Fiona M. Doyle

A Silica-Supported Iron Oxide Catalyst Capable of Activating Hydrogen Peroxide at Neutral pH Values

Iron oxides catalyze the conversion of hydrogen peroxide (H(2)O(2)) into oxidants capable of transforming recalcitrant contaminants. Unfortunately, the process is relatively inefficient at circumneutral pH values because of competing reactions that decompose H(2)O(2) without producing oxidants. Silica- and alumina-containing iron oxides prepared by sol-gel processing of aqueous solutions containing Fe(ClO(4))(3), AlCl(3), and tetraethyl orthosilicate efficiently catalyzed the decomposition of H(2)O(2) into oxidants capable of transforming phenol at circumneutral pH values. Relative to hematite, goethite, and amorphous FeOOH, the silica-iron oxide catalyst exhibited a stoichiometric efficiency, defined as the number of moles of phenol transformed per mole of H(2)O(2) consumed, which was 10-40 times higher than that of the iron oxides. The silica-alumina-iron oxide catalyst had a stoichiometric efficiency that was 50-80 times higher than that of the iron oxides. The significant enhancement in oxidant production is attributable to the interaction of Fe with Al and Si in the mixed oxides, which alters the surface redox processes, favoring the production of strong oxidants during H(2)O(2) decomposition.

The Role of Glycine in the Chemical Mechanical Planarization of Copper

Chemical mechanical polishing (CMPI is an essential process in the production of integrated circuits containing copper interconnects. The role of glycine in reactive slurries representative of those that might be used in copper CMP was studied with the aim of improving our understanding of the mechanisms at play. The electrochemical processes involved in the oxidative dissolution of copper were investigated by potentiodynamic polarization studies. To delineate the specific effect of glycine on the dissolution and polishing of copper, electrochemical tests were conducted in aqueous media with and without glycine. Experimentally measured polarization curves agreed well with potential-pH diagrams for the copper-water and copper-water-glycine systems. In situ electrochemical polarization experiments and open-circuit potential measurements were then conducted during polishing using slurries containing either no glycine or 10 -2 M glycine. These tests indicated that glycine curtails the formation of copper oxide films during polishing and hence may improve the CMP process by controlling the oxide thickness.

In Situ Chemical Oxidation of Contaminated Groundwater by Persulfate: Decomposition by Fe(III)- and Mn(IV)-Containing Oxides and Aquifer Materials

Persulfate (S2O82–) is being used increasingly for in situ chemical oxidation (ISCO) of organic contaminants in groundwater, despite an incomplete understanding of the mechanism through which it is converted into reactive species. In particular, the decomposition of persulfate by naturally occurring mineral surfaces has not been studied in detail. To gain insight into the reaction rates and mechanism of persulfate decomposition in the subsurface, and to identify possible approaches for improving its efficacy, the decomposition of persulfate was investigated in the presence of pure metal oxides, clays, and representative aquifer solids collected from field sites in the presence and absence of benzene. Under conditions typical of groundwater, Fe(III)- and Mn(IV)-oxides catalytically converted persulfate into sulfate radical (SO4•–) and hydroxyl radical (HO•) over time scales of several weeks at rates that were 2–20 times faster than those observed in metal-free systems. Amorphous ferrihydrite was the most reactive iron mineral with respect to persulfate decomposition, with reaction rates proportional to solid mass and surface area. As a result of radical chain reactions, the rate of persulfate decomposition increased by as much as 100 times when benzene concentrations exceeded 0.1 mM. Due to its relatively slow rate of decomposition in the subsurface, it can be advantageous to inject persulfate into groundwater, allowing it to migrate to zones of low hydraulic conductivity where clays, metal oxides, and contaminants will accelerate its conversion into reactive oxidants.

Electrochemistry of Copper in Aqueous Glycine Solutions

Potential-pH equilibria and potentiodynamic polarization studies were used to examine the electrochemical behavior of copper in aqueous glycine solutions. Potential-pH diagrams for the copper-water-glycine system were derived at different total copper {Cu T } and glycine {L T } activities. The diagrams show that glycine significantly extended the solubility range of copper. Polarization experiments were conducted in deaerated and aerated aqueous solutions of 10 -2 M glycine with 10 -3 M cupric nitrate and 10 -1 M glycine with 10 -4 M cupric nitrate at pH values between 9 and 12. The results of these experiments are discussed in terms of the relevant potential-pH diagrams. Good correlations were observed.

Kinetics and efficiency of H2O2 activation by iron-containing minerals and aquifer materials

To gain insight into factors that control H(2)O(2) persistence and ·OH yield in H(2)O(2)-based in situ chemical oxidation systems, the decomposition of H(2)O(2) and transformation of phenol were investigated in the presence of iron-containing minerals and aquifer materials. Under conditions expected during remediation of soil and groundwater, the stoichiometric efficiency, defined as the amount of phenol transformed per mole of H(2)O(2) decomposed, varied from 0.005 to 0.28%. Among the iron-containing minerals, iron oxides were 2-10 times less efficient in transforming phenol than iron-containing clays and synthetic iron-containing catalysts. In both iron-containing mineral and aquifer materials systems, the stoichiometric efficiency was inversely correlated with the rate of H(2)O(2) decomposition. In aquifer materials systems, the stoichiometric efficiency was also inversely correlated with the Mn content, consistent with the fact that the decomposition of H(2)O(2) on manganese oxides does not produce ·OH. Removal of iron and manganese oxide coatings from the surface of aquifer materials by extraction with citrate-bicarbonate-dithionite slowed the rate of H(2)O(2) decomposition on aquifer materials and increased the stoichiometric efficiency. In addition, the presence of 2 mM of dissolved SiO(2) slowed the rate of H(2)O(2) decomposition on aquifer materials by over 80% without affecting the stoichiometric efficiency.

