Anisotropic Morphological Changes in Goethite during Fe²⁺-Catalyzed Recrystallization

When goethite is exposed to aqueous Fe²⁺, rapid and extensive Fe atom exchange can occur between solid-phase Fe³⁺ and aqueous Fe²⁺ in a process referred to as Fe²⁺-catalyzed recrystallization. This process can lead to the structural incorporation or release of trace elements, which has important implications for contaminant remediation and nutrient biogeochemical cycling. Prior work found that the process did not cause major changes to the goethite structure or morphology. Here, we further investigated if and how goethite morphology and aggregation behavior changed temporally during Fe²⁺-catalyzed recrystallization. On the basis of existing literature, we hypothesized that Fe²⁺-catalyzed recrystallization of goethite would not result in changes to individual particle morphology or interparticle interactions. To test this, we reacted nanoparticulate goethite with aqueous Fe²⁺ at pH 7.5 over 30 days and used transmission electron microscopy (TEM), cryogenic TEM, and ⁵⁵Fe as an isotope tracer to observe changes in particle dimensions, aggregation, and isotopic composition over time. Over the course of 30 days, the goethite particles substantially recrystallized, and the particle dimensions changed anisotropically, resulting in a preferential increase in the mean particle width. The temporal changes in goethite morphology could not be completely explained by a single mineral-transformation mechanism but rather indicated that multiple transformation mechanisms occurred concurrently. Collectively, these results demonstrate that the morphology of goethite nanoparticles does change during recrystallization, which is an important step toward identifying the driving force(s) of recrystallization

Low Energy Desalination Using Battery Electrode Deionization

New electrochemical technologies that use capacitive or battery electrodes are being developed to minimize energy requirements for desalinating brackish waters. When a pair of electrodes is charged in capacitive deionization (CDI) systems, cations bind to the cathode and anions bind to the anode, but high applied voltages (>1.2 V) result in parasitic reactions and irreversible electrode oxidation. In the battery electrode deionization (BDI) system developed here, two identical copper hexacyanoferrate (CuHCF) battery electrodes were used that release and bind cations, with anion separation occurring via an anion exchange membrane. The system used an applied voltage of 0.6 V, which avoided parasitic reactions, achieved high electrode desalination capacities (up to 100 mg-NaCl/g-electrode, 50 mM NaCl influent), and consumed less energy than CDI. Simultaneous production of desalinated and concentrated solutions in two channels avoided a two-cycle approach needed for CDI. Stacking additional membranes between CuHCF electrodes (up to three anion and two cation exchange membranes) reduced energy consumption to only 0.02 kWh/m³ (approximately an order of magnitude lower than values reported for CDI), for an influent desalination similar to CDI (25 mM decreased to 17 mM). These results show that BDI could be effective as a very low energy method for brackish water desalination

Electrochemical Analyses of Redox-Active Iron Minerals: A Review of Nonmediated and Mediated Approaches

Redox-active minerals are ubiquitous in the environment and are involved in numerous electron transfer reactions that significantly affect biogeochemical processes and cycles as well as pollutant dynamics. As a consequence, research in different scientific disciplines is devoted to elucidating the redox properties and reactivities of minerals. This review focuses on the characterization of mineral redox properties using electrochemical approaches from an applied (bio)­geochemical and environmental analytical chemistry perspective. Establishing redox equilibria between the minerals and working electrodes is a major challenge in electrochemical measurements, which we discuss in an overview of traditional electrochemical techniques. These issues can be overcome with mediated electrochemical analyses in which dissolved redox mediators are used to increase the rate of electron transfer and to facilitate redox equilibration between working electrodes and minerals in both amperometric and potentiometric measurements. Using experimental data on an iron-bearing clay mineral, we illustrate how mediated electrochemical analyses can be employed to derive important thermodynamic and kinetic data on electron transfer to and from structural iron. We summarize anticipated methodological advancements that will further contribute to advance an improved understanding of electron transfer to and from minerals in environmentally relevant redox processes

Thermodynamic Controls on the Microbial Reduction of Iron-Bearing Nontronite and Uranium

