Speciation of Polysulfides and Zerovalent Sulfur in Sulfide-rich Water Wells in Southern and Central Israel

Zerovalent sulfur and inorganic polysulfides were determined in nine sulfide-rich water wells in central and southern Israel. Although the two locations belong to the same aquifer, they are characterized by different pH and hydrogen sulfide levels. Hydrogen sulfide in the central Israel wells ranged between 19 and 32 μM, and the pH was 7.26 ± 0.07. The southern basin is characterized by lower water circulation, lower pH (around 6.8), and higher hydrogen sulfide levels (>470 μM). Polysulfides were determined by a rapid single-phase methylation using methyl trifluoromethanesulfonate (methyl triflate) reagent. The summary polysulfide concentration for S₄²⁻–S₇²⁻ species was found to be around 0.14–0.75 μM in the central region of Israel and substantially higher, 2.3–4.6 μM in the southern region. The sum of polysulfide zerovalent sulfur and colloidal sulfur was quantitatively detected by cyanide derivatization and compared to polysulfide sulfur determined by methyl triflate derivatization and to the chloroform extraction of zerovalent sulfur. A method for the determination of sulfur undersaturation level—the ratio between dissolved elemental sulfur and its equilibrium concentration in the presence of solid sulfur—based on the observed levels of the major polysulfide species is described. The observed polysulfide speciation was compared with the predicted speciation under sulfur saturation conditions taking into account the water temperature, its ionic strength, and pH. Criteria for sulfur saturation versus unsaturated conditions were established based on (1) the chain length dependence of the ratio between the observed polysulfide concentrations and their predicted value under sulfur saturated conditions, and (2) the difference between the concentration of zerovalent sulfur, as determined by cyanolysis, and the total polysulfide sulfur. According to this dual criterion five of the water wells were classified as being undersaturated with respect to sulfur, though for all the examined water wells the majority of the zerovalent sulfur was in the form of polysulfide sulfur.     

Effect of surface oxide/hydroxide products on the collectorless flotation of copper-activated sphalerite

The change in collectorless flotation of sphalerite with pH and Cu(II) concentration was correlated with the type and proportion of species present on the sphalerite surface. The solution and surface species were determined using a combination of analytical techniques including zeta potential measurement and X-ray photoelectron spectroscopy. An optimum copper concentration for maximum sphalerite flotation was identified, beyond which flotation decreased. This decrease in flotation coincided with the precipitation of copper hydroxide in neutral to mildly alkaline pH conditions. The hydrophobic polysulfide and hydrophilic copper hydroxide species were the main surface species influencing sphalerite flotation.     

Marcasite precipitation from hydrothermal solutions

Pyrite and marcasite were precipitated by both slow addition of aqueous Fe²⁺ and SiO₃²⁻ to an H₂S solution and by mixing aqueous Fe²⁺ and Na₂S₄ solutions at 75°C. H₂S₂ or HS₂⁻ and H₂S₄ or HS₄⁻ were formed in the S₂O₃²⁻ and Na₂S₄ experiments, respectively. Marcasite formed at pH < pK₁ of the polysulfide species present (for H₂S₂, pK₁ = 5.0; for H₂S₄, pK₁ = 3.8 at 25°C). Marcasite forms when the neutral sulfane is the dominant polysulfide, whereas pyrite forms when mono-or divalent polysulfides are dominant. In natural solutions where H₂S₂ and HS₂ are likely to be the dominant polysulfides, marcasite will form only below pH 5 at all temperatures.

The pH-dependent precipitation of pyrite and marcasite may be caused by electrostatic interactions between polysulfide species and pyrite or marcasite growth surfaces: the protonated ends of H₂S₂ and HS₂ are repelled from pyrite growth sites but not from marcasite growth sites. The negative ions HS₂ and S₂²⁻ are strongly attracted to the positive pyrite growth sites. Masking of 1π_g^* electrons in the S₂ group by the protons makes HS₂ and H₂S₂ isoelectronic with AsS²⁻ and As₂²⁻, respectively ( et al., 1981). Thus, the loellingitederivative structure (marcasite) results when both ends of the polysulfide are protonated.

