Collaborative Proposal: Probing the Reductive Potential of Wetland Sediments and Pore Waters

University Of California - Merced, Merced CA

Investigators

Abstract

Traina 0337339 Freshwater wetland sediments are complex biogeochemical systems, which contain a host of natural reductants formed from microbial activity. These natural reductants such as Fe(II) in various forms and natural organic matter (NOM) may be capable of being involved in a number of environmental and biogeochemical reactions. Moreover, many wetlands are capable of removing organic pollutants from surface runoff by sedimentation. To date the fate of many of these contaminants are assumed to be recalcitrant or undergo transformation through microbial processes. Because of the variety and abundance of natural reductants present in sediments and their porewaters many of these pollutants may react through abiotic pathways. We hypothesize that two different groups of environmental "reagents" in wetland sedimentary environments may be particularly potent in promoting the transformation of organic compounds susceptible to reductive processes. These include (1) Fe(II) species that have recently been proven highly reactive reductants of nitroaromatic and azo compounds (particularly when adsorbed to Fe(III) oxides), and (2) natural organic matter that is capable of acting as reductants by themselves or synergistically with Fe(II). We propose to probe the reductive potential of wetland sediments and porewaters through a multifaceted investigation using nitroaromatic and azo compounds that span a range of physicochemical properties (from polarity to reactivity). The rates at which well-defined adsorbed Fe(II) and NOM isolated from porewaters will react with representative groups of our probes will be determined in well-controlled batch experiments. Relevant parameters will be systematically varied to aid in the identification of the reactive species and to facilitate understanding kinetic processes and reaction pathways. Careful attention will be paid to identifying reaction products. Further experiments will be conducted in real systems with porewaters and sediments isolated anoxically from a site in Ohio (Old Woman Creek: a NOAA site) and in Georgia (Bishop Pond: a USDA site). We will conduct probe reactivity studies in both native porewaters and sediment slurries. To elucidate between sediment/surface driven processes as oppose to reactions in the solution phase, we will use azo-probes covalently bonded to Sepharose beads. These bonded probes will be separated from particle phases by dialysis membranes. Finally, we will investigate these processes in situ at both sites using the azo probes. In order to differentiate between porewater driven processes to those catalyzed by the sediments, the azo probes will be buried in the sediments using a "peeper" that will prevent direct contact with any solid phases. Parallel experiments using buried "peepers" containing sediments will be conducted to determine the overall rate of reduction. The results of these investigations will provide information useful in assessing the reductive potential of wetland sediments to abiotically transform those organic compounds susceptible to reduction that are of environmental interest. These studies will improve our understanding of biogeochemical redox processes in wetland sediments. Finally, we have included a comprehensive outreach plan where middle school students from diverse backgrounds will be actively involved in the proposed research project.