Activity of zero-valent sulfur in sulfidic natural waters

BACKGROUND

Ionic and molecular carriers of dissolved (filter-passing) zero-valent sulfur (S⁰) in anaerobic natural waters include polysulfides, S_n^2-, molecular S₈(aq), organic macromolecules and certain higher valent thioanions. Because S⁰ is rapidly transferred among these various carriers, its biogeochemical roles in such processes as dehalogenation of organic compounds, chelation of trace metals, and anaerobic microbial metabolism are not determined solely by one ionic or molecular species. Here, S⁰ is treated collectively as a virtual thermodynamic component, and computational as well as graphical methods for quantifying its activity (a_S0) in natural waters are presented. From a_S0, concentrations of the ionic and molecular carriers of S⁰ can be calculated easily.

RESULTS

Concentration ratios of any two polysulfide ions define a_S0 (Method I). Unfortunately these concentrations are often too low in nature for accurate quantification with current methods. Measurements of total divalent sulfur (ΣS^-II), zero-valent sulfur (ΣS⁰) and pH provide a more widely applicable approach (Method II). Systematic errors in ΣS⁰ measurements are the main limit to accuracy of this method at the present time. Alternative methods based on greigite solubility and potentiometry are discussed. A critical comparison of Methods I and II reveals inconsistencies at low ΣS⁰/ΣS^-II that imply errors in the thermodynamic data for HS₂^- and S₂^-. For samples having low ΣS⁰/ΣS^-II, an interim remedy is recommended: letting pK_a2 = 6.3 for all HS_n^- ions.

CONCLUSIONS

Newly assembled data for a_S0 in a selection of anaerobic natural waters indicate that S⁰ is always metastable in the surveyed samples with respect to disproportionation to sulfide and sulfate. In all the surveyed environments, sulfur-rich minerals, such as greigite, covellite and orpiment, are stable in preference to their sulfur-poor cohorts, mackinawite, chalcocite and realgar. The a_S0 values in the dataset span conditions favoring Hg-polysulfide complexes vs. Hg-sulfide complexes, implying that a_S0 could affect Hg-methylation rates in nature. No support is found for the common assumption that a_S0 = 1 in reducing natural waters. This paper calls attention to an urgent need for improved measurement methods, especially for total zero-valent sulfur, as well as new determinations of ionization constants for all HS_n^- species.

Trace element profiles in sediments as proxies of dead zone history; rhenium compared to molybdenum

Warm-season dead zones-volumes of coastal water containing too little O(2) to support macrofauna-are a growing global menace. Trace elements that are deposited in sediments in response to reducing or sulfidic conditions can provide proxy records for reconstructing dead zone evolution. Based on relative enrichment in reduced vs oxidized marine sediments, Re seems promising as a dead zone proxy. Here, Re is determined by isotope dilution mass spectrometry in sediments underlying the summertime dead zone in Chesapeake Bay. Contrary to expectation, Re becomes only modestly (∼2-fold) elevated during the 20th century and fails to track the historic record of summertime O(2) depletion. Rhenium enrichments are watershed-specific and apparently controlled by anthropogenic sources, not by redox-linked authigenic processes. In contrast, Mo enrichments do track historic O(2) depletion. Three factors cause redox control to override anthropogenic control in the case of Mo: relative to weathering fluxes, anthropogenic Mo fluxes are weaker than Re fluxes; during anoxic periods, Mn refluxing amplifies Mo but not Re concentrations near the sediment surface; and high pore water sulfide-polysulfide promotes Mo fixation in pyrite while promoting formation of organo-Re adducts; the latter are too mobile and reactive to preserve a reliable historic record under seasonally fluctuating redox conditions.

Extraordinary stability of copper(I)-tetrathiomolybdate complexes: possible implications for aquatic ecosystems

An extraordinary affinity of MoS₄²⁻ for Cu accounts for Mo-induced Cu deficiency in ruminants (molybdenosis) and offers an approach to treating Wilson's disease in humans. Evidence of thiomolybdates in sulfidic natural waters, and possibly even as metastable traces in oxic natural waters, raises the question of how Cu-Mo affinity might affect Cu availability or toxicity in aquatic ecosystems. Stabilities of inorganic Cu-MoS₄²⁻ complexes are characterized and quantified here for the first time. Two remarkably stable Cu(I) dissolved complexes are identified (T = 23°C ± 2°C): Cu₂(HS)₂MoS₄²⁻ and Cu₂S₂MoS₄⁴⁻. In addition, the solubility constant for a precipitate (NH₄CuMoS₄) was measured. Under the extremely reducing conditions in rumen fluids, these complexes will greatly suppress Cu(+) activity, supporting prior conclusions about the mechanism of molybdenosis. In sulfidic natural waters, they help to prevent complete Cu impoverishment, as might otherwise occur by sulfide mineral precipitation. On the other hand, the complexes discovered here are HS⁻-dependent and could not be important in oxic natural waters (with HS⁻ concentrations < 10⁻⁹ M) even if metastable, biogenic MoS₄²⁻ indeed were present as previously conjectured.

