Complexes bisulfide complex

According to these previous studies, the most dominant dissolved states of Au and Ag in ore fluids are considered to be bisulfide and chloride complexes, depending on the chemistry of ore fluid (salinity, pH, redox state, etc.). However, very few experimental studies of Au solubility due to chloride complex and Ag solubility due to bisulfide complexes under hydrothermal conditions of interest here have been conducted. Thus, it is difficult to evaluate the effects of these important species on the Ag/Au of native gold and electrum. Other Au and Ag complexes with tellurium, selenium, bismuth, antimony, and arsenic may be stable in ore fluids but are not taken into account here due to the lack of thermochemical data. 

Table II. Bisulfide Complexation Constants (K in Equation 7) for Selected Oceanic Trace Metals, all as Base 10 Logarithms. Parenthetical Entries have been Estimated Through linear Free Energy Comparisons Others are Taken from References (191 or (311. for 25°C, and an Ionic Strength of 1.0 in Most Cases...

The metals Cu, Ag most commonly occur as monovalent and Zn, Cd, Hg as divalent cations in natural hydrothermal systems, thus having the configuration and forming highly stable chloride and bisulfide complexes (Barnes, 1979). 

Considerable attention has been paid to complexing of metals by reduced sulfur species. Some of the complexes of zinc and lead which have been considered include Zn(HS)f, Zn(HS)4, Pb(HS)3, Pb(HS)°, and PbS(H2S)° (Barnes, 1979 Kharaka et al., 1987). Giordano and Barnes (1981) concluded, on the basis of experimental studies, that ore-forming solutions at temperatures less than 200 °C with total dissolved sulfur contents of less than 1 m (ca. 3.2 X 10" mg L ) cannot transport significant quantities of lead as bisulfide complexes. Extensive metal complexing by the bisulfide complexes requires much higher pH values than those found in saline formation waters (Kharaka et al., 2000). 

The metals are transported as chloride complexes at high temperatures and, in some cases, as bisulfide complexes (in particular Au) at lower temperatures. 

Bourcier, W.L. and Barnes, H.L., 1987. Ore solution chemistry Vll. Stabilities of chloride and bisulfide complexes of zinc to 350°C. Economic Geology, 82 1839-1863. 

Biulfidecmplx ZfiCd adHg. In addition to chloride complexes, bisulfide... 

HS ) complexes are also invoked to explain metal solubilities in hydrothermal ore-forming solutions (Hayashi et al. 1990). Unfortunately, there is much less data for gas-and aqueous-phase bisulfide complexes of metals. Table 6 gives some comparison between calculated and experimental bond lengths and vibrational frequencies of zinc... 

The calculated thermodynamic properties (Table 7) show that, in the presence of chloride ligands, bisulfide complexation of Zn, Cd and Hg is unimportant even up to 300°C. The thermodynamics of reactions between simple gas-phase molecules can be calculated easily from first-principles and may provide an approach to understanding metal speciation in supercritical fluids where solvation is minimal. 

Bourcier WL, Barnes HL (1987) Ore solution chemistiy. VII. Stabihties of chloride and bisulfide complexes of zinc to 350°C. Econ Geol 82 1839-1863 Bowers TS, Helgeson HC (1983). Calculation of the thermodynamic and geochemical consequences of nonideal mixing in the system H20-C02-NaCl on phase relations in geologic systems Equation of state for H20-C02-NaCl fluids at high temperatures and pressures. Geochim Cosmochim Acta 47 1247-1275... 

Helz GR, Vaughan DJ, Gamer CD (1992) Multinuclearity of aqueous copper and zinc bisulfide complexes andEXAFS investigation. Geochim Cosmochim Acta 57 15-25 Hehre WJ, Radom L, Schleyer PVR, Pople JA (1986) Ab initio Molecular Orbital Theory. Wiley Interscience, New York... 

Fig. 1.32 Cu and Fe concentration in hydrothennal solution in eqnilibrinm with chalcopyrite-bomite-pyrite assemblage (Crerar and Barnes 1976). Solid line-, solnbility dne to chloro complexes, dotted line solubility due to bisulfide complexes, dotted region foj-pH range of porphyry copper deposits...

Assuming that Au is transported dominantly as bisulfide or chloride complexes, the following reaction can be used to determine which species are dominant under near neutral conditions. 

Equilibrium calculations suggested that Hg complexation varies greatly among redox and pH levels typical of the regions of lakes sampled during this study. In an oxic lake, pore water, and groundwater, Hg complexation with organic matter most likely dominates. Under anoxic conditions in the hypolimnion and pore waters, Hg most likely forms soluble bisulfide and polysulfide complexes. 

For these reasons, it is useful to evaluate the effects of forming or not forming aqueous sulfide complexing agents and sulfide precipitates on radionuclide solubilities and aqueous speciation. Therefore, calculations were performed for key radionuclides for two cases (1) reduction of sulfate to sulfide and bisulfide was allowed to occur as Eh was lowered and (2) sulfate reduction as a function of Eh was prohibited so that sulfate was the only sulfur complexing agent available at all Eh values. 

Table III. The Ratio of Concentrations for Selected Complexes to those of the Aquo Ion M24, (17.31.321. and Free Bisulfide Ion Concentrations (Molar) Required to Sustain Like Stability, all as Base 10 Logarithms.

Figure 3. Rough ratios of equilibrium concentrations for inorganic (31.321 and organic (20-221 copper species to those of the aquo ion Cu2+, with sulfide complexes plotted as functions of free bisulfide ion level and set at the lower limit in Table II.

Polysulfides and Sulfide. The polysulfide complexes of Ag and Cu have been added to the model in an attempt to reduce the apparent oversaturation with Ag2S(s) calculated for San Francisco Bay waters (12). Calculation of the activity of polysulfide iotis requires the assumptions 1) the quantity of Sg (free sul-fur) is not a limitation on its reaction with bisulfide (HS ) to form polysulfides and 2) polysulfides are in equilibrium with bisulfide. 

In this context, pioneering investigations of the metal behavior under anoxic conditions in the Gotland Basin have been described in detail by Kremling (1983). Models have been developed by comparing field measurements with equilibrium calculations to predict the maximum solubility of metals in anoxic waters. Furthermore, it has been demonstrated that the solubility of the individual trace metals is controlled by the formation of bisulfide or poly sulfide complexes. 

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