Electron Transfer Between Sulfide and Humic Acid: Electrochemical Evaluation of the Reactivity of Sigma-Aldrich Humic Acid Toward Sulfide

Little is known of potential reactivity and redox properties of reduced dissolved organic matter (DOM), although DOM in anoxic environments, e.g., groundwater, peat soils, or lake sediments, can be expected to differ from DOM of oxidized environments. We therefore investigated the impact of electrochemical and wet chemical [hydrogen (H₂)/Pd catalyst] reduction in Sigma-Aldrich humic acid (HA) as a model DOM for high salinity, high ionic strength, or iron-rich systems on its reactivity toward sulfide. Mediated electrochemical measurement showed that the reactivity of HA toward sulfide decreased in the order non-reduced HA > electrochemically reduced (?0.1 V) HA > H₂/Pd-reduced HA > electrochemically reduced (?0.4 V) HA. Results indicated that measured initial values of electron-accepting capacities of HA had a strongly positive correlation with the sulfide transformation, except for the H₂/Pd treatment of HA. This latter treatment obviously changed HA structures and lead to a different reactivity toward sulfide, limiting a direct comparison to electrochemically reduced organic matter. Our result confirmed that reduced HA was still reactive toward sulfide, although to a lower extent compared with oxidized HA. Compared to electrochemical reduction, H₂/Pd pre-treatment of HA alters redox properties and reactivity of organic matter and may therefore lead to results that cannot be transferred to natural systems.     

Determination of the conditional interaction constant between colloidal technetium(IV) and Gorleben humic substances

Varying pertechnetate (Tc(VII)) doses were reduced to Tc(IV) in the presence and absence of Gorleben humic substances with the aid of magnetite, a reducing Fe(II)-containing surface. In absence of humic substances dissolved Tc(IV) concentrations are over-saturated with respect to the known TcO₂ · nH₂O solubility and increase with increasing Tc(VII) dose due to the formation of a range of mononuclear to colloidal Tc(IV) species. In presence of dissolved humic substances, the Tc solubility is enhanced due to the additional interaction of dissolved Tc(IV) species with humic substances. Both in the absence and the presence of dissolved humic substances a sorption mechanism controls the distribution of the range of mononuclear to colloidal Tc(IV) species between the solid and the liquid phase. The proposed reaction mechanism between Tc(IV) and HS is represented by Σ[TcO(OH)₂]_n+HS = [TcO(OH)₂]_n − HS in which Σ[TcO(OH)₂]_n stands for the sum of monomeric and polynuclear (colloidal) Tc(IV) species present in the equilibrium solution. A log K-value of 2.9 (±0.3) was quantified from a modified Schubert approach which is based on the competition of HS and magnetite for all dissolved Tc(IV) species and was found independent of Tc–HS loading, Tc–magnetite loading and pH.     

Lanthanide-humic substances complexation. I. Experimental evidence for a lanthanide contraction effect

The interaction of the lanthanides (Ln) with humic substances (HS) was investigated with a novel chemical speciation tool, Capillary Electrophoresis-Inductively Coupled Plasma Mass Spectrometry (CE-ICP-MS). By using an EDTA-ligand competition method, a bi-modal species distribution of LnEDTA and LnHS is attained, separated by CE, and detected online by sector field ICP-MS. We quantified the binding of all 14 rare earth elements (REEs), Sc and Y with Suwannee river fulvic acid, Leonardite coal humic acid, and Elliot soil humic acid under environmental conditions (pH 6-9, 0.001-0.1 mol L⁻¹ NaNO₃, 1-1000 nmol L⁻¹ Ln, 10-20 mg L⁻¹ HS). Conditional binding constants for REE-HS interaction (K_c) ranged from 8.9 < log K_c < 16.5 under all experimental conditions, and display a lanthanide contraction effect, Δ_LK_c: a gradual increase in K_c from La to Lu by 2-3 orders of magnitude as a function of decreasing ionic radius. HS polyelectrolyte effects cause K_c to increase with increasing pH and decreasing ionic strength. Δ_LK_c increases significantly with increasing pH, and likely with decreasing ionic strength. Based on a strong correlation between Δ_LK_c values and denticity for organic acids, we suggest that HS form a range of tri- to tetra-dentate complexes under environmental conditions. These results confirm HS to be a strong complexing agent for Ln, and show rigorous experimental evidence for potential REE fractionation by HS complexation.     

