Voltammetry of Sulfide Nanoparticles and the FeS(aq) Problem - ACS

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Chapter 13

Voltammetry of Sulfide Nanoparticles and the FeS(aq) Problem G. R. Helz,*,1 I. Ciglenečki,2 D. Krznarić,2 and E. Bura-Nakić2 1Department

of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA 2Center for Marine and Environmental Research, Ruđer Bošković Institute, Bijenička 54, 1000 Zagreb, Croatia *[email protected]

Voltammetry at Hg drop electrodes is a promising method for detecting sulfide nanoparticles in natural waters. Recent research suggests that such nanoparticles might affect organisms in unforeseen ways. Sulfide nanoparticles diffusing to Hg surfaces are captured selectively even from unfiltered waters that contain larger amounts of other nanoscale materials, such as organic macromolecules or clay minerals. Optimum size sensitivity for capture is roughly 5-100 nm at Hg drop electrodes. Sulfide nanoparticles are stabilized at Hg surfaces by transformation to adlayers, whose accumulation can be quantified electrochemically. Study of FeS adlayers has led to new insights regarding the puzzling -1.1 V vs. Ag/AgCl signal observed in sulfidic natural waters. This signal has been attributed previously to Fe sulfide clusters or complexes. New evidence shows that it arises from reduction of Fe2+ at FeS adlayers formed by sorption of FeS nanoparticles on Hg electrodes. Partial coverage of Hg with FeS creates in essence two electrodes. These reduce Fe2+ at separate potentials.

Introduction Aquatic and marine chemists by convention have relied on filtration to divide samples of natural waters into dissolved and particulate fractions prior to analysis. For many decades, 0.45 μm pore size filters were preferred, although © 2011 American Chemical Society In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


0.2 μm or smaller pore sizes have been also in common use. It has always been recognized that this approach provides a less than ideal way of classifying analytes. The filtration process itself is procedure-dependent and problematic (1). More importantly, solutes in the so-called dissolved (filterable) fraction consist not solely of hydrated ions and molecules, but include nanoparticles. Recently, nanoparticles have become the focus of increasing interest. Exactly what constitutes a nanoparticle has received much discussion. Interested readers are referred to a review by Lead and Wilkinson (2). Some physicists and chemists define nanoparticles as objects so small (usually < 5 nm) that they possess optical properties influenced by quantum confinement. In natural waters, this definition is operationally impractical because of the presence of optically interfering substances, especially macromolecular organic materials. Additionally, the relevant optical properties are well-defined only for nanoparticles having narrow ranges of composition and size, an unlikely situation in nature. Many environmental scientists now prefer a size-based definition that nanoparticles simply have at least one dimension in the 1 to 100 nm range (2). Although neither filtration nor ultrafiltration achieve precise size-based separations, sizes of 1 to 100 nm are roughly consistent with an operation-based definition that nanoparticles pass filters with nominal pore sizes in the tenths of a micron range but are retained by ultrafiltration membranes with cutoffs of a few kDa (3). Some nanoparticles may possess unique toxicological properties that differ from those of the same components as bulk solids or as ions or molecules in solution (4–6). A subclass of nanoparticles, the chalcogenide quantum dots, has attracted particular attention. These are sulfide-, selenide- or telluride-containing semiconductors in the quantum confinement size range. Some quantum dots appear to be able to enter cells and cause damage that differs from that caused by the same components as ions in solution (7–10). Whether this behavior is a general property of chalcogenide quantum dots is not yet clear (11). Filterable (0.2 μm) HgS nanoparticles are sufficiently hydrophobic to be extracted into octanol and may be responsible for the biological activity previously attributed to HgS0 complexes or clusters (12). As discussed in the next section, good evidence indicates that sulfide nanoparticles of natural origin exist in the environment. The possibility that such nanoparticles might also exhibit unusual biological behavior creates a need for analytical methods to characterize and quantify them in natural waters. The needed methods should be able to detect sulfide nanoparticles in the presence of more abundant and common nanoscale materials, including organic macro-molecules, Fe and Mn oxyhydroxides and clay minerals. In this chapter, we first review evidence that metal sulfide nanoparticles probably exist in natural waters, possibly even in the presence of O2. Then we describe recent research at the Ruđer Bošković Institute aimed at characterizing their voltammetric behavior. The strong affinity of Hg for sulfide makes the Hg drop electrode an attractive device for selectively capturing sulfide particles from samples containing an abundance of other nanoparticulate materials. Finally we present some preliminary results to show that this approach might prove useful for direct determination of sulfide nanoparticles in natural waters. 266 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

