Environ. Sci. Technol. 2002, 36, 394-402
Aqueous Copper Sulfide Clusters as Intermediates during Copper Sulfide Formation G E O R G E W . L U T H E R , I I I , * ,† S T E P H E N M . T H E B E R G E , †,‡ TIM F. ROZAN,† DAVID RICKARD,§ C. C. ROWLANDS,| AND ANTHONY OLDROYD§ College of Marine Studies, University of Delaware, Lewes, Delaware 19958, Department of Earth Sciences, Cardiff University, Cardiff, CF1 3YE Wales, U.K., and Department of Chemistry, Cardiff University, Cardiff CF1 3YB, Wales, U.K.
Using a combination of experimental techniques, we show that Cu(II) reduction by sulfide to Cu(I) occurs in solution prior to precipitation. EPR and 63Cu NMR data show that reduction to Cu(I) occurs during the reaction of equimolar amounts of Cu(II) with sulfide. 63Cu solution NMR data show that Cu(I) is soluble when bound to sulfide and is in a site of high symmetry. EPR data confirm that Cu(I) forms in solution and that the mineral covellite, CuS, contains only Cu(I). Mass spectrometry data from covellite as well as laboratory prepared solid and solution CuS materials indicate that Cu3S3 six-membered rings form in solution. These trinuclear Cu rings are the basic building blocks for aqueous CuS molecular clusters, which lead to CuS precipitation. In controlled titration experiments where sulfide is slowly added to Cu(II), Cu3S3 rings and tetranuclear Cu molecular clusters (Cu4S5, and Cu4S6) form; the rings are composed primarily of Cu(II). During cluster formation from Cu3S3 condensation, some Cu(II) is released back into solution, indicating that Cu(II) reduction does not occur until after Cu-S bond and higher order cluster formation. Analysis of the frontier molecular orbitals for Cu(II) and sulfide indicate that an outer-sphere electron transfer is symmetry forbidden. These results are consistent with the formation of CuS bonds prior to electron transfer, which occurs via an inner-sphere process.
Introduction The organic and inorganic complexation of copper has received significant attention in environmental chemistry. Copper’s speciation is complicated by the reduction of Cu(II) to Cu(I) in both oxic and anoxic environments (1). In sulfidic environments, Cu(II) reacts quickly with sulfide, but it is not clear when reduction to Cu(I) occurs (2). This can complicate speciation studies at oxic-anoxic interfaces. At millimolar concentrations, reduction occurs quickly with the formation of amorphous Cu-S precipitates. The nature of these * Corresponding author e-mail:
[email protected]; phone: (302)645-4208; fax: (302)645-4007. † University of Delaware. ‡ Present address: Department of Chemistry N7, Merrimack College, 315 Turnpike Street, North Andover, MA 01845. § Department of Earth Sciences, Cardiff University. | Department of Chemistry, Cardiff University. 394
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 3, 2002
precipitates depends on the pH of the solutions. Both brown sols (3) and blue precipitates (4, 5) have been found on reaction. Depending on reaction conditions, these amorphous precipitates will form “blaubleibender covellite” (i.e., mixtures of spionkopite, Cu1.4S, and yarrowite, Cu9S8) and stoichiometric covellite, CuS (6). Pattrick et al. (2) concluded from X-ray absorption studies that the amorphous precipitates show the presence of S22- in the primitive structure. Thus, redox reactions and structural reordering occur before the development of the primitive amorphous precipitate. Because these phases are close to a 1:1 Cu:S ratio, they concluded that the precursor must have a similar stoichiometry. The classical crystallographic structure of covellite was redefined by Evans and Konnert (7). It can be described as linked six-membered rings of alternating Cu-S atoms, with the Cu in trigonal coordination, connected vertically by tetrameric CuS groups, where the Cu is tetrahedrally coordinated (Figure 1; 8). The six Cu atoms of the unit cell are thus divided into four trigonally coordinated Cu atoms, which have been classically assigned to Cu(I) and two tetrahedrally coordinated Cu atoms assigned to Cu(II). The S-S bond distance in the trigonal groups is 2.09 Å, suggesting S22- groups. The assignment leads to covellite being a mixed Cu(I)/Cu(II) sulfide with a composition Cu(I)4S(-I)4Cu(II)2S(-II)2. However, X-ray absorption spectroscopic studies (3, 9) and theoretical studies (10) suggested that all the Cu in the mineral covellite was in the