Activated carbon as an insoluble electron shuttle to enhance the anaerobic ammonium oxidation coupled with Fe(III) reduction process

Soils and sediments worldwide contain appreciable amounts of thermally altered organic matter (chars). Chars contain electroactive quinoid functional groups and polycondensed aromatic sheets that were recently shown to be of biogeochemical and envirotechnical relevance. However, so far no systematic investigation of the redox properties of chars formed under different pyrolysis conditions has been performed. Here, using mediated electrochemical analysis, we show that chars made from different feedstock and over a range of pyrolysis conditions are redox-active and reversibly accept and donate up to 2 mmol electrons per gram of char. The analysis of two thermosequences revealed that chars produced at intermediate to high heat treatment temperatures (HTTs) (400-700 °C) show the highest capacities to accept and donate electrons. Combined electrochemical, elemental, and spectroscopic analyses of the thermosequence chars provide evidence that the pool of redox-active moieties is dominated by electron-donating, phenolic moieties in the low-HTT chars, by newly formed electron accepting quinone moieties in intermediate-HTT chars, and by electron accepting quinones and possibly condensed aromatics in the high-HTT chars. We propose to consider chars in environmental engineering applications that require controlled electron transfer reactions. Electroactive char components may also contribute to the redox properties of traditionally defined "humic substances".

Direct interspecies electron transfer (DIET) is potentially an effective form of syntrophy in methanogenic communities, but little is known about the diversity of methanogens capable of DIET. The ability of Methanosarcina barkeri to participate in DIET was evaluated in coculture with Geobacter metallireducens. Cocultures formed aggregates that shared electrons via DIET during the stoichiometric conversion of ethanol to methane. Cocultures could not be initiated with a pilin-deficient G. metallireducens strain, suggesting that long-range electron transfer along pili was important for DIET. Amendments of granular activated carbon permitted the pilin-deficient G. metallireducens isolates to share electrons with M. barkeri, demonstrating that this conductive material could substitute for pili in promoting DIET. When M. barkeri was grown in coculture with the H2-producing Pelobacter carbinolicus, incapable of DIET, M. barkeri utilized H2 as an electron donor but metabolized little of the acetate that P.carbinolicus produced. This suggested that H2, but not electrons derived from DIET, inhibited acetate metabolism. P. carbinolicus-M. barkeri cocultures did not aggregate, demonstrating that, unlike DIET, close physical contact was not necessary for interspecies H2 transfer. M. barkeri is the second methanogen found to accept electrons via DIET and the first methanogen known to be capable of using either H2 or electrons derived from DIET for CO2 reduction. Furthermore, M. barkeri is genetically tractable,making it a model organism for elucidating mechanisms by which methanogens make biological electrical connections with other cells.

Many iron (Fe) redox processes that were previously assumed to be purely abiotic, such as photochemical Fe reactions, are now known to also be microbially mediated. Owing to this overlap, discerning whether biotic or abiotic processes control Fe redox chemistry is a major challenge for geomicrobiologists and biogeochemists alike. Therefore, to understand the network of reactions within the biogeochemical Fe cycle, it is necessary to determine which abiotic or microbially mediated reactions are dominant under various environmental conditions. In this Review, we discuss the major microbially mediated and abiotic reactions in the biogeochemical Fe cycle and provide an integrated overview of biotic and chemically mediated redox transformations.