Environ. Sci. Technol. 1993, 27, 1154-1 163
Voltammetric Characterization of Iron( I I)Sulfide Complexes in Laboratory Solutions and in Marine Waters and Porewaters George W. Luther, 111,’ and Timothy 0. Ferdelman College of Marine Studies, University of Delaware, Lewes, Delaware 19958
Thus, sulfide and oxygen should be able to coexist in water at pH values greater than 7. In oxic seawater, the thermodynamically stable form of iodine is iodate, which oxidizes sulfide ( 4 , 5 ) . Thus, sulfide should not exist in significant quantities in oxic seawater. In oceanic field studies, Cutter and Krahforst (6)(who studied the photic zone of the ocean) and Luther and Tsamakis (5)(who studied the entire water column of the ocean) have reported that trace amounts of sulfide do exist in oxic seawater. Also, Luther and Tsamakis (5)reported that purging of seawater did not remove all the sulfide from oxic seawater. Based upon existing thermodynamic data and ligand field theory, Luther and Tsamakis (5) suggested that Cu and ligand field stabilized metals would preferentially complex sulfide throughout the water col-
umn. In Black Sea anoxic waters, Luther et al. (7), using voltammetric techniques to measure the sulfide peak, provided evidence for a MnS-type complex at the sulfide interface zone where the manganese to sulfide ratio was greater than 50. In freshwater lakes, Davison et al. (8) and Buffle et al. (9) have provided evidence for a FeS dimer using polarographic data based on the Fe(I1) reduction wave of a “FeS” complex which is less sensitive than the sulfide electrochemical oxidation wave. Dyrssen and co-workers (10, 11) and Elliot (12)have estimated the stability constants of a variety of simple inorganic metal sulfide complexes to understand the possible stability of hydrogen sulfide in oxic seawater. These studies predict that Cu(I1) is the thermodynamically favored complex in oxic seawater. Vazquez et al. (2)have reported a new value for the ZnS complex, which is unreactive with oxygen, based on kinetic information from trace metal catalyzed sulfide oxidation experiments. For marine systems, all of the thermodynamic data to date generally consider simple inorganic MS-, MSH-, or M(SH)z-type complexesonly. There has been no evidence for complexes enriched with iron such as [Fe2(SH)l3+type complexes or for the influence of organic chelates on the kinetic stability of iron and other metal sulfide complexes in environmental systems (13, 14). Some systems such as marine porewaters and anoxic/sulfidic basins have high concentrations of dissolved organic matter (151, which can complex metals, and higher Fe(I1) concentrations than sulfide at the oxic/anoxic interface. In anaerobic laboratory studies conducted to understand iron sulfide mineral formation, Rickard (16) and Luther (17 ) have shown that the first product of the reaction of sulfide with Fe(1I) at near neutral pH and equal molar ratios is Fe(SH)2 or Fe(SH)+. Luther (17)has also shown that the reaction of solid FeS or soluble Fe(SH)+ with polysulfides is a mixed ligand complex of form [Fe(SH)(&)I- which decomposes to form pyrite, FeS2. This polysulfide complex, [Fe(SH)(Sdl-, has similar polarographic behavior to the ferrodoxin complexes which contain the stoichiometric unit, [Fe(S)(RS)l-, and which are important in electron transport processes. The objectives of this study are (i) to demonstrate the existence of metal sulfide complexes in marine waters and porewaters, where Fe(I1) complexation with sulfide is likely, via polarographic and voltammetric methods; (ii) to ascertain the relative thermodynamic strength of these complexes; and (iii) to show that Fe complexes may relate to FeSz formation based upon recent synthesis results in our laboratory (17). The waters studied were obtained from local salt marsh creek waters and the Chesapeake Bay. The porewaters were obtained from the continental slope of the Mid-Atlantic Bight and the Great Marsh, DE, salt marsh. These waters contain more dissolved organic matter, which can complex trace metals and affect metal sulfide stability, than does the oceanic water column. The voltammetric approach used takes advantage of the sensitive sulfide electrochemical oxidation wave at the Hg