Marcasite occurs abundantly only for conditions below pH 5 and where H₂S₂ was formed near the site of deposition by either partial oxidation of aqueous H₂S by O₂ or by the reaction of higher oxidation state sulfur species that are reactive with H₂S at the conditions of formation e.g., S₂O₃²⁻ but not SO₄²⁻. The temperature of formation of natural marcasite may be as high as 240°C ( and , 1985), but preservation on a multimillion-year scale seems to require post-depositional temperatures of below about 160°C ( , 1973; and , 1985).     

Synchrotron XPS studies of solution exposed chalcopyrite,bornite, and heterogeneous chalcopyrite with bornite

Chalcopyrite, CuFeS₂, is an important source of copper and is recovered from ore by the flotation process. Chalcopyrite is commonly associated with other metal sulfides, e.g. bornite, Cu₅FeS₄. In this study the effect of bornite on the oxidation and leaching of chalcopyrite has been investigated by probing the surface evolution of pure chalcopyrite, bornite, and heterogeneous samples containing both chalcopyrite and bornite using Synchrotron X-ray Photoelectron Spectroscopy (SXPS). Samples were freshly fractured in a N₂ atmosphere and the resulting surfaces were oxidised in pH 9 KOH for 30 min or leached in pH 1 HCl for 2 h before being transferred into vacuum without leaving the N₂ atmosphere. Analysis of the chalcopyrite region of each sample indicates that exposure to pH 9 for 30 min when bornite is present results in a decreased concentration of hydrophobic polysulfide species (from 43% to 36% of the total S 2p spectrum). In addition to this decrease in hydrophobic species, there is an increase in the amount of hydrophilic sulfate on the surface, from trace amounts to 3%. For those samples leached at pH 1 there was a small decrease in the amount of polysulfide species (43% to 39%), but also a slight increase in disulfide species (16% to 19%) indicating an alteration to the oxidation process at low pH in the presence of bornite.     

Comparison of dissolution under oxic acid drainage conditions for eight sedimentary and hydrothermal pyrite samples

The abiotic oxidative dissolution behaviors of eight natural pyrite samples, five sedimentary and three hydrothermal, from various geological environments were compared under oxic conditions at pH 3 and 6 in a highly controlled batch reactor dissolution system. The three sedimentary pyrite samples associated with coal had greater specific surface areas and also exhibited greater apparent dissolution rates and extent than the other two sedimentary and three hydrothermal samples under both pH conditions. However, after normalizing for surface area, the dissolution rate constants for the different pyrite samples were similar; the greatest difference was between the two non-coal sedimentary pyrite samples. Pyrite morphology and the presence of trace metals could contribute to the differences in dissolution behavior as reflected in the normalized dissolution rates. The sulfur:iron ratio observed in the aqueous solution at pH 3 increased with time, but was always less than 2.0 (predicted from the stoichiometry of dissolution) for all the pyrite samples during the 24-h experimental duration. This can be explained by the disproportionation dissociation of thiosulfate, an initial product of pyrite dissolution, to elemental sulfur and sulfate which does not occur in a 1:1 ratio. The results of this work indicate the importance of extracting and using the specific pyrite(s) relevant to particular mining areas in order to understand pyrite dissolution rates and the influence of environmental conditions on those rates.     