Cu(I) state. In contrast, 63Cu NMR studies apparently showed the presence of Cu(II) (11). In this paper, we confirm that covellite is a copper(I) sulfide and track the formation of the phase from discrete aqueous Cu(II) and S(-II) species through the formation of aqueous clusters to the solid phase. We discuss and describe mechanistic details on (i) the transition from soluble species or complexes to multinuclear species that are the building blocks for solid phases and (ii) the redox process for CuS during the transition. Previous studies (12, 13) of the Cu-S system were performed on the dissolution of minerals with millimolar levels of sulfide as a dissolution agent or with concentrations of reactants 0.1 mM or higher (2, 3) so that spectroscopic studies can be performed for structural elucidation of the clusters. Structural studies have used primarily UV-Vis spectrophotometry (3) and X-ray methods (2, 3, 12). None of these methods yield significant information below concentrations of 0.1 mM. In this study, we use several methods (voltammetry, UV-Vis spectroscopy, 63Cu NMR spectroscopy, mass spectrometry, EPR spectroscopy, molecular mechanic modeling) to assess the stoichiometry and redox state of Cu-S complexes formed in aqueous solution at concentrations ranging from 1 to 100 µM. 63Cu NMR will only give signals for diamagnetic Cu(I) (d10) whereas EPR will give signals for paramagnetic Cu(II) (d9). Both solid state and solution 63Cu NMR are used; solution phase 63Cu NMR data clearly indicate that Cu(I)S phases are dissolved. We show that soluble clusters are built up readily in aqueous solution and that the best structural entity for the 1:1 ratio is Cu3S3, which is a six-membered ring of alternating Cu and S atoms. Reduction occurs after formation of Cu-S bonds.
Materials and Methods Materials. ACS reagent grade CuSO4‚5H2O was used to make up standard Cu solutions. Sodium sulfide nonahydrate (ACS grade) or anhydrous Na2S (Alfa) was used for sulfide standards after iodometric titration. All solutions were made with deionized water (DIW) that had been purged with high-purity argon for at least 1 h; the argon was also purged through a 10.1021/es010906k CCC: $22.00
2002 American Chemical Society Published on Web 01/04/2002
FIGURE 1. Structure of covellite (A) showing six-membered rings with alternating Cu and S atoms. The Cu3S-CuS3 layer serves as a base for tetrameric Cu4S6 (B) with Cu bound to S and as a top for the Cu4S6 with S bound to S (C), which leads to the S2 layer. pyrogallol trap to remove traces of O2 from the gas cylinder. Solutions were prepared fresh daily, and aliquots were transferred with anaerobic syringe techniques. Sargasso seawater (pH 8.1, I ) 0.70) and 0.100-0.545 M NaCl solutions adjusted to the appropriate pH with carbonate or acetate buffers (typical range 7-8.3) were used as stock electrolyte solutions. All titration experiments were performed at 25 °C in Ar-purged solutions and were completed within 20-30 min. Voltammetric Analyses. Both an EG&G Princeton Applied Research model 384B polarographic analyzer in conjunction with a model 303A static dropping mercury electrode and an Analytical Instrument Systems, Inc. (AIS) model DLK-100 voltammetric analyzer were used for all electrochemical work. The electrode stands were modified to use a saturated calomel electrode (SCE) reference rather than the Ag/AgCl reference supplied. Instrumental parameters for the square wave mode were typically 200 mV/s scan rate over the potential range of -0.1 to -1.3 V with a 25 or 50 mV pulse height. The copper sulfide titration experiments were performed at 1-15 µM quantities of reactants. Electrochemical experiments were generally performed without a deposition step unless indicated otherwise. This allows any sulfide dissociating from the metal (or vice versa) to be detected at the electrode, which is an indication of the lability of the complex (14). Both Cu(II) and bisulfide are measurable with this method (14). The electrochemical titration methods employed are similar to those described previously (14). High-purity argon was used for purging of the voltammetric cell. Titrations of sulfide solutions with Cu(II) were performed in two ways, and reaction was complete within 30 s of titrant addition. First, Cu(II) was added to sulfide solutions held at fixed concentration usually between 1 and 10 µM to determine if S to Cu complexes of stoichiometry
