Biodegradation of crude oil hydrocarbons supported on clay minerals

Abstract

Clay minerals are the most abundant minerals near the earth’s surface and play very important roles in biogeochemical processes. They have been found very useful in various industrial applications. The surface properties of clay minerals such as high specific surface area (SSA) and cation exchange capacity (CEC) make them able to act as catalysts, supports and sorbents of toxic and radioactive chemicals. However, their role during the biodegradation of crude oil hydrocarbons is not well understood. The main aim of this research project was to investigate the capabilities of the various forms and types of clay minerals in supporting the microbial degradation of crude oil hydrocarbons so as to gain better understanding of their potential role in the bioremediation of oil polluted sites. The role of clays in hydrocarbon removal was investigated in aqueous clay/oil microcosm experiments with a hydrocarbon degrading microorganism community. The clays used for this study were bentonite, palygorskite, saponite and kaolinite. Clays were treated to produce acid activated clays and organoclays; homoionic interlayer bentonites were also used in this study. The study identified volatilization and adsorption as processes that will take place alongside biodegradation and therefore needed to be accounted for in the assessment of the effects of the clays. The study indicated that acid activated clays, organoclays, untreated kaolinite, K-bentonite, Zn-bentonite and Cr-bentonite were inhibitory to biodegradation of the hydrocarbons, via different mechanisms, whereas Ca-bentonite and Fe-bentonite were stimulatory to biodegradation with about 80% removal of the total petroleum hydrocarbons (TPH) due to biodegradation. The ‘local bridging effect’ and polarization of the interlayer water were identified as two opposing influences arising from the interlayer cations of clay minerals that probably determine the extent of biodegradation of the hydrocarbons. Adsorption of hydrocarbons was significant during biodegradation especially with unmodified palygorskite, Zn-bentonite and K-bentonite as each of them caused more than 40% removal of TPH by adsorption in the experimental microcosm containing 5:1 ratio (w/w) of clay to oil. The process of adsorption of aromatic compounds in the crude oil was believed to take place via cation-π interactions. The correlation between extent of biodegradation and surface area is more robust than that between extent of biodegradation and CEC. The same trend applies with adsorption indicating that both biodegradation and adsorption are more surface area dependent than CEC.