Iron-bearing phyllosilicate minerals help establish the hydrogeological and geochemical conditions of redox transition zones because of their small size, limited hydraulic conductivity, and redox buffering capacity. The bioreduction of soluble U­(VI) to sparingly soluble U­(IV) can promote the reduction of clay-Fe­(III) through valence cycling. The reductive precipitation of U­(VI) to uraninite was previously reported to occur only after a substantial percentage of clay-Fe­(III) had been reduced. Using improved analytical techniques, we show that concomitant bioreduction of both U­(VI) and clay-Fe­(III) by <i>Shewanella putrefaciens</i> CN32 can occur. Soluble electron shuttles were previously shown to enhance both the rate and extent of clay-Fe­(III) bioreduction. Using extended incubation periods, we show that electron shuttles enhance only the rate of reduction (overcoming a kinetic limitation) and not the final extent of reduction (a thermodynamic limitation). The first 20% of clay-Fe­(III) in nontronite NAu-2 was relatively “easy” (i.e., rapid) to bioreduce; the next 15% of clay-Fe­(III) was “harder” (i.e., kinetically limited) to bioreduce, and the remaining 65% of clay-Fe­(III) was effectively biologically unreducible. In abiotic experiments with NAu-2 and biogenic uraninite, 16.4% of clay-Fe­(III) was reduced in the presence of excess uraninite. In abiotic experiments with NAu-2 and reduced anthraquinone 2,6-disulfonate (AH₂DS), 18.5–19.1% of clay-Fe­(III) was reduced in the presence of excess and variable concentrations of AH₂DS. A thermodynamic model based on published values of the nonstandard state reduction potentials at pH 7.0 (<i>E</i>′_H) showed that the abiotic reactions between NAu-2 and uraninite had reached an apparent equilibrium. This model also showed that the abiotic reactions between NAu-2 and AH₂DS had reached an apparent equilibrium. The final extent of clay-Fe­(III) reduction correlated well with the standard state reduction potential at pH 7.0 (<i>E</i>°′_H) of all of the reductants used in these experiments (AH₂DS, CN32, dithionite, and uraninite)

A pH-Gradient Flow Cell for Converting Waste CO₂ into Electricity

The CO₂ concentration difference between ambient air and exhaust gases created by combusting fossil fuels is an untapped energy source for producing electricity. One method of capturing this energy is dissolving CO₂ gas into water and then converting the produced chemical potential energy into electrical power using an electrochemical system. Previous efforts using this method found that electricity can be generated; however, electrical power densities were low, and expensive ion-exchange membranes were needed. Here, we overcame these challenges by developing a new approach to capture electrical power from CO₂ dissolved in water, the pH-gradient flow cell. In this approach, two identical supercapacitive manganese oxide electrodes were separated by a nonselective membrane and exposed to an aqueous buffer solution sparged with either CO₂ gas or air. This pH-gradient flow cell produced an average power density of 0.82 W/m², which was nearly 200 times higher than values reported using previous approaches

Harvesting Energy from Salinity Differences Using Battery Electrodes in a Concentration Flow Cell

Salinity-gradient energy (SGE) technologies produce carbon-neutral and renewable electricity from salinity differences between seawater and freshwater. Capacitive mixing (CapMix) is a promising class of SGE technologies that captures energy using capacitive or battery electrodes, but CapMix devices have produced relatively low power densities and often require expensive materials. Here, we combined existing CapMix approaches to develop a concentration flow cell that can overcome these limitations. In this system, two identical battery (i.e., faradaic) electrodes composed of copper hexacyanoferrate (CuHCF) were simultaneously exposed to either high (0.513 M) or low (0.017 M) concentration NaCl solutions in channels separated by a filtration membrane. The average power density produced was 411 ± 14 mW m^–2 (normalized to membrane area), which was twice as high as previously reported values for CapMix devices. Power production was continuous (i.e., it did not require a charging period and did not vary during each step of a cycle) and was stable for 20 cycles of switching the solutions in each channel. The concentration flow cell only used inexpensive materials and did not require ion-selective membranes or precious metals. The results demonstrate that the concentration flow cell is a promising approach for efficiently harvesting energy from salinity differences

Linking Thermodynamics to Pollutant Reduction Kinetics by Fe²⁺ Bound to Iron Oxides

Numerous studies have reported that pollutant reduction rates by ferrous iron (Fe²⁺) are substantially enhanced in the presence of an iron (oxyhydr)­oxide mineral. Developing a thermodynamic framework to explain this phenomenon has been historically difficult due to challenges in quantifying reduction potential (<i>E</i>_H) values for oxide-bound Fe²⁺ species. Recently, our group demonstrated that <i>E</i>_H values for hematite- and goethite-bound Fe²⁺ can be accurately calculated using Gibbs free energy of formation values. Here, we tested if calculated <i>E</i>_H values for oxide-bound Fe²⁺ could be used to develop a free energy relationship capable of describing variations in reduction rate constants of substituted nitrobenzenes, a class of model pollutants that contain reducible aromatic nitro groups, using data collected here and compiled from the literature. All the data could be described by a single linear relationship between the logarithms of the surface-area-normalized rate constant (<i>k</i>_SA) values and <i>E</i>_H and pH values [log­(<i>k</i>_SA) = −<i>E</i>_H/0.059 V – pH + 3.42]. This framework provides mechanistic insights into how the thermodynamic favorability of electron transfer from oxide-bound Fe²⁺ relates to redox reaction kinetics

