Gretel K. Parker

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 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 Raman Spectroscopic Investigation of Pyrite Oxidation and Flotation Reagent Interaction

The oxidation of pyrite and the interaction of flotation collectors, xanthate and hydroxamate, with the pyrite surface were investigated via normal- and surface-enhanced Raman scattering (SERS) techniques. SERS was facilitated via vapour deposition of a discontinuous gold island film or through the application of silver colloids to the pyrite surface. The oxidation of pyrite produced a Raman-inactive solid phase, in addition to octasulfur. 442 nm irradiation could induce the formation of polymeric sulfur from the pyrite oxidation product. Dixanthogen was the major species present on pyrite conditioned with xanthate solutions under oxidizing conditions, although a bound xanthate species was also evident, particularly at reducing potentials. The interaction of hydroxamate with pyrite under oxidizing conditions produced a ferric hydroxamate compound

Interaction of Hydroxamates with Malachite

The interaction of alkyl hydroxamate in 0.1 M KOH with a malachite or azurite mineral surface has been investigated using Raman spectroscopy. Cupric hydroxamate was detected on the mineral surface following exposure in aerated solutions. Gold film coating or colloid deposition was not successful in the enhancement of spectra from the exposed mineral surfaces. Surface enhanced Raman scattering was obtained from roughened gold electrodes or gold colloid particles exposed to aerated hydroxamates. These spectra exhibit potential dependent formation of a species with a carbon nitrogen triple bond, indicative of oxidative decomposition of the hydroxamate functional group.

A SERS Study of the Interaction of n-Octanohydroxamate with a Copper Electrode

SERS spectroelectrochemical investigations have demonstrated that the flotation collector n-octanohydroxamate specifically adsorbs on copper surfaces in the region of thermodynamic stability of the metal and, at low ionic strength, this gives rise to a contact angle of ~ 70஠The contact angle fell to zero when Cu2O was formed on the copper surface and increased again to ~ 70when the potential was increased further. Voltammetry established that, in 0.01 M KOH, the presence of hydroxamate in concentrations of 10-3 M and above retards the rate of formation of Cu2O and indicates that a stability zone of copper n-octanohydroxamate replaces that of hydrated CuO. It is concluded that the development of a copper n-octanohydroxamate layer is responsible for the copper oxide surface being rendered hydrophobic at high potentials. SERS investigations of the copper/hydroxamate system is considered not to be an appropriate model for advancing understanding of the interaction of this collector with oxidized mineral surfaces.

A Spectroelectrochemical Investigation of the Interaction of Gold with Cyano-Containing Ligands

Surface-enhanced Raman spectroelectrochemical (SERS) investigations of cyanamide, cyanoacetamide and malononitrile interaction with a roughened gold electrode were undertaken. The reagents were readily adsorbed on the surface and large shifts in the ?(C=N) band were observed on adsorption. The band underwent Stark tuning as the potential was scanned and, at high potentials, bands consistent with an oxidized gold complex were observed. It did not appear that the cyano functional group dissociated from the ligands and, together with the large shift in the ?(C=N) frequency on adsorption, this is indicative of p-type coordination. The 1st and subsequent voltammetric cycles in the malononitrile system showed quite different characteristics, indicative of bonding of only one of the two nitriles of the molecule during the 1st oxidation sweep, followed by bonding of both the nitriles in subsequent sweeps.

Gold Enhanced Observation of Surface Products in Chalcopyrite Dissolution

Gold decoration of chalcopyrite surfaces has been shown to stimulate surface enhancement of Raman scattering and this enables sulfur oxidation products to be investigated when they are undetectable by normal Raman spectroscopy. Solution cementation, sputtering and evaporative deposition of gold each facilitated SERS. The last of these techniques was selected for studies of chalcopyrite leaching since it has the least possibility of affecting the mineral surface. Similar oxidation products and electrochemical behavior were observed on potential cycling of chalcopyrite electrodes in H 2 SO 4 and HCl. SERS spectroscopy indicated that S-S bonding was present in the major product, indicative of sulfur in a somewhat amorphous structure. It is proposed that this species results from aging of a metal-deficient remnant sulfide lattice formed when metal atoms are dissolved. Ex situ investigations of the oxidation of chalcopyrite substantiated the conclusion that the species detected after gold deposition were oxidation products of chalcopyrite.