| The toxicity and solubility of chromium are a function of its valence state. Hexavalent chromium is highly toxic and very soluble in groundwater systems. Trivalent chromium is relatively non-toxic and is relatively insoluble in groundwater systems. Other research has identified bacteria that are capable of reducing toxic, mobile hexavalent chromium to non-toxic, immobile trivalent chromium. This suggests an in situ bioremediation strategy for soils and groundwater contaminated with hexavalent chromium. Mixed cultures were enriched from three diverse soil sources: subsurface soil from Hanford Works, Washington; surface soil from Joseph, Oregon; and river sediment from the Nara River, Russia. All mixed cultures exhibited chromium reduction capability. Kinetic parameters were estimated under growth and resting cell conditions for the mixed cultures and for a widely studied chromium-reducing pure culture. Kinetic experiments were conducted in batch reactors under anaerobic conditions. Cells were suspended in a buffered simulated groundwater minimal salts medium, augmented with molasses as the carbon source and nitrate as the terminal electron acceptor. Biomass, substrate, nitrate, and chromium data were fit using a Monod model. Optimized values of kinetic parameters were compared between the cultures. Although there were statistically significant differences between cultures, the range of values of the kinetic parameters was small. The data may be useful for engineering design of an in situ bioreactor or for estimating natural attenuation rates. Part of the population of the Nara River mixed culture consisted of ferminting microorganisms that neither reduced chromium nor were inhibited by chromium. This population's activity supports the hypothesis that bacterial reduction of chromium is a function of the electron transport system. |
