Iron compounds and oil biodegradation in overmoistened contaminated soils: A review of publications

In Russia, the areas of oil pollution gradually shift toward the north into the zone of increased moistening with widespread hydromorphic soils. In this zone, the role of the rapid aerobic degradation of hydrocarbons decreases, while that of slow anaerobic degradation increases. The biological reduction of Fe(III) only proceeds at the expense of the energy of oxidation of organic substances, including oil hydrocarbons, in the oil-polluted soils. This favors the development of technogenic gleying. In contrast to the uncontaminated background soils, in which gleying is correctly considered a degradation process, the same process in the oil-contaminated mineral soils plays a positive role, because it is accompanied by the oxidation of organic pollutants, which otherwise penetrate into rivers and lakes with water flows. The role of Fe(III) reduction may be significant: at one of the oil-spill sites, one-third of the organic pollutants degraded within twelve years after an accident in the anaerobic zone due to Fe(III) reduction. Both iron hydroxides and clay minerals enriched in Fe(III) participate in the reduction processes. In the anaerobic zone, the destruction of organic pollutants begins several years after the relevant natural microorganisms become active. The reduction of Fe(III) reaches its maximum faster than the process of methanogenesis. Upon the soil’s cooling in the winter, the reduction of Fe(III) is replaced by the spontaneous formation of iron oxides (oxidogenesis). Thus, alternating reduction ↔ oxidation reactions proceed in the soils with a contrasting temperature regime. Iron oxides formed in the winter are reduced to Fe(II) in the summer and, thus, resume the associated oxidation of organic pollutants upon the stagnant moisture regime. Therefore, upon monitoring of hydromorphic oilcontaminated soils, special attention should be paid to the forms of iron compounds.