Marek Odziemkowski

Mechanism of oxide film formation on iron in simulating groundwater solutions : Raman spectroscopic studies

Abstract In the use of iron for reductive dehalogenation of chlorinated solvents in ground water, the formation of surface films may cause long-term problems by reducing the activity of the metal surfaces or by causing the clogging of pores. Normal (NRS) and enhanced (SERS) Raman spectroscopy was used to identify the surface film(s) formed during contact of iron particles with solvent-free simulated ground water solutions in column experiments. It was found that anaerobic corrosion of iron leads to the initial formation of ferrous hydroxide at the beginning of the reaction. Independent of the ground water composition, however, the final corrosion product is magnetite. The spontaneous (no current applied) formation of magnetite takes place by a dissolution/precipitation mechanism with the separation of anodic and cathodic sites across the surface film. The cathodic reaction, which takes place at the porous film/solution interface, requires the film to be electron conducting.

An in situ study of the role of surface films on granular iron in the permeable iron wall technology

Permeable walls of granular iron are a new technology developed for the treatment of groundwater contaminated with dissolved chlorinated solvents. Degradation ofthe chlorinated solvents involves a charge transfer process in which they are reductively dechlorinated, and the iron is oxidized. The iron used in the walls is an impure commercial material that is covered with a passive layer of Fe2O3, formed as a result of a high-temperature oxidation process used in the production of iron. Understanding the behaviour of this layer upon contact with solution is important, because Fe2O3 inhibits mechanisms involved in contaminant reduction, including electron transfer and catalytic hydrogenation. Using a glass column specially designed to allow for in situ Raman spectroscopic and open circuit potential measurements, the passive layer of Fe2O3 was observed to be largely removed from the commercial product, Connelly iron, upon contact with Millipore water and with a solution of Millipore water containing 1.5 mg/l trichloroethylene (TCE). It has been previously shown that Fe2O3 is removed from iron surfaces upon contact with solution by an autoreduction reaction; however, prior to this work, the reaction has not been shown to occur on the impure commercial iron products used in permeable granular iron walls. The rate of removal was sufficiently rapid such that the initial presence of Fe2O3 at the iron surface would have no consequence with respect to the performance of an in situ wall. Subsequent to the removal of Fe2O3 layer, magnetite and green rust formed at the iron surface as a result of corrosion in both the Millipore water and the solution containing TCE. The formation of these two species, rather than higher valency iron oxides and oxyhydroxides, is significant for the technology. The former can interfere with contaminant degradation because they inhibit electron transfer and catalytic hydrogenation. Magnetite and green rust, in contrast, will not inhibit the mechanisms involved in contaminant reduction, and hence their formation is beneficial to the long-term performance of the iron material.

Stable solvates in solution of lithium bis(trifluoromethylsulfone)imide in glymes and other aprotic solvents: Phase diagrams, crystallography and Raman spectroscopy

Lithium bis(trifluoromethylsulfone)imide (LiTFSI), a promising electrolyte for high energy lithium batteries, forms several stable solvates having low melting points in aprotic solvents. In a previous study (D. Brouillette, G. Perron and J. E. Desnoyers, J. Solution Chem., 1998, 27, 151), it was suggested, based on thermodynamic studies, that such stable solvates may persist in solution and influence their properties. To verify this hypothesis, phase diagrams and Raman spectra have been measured for solutions of LiTFSI in acetonitrile, propylene carbonate and glymes (n(ethyleneglycol) dimethyl ether or Gn), which have the chemical structure CH3–O–(CH2–CH2–O)n–CH3 for n = 1 to 4 and 10. The relative intensities of the LiTFSI and solvent Raman bands are proportional to the concentration for systems without solvates. The systems for which stable solvates were identified in the phase diagram show important changes in the relative intensities for both the LiTFSI and the solvent Raman bands at concentrations corresponding to particular stoichiometries and support the conclusion that stable solvates are present in the solutions. The structure of the crystalline G1:LiTFSI solvate was determined by X-ray crystallography. Structures for (G2)2:LiTFSI and (G1)3:LiTFSI solvates are proposed.

