Iron cycling in the environment: new insights from the micro to the macro-scale

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Understanding the iron cycle in the environment is of prime importance because iron oxides are highly efficient at sorbing soluble contaminants, such as toxic metals. At first glance, iron cycling appears rather straight forward, i.e., microbial and/or chemical oxidation of ferrous iron and subsequent precipitation of iron oxides and abiotic and/or biotic reduction of ferric iron oxides and subsequent reductive dissolution of the oxides. Recent studies have however shown that the iron cycle is far more complex than we thought and that iron behavior in the environment is closely linked to the presence of other solutes, such as arsenic, aluminum, nitrogen, silicon, sulfur, phosphate, etc. The present proposal therefore aims at investigating iron cycling in natural and synthetic biogenic iron oxides (BIOS) in the presence of several solutes known to affect iron transformation and/or stability. The 3 objectives of this proposal are 1) How do chemical changes affect natural BIOS mineralogy and microbial diversity?, 2) Which chemical factors control the reduction of BIOS? and 3) Does organic matter stabilize BIOS and prevent mineralogical transformation over time? Natural BIOS will be collected from several freshwater sites near Ottawa (pristine and metal contaminated), whereas artificial BIOS will be synthesized in the laboratory. The overarching goal is to bring the “environment”, i.e., BIOS, to the laboratory and modify it by changing a wide range of physico-chemical parameters (i.e., pH, redox, etc.) in order to tease apart the role of other solutes (Al, As, N, S, Si, etc.) in iron cycling at the micro scale using in situ sensors and bulk analyses. The redox stability of BIOS will also be determined using environmentally relevant conditions. In order to elucidate the intricate relationship between iron and other solutes, synthetic BIOS will also be synthesized and used in well controlled systems submitted to the same manipulations as natural BIOS in order to isolate specific chemical and microbial transformations. The various systems will be analyzed for aqueous (using an in situ micro-profiler and micro-electrodes) and solid phase geochemistry, along with microbial activity and diversity. This research project will train several HQPs (7 B.Sc., 2 M.Sc. and 3 Ph.D.) who will be exposed to state-of-the-art techniques in the field of geochemistry and geomicrobiology and gain valuable skills to enter the work force. Results from this proposal will enable us to better predict the fate of toxic metals in the environment where redox and chemical fluctuations occur due to seasonal changes.**