Ten year performance evaluation of a field-scale zero-valent iron permeable reactive barrier installed to remediate trichloroethene contaminated groundwater

Core-shell structure dependent reactivity of Fe@Fe₂O₃ nanowires on aerobic degradation of 4-chlorophenol

In this study, core-shell Fe@Fe₂O₃ nanowires with different iron oxide shell thickness were synthesized through tuning water-aging time after the reduction of ferric ions with sodium borohydride without any stirring. We found that these Fe@Fe₂O₃ nanowires exhibited interesting core-shell structure dependent reactivity on the aerobic degradation of 4-chlorophenol. Characterization results revealed that the core-shell structure dependent aerobic oxidative reactivity of Fe@Fe₂O₃ nanowires was arisen from the combined effects of incrassated iron oxide shell and more surface bound ferrous ions on amorphous iron oxide shell formed during the water-aging process. The incrassated iron oxide shell would gradually block the outward electron transfer from iron core for the subsequent two-electron molecular oxygen activation, but more surface bound ferrous ions on iron oxide shell with prolonging aging time could favor the single-electron molecular oxygen activation, which was confirmed by electron spin resonance spectroscopy with spin trap technique. The mineralization of 4-chlorophenol was monitored by total organic carbon measurement and the oxidative degradation intermediates were analyzed by gas chromatography-mass spectrometry. This study provides new physical insight on the molecular oxygen activation mechanism of nanoscale zerovalent iron and its application on aerobic pollutant removal.

Reductive sequestration of pertechnetate (⁹⁹TcO₄⁻) by nano zerovalent iron (nZVI) transformed by abiotic sulfide

Under anoxic conditions, soluble pertechnetate (⁹⁹TcO₄⁻) can be reduced to less soluble TcO₂·nH₂O, but the oxide is highly susceptible to reoxidation. Here we investigate an alternative strategy for remediation of Tc-contaminated groundwater whereby sequestration as Tc sulfide is favored by sulfidic conditions stimulated by nano zerovalent iron (nZVI). nZVI was pre-exposed to increasing concentrations of sulfide in simulated Hanford groundwater for 24 h to mimic the onset of aquifer biotic sulfate reduction. Solid-phase characterizations of the sulfidated nZVI confirmed the formation of nanocrystalline FeS phases, but higher S/Fe ratios (>0.112) did not result in the formation of significantly more FeS. The kinetics of Tc sequestration by these materials showed faster Tc removal rates with increasing S/Fe between 0 and 0.056, but decreasing Tc removal rates with S/Fe > 0.224. The more favorable Tc removal kinetics at low S/Fe could be due to a higher affinity of TcO₄⁻ for FeS than iron oxides, and electron microscopy confirmed that the majority of the Tc was associated with FeS phases. The inhibition of Tc removal at high S/Fe appears to have been caused by excess HS(-). X-ray absorption spectroscopy revealed that as S/Fe increased, the pathway for Tc(IV) formation shifted from TcO₂·nH2₂ to Tc sulfide phases. The most substantial change of Tc speciation occurred at low S/Fe, coinciding with the rapid increase in Tc removal rate. This agreement further confirms the importance of FeS in Tc sequestration.

Permeability of iron sulfide (FeS)-based materials for groundwater remediation

Iron sulfide (FeS) has been extensively assessed as a reactive medium to remove both metals and halogenated organics from groundwater. However, to address its suitability as a material for permeable reactive barriers (PRBs), its propensity for solids and gas production, which result in reduced permeability, must be evaluated. The reduction in permeability for sands coated with FeS (as mackinawite), under the anoxic conditions often encountered at contaminated groundwater sites, was examined through column experiments and geochemical modeling under conditions of high calcium and nitrate, which have been previously shown to cause significant permeability reduction in zero-valent iron (ZVI) systems. The column experiments showed negligible production of both solids and gases. The geochemical modeling predicted a maximum reduction in permeability of 1% due to solids and about 30% due to gas formation under conditions for which a complete loss of permeability was predicted for ZVI systems. This difference in permeability reduction is driven by the differences in thermodynamic stability of ZVI and FeS in aqueous solutions. The results suggest that geochemical conditions that result in high permeability losses for ZVI systems will likely not be problematic for FeS-based reactive materials.