Sediment profiles of less commonly determined elements measured by Laser Ablation ICP-MS

Anthropogenic influences on trace element profiles in dated sediments from estuaries have been often documented, with the vast majority of studies focusing on a short list of high-abundance trace elements. Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) provides a new approach that minimizes sample preparation and contamination while yielding data on a much larger list of elements simultaneously. We present concentrations and enrichment factor profiles for 22 elements at a locality that is 50 km southeast of Baltimore, the principal industrial city on Chesapeake Bay. Samples representing deposition over almost the entire 20th century were obtained from two archived cores collected 20 years apart. The following elements exhibit profiles consistent with a strong anthropogenic influence, i.e. enrichment after 1920 followed by decline after ca.1980, possibly reflecting increased regulatory efforts: Mn, Co, Cu, Zn, Ag, Cd, In, Sn, Sb, Te, Tl, Pb and Bi. As expected, the redox-sensitive elements: Mo, Re and U have similar profiles to one another. Previously, the potentially hazardous elements, Ag, In, Sb, Te, Tl and Bi, have been measured only rarely in estuarine sediments and never in Chesapeake Bay. Our discovery that their profiles track those of well-known pollutants underscores a need to investigate their sources, transport and biogeochemical behavior. Several rarely determined trace elements, Ga, Ge and Nb, exhibit trendless profiles, as do the major elements, Ti and Fe.

Accumulation mechanism for metal chalcogenide nanoparticles at Hg0 electrodes: copper sulfide example

Mercury electrodes preconcentrate metal chalcogenide nanoparticles effectively, enabling their detection at submicromolar concentrations (as Sigma chalcogenide) by adsorptive cathodic stripping voltammetry. Understanding the unique behavior of nanoparticle analytes during preconcentration is critical for lowering detection limits and for quantification. A multistep mechanism is proposed on the basis of accumulation experiments with polydisperse copper sulfide (CuxS) nanoparticles. Particles first diffuse and adsorb at the Hg0 surface. When both the electrode and particles have negative surface potentials, this process resembles charge-impeded coagulation, obeying the Schulze-Hardy rule at various electrolyte strengths. Consequently, accumulation rates are surprisingly sensitive to electrolyte concentration. Choosing accumulation potentials where the electrode and particles have opposite surface potentials greatly improves collection efficiency, especially for the smallest particles. After adsorption, particles undergo transformations. One product is a more stable (harder to reduce) form of CuxS, interpreted to consist of adclusters or adlayers. A very significant (approximately 0.3 V) negative shift in reduction potential results from this transformation. Loss of analyte to at least one nonelectroactive product is also observed. Loss is greatest for the smallest particles and is sensitive to choice of accumulation potential. To improve accumulation efficiency, accumulation potentials more positive that the potential of zero charge of Hg electrodes are advantageous but care must be taken to remove dissolved chalcogenides under these conditions in order to avoid artifacts.

Voltammetric characterization of metal sulfide particles and nanoparticles in model solutions and natural waters

Voltammetric scans in sulfidic natural waters often reveal reduction peaks in the range -0.9 to -1.35 V versus Ag/AgCl. These peaks have been attributed to iron sulfide complexes or clusters. However, sols containing CuS nanoparticles now also are known to produce reduction peaks in this range. Here we investigate the voltammetric behavior of two additional metal sulfides at the Hg electrode in 0.55 M NaCl + 0.03 M NaHCO3 electrolyte, pH=8.5. We show that Pb and Hg sulfides, either as suspended powders or as precipitated nanoparticles, also yield cathodic peaks between -0.9 and -1.35 V, similar to peaks obtained with CuS and FeS. For precipitated nanoparticles, the position and shape of these reduction peaks change with ageing. Freshly formed nanoparticles produce less negative reduction peaks than aged nanoparticles. Peaks from aged nanoparticles often consist of two or more superimposed reduction peaks. When all other experimental parameters are held constant, the amount of nanoparticle analyte accumulated on the electrode increases with the amount of ageing (< or = 1 h). Addition of EDTA or acidification followed by purging can be used to distinguish PbS nanoparticles and Fe sulfide clusters from CuS and HgS nanoparticles or from colloidal S. This test was applied to interpret -0.9 to -1.35 V reduction peaks observed in two meromictic lakes. In conjunction with other evidence, this test suggests that FeS clusters are present in one case whereas colloidal S is present in the other. Interpreting -0.9 to -1.35 V voltammetric peaks observed in sulfidic natural waters requires caution, but these peaks are potentially rich sources of information about trace metal speciation.

