The improvement of pore characteristics, remediation efficiency, and biotoxicity of petroleum-contaminated soil with the addition of bulking agent on field-scale biopile treatment

Purpose The objectives of this study were to explore the effects of bulking agent and nutrients on the pore characteristics, remediation efficiency, and biotoxicity of petroleum hydrocarbon-contaminated soil. Furthermore, the correlation among the three characteristic indices was studied. Methods Three pilot-scale biopiles were constructed to treat aged petroleum-contaminated soil. Micro-CT was applied to obtain the pore characteristics of piles. Results The results showed that the addition of bulking agent had a positive effect on the pore development of the biopile. The porosity of the pile was increased from 0.33 to 3.32% of the control pile to 4.41 to 26.93%, and the addition of bulking agent showed a significant positive effect on the Feret diameter. However, the addition of nutrients had no significant effect on the porosity and Feret diameter. Besides, the total petroleum hydrocarbon removal rate of bulking agent pile (36.82–49.18%) was significantly higher than that of nutrient piles (20.07–23.43%) and control piles (17.57–27.08%). The bulking agent pile had the lowest toxicity. The porosity of the pile, total petroleum hydrocarbons removal rate, and the characteristic values of biotoxicity (EC50) showed a positive correlation. Conclusion Bulking agents might promote gas circulation by improving the pore structure of the biopile. And the metabolic activity of microorganisms increased as a result of the promotion of the microbial habitat. The higher biodegradation of petroleum hydrocarbons was achieved. However, the main finding of this research is that adding bulk agent to petroleum-contaminated soil significantly increases biodegradation of petroleum. This may be inapplicable to other soils with a different clay content or porosity.