Characterization of Bioaccessible Porosity in Soil Aggregates by Micro-CT Scanning

English Abstract:

In this study we characterized soil aggregate micro-structures in terms of pore network characteristics and evaluated the potential role of the micro-structure on biodegradation of petroleum hydrocarbons in a fine-grained soil obtained from an oil-contaminated site. Bioremediation experiments indicated significant biodegradation of about 50% of the total petroleum hydrocarbons and a relatively high remediation endpoint of about 620 mg/kg soil. It is likely that the residual hydrocarbons were entrapped in pores that were inaccessible to hydrocarbon-degrading bacteria. The intra-aggregate bioaccessible porosity, the fraction of aggregate volume constituted by pores that bacteria can access, was determined by X-ray CT scanning. A new image analyses procedure was developed to threshold the images, determine the boundary of aggregate and extract the pore network information. The CT scanning results showed that about 26% of soil aggregate volume was potentially accessible to the hydrocarbon degrading bacteria, whereas, about 1.5% of aggregate volume was attributed to inaccessible pores. The volume of pores smaller than 1 µm could not be determined by CT scanning and was determined by BET analyses. The significant volume of potentially non-bioaccessible pores can explain the observed relatively high remediation endpoint in the studied soil. The procedure for CT image analysis of intra-aggregate porosity developed in this study can be a new approach for a priori assessment of bioremediation performance in oil-contaminated soils.