Voltammetry of copper sulfide particles and nanoparticles: investigation of the cluster hypothesis

An association of Cu with sulfide in aerobic natural waters has been attributed to these components' coexistence in clusters of sizes intermediate between mononuclear complexes and colloidal particles. This hypothesis is investigated here. Copper sulfide solid phases display size-related voltammetric behavior at Hg electrodes. Suspensions of copper sulfide powders held at accumulation potentials of 0 to -0.2 V (vs Ag/AgCl) produce voltammetric peaks near -0.15, -0.65, and -0.95 V during subsequent cathodic scans. The first two peaks arise from electrochemically generated Cu-oxyhydroxides and HgS; the -0.95 V peak arises from reduction of sorbed copper sulfide particles. Nanoparticles of radius approximately 10(-8) m produce the third peak even without stirring or accumulation. Still smaller analytes give only the first two peaks. Published evidence alleging production of subnanometer copper sulfide clusters during titrations of Cu2+ and HS- was not reproduced when sulfide oxidation was avoided. Instead, such titrations apparently generate nanoparticles. The titration stoichiometry is 1/1, consistent with previous descriptions of this process: Cu2+ + HS- --> 1/2Cu2S x S0 (brown sol) --> CuS (green sol). Titrating Cu2+ into organic-rich (muscilaginous) Adriatic Sea water, which contains 10(-7) M natural thiols and sulfide, produces solid products. In the future, voltammetry might prove useful for studying semiconductor sulfide nanoparticles in nature.

Making chlorine greener: performance of alternative dechlorination agents in wastewater

The residual chlorine in chlorine-disinfected and dechlorinated wastewater was characterized using a liquid chromatograph that was switched between reversed-phase separation and flow injection analysis modes, permitting measurement of fractionated and total residual chlorine, respectively. Residuals were detected in the effluent of an operating wastewater treatment plant employing chlorine disinfection and sulfite dechlorination. Despite dechlorination, an estimated total residual chlorine of 3 microM (0.2 ppm as Cl2) was detected in the effluent. To improve dechlorination effectiveness, four alternative agents (ascorbic acid, iron, sulfite plus iodide mediator, thiosulfate) were compared to sulfite on laboratory-chlorinated wastewater. Listed in order of decreasing relative effectiveness, we found: iron metal >> sulfite plus iodide approximately = thiosulfate > sulfite >> ascorbic acid. Only the iron metal column was completely effective at rapidly removing all traces of residual chlorine.

Molybdenum scavenging by iron monosulfide

Molybdenum profiles in dated sediment cores provide useful historical information about anoxia in anthropogenically impacted natural waters but would be of greater service if Mo fixation mechanisms were better understood. Here, we explore Mo scavenging by precipitated FeS in a model system consisting of an FeIII-bearing kaolinite (KGa-1B) dispersed in NaHS solutions. Test solutions contain 18 microM thiomolybdates (mainly MoOS3(2-)). Optically measuring dissolved polysulfides monitors the rate of FeS production from FeIII minerals. Even though the exposed clay surface area is large (450 m2/L), the clay itself sorbs little Mo at pH 8.6. As FeS forms, Mo is taken up in initial Mo/Fe mole ratios of 0.04-0.06, irrespective of HS- concentration (4-40 mM range). After about a day, Mo expulsion from the solids begins, accompanied by net polysulfide consumption. These changes reflect recrystallization of amorphous FeS to more ordered products such as greigite. FeS captures some MoO4(2-) but captures thiomolybdates more effectively. Kaolinite accelerates conversion of MoOS3(2-) to MoS4(2-), as predicted previously, and thiomolybdates facilitate reduction of FeIII minerals in the clay compared to Mo-free solutions. FeS is a potentially effective, transient scavenging agent for Mo in sulfidic environments, although FeS2 and organic matter appear to be the ultimate sedimentary hosts.

