Kym Marjorie Watling

Sorption of arsenic(V) and arsenic(III) to schwertmannite

This study describes the sorption of As(V) and As(III) to schwertmannite as a function of pH and arsenic loading. In general, sorption of As(V) was greatest at low pH, whereas high pH favored the sorption of As(III). The actual pH of equivalent As(V) and As(III) sorption was strongly loading dependent, decreasing from pH approximately 8.0 at loadings <120 mmol(As) mol(Fe)(-1) to pH approximately 4.6 at a loading of 380 mmol(As) mol(Fe)(-1). Sorption isotherms for As(V) were characterized by strong partitioning to the schwertmannite solid-phase at low loadings and sorption capacities of 225-330 mmol(As(V)) mol(Fe)(-1) at high loadings. In contrast, the As(III) isotherms revealed a weak affinity for sorption of As(III) versus As(V) at low loadings yet a greater affinity for As(III) sorption compared with As(V) at high loadings (when pH > 4.6). Sorption of As(V) and As(III) caused significant release of SO(4)(2-) from within the schwertmannite solid-phase, without major degradation of the schwertmannite structure (as evident by X-ray diffraction and Raman spectroscopy). This can be interpreted as arsenic sorption via incorporation into the schwertmannite structure, rather than merely surface complexation at the mineral-water interface. The results of this study have important implications for arsenic mobility in the presence of schwertmannite, such as in areas affected by acid-mine drainage and acid-sulfate soils. In particular, arsenic speciation, arsenic loading, and pH should be considered when predicting and managing arsenic mobility in schwertmannite-rich systems.

A SERS spectroelectrochemical investigation of the interaction of 2-mercaptobenzothiazole with copper, silver and gold surfaces

The interaction of the sulfide mineral flotation collector, 2-mercaptobenzothiazole, with silver, copper and gold surfaces has been investigated by surface enhanced Raman scattering (SERS) spectroscopy. 2-mercaptobenzothiazole, the copper, silver and gold compounds of this species, and the dithiolate, 2,2′-dithiobis(benzothiazole) were characterised by 13C NMR and Raman spectroscopy to provide a basis for identifying surface species. SERS investigations showed that, at pH 4.6 where the solution species is in the protonated form, and at 9.2, where it is present as the ion, adsorption on each metal occurs over a wide potential range. Attachment of the organic compound occurs through bonding between the exocyclic sulfur atom and metal atoms in the surface. X-ray photoelectron spectroscopy confirmed that the adsorbed layer was of monolayer thickness. Adsorption of the protonated 2-mercaptobenzothiazole occurs on copper at pH 4.6 at potentials below that at which charge transfer adsorption commences.

Arsenic effects and behavior in association with the Fe(II)-catalyzed transformation of schwertmannite.

In acid-mine drainage and acid-sulfate soil environments, the cycling of Fe and As are often linked to the formation and fate of schwertmannite (Fe(8)O(8)(OH)(8-2x)(SO(4))(x)). When schwertmannite-rich material is subjected to near-neutral Fe(III)-reducing conditions (e.g., in reflooded acid-sulfate soils or mining-lake sediments), the resulting Fe(II) can catalyze transformation of schwertmannite to goethite. This work examines the effects of arsenic(V) and arsenic(III) on the Fe(II)-catalyzed transformation of schwertmannite and investigates the associated consequences of this mineral transformation for arsenic mobilization. A series of 9-day anoxic transformation experiments were conducted with synthetic schwertmannite and various additions of Fe(II), As(III), and As(V). X-ray diffraction (XRD) and Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy demonstrated that, in the absence of Fe(II), schwertmannite persisted as the dominant mineral phase. Under arsenic-free conditions, 10 mM Fe(II) catalyzed rapid and complete transformation of schwertmannite to goethite. However, the magnitude of Fe(II)-catalyzed transformation decreased to 72% in the presence of 1 mM As(III) and to only 6% in the presence of 1 mM As(V). This partial Fe(II)-catalyzed transformation of As(III)-sorbed schwertmannite did not cause considerable As(III) desorption. In contrast, the formation of goethite via partial transformation of As(III)- and As(V)-sorbed schwertmannite significantly decreased arsenic mobilization under Fe(III)-reducing conditions. This implies that the Fe(II)-catalyzed transformation of schwertmannite to goethite may help to stabilize solid-phase arsenic and retard its subsequent release to groundwater.

