Metal–sulfide complexation in seawater

Abstract

The conditional stability constants of metal–sulfide complexes were determined in pH 8 seawater at various salinities, by flow-analysis with detection by cathodic stripping voltammetry (FA-CSV). Two methods were used. The first method was titration of the sulfide by metal with detection of the free sulfide by FA-CSV. The titrant metals were Ag+, Cd2+, Co2+, Fe2+, Mn2+, Pb2+, Zn2+, Cr3+ and Al3+. This method was suitable for comparatively weak complexes (log K<∼8). The second method took advantage of ligand competition between sulfide and 8-hydroxyquinoline (oxine) for free metal ions. The ability of the oxine to form electroactive complexes with Cu2+, Cd2+, Pb2+, Zn2+, Co2+ and Ni2+, was used to detect the concentration of the metal–oxine complexes in equilibrium with sulfide. More stable complexes could be detected by this method. The two independent methods could be compared for some metals showing good agreement. The stability of the sulfide complexes with copper was found to be much greater than previously determined, in line with theoretical prediction. The high stability, greater than that for Ag+, suggests that copper may be complexed by sulfide as copper(I) rather than copper(II). Complex stability with Co2+ and Cd2+ is also greater than found before, but the difference is smaller. The stability constants were used to calculate the speciation of sulfide at realistic sulfide and metal concentrations similar to those present in the open ocean. At a sulfide concentration of 0.5 nM and a copper concentration of 1 nM in seawater, the major form of sulfide is copper(II) monobisulfide (Cu(HS)+) representing 99% of the total sulfide. In the presence of natural organic complexing ligands a smaller percentage (28%) of sulfide was bound to copper, the remainder occurring as bisulfide, whereas copper was then 71% complexed by organic matter. The new data and the calculations confirm the potential importance of sulfide as a complexing ligand in seawater, more important than major anions and competing effectively with natural organic ligands.