Effects of Solution Chemistry on the Dechlorination of 1,2,3-Trichloropropane by Zero-Valent Zinc

The reactivity of zerovalent zinc (ZVZ) toward 1,2,3-trichloropropane (TCP) was evaluated under a variety of solution conditions, including deionized water, groundwater, and artificial groundwater, over a pH range of about 6.5–12. In deionized water, first-order rate constants for TCP disappearance (<i>k</i>_obs) exhibit a broad minimum between pH 8 and 10, with increasing <i>k</i>_obs observed at lower and higher pH. The similarity between this trend and zinc oxide (ZnO) solubility behavior suggests pH related changes to the ZnO surface layer strongly influence ZVZ reactivity. Values of <i>k</i>_obs measured in acidic groundwater are similar to those measured in DI water, whereas values measured in alkaline groundwater are much smaller (>1 order of magnitude at pH values >10). Characterization of the surfaces of ZVZ exposed to deionized water, acidic groundwater, and alkaline groundwater suggests that the slower rates obtained in alkaline groundwater are related to the presence of a morphologically distinct surface film that passivates the ZVZ surface. TCP degradation rates in artificial groundwater containing individual solutes present in groundwater suggest that silicate anions contribute to the formation of this passivating film

Novel Contaminant Transformation Pathways by Abiotic Reductants

Environmentally relevant abiotic reductants, such as zerovalent iron (ZVI) and minerals containing divalent iron (DVI), react predominantly by electron-transfer mechanisms with a variety of contaminant and probe compounds. Other reduction mechanisms involving activated forms of hydrogen (H_ads or H^–) have been suggested, but most evidence for these is only from systems containing noble metals that catalyze hydrogen activation (e.g., Pd). Here, 2-chlorophenylethanol and relatives of this aromatic halohydrin are used as probe compounds to show that ZVI can affect reduction by several novel pathways that are not observed with DVI minerals. These pathways include dechlorination by intramolecular nucleophilic substitution and epoxide ring opening by reduction. The former appears to be catalyzed by hydroxyl groups associated with oxides on actively corroding ZVI, and the latter can arise from hydride transfer (from NaBH₄) or electron transfer (from ZVI)

Effects of Nano Zero-Valent Iron on Oxidation−Reduction Potential

Oxidation−reduction potential (ORP) measurements have been widely used to assess the results of injection of nano zerovalent iron (nZVI) for groundwater remediation, but the significance of these measurements has never been established. Using rotating disk electrodes (RDE) in suspensions of nZVI, we found the electrode response to be highly complex but also a very sensitive probe for a range of fundamentally significant processes. The time dependence of the electrode response reflects both a primary effect (attachment of nZVI onto the electrode surface) and several secondary effects (esp., oxidation of iron and variations in dissolved H₂ concentration). At nZVI concentrations above ∼200 mg/L, attachment of nZVI to the electrode is sufficient to give it the electrochemical characteristics of an Fe⁰ electrode, making the electrode relatively insensitive to changes in solution chemistry. Lower nZVI concentrations give a proportional response in ORP, but much of this effect is mediated by the secondary effects noted above. Coating the nZVI with natural organic matter (NOM), or the organic polymers used to make stabile suspensions of nZVI, moderates its effect on ORP measurments. Our results provide the basis for interpretating ORP measurements used to characterize the results of injecting nZVI into groundwater

Mechanochemically Sulfidated Microscale Zero Valent Iron: Pathways, Kinetics, Mechanism, and Efficiency of Trichloroethylene Dechlorination