A Spectroelectrochemical Study of Surface Species Formed in the Gold/Thiosulfate System

Surface-enhanced Raman scattering (SERS) spectroscopy has been applied to identify species formed during gold leaching in thiosulfate media. Surface coverages of copper and sulfur for various thiosulfate dissolution systems were determined electrochemically and by X-ray photoelectron spectroscopy. A gold sulfide monolayer was shown to be formed slowly in the potential region where leaching occurs. At higher potentials, more sulfur is adsorbed but, rather than creating a multilayer, sulfur-sulfur linkages are formed with the sulfur atoms no longer being covalently bonded to gold. SERS spectra show the presence of polythionates on the gold surface in addition to gold sulfide and S°. At longer exposure times, thiosulfate is oxidized to elemental sulfur and sulfate, both products being identified by normal Raman spectroscopy from the leach solution. In typical thiosulfate/ ammonia/copper(II) leach solutions, copper is present on the surface of gold in addition to sulfur. In these situations, ammonia is coadsorbed.

A SERS spectroelectrochemical investigation of the interaction of isopropyl, isobutyl and isoamyl xanthates with silver

The interaction of isopropyl, isobutyl and isoamyl xanthates with silver surfaces has been investigated by voltammetry and by in situ surface enhanced Raman scattering (SERS) spectroscopy. Voltammograms for each homologue are characterised by a prewave occurring at potentials below that at which the silver xanthate compound develops. The prewave was found to shift to more negative potentials by 0.028 V for each additional carbon atom in the alkyl chain between ethyl and isoamyl xanthates. The potassium, sodium and silver xanthates were characterised by Raman and 13C NMR spectroscopies to provide a basis for identifying surface species. The SERS investigations showed that xanthate chemisorbs in the prewave potential region predominantly with the alkyl groups in the all-trans conformation. The xanthates were found to reduce at low potentials.

Surface enhanced raman scattering spectroscopic studies of the adsorption of flotation collectors

Abstract Surface enhanced Raman scattering (SERS) spectroscopy at surfaces under electrochemical control has been applied to elucidate the adsorption of thiol collectors. Voltammetry has shown that charge transfer chemisorption of ethyl, isopropyl, isobutyl and isoamyl xanthates occurs on silver surfaces at potentials below the reversible value for the formation of the silver xanthate. SERS spectroscopy has identified the species formed at underpotentials to be xanthate bonded to silver atoms through the sulfur atoms. Application of this technique has also shown that the adsorption of O-isopropyl-N-ethylthionocarlpamate on copper involves a charge transfer process and rest potential measurements indicate that this adsorption occurs at underpotentials. SERS spectra also establish that 2-mercaptobenzothiazole undergoes charge transfer chemisorption on copper, silver and gold over a wide potential range by bonding through the exocyclic sulfur atom.

