Sorption Strength of Persistent Organic Pollutants in Particle-size Fractions of Urban Soils

Abstract

The bioavailability of persistent organic pollutants in soils depends on their sorption strength that may vary among different pools. We hypothesized that polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) had different soil organic C-water partition coefficients (KOC) among particle-size fractions. We determined the concentrations of 20 PAHs and 12 PCBs in coarse-sand, fine-sand, silt, and clay fractions of 11 urban topsoils (0–5 cm). The KOC values were determined using sequential extraction with methanol-water mixtures (35 and 65% methanol) at 60°C. The ∑20 PAHs concentrations ranged from 0.3 to 186 mg kg−1, the ∑12 PCBs concentrations from 1.2 to 158 μg kg−1 In most soils, the PAH concentrations decreased in the order, silt > clay ≥ fine sand > coarse sand, and those of the PCBs in the order, clay > silt ≥ fine sand > coarse sand. The distribution of PAHs among particle-size fractions was more heterogeneous than reported in the literature because the soils received PAH-contaminated wastes (ashes, slags, rubble) with varying texture. In all soils, the proportions of two- or three-ring PAHs decreased with decreasing particle size, indicating that the PAH mixture was increasingly altered. The KOC values of the PAHs were three to 10 times higher than those of the PCBs with similar octanol-water partition coefficients (KOW). The mean KOC values of all individual PAHs were highest in silt. For all individual PCBs, mean KOC values were highest in clay. The KOC values of PAHs and PCBs varied up to a factor of 100 among the studied soils and particle-size fractions. Particle-size fractions with highest PAH and PCB concentrations also showed highest KOC values indicating low bioavailability.