Effects of source materials on desorption kinetics of PAHs from contaminated soils

Polycyclic aromatic hydrocarbons (PAHs) are a group of ubiquitous soil contaminants generated mostly from anthropogenic activities. Remediation and management of contaminated land with PAHs relies on robust risk assessment, which is underpinned by bioavailability and bioaccessibility of these pollutants. A significant volume of research in the past few decades has identified a number of factors controlling bioavailability and bioaccessibility of PAHs in soil. In general, the quantity and quality of soil organic matter (SOM) determine the sorption and desorption of PAHs, which control their bioavailability and bioaccessibility. Unlike other organic contaminants such as pesticides and polychlorinated biphenyls (PCBs), PAHs are unintentionally produced and released to soil in various source materials including non-aqueous phase liquids (NAPLs), solids, and semi-solids. The routine approach adopted in investigations on bioavailability and bioaccessibility of soil PAHs, which is to deliver PAHs in volatile organic solvents, overlooks the effects of these source materials. Recent studies have demonstrated higher solid-liquid partition coefficients (Kd) and lower dermal uptake of PAHs in artificial soils contaminated by different source materials, compared to those by PAHs in organic solvent (Xia et al., 2016). It was suggested that the carbonaceous source materials possess high affinity to PAHs in general, leading to decreased bioavailability and bioaccessibility. Steric hindrance in pore structures and intense surface adsorption in solid source materials, as well as retarded interfacial transfer and internal diffusion in NAPLs, are believed to contribute to the enhanced sorption. As a result, these materials have been more considered as sorption phases for PAHs, while their other role as sources is not well characterised. It is important to investigate the release of PAHs from soils contaminated with different source materials as rapidly desorbing fraction of organic pollutants drives the risks to receptors in risk assessment (Ortega-Calvo et al., 2015). This ongoing research aims to study the desorption kinetics of PAHs from soils contaminated with different source materials including pitched coal tar (semi-solid), liquid coal tar (NAPL) and traffic soot (solid), and elucidate how soil total organic carbon (TOC) and texture affect the desorption processes. (author)