Photoreductive Dissolution of Iron (Hydr)oxides and Its Geochemical Significance

Abstract

Iron (hydr)­oxides are the most abundant metal oxides, which are widespread on Earth’s surface in the major form of micro/nanoparticles. Dissolution of iron (hydr)­oxides significantly controls their compositions on Earth’s surface and is a critical step for the global Fe cycling. Photoreductive dissolution of iron (hydr)­oxides is recognized as one of the most important process for generating Fe2+ in surface water and is also a common pathway for transforming solar energy into chemical energy. This review article dissects the main characteristics of photoreductive dissolution of iron (hydr)­oxides and discusses its geochemical and environmental significance. We categorize the mechanisms for photoreduction of iron (hydr)­oxides into three types: reduction by intrinsic photogenerated electrons, reduction by ligand to metal electron transfer (LMCT), and reduction by direct injection of exogenous photoelectrons. The efficiency of photoreductive dissolution is constrained by both the structure of iron (hydr)­oxides (e.g., crystal structure and particle size) and environmental conditions (e.g., light, pH, and concurrent chemicals). Therefore, different iron (hydr)­oxides may exhibit quite distinctive photoreductive dissolution characteristics because of their unique crystal structures and physicochemical properties. Iron (hydr)­oxides with low crystallinity (e.g., ferrihydrite, lepidocrocite) are subject to direct photoreductive dissolution, while those with high crystallinity (e.g., goethite, hematite) generally need ligands to proceed with photoreductive dissolution. The photoreductive dissolution of iron (hydr)­oxides is involved in many important geochemical and environmental processes, such as the Fe availability to primary producers, the generation of reactive oxygen species, the transportation and fate of contaminants, and the phase transformation of iron (hydr)­oxides. Given the ubiquitous occurrence of photoreductive dissolution of iron (hydr)­oxides, this review will advance our understanding of the role of this process in Earth’s surface environments.

Sunlight Significantly Enhances Soil Denitrification via an Interfacial Biophotoelectrochemical Pathway

“…Fe­(II) generation in the dark occurred due to the microbial reduction process (Figure c) because almost no Fe­(II) generation was detected in the dark after autoclaving the soil (121 °C for 20 min) (Figure d). Moreover, the Fe­(II) generation in the autoclaved soil during irradiation (Figure d) confirmed that part of the Fe­(II) generation was via the abiotic photochemical processes, which has been demonstrated by previous work …”

“…Figure a demonstrates that more NO 3 – -N was reduced in the light (13.2 mg kg –1 ) than in the dark (7.2 mg kg –1 ), inferring that the contribution of photoinduced electrons to denitrifers was 45.5%, while the contribution of electrons generated in the dark was 54.5%. The enhanced NO 3 – -N reduction may be due to photoinduced Fe­(II) generation or photoelectrons …”

“…In addition, soil organic matter is photosensitive as it contains a number of chromophore species, like humic acid . In addition to generating photoexcited electrons, the soil is also capable of generating ferrous species (Fe­(II)) via the photoreduction of iron oxide to Fe­(II) by either direct ligand-to-metal charge transfer or radicals . Fe­(II) is the electron donor for iron-based microbial denitrification .…”

Molecular investigation of the multi-phase photochemistry of Fe(iii)–citrate in aqueous solution

“…109 For mechanistic interpretation of Fe(III)-citrate photochemical degradation discussed here, special consideration of multiphase photoreductive dissolution processes of colloidal surface-bound Fe III -citrate and other Fe-organic complexes in the aqueous solution should be accounted for. 108,[110][111][112] The photodegradation of oxide bound Fe III -citrate complexes leads to dissolution of Fe III -citrate, followed by Fe(III) reduction and immediate release of soluble Fe 2+ and citrate from the colloid to the aqueous solution. 108 Overall, photodegradation of the soluble and surface bound Fe III -citrate complexes was initiated through LMCT reactions 31 R1-R4 (Scheme 1) reactions are very fast, resulting in rapid reduction of Fe III to Fe II over the rst 5 min of photolysis.…”

Virtual Special Issue of ACS Earth and Space Chemistry in Honor of Prof. Michael F. Hochella, Jr.

“…In addition, the three Hochella Symposium organizers were invited by Prof. Joel Blum, Editor-in-Chief of ACS Earth and Space Chemistry , to organize and serve as Guest Editors of a virtual special issue of this journal in honor of Mike Hochella. A total of 20 peer-reviewed manuscripts were accepted for publication in this virtual special issue (), with topics ranging from mineral/water interface chemistry and mineral/organic matter nanoparticles to organic matter/soil chemistry and microbial geochemistry studies. − These papers highlight the importance of nanogeoscience, mineral/aqueous solution interface chemistry, biogeochemistry, and organic matter/soil chemistry in understanding the Earth system.…”