An in situ study of the effect of nitrate on the reduction of trichloroethylene by granular iron

The effect of nitrate on the reduction of TCE by commercial granular iron was investigated in column experiments designed to allow for the in situ monitoring of the iron surface film with Raman spectroscopy. Three column experiments were conducted; one with an influent solution of 100 mg/l nitrate+1.5 mg/l TCE, and two control columns, one saturated directly with 100 mg/l nitrate solution, the other pre-treated with Millipore water prior to the introduction of a 100 mg/l nitrate solution. In the presence of nitrate, TCE adsorbed onto the iron, but there was little TCE reduction to end-products ethene and ethane. The iron used (Connelly, GPM, Chicago) is a product typical of those used in permeable granular iron walls. The material is covered by an air-formed high-temperature oxidation film, consisting of an inner layer of Fe(3)O(4), and an outer, passive layer of Fe(2)O(3). In the control column pre-treated with Millipore water, the passive Fe(2)O(3) layer was removed upon contact with the water in a manner consistent with an autoreduction reaction. In the TCE+nitrate column and the direct nitrate saturation column, nitrate interfered with the removal of the passive layer and maintained conditions such that high valency protective corrosion species, including Fe(2)O(3) and FeOOH, were stable at the iron surface. The lack of TCE reduction is explained by the presence of these species, as they inhibit both mechanisms proposed for TCE reduction by iron, including catalytic hydrogenation, and direct electron transfer.

The Electrochemical Response of Preoxidized Copper in Aqueous Sulfide Solutions

The conversion of a Cu 2 O film on copper to Cu 2 S in aqueous sulfide solutions has been followed using a combination of electrochemical techniques and in situ Raman spectroscopy. Oxide films were electrochemically grown in alkaline solutions and their composition and morphology determined using Raman spectroscopy and scanning electron microscopy. Although corrosion potential measurements indicate that the aqueous sulfide solution rapidly penetrates the porous Cu 2 O layer, the oxide-to-sulfide reaction appears to proceed chemically at the oxide/solution interface rather than via the galvanic coupling of Cu oxidation to Cu 2 S and Cu 2 O reduction to Cu. In situ Raman spectroscopy confirms that the sulfide formed is Cu 2 S, and cathodic stripping voltammetry shows that the reaction is initially rapid and then proceeds at a constant rate until the conversion is complete. Comparison of the amounts of oxide initially present and sulfide eventually formed demonstrates that the conversion is 100% efficient. These studies are part of a larger project to determine the important corrosion processes on copper high-level nuclear waste containers exposed to anoxic aqueous sulfide containing groundwaters.

Electrochemically Activated Copper Electrodes Surface Characterization, Electrochemical Behavior, and Properties for the Electroreduction of Nitrate

A polycrystalline copper electrode was activated by creating a nanostructured and highly electrocatalytic surface through an appropriate electrochemical treatment in 1 M NaOH. It was demonstrated that a thick layer (∼2 μm) of Cu(OH) 2 nanoneedles can be formed on copper substrate after 3000 cycles (scan rate 10 V s -1 ) between -1650 and 1000 mV vs Hg/HgO or by anodization for 15 min at -100 mV. Energy dispersive X-ray analysis, X-ray diffraction, X-ray photoelectron spectroscopy, and in situ Raman spectroscopy analyses revealed that the conversion of orthorhombic Cu(OH) 2 to face-centered-cubic Cu is completed after 20 cycles between -450 and -1650 mV at 20 mV s -1 . However, copper obtained from the reduction of the thickest Cu(OH) 2 nanoneedle films formed by a repetitive fast cycling or by anodization at -100 mV appeared as nanowires, whereas the electrode anodized at 0 or 700 mV recovered a quasi-smooth original copper surface. The presence of high-energy sites on these Cu nanostructures was highlighted by cyclic voltammetry in the pseudo-capacitive potential region, where premonolayer oxidation was observed with an unusually high magnitude at unusually low potentials. As a result, a remarkable improvement of the electrocatalytic activity of the activated Cu electrodes for the nitrate electroreduction was observed.

