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- The effect of adsorbed lipid on pyrite oxidation under biotic conditions

It is well-known that several species of prokaryotes are able to catalyze the oxidation of pyrite and play an important role in the development of Acid Mine Drainage (AMD), a severe environmental problem. Acidithiobacillus ferrooxidans, for example, utilize reduced Fe2+ in AMD environments as an electron donor for energy production at low pH [1]. Due to the importance of microbes in the chemistry of such environments, a significant amount of research activity has been focused on understanding the role of microbes in the oxidation of sulfur-bearing minerals such as pyrite [2-11]. With regard to microbial-induced pyrite oxidation it is the general consensus that these microorganisms exert their impact on pyrite dissolution to a large degree by increasing the amount of available Fe3+ reactant [i.e., convert Fe2+ to Fe3+], which increases the rate of pyrite oxidation [3,12,13]. A goal of our research was to build on this prior research and to extend our understanding of microbial-accelerated pyrite oxidation to surfaces having adsorbed organic layers. With regard to this last point, the inhibition of pyrite oxidation using lipid having two hydrophobic tails (per polar head) has been studied as an AMD abatement strategy [14-16]. However, the lipid-induced inhibition of pyrite oxidation in the presence of microbes relevant to AMD has not been investigated. Toward this end, the experiments reported here were designed to investigate the effect of A. ferrooxidans microorganisms on pyrite oxidation in the presence and absence of adsorbed lipid layer at a solution pH near 2.

Our studies show that surface-bound lipid inhibits the oxidation of pyrite, even in the presence of bacteria. The lipid layers that suppress the oxidation limit the colonization of the bacteria on the pyrite, compared to lipid-free samples. This effect is observed throughout the duration of the experiment (about 25 days in all cases). Hence, in the presence of A. ferrooxidans the lipid coating remains in tact for at least 25 days and probably much longer. Furthermore, our study supports the notion that the rate of pyrite oxidation process is significantly controlled by the bacteria that colonize the pyrite surface.

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