Evaluation of Non-Growth Biokinetic Models for Chromate Reduction with Shewanellae

                Hexavalent chromium is a carcinogenic pollutant often associated with leather tanning and electroplating industries. Additional applications have included its use as a corrosion inhibitor in cooling water and as a colorant in dye and pigment compounds. Its widespread use in industrial application has often been met with poor waste management practices, leading to its release into the natural environment.

                Bioremediation has been proposed as a viable remediation tool to reduce hexavalent chromium to its trivalent state. The rationale behind this approach is that while the hexavalent form is very soluble and mobile in the environment, the trivalent form is known to form insoluble precipitates under most environmental conditions. Such a transformation results in chromium immobility. Moreover, by utilizing microbes encountered in the subsurface environment, the potential of designing an in situ treatment approach is much more appealing when compared to more invasive ex situ approaches.

                In particular, Shewanella species have been proposed as model organisms capable of hexavalent chromium reduction. The widespread abundance of Shewanella in nature combined with the breadth of genomic and physiological knowledge make it an ideal organism to study in order to understand mechanisms of chromium reduction in the environment. To this end, several Shewanella species were evaluated for their ability to reduce chromium under anaerobic conditions in batch reactors.

                Data generated from these experiments was used to estimate biokinetic parameters for models proposed in literature for remediation of other types of pollutants. In this manner, we have evaluated the application of these models for hexavalent chromium reduction. A “transformation capacity” model was selected based on its agreement with experimental data. Furthermore, the sensitivity of this model to changes in biokinetic parameters was determined, as well as the effects of pH, temperature, and electron donor. The data generated from these studies suggests that biomass, pH, and electron donor are among the key parameters to consider when using designing bioremediation based treatment systems.