Environ. Sci. Technol. 2003, 37, 4374-4381
Simultaneous Release of Metals and Sulfide in Lacustrine Sediment MIKAEL MOTELICA-HEINO,† CHRIS NAYLOR, HAO ZHANG, AND WILLIAM DAVISON* Department of Environmental Sciences, IENS, Lancaster University, Lancaster LA1 4YQ, U.K.
A single DGT (diffusive gradient in thin films) probe that could measure metals and sulfide simultaneously and at the same location was deployed in the surface sediment of a productive lake (Esthwaite Water). It contained a layer of AgI that binds sulfide overlying a layer of chelating resin that binds metals. Analysis for sulfide in two dimensions showed local sources of sulfide, 1-5 mm in diameter, at 8-11 cm depth within the sediment. A transect of trace metals measured at 100-µm intervals through the largest sulfide “hot spot” demonstrated concomitant release of Fe, Mn, Cu, Ni, and Co. Substantial supersaturation with respect to metal sulfides was observed for Fe and Co at the site of metal generation, but at a distance of less than 1 mm, solution concentrations were consistent with equilibration with amorphous FeS and CoS phases. Simple mass balance calculations were consistent with Fe being supplied from reductive dissolution of its oxides and with sulfide being supplied from reduction of sulfate. The observed concentrations of Cu, Ni, Co, and Mn could be accounted for by their release from iron oxides without invoking Mn reduction. The metals are removed rapidly (∼1 min) at the edge of the hot spot. These first observations of the simultaneous release of trace metals and sulfide are consistent with the known removal of metals by formation of their insoluble sulfides if the in situ kinetics of metal sulfide formation is on this time scale. The coproduction of reduced Fe and S suggests that iron- and sulfate-reducing bacteria may exist together in the same localized zone of actively decomposing organic matter.
Sediments are heterogeneous mixtures of mineral phases and organic matter. The distribution of metals between chemical forms and phases is controlled by chemical interactions [redox reactions, (de)sorption, and solubility] that take place at the interface between solid phases and water (e.g., minerals and solution, organisms and solution, etc.) (8). Most redox reactions are mediated by microorganisms that control metal release directly or indirectly via, for example, mineral dissolution or release from iron and manganese oxyhydroxides during their reduction (9). Metals can also be removed by biological uptake and released during decomposition (10). As yet, the precise mechanisms responsible for the highly localized release of metals within the sediment are poorly known. To advance knowledge and understanding, it is important to measure processes simultaneously at a fine (sub-millimeter) scale and at exactly the same location. DGT (diffusive gradients in thin films) is an emerging technique for the in-situ measurement of solutes (metals, phosphate, and sulfide) in waters, sediments, and soils (11-15). In contrast to equilibrium probes such as peepers, DGT is a dynamic measurement that is based on the accumulation of solutes from porewaters by establishing a concentration gradient from the solution to a binding agent. When a DGT probe for metals is inserted in the sediment, metal ions bind to a chelating resin after diffusion through a layer of polyacrylamide hydrogel. By measuring the accumulated metal, the flux of metal to the probe and the concentration in the porewaters at the probe’s surface can be calculated. This surface concentration reflects the bulk porewater concentration and the supply from solid phase to solution, as shown by numerical modeling of the DGTsediment system (16). Similarly, DGT with AgI as the binding agent can be used to measure the flux of dissolved sulfide (17). In this study, a composite DGT probe with distinct layers of both AgI and Chelex was used for simultaneous measurements of concentrations of Fe, Mn, Co, Cu, Ni, and S(-II). This is the first time that the release of trace metals and sulfide has been measured simultaneously on a small scale at exactly the same location in sediments. It provides new insights into the mechanisms of localized release processes.
Materials and Methods Introduction Accumulation of inorganic and organic material at surface sediments and subsequent systematic decomposition of the organic fraction gives rise to a layered structure of redox processes, as demonstrated by measurements of solutes in porewater (1). Oxidation of natural organic carbon and the associated reduction of electron acceptors such as O2, NO3-, Mn(IV), Fe(III), and SO42- results in metal remobilization at the sediment-water interface, while the concentration of sulfide and pH are major controls of the dissolved concentrations of trace metals in the underlying sediment porewaters (2). More recently, it has become evident that there can also be small-scale remobilization of metals within a threedimensional framework (microniches), superimposed on the relatively macro features of systematic vertical changes associated with redox zones (3-7). * Corresponding author e-mail:
[email protected]; telephone: +44 1524 593935; fax: +44 1524 593985. † Present address: BRGM, 3 ave Claude Guillemin, Orle ´ ans, France. 4374
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 19, 2003
Principle of DGT. DGT probes use thin films (
