Effect of soil properties, compound aging, and presence of cosolute on sorption, desorption, and biodegradation of polycyclic aromatic hydrocarbons in natural soils

Serious health concerns arise when polycyclic aromatic hydrocarbons (PAHs) are present as environmental pollutants due to their carcinogenicity and mutagenicity. Sorptive interaction of PAHs with geosorbents has been known to be the most important factor governing PAH biodegradability. To better understand complex bioavailability issues, this research was conducted to evaluate the contribution of soil organic matter (SOM) and clay minerals and the effects of compound aging and cosolutes to the sorption, desorption and subsequent biodegradation of PAHs.; Experimental results showed that both SOM and clay minerals played a significant role in sorption and desorption of PAHs. Their contribution depended on their quality (SOM fraction and clay mineralogy) and quantity (concentration), as well as the type and concentration of PAHs. More extensive studies showed that, for both sorption and desorption, the presence of a co-solute modified the solute's sorptive characteristics and depended on the compounds concentration. Furthermore, the extent of PAH sorption and desorption was very sensitive to the change in the sorptive contact time. Under all three different sorption/desorption contact times investigated, it was found that a substantial fraction of the sorbed pyrene (PYR) was not desorbed within the allotted time (i.e., <12% for 3 mg PYR/L and 240-hr contact time). This desorption hysteresis was attributed to the sorption enhancement due to soil hydration by which more sorption sites, such as the internal sites of the expandable clays, became available.; Compound aging for 200 days resulted in higher PAH concentrations and lower bacterial activity in the solution phase. Surprisingly, the total biodegraded extent was greater in the 200-day aged soil system (78% PYR biodegradation) than in the freshly spiked system (65%). These findings were attributed to indigenous Pseudomonas aeruginosa entering a stationary phase and producing rhamnolipid biosurfactants. Furthermore, cometabolism enhanced PYR degradation when phenanthrene (PHE) was present as a primary substrate. The dissimilar soils also showed different PAH biodegradation efficiencies. For instance, 90% PHE biodegradation was achieved in the Colombia soil whereas a 98% reduction was obtained in the New Mexico soil. This was due to the higher amount of the total expandable clays governing the substrate availability and microbial accessibility. The most significant aging effect was present in the field aged soil where the waste oil was aged for 30+ yrs, resulting in more resistant biodegradation.; The most meaningful result of this study is that clay minerals, especially the expandable clays, played an important role in the sorption, desorption, and biodegradation of PAHs. Therefore, the contribution of the SOM and clay minerals to the abiotic and biotic fate and transport of PAHs should be taken equally into account in order to better understand complex bioremediation issues.