Abiotic nitrate reduction by redox activated iron-bearing smectites

Nitrate is a contaminant of interest in soils and water. It has been linked to chronic human illness and a variety of environmental problems, including hypoxia in coastal waters. Previous work has shown that nitrate is naturally removed from soils below the redox interface of the soil profile. A lack of denitrifying bacteria in the studied profiles points to an abiotic source of nitrate reduction that likely involves mineral phases containing iron. The ability of reduced, iron-bearing clay minerals to in turn reduce nitrate was investigated using SWa-1 chemically reduced by sodium dithionite. Analysis of ferrous and total iron concentrations in the reduced clay was performed with a colorimetric 1,10-phenanthroline method before and after reaction with a dilute nitrate solution to observe reoxidation of the clay structure. This was paired with chemiluminescent nitrogen analysis of the dilute nitrate solution to determine that not only are these reduced clay samples capable of reducing nitrate, but also that the extent of reduction plays a key role in the reductive capacity. A strikingly linear response was found between the amount of ferrous iron present in the clay structure and the amount of nitrate that was removed from the system. An additional approach for monitoring such reactions as they occur with an oxygen-free flow reactor connected to a UV-Vis spectrometer was also evaluated. These flow reactor experiments sought to investigate the differences in nitrate reductive capacity between the chemically reduced clay, bacteria reduced clay, and clay reduced in the presence of zero-valent iron. This method proved inferior to the more rigorous approach of chemically analyzing iron and nitrate concentrations. Experiments were also performed to test the effects of an electron shuttle by addition of anthraquinone-2,6-disulfonate to samples of bacteria and dithionite reduced clays using the phenanthroline and nitrogen analysis combination. Results of this study are proof of principle that the redox active iron-bearing mineral phases within soils are capable of removing nitrate from soils