1154 Environ. Sci. Technol., Vol. 27, No. 6,1993
0013-936X/93/0927-1154$04.00/0
We report on metal sulfide complexes in marine waters and porewaters using voltammetric methods and compare field data with standard solutions containing Fe(1I) and sulfide. Field data indicate that sulfide is complexed and not free at the sulfidictnonsulfidic interface when the Fe(11) content 1 0.1 of the sulfide content of the sample. Acid-base titration data for field samples indicate that the forms of the sulfide complex are FeSH+ and [Fez(SH)13+at seawater pH. From pH 7-10, the complexes FeS and Fe(H2SI2+are not consistent with the data. Salt marsh creek waters obtained at low tide and Chesapeake Bay sulfidic waters contain strong Fe(1I) sulfide complexes [log Kf near 6.95 for Fe(SH)+l. This compares well with the conditional stability constants evaluated for Fe(SH)+ (log PI = 5.50 f 0.24) and [Fez(SH)J3+(log p2 = 11.08 f 0.25) by electrochemical methods with standard iron(I1) sulfide solutions in seawater. The environmental complexes can be kinetically inert to dissociation of sulfide whereas the laboratory solutions are labile. Thus, organic chelates may stabilize the complexes. In salt marsh creek waters, strong sulfide complexes may be transported over reasonable distances. In porewaters and waters in enclosed basins, the complexes are likely important in pyrite synthesis.
Introduction Metal sulfide complexes have been the subject of much recent laboratory and field interest in the environmental literature. Part of this interest stems from the known reactivity of hydrogen sulfide with oxidants found in nature. Hydrogen sulfide has been reported to undergo oxidation reactions with 0 2 in the absence and in the presence of some trace metals (1, 2). The reaction is generally faster when trace metals are present near the micromolar level. However, based on the recent compilation of reduction potentials for free radicals in aqueous solution (31,the uncatalyzed reaction of sulfide with oxygen (eq 1)is thermodynamically unfavorable: HS-
+ 0,
-
SH + 0,-
E” = -1.24 V
(1)
0 1993 American Chemical Society
electrode. In addition, the hydrogen sulfide system (HzS, HS-, S2-) has acid-base properties which can be used effectively to monitor peak shifts during titrations. Plots of peak shifts versus pH and equivalents of acid-base added to samples also provide useful stoichiometric and thermodynamic data for the sulfide system. Lastly, titrations of sulfide with Fe(I1) provide data to calculate the conditional stability constants of iron sulfide complexes in seawater. Experimental Section
Porewaters from the Great Marsh, DE, were obtained as described in previous work (18). Local salt marsh creek waters were obtained from the bow of the boat at low tide when the salt marsh sediments drain their porewaters into the creek. Precautions were taken to minimize trace metal contamination by obtaining the water in trace metal cleaned bottles or syringes using trace metal clean techniques. Porewaters from the continental slope of the MidAtlantic Bight were obtained from a whole core squeezer (19). The squeezer was pressurized with nitrogen to express the porewaters into trace metal clean polypropylene syringes which had been previously purged with high purity argon. Chesapeake Bay waters were obtained June 24-27,1991, with Go-Flo bottles, which were pressurized with nitrogen to minimize oxidation. All samples were filtered through 0.40-pm Nuclepore filters and typically analyzed for sulfide, Fe, Mn, pH, alkalinity, 0 2 , thiosulfate, iodide, polysulfides, chloride, and sulfate. In this study we report those results necessary to describe metal sulfide complexes. For field samples, free and complexed sulfide were measured by three different voltammetric methods depending on the concentration of sulfide in seawater. For >10 pM sulfide levels, the sampled DC polarographic method (20) was performed. For 50 nM-10 pM sulfide concentrations, the square wave voltammetric method (21) was used. In these two methods, an aliquot of sample can be run directly or is added to a base matrix for analysis. For sulfide levels