Field-scale transport and transformation of carboxymethylcellulose-stabilized nano zero-valent iron

The fate of nano zerovalent iron (nZVI) during subsurface injection was examined using carboxymethylcellulose (CMC) stabilized nZVI in a very large three-dimensional physical model aquifer with detailed monitoring using multiple, complementary detection methods. A fluorescein tracer test in the aquifer plus laboratory column data suggested that the very-aggressive flow conditions necessary to achieve 2.5 m of nZVI transport could be obtained using a hydraulically constrained flow path between injection and extraction wells. However, total unoxidized nZVI was transported only about 1 m and <2% of the injected nZVI concentration reached that distance. The experimental data also indicated that groundwater flow changed during injection, likely due to hydrogen bubble formation, which diverted the nZVI away from the targeted flow path. The leading edge of the iron plume became fully oxidized during transport. However, within the plume, oxidation of nZVI decreased in a fashion consistent with progressive depletion of aquifer "reductant demand". To directly quantify the extent of nZVI transport, a spectrophotometric method was developed, and the results indicated that deployment of unoxidized nZVI for groundwater remediation will likely be difficult.

Sulfur and oxygen isotope tracing in zero valent iron based In situ remediation system for metal contaminants

In the present study, controlled laboratory column experiments were conducted to understand the biogeochemical changes during the microbial sulfate reduction. Sulfur and oxygen isotopes of sulfate were followed during sulfate reduction in zero valent iron incubated flow through columns at a constant temperature of 20±1°C for 90 d. Sulfur isotope signatures show considerable variation during biological sulfate reduction in our columns in comparison to abiotic columns where no changes were observed. The magnitude of the enrichment in δ(34)S values ranged from 9.4‰ to 10.3‰ compared to initial value of 2.3‰, having total fractionation δS between biotic and abiotic columns as much as 6.1‰. Sulfur isotope fractionation was directly proportional to the sulfate reduction rates in the columns. Oxygen isotopes in this experiment seem less sensitive to microbial activities and more likely to be influenced by isotopic exchange with ambient water. A linear relationship is observed between δ(34)S and δ(18)O in biotic conditions and we also highlight a good relationship between δ(34)S and sulfate reduction rate in biotic columns.

Treatment of trichloroethene and hexavalent chromium by granular iron in the presence of dissolved CaCO3

Column experiments and numerical simulations were conducted to evaluate the effects of Cr(VI) and dissolved CaCO(3) on the iron reactivity towards trichloroethene (TCE) and Cr(VI) reduction. Column experiments included measurements of iron corrosion potential and characterization of surface film composition using Raman spectroscopy. Three columns received different combinations of TCE (5 mg L(-1)), Cr(VI) (10 mg L(-1)) and dissolved CaCO(3) (300 mg L(-1)), after short periods of conditioning with Millipore water followed by 10 mg L(-1) TCE in Millipore water, for a total of 8 months. The results showed that co-existence with TCE did not affect Cr(VI) reduction kinetics, however, the presence of Cr(VI) reduced TCE degradation rates significantly. The formation of Fe(III)/Cr(III) products caused progressive passivation of the iron and was consistent with the increase in corrosion potential. The presence of dissolved CaCO(3) resulted in a stable corrosion potential and faster degradation rates of TCE and Cr(VI). Over time, however, the accumulation of secondary carbonate minerals on the iron surface decreased the iron reactivity. Numerical simulation using a reactive transport model reproduced the observations from the column experiments reasonably well. The simulation can be valuable in the design of PRBs or in the development of effective maintenance procedures for PRBs treating groundwater co-contaminated with Cr(VI) and TCE in the presence of dissolved CaCO(3).

Electrochemical depassivation of zero-valent iron for trichloroethene reduction

Permeable reactive barriers (PRBs) composed of zero-valent iron (ZVI) are susceptible to passivation, resulting in substantially decreased rates of chlorinated solvent removal over time. In this study, the application of low electrical direct current (DC) to restore the reductive capacity of passivated ZVI was examined. Electrical current was applied to a laboratory column reactor filled with a mixture of pre-passivated ZVI and sand. Variable voltage settings (0-12 V) were applied through two stainless steel electrodes placed at the ends of the reactor. While only partial restoration of the reductive capacity of the passivated ZVI was observed, higher rates of trichloroethene (TCE) removal were always obtained when current was applied, and the rates of TCE removal were roughly proportional to the voltage level. Although differences were observed between the rates and extent of TCE removal within the column, it is noteworthy that TCE removal was not restricted to that region of the column where the electrons entered (i.e., at the cathode). While complete "depassivation" of ZVI may be difficult to achieve in practice, the application of DC demonstrated observable restoration of reactivity of the passivated ZVI. This study provides evidence that this approach may significantly extend the life of a ZVI PRB.