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 H₂ 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^bm) exhibits reduced aggregation, relatively homogeneous distribution of Fe and S throughout the particle (not core–shell structure), enhanced reactivity with trichloroethylene (TCE), less H₂ formation, and therefore greatly improved electron efficiency of TCE dechlorination (ε_e). Under ZVI-limited conditions (initial Fe⁰/TCE = 1.6 mol/mol), S-mZVI^bm gave surface-area normalized reduction rate constants (<i>k</i>′_SA) and ε_e that were ∼2- and 10-fold greater than the unsulfidated ball-milled control (mZVI^bm). Under TCE-limited conditions (initial Fe⁰/TCE = 2000 mol/mol), sulfidation increased <i>k</i>_SA and ε_e ≈ 5- and 50-fold, respectively. The major products from TCE degradation by S-mZVI^bm 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 Fe₃O₄, mostly likely significant coverage of the surface with FeS

Predicting Reduction Rates of Energetic Nitroaromatic Compounds Using Calculated One-Electron Reduction Potentials

The evaluation of new energetic nitroaromatic compounds (NACs) for use in green munitions formulations requires models that can predict their environmental fate. Previously invoked linear free energy relationships (LFER) relating the log of the rate constant for this reaction (log­(<i>k</i>)) and one-electron reduction potentials for the NAC (<i>E</i>¹_NAC) normalized to 0.059 V have been re-evaluated and compared to a new analysis using a (nonlinear) free-energy relationship (FER) based on the Marcus theory of outer-sphere electron transfer. For most reductants, the results are inconsistent with simple rate limitation by an initial, outer-sphere electron transfer, suggesting that the linear correlation between log­(<i>k</i>) and <i>E</i>¹_NAC is best regarded as an empirical model. This correlation was used to calibrate a new quantitative structure–activity relationship (QSAR) using previously reported values of log­(<i>k</i>) for nonenergetic NAC reduction by Fe­(II) porphyrin and newly reported values of <i>E</i>¹_NAC determined using density functional theory at the M06-2X/6-311++G­(2d,2p) level with the COSMO solvation model. The QSAR was then validated for energetic NACs using newly measured kinetic data for 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4-DNT), and 2,4-dinitroanisole (DNAN). The data show close agreement with the QSAR, supporting its applicability to other energetic NACs

Mechanisms and Kinetics of Alkaline Hydrolysis of the Energetic Nitroaromatic Compounds 2,4,6-Trinitrotoluene (TNT) and 2,4-Dinitroanisole (DNAN)

The environmental impacts of energetic compounds can be minimized through the design and selection of new energetic materials with favorable fate properties. Building predictive models to inform this process, however, is difficult because of uncertainties and complexities in some major fate-determining transformation reactions such as the alkaline hydrolysis of energetic nitroaromatic compounds (NACs). Prior work on the mechanisms of the reaction between NACs and OH^– has yielded inconsistent results. In this study, the alkaline hydrolysis of 2,4,6-trinitrotoluene (TNT) and 2,4-dinitroanisole (DNAN) was investigated with coordinated experimental kinetic measurements and molecular modeling calculations. For TNT, the results suggest reversible formation of an initial product, which is likely either a Meisenheimer complex or a TNT anion formed by abstraction of a methyl proton by OH^–. For DNAN, the results suggest that a Meisenheimer complex is an intermediate in the formation of 2,4-dinitrophenolate. Despite these advances, the remaining uncertainties in the mechanisms of these reactionsand potential variability between the hydrolysis mechanisms for different NACsmean that it is not yet possible to generalize the results into predictive models (e.g., quantitative structure–activity relationships, QSARs) for hydrolysis of other NACs

Disinfection of Ballast Water with Iron Activated Persulfate

The treatment of ballast water carried onboard ships is critical to reduce the spread of nonindigenous aquatic organisms that potentially include noxious and harmful taxa. We tested the efficacy of persulfate (peroxydisulfate, S₂O₈^2–, PS) activated with zerovalent iron (Fe⁰) as a chemical biocide against two taxa of marine phytoplankton grown in bench-scale, batch cultures: the diatom, Pseudonitzshia delicatissima and the green alga, Dunaliella tertiolecta. After testing a range of PS concentrations (0–4 mM activated PS) and exposure times (1–7 days), we determined that a dosage of 4 mM of activated PS was required to inactivate cells from both species, as indicated by reduced or halted growth and a reduction in photosynthetic performance. Longer exposure times were required to fully inactivate D. tertiolecta (7 days) compared to P. delicatissima (5 days). Under these conditions, no recovery was observed upon placing cells from the exposed cultures into fresh media lacking biocide. The results demonstrate that activated PS is an effective chemical biocide against species of marine phytoplankton. The lack of harmful byproducts produced during application makes PS an attractive alternative to other biocides currently in use for ballast water treatments and merits further testing at a larger scale