Sulfide Nanoparticles in Nature Expectations from Thermodynamics


Figure 1 shows thermodynamic speciation calculations for two sulfidereactive metals, Cu and Fe. Saturation with respect to elemental S is assumed, resulting in redox control by the S(-II)/S(0) couple. Notice that even quite low sulfide concentrations cause the principal dissolved complexes in both cases to be in lower oxidation states at equilibrium.

Figure 1. Equilibrium speciation and degree of saturation (Ω) in waters containing 10-9M total Cu (left panel) and 10-5M total Fe (right panel) as a function of free sulfide (H2S+HS-)concentration. Metal concentrations are representative for suboxic or anoxic natural waters. Ω = [M2+][HS-]/10-pH/Ksp. Assumed conditions: pH 7.5; major inorganic anions at seawater concentrations; saturation with Srhombic. (Data: (13–17)). The species distributions assume that no sulfide phases precipitate, but the figure indicates that very high degrees of supersaturation would exist if this were actually possible. To the contrary, huge supersaturations near the maxima in the Ω curves imply negligible barriers to sulfide phase nucleation; rates of nucleation should occur nearly at diffusion limits. Rapid nucleation produces numerous small primary particles, usually having nanoscale dimensions (18). For example, FeS precipitation in the laboratory produces primary particles < 10 nm (19–22). Copper sulfide precipitates are similar (23, 24). In nature, huge degrees of supersaturation that would drive nanoparticle precipitation can be created wherever sharp redox gradients exist. Examples of such places include pycnoclines above euxinic water columns and sediment-water interfaces. At the microscale, sharp gradients occur around reducing microniches in what might appear to be homogeneously oxidized media (25). 267 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 1 shows that huge supersaturations with respect to CuS (covellite) exist already at the lowest sulfide concentrations shown. In this range, dissolved Cu(I) complexes involving sulfide and polysulfide are not yet competitive with Cu(II)organic matter complexes, but Cu sulfide particles are nonetheless stable. Thus if Cu is involved in preserving traces of filterable sulfide in oxic natural waters, as has been proposed, then Cu sulfide nanoparticles rather than dissolved Cu complexes are most likely the agent (26). As sulfide increases in Figure 1, the degree of supersaturation passes through a maximum and then decreases as Cu(I) sulfide and polysulfide complexes start to dominate in the aqueous phase. Under S0-saturation conditions, Cu polysulfide complexes are most abundant, but they would be replaced by sulfide complexes or bidentate thioanion complexes (27, 28) at lower activity of zero-valent sulfur. For Cu, the greatest driving force for precipitation occurs near 0.01 μM sulfide. In the case of Fe, sulfide complexes do not become significant until sulfide concentrations exceed those normally found in euxinic waters; this prediction is consistent with field evidence (29, 30). The supersaturation curve for Fe is qualitatively similar to that of Cu, but the greatest driving force for FeS (mackinawite) precipitation is centered at much higher sulfide. A very important implication is that Cu and Fe sulfide nanoparticles are most likely to be found in different sulfide concentration regimes in nature. Remarkably, thermodynamics suggest that Cu sulfide nanoparticles are most likely to be found in waters that would be judged non-sulfidic when assessed by conventional sulfide analytical methods, which have detection limits near 10-6 M. Field Evidence To date, evidence that filterable (submicron) FeS particles indeed exist in sulfidic natural waters is indirect. Based on sampling with size exclusion columns, as much as a third of the filterable Fe below the chemocline in the Black Sea consists of particles that are less than 50 nm in size (31). This material is soluble at pH 5.5 and is inferred to be FeS. By comparing colorimetrically and electrochemically determined concentrations of filterable Fe(II) and S(-II) in Lake Bret, Buffle et al. (32) established that both components occur in submicron colloids. Curiously, colloidal Fe exceeded colloidal S by about 2-fold on a molar basis. If this is not an analytical artifact related to the FeSaq problem (see below), then mackinawite cannot be the only Fe-bearing nanoscale precipitate. Bura-Nakić et al. (33) concluded that roughly half the methylene blue determined S(-II) in Lake Pavin resides in a filterable form that is not free sulfide. This form is most likely FeS based on thermodynamic arguments as well as analytical constraints. Similar evidence from natural waters supports existence of sulfide nanoparticles containing other metals. Skei et al. (34) captured aggregates of ZnS and CuS at the top of the sulfidic water column of Framvaren Fjord by 0.4 μm filtration. The aggregates themselves exceeded 1 μm in size, but consisted of smaller particles. Spherical aggregates consisting of 1-5 nm primary ZnS particles have been observed in biofilms in a flooded Pb-Zn mine (35, 36). Similar aggregates have been found in H2S-rich pore waters in wetlands (37). Copper-rich 268 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


