Technical description.The biogeochemical cycling of iron in aqueous geochemical environments is intimately linked to the cycling of carbon, nitrogen, phosphorus and sulfur, and strongly impacts the solubility and speciation of trace metals and metalloids in these systems. The research proposed here focuses on coupled Fe and trace metal cycling in riparian soils, which are located at the interface between dryland habitats and aquatic environments and play a key role in the transfer of nutrients and contaminants between upland and aquatic ecosystems. We observe the formation of layered Fe(II)-Al(III)-hydroxide minerals during reaction of aqueous Fe(II) with Al-oxide and clay mineral substrates under geochemical conditions common to submerged soils. We hypothesize that these previously unrecognized Fe(II) phases play a critical role in the biogeochemical cycling of Fe and trace metals in riparian environments. These secondary minerals form fast (on a time scale of hours in model systems, and within several days in experiments performed with wetland soil) and are therefore expected to be a major sink for Fe(II) released during reductive dissolution of Fe(III)-oxides. In addition, owing to small particle size, layered structure, and high Fe(II) content, these Fe(II) minerals are likely to be highly reactive towards redox-active contaminants such as Cr(VI) and may control retention of divalent metals such as Ni(II) and Zn(II) through adsorption and coprecipitation reactions. We hypothesize that the Fe(II)-Al(III)-hydroxide phases formed during initial reaction of Fe(II) with Al-bearing substrates are metastable transitional phases which over time will age into more crystalline Fe sorption products with reduced reactivity towards trace metal(loid)s. We will study the thermodynamic, kinetic and mechanistic aspects involved in the formation of these novel Fe(II) phases, and to characterize their structure and reactivity over a time span ranging from seconds to years. A suite of state-of-the-art spectroscopic techniques, including Q-XAS, bulk XAS, and Mossbauer analyses, will be used to address these issues. This project will fill a major gap in our knowledge of Fe cycling in reducing and riparian environments, and will improve our understanding of contaminant fate and transport in these dynamic systems.Broader significance and importance.About 4-6% of the Earth's land surface is intermittently or permanently submerged. Soil flooding causes drastic changes in the chemistry of soil pore waters, driven mostly by microbial activity as soil microbes are forced to switch from using oxygen to alternative electron acceptors for respiration of organic carbon. Use of Fe(III)-oxide minerals in microbial respiration causes reductive dissolution of these minerals, which leads to the build-up of high aqueous concentrations of dissolved Fe(II) and release of toxic metal(loid) impurities associated with the Fe(III)-oxide minerals. The research addresses the fate of Fe(II) and metalloid pollutants released to solution during flooding. We have identified a previously unknown precipitation mechanism which may repartition released Fe(II) and trace metals back to the solid phase. The precipitation process is activated by reaction of dissolved Fe(II) with Al-bearing soil minerals causing precipitation of secondary Fe(II)-Al(III)-hydroxide minerals. Precipitation of these new Fe(II) phases occurs rapidly and extensively under conditions typical of flooded soils, and is therefore likely to be an important process governing the fate of released Fe(II) in these systems. Formation of the Fe(II) minerals removes toxic metals from solution as well as metals are incorporated into the structure or adsorb onto the surface of the new phases. The research proposed here will characterize the main geochemical parameters controlling the formation and reactivity of these Fe(II) phases in flooded soils, and provide quantitative thermodynamic data allowing for prediction of their occurrence in natural systems. The results of this project will fill a major gap in our understanding of the geochemical processes controlling soil and water quality in riparian systems, which includes environments as diverse as polar bogs and fens, tropical swamps, coastal and freshwater wetlands, paddy rice fields, and floodplain soils. The work is of importance in assessing the restoration of former wetlands (through re-establishment of riparian conditions) as a management option for floodwater control and restoration of biodiversity at sites where (re)mobilization of previously accumulated pollutants is a concern. Our work is also expected to be of major significance to remediation strategies involving biostimulation of metal-reducing microbial populations to immobilize subsurface contaminants.
|Effective start/end date||9/1/12 → 8/31/15|
- National Science Foundation (National Science Foundation (NSF))