Effects of Second Metal Oxides on Surface-Mediated Reduction of Contaminants by Fe(II) with Iron Oxide

Abstract

This work examined the effects of two second metal oxides (SiO2 and TiO2) on the reductive reactivity of Fe­(II)/goethite, an important natural reductant. SiO2 significantly inhibited the reductive reactivity, as quantified by the reduction kinetics of p-cyanonitrobenzene (pCNB) as a probe compound, while TiO2 greatly enhanced the reactivity. Silicate showed comparable inhibitory effects as SiO2 particles, indicating that the inhibition effect of SiO2 was dominated by its dissolution. Pseudo-first-order rate constants (k) of Fe­(II)/goethite + TiO2 mixtures were higher than the sum of the k values of the respective single oxide. For the mixtures of Fe­(II)/goethite + TiO2, the k values followed rutile > TiO2–P25 > anatase, despite a different trend in the adsorbed Fe­(II) amount. This reactivity trend agreed well with their conduction band potentials. Higher loadings of TiO2 also led to higher reactivity. When TiO2 was physically separated from goethite by confining it in a dialysis bag, k of Fe­(II)/goethite + TiO2 was comparable to the sum of the k values of the respective single oxide. We believe that the conduction bands of goethite and TiO2 were used as conduits for electron transfer from Fe­(II) through TiO2 to goethite and eventually to reduce pCNB. This type of interparticle electron transfer was for the first time discovered for dark conditions, which might play a previously unrecognized yet important role in contaminant reduction and Fe­(II)/Fe­(III) redox cycling in the environment.

Characterizing Reactive Iron Mineral Coatings in Redox Transition Zones

“…On the other hand, because of its nanometer-scale particle size, magnetite may not have been distinguishable. In a number of studies, − abiotic dehalogenation of chlorinated solvents has been investigated using magnetite and dissolved Fe­(II), where the Fe 2+ ion adsorbs on the mineral surface through a ≡ Fe–O–Fe 2+ structure (≡Fe refers to structural iron in minerals). Dehalogenation has also been attributed to structural Fe 2+ on the surface of magnetite. , Magnetite shares the same atomic structure with greigite; both are inverse spinels.…”

Fe(II) Redox Chemistry in the Environment

“…Iron (Fe) is the fourth most abundant element in the Earth’s crust and is present in virtually all aquatic environments. , Average quantities of Fe in sedimentary rocks are approximately 5–6% by weight, with approximately 3.5 × 10 12 mol/year of Fe involved in redox reactions in the environment . Iron plays an important role in the global biogeochemical cycles of many other major and minor elements (e.g., C, O, N, and S) − and also has direct and indirect impacts on corrosion, degradation of organic and inorganic compounds, , mobility of metals, , evolution and sequestration of natural organic matter (NOM), − mineral dissolution, nutrient availability, and the weathering of rock and diagenesis, , in addition to microbial activity . Iron is also central to many chemical aspects of the built or human-impacted environments including catalysis, − corrosion, environmental remediation, medical diagnosis and therapy, pigments manufacture, , sensors, solar cell operations, , water treatment, , and development of cost-effective iron-based materials for environmental and energy applications. − Iron redox chemistry is involved in all of the above processes.…”

Role of Carbonate in Thermodynamic Relationships Describing Pollutant Reduction Kinetics by Iron Oxide-Bound Fe2+

“…Ferrous iron (Fe 2+ ) can reduce and simultaneously alter the toxicities and/or solubilities of several classes of environmental pollutants found in groundwater. − Consequently, Fe 2+ is a critical reductant to account for in naturally attenuated and engineered remediation systems. − Prior work has established that aqueous Fe 2+ in the presence of an iron (oxyhydr)­oxide (i.e., an “iron oxide”) reduces pollutants far more quickly than aqueous Fe 2+ alone, ,,,− with reduction rates depending on the redox properties of the iron oxide present. ,− ,,,,− This effect is a result of the iron oxide influencing what Fe 3+ oxidation product forms. ,,, When aqueous Fe 2+ is oxidized in the absence of an iron oxide, it tends to form an aqueous Fe 3+ complex or ferrihydrite . When Fe 2+ is oxidized in the presence of a crystalline iron oxide, however, it tends to form a more thermodynamically stable iron oxide phase, typically via homoepitaxial growth. ,, Thus, the presence of an iron oxide alters the reduction potential ( E H ) value of the Fe 3+ /Fe 2+ redox couple by changing the Fe 3+ speciation (i.e., the iron oxide that forms).…”