Susceptibility of Goethite to Fe²⁺-Catalyzed Recrystallization over Time

Recent work has shown that iron oxides, such as goethite and hematite, may recrystallize in the presence of aqueous Fe²⁺ under anoxic conditions. This process, referred to as Fe²⁺-catalyzed recrystallization, can influence water quality by causing the incorporation/release of environmental contaminants and biological nutrients. Accounting for the effects of Fe²⁺-catalyzed recrystallization on water quality requires knowing the time scale over which recrystallization occurs. Here, we tested the hypothesis that nanoparticulate goethite becomes less susceptible to Fe²⁺-catalyzed recrystallization over time. We set up two batches of reactors in which ⁵⁵Fe²⁺ tracer was added at two different time points and tracked the ⁵⁵Fe partitioning in the aqueous and goethite phases over 60 days. Less ⁵⁵Fe uptake occurred between 30 and 60 days than between 0 and 30 days, suggesting goethite recrystallization slowed with time. Fitting the data with a box model indicated that 17% of the goethite recrystallized after 30 days of reaction, and an additional 2% recrystallized between 30 and 60 days. The decreasing susceptibility of goethite to recrystallize as it reacted with aqueous Fe²⁺ suggested that recrystallization is likely only an important process over short time scales

Mediated Electrochemical Reduction of Iron (Oxyhydr-)Oxides under Defined Thermodynamic Boundary Conditions

Iron (oxyhydr-)­oxide reduction has been extensively studied because of its importance in pollutant redox dynamics and biogeochemical processes. Yet, experimental studies linking oxide reduction kinetics to thermodynamics remain scarce. Here, we used mediated electrochemical reduction (MER) to directly quantify the extents and rates of ferrihydrite, goethite, and hematite reduction over a range of negative reaction free energies, Δ_r<i>G</i>, that were obtained by systematically varying pH (5.0 to 8.0), applied reduction potentials (−0.53 to −0.17 V vs SHE), and Fe²⁺ concentrations (up to 40 μM). Ferrihydrite reduction was complete and fast at all tested Δ_r<i>G</i> values, consistent with its comparatively low thermodynamic stability. Reduction of the thermodynamically more stable goethite and hematite changed from complete and fast to incomplete and slow as Δ_r<i>G</i> values became less negative. Reductions at intermediate Δ_r<i>G</i> values showed negative linear correlations between the natural logarithm of the reduction rate constants and Δ_r<i>G</i>. These correlations imply that thermodynamics controlled goethite and hematite reduction rates. Beyond allowing to study iron oxide reduction under defined thermodynamic conditions, MER can also be used to capture changes in iron oxide reducibility during phase transformations, as shown for Fe²⁺-facilitated transformation of ferrihydrite to goethite

Thermodynamic Characterization of Iron Oxide–Aqueous Fe²⁺ Redox Couples

Iron is present in virtually all terrestrial and aquatic environments, where it participates in redox reactions with surrounding metals, organic compounds, contaminants, and microorganisms. The rates and extent of these redox reactions strongly depend on the speciation of the Fe²⁺ and Fe³⁺ phases, although the underlying reasons remain unclear. In particular, numerous studies have observed that Fe²⁺ associated with iron oxide surfaces (i.e., oxide-associated Fe²⁺) often reduces oxidized contaminants much faster than aqueous Fe²⁺ alone. Here, we tested two hypotheses related to this observation by determining if solutions containing two commonly studied iron oxideshematite and goethiteand aqueous Fe²⁺ reached thermodynamic equilibrium over the course of a day. We measured reduction potential (<i>E</i>_H) values in solutions containing these oxides at different pH values and aqueous Fe²⁺ concentrations using mediated potentiometry. This analysis yielded standard reduction potential (<i>E</i>_H⁰) values of 768 ± 1 mV for the aqueous Fe²⁺–goethite redox couple and 769 ± 2 mV for the aqueous Fe²⁺–hematite redox couple. These values were in excellent agreement with those calculated from existing thermodynamic data, and the data could be explained by the presence of an iron oxide lowering <i>E</i>_H values of aqueous Fe³⁺/Fe²⁺ redox couples