Adsorption of Fe(II) and U(VI) to carboxyl-functionalized microspheres: The influence of speciation on uranyl reduction studied by titration and XAFS

The chemical reduction of U(VI) by Fe(II) is a potentially important pathway for immobilization of uranium in subsurface environments. Although the presence of surfaces has been shown to catalyze the reaction between Fe(II) and U(VI) aqueous species, the mechanism(s) responsible for the enhanced reactivity remain ambiguous. To gain further insight into the U-Fe redox process at a complexing, non-conducting surface that is relevant to common organic phases in the environment, we studied suspensions containing combinations of 0.1 mM U(VI), 1.0 mM Fe(II), and 4.2 g/L carboxyl-functionalized polystyrene microspheres. Acid-base titrations were used to monitor protolytic reactions, and Fe K-edge and U L-edge X-ray absorption fine structure spectroscopy was used to determine the valence and atomic environment of the adsorbed Fe and U species. In the Fe + surface carboxyl system, a transition from monomeric to oligomeric Fe(II) surface species was observed between pH 7.5 and pH 8.4. In the U + surface carboxyl system, the U(VI) cation was adsorbed as a mononuclear uranyl-carboxyl complex at both pH 7.5 and 8.4. In the ternary U + Fe + surface carboxyl system, U(VI) was not reduced by the solvated or adsorbed Fe(II) at pH 7.5 over a 4-month period, whereas complete and rapid reduction to U(IV) nanoparticles occurred at pH 8.4. The U(IV) product reoxidized rapidly upon exposure to air, but it was stable over a 4-month period under anoxic conditions. Fe atoms were found in the local environment of the reduced U(IV) atoms at a distance of 3.56 Å. The U(IV)-Fe coordination is consistent with an inner-sphere electron transfer mechanism between the redox centers and involvement of Fe(II) atoms in both steps of the reduction from U(VI) to U(IV). The inability of Fe(II) to reduce U(VI) in solution and at pH 7.5 in the U + Fe + carboxyl system is explained by the formation of a transient, “dead-end” U(V)-Fe(III) complex that blocks the U(V) disproportionation pathway after the first electron transfer. The increased reactivity at pH 8.4 relative to pH 7.5 is explained by the reaction of U(VI) with an Fe(II) oligomer, whereby the bonds between Fe atoms facilitate the transfer of a second electron to the hypothetical U(V)-Fe(III) intermediate. We discuss how this mechanism may explain the commonly observed higher efficiency of uranyl reduction by adsorbed or structural Fe(II) relative to aqueous Fe(II).     

Effect of the disproportionation reaction of ferrous iron in impact-evaporation processes

Evidence for the disproportionation of iron was found in model experiments imitating impact melting, evaporation, and condensation. The experiments were carried out using a laser system at a characteristic temperature of ～3000–4000 K and a pulse duration of ～10^?3 s in a He atmosphere (P = 1 atm). Augite and mixtures of peridotite with MnO₂ and WO₃ were used as starting target materials. Experimental products (condensed vapor phase) were analyzed by X-ray photoelectron spectroscopy. The results of condensate analysis provided compelling evidence for the presence of iron in three oxidation states (Fe⁰, Fe²⁺, and Fe³⁺). In an experiment with augite, the proportions of iron species of different valences were similar to the stoichiometry of the disproportionation reaction. Similar evidence for this reaction was first found in a condensate from the samples of the fine fraction of the Luna 16 regolith. In the layers of the lunar condensate, the proportions of the valence states of iron were on average Fe⁰:Fe²⁺:Fe³⁺ = 1.2: 1.9: 0.7.     

The dissolution of calcite in CO2-saturated solutions at 25°C and 1 atmosphere total pressure

The dissolution of Iceland spar in CO₂-saturated solutions at 25°C and 1 atm total pressure has been followed by measurement of pH as a function of time. Surface concentrations of reactant and product species have been calculated from bulk fluid data using mass transport theory and a model that accounts for homogeneous reactions in the bulk fluid. The surface concentrations are found to be close to bulk solution values. This indicates that calcite dissolution under the experimental conditions is controlled by the kinetics of surface reaction. The rate of calcite dissolution follows an empirical second order relation with respect to calcium and hydrogen ion from near the initial condition (pH 3.91) to approximately pH 5.9. Beyond pH 5.9 the rate of surface reaction is greatly reduced and higher reaction orders are observed. Calculations show that the rate of calcite dissolution in natural environments may be influenced by both transport and surface-reaction processes. In the absence of inhibition, relatively short times should be sufficient to establish equilibrium.     