Chemical Reactivity Probes for Assessing Abiotic Natural Attenuation by Reducing Iron Minerals

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 (<i>E</i>_CRP) of suspended minerals by spectrophotometric determination of equilibrium CRP speciation and calculations using the Nernst equation. <i>E</i>_CRP varied as expected with mineral type, mineral loading, and Fe­(II) concentration. Comparison of <i>E</i>_CRP with reduction potentials measured potentiometrically using a Pt electrode (<i>E</i>_Pt) showed that <i>E</i>_CRP was 100–150 mV more negative than <i>E</i>_Pt. When <i>E</i>_Pt was measured with small additions of CRPs, the systematic difference between <i>E</i>_Pt and <i>E</i>_CRP 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 <i>E</i>_CRP for mineral suspensions. Using the rate constants compiled from literature for contaminants and relative mineral reduction potentials based on <i>E</i>_CRP measurements, qualitatively consistent trends were obtained, suggesting that CRP-based assays may be useful for estimating abiotic NA rates of contaminants in groundwater

Coupled Effects of Aging and Weak Magnetic Fields on Sequestration of Selenite by Zero-Valent Iron

The sequestration of Se­(IV) by zero-valent iron (ZVI) is strongly influenced by the coupled effects of aging ZVI and the presence of a weak magnetic field (WMF). ZVI aged at pH 6.0 with MES as buffer between 6 and 60 h gave nearly constant rates of Se­(IV) removal with WMF but with rate constants that are 10- to 100-fold greater than without. XANES analysis showed that applying WMF changes the mechanism of Se­(IV) removal by ZVI aged for 6–60 h from adsorption followed by reduction to direct reduction. The strong correlation between Se­(IV) removal and Fe²⁺ release suggests direct reduction of Se­(IV) to Se(0) by Fe⁰, in agreement with the XANES analysis. The numerical simulation of ZVI magnetization revealed that the WMF influence on Se­(IV) sequestration is associated mainly with the ferromagnetism of ZVI and the paramagnetism of Fe²⁺. In the presence of the WMF, the Lorentz force gives rise to convection in the solution, which narrows the diffusion layer, and the field gradient force, which tends to move paramagnetic ions (esp. Fe²⁺) along the higher field gradient at the ZVI particle surface, thereby inducing nonuniform depassivation and eventually localized corrosion of the ZVI surface

Remediation of Trichloroethylene by FeS-Coated Iron Nanoparticles in Simulated and Real Groundwater: Effects of Water Chemistry

The reactivity of FeS-coated iron nanoparticles (nFe/FeS) toward trichloroethylene (TCE) reduction was examined in both synthetic and real groundwater matrices to evaluate the potential performance of nFe/FeS in field treatment. The rate of TCE reduction increased with increasing pH, which is consistent with the pH effect reported previously for iron sulfide systems, but opposite that has been observed for (nonsulfidic) Fe⁰ systems. The rates of TCE reduction were unaffected by ionic strength over the range of 0.1–10 mM NaCl, increased with Ca²⁺ or Mg²⁺ concentrations, and inhibited by the presence of humic acid. The inhibitory effect of humic acid on the reactivity of nFe/FeS was largely alleviated when humic acid was combined with Ca²⁺/Mg²⁺, presumably due to decreased adsorption of humic acid onto nFe/FeS surface by the formation of humic acid–Ca²⁺/Mg²⁺ complexes