Environ. Sci. Technol. 1996, 30, 671-679
Determination of Metal (Bi)Sulfide Stability Constants of Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+ by Voltammetric Methods G E O R G E W . L U T H E R , I I I , * ,† DAVID T. RICKARD,‡ STEPHEN THEBERGE,† AND ANTHONY OLROYD‡ College of Marine Studies, University of Delaware, Lewes, Delaware, 19958, and Department of Earth Sciences, University of Wales, Cardiff, CF1 3YE, Wales, U.K.
The stoichiometry as well as the conditional and thermodynamic stability constants for the (bi)sulfide complexes of the +2 cations of Mn, Fe, Ni, Co, Cu, and Zn have been determined by voltammetric methods in seawater and chloride solutions of varying ionic strength. Acid-base titrations allowed for the determination of the proton stoichiometry of the complexes. Mn, Fe, Ni, and Co form bisulfide, HS-, complexes of stoichiometry MSH+, M2(SH)3+, and M3(SH)5+, which are labile under diffusion control conditions, in seawater at pH values > 7. These complexes dissociate below pH ) 7, releasing H2S from solution. Evidence for sulfide-rich complexes of form M(SH)2 was not found. Cu and Zn form sulfide, S2-, complexes of stoichiometry MS and M2S32-, which are inert (nonlabile) under diffusion control conditions, in seawater at pH values > 7. There is no evidence for metal-rich sulfide complexes of stoichiometry [MxS]2x-2 from the titration data. The M2S32- complexes are tetrameric structures (M4S6) with tetrahedral metal coordination based on known thiolate complex and mineral geometries. The Zn complexes dissociate below pH ) 6.7, releasing H2S from solution. However, the Cu complex does not fully dissociate below pH ) 2 because of Cu(II) reduction and production of polysulfide, which do not appear to be quantitative. At seawater pH, both Zn and Cu sulfide complexes can be deposited on a mercury drop, indicating that these metal complexes are likely responsible for the presence of nanomolar levels of sulfide in oxic seawater.
Introduction Metal (bi)sulfide complexes are important soluble species which are of great environmental interest. Two principal * Author to whom correspondence should be addressed. † University of Delaware. ‡ University of Wales, Cardiff.
0013-936X/96/0930-0671$12.00/0
1996 American Chemical Society
reasons for interest in these species are (i) metal concentrations in the environment are frequently higher than predicted on the basis of metal sulfide solubility products (1); and (ii) complexes are intermediates in the formation of sulfide minerals and are products during mineral dissolution (2, 3). In precipitation reactions, stoichiometric and structural variations in the type of complex(es) formed may lead to different solid products for a given metal ion. Interest in metal sulfide complexes is also due in part to the determination of nanomolar levels of sulfide in oxic seawater (4, 5), to the identification of a FeSH+ species in oxic salt marsh creek waters and anoxic pore waters (6), and to the existence of a Fe2S2 species in anoxic lake waters (7) and in marine porewaters (8). In oxic seawater, Elliott et al. (9) suggested that OCS hydrolysis could release sulfide into oxygenated natural waters. The exact mechanism for sulfide production in oxic seawater remains unknown; although, in culture, phytoplankton can produce H2S particularly when trace metal content is high (10). The sulfide produced is stable and related to metal sulfide complexes which have high stability constants and are kinetically inert (5). These results are in contrast to the oxidation of (bi)sulfide in lab studies (11) and in field samples from anoxic basins (12-16) which occur at reasonable rates. The determination of metal (bi)sulfide complexes in solution has received renewed attention in the last several years. The experimental methods recently used to determine the stoichiometry, structure, and stability constants of sulfide complexes include (i) titrations of low level concentrations of metal with sulfide (17) and sulfide with metal (6) while monitoring the sulfide by voltammetric methods, and (ii) the dissolution of sulfide minerals with excess amounts of dissolved sulfide (usually millimolar) with subsequent experimental determination of total sulfide and metal followed by calculations to predict the resulting species. Linear free energy relationship techniques have also been employed to estimate the stability constants of metal sulfide complexes (18, 19) when experimental data was sparse. Recently in laboratory studies, Luther and Ferdelman (6) and Zhang and Millero (17) used square wave voltammetry at mercury electrodes to monitor the sulfide peak in order to determine stability constants for metal sulfide complexes in seawater. These studies were performed at micromolar sulfide concentrations. Luther and Ferdelman (6) did not deposit sulfide species onto the Hg drop and used both the current and potential data to determine FeSH+ (log β ) 5.5) and [Fe2(SH)]3+ (log β ) 11.1) from the titration of Fe(II) with sulfide at pH 8.1 in seawater. They also titrated the solutions with acid and base and found that the complex is a bisulfide (HS-) ion complex over the pH range 7-10. Below 7, a complex of Fe(H2S) forms which dissociates and releases H2S. Zhang and Millero (17) did deposit sulfide species onto the Hg drop and used only the current data of the sulfide peak when titrating the divalent metal ions of Mn, Fe, Co, Ni, Cu, Zn, Cd, Hg, and Cu+ with bisulfide in seawater. They found log β ) 5.3 for FeSH+ but could not detect any metal-rich sulfide species. Because they did not perform acid/base titrations, they assumed that all their constants were for the bisulfide species (HS-)
VOL. 30, NO. 2, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
671
only. Thus, their constants are conditional stability constants without experimental verification of the metal, sulfur, and proton stoichiometry of the complexes. Metal sulfide minerals and pure metals which are exposed to sulfidic solutions can be dissolved to form sulfide complexes. In hydrothermal systems these complexes can be transported over long distances (20). Several groups have used the mineral dissolution approach to determine the solubility of minerals and to infer the form of metal sulfide complex released based on speciation calculations from total metal and sulfide data and available thermodynamic data (20-24). Pertinent for comparison to the work we report, Helz and co-workers (25-28) have studied the minerals CdS, ZnS, CuS, and Cu2S. They concluded that Zn and Cd do form mononuclear bisulfide complexes. However, Cu(II) is more complex because of its tendency to reduce in sulfidic solutions. Thus, Cu forms a variety of bisulfide, sulfide, and polysulfide complexes which may be mixed ligand type as well as mononuclear and multinuclear complexes. To better ascertain the multinuclearity of Zn and Cu bisulfide complexes, Helz et al. (29) studied the dissolution of ZnS and CuS minerals with molar quantities of NaHS using extended X-ray absorption fine structure spectroscopy (EXAFS). These results further demonstrated the multinuclear character of Cu and Zn sulfide complexes and the mononuclear character of Zn complexes. Helz et al. (29) could not perform these experiments at lower environmental bisulfide concentrations since the signal to noise level did not provide useful EXAFS spectra. Thus, they concluded that the species may have been fine particles which are