Solubility of Pt and Pd sulfides and Au metal in aqueous bisulfide solutions

An experimental study of the solubility of Pt and Pd sulfides and Au metal in aqueous bisulfide solutions was conducted at temperatures from 200° to 350 °C and at saturated vapor pressure. A 500-mL Bridgemantype pressure vessel constructed of titanium, and equipped with a motor-driven magnetic stirrer was employed. The pH and the oxidation state were buffered by the coexistence of H₂S/HS^–/SO _inf4 ^sup2– . The pH at temperature was calculated to be in the range 5.91–9.43, and S was 0.3–2.2 m. Under the experimental conditions, the measured solubility of gold is about two to three orders of magnitude greater than that of either platinum and palladium, and the measured solubility of platinum is, in general, approximately equal to that of palladium, in molal units. The solubilities are found to be in the range: platinum 4–800 ppb, palladium 1–400 ppb, and gold 2–300 ppm. The solubility data can be modeled adequately using the following reactions: Au+H₂S+HH^–=Au(HS) ₂ ^– +1/2H₂ (K₁₄); PtS+HS^–+H⁺=Pt (HS) ₂ ⁰ (K₁₅); PdS+HS^–+H⁺=Pd (HS) ₂ ⁰ (K₁₆); PtS₂+H₂=Pt (HS) ₂ ⁰ (K₂₁).With equilibrium constants determined as follows (errors represent two standard deviations): Preliminary measurements of the solubilities of metallic Pt, Pd and Au as hydroxide complexes were also conducted using a second titanium pressure vessel, at temperatures of 200° to 350 °C and vapor saturation pressure, with pH and the oxidation state controlled or buffered by adding known amounts of NaOH and H₂ gas. The concentration of NaOH was in the range 0.01–1.3 m, and the partial pressure of H₂ at 200 °C was 62–275 bars, initially. Under the temperature and pressure conditions of these experiments, the solubility of platinum in 1 m NaOH solution is less than 100 ppb, that of palladium is less than 10 ppb and that of gold is less than 0.2 ppm; and in 0.01 m NaOH solutions, both Pt and Pd solubilities are less than 1 ppb. These data indicate that the contributions of hydroxide complexes to the total solubilities in the bisulfide runs, where the pH was in the range of 5.9–9.4, are negligible. The concentrations of both Pt and Pd as bisulfide complexes in the Salton Sea geothermal system predicted using the stability constants determined in this work agree very well with those values measured by McKibben et al. (1990). This calculation strongly suggests that the PGE are transported in moderately reducing, near neutral hydrothermal fluids as bisulfide complexes, as is gold. However, the much lower maximum solubility of the PGE relative to gold severely constrains models of re genesis, and may explain the relative rarity of hydrothermal PGE deposits compared to the relative abundance of hydrothermal Au deposits.     

Nonaqueous titration of functional groups in humic acid

Humic acid was titrated by sodium methoxide in dimethylsulfoxide using platinum-calomel electrode systems. Adding benzoic acid and phenol as internal standards to humic acid yielded two inflections. The titer at the first inflection point was equivalent to the carboxyl groups whose pK_a (H₂O) values were less than 7. The difference between the titers at the two inflection points was equivalent to the phenolic hydroxyl groups whose pK_a (H₂O) values were 7–10. Calculated results for the carboxyl and phenolic hydroxyl groups in humic acid obtained by the nonaqueous titration method agreed closely with those obtained by conventional methods.     

Phase Equilibria in the Ternary Systems KBr–K2B4O7–H2O and KCl–K2B4O7–H2O at 373 K

According to the compositions of the underground gasfield brines in the west of Sichuan Basin,the phase equilibria in the ternary systems KBr-K2B4O7-H2O and KCl-K2B4O7-H2O at 373 K were studied using the isothermal dissolution equilibrium method.The solubilities of salts and the densities of saturated solutions in these ternary systems were determined.Using the experimental data,phase diagrams and density-composition diagrams were constructed.The two phase diagrams were simple co-saturation type,each having an invariant point,two univariant curves and two crystallization regions.The equilibrium solid phases in the ternary system KBr-K2B4O7-H2O are potassium bromide （KBr） and potassium tetraborate tetrahydrate （K2B4O7·4H2O）,and those in the ternary system KCl-K2B4O7-H2O are potassium chloride （KCl） and potassium tetraborate tetrahydrate （K2B4O7·4H2O）.Comparisons of the phase diagrams of the two systems at different temperatures show that there is no change in the crystallization phases,but there are changes in the size of the crystallization regions.As temperature increases,the solubility of K2B4O7·4H2O increases rapidly,so the crystallization field of K2B4O7·4H2O becomes smaller.     

Interactions between magnetite and humic substances: redox reactions and dissolution processes

Humic substances (HS) are redox-active compounds that are ubiquitous in the environment and can serve as electron shuttles during microbial Fe(III) reduction thus reducing a variety of Fe(III) minerals. However, not much is known about redox reactions between HS and the mixed-valent mineral magnetite (Fe₃O₄) that can potentially lead to changes in Fe(II)/Fe(III) stoichiometry and even dissolve the magnetite. To address this knowledge gap, we incubated non-reduced (native) and reduced HS with four types of magnetite that varied in particle size and solid-phase Fe(II)/Fe(III) stoichiometry. We followed dissolved and solid-phase Fe(II) and Fe(III) concentrations over time to quantify redox reactions between HS and magnetite. Magnetite redox reactions and dissolution processes with HS varied depending on the initial magnetite and HS properties. The interaction between biogenic magnetite and reduced HS resulted in dissolution of the solid magnetite mineral, as well as an overall reduction of the magnetite. In contrast, a slight oxidation and no dissolution was observed when native and reduced HS interacted with 500 nm magnetite. This variability in the solubility and electron accepting and donating capacity of the different types of magnetite is likely an effect of differences in their reduction potential that is correlated to the magnetite Fe(II)/Fe(III) stoichiometry, particle size, and crystallinity. Our study suggests that redox-active HS play an important role for Fe redox speciation within minerals such as magnetite and thereby influence the reactivity of these Fe minerals and their role in biogeochemical Fe cycling. Furthermore, such processes are also likely to have an effect on the fate of other elements bound to the surface of Fe minerals.

