Enhanced anoxic biodegradation of polycyclic aromatic hydrocarbons (PAHs) in a highly contaminated aged soil using nitrate and soil microbes

Biodegradation of polycyclic aromatic hydrocarbons (PAHs) is an alternative method for the removal of these compounds from polluted environments. However, there is a lack of knowledge regarding anoxic degradation of the 16 United States Environmental Protection Agency (USEPA) priority PAHs by indigenous bacteria in a highly contaminated aged soil with PAHs. Accordingly, we hypothesized that nitrate, as an electron acceptor, can enhance the anoxic biodegradation of PAHs in the presence of soil microbes, in contaminated soils. The objective was to investigate if the use of nitrate and soil microbes can enhance the anoxic biodegradation of PAHs in an aged soil from a former steel-making factory. The addition of nitrate did not improve the anoxic biodegradation of 5- and 6-ring PAHs; however, it facilitated anoxic biodegradation of 3- and 4-ring PAHs in soil (p < 0.05). After 300 days of anoxic incubation, the removal efficiency of 3- and 4-ring PAHs ranged from 45 to 73% and 32-63%, respectively. As more nitrate was added, the percentage of 3- and 4-ring PAHs biodegraded increased. Bacteria capable of degrading PAHs under anoxic conditions were dominated by bacterial strains belonging to Proteobacteria and Firmicutes, which accounted for 55% of the total number of indigenous bacteria in soil. Moreover, the total number of Proteobacteria and Firmicutes increased with nitrate added, facilitating the degradation of 3- and 4-ring PAHs. The results in this study also showed that the recalcitrance of PAHs increased with increasing number of benzene rings, molecular weight and organic carbon/water partition coefficient (Koc) of PAHs. However, the recalcitrance of PAHs increased with decreasing solubility of the PAHs. Overall, the results of this study offer an attractive approach to the removal of PAHs from polluted subsurface soils in industrial sites using nitrate and the consortium of soil bacteria.