Metal sulfides in oxidizing freshwater systems

Abstract

<p>The overall goal of this research project was to examine the occurrence and fate of S(II-) and associated trace metals in aqueous systems under oxidizing conditions, specifically freshwater sediments and surface waters. Three studies were conducted that examined the following: (a) the fate of Ag(I) sorbed to FeS(s) and the stabilization of S(II-) by Ni(II), Mn(II), Cu(II) and Zn(II) upon oxidation of an FeS(s) suspension, (b) the stabilization of S(II-) in oxic solution by Zn(II), Fe(II), Fe(III), Cu(II) and Ag(I) in association with Suwannee River fulvic acid (SRFA), and (c) the applicability of the colourimetric methylene blue-sulfide (MBS) method to the determination of trace S(II-) in oxic freshwaters. Silver(I) added to an aqueous FeS(s) suspension was scavenged from solution by the solid phase. Upon oxidation of the Ag-FeS(s) suspension, S(II-) concentration in solution rapidly decreased while Ag(I) was released back into solution. Similar experiments were repeated with added amounts of Ni(II), Mn(II), Cu(II) and Zn(II) to examine the effect of these metals on S(II-) in solution during FeS(s) oxidation. Zinc(II) in the presence of FeS(s) retarded the loss of S(II-) from solution, suggesting the stabilization of S(II-) by Zn(II) under oxic conditions. Sulfide did not appear to be stabilized by the other metals as concentrations decreased below detectable levels. In the case of Cu(II) and Ni(II) however, stable metal sulfide species may have formed and persisted under oxic conditions, but were not reacted and detected in the MBS analysis. Sulfide in aqueous solution in the absence of metals, with and without SRFA, decreased over time under oxic conditions due to reaction with oxygen and/or NOM. Zinc(II) was able to bind and stabilize S(II-) in the presence of NOM under oxic conditions, and this effect was attained even when other S(II-) binding metals, Fe(III) or Ag(I), were added to the sample. It was not, however, clear whether the association of Zn(II) with SRFA offered more stability to S(II-) than the Zn(II) alone. Iron in either oxidation state (II, III) did not stabilize S(II-) under oxic conditions regardless of whether NOM was present. Although Ag(I) and Cu(II) were both able to bind S(II-), even in the presence of NOM, it was not possible to assess whether these metals stabilized S(II-) under oxic conditions. In the presence of Ag(I) and Cu(II), MBS formation was inhibited in the acidic colourimetric reagent (MDR) and S(II-) could not be determined. Using the methylene blue method of S(II-) determination (MBS), S(II-) was measured at low nanomolar concentrations in a number of natural freshwater samples of varying organic carbon content. This study revealed a number of limitations in the application of the MBS method to the measurement of S(II-) in natural samples. Most significantly, total S(II-) concentration was underestimated by this method, as S(II-) bound to Ag(I) and Cu(II) and possibly other metals, was not readily reactive in the colourimetric reagent to form the methylene blue complex. Pretreatment of the sample with Cr(II) to reduce such metal sulfide species, resulted in higher S(II-) recoveries relative to MBS. Particulates and NOM contained in samples interfered with absorbance measurements, while adsorption of metal sulfide species to container surfaces caused S(II-) concentrations to be underestimated.</p>