Introduction: This proposal is for research to investigate complexation of heavy metals (Cu, Cd, Zn, Pb, Ag) by dissolved, reduced sulfur compounds in fully oxygenated surface waters. This project will be the first systematic investigation of dissolved reduced sulfides in oxic freshwater environments. Preliminary research suggests that metal-sulfide complexes may be the dominant form of many metals in rivers. Yet, almost nothing is known about the identity, distribution, sources, and cycling of dissolved reduced sulfur compounds and metal-sulfide complexes in this environment. While the research builds on what is known about in the marine realm, fresh waters constitute a different, and possibly more complex, biogeochemical system containing groundwater, anoxic sediments, wetlands, and pollution.
Research Overview: Little is known about sulfides and their interaction with metals in oxic fresh waters. Consequently, the research strategy proceeds in two distinct stages, first, basic, then, more process oriented. In the first stage, extensive field sampling will be used to document the spatial and temporal variability of sulfides, and new analytical methods will be developed in the laboratory. In the second stage, the refined analytical techniques will be applied to samples collected from two carefully selected sites, to clarify the biogeochemical cycle of sulfides and their metal complexes in this new environment. Goals are to identify sources, elucidate composition, and better understand processes that control the behavior, transport, and fate of the dissolved reduced sulfur compounds and the complexes that they form with heavy metals in fresh oxic surface waters. The following objectives will be achieved: (1) identification of pools and pathways of interconversion of dissolved reduced sulfur in oxic fresh waters, (2) evaluation of the relative magnitude of sulfide complexation compared to other ligands, especially DOM, (3) appraisal of the nature and magnitude of dissolved reduced sulfur sources, (4) assessment of levels of total dissolved sulfides in 20 rivers and lakes covering a broad range of biogeochemical characteristics, and (5) development and refinement of analytical techniques to identify and quantify a spectrum of reduced sulfur and sulfide-metal species.
Methods: Multiple analytical methods will be used to provide reliable identification and quantification of several classes of sulfur compounds, including: (1) five sulfides and individual metal-sulfide complexes (voltammetry), (2) free sulfides, precursor volatile reduced sulfur species, and dissolved acid-volatile metal sulfides (gas chromatography), (3) dissolved reduced sulfur associated with DOM (HPLC with combined polarographic/spectroscopic detection), and (4) total dissolved sulfides (preconcentration and methylene blue spectrophotometry). Clean techniques will be used at all stages of sample collection, pretreatment, and analysis to avoid contamination artifacts.
Significance: Complexation can greatly reduce the reactivity of metals, thereby decreasing their bioavailability and tendency to function as micronutrients or toxins. Dissolved sulfides may dominate metal speciation in many rivers and lakes, playing a greater role than even DOM.
