The solubility of copper in sulfidic waters: Sulfide and polysulfide complexes in equilibrium with covellite

Abstract

The solubility of covellite has been measured in 0.5–2.0 M NaCl at 25°C. Solutions containing various concentrations of total sulfide and zerovalent sulfur have been studied over the pH range 6–11. The data are interpreted in terms of the following complexes, for which stability constants are presented: CuS(HS) 2 2−, CuS(HS) 3 3−, Cu(S 5) 2 3−, Cu(S 4)(S 5) 3−, and CuS(S 5) 2−. Three of these five species have not been reported before and are probably of importance in anoxic natural waters. The three complexes with the general formula, CuS( X) n , do not display optical absorption spectra characteristic of Cu(II) even though the oxidation state of copper, by convention, is divalent. When Cu 2+ and S 2− combine to form the CuS moiety in these complexes, internal electron transfer from S to Cu occurs. Because polysulfide complexes are extremely stable, the solubility of copper in sulfidic waters is strongly dependent on the activity of solid sulfur, a s 0 . For natural waters, a s 0 can be calculated from measurements of pH, total zerovalent and total divalent sulfur. At a s 0 > 0.1, polysulfide complexes (especially CuS(S 5) 2−) are probably the predominant copper species in most sulfidic marine waters. Calculated total copper concentrations in equilibrium with covellite agree satisfactorily with published measurements for some Delaware salt marsh pore waters. This agreement is achieved by postulating that Cu-Fe-S phases, such as chalcopyrite, fail to nucleate at low temperatures in nature.