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Speciation of gold in hydrosulphide-rich ore-forming fluids: Insights from first-principles molecular dynamics simulations

To shed light on gold speciation in sulfur-containing ore-forming fluids, we perform first principles molecular dynamics (FPMD) simulations to investigate gold-hydrosulphide complexing under representative geological conditions. With this advanced technique, the electronic structures of solutes and solvents are calculated with density functional theory and the thermal motions are sampled with molecular dynamics. The molecular structures, solvated structures and stabilities of possible complexes are characterized in detail and the following insights have been gained. (1) The previously hypothesized species Au(HS)(H₂S)₃ and Au(HS) are found unstable under ore-forming conditions. Au(HS)(H₂S)₃ would dissociate to LAu(HS) (L = H₂S or H₂O) and free H₂S molecules spontaneously. Au(HS) is highly reactive and tends to capture a second ligand to form a double-coordinated complex. (2) In the thin vapor-like phases of low pressures, the stable complexes include Au(HS)(H₂O), Au(HS)(H₂S) and Au(HS)₂⁻ and their relative stability is Au(HS)₂⁻ > Au(HS)(H₂S) > Au(HS)(H₂O). In dense aqueous phases of high pressures, Au(HS)(H₂S) would spontaneously deprotonate to Au(HS)₂⁻ and thus Au(HS)(H₂O) and Au(HS)₂⁻ are the stable forms. All of these complexes can retain to the upper-limit of ore-forming temperatures. (3) The gold ions in the complexes do not favor coordinating more molecules and therefore the solvations happen mainly through H-bonding interactions between the ligands and environmental waters. H-bonds are found in vapor, liquid, and dense supercritical phases, whereas in the thin supercritical phase the hydration is very weak. These results provide quantitative and microscopic basis for understanding the speciation of gold in hydrothermal fluids.     

Aggregation of aquatic humic substances

Humic substances (HS) were isolated from Penwhirn Reservoir (PR) and Esthwaite Water (EW) and their removal from solution by centrifugation was studied as a function of pH, humic concentration and molecular weight, and CaCl₂ concentration. Large amounts (up to 50%) of PR HS could be removed but only small amounts (? 3%) of EW HS. At pH ? 5 removal of PR HS by Ca²⁺ can be explained satisfactorily in terms of decreases in humic solubilities induced by complexation with the cation. However, removal induced by protonation of the PR HS is unusual in that it decreases with increasing humic concentration.The results suggest that PR HS comprise a range of molecules differing in solubility, with the high-molecular-weight (40,000) components being the least soluble. The EW HS consist of molecules of weight-average molecular weight 5000 and resemble similarly sized PR HS in that they remain unaggregated in solution even when highly complexed with Ca²⁺.     

Influence of Mn oxides on the reduction of uranium(VI) by the metal-reducing bacterium Shewanella putrefaciens

The potential for Mn oxides to modify the biogeochemical behavior of U during reduction by the subsurface bacterium Shewanella putrefaciens strain CN32 was investigated using synthetic Mn(III/IV) oxides (pyrolusite [β-MnO₂], bixbyite [Mn₂O₃] and K⁺-birnessite [K₄Mn₁₄O₂₇ · 8H₂O]). In the absence of bacteria, pyrolusite and bixbyite oxidized biogenic uraninite (UO₂[s]) to soluble U(VI) species, with bixbyite being the most rapid oxidant. The Mn(III/IV) oxides lowered the bioreduction rate of U(VI) relative to rates in their absence or in the presence of gibbsite (Al[OH]₃) added as a non-redox-reactive surface. Evolved Mn(II) increased with increasing initial U(VI) concentration in the biotic experiments, indicating that valence cycling of U facilitated the reduction of Mn(III/IV). Despite an excess of the Mn oxide, 43 to 100% of the initial U was bioreduced after extended incubation. Analysis of thin sections of bacterial Mn oxide suspensions revealed that the reduced U resided in the periplasmic space of the bacterial cells. However, in the absence of Mn(III/IV) oxides, UO₂(s) accumulated as copious fine-grained particles external to the cell. These results indicate that the presence of Mn(III/IV) oxides may impede the biological reduction of U(VI) in subsoils and sediments. However, the accumulation of U(IV) in the cell periplasm may physically protect reduced U from oxidation, promoting at least a temporal state of redox disequilibria.     

Multi-method study of the characteristic chemical nature of aquatic humic substances isolated from the Han River,Korea

Natural organic matter (NOM) from the Han River, Korea was fractionated into humic and non-humic fractions by absorbing onto XAD-7HP, and these fractions were analyzed using UV-absorption, and for dissolved