THERMODYNAMIC CONSTRAINTS ON THE OXIDATION OF HYDROCARBONS BY IRON OXIDES AT A CRUDE OIL SPILL SITE

Multicomponent reactive solute transport simulations were performed to simulate groundwater contamination at Bemidji, MN where crude oil infiltrated into the groundwater after a pipeline break in 1979. Dissolved organic carbon that entered the groundwater from crude oil dissolution is known to be undergoing aerobic biodegradation and anaerobic biodegradation processes including Mn(IV) and Fe(III) reduction, and methanogenesis. Biodegradation of groundwater contaminants coupled to Fe(III) reduction is an important process at this site and many other sites because the sediment Fe(III) may be present at high concentrations relative to other terminal electron acceptors. Thus, it is important to understand the factors that control the availability of Fe(III) in the subsurface. The oxidation of hydrocarbons coupled with the reduction of Fe(III) is complex because of the diverse nature of the hydrocarbons derived from the crude oil and because Fe(III) may be present in multiple phases with varying thermodynamic stabilities. Previous studies have shown that near the oil body both ferrihydrite and crystalline Fe(III) phases were depleted relative to background sediments. This Fe(III) reduction produces dissolved Fe(II) exceeding 1mM which causes the Fe(III) reduction to be less energetically favorable. Speciation calculations using measured groundwater geochemistry were used to evaluate the thermodynamic favorability of the oxidation of benzene, an important constituent in the dissolved hydrocarbon plume, by Fe(III) solid phases with varying thermodynamic stabilities represented by ferrihydrite and goethite. The calculations show that the oxidation of benzene is by ferrihydrite and goethite is favorable in the background sediments and in the Fe(III) reducing zone. In contrast, for the methanogenic zone, benzene oxidation coupled to the reduction of ferrihydrite and goethite was either unfavorable or yielded less than 2 kJ/mole per electron transferred suggesting that thermodynamics limits the oxidation of benzene in this zone. The calculated free energy of methane formation from benzene was favorable in all zones. Reactive transport simulations that incorporated thermodynamically correct rate or equilibrium controlled reactions successfully reproduced the observed geochemical data.