Modeling gas formation and mineral precipitation in a granular iron column

In granular iron permeable reactive barriers (PRBs), hydrogen gas formation, entrapment and release of gas bubbles, and secondary mineral precipitation have been known to affect the permeability and reactivity. The multicomponent reactive transport model MIN3P was enhanced to couple gas formation and release, secondary mineral precipitation, and the effects of these processes on hydraulic properties and iron reactivity. The enhanced model was applied to a granular iron column, which was studied for the treatment of trichloroethene (TCE) in the presence of dissolved CaCO(3). The simulation reasonably reproduced trends in gas formation, secondary mineral precipitation, permeability changes, and reactivity changes observed over time. The simulation showed that the accumulation of secondary minerals reduced the reactivity of the granular iron over time, which in turn decreased the rate of mineral accumulation, and also resulted in a gradual decrease in gas formation over time. This study provides a quantitative assessment of the evolving nature of geochemistry and permeability, resulting from coupled processes of gas formation and mineral precipitation, which leads to a better understanding of the processes controlling the granular iron reactivity, and represents an improved method for incorporating these factors into the design of granular iron PRBs.

Electrogeneration of H(2) for Pd-catalytic hydrodechlorination of 2,4-dichlorophenol in groundwater

A novel electrolytic groundwater remediation process, which used the H(2) continuously generated at cathode to achieve in situ catalytic hydrodechlorination, was developed for the treatment of 2,4-dichlorophenol (2,4-DCP) in groundwater. Catalytic hydrodechlorination using Pd supported on bamboo charcoal and external H(2) showed that 2,4-DCP was completely dechlorinated to phenol within 30 min at pH ≤ 5.5. In a divided electrolytic system, the catalytic hydrodechlorination of 2,4-DCP in cathodic compartment by H(2) generated at the cathode under 20 and 50 mA reached 100% at 120 and 60 min, respectively. Two column experiments with influent pHs of 5.5 (unconditioned) and 2 were conducted to evaluate the feasibility of this process. The 2,4-DCP removal efficiencies were about 63% and nearly 100% at influent pHs of 5.5 and 2, respectively. Phenol was solely produced by 2,4-DCP hydrodechlorination, and was subsequently degraded at the anode. A low pH could enhance the hydrodechlorination, but was not necessarily required. This study provides the preliminary results of a novel effective electrolytic process for the remediation of groundwater contaminated by chlorinated aromatics.

Rapid dechlorination of chlorophenols in aqueous solution by [Ni|Cu] microcell

The [Ni|Cu] microcell was prepared by mixing the Ni(0) and Cu(0) particles. The composition and crystal form were characterized by X-ray diffraction (XRD) and scanning electron microscope. The results evidenced the zero-valence metals Ni and Cu were exposed on the surface of particles mixture. The [Ni|Cu] microcell was employed to decompose chlorophenols in aqueous solution by reductive dechlorination. The dechlorination rates of chlorophenols by [Ni|Cu] were >10 times faster than those by [Fe|Cu], [Zn|Cu], [Sn|Cu], and [Fe|Ni] mixtures under the same conditions. [Ni|Cu] is different from other zero valent metals (ZVMs) in that it performed the best at neutral pH. The main products of chlorophenol dechlorination were cyclohexanol and cyclohexanone. The reduction kinetics was between pseudo zero-order and first-order, depending on the pH, concentration, and temperature. These results, combined with electrochemical analysis, suggested that Ni(0) acted as a reductant and catalyst in dechlorination reaction. The H* corridor mechanism from Ni(0) to Cu(0) was also proposed based on hydrogen spillover. The inhibition on the release of Ni(2+) by adding natural organic matters and adjusting pH was investigated.

