Formation of non-extractable residues as a potentially dominant process in the fate of PAHs in soil: Insights from a combined field and modeling study on the eastern Tibetan Plateau

Whereas non-extractable residue (NER) formation is recognized as an important process affecting the ecological risk of organic contaminants in soils, it is commonly neglected in regional-scale multi-media models assessing chemical environmental fate and risk. We used a combined field and modeling study to elucidate the relative importance of NER formation to the reduction in available organic contaminants compared with fate processes commonly considered in risk assessment models (volatilization, leaching, and biodegradation). Specifically, four polycyclic aromatic hydrocarbons (PAHs), i.e., phenanthrene (Phe), pyrene (Pyr), benzo[a]pyrene (BaP), and benzo[ghi]perylene (BghiP), were spiked and measured in a one-year field pot experiment at four sites with diverse environmental conditions on the eastern Tibetan Plateau. The rate of NER formation was derived as the difference between the overall rate of decline in total-extractable PAH concentrations, obtained by fitting a biphasic first-order model to the measured concentrations, and the sum of the calculated rates of volatilization, leaching, and biodegradation. Our work shows that the total-extractable PAH concentration undergoes a rapid decline and a slow decline, with shorter overall half-lives (especially for BaP and BghiP) than those observed in earlier studies. Generally, NER formation was assessed to be the dominant contributor (64 ± 33%) to the overall decline of PAHs, followed by biodegradation (35 ± 32%); volatilization and leaching were the smallest contributors. In particular, heavier PAHs (i.e. BaP and BghiP) tend to have shorter half-lives in the rapid and the overall decline phase, indicating that the erroneous estimation of environmental fate and risks might be more pronounced for organic contaminants with a large molecular size. The trend of overall decline rates of PAHs displayed a combined effect of NER formation and biodegradation. This work indicates the need to consider NER formation as a process in multi-media models of chemical fate and risk.