Distribution and biodegradation of nonextractable polycyclic aromatic hydrocarbons in particle-size aggregates of field-contaminated soils

PurposeAdsorption on soil organic matter (SOM) and physical entrapment are the main mechanisms of nonextractable residue (NER) formation of polycyclic aromatic hydrocarbons (PAHs) in soil. Due to the SOM characteristics and pore structure in particle-size aggregates being different, the environmental fates of PAH NERs should be different as well. It's valuable to understand the distribution and biodegradation of PAH NERs in soil particle-size aggregates.Materials and methodsThree field-contaminated soils (Phaeozems, Anthrosols, and Calcareous soil) were separated into four particle-size aggregates (coarse sand, fine sand, coarse silt, and fine silt). The contents, compositions, and biodegradation of parent PAH NERs in particle-size aggregates were studied. Moreover, the effects of soil physico-chemical properties on the distribution and biodegradation of PAH NERs in particle-size aggregates were analyzed.ResultsThe percentages of PAH NERs to total PAHs in particle-size aggregates were in the range of 6.7-9.8%, 2.8-6.0%, and 24.3-35.9% for Phaeozems, Anthrosols, and Calcareous soil, respectively. Comparing with the composition of extractable PAHs, the proportions of two- and three-ring PAHs increased and five- and six-ring PAHs decreased of PAH NERs in all particle-size aggregates for each soil. Moreover, the proportions of PAH NERs decreased with the increase of PAH ring numbers overall. The contents of PAH NERs had significantly positive correlation with the contents of organic carbon (OC) (p < 0.05) in particle-size aggregates; however, the proportions of PAH NERs had significantly positive correlations with specific surface area, pore area, and volume (p < 0.05). The biodegradation rates of PAH NERs in particle-size aggregates were different in each soil and the degradation rates of total PAH NERs were lowest in Phaeozems owning to the highest OC content.ConclusionsThe content of OC was the main factor controlling the distribution and biodegradation of PAH NERs in particle-size aggregates, and the entrapment in soil pores was the main factor controlling the proportions of PAH NERs. The proportions of PAH NERs were higher in low-molecular-weight PAHs than in high-molecular-weight PAHs. Our findings help deeply understand the environmental fates of PAH NERs in soils.