Determination of rate constants and branching ratios for TCE degradation by zero-valent iron using a chain decay multispecies model

Significance of temperature as a key driver in ZVI PRB applications for PCE degradation

We report on the potential of elevated groundwater temperatures and zero-valent iron permeable reactive barriers (ZVI PRBs), for example, through a combination with underground thermal energy storage (UTES), to achieve enhanced remediation of chlorinated hydrocarbon (CHC) contaminated groundwater. Building on earlier findings concerning deionized solutions, we created a database for mineralized groundwater based on temperature dependence of tetrachloroethylene (PCE) degradation using two popular ZVIs (i.e., Gotthart-Maier cast iron [GM] and ISPAT sponge iron [IS]) in column experiments at 25 °C-70 °C to establish a temperature-dependent ZVI PRB dimensioning approach. Scenario analysis revealed that a heated ZVI PRB system in a moderate temperature range up to 40 °C showed the greatest efficiency, with potential material savings of ~55% to 75%, compared to 10 °C, considering manageability and longevity. With a 25 °C-70 °C temperature increase, rate coefficients of PCE degradation increased from 0.4 ± 0.0 h-1 to 2.9 ± 2.2 h-1 (GM) and 0.1 ± 0.1 h-1 to 1.8 ± 0.0 h-1 (IS), while TCE rate coefficients increased from 0.6 ± 0.1 h-1 to 5.1 ± 3.9 h-1 at GM. Activation energies for PCE degradation yielded 32 kJ mol-1 (GM) and 56 kJ mol-1 (IS). Temperature-dependent anaerobic iron corrosion was key in regulating mineral precipitation and passivation of the iron surface as well as porosity reduction due to gas production.

Mobilization pilot test of PCE sources in the transition zone to aquitards by combining mZVI and biostimulation with lactic acid

The potential toxic and carcinogenic effects of chlorinated solvents in groundwater on human health and aquatic ecosystems require very effective remediation strategies of contaminated groundwater to achieve the low legal cleanup targets required. The transition zones between aquifers and bottom aquitards occur mainly in prograding alluvial fan geological contexts. Hence, they are very frequent from a hydrogeological point of view. The transition zone consists of numerous thin layers of fine to coarse-grained clastic fragments (e.g., medium sands and gravels), which alternate with fine-grained materials (clays and silts). When the transition zones are affected by DNAPL spills, free-phase pools accumulate on the less conductive layers. Owing to the low overall conductivity of this zone, the pools are very recalcitrant. Little field research has been done on transition zone remediation techniques. Injection of iron microparticles has the disadvantage of the limited accessibility of this reagent to reach the entire source of contamination. Biostimulation of indigenous microorganisms in the medium has the disadvantage that few of the microorganisms are capable of complete biodegradation to total mineralization of the parent contaminant and metabolites. A field pilot test was conducted at a site where a transition zone existed in which DNAPL pools of PCE had accumulated. In particular, the interface with the bottom aquitard was where PCE concentrations were the highest. In this pilot test, a combined strategy using ZVI in microparticles and biostimulation with lactate in the form of lactic acid was conducted. Throughout the test it was found that the interdependence of the coupled biotic and abiotic processes generated synergies between these processes. This resulted in a greater degradation of the PCE and its transformation products. With the combination of the two techniques, the mobilization of the contaminant source of PCE was extremely effective.

Temperature Effects on PCE Degradation on ZVI in Column Experiments with Deionized Water

The effects of rising groundwater temperatures on zerovalent iron (ZVI)-based remediation techniques will be critical in accelerating chlorinated hydrocarbon (CHC) degradation and side reactions. Therefore, tetrachloroethylene (PCE) degradation with three ZVIs widely used in permeable reactive barriers (Gotthart-Maier cast iron [GM], Peerless cast iron [PL], and ISPAT sponge iron [IS]) was evaluated at 10-70 °C in deionized water. From 10 to 70 °C, PCE degradation half-lives decreased from 25 ± 2 to 0.9 ± 0.1 h (PL), 24 ± 3 to 0.7 ± 0.1 h (GM), and 2.5 ± 0.01 to 0.3 ± 0.005 h (IS). Trichloroethylene (TCE) degradation half-lives at PL and GM decreased from 14.3 ± 3 to 0.2 ± 0.1 h (PL) and 7.6 ± 2 to 0.4 ± 0.1 h (GM). This acceleration of CHC degradation and the stronger shift toward reductive β-elimination reduced the concentration of potentially harmful metabolites with increasing temperatures. PCE and TCE degradation yields an activation energy of 28 (IS), 58 and 40 kJ mol^-1 (GM), and 62 and 53 kJ mol^-1 (PL). Hydrogen gas production by ZVI corrosion increased by 3 orders of magnitude from 10 to 70 °C, and an increased chance of gas clogging was observed at high temperatures.

