Copper speciation in sulfidic solutions at low sulfur activity: Further evidence for cluster complexes?

Abstract

The solubility of two a s 0 -buffering assemblages in the Cu-S system have been studied: chalcocite-djurleite (Cc-Dj) and anilite-covellite (An-Cv). Ion activity products, [Cu +]HS −] 1 2 [H +] − 1 2 (25°C, I = 0) at equilibrium, derived from solubility measurements in penicillamine solutions, are 10 −17.01 ± 0.05 (Cc-Dj) and 10 −17.14 ± 0.10 (An-Cv), from which ΔG° f = −82.11 kJ/mol for Cc and −74.77 kJ/mol for An. In the An-Cv assemblage, a Cu 2 S = 0.55 (2 σ = 0.2) vs. 1.00 in the Cc-containing assemblage. The difference in a Cu 2 S between the two assemblages is used in a novel way to estimate stoichiometry of Cu-HS complexes. The solubility of both assemblages (0.7-0.01 M NaHS, pH 7–12.5, 25°C) can be fit with a model incorporating the same two chemical species, one containing an odd number of Cu atoms (Cu(HS) 2− 3, CU 3S 4H 2− 3, or a higher multimer) and the other containing an even number of Cu atoms (Cu 2S(HS) 2 2−, Cu 4S 4H 2 2−, etc.). The trimer-tetramer model fits the combined data for the two assemblages distinctly better than the monomer-dimer model, but this result is very sensitive to uncertainty in a Cu 2 S. Along with EXAFS results, the weight of the evidence favors small cluster complexes (2–5 Cu atoms), but is inconclusive at the present level of resolution. Multimers can be rationalized because condensation of metal-centered monomers to clusters provides a means for soft acid/base elements to maintain favored coordination geometries at low ligand to metal ratios. Based on the fitting methods developed here, previous covellite solubility data from this laboratory are reinterpreted in terms of Cu 2S 2(HS) 3 3−, Cu 2S 3)(S 4) 2−, and Cu(S 9)S 10) 3−; the last of these could also be represented by the trimer, Cu 3(S 7) 3 3−, which is homologous with a known complex. With the measured equilibrium constants, the speciation of Cu in the sulfidic zone of the Black Sea is calculated. Covellite is the stable Cu-S mineral, but the sulfidic water column is vastly supersaturated with respect to it. Most of the sulfidic water column is modestly (2.5–5.5 times) supersaturated with respect to Cc, hinting that this mineral metastably controls ΣCu. The slight supersaturation suggests that Cc occurs as 10–100 nm particles.