sulfide particles < 50 nm in diameter, as well as hollow spheres 50 – 150 nm, were produced in experiments with flooded soils (38). On the other hand, the highly sulfidic Black Sea water column contains copper mainly in an anionic form, not as nanoparticles (31). This supports the inference from thermodynamics that Cu sulfide and polysulfide anionic complexes are more likely than nanoparticles at higher sulfide concentrations (Figure 1). Even in oxic environments, tentative evidence exists for sulfide nanoparticles. Wastewaters contain filterable forms of Ag having properties consistent with Ag2S nanoparticles, including retention by ultrafiltration and resistance to oxidation (39). Nanoscale Ag2S (5-20 nm) now actually has been imaged by transmission electron microscopy in wastewater sludge (40). Oxic surface waters and sewage treatment plant effluents contain filterable forms of bound S(-II) that are partly retained by ultrafilters; these behave analytically like Cu and Zn sulfide particles (41). Counter-intuitively, Sukola et al. (26) demonstrate that Cu, Zn and Cd sulfide species that are probably nanoparticles persist in oxic water for weeks. A shortcoming of this evidence is that filtration is necessary in most cases to qualify analytes as nanoparticulate. Experiments with recovery of synthesized sulfide nanoparticles suggest that they readily adhere to surfaces and therefore are likely to be underdetermined in filtered samples (26, 42, 43). Another shortcoming is that the evidence is largely indirect. Nanoparticles are considered to be what is left after other analytes have been accounted for. In many cases, nanoparticles themselves are not being observed nor are their properties being measured.

