Bisulfide metal complexes

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). 

From the reactions (1.86) and (1.87) and chemical equilibrium involving Cu-and Fe-chloro-complexes, iso-concentration contours for Cu- and Fe-minerals are drawn on logfo -pH diagram (Fig. 1.32 Crerar and Barnes 1976). Above calcida-tions were carried out, assuming that Cu- and Fe-chloro-complexes are dominant Cu- and Fe-species (Crerar and Barnes 1976). Bisulfide-, thio- and carbonate-complexes are also important in determining solubility of sulfides as well as chloro-complexes (Table 1.5, Brimhall and Crerar (1987)). Above argument clearly indicates that dominant base metal complexes in hydrothermal solution depend on concentration of ligand, pH, foj, temperature and so on. 

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). 

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. 

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

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. 

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