A spectroelectrochemical investigation of the interaction of diisobutyldithiophosphinate with copper, silver and gold surfaces: II. Electrochemistry and Raman spectroscopy

The interaction of the sulphide mineral flotation collector, diisobutyldithiophosphinate (DIBDTPI), with silver, copper, and gold surfaces has been investigated by voltammetry and Raman scattering spectroscopy. Voltammetry showed that DIBDTPI is oxidised to its disulphide on gold whereas the metal-collector compounds are formed on silver and copper. Surface enhanced Raman spectra (SERS) were observed for silver in the presence of DIBDTPI over a wide potential range and demonstrated that charge transfer chemisorption of the collector takes place. At high potentials, Raman spectra from AgDIBDTPI were also detected. No SERS spectra were evident with copper electrodes, but the formation of CuDIBDTPI was confirmed from Raman spectra. Similar results were observed with diethyldithiophosphate on copper, and this indicates that copper metal does not behave in the same manner as copper sulphide minerals with regard to interaction with thiophosphate collectors. At high potentials on gold, Raman spectra were observed from the disulphide and AuDIBDTPI. SERS spectra were also found on gold under laser illumination that were characteristic of the development of layer of sulfur and this is explained in terms of photolysis of DIBDTPI radicals formed as intermediates in the oxidation of DIBDTPI to its disulphide.

SERS Investigation of Gold Dissolution in Chloride and Cyanide Media

Raman vibrational bands from surface and solution species formed during potential cycles at gold electrodes in chloride and cyanide solutions were examined using surface-enhanced Raman scattering (SERS) spectroscopy. In acidic or neutral solution, chloride ion is adsorbed prior to gold dissolution and νv A u C i - a d s Stark-shifts to higher wavenumbers with increasing electrode potential, but the SERS intensity of this band is significantly diminished when leaching commences. In cyanide solutions, the presence of specifically adsorbed cyanide ions at the electrode surface at the lower potential limit was indicated by Stark-shifted Raman bands characteristic of carbon-nitrogen stretching. Bond formation between cyanide and gold, evidenced by vibrational modes due to gold-carbon stretching and gold-carbon-nitrogen bending, commenced near the potential at which voltammetric currents consistent with gold dissolution were observed. At high potentials, cyanide was displaced from the surface and a Raman band characteristic of gold-oxygen stretching appeared. Oxidation of cyanide to cyanate was also indicated in this region by a Raman band characteristic of the carbon-nitrogen stretch of the cyanate ion. The voltammetric current was depressed when gold oxide was formed on the surface. Reductive removal of the oxide layer on the reverse sweep was followed by facile gold dissolution and accompanied by a rapid coverage of cyanide species.

A SERS Investigation of the Interaction of Sulfur with Gold

The interaction of S -II solution species with gold has been studied using surface-enhanced Raman spectroelectrochemical techniques to provide a basis for understanding sulfur layers formed during the oxidation of sulfide minerals and the leaching of gold in thiosulfate. Underpotential deposition in acid and basic media is characterized by a ν (Au-S) band. This is explained by a monoatomic S layer bonded to gold atoms in the electrode surface as in the (3 x 3) R30° lattice reported from STM studies. The surface coverage in this potential region is limited to 0.35 ML. At potentials above the S -II /S 0 reversible value, the surface coverage increases and ν (S-S) bands are observed indicating the formation of S n species. As coverage increases further, δ (S-S-S) bands develop that are characteristic of the stable 5 8 structure. Bands were found to be absent that would have indicated the adsorption of SH species as has been reported in the literature.

A spectroelectrochemical investigation of the interaction of diisobutyldithiophosphinate with copper, silver and gold surfaces: I. Raman and NMR spectra of diisobutyldithiophosphinate compounds.

NMR and Raman spectroscopies have been applied to characterise the diisobutyldithiophosphinate (DIBDTPI) compounds of sodium, copper, silver and gold, diisobutyldithiophosphinic acid (DIBDTPIH), and the oxidation product bis(diisobutylthiophosphoryl)disulfide (DIBDTPI)2 in order to provide a basis for investigating the interaction of the flotation collector, NaDIBDTPI, with the coinage metal surfaces. Spectra for the various species in the solid state and in solution obtained by both techniques are interpreted in terms of the symmetry and conformation of the molecule. Raman spectroscopy provides an effective means of distinguishing between coinage metal DIBDTPI compounds and (DIBDTPI)2.