Effect of Hydrogen Peroxide on Oxidation of Copper in CMP Slurries Containing Glycine

This study compares the oxidative dissolution, passivation, and polishing behavior of copper in chemical mechanical polishing (CMP) the presence of glycine when the potential was controlled electrochemically, and when thepotential was controlled using hydrogen peroxide, the most common oxidizer used in commercial slurries. The dissolution behavior was investigated with the help of potential/pH diagrams for the copper-water-glycine and copper-water systems, potentiodynamic polarization measurements and weight loss experiments. In situ polarization measurements and polishing experiments were conducted to study the electrochemical behavior of copper during polishing. With electrochemical control, passivation was only observed at high pH. When peroxide was used as an oxidant, passivation was observed at neutral pH values, when the H 2 O 2 concentration exceeded relatively low threshold concentrations, suggesting the formation of a protective film. This phenomenon could have important utility for simultaneously achieving high selectivity and good planarization during copper CMP.

EFFECT OF pH ON THE ADSORPTION OF SELECTED HEAVY METAL IONS FROM CONCENTRATED CHLORIDE SOLUTIONS BY THE CHELATING RESIN DOWEX M-4195

The effect of pH on the adsorption of nickel (II), copper (II), cobalt (II), lead (II), manganese (II), and iron (III) from concentrated chloride solutions onto Dowex M-4195 is reported for the first time. This chelating resin has an unusual ability to adsorb many of these metals, most notably copper, even at very low pH. The selectivity for many of these metals makes this resin particularly attractive for detoxifying concentrated acidic chloride solutions generated during the processing of many manganese ores and minerals. The uptake behavior at these high-chloride concentrations differs somewhat from that at the lower chloride concentrations previously reported in the literature. The behavior is discussed in terms of the chemistry of the bis(2-pyridylmethyl) amine (bispicolylamine) functional group.

Ion flotation—its potential for hydrometallurgical operations

Abstract Ion flotation is a separation technology for recovering and removing metal ions from dilute aqueous solutions. In this process, an ionic collector is utilized to transport non-surface active colligend ions of the opposite charge from a bulk solution to the solution–vapor interface. If a sufficiently large solution–vapor interfacial area can be provided by sparging gas through the solution, the colligend can be concentrated and removed along with the collector in a foam phase. Ion flotation has a long history for laboratory-scale metal separations, and has been tested at the pilot scale for recovering gold from leach solutions. Although ion flotation has not yet been adopted in commercial hydrometallurgical operations, the process has many attractive features that are promising for treating dilute solutions and effluents. Here, the fundamental characteristics of ion flotation are reviewed in the context of their implications for commercial applications. These characteristics include the kinetics of metal ion removal, the ability to separate target ions selectively from mixed solutions, the ability to manipulate selectivity, the ability to recover metal values from the foam product and a comparison of the performance of ion flotation with widely adopted separation methods such as solvent extraction, ion exchange and precipitation. It is evident from consideration of the kinetics of ion flotation that the equipment traditionally used for ion flotation is far from optimized, and that significant improvements in throughput could be achieved with redesigned equipment.

Oxidation of Benzene by Persulfate in the Presence of Fe(III)- and Mn(IV)-Containing Oxides: Stoichiometric Efficiency and Transformation Products

Sulfate radical (SO4(•-)) is a strong, short-lived oxidant that is produced when persulfate (S2O8(2-)) reacts with transition metal oxides during in situ chemical oxidation (ISCO) of contaminated groundwater. Although engineers are aware of the ability of transition metal oxides to activate persulfate, the operation of ISCO remediation systems is hampered by an inadequate understanding of the factors that control SO4(•-) production and the overall efficiency of the process. To address these shortcomings, we assessed the stoichiometric efficiency and products of transition metal-catalyzed persulfate oxidation of benzene with pure iron- and manganese-containing minerals, clays, and aquifer solids. For most metal-containing solids, the stoichiometric efficiency, as determined by the loss of benzene relative to the loss of persulfate, approached the theoretical maximum. Rates of production of SO4(•-) or hydroxyl radical (HO(•)) generated from radical chain reactions were affected by the concentration of benzene, with rates of S2O8(2-) decomposition increasing as the benzene concentration increased. Under conditions selected to minimize the loss of initial transformation products through reaction with radicals, the production of phenol only accounted for 30%-60% of the benzene lost in the presence of O2. The remaining products included a ring-cleavage product that appeared to contain an α,β-unsaturated aldehyde functional group. In the absence of O2, the concentration of the ring-cleavage product increased relative to phenol. The formation of the ring-cleavage product warrants further studies of its toxicity and persistence in the subsurface.

Electrochemistry of Copper in Aqueous Ethylenediamine Solutions

Potential-pH equilibria and potentiodynamic polarization studies were used to examine the electrochemical behavior of copper in aqueous ethylenediamine solutions. Potential-pH diagrams for the copper-water-ethylenediamine system were derived at different total copper {Cu T } and ethylenediamine {En T } activities. The diagrams show that the solubility range of copper is significantly extended in the presence of ethylenediamine. Polarization experiments were conducted in deaerated and aerated aqueous solutions of 10 -2 M ethylenediamine with 10 -3 M cupric sulfate, and 10 -4 M ethylenediamine with 10 -5 M cupric sulfate over a wide pH range. The results of these experiments are discussed in terms of relevant potential-pH diagrams. Good correlations were observed.