Making chlorine greener: investigation of alternatives to sulfite for dechlorination

Inorganic and organic chloramines pose a threat to aquatic ecosystems that are exposed to discharges of treated and disinfected wastewater. Conventionally practiced dechlorination with sulfite reduces the most refractory organic chloramines too slowly to produce wastewater effluents that meet current ecosystem protection criteria in the United States (i.e. total residual chlorine < or =0.011mg Cl(2)/L in freshwaters). Seeking faster dechlorinating agents, we have measured the rates that four test chloramines (NH(2)Cl, N-Cl-piperidine, N-Cl-leucylalanine and N-Cl-alanylalanine) react with 10 selected reducing agents at pH 7.4 and pH 8.4. The aqueous-phase reducing agents that offer speed advantages over sulfite alone include dithionite, thiosulfate, and iodide-mediated sulfite. Ascorbic acid was the most reactive of the sulfur-free agents but was found to be slow relative to sulfite. The potential biological oxygen demand might constrain the choice of aqueous reductants. Metallic iron is shown to reduce inorganic and organic chloramines effectively. The implications of these results for wastewater chlorine reduction and analysis are discussed.

Production of macromolecular chloramines by chlorine-transfer reactions

Chlorination of treated wastewaters is undertaken to prevent dispersal of human pathogens into the environment. Except in well-nitrified effluents, the primary agents in chlorination, Cl2(g) or NaOCl(aq), are short-lived and quickly transfer oxidative chlorine to secondary agents (N-chloramines), which then participate in the disinfection process. Maturation of residual chlorine resulting from chlorine-transfer reactions is still poorly characterized. Using gel permeation and reversed-phase liquid chromatography combined with a novel, oxidant-specific detector, unanticipated trends during the maturation of residual chlorine in wastewater are identified. Within 2 min after addition of NaOCl, and continuing for several hours at least, significant amounts of oxidative chlorine are transferred to secondary agents that are moderately to strongly hydrophobic and to agents that have high relative molecular masses (Mr 1300-25000). It is hypothesized that hydrophobic stabilization of organic chloramines (RNHCl(o)) thermodynamically drives these transfers, making macromolecular chloramines the ultimate oxidative chlorine carriers. Macromolecular chloramines are expected to be sluggish oxidants, as observed in their reduction by sulfite, and are expected to be poor disinfectants. If transfer of oxidative chlorine to high Mr components occurs widely at treatment plants, then this phenomenon offers a new, physicochemical explanation for the well-known impotency of organic chloramines in wastewater disinfection.

Differential adsorption of molybdate and tetrathiomolybdate on pyrite (FeS2)

Molybdenum is a nutrient important for a variety of biological functions, most notably nitrogen fixation. Molybdenum availability is limited through sorption reactions, particularly in environments rich in sulfide minerals. This study examines the sorption of two major molybdenum species, molybdate (MoO4(2-)) and tetrathiomolybdate (MoS4(2-)), on synthetic pyrite (FeS2) as a function of solution composition. Both MoO4(2-) and MoS4(2-) partitioned strongly on FeS2 under a range of conditions and ionic strengths. Molybdate and tetrathiomolybdate adsorption obeyed a Langmuir isotherm with a calculated site density between 2 and 3 sites/nm2 under acidic and circumneutral conditions, which decreased to less than 1 site/ nm2 at pH 9. Although both MoO4(2-) and MoS4(2-) adsorbed most strongly under moderately acidic conditions, MoO4(2-) readily desorbed while MoS4(2-) remained adsorbed even at high pH. The reversibility of MoO4(2-) adsorption suggests the formation of labile surface complexes while MoS4(2-) likely forms strong inner-sphere complexes. X-ray absorption spectroscopy was used to determine the structure of the surface complexes. Molybdate formed bidentate, mononuclear complexes on FeS2. The Mo-S and Mo-Fe distances for tetrathiomolybdate on pyrite are consistent with the formation of Mo-Fe-S cubane-type clusters. The high affinity of MoS4(2-) for FeS2, as well as its resistance to desorption, supports the hypothesis that thiomolybdate species are the reactive Mo constituents in reduced sediments and may control Mo enrichment in anoxic marine environments.

