Gold speciation

When HA11CI4 is dissolved in water, the chloroauric anion hydrolyses to form anionic hydroxychloro-gold(III) complexes [Au(OH)xCl4 x] . The 

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Table 4.1 Gold speciation in HA11CI4 solutions at room temperature as a function of pH.

The explanations tentatively proposed in the literature [20, 25, 41, 61] for the variations of gold uptake with pH as shown in Figure 17.1, depend on the gold speciation that is considered. Some groups consider the presence of hydroxo-chloro anions as identified by Raman and XAFS [27-29], while others take into account the speciation deduced from thermodynamic data, that is the presence of neutral complexes, such as AuCfi at pH 2-5 [35] (see Section 17.1). 

Vlassopoulous D, Wood SA, Mucci A (1990) Gold speciation in natural waters. II. The importance of organic complexing - experiments with some simple model ligands. Geochim Cosmochim Acta 54 1575-1586... 

Ferrous ferric oxide showed a different concentration-time profile with change of pH in comparison to the two other materials - it was linear, while the decline in gold concentration was rather steep for FeOOH and AI2O3 within the first minutes of the experiment. This behavior was observed under all pH conditions. Furthermore, in the case of Fe304 it was seen that an increase in pH resulted in a gradual increase in speed of gold removal from solution, e.g., while it took 90 min to remove all gold present at pH=2 and 3 it took about 60 min at pH=4 and had finished after 5 min at pH=5 and 6, which can also be explained with the mentioned speciation effect in connection with the associated different complex stabilities. 

The amount of gold removed from solution was a function of pH, with near neutral pH resulting in more complete gold removal from solution in comparison to lower pH values. This behavior is caused by speciation changes associated with the successive replacement of a CL ligand with a OH- ion upon pH increase. 

Murphy, P. J., LaGrange, M. S. (1998). Raman spectroscopy of gold chloro-hydroxy speciation in fluids at ambient temperature and pressure a re-evaluation of the effects of pH and chloride concentration Geochimica et Cosmochimica Acta, 62(21-22), 3515-3526. doi 10.1016/S0016-7037(98)00246-4... 

Paktunc, D. 2008. Speciation of arsenic in an anaerobic treatment system at a Pb-Zn smelter site, gold roaster products, Cu smelter stack dust and impacted soil. Proc. 9th Int. Cong. Applied Mineralogy (ICAM2008) 8-10 Sep 2008, Brisbane, Queensland, The Australasian Institute of Mining and Metallurgy, 8/2008, 343-348. 

Walker, S.W. Jamieson, H.E., Lanzirotti, A., Andrade, C.F. 2005. Determining arsenic speciation in iron oxides derived from a gold-roasting operation Application of... 

An interesting application of speciation is in the study of changes in the distribution of certain metals after administration of drugs. Falchuk used gel filtration combined with flame atomic absorption spectrometry to study effects of the administration of ACTH on the zinc distribution in serum. Kamel et ai. developed methods to follow the distribution of gold. 

Savage, K.S., Tingle, T.N., O Day, P.A. et al. (2000) Arsenic speciation in pyrite and secondary weathering phases, Mother Lode gold district, Tuolumne County, California. Applied Geochemistry, 15(8), 1219-44. 

A study by Rasemann et al. demonstrated to what extent mercury concentrations depend on the method of handling soil samples between sampling and chemical analysis for samples from a nonuniformly contaminated site [152], Sample pretreatment contributed substantially to the variance in results and was of the same order as the contribution from sample inhomogeneity. Welz et al. [153] and Baxter [154] have conducted speciation studies on mercury in soils. Lexa and Stulik [155] employed a gold film electrode modified by a film of tri-n-octylphosphinc oxide in a PVC matrix to determine mercury in soils. Concentrations of mercury as low as 0.02 ppm were determined. 

Salattn, P., B. Planer-Friedrich, and C.M.G. van den Berg. 2007. Inorganic arsenic speciation in water and seawater by anodic stripping voltammetry with a gold microelectrode. Anal. Chim. Acta 585 312-322. 

Nuzzio D. B. (2001) Sulfur speciation monitored in situ with solid state gold amalgam voltammetric microelectrodes polysulfides as a special case in sediments, microbial mats and hydrothermal vent waters. J. Environ. Monitor. 3, 61-66. 

Experience with bioleaching of arsenic-rich gold ores has shown that the ratio of pyrite to FeAsS is an important factor controlling the speciation of the arsenic released (Nyashanu et al., 1999). In the absence of pyrite, —72% of the arsenic released was As(III), whereas in the presence of pyrite and Fe(III), 99% of the arsenic was As(V). It appears that pyrite catalyzed the oxidation of As(III) by Fe(III), since Fe(III) alone did not oxidize the arsenic (Nyashanu et al., 1999). 

Nyashanu R., Monhemius A., and Buchanan D. (1999) The effect of ore mineralogy on the speciation of arsenic in bacterial oxidation of refractory arsenical gold ores. In International Biohydrometallurgy of Symposium IBS 99, Madrid (eds. R. Amils and A. Ballester). Elsevier, Oxford, pp. 431-441. 

Borch R. S., Hastings L. L., Tingle T. N., and Verosub K. L. (1994) Speciation and in vitro gastrointestinal extractabihty of arsenic in two California gold mine tailings. EOS 75, 190. 

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