The investigation of dissolved and suspended-particulate trace metal fractionation in the Black Sea

Abstract

A sequential filtration-ion-exchange column scheme has been developed for the investigation of dissolved trace metal fractionation. The method is designed to separate water column metals rapidly into particulate and colloidal-sized species, anionic dissolved metal-organic and/or metal-sulfide complexes, and ‘free’ (hydrated) metal cations and/or labile major ion complexes. The suspended-particulate matter (0.4 μm filterable material) is further subjected to a sequential leaching procedure to separate particulate metals into weak-acid leachable, strong-acid leachable, and refractory fractions. These methods are evaluated with respect to laboratory experiments and field results. These methods have been applied to the study of trace metal chemical fractionation in the Black Sea, the world's largest anoxic basin. We present here the results for Co, Ni, Cu, Zn, Cd, and Pb. The dissolved Co distribution is best explained in terms of a scavenging-regeneration cycle with Mn-oxyhydroxides across the sulfide interface and coprecipitation of Co with Fe-sulfides in the deep waters. Nickel displays nearly constant dissolved metal concentrations with depth and is apparently unaffected by redox processes. The Class B metals (Cu, Zn, Cd, and Pb) have high dissolved metal concentrations in the surface waters then decrease rapidly across the sulfide interface to low deep water concentrations, consistent with metal-sulfide precipitation below the interface. The dissolved Class B metal fractionation was generally dominated in the oxic zone by ‘free’ metal species, shifting below the interface to anionic species, probably dissolved metal-sulfide complexes. The suspended matter trace metal fractionation is dominated by weak-acid soluble forms throughout most of the water column. For particulate Co, strong-acid leachable forms, probably metal-sulfide phases, are important in the anoxic deep waters. The Class B metals form relatively reactive (weak-acid leachable) metal-sulfide precipitates just below the sulfide interface.