Electrodeposition of metal cations from the wet ionic liquid [EMIM][TFSI]

We investigated the deposition of silver, copper, and lead from the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][TFSI] under potentiostatic conditions in the presence of water. This was part of a larger project involving the extraction of metal ions from mining waste into an ionic liquid followed by electrodeposition so that the ionic liquid could be recycled. All three elements were deposited in metallic form by electrolysis in the ionic liquid, and the process was enhanced rather than hindered by the presence of water. The deposited metals did not adhere strongly to the cathode of the electrochemical cell, especially when Ebonex® was used as the cathode. The deposition of silver showed little temperature dependence, and at temperatures close to ambient, the ionic liquid was not adversely affected. The deposits of copper and lead gradually re-dissolved after electrodeposition, suggesting that chemical re-oxidation of these metals by air is more facile in the ionic liquid than in water. Copper showed strong evidence of formation of a Cu+ species upon reduction of Cu2+ (not seen in water); lead (Pb2+) showed evidence of a time-dependent complexation with the anions of the ionic liquid.

Electrochemical reduction of aqueous nitrate ion at tin cathodes

The remediation of nitrate-contaminated water using electrochemical reduction at a tin cathode has previously been shown to give almost quantitative denitrification (removal of dissolved nitrogen species) under highly cathodic polarization. A particular focus of this project was to identify specific role(s) for tin in the reaction in the context of the previous literature. The current efficiency for denitrification was enhanced in alkaline solution, and the reaction was accelerated by the presence of small concentrations of Sn(II) salts, which are in a dynamic exchange between cathodic deposition and corrosion of the cathode. Literature precedent indicates that Sn(II) salts promote the “dimerization” pathway of NO to hyponitrite in preference to reduction to ammonia. Hyponitrite is a known intermediate in the electrochemical reduction of nitrate, but its spontaneous decomposition gives predominantly N2O, which does not reduce further to N2. We have shown that hyponitrite is reduced electrochemically in competition with its thermal decomposition, which provides a pathway to N2 via the spontaneous dehydration of HO−NH−NH−OH. The possible role of surface-bound Sn−H species in the reduction mechanism is discussed, but further work is needed to substantiate this proposal.

Electrochemical activation of chemotherapeutic prodrugs that mimic P450-catalyzed oxidation: proof-of-concept for a focal approach to chemical cancer treatment

The electrochemical oxidation of substrates cyclophosphamide and acetaminophen at Ti/RuO2 or (preferably) graphite anodes parallels P450-catalyzed oxidation in that both mechanisms involve transfer of an oxygen atom to the substrate. The aim of this work was to use this parallel to provide proof-of-concept for a localized approach to tumor chemotherapy using electrochemical oxidation to activate chemotherapeutic prodrugs ex situ. Cyclophosphamide and acetaminophen were electroactivated in batch and flow electrolytic cells and the products were tested against EMT6 mouse mammary adenocarcinoma cells. Cell viability was determined using a tetrazolium dye assay that monitored NADPH concentrations; microscopic examination showed consistent morphological differences between viable and nonviable cells. No cytotoxicity was observed in nonelectrolyzed control samples. The electrolyzed prodrugs demonstrated cytotoxicity up to the IC99 level at 5 mmol L−1 initial prodrug concentrations but not at 1 mmol L−1. The long-term objective of the work is to develop an ex situ electrochemical system for activating prodrugs to cause lethal toxicity to the cells of solid tumors, many of which lack sufficient P450s to bioactivate the toxicant themselves.

The use of Ebonex electrodes for the electrochemical removal of nitrate ion from water

This work addresses the remediation of nitrate-contaminated water using electrodes made of Ebonex (a titanium oxide ceramic with a wide range of potential stability). The objective was the complete denitrification of solutions containing nitrate ion. Denitrification was achieved in about 50% yield with unreactive supporting electrolytes when Ebonex was used as both cathode and anode, the remaining product being ammonia. Ammonia could be re-oxidized at the Ebonex anode, but this was much less efficient than the reduction step. A more efficient electrolytic denitrification was possible for solutions containing chloride; this is oxidized anodically to hypochlorite, which then oxidizes ammonia chemically to N2. The overall rate of denitrification was highest at moderate concentrations of chloride ion, because hypochlorite also re-oxidizes reduction intermediates such as nitrite back to nitrate. Complete denitrification was achieved at all stages of the reaction using Ebonex cathode and a dimensionally stable anode based on Ti/IrO2 or Ti/RuO2, because the DSA oxidizes chloride ion more efficiently than Ebonex. Cathode fouling by water sources that are high in hardness cations can be prevented by using one DSA and a pair of Ebonex electrodes that undergo periodic polarity reversal.