Low-temperature, polysulfide reactions of conjugated ene carbonyls: A reaction model for the geologic origin of S-heterocycles

The theoretical basis for a type of reaction of sulfur-acceptor carbon molecules with donor catenated forms of sulfur is presented. The reactions between conjugated ene carbonyls as sulfur-acceptors and polysulfide as a sulfur donor are described. The scope of this reaction is broadened by demonstrating that the room-temperature action of polysulfide on dienones produces thiophenes. Evidence is given for the structure of 2-benzoyl-5-phenylthiophene, resulting from 1,5-diphenyl-2,4-pentadien-1-one, and 2-acetyl-5-phenylthiophene, resulting from 6-phenyl-3,5-hexadien-2-one. Evidence is also presented for the structure of 2,4-dibenzoyl-3,5-diphenylthiolane, the product resulting from the action of polysulfide on the mixture of benzaldehyde and acetophenone. The same thiolane is shown to result as the minor thiolane component when chalcone reacts with polysulfide in 80% water/20% ethanol. The major component is a known rearranged thiolane identified by its physical properties. The reactiol of chalcone and polysulfide in ethanol at 0 or 25°C results in thiolane formation as evidenced by the isolation of 2,4-dibenzoyl-3,5-diphenylthiolane. However, the reaction of the same two compounds in boiling methanol (65°C) results in a competitive hydrogenation of chalcone giving dihydrochalcone in addition to the formation of thiolane. The demonstration of the low-temperature reaction of polysulfide with conjugated ene carbonyls in the present work adds an additional pathway for possible formation of S-heterocycles in the geosphere.     

Partitioning of major and trace inorganic contaminants in fly ash acid mine drainage derived solid residues

Acid mine drainage was reacted with coal fly ash over a 24 h reaction time and species removal trends evaluated. The evolving process water chemistry was modeled by the geochemical code PHREEQC using WATEQ4 database. Mineralogical analysis of the resulting solid residues was done by X-ray diffraction analysis. Selective sequential extraction was used to evaluate the transfer of species from both acid mine drainage and fly ash to less labile mineral phases that precipitated out. The quantity of fly ash, volume of acid mine drainage in the reaction mixture and reaction time dictated whether the final solution at a given contact time will have a dominant acidic or basic character. Inorganic species removal was dependent on the pH regime generated at a specific reaction time. Sulphate concentration was controlled by precipitation of gypsum, barite, celestite and adsorption on iron-oxy-hydroxides at pH > 5.5. Increase of pH in solution with contact time caused the removal of the metal ions mainly by precipitation, co-precipitation and adsorption. PHREEQC predicted precipitation of iron, aluminium, manganese-bearing phases at pH 5.53–9.12. An amorphous fraction was observed to be the most important in retention of the major and minor species at pH > 6.32. The carbonate fraction was observed to be an important retention pathway at pH 4–5 mainly due to initial local pockets of high alkalinity on surfaces of fly ash particles. Boron was observed to have a strong retention in the carbonate fraction.     

Equilibria in a sulfide rich water from Enghien-les-Bains,France

A complete analysis of a sulfide rich water from a sedimentary area has been achieved. The formation of metastable sulfur species (polysulfide ions, colloidal sulfur and thiosulfate) is very important. The relative concentrations of the sulfur species is controlled by bacterial processes (Desulfovibrio and Thiobacteriaceae). Electrochemical measurements and results of the analyses are in agreement. A possible repartition of polysulfide ions is S^2?₆ ≈- S^2?₅ >S^2?₄. This repartition, although out of equilibrium, is characteristic of the processes leading to the formation of the metastable sulfur species.The water is in equilibrium with amorphous FeS formation. When sulfide, polysulfide and thiosulfate complexing of trace metals Cu, Cd and Pb is taken into account, an agreement is reached between their concentrations in water and their concentrations in the FeS precipitate.     

