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Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.

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The silver-nanoparticle-catalyzed decomposition of hydrogen peroxide (H2O2) in pH 9.5 bicarbonate buffer is investigated here with attention given to (i) the mechanism of decomposition, (ii) the role of superoxide in mediating silver nanoparticle re-formation, and (iii) the effect of nanoparticle size on decomposition rate. Silver nanoparticles (AgNPs) of average size between 25.0 and 69.4 nm were synthesized via the reduction of Ag+ [the dominant Ag(I) species present] by photochemically produced superoxide at pH 9.5 and characterized by UV−visible spectroscopy and dynamic light scattering. The ability of these particles to catalytically decompose H2O2 was examined by measuring the decay of H2O2 and the approach to steady state in AgNP and Ag+ concentrations. Additionally, the generation of superoxide on reaction of AgNPs with H2O2 was monitored using a chemiluminescence-based method. The second-order rate constants for reaction between AgNPs and H2O2 correlated linearly with their average particle size ranging from 35.0 to 3.0 × 102 M−1 s−1 for average sizes between 69.4 and 25.0 nm. A sensitive trap-and-trigger chemiluminescence-based method for hydroxyl radical detection showed no evidence for the presence of hydroxyl radicals, though an inhibitory effect of tert-butyl alcohol suggested the presence of a strongly oxidizing species. A process involving the superoxide-mediated charging of silver nanoparticles with subsequent discharge by reaction with oxygen and Ag+ leading to regeneration of Ag0 and superoxide is proposed to account for the results obtained.

Contemporary studies indicate that reactive oxygen species (ROS) such as superoxide play a key role in the toxicity and behavior of silver nanoparticles (AgNPs). While there have been suggestions that superoxide is able to reduce silver(I) ions with resultant production of AgNPs, no experimental evidence that this process actually occurs has been produced. Here we present definitive experimental evidence for the reduction of silver(I) by superoxide. A second-order rate constant of 64.5 ± 16.3 M(-1)·s(-1) is determined for this reaction in the absence of AgNPs. The overall rate constant, however, increases by at least 4 orders of magnitude in the presence of AgNPs. A model based on electron charging and discharging of AgNPs satisfactorily describes the kinetics of this process. The ability for AgNPs to undergo catalytic cycling provides a pathway for the continual generation of ROS and the regeneration of AgNPs following oxidation.