In water treatment processes that involve contaminant reduction by zerovalent iron (ZVI), reduction of water to dihydrogen is a competing reaction that must be minimized to maximize the efficiency of electron utilization from the ZVI. Sulfidation has recently been shown to decrease H formation significantly, such that the overall electron efficiency of (or selectivity for) contaminant reduction can be greatly increased. To date, this work has focused on nanoscale ZVI (nZVI) and solution-phase sulfidation agents (e.g., bisulfide, dithionite or thiosulfate), both of which pose challenges for up-scaling the production of sulfidated ZVI for field applications. To overcome these challenges, we developed a process for sulfidation of microscale ZVI by ball milling ZVI with elemental sulfur. The resulting material (S-mZVI) exhibits reduced aggregation, relatively homogeneous distribution of Fe and S throughout the particle (not core-shell structure), enhanced reactivity with trichloroethylene (TCE), less H formation, and therefore greatly improved electron efficiency of TCE dechlorination (ε). Under ZVI-limited conditions (initial Fe/TCE = 1.6 mol/mol), S-mZVI gave surface-area normalized reduction rate constants (k') and ε that were ∼2- and 10-fold greater than the unsulfidated ball-milled control (mZVI). Under TCE-limited conditions (initial Fe/TCE = 2000 mol/mol), sulfidation increased k and ε ≈ 5- and 50-fold, respectively. The major products from TCE degradation by S-mZVI were acetylene, ethene, and ethane, which is consistent with dechlorination by β-elimination, as is typical of ZVI, iron oxides, and/or sulfides. However, electrochemical characterization shows that the sulfidated material has redox properties intermediate between ZVI and FeO, mostly likely significant coverage of the surface with FeS.

Increasing recognition that abiotic natural attenuation (NA) of chlorinated solvents can be important has created demand for improved methods to characterize the redox properties of the aquifer materials that are responsible for abiotic NA. This study explores one promising approach: using chemical reactivity probes (CRPs) to characterize the thermodynamic and kinetic aspects of contaminant reduction by reducing iron minerals. Assays of thermodynamic CRPs were developed to determine the reduction potentials (ECRP) of suspended minerals by spectrophotometric determination of equilibrium CRP speciation and calculations using the Nernst equation. ECRP varied as expected with mineral type, mineral loading, and Fe(II) concentration. Comparison of ECRP with reduction potentials measured potentiometrically using a Pt electrode (EPt) showed that ECRP was 100-150 mV more negative than EPt. When EPt was measured with small additions of CRPs, the systematic difference between EPt and ECRP was eliminated, suggesting that these CRPs are effective mediators of electron transfer between mineral and electrode surfaces. Model contaminants (4-chloronitrobenzene, 2-chloroacetophenone, and carbon tetrachloride) were used as kinetic CRPs. The reduction rate constants of kinetic CRPs correlated well with the ECRP for mineral suspensions. Using the rate constants compiled from literature for contaminants and relative mineral reduction potentials based on ECRP measurements, qualitatively consistent trends were obtained, suggesting that CRP-based assays may be useful for estimating abiotic NA rates of contaminants in groundwater.

The mixed and variable valence of iron in magnetite (Fe­(III)tet[Fe­(II),Fe­(III)]octO4 2–) give this mineral unique properties that make it an important participant in redox reactions in environmental systems. However, the variability in its stoichiometry and other physical properties complicates the determination of its effective redox potential. To address this challenge, a robust method was developed to prepare working electrodes with mineral powders of diverse characteristics and agarose-stabilized pore waters of controlled composition. This second-generation powder-disk electrode (PDEv2) methodology was used to characterize the electrochemical properties of magnetite samples from a wide variety of sources (lab-synthesized, commercial, and magnetically separated from environmental samples) using a sequence of complementary potentiometric methods: chronopotentiometry (CP), linear polarization resistance (LPR), and then linear sweep voltammetry (LSV). The passive method CP gave open-circuit potentials (E OC) and the active method LPR gave corrosion potentials (E 0,LPR) that agree closely with each other but vary over a wide range for the magnetite samples tested (ca. 520 mV, from −267 to +253 mV vs SHE). The active method LSV gave values of E 0,LSV that become increasingly more negative than E OC for the samples with more positive potentials (by up to 189 mV). This effect is consistent with the cathodic polarization applied at the beginning of the LSV scan and suggests there is convergence of substoichiometric magnetites to the potential of stoichiometric magnetite after polarization. By all methods, lab-synthesized magnetites gave more negative potentials and smaller polarization resistances (R p) than magnetite from commercial sources or magnetic separation of environmental samples. This is consistent with the common notion that freshly synthesized minerals are more reactive, but clear correlations were not found between the measured redox potentials and surface area, iron stoichiometry, or magnetic susceptibility. All the measured potentials for magnetite fall in a range between calculated thermodynamic values for redox couples involving relevant iron species, which is consistent with the measured values being mixed potentials. The wide range in effective redox potential of magnetite is likely to influence its role in biogeochemistry and contaminant fate.