Sorption isotherms: A review on physical bases,modeling and measurement

The retention (or release) of a liquid compound on a solid controls the mobility of many substances in the environment and has been quantified in terms of the “sorption isotherm”. This paper does not review the different sorption mechanisms. It presents the physical bases underlying the definition of a sorption isotherm, different empirical or mechanistic models, and details several experimental methods to acquire a sorption isotherm. For appropriate measurements and interpretations of isotherm data, this review emphasizes 4 main points: (i) the adsorption (or desorption) isotherm does not provide automatically any information about the reactions involved in the sorption phenomenon. So, mechanistic interpretations must be carefully verified. (ii) Among studies, the range of reaction times is extremely wide and this can lead to misinterpretations regarding the irreversibility of the reaction: a pseudo-hysteresis of the release compared with the retention is often observed. The comparison between the mean characteristic time of the reaction and the mean residence time of the mobile phase in the natural system allows knowing if the studied retention/release phenomenon should be considered as an instantaneous reversible, almost irreversible phenomenon, or if reaction kinetics must be taken into account. (iii) When the concentration of the retained substance is low enough, the composition of the bulk solution remains constant and a single-species isotherm is often sufficient, although it remains strongly dependent on the background medium. At higher concentrations, sorption may be driven by the competition between several species that affect the composition of the bulk solution. (iv) The measurement method has a great influence. Particularly, the background ionic medium, the solid/solution ratio and the use of flow-through or closed reactor are of major importance. The chosen method should balance easy-to-use features and representativity of the studied natural conditions.     

热活化过硫酸盐降解氧氟沙星特性及响应面优化

为了探究热活化过硫酸盐（PS）技术对水中氧氟沙星（OFX）的氧化降解作用,考察了反应温度、体系的初始pH、PS的初始浓度、OFX的初始浓度对OFX降解效果的影响;并在单因素实验的基础上,选取反应时间、体系的初始pH、PS的初始浓度和OFX的初始浓度4个因素进行了响应面优化实验。结果表明：最佳降解条件为,反应温度60℃、PS初始浓度4.0 mmol/L、pH=4.7、OFX初始浓度0.03 mmol/L、反应时间60 min,此时OFX的降解率为81.29%;4个因素对热活化PS降解OFX均有影响,其影响显著性从大到小为反应时间、OFX的初始浓度、PS的初始浓度、初始pH。利用响应曲面法模拟出反应体系的最佳条件,经实验验证,OFX降解率为93.78%,与预测最佳结果95.00%基本相符,表明模型可靠有效。     

A comparison between heavy metals released from soil and its efficient speciation extracted by sequential extraction procedure

A simulating experiment was carried out on the interaction between natural precipitation and soil. The results demonstrated that the contents of heavy metals （V, Co, Cr, and Ni） released from soil into the solution under Earth＇s surface conditions are higher than the contents of those metals bonded to exchangeable species, which were extracted by sequential extraction procedure recommended by Tessier and others in 1979. It is demonstrated that the metals bonded to other 3 species （carbonate, Fe-Mn oxide, and organic matter） except those bonded to the exchangeable species in efficient speciation can be released under the Earth＇s surface conditions, when pH=4 in the reaction system, and the higher correlation coefficient indicated that the concentrations of heavy metals released from soil into the solution vary approximately with reaction time in terms of index regulations.     