Behavior of Sulfide Nanoparticles at Hg Electrode Surfaces Sulfide macro- and nanoparticles readily sorb to Hg surfaces owing to the great affinity of Hg0 for reduced S in almost any form (44, 45). As a consequence, metallic Hg is an effective and selective sample collection device. For example, we have shown that an Hg drop exposed to stirred, unfiltered Adriatic Sea water can be used as a sulfide nanoparticle collector (44). Sulfide Adlayers at Hg0 Surfaces To understand the voltammetry of nanoparticles, it is necessary to understand the adlayers that form on Hg electrode surfaces. Adlayer is a portmanteau word that simply means adsorbed layer; the word is used throughout surface science, not simply with regard to electrode surfaces. In metallurgy, oxide adlayers are commonly used to impart corrosion resistance to metal surfaces; bluing of gunmetal and anodizing of aluminum are familiar examples. It has been known for more than a century that HgS adlayers accumulate spontaneously under certain conditions on Hg electrodes that are exposed to sulfide solutions (45). For this to happen the electrode potential must be held within a window of approximately +0.15 V to -0.65 V. (All potentials quoted in this paper are vs. a Ag/AgCl, 3 M KCl reference electrode; for electrochemical reactions that are H+- or HS--dependent, potentials are approximate and assume near-neutral pH, ~105 M HS-, which are common conditions in sulfidic natural waters.) 269 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Outside this window, HgS adlayers are unstable. At more positive potentials, HgS in the adlayer is transformed to HgO (or calomel in chloride solutions); at more negative potentials, HgII is reduced to Hg0 with release of dissolved sulfide (45). The reduction reaction (HgS + H+ + 2e- → Hg0 + HS-) is often exploited in the voltammetric determination of dissolved sulfide. To reduce completely an HgS monolayer, a charge of about 180 microcoulombs per square centimeter of electrode surface must be provided (45). When HgS adlayers exceed monolayer thicknesses, the necessary reduction charge per layer is greater and the reduction potential shifts to more negative values. This implies that multilayers are more dense and thermodynamically stable than monolayers. When metal sulfide (MS) nanoparticles are present in sulfidic solution, MS adlayers as well as HgS adlayers are a possibility on Hg electrodes. We have explored CuS and FeS adlayers in some detail (24, 46, 47), and explored adlayers of a few other metals to a limited extent (48). The potential window for CuS adlayer stability reaches from an anodic limit of about -0.3 V to a cathodic limit of about -0.95 V. The anodic reaction involves oxidation of Hg0 to HgS with release of Cu2+ or a CuII complex, depending on solution composition (46). Notice that this limit does not involve oxidation of CuS, itself, which occurs at much more positive potentials (49). The cathodic limit, around -0.95 V, is nominally due to CuS + H+ + 2e- → Cu0 + HS-, but Cu0 in this case represents an amalgam or intermetallic Cu-Hg compound on the electrode surface. As in the case of HgS, the reduction potential of CuS shifts in the negative direction as the adlayer becomes thicker (24). The potential window for stable FeS adlayers is broader, reaching from about -0.45 V to beyond -1.50 V. The anodic limit is established by FeS + Hg0 → HgS + Fe2+ + 2e- (47). The cathodic side of this window, which has not been investigated carefully, presumably is established by reduction of FeS to Fe0 with release of sulfide to solution. The cathodic limit is hundreds of millivolts more negative than the thermodynamic reduction potential of FeS, which is estimated to be -1.2 V (50). This overpotential is very similar to the known overpotential for Fe2+ + 2e- → Fe0 at Hg electrodes (51). Both overpotentials are related to the difficulty of nucleating Fe0, a non-amalgamating metal, on Hg electrodes. It is important to note that the stability windows for FeS and CuS adlayers overlap partly, but not entirely, with the stability window of HgS adlayers. The key to voltammetric determination of MS nanoparticles is to accumulate them in the form of MS adlayers on an electrode. This must be done at potentials where no interfering HgS adlayer can form. After accumulation, the amount of MS adlayer can be quantified by scanning past either the anodic or cathodic limit of stability of the MS adlayer and measuring the current as the adlayer decomposes electrochemically. The strategy is analogous to the time-honored one of determining dissolved sulfide by accumulating an HgS adlayer and then scanning past the potential where that adlayer is reduced. Sulfide Nanoparticle Interactions with Hg0 Electrodes Figure 2 presents a conceptualization of what happens if an Hg surface is exposed to FeS nanoparticles in suspension. It is assumed that the electrode 270 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


potential is being held at a value that lies beyond the negative limit of HgS adlayer stability.

Figure 2. Schematic of the interaction of FeS nanoparticles with a mercury electrode surface. The FeS nanoparticles diffuse by Brownian motion and bind to Hg on the surface (A in Figure 2). The electrode has a negative surface charge at this potential. Sulfide particles usually have negative zeta potentials at near-neutral pH if kept out of contact with O2 (52, 53). In this situation, the incoming particles must overcome an electrostatic potential energy barrier in order to reach the electrode. Particle collection at the Hg drop electrode then is analogous to coagulation; collection efficiency is sensitive to cation charge and concentration in the electrolyte as predicted by the Schulze-Hardy Rule (24). If the electrode and particles have opposite surface charges, then particle collection is facilitated by electrostatics, rather than impeded. Collection efficiency becomes comparatively insensitive to electrolyte composition (24). This colloid-like behavior during accumulation of an adlayer demonstrates that the nanoparticles, themselves, are transporting the analyte to the electrode surface. Transport is not due to a minor dissolved intermediate. Coagulation-like behavior discriminates against very small nanoparticles (less than a few nm). They are less able to penetrate electrostatic energy barriers and more likely to escape back to solution (54). In some cases small particles also may be lost by reduction during the accumulation period and thus not assayed during the subsequent scan (24). The mass accumulation rate of spherical particles by Brownian diffusion is proportional to radius-1, resulting in discrimination against the largest particles (i.e. particle radii on the order of 100 nm or larger). Although we have shown that particles in the micron size range, such as found in powder slurries, will accumulate readily on Hg electrodes (44, 48) the accumulation process is most sensitive to particles in the 5-100 nm diameter range. This size selection effect is easily observed by repeatedly analyzing an equimolar metal-sulfide mixture (46, 48). Typically, no metal sulfide deposit 271 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