Improving the recoveries of unstable N-chloramines determined by liquid chromatography-postcolumn electrochemical detection

Liquid chromatographic (LC) measurement of individual N-chloramines, which are key byproducts of wastewater and drinking water chlorination, could lead to more effective control of water disinfection. Such measurements are challenging because of analyte instability. A detector selective for N-chloramines is constructed based on postcolumn derivatization with iodide followed by reductive detection of the iodine product at a glassy carbon electrode. In flow injection (FIA) mode, the detector gives identical responses for a test set of four chemically diverse N-chloramines. In the LC mode, losses of the test compounds are observed when LC and FIA responses are compared and quantitated by introducing a relative response factor (RRF). Using the RRF, N-chloramine recoveries are evaluated as a function of multiple LC separation parameters. The highest recoveries are obtained using a reversed-phase (C18) column with an acetonitrile mobile phase and a pH 7.02 aqueous phosphate buffer. With these conditions, linear calibration curves are obtained for all test N-chloramines. The detection limits obtained are in the low 10(-7)-mol/L range, which is nearly tenfold better than previously reported and 10-1000-fold lower than total residual chlorine concentrations typically found in disinfected water and wastewater.

Antimony speciation in alkaline sulfide solutions: role of zerovalent sulfur

Antimony is subject to a lower drinking water standard than arsenic, its notorious group 15 cohort in the periodic table. Both elements often co-vary in nature and are fairly soluble under reducing, alkaline conditions. Of the two, much less is known about the environmental chemistry of Sb. Measurements of Sb solubility in sulfidic solutions equilibrated with stibnite (Sb2S3) and orthorhombic sulfur reveal the existence of two new complexes that may control Sb behavior in many reducing environments. Formation reactions and stability constants (23 +/- 2 degrees C) are HS- + S(s) + Sb2S3(s) <==> HSb2S5-, log K = -1.47 +/- 0.17; and HS- + 2S(s) + Sb2S3(s) <==> Sb2S6(2-) + H+, log K = -9.55 +/- 0.07. The first complex is a mixed-valence Sb(III,V) complex; the second is an Sb(V) complex. Their stability in sulfidic solutions may explain previously puzzling evidence of Sb(V) in natural anoxic environments. Owing to these complexes, zerovalent S can enhance stibnite solubility up to 3 orders of magnitude. In neutral-to-alkaline, reducing environments, less than 7 microM HS- will transform O-coordinated, electrically neutral Sb(OH)3o to predominantly anionic S-coordinated complexes. This transformation could diminish the adsorption of Sb to negatively charged mineral surfaces, lowering retardation factors in anoxic aquifers.

Hydrogen peroxide effects on chromium oxidation state and solubility in four diverse, chromium-enriched soils

High concentrations of H2O2 are being tested for in situ oxidation and remediation of buried organic contaminants in soils and groundwater. Peroxide is being considered as a direct chemical oxidant in Fenton-type reactions or as a source of oxidizing equivalents in bioremediation schemes. How H2O2 affects the oxidation state and solubility of Cr(III) and Cr(VI), common co-contaminants with organic chemicals, is explored here in four chemically diverse soils containing elevated levels of Cr. Soil contaminated with soluble Cr(VI) from chromite ore processing residue and soil containing high levels of recently reduced Cr (III) from electroplating waste both released dissolved Cr(VI) after single applications of up to 24 mM H2O2. In no case was there evidence that H202 reduced preexisting Cr(VI) to Cr(III), even though this would be allowed thermodynamically. Chromate in the leachates exceeded the U.S. EPA drinking water standard for total dissolved Cr (2 microM) by a factor of 10-1000. Anaerobic conditions in an organic-rich, tannery waste-contaminated soil protected Cr(III) from oxidation and mobilization. Mineral forms of Cr in serpentinitic soil near a former chromite mine also resisted oxidation on the time scale of days. Mobilization of Cr(VI) could be a hazardous consequence of using H2O2 for in situ remediation of chemically complex wastes, but H2O2 could prove attractive for ex situ treatment (i.e., soil washing). This paper demonstrates marked differences among Cr-contaminated soils in their capacity to release Cr(VI) upon chemical treatment with H2O2.

Investigation of the transport of metals and orthophosphate away from a sewage treatment plant outfall

The distribution of O2, chlorophyll a, alkalinity, phosphate, Ca, Mg, Mn, Fe, Cu, Zn, Cd and Pb has been measured in the discharge plume of the large sewage treatment plant on Back River, Maryland. The concentrations of phosphate and the trace metals decrease downstream more rapidly than can be accounted for by conservative dilution. Solubility calculations suggest that saturation with respect to phosphate or oxyhydroxide phases could limit the mobility of phosphate, Fe, Mn, and Pb. For Cu, Zn, and Cd, saturation is not a feasible control mechanism, and other processes such as biofixation or adsorption are probably involved.