Carbon isotopic fractionation of CFCs during abiotic and biotic degradation

Carbon stable isotope ((13)C) fractionation in chlorofluorocarbon (CFC) compounds arising from abiotic (chemical) degradation using zero-valent iron (ZVI) and biotic (landfill gas attenuation) processes is investigated. Batch tests (at 25 °C) for CFC-113 and CFC-11 using ZVI show quantitative degradation of CFC-113 to HCFC-123a and CFC-1113 following pseudo-first-order kinetics corresponding to a half-life (τ(1/2)) of 20.5 h, and a ZVI surface-area normalized rate constant (k(SA)) of -(9.8 ± 0.5) × 10(-5) L m(-2) h(-1). CFC-11 degraded to trace HCFC-21 and HCFC-31 following pseudo-first-order kinetics corresponding to τ(1/2) = 17.3 h and k(SA) = -(1.2 ± 0.5) × 10(-4) L m(-2) h(-1). Significant kinetic isotope effects of ε(‰) = -5.0 ± 0.3 (CFC-113) and -17.8 ± 4.8 (CFC-11) were observed. Compound-specific carbon isotope analyses also have been used here to characterize source signatures of CFC gases (HCFC-22, CFC-12, HFC-134a, HCFC-142b, CFC-114, CFC-11, CFC-113) for urban (UAA), rural/remote (RAA), and landfill (LAA) ambient air samples, as well as in situ surface flux chamber (FLUX; NO FLUX) and landfill gas (LFG) samples at the Dargan Road site, Northern Ireland. The latter values reflect biotic degradation and isotopic fractionation in LFG production, and local atmospheric impact of landfill emissions through the cover. Isotopic fractionations of Δ(13)C ∼ -13‰ (HCFC-22), Δ(13)C ∼ -35‰ (CFC-12) and Δ(13)C ∼ -15‰ (CFC-11) were observed for LFG in comparison to characteristic solvent source signatures, with the magnitude of the isotopic effect for CFC-11 apparently similar to the kinetic isotope effect for (abiotic) ZVI degradation.

Integrated evaluation of the performance of a more than seven year old permeable reactive barrier at a site contaminated with chlorinated aliphatic hydrocarbons (CAHs)

An important issue of concern for permeable reactive iron barriers is the long-term efficiency of the barriers due to the long operational periods required. Mineral precipitation resulting from the anaerobic corrosion of the iron filings and bacteria present in the barrier may play an important role in the long-term performance. An integrated study was performed on the Vapokon permeable reactive barrier (PRB) in Denmark by groundwater and iron core sample characterization. The detailed field groundwater sampling carried out from more than 75 well screens up and downstream the barrier showed a very efficient removal (>99%) for the most important CAHs (PCE, TCE and 1,1,1-TCA). However, significant formation of cis-DCE within the PRB resulted in an overall insufficient efficiency for cis-DCE removal. The detailed analysis of the upstream groundwater revealed a very heterogeneous spatial distribution of contaminant loading into the PRB, which resulted in that only about a quarter of the barrier system is treating significant loads of CAHs. Laboratory batch experiments using contaminated groundwater from the site and iron material from the core samples revealed that the aged iron material performed equally well as virgin granular iron of the same type based on determined degradation rates despite that parts of the cored iron material were covered by mineral precipitates (especially iron sulfides, carbonate green rust and aragonite). The PCR analysis performed on the iron core samples indicated the presence of a microbial consortium in the barrier. A wide range of species were identified including sulfate and iron reducing bacteria, together with Dehalococcoides and Desulfuromonas species indicating microbial reductive dehalogenation potential. The microbes had a profound effect on the performance of the barrier, as indicated by significant degradation of dichloromethane (which is typically unaffected by zero valent iron) within the barrier.

Predictions of long-term performance of granular iron permeable reactive barriers: field-scale evaluation

Long-term performance is a key consideration for the granular iron permeable reactive barrier (PRB) technology because the economic benefit relies on sustainable operation for substantial periods of time. However, predictions on the long-term performance have been limited mainly because of the lack of reliable modeling tools. This study evaluated the predictive capability of a recently-developed reactive transport model at two field-scale PRBs, both having relatively high concentrations of dissolved carbonate in the native groundwater. The first site, with 8 years of available monitoring data, was a funnel-and-gate installation, with a low groundwater velocity through the gate (about 0.12 m d(-1)). The loss in iron reactivity caused by secondary mineral precipitation was small, maintaining relatively high removal rates for chlorinated organics. The simulated concentrations for most constituents in the groundwater were within the range of the monitoring data. The second site, with monitoring data available for 5 years, was a continuous wall PRB, designed for a groundwater velocity of 0.9 m d(-1). A comparison of measured and simulated aqueous concentrations suggested that the average groundwater velocity through the PRB could be lower than the design value by a factor of two or more. The distribution and amounts of carbonate minerals measured in core samples supported the decreased groundwater velocity used in the simulation. The generally good agreement between the simulated and measured aqueous and solid-phase data suggest that the model could be an effective tool for predicting long-term performance of granular iron PRBs, particularly in groundwater with high concentrations of carbonate.