The reaction of aqueous Fe(II) with Fe(III) oxides is a complex process, comprising sorption, electron transfer, and in some cases, reductive dissolution and transformation to secondary minerals. To better understand the dynamics of these reactions, we measured the extent and rate of Fe isotope exchange between aqueous Fe(II) and goethite using a 57Fe isotope tracer approach. We observed near-complete exchange of Fe atoms between the aqueous phase and goethite nanorods over a 30-day time period. Despite direct isotopic evidence for extensive mixing between the aqueous and goethite Fe, no phase transformation was observed, nor did the size or shape of the goethite rods change appreciably. High-resolution transmission electron microscopy images, however, appear to indicate that some recrystallization of the goethite particles may have occurred. Near-complete exchange of Fe between aqueous Fe(II) and goethite, coupled with negligible change in the goethite mineralogy and morphology, suggests a mechanism of c...

Atom Exchange between Aqueous Fe(II) and Goethite: An Fe Isotope Tracer Study

Direct interspecies electron transfer via conductive materials: A perspective for anaerobic digestion applications.

Anaerobic digestion (AD) is an effective biological treatment for stabilizing organic compounds in waste/wastewater and in simultaneously producing biogas. However, it is often limited by the slow reaction rates of different microorganisms’ syntrophic biological metabolisms. Stable and fast interspecies electron transfer (IET) between volatile fatty acid-oxidizing bacteria and hydrogenotrophic methanogens is crucial for efficient methanogenesis. In this syntrophic interaction, electrons are exchanged via redox mediators such as hydrogen and formate. Recently, direct IET (DIET) has been revealed as an important IET route for AD. Microorganisms undergoing DIET form interspecies electrical connections via membrane-associated cytochromes and conductive pili; thus, redox mediators are not required for electron exchange. This indicates that DIET is more thermodynamically favorable than indirect IET. Recent studies have shown that conductive materials (e.g., iron oxides, activated carbon, biochar, and carbon fibers) can mediate direct electrical connections for DIET. Microorganisms attach to conductive materials’ surfaces or vice versa according to particle size, and form conductive biofilms or aggregates. Different conductive materials promote DIET and improve AD performance in digesters treating different feedstocks, potentially suggesting a new approach to enhancing AD performance. This review discusses the role and potential of DIET in methanogenic systems, especially with conductive materials for promoting DIET.

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Role and Potential of Direct Interspecies Electron Transfer in Anaerobic Digestion

A pulsed eddy current (PEC) system has been used to characterize atmospheric corrosion on steel samples. International Paint has supplied coated and uncoated mild steel (S275) samples with marine atmospheric corrosion (exposure time between 1 and 10 months). The PEC response due to corrosion is a complex mix of many factors, including conductivity, permeability and material thickness variation, which are all taken into account through experimental studies and the extraction of signal features. Considering the conductivity and permeability variation in the corrosion layer or the actual rust region, two time-domain features, each representing the conductivity and permeability, are extracted and used to characterize corrosion. The relationship between PEC features and exposure time has been derived, which can be useful for corrosion rate measurement and early-stage corrosion evaluation and prediction. In conclusion, PEC testing has the potential for corrosion characterization and monitoring in areas such as the marine industry.

Steel Corrosion Characterization Using Pulsed Eddy Current Systems

Carbon nanotubes (CNTs)−magnetite nanocomposite with 20−30-nm magnetite particles attached onto CNTs has been successfully prepared for the first time by in situ solvothermal synthesis from the precursor of Fe−urea coordination complex (Fe[(NH2)2CO]6(NO3)3) and CNTs. The effects of CNTs and various processing parameters on ultimate composites characteristics were investigated. It has been found that pure CNTs−magnetite nanocomposite was obtained in an ethylenediamine medium at 200 °C for 50 h with the weight ratio of Fe[(NH2)2CO]6(NO3)3:CNTs = 10:1. XRD, BET, TEM, EDS, and a Mossbauer spectrum were used to characterize the final product. A possible formation mechanism of the CNTs−magnetite nanocomposite was suggested. It has been concluded that a suitable amount of water introduced by CNTs is critical for preparing CNTs−magnetite nanocomposite. The addition of CNTs in the composite increased the electrical conductivity by about 32% from 1.9 to 2.5 S cm-1, compared with the composite without CNTs. The rela...

Carbon Nanotubes−Magnetite Nanocomposites from Solvothermal Processes: Formation, Characterization, and Enhanced Electrical Properties

Photoacoustic, EPR and electrical conductivity investigations of three synthetic mineral pigments: hematite, goethite and magnetite

The magnetic and electronic properties of composites consisting of oxidized multiwall carbon nantubes (MWNTs) dispersed in elastomeric poly(ether-ester) segmented block copolymer have been studied by means of electron spin resonance (ESR) and dc magnetization measurements. A marked reduction in the MWNT diamagnetic response is identified, indicative of substantial hole doping related to the oxygen functional groups on the oxidized carbon nanotube’s surface. Both ESR and the static magnetization reveal considerable enhancement of the spin susceptibility due to an excessive increase in the density of paramagnetic defects, which are sensitive to the dynamical polymer relaxation and thus to the MWNT-polymer interfacial coupling.

D. Yarmis

Papers

Photoacoustic, EPR and electrical conductivity investigations of three synthetic mineral pigments: hematite, goethite and magnetite

Magnetic properties of carbon nanotube poly(ether-ester) nanocomposites