Clay mineral-enhanced bioremediation of marine oil pollution

Three year old laboratory mixtures of Prestige oil, Atlantic sea water and various types of clay mineral powder (Na- and Ca-smectites, palygorskite, synthetic mica-montmorillonite and a low defect kaolinite) contained significantly lower concentrations of extracted oil than the non-clay containing control. As no significant quantities of oil could be detected in the clay samples following solvent extraction, and no expansion of the viscous oil-treated smectite interlayers could be detected by X-ray diffraction, the measured hydrocarbon content is suggested to provide a reasonable estimate of the total oil present. The observed decrease in the concentration of oil over the reaction period is attributed largely to bacterial digestion rather than to hydrocarbon adsorption, with denser populations of cultured oleophilic bacteria in the more strongly digested samples. Overall, the smectites with high specific surface areas and cation exchange capacities induced the highest degree of oil breakdown, and also stimulated the digestion of the heavier aromatic and resin–asphaltene compounds. The presence of bivalent cations at interlayer sites and hydrated surfaces, including abundant Mg2+ derived from sea water, is suggested to minimize the thickness of the diffuse ion layer and thus enhance both inorganic and organic nutrient supply from mineral surface to bacterial cell. Adding clay minerals to marine oil pollution is therefore suggested as an effective and economically interesting way of enhancing the bacterial digestion of contaminating spills.