Scope of Work for Initial projects Related to Cr Contamination
The focus of the initial Honeywell supported projects within the Center will be on the conditions conducive to the reduction of Cr(VI) to Cr(III) in aquatic environments and sediment, the rates of the reduction processes at specific sites (e.g., Baltimore Harbor), and the likelihood that the reduction of Cr(VI) to Cr(III) will be a permanent process. The primary, practical objective of the Center’s proposed research is to provide EpA, state regulatory agencies, NGOs and others with a sound scientific basis for regulating Cr in sediments. Final reports for each of these projects will be written in the format of peer reviewed journal articles. The scope of work for each of the two initial research projects is discussed below.
A. Study to Determine Conditions Conducive to the Reduction of Cr(VI) to Cr(III) in Aquatic Environments and Sediment.
Objective: The objective of this research project is to identify the chemical conditions pertinent to Baltimore Harbor that govern the extent and rates of transformation of Cr(VI) to Cr(III). Specific sub-objectives of this work include (1) determine the significance of reduced iron and sulfur in the transformation of Cr(VI) to Cr(III), (2) determine the relative significance of microbial reduction of Cr(VI) to Cr(III), and (3) evaluate the rates of Cr(VI) reduction to Cr(III) by the chemical and microbial processes.
Approach: It has been established that Cr(VI) is readily reduced to Cr(III) by various naturally occurring systems including Fe(II), S(-II), organic compounds, wetland plants, and several species of microorganisms. Ferrous iron and hydrogen sulfide may compete with biological pathways in the reduction of Cr(VI), although they tend to be dominant in anaerobic environments. Ferrous iron dominates reduction at pH values greater than 5.5, while hydrogen sulfide will dominate the reduction reaction of Cr(VI) at pH values below 5.5. The ferrous iron reduction reaction occurs relatively fast with reaction going to completion in less than 5 minutes, even in the presence of dissolved oxygen. Therefore, it is possible for natural systems to rapidly convert Cr(VI) to Cr(III).
A literature review on pertinent reduction processes for Cr will be conducted to guide the subsequent chemical and biological characterizations. Sediment and pore water samples from the Baltimore Harbor will be taken to our laboratory and spiked with Cr(VI) to determine reaction processes capable of reducing the Cr(VI) to Cr(III). The systems will be characterized for reductants (e.g., Fe(II) and S(-II)) and microorganisms capable of Cr transformations. Several samples will be studied to provide insight into the variability in the reduction processes across the Baltimore Harbor. The controlled laboratory experiments will permit a mass balance on the Cr species and will be used to determine the extent and rates of Cr(VI) reduction to Cr(III). The laboratory conditions will mimic the conditions in the Baltimore Harbor so that the results have direct relevance to the current environmental settings.
B. Study to Determine the Likelihood that the Reduction of Cr(VI) to Cr(III) will be a permanent process.
Objective: The objective of this research project is to determine the stability of Cr(III) in sediments. Specific sub-objectives of this work include (1) determine the significance of oxidants capable of transforming Cr(III) to Cr(VI) in sediments and the water column, such as manganese oxides and oxygen, (2) determine the rates of Cr(III) oxidation in sediments, even under conditions where it is thermodynamically unstable, and (3) determine the impact of introducing oxygen into anaerobic sediments on the stability of Cr(III).
Approach: previous studies suggest that Cr(III) is stable in anaerobic sediments. Direct oxidation of Cr(III) to Cr(VI) by dissolved oxygen is very slow and does not appear to play a significant role in oxidizing Cr. Manganese (hydr)oxides are the only important naturally occurring oxidant of Cr(III). Elevated levels of manganese (hydr)oxides are not likely to be present in anaerobic sediments, especially under conditions that form acid volatile sulfides (AVS). Consequently, Cr(VI) reduction to Cr(III) dominates over the potential for Cr(III) oxidation. This research project will examine the extent to which sediments will remain a sink for Cr(III).
Sediment and pore water samples from the Baltimore Harbor will be taken to our laboratory and spiked with Cr(VI) to facilitate reduction to Cr(III). Some of these sediments will be incubated for long periods (up to two years) to monitor for presence of any detectable Cr(VI). An oxidation rate of Cr(III) can be calculated only if some conversion of Cr(III) to Cr(VI) can be observed. Another set of experiments will determine the redox buffering capacity of the sediments by measuring a series of redox species (e.g., AVS, Fe(II), Mn(II), redox potential). This information will be used to determine the quantity of oxygen required to convert the system from an anaerobic sediment to an aerobic sediment. A final set of experiments will examine the rates of Cr(III) oxidation in the presence of aerobic sediments and in the presence of manganese (hydr)oxides. The laboratory conditions will mimic the conditions in the Baltimore Harbor so that the results have direct relevance to the current environmental settings.