Complexation and Redox Buffering of Iron(II) by Dissolved Organic Matter

Abstract

Iron (Fe) bioavailability depends upon its solubility and oxidation state, which are strongly influenced by complexation with natural organic matter (NOM). Despite observations of Fe(II)-NOM associations under conditions favorable for Fe oxidation, the molecular mechanisms by which NOM influences Fe(II) oxidation remain poorly understood. In this study, we used X-ray absorption spectroscopy to determine the coordination environment of Fe(II) associated with NOM (as-received and chemically reduced) at pH 7, and investigated the effect of NOM complexation on Fe(II) redox stability. Linear combination fitting of extended X-ray absorption fine structure (EXAFS) data using reference organic ligands demonstrated that Fe(II) was complexed primarily by carboxyl functional groups in reduced NOM. Functional groups more likely to preserve Fe(II) represent much smaller fractions of NOM-bound Fe(II). Fe(II) added to anoxic solutions of as-received NOM oxidized to Fe(III) and remained organically complexed. Iron oxidation experiments revealed that the presence of reduced NOM limited Fe(II) oxidation, with over 50% of initial Fe(II) remaining after 4 h. These results suggest reduced NOM may preserve Fe(II) by functioning both as redox buffer and complexant, which may help explain the presence of Fe(II) in oxic circumneutral waters.

Mechanisms Responsible for Soil Phosphorus Availability Differences between Sprinkler and Furrow Irrigation

“…However, furrow-irrigated soil contained greater concentrations of moderately and nonlabile organic P. This may have been due to decreased organic matter mineralization rates under reducing conditions, leading to an accumulation of P in these organic fractions (González-Alcaraz et al, 2012;Zhang et al, 1994). Or, as suggested by Darke and Walbridge (2000), Jansen et al (2011), andDaugherty et al (2017), this may have been due to the formation of Al-and/or Fe-organic material complexes under flooded conditions, leading to greater P retention in the moderately labile and nonlabile P pools.…”

Microbial anaerobic Fe(II) oxidation – Ecology, mechanisms and environmental implications

“…Under oxidizing conditions, iron is found mainly in Fe(III) (oxyhydr) oxide minerals such as ferrihydrite (Fe(OH) 3 ), goethite (α-FeOOH) and haematite (α-Fe 2 O 3 ), as well as, to a lesser extent, in dissolved Fe(III)-natural-organic-matter complexes (Fe(III)-NOM complexes) (Carlson and Schwertmann, 1981;Schwertmann and Murad, 1988;Kostka and Luther, 1994). Under reducing conditions iron is mostly found in mixedvalent Fe minerals such as magnetite and green rust, or in Fe(II) minerals such as vivianite (Fe 3 (PO 4 ) 2 Á 8H 2 O) and siderite (FeCO 3 ), or even as dissolved Fe 2+ ions (Thompson et al, 2011;Daugherty et al, 2017;Ginn et al, 2017;Herndon et al, 2017). Under both oxidizing and reducing conditions, iron can be found as structural components of silicate minerals such as clays (Pentráková et al, 2013) and in iron sulfide minerals.…”

Electrochemical characterization of natural organic matter by direct voltammetry in an aprotic solvent

“…40,[45][46][47][48][49][50][51] The other major applications of NOM characterization are to its role in biogeochemistry, 46,[52][53][54][55][56] and contaminant fate. [57][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73] In both of these contexts, NOM participates in reactions as a ligand, [74][75][76][77][78] electron donor/acceptor, 53,56,[79][80][81][82][83][84][85][86][87] electron shuttle (i.e., electron-transfer mediator), [88][89][90]…”