is formed on an Hg electrode immediately after preparing the mixture. At this stage, particle sizes usually are in the 1-10 nm range (24), and most particles are too small to be captured efficiently. As assays continue, however, signals due to metal sulfide adlayers gradually grow because the primary particles coagulate and recrystallize, becoming more efficiently captured by Hg0. Eventually though, the signals decay as the particles pass out of the optimum size range for capture. Nanoparticles that adhere to Hg surfaces apparently transform spontaneously to adlayers (A→B in Figure 2). The thermodynamic drive for this involves maximizing the exergonic sulfide-Hg0 interfacial interaction. In the case of CuS, a large shift in reduction potential occurs during this transformation, implying that the adlayer is much more stable than the adsorbed particle (24). Once formed, the adlayer can be destabilized and its mass quantified by either anodic (B→D) or cathodic (B→C) scans. In the case of FeS, the cathodic reaction lies at extremely negative potentials where many interfering reactions are possible; therefore the anodic reaction is best for quantification. On the other hand, CuS is more conveniently quantified by its cathodic reaction. Within the stability window of FeS adlayers, cathodic reactions with Fe2+ and its labile complexes (FeCl+, FeSO40 etc.) occur (B→D in Figure 2). As discussed in the next section, if the electrode surface consists partly of an FeS adlayer and partly of bare Hg0, then Fe2+ can be reduced at both surfaces, but at different potentials. An alternate mechanism of producing an FeS adlayer on Hg involves first accumulating an HgS adlayer at a relatively positive potential, such as -0.2 V (i.e. within the HgS stability window but outside the FeS stability window). FeS nanoparticles diffusing to the electrode will oxidize Hg0 to HgS with release of Fe2+ rather than accumulating as an FeS adlayer. Then, if a cathodic scan is made in Fe2+-containing solution, the HgS adlayer can be transformed to an FeS adlayer (D→B) (47). By the same kind of replacement reaction, HgS can produce CuS adlayers (24, 44, 46). Thus an analyst who wishes to detect nanoparticles by the adlayer they deposit on Hg0 must avoid accumulation potentials where HgS adlayers form.

The FeSaq Problem Our study of FeS adlayers has led to an interesting new explanation for the mysterious analyte that is commonly designated FeSaq (50, 55). This putative analyte yields a reduction peak, which is sometimes a doublet, near -1.1 V in sulfidic natural waters and in synthetic solutions containing Fe and sulfide. Davison et al. (55) presented a detailed summary of the properties of FeSaq but were unable to deduce its actual composition. They did show that it has a diffusion coefficient similar to Fe2+ and therefore must have a low molecular mass. However it also seemed to display coagulation behavior, disappearing at ionic strengths above 0.1 M. Other authors have proposed that FeSaq is a charge-neutral iron sulfide cluster of uncertain composition (50). The FeSaq signal appears only in solutions that are near saturation with respect to FeS precipitates (33, 47, 50, 55). Such solutions can be suspected of containing FeS nanoparticles. 272 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


The electrochemical landscape associated with the putative FeSaq signal is illustrated in Figure 3. This figure shows both anodic and cathodic scans starting from an accumulation potential of -0.75 V in solutions containing mixtures of Fe(II) and sulfide. The mixtures were initially supersaturated with mackinawite and contain suspended nanoparticulate FeS (diameter

Voltammetry of Sulfide Nanoparticles and the FeS(aq) Problem - ACS

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