Isotopic fractionation and reaction kinetics between Cr(III) and Cr(VI) in aqueous media

The redox-sensitive stable isotope geochemistry of chromium bears the potential to monitor the attenuation of chromate pollution and to investigate changes in environmental conditions in the present and the past. The use of stable Cr isotope data as a geo-environmental tracer, however, necessitates an understanding of the reaction kinetics and Cr fractionation behaviour during redox transition and isotope exchange. Here, we report stable chromium isotope fractionation data for Cr(VI) reduction, Cr(III) oxidation and isotopic exchange between soluble Cr(III) and Cr(VI) in aqueous media. The reduction of Cr(VI) to Cr(III) with H₂O₂ under strongly acidic conditions shows a near-equilibrium isotope fractionation of Δ^53/52Cr_{(Cr(III)-Cr(VI))} of −3.54 ± 0.35‰. At pH neutrality, however, the reduction experiments show a kinetic isotope fractionation Δ^53/52Cr_{(Cr(III)-Cr(VI))} of −5‰ for the extent of reduction of up to 85% of the chromium. The oxidation of Cr(III) to Cr(VI) in alkaline media, using H₂O₂ as the oxidant, cannot be explained by a single, unidirectional reaction. Our experiments indicate that the involvement of the unstable intermediates Cr(IV) and Cr(V) and their disproportionation during redox reactions between Cr(III) and Cr(VI) influence the overall fractionation factor, depending on the prevailing pH conditions and the reaction rates. No detectable isotope exchange between soluble Cr(VI) and Cr(III) species at pH values of 5.5 and 7 was revealed over a timescale of days to weeks. This means that, at least within such a time frame, the isotopic composition of Cr(VI) in a natural system will not be influenced by equilibration with any Cr(III) and thus reveal the true extent of reduction, given that the Cr isotope composition of the source Cr(VI) and the fractionation factor for the prevailing conditions are known.     

Valence state of iron in a condensate from the Luna 16 regolith

This paper reports the results of an X-ray photoelectron spectroscopic study of the condensate phase of regolith sample L1639 returned by the Luna 16 mission. The reduced Si⁰, Si²⁺, Al⁰, Ti²⁺, and Ti³⁺ forms were detected in the sample. Iron occurs in all valence states, and Fe³⁺ species were detected for the first time in the condensate. Minor Fe³⁺ concentrations were observed in the upper layers of the sample containing the maximum amounts of condensate products. The fraction of ferric Fe is 22%, and the Fe⁰: Fe²⁺: Fe³⁺ proportion is 33: 45: 22. The appearance of ferric Fe in the lunar condensate is explained by the reaction of FeO disproportionation occurring either at the stage of the expansion and cooling of impact-related vapor or directly in the condensed phase on the surface of regolith particles. This interpretation is supported by the results of a model experiment on augite vaporization and condensation. The experiment simulating impact vaporization was carried out on a laser set-up at a temperature of ∼3000–4000 K and a pulse duration of ∼10⁻³ s in a He atmosphere (P = 1 atm). The results of analyses provided compelling evidence that the condensate produced after augite vaporization contains Fe in all oxidation states, and the proportions of different valence forms approach the stoichiometry of the disproportionation reaction.     

Protocol for Quantitative Detection of Elemental Sulfur and Polysulfide Zero-Valent Sulfur Distribution in Natural Aquatic Samples

A HPLC-based protocol has been developed for the determination of zero-valent sulfur (ZVS) speciation, including solid, colloidal elemental sulfur and individual inorganic polysulfides in natural aquatic samples. The protocol includes four experimental procedures: (1) determination of polysulfide speciation by rapid single-phase derivatisation with methyl trifluoromethanesulfonate; (2) determination of the sum of polysulfide and colloidal sulfur by reaction with hydrogen cyanide (cyanolysis); (3) determination of total zero-valent sulfur by treatment with zinc chloride followed by extraction with chloroform; and (4) chromatographic determination of polythionates without sample pre-treatment. With proper sampling and preservation techniques in the field or on board ship, this combination of methods allowed the quantitative determination of: (a) individual polysulfide species; (b) dispersed colloidal sulfur; (c) dispersed solid elemental sulfur; and (d) tetra-, penta- and hexathionates. With minor modification, the method could be expanded to include other polythionates. Sixteen various wet chemical and liquid chromatographic methods were tested on nine synthetic reference samples (including solid elemental sulfur, colloidal elemental sulfur, inorganic polysulfides and polythionates) to establish the optimal protocol. The protocol was further evaluated by analysing the zero-valent sulfur content in microbially-produced sulfur and in sulfur from two natural samples of sulfide-rich seawater from tidal flats pools of the Wadden Sea (Germany).     

