorption from soil declined markedly with aging. In a ddition, the percentage of naphthalene that desorbed from soil was greater for the higher incubation concentration. The desorption of naphthalene from the pear and soil was higher into the neutral solution than into the gastr ic solution. These results suggest that aging, exposure conditions, concent ration effect, and organic matter content should be taken into account in p redictive models and risk assessments.