Adsorptive bioremediation of soil highly contaminated with crude oil.

Due to the extended oil extraction and transportation in Russia and other oil-producing countries, many lands remain contaminated because of accidental spills. This situation requires the cost-effective and efficient remediation of petroleum-contaminated soils. Bioremediation of soils contaminated with high concentrations of crude oil is usually hampered by high toxicity thresholds for microbial degraders. We have performed a two-year microfield experiment on the influence of a mixed adsorbent (ACD) composed of granular activated carbon and diatomite on bioremediation of a grey forest soil contaminated with crude oil at concentrations (5-15 % w/w) that would theoretically not result in a successful pollutant removal due to toxicity. Remediation of these soils was evaluated after treating with the ACD adsorbent (from 4 to 12% w/w) and a biopreparation (BP) containing hydrocarbon-degrading bacteria, separately or in combination. Reduction of total petroleum hydrocarbons content was significantly greater in highly contaminated soils with the combined amendments than in the respective controls (through the activation of indigenous degrading microorganisms by fertilizing and mixing) by 9-10% and 5-8% at the end of the first and second years, respectively, depending on the contamination level. Significantly higher counts of petroleum-degrading microorganisms (as indigenous and introduced by the BP), as well as much less phytotoxicity was detected in the ACD-amended soils, as compared with the samples without adsorbent. In addition, the ACD mixture drastically reduced the wash-out of polar petroleum metabolites (evidently oxidized hydrocarbons) and the phytotoxicity of the lysimetric waters, especially in highly contaminated soils. The results indicate that the mixture of activated carbon and diatomite is a prospective adsorbent for the in situ bioremediation of soils highly contaminated with crude oil.