Electrochemical and Raman spectroscopic studies of the influence of chlorinated solvents on the corrosion behaviour of iron in borate buffer and in simulated groundwater

The remediation, by contact with granular iron, of groundwater contaminated with chlorinated halocarbons necessitates a flow of electrons at the iron/solution interface. To refine our understanding of the mechanism and kinetics of the charge transfer process, electrochemical and spectroscopic measurements were performed on iron electrodes in borate buffer and in simulated groundwater solutions of calcium carbonate and potassium bromide, before and after exposure to carbon tetrachloride. The results of these measurements highlighted the combined influence of the organic contaminant and inorganic ions on the corrosion behaviour of iron as well as on the nature of the films formed in their presence.

Influence of chlorinated solvents on polarization and corrosion behaviour of iron in borate buffer

Abstract A new remediation technology for groundwater contaminated with chlorinated solvents is through contact with granular iron. To refine our understanding of the reaction mechanism, electrochemical and spectroscopic measurements were performed on iron electrodes in deaerated borate buffer containing an amount of a degradable (carbon tetrachloride) or non-degradable (dichloromethane) compound. The results of polarization measurements indicated that carbon tetrachloride acts as an oxidizer towards iron while dichloromethane is nonreactive. Magnetite and hydrated magnetite, identified by Raman spectroscopy, are the final products of the surface redox reactions. Based on electrochemical and spectral evidence, a new conceptual model for the reductive reactions is proposed.

An In Situ Raman-Electrochemical Investigation of Carbon Steel Corrosion in Na2CO3 ∕ NaHCO3, Na2SO4, and NaCl Solutions

The anoxic corrosion of carbon steel liners inside failed copper nuclear waste containers is dependent on the composition of the groundwater to which it is exposed. The influence of carbonate/bicarbonate, sulfate, and chloride in solutions simulating concentrated groundwaters at pH 8.9 on the composition of the corrosion products formed on A 516 Gr 70 carbon steel has been investigated at room temperature. In situ Raman spectroscopic identification of the corrosion products formed during polarization at constant potential was conducted in a spectroelectrochemical cell. Siderite was established as the main product in sodium carbonate/bicarbonate solutions, and the observance of iron carbide indicates extensive steel dissolution in this environment. In mixed carbonate, sulfate, chloride solutions, compact deposits of carbonate-containing, and to a lesser degree, sulfate-containing green rusts were formed, along with small amounts of magnetite. In chloride-dominated solutions very thin, compact films, undetectable by Raman spectroscopy, were formed. Ex situ Raman analysis suggests this film may be magnetite.

In Situ Identification of Carbonate-Containing Green Rust on Iron Electrodes in Solutions Simulating Groundwater

Open-circuit potential-time and spectral measurements were performed on ironelectrodes in aqueous solutions containing calcium carbonate to simulateground-water, to which an amount of carbon tetrachloride was added. In the case of apreoxidized iron electrode, the injection of the chlorinated aliphatic hydrocarbonresulted in the formation of carbonate-containing green rust. In situ identification,performed by Raman spectroscopy, was based on bands at ca. 433, 509, and1053 cm−1, which were assigned, respectively, to the Fe2+—OH stretching modeof green rust, the Fe3+—OH stretching mode of green rust, and the stretchingvibrations of carbonate ions in the interlayer regions of the green rust. Theassignment of the Fe2+—OH and Fe3+−OH stretching mode bands was confirmedby parallel experiments using D2O solution. The results of the open-circuitpotential-time experiments are in good agreement with literature thermodynamic datafor iron in carbonate-containing aqueous solutions.