Continuum model for simultaneous chemical reactions and mass transport in hydrothermal systems

A time-space continuum model for transport of hydrothermal fluids in porous media is presented which provides for simultaneous, reversible and irreversible chemical reactions involving liquids, gases and minerals. Homogeneous and heterogeneous reactions are incorporated in the model in a similar fashion through source/sink terms added to the continuity equation. The model provides for moving reaction fronts through surfaces of discontinuity across which occur jump discontinuities in the various field variables satisfying generalized Rankine-Hugoniot relations. Reversible reactions including aqueous complexing, oxidation-reduction reactions, mineral precipitation and dissolution reactions and adsorption are explicitly accounted for by imposing chemical equilibrium constraints in the form of mass action equations on the transport equations. This is facilitated by partitioning the reacting species into primary and secondary species corresponding to a particular representation of the stoichiometric reaction matrix referred to as the canonical representation. The transport equations for the primary species combined with homogeneous and heterogeneous equilibria result in a system of coupled, nonlinear algebraic/partial differential equations which completely describe the evolution of the system in time. Spatially separated phase assemblages are accommodated in the model by altering the set of independent variables across surfaces of discontinuity. Constitutive relations for the fluid flux corresponding to primary species are obtained describing transport of both neutral and charged species by advection, dispersion and diffusion. Numerical implementation of the transport equations is considered and both explicit and implicit finite difference algorithms are discussed. Analytical expressions for the change in porosity and permeability with time are obtained for an assemblage of minerals reacting reversibly with a hydrothermal fluid under quasi-steady state conditions. Fluid flow is described by Darcy's law employing a phenomenological expression relating permeability and porosity. Finally an expression for the local retardation factor of solute species is derived for the case of advective transport in a single spatial dimension which accounts for the effects of homogeneous and heterogeneous equilibria including adsorption on the rate of advance of a reaction front. The condition for the formation of shock waves is given.     

Formation of sulfur and nitrogen cross-linked macromolecules under aqueous conditions

Polysulfides and ammonia are abundant in young depositional environments and play an important role in the formation of macromolecular structures such as protokerogen and humics. In this work, we study the co-incorporation of polysulfides and ammonia into simple carbonyl model compounds, octanal and trans 2-octenal, in order to study their effect on the formation of a cross-linked macromolecule and suggest a feasible mechanism. The reactions, performed in aqueous solutions at ambient temperature and pH ∼6 to 9, simulate formation of S and N cross-linked polymers in the natural environment. The complex S and N containing polymer was studied by ¹⁵N enrichment coupled to 2D NMR (¹H, ¹³C, ¹⁵N) techniques and chemical degradation of S-S bonds followed by deuterium labeling and GC-MS analyses. In addition, molecular modeling techniques were used to provide theoretical interpretations and important insights at the molecular level. The results indicate that polysulfide out competes ammonia in the formation of Michael adducts while ammonia is equally competitive with polysulfides when the reaction is addition to the carbonyl position. The co-incorporation of ammonia and polysulfides into carbonyls rapidly forms N and S cross-linked polymers. The effects of ammonia and amines on the polymerization processes are by two means: (i) reaction with carbonyls through an imine functionality to form oligomers and polymers and (ii) catalysis of sulfur nucleophiles onto carbonyls by transfer of a proton which enhances the rate of polymerization. A similar catalytic effect is observed when glycine is used instead of ammonia. This mechanism is especially important under basic to neutral conditions like those that prevail in marine environments. The results show that ammonia and glycine or possibly other amino acids and/or peptides are intimately involved with sulfur nucleophiles throughout the polymerization processes that occur at low temperatures and thus are suggested as key reactants in diagenetic formation of protokerogen and humics.