Abstract
The solubility of both synthetic and natural sphalerite have been measured at 25°, pH 2.2 to 9.1, and ΣS(-II) 0.1 to 0.0004 M. Inversion of wurtzite to sphalerite during equilibration precluded measuring wurtzite solubilities. To hinder colloid formation, we used well-crystallized ZnS preleached with EDTA. Run durations were several months. Clean-room analytical procedures were used. Measured solubilities are much lower than those of Gübeli and Ste-Marie (1967), whose solutions probably contained colloids. Colloidal ZnS sols are shown not to equilibrate with sphalerite even over several months. The new data are consistent with, and complementary to those of Hayashi et al. (1990). The solubility of Zn 2+ in equilibrium with sphalerite is given by a Zn 2+a 2 HS− a H2 S = 10 18.47 +− 0.01 . Aqueous Zn-polysulfides appear to be unimportant under the conditions studied. In terms of anhydrous, mononuclear complexes, Zn solubility can be described by a series of complexes whose formation is represented by: ZnS( sp) + (2 y− z−2) HS − + ( z− y+1) H 2 S aq ZnS y H z −(2 y− z−2) , K yz . Three complexes are essential to model the results adequately: Zn( HS) −2 4, log K 44 = −3.83 ± 0.17; ZnS( HS) −, log K 21 = −4.64 ± 0.08; and ZnS( HS) −2 2, log K 32 = −5.33 ± 0.07. Additional complexes improve the fit marginally. Others have suggested that ZnS(HS) −2 2 should be represented by its hydrated formula, Zn(OH)(HS) −2 3. The observed stability of this complex is much greater than predicted for a mixed ligand complex derived from Zn(HS) −2 4 and Zn(OH) −2 4. Based on recent calculations, less than 3 4 of the discrepancy can be accounted for by molecular orbital stabilization. A tetranuclear formula suggested by known Zn-thiolate structures and supported by EXAFS evidence of multinuclearity is a possible alternative. The solubility of Zn in anoxic marine waters is determined largely by ZnS(HS) −, which dominates in mildly alkaline solutions at low σS(-II). According to the new data, anoxic zones in Framvaren Fjord and the Black Sea are undersaturated with respect to sphalerite, except immediately below the oxic/anoxic interface. This suggests that scavenging, rather than precipitation, controls Zn.
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