Electrochemical oxidation of sulfide ion in synthetic sour brines using periodic polarity reversal at Ebonex® electrodes

The Magneli phase Ti4O7 (Ebonex®) was used as both anode and cathode in the electrochemical oxidation of sulfide ion in alkaline solution in the absence and presence of chloride and naphthenate ions. Ebonex anodes gradually lost their activity through the formation of an over-oxidized surface layer, but their activity could be maintained by periodic polarity reversal. In the context of the current paradigm for the mechanistic behaviour of oxide-based anodes, Ti4O7 has properties that combine those of “inactive” anodes (formation of hydroxyl radicals) and “active” anodes (formation of a higher oxide at the surface), with the exception that the higher oxide in the case of Ti4O7 is TiO2, which is incapable of substrate oxidation. Sulfate is the major oxidation product, especially in the presence of chloride, an ubiquitous component of sour brines, via mediated electro-oxidation to hypochlorite. Unlike at boron-doped diamond anodes, at which sulfide is oxidized with near-quantitative current efficiency, significant parasitic oxidation of water to O2 occurs at Ebonex, and oxidation of sulfide requires ~16 F mol–1, corresponding to a 50% current efficiency.

Competition between electrochemical advanced oxidation and electrochemical hypochlorination of acetaminophen at boron-doped diamond and ruthenium dioxide based anodes

This work was undertaken to distinguish four pathways for the electrochemical oxidation of acetaminophen as a model organic substrate: (i) direct electron transfer from the substrate to the anode, (ii) reaction of the substrate with HO• at boron-doped diamond anodes, (iii) non-radical (two-electron) oxidation of the substrate at Ti/RuO2 anodes, and (iv) electrochemical hypochlorination if Cl– is present. Pathway (i) was isolated as a slow reaction when boron-doped diamond (BDD) was used as the anode in the range of water stability, whereas in the corresponding reaction with Ti/RuO2 only pathway (iii) could be detected. Pathway (ii) predominated for BDD in the potential range of water oxidation, and was the only mechanism leading to mineralization of the substrate. Comparison between chemical hypochlorination and electrochemical oxidation at Ti/RuO2 in the presence of chloride ion indicated that the latter process principally involves mediated hypochlorination. Oxidation at boron-doped diamond anodes in the presence of chloride was the most complex mechanistically, with competition between hypochlorination and the electrochemical “advanced oxidation process”; this led to the formation of chlorinated byproducts. The observation of mineralization under these conditions demonstrated cross-over between reaction pathways (ii) and (iv), even though hypochlorination appeared to be the initial pathway for loss of acetaminophen. The presence of chloride ion did not significantly retard mineralization of acetaminophen in the initial stages of oxidation, but significantly increased the energy requirement for complete mineralization. The results are discussed in the context of the use of electrochemical oxidation in waste management.

Electrolytic debromination of PBDEs in DE-83 technical decabromodiphenyl ether

Electrochemical debromination of the commercial decabromodiphenyl ether flame retardant DE-83 in partly aqueous tetrahydrofuran (THF) solution gave lower brominated congeners by sequential loss of bromine atoms. Hydrodebromination was most facile for the most heavily brominated congeners. It involves initial electron transfer and proton transfer from water, rather than hydrogen atom abstraction from THF, as shown by experiments with deuterated water. The product distribution from electrolysis involves preferential loss of bromine meta- and para- to the ether linkage, comparable with the products of metabolism of BDE-209 in various organisms. Significantly, the environmentally relevant congeners BDE-47, BDE-99, and BDE-154 were not major products of debromination of BDE-209 by the electron transfer mechanism.

Mass spectrometric and toxicological assays of Athabasca oil sands naphthenic acids

This work concerns the analysis of model naphthenic acids and authentic naphthenic acids from the tailings ponds of the Athabasca tar sands. A first objective was to compare atmospheric pressure chemical ionization mass spectrometry (APCI-MS) with the previously studied electrospray mass spectrometry (ESI-MS) in this analysis. APCI-MS had a wider range of quantitation than ESI-MS, but its detection limit was poorer and model compounds showed greater variation in calibration sensitivity. A second objective was fractionation of naphthenic acids from tailings pond water and analysis by the Microtox toxicity assay. Fractionation on the basis of solubility gave fractions that did not differ significantly either in their congener distribution by ESI-MS or in their response to the Microtox assay. When partial separation was achieved by anion exchange chromatography, fractions with a higher proportion of multi-ring structures exhibited lower toxic potency. This finding is consistent with field observations that indicate that the toxic potency of tailings ponds water declines as the samples age-multi-ring structures are more highly branched and therefore more resistant to microbial degradation.