Thermodynamic controls on rates of iron oxide reduction by extracellular electron shuttles

Anaerobic microbial respiration in suboxic and anoxic environments often involves particulate ferric iron (oxyhydr-)oxides as terminal electron acceptors. To ensure efficient respiration, a widespread strategy among iron-reducing microorganisms is the use of extracellular electron shuttles (EES) that transfer two electrons from the microbial cell to the iron oxide surface. Yet, a fundamental understanding of how EES–oxide redox thermodynamics affect rates of iron oxide reduction remains elusive. Attempts to rationalize these rates for different EES, solution pH, and iron oxides on the basis of the underlying reaction free energy of the two-electron transfer were unsuccessful. Here, we demonstrate that broadly varying reduction rates determined in this work for different iron oxides and EES at varying solution chemistry as well as previously published data can be reconciled when these rates are instead related to the free energy of the less exergonic (or even endergonic) first of the two electron transfers from the fully, two-electron reduced EES to ferric iron oxide. We show how free energy relationships aid in identifying controls on microbial iron oxide reduction by EES, thereby advancing a more fundamental understanding of anaerobic respiration using iron oxides.

Mechanochemical Activation Effect on Technogenic Iron Oxide Reduction Kinetics

Understanding the reaction kinetics of iron oxide reduction by carbon is a key task of the theory of metallurgical processes. One of the understudied features of the reaction kinetics of iron oxide solid-phase reduction by carbon is the discrepancy between the reacting substances’ small contact area and the process’s high rate. A convincing theoretical and experimental explanation of this effect has not yet been obtained. The data obtained earlier show that an increase in the scale of the briquetting pressure from 0 to 300 MPa increases the degree of its metallization during heating two-fold, and the metallization temperature decreases by more than 40 °C. Therefore, it was assumed that these effects during heating are a consequence of the mechanochemical activation (MCA) of iron oxides in the scale during its pressing. This paper presents the results of experimental studies on the influence of two types of scale MCA (grinding and pressing) on iron oxide reduction. The study of the MCA effect on the reaction kinetics of scale iron oxide reduction by carbon is a promising way to assess the criteria for scale phase composition changes under external factors. The presented results indicate a decrease in the amount of trivalent iron oxide (Fe2O3) after the MCA and an increase in the amount of one-and-a-half oxide (Fe3O4) and bivalent iron oxide (FeO). The obtained experimental data show that the initial stage of iron oxide reduction, consisting in the transition from higher iron oxides to lower ones, is possible at room temperature without carbon presence.

Co-Conversion Mechanisms of Boron and Iron Components of Ludwigite Ore during Reductive Soda-Ash Roasting

Ludwigite ore is a typical intergrown mineral resource found in China. Reductive soda-ash roasting followed by water leaching is an innovative process for the high-efficiency separation and recovery of boron and iron. In this study, the co-conversion mechanism of boron activation and iron reduction during soda-ash reductive roasting for boron-bearing iron concentrate was clarified. When the boron-bearing iron concentrate was reduced in the presence of Na2CO3, szaibelyite (Mg2(OH)(B2O4) (OH)) was activated to sodium metaborate (NaBO2) and, meanwhile, magnetite (Fe3O4) was reduced to metallic iron (MFe). Boron activation promoted iron-oxide reduction effectively, while the latter could only slightly influence the former. The promotion occurred through (1) a facilitated generation of sodium magnesium silicate (Na2MgSiO4) and a hindering of the formation of olivine (MgxFe2-x(SiO4)). (2) The newly generated NaBO2 promoted iron-oxide reduction. (3) The low melting point of the NaBO2 (966 °C) favored particle migration, which accelerated metallic iron particle aggregation.

Influence of Cerium on the Reduction Behaviour of Iron Oxide by Using Carbon Monoxide: TPR and Kinetic Studies

Reduction of iron oxide is one of the most studied topics owing to the importance of iron/steel industry and also has been used as a precursor and active component in a number of important chemical processes. The interaction between iron oxide and other metal additive have gained interest in the past two decades due to the ability on enhancing the reduction performance of the iron oxide. Therefore, this study was undertaken to investigate the influence of cerium on the reduction behaviours of iron oxide by (10%, v/v) carbon monoxide in nitrogen. The cerium doped (Ce-Fe2O3) and non-doped iron oxide reduction behaviour and the kinetic studies have been studied by temperature programmed reduction (TPR) and the phases formed of partially and completely reduced samples were characterized by X-ray diffraction spectroscopy (XRD) while the activation energy values were calculated from Arrhenius equation using Wimmer’s method. TPR results indicate that the reduction of doped and undoped iron oxide proceeds in three steps reduction (Fe2O3 ? Fe3O4 ? FeO ? Fe), while doped iron oxide showed a large shifted towards lower temperature especially in the transition steps of FeO ? Fe. Furthermore, TPR results also suggested that by adding Ce metal into iron oxide the reduction of metal iron completed at lower temperature (700 ?C) compared to non-doped iron oxide (900 ?C). Meanwhile, XRD analysis indicated that doped iron oxide composed of Fe2O3 and a small amount of FeCe2O4. The increase in the rates of iron oxide reduction may relate to the presence of cerium species in the formed of FeCe2O4 and was confirmed by the decrease in the activation energy regarding to all transition phases (Fe2O3 ? Fe3O4 ? FeO ? Fe) during the reduction process