Groundwater geochemical constituents controlling the reductive dechlorination of TCE by nZVI: Evidence from diverse anaerobic corrosion mechanisms of nZVI

The corrosion mechanisms of nanoscale zero-valent iron (nZVI) vary with different geochemical constituents, which affect the reductive dechlorination process of trichloroethylene (TCE). In this study, the effect of nZVI anaerobic corrosion on the reductive dechlorination of TCE with different groundwater geochemical constituents (Ca²⁺-SO₄^2-, Ca²⁺-HCO₃^-, Na⁺-NO₃^-) was investigated. Microscopic characterization by X-ray diffraction (XRD) and transmission electron microscopy (TEM) combined with pH, oxidation-reduction potential (ORP) and dissolved Fe²⁺ in solutions to illustrate the corrosion mechanism of nZVI. In the four systems including ultrapure water (UPW), the reduction of TCE conformed to pseudo-first-order kinetics, the generation of Cl^- accorded with zero-order kinetics, and multi-step reaction kinetics was used to fit the generation and degradation of chlorinated byproducts (Dichloroethylene, DCEs). Compared with UPW system, the dissolution corrosion of Ca²⁺-HCO₃^- and Ca²⁺-SO₄^2- promoted the reductive dechlorination of TCE (k_{obs, TCE} = 0.658 ± 0.010 & 0.245 ± 0.028 d^-1 and k_{obs, Cl-} = 41.682 ± 1.016 & 20.623 ± 1.923 μM⋅d^-1 for Ca²⁺-HCO₃^- & Ca²⁺-SO₄^2-, respectively) and the degradation of DCEs (0.444 ± 0.036 & 0.244 ± 0.040 μM⋅d^-1 for Ca²⁺-HCO₃^- & Ca²⁺-SO₄^2-, respectively); redox-active NO₃^- competed for electrons and passivated the surface of nZVI, which limited the reductive dechlorination of TCE (k_{obs, TCE} = 0.111 ± 0.025 d^-1 & k_{obs, Cl-} = 14.943 ± 0.664 μM⋅d^-1) and the degradation of DCEs (0.078 ± 0.018 μM⋅d^-1), and the passivation layer promoted the adsorption of TCE. This study from the perspective of nZVI corrosion provides a theoretical basis for the long-term application of nZVI technology in the remediation of TCE-contaminated sites with different groundwater geochemical types.

Geochemical and Isotope Study of Trichloroethene Degradation in a Zero-Valent Iron Permeable Reactive Barrier: A Twenty-Two-Year Performance Evaluation

This study provides a twenty-two-year record of in situ degradation of chlorinated organic compounds by a granular iron permeable reactive barrier (PRB). Groundwater concentrations of trichloroethene (TCE) entering the PRB were as high as 10670 μg/L. Treatment efficiency ranged from 81 to >99%, and TCE concentrations from <1 μg/L to 165 μg/L were detected within and hydraulically down-gradient of the PRB. After 18 years, effluent TCE concentrations were above the maximum contaminant level (MCL) along segments of the PRB exhibiting upward trending influent TCE. Degradation products included cis-dichloroethene ( cis-DCE), vinyl chloride (VC), ethene, ethane, >C4 compounds, and possibly CO_2(aq) and methane. Abiotic patterns of TCE degradation were indicated by compound-specific stable isotope data and the distribution of degradation products. δ¹³C values of methane within and down-gradient of the PRB varied widely from -94‰ to -16‰; these values cover most of the isotopic range encountered in natural methanogenic systems. Methanogenesis is a sink for inorganic carbon in zerovalent iron PRBs that competes with carbonate mineralization, and this process is important for understanding pore-space clogging and longevity of iron-based PRBs. The carbon isotope signatures of methane and inorganic carbon were consistent with open-system behavior and 22% molar conversion of CO_2(aq) to methane.