Use of natural sorbents for accelerated bioremediation of grey forest soil contaminated with crude oil.

Due to the extensive oil extraction and transportation that occurs in oil-producing countries, many lands remain contaminated because of accidental leakages. Despite its low cost and environmentally safe nature, bioremediation technology is not always successful, mainly because of the soil toxicity to the degrading microbial populations and plants. Here we report a three-year microfield experiment on the influence of natural sorbents of mineral (zeolite, kaolinite, vermiculite, diatomite), organic (peat), carbonaceous (biochar) origin, and a mixed sorbent ACD (composed of granular activated carbon and diatomite) on the bioremediation of grey forest soil contaminated with weathered crude oil (40.1 g total petroleum hydrocarbons (TPH) kg-1). Optimal doses of the sorbents significantly accelerated bioremediation of petroleum-contaminated soil through bioaugmentation followed by phytoremediation. The main reason for the influence of the sorbent amendments relied upon the creation of optimal conditions for the activation of hydrocarbon-utilizing bacteria and plant growth due to the reduction of soil toxicity, as well as maintaining an optimal pH and water-air regime in the soil. That happened because of reducing the soil hydrophobicity, increasing porosity and water holding capacity. The content of the TPH in the best samples (2% biochar or ACD) reduced to their local permissible concentration accepted for remediated soils in the Russian Federation (≤5 g kg-1) after two warm seasons compared to that after three warm seasons in the other samples. Although some sorbents decelerated biodegradation of highly condensed polycyclic aromatic hydrocarbons (PAHs, including benzo(a)pyrene) in the soil, the overall risk from the residual contaminants present in the remediated soil and plants was minimized. The final total content of the main PAHs in the sorbent-amended soils did not exceed the maximal permissible levels that are accepted in most EU countries (1000-40,000 µg kg-1), and they did not accumulate in the aboveground phytomass of grasses in dangerous concentrations.

Express-phytotest for choosing conditions and following process of soil remediation.

Phyto- and bioremediation are perspective methods for soil recultivation. In spite of resistance of plant-hyperaccumulators and degrading microorganisms to some contaminants, there are soil toxicity limits for their growth and activity. Therefore, simple and express methods are needed to estimate the soil phytotoxicity. This article is devoted to description of an express-phytotest evaluated by germination rate of white clover (Trifolium repens) (PhCG) for estimating phytotoxicity of contaminated soils. This phytotest was developed on the example of grey forest soil contaminated with diesel fuel or copper(II) and approbated during our long-year experiments on adsorptive bioremediation of petroleum-contaminated soils. The sensitivity of the phytotest values PhCG to these contaminants is much higher compared to those phytotests evaluated by germination of larger seeds: cress (Lepidium sativum), and wheat (Triticum vulgare). A significant increase of PhCG in those soils by 10% was already recorded at 50-100 mg of available Cu2+ kg-1 and 1-5 g total petroleum hydrocarbons kg-1, depending on the hydrocarbon composition. The sensitivity of the standard phytotests evaluated by root length of wheat seedlings or by plant (T. vulgare or T. repens) biomass is higher than that of PhCG determination. However, bio- and phytoremediation are mostly applied for heavily contaminated soils. Therefore, use of the simple and cheap express phytotest for choosing optimal conditions of the soil remediation and following the process is quite justified. Besides, measuring an additional parameter-root length of the white clover seedlings may significantly increase the sensitivity of the express phytotest for lower contaminated soils.

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.

Dynamics of PCB removal and detoxification in historically contaminated soils amended with activated carbon.

Activated carbon (AC) can help overcome toxicity of pollutants to microbes and facilitate soil bioremediation. We used this approach to treat a Histosol and an Alluvial soil historically contaminated with PCB (4190 and 1585 mg kg(-1), respectively; primarily tri-, tetra- and pentachlorinated congeners). Results confirmed PCB persistence; reductions in PCB extractable from control and AC-amended soils were mostly due to a decrease in tri- and to some extent tetrachlorinated congeners as well as formation of a bound fraction. Mechanisms of PCB binding by soil and AC were different. In addition to microbial degradation of less chlorinated congeners, we postulate AC catalyzed dechlorination of higher chlorinated congeners. A large decrease in bioavailable PCB in AC-amended soils was demonstrated by greater clover germination and biomass. Phytotoxicity was low in treated soils but remained high in untreated soils for the duration of a 39-month experiment. These observations indicate the utility of AC for remediation of soils historically contaminated with PCB.