Methanotrophic Bacteria and Facilitated Transport of Pollutants in Aquifer Material

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In situ stimulation of methanotrophic bacteria has been considered as a methodology for aquifer remediation. Chlorinated aliphatic hydrocarbons such as trichloroethylene are fortuitously oxidized by the methane monooxygenase produced by methanotrophic bacteria. Experimental results are presented that indicate that both colloidal suspensions containing methanotrophic cells and the soluble extracellular polymers produced by methanotrophic cells have the potential to enhance the transport and removal of other environmental contaminants such as polynuclear aromatic hydrocarbons and transition metals in aquifer material. Three well-characterized methanotrophic bacteria were used in the experiments: Methylomonas albus BG8 (a type I methanotroph), Methylosinus trichosporium OB3b (a type II methanotroph), and Methylocystis parvus OBBP (a type II methanotroph). Isotherms were obtained for sorption of two radiolabeled pollutants, [14C] phenanthrene and 109Cd, onto an aquifer sand in the presence and absence of washed cells and their extracellular polymer. Column transport experiments were performed with the washed methanotrophic cells and phenanthrene. The distribution coefficients for Cd with extracellular polymers were of the same order as that obtained with the aquifer sand, indicating that polymers from the methanotrophic bacteria could act to increase the transport of Cd in a porous medium. Polymer from BG8 significantly reduced the apparent distribution coefficient for Cd with an aquifer sand. [14C] phenanthrene also sorbed to extracellular polymer and to washed, suspended methanotrophic cells. The exopolymer of BG8 and OBBP significantly reduced the apparent distribution coefficient (Kd) for phenanthrene with aquifer sand. The distribution coefficients for phenanthrene with the methanotrophic cells were an order of magnitude greater than those previously reported for other heterotrophic bacteria. Cells of the methanotrophs also significantly reduced the apparent Kd for phenanthrene with an aquifer sand. The three strains of methanotrophs tested displayed mobility in a column of packed sand, and strain OBBP reduced the retardation coefficient of phenanthrene with an aquifer sand by 27%. These data indicate that both extracellular polymer and mobile cells of methanotrophic bacteria display a capacity to facilitate the mobility of pollutant metals and polynuclear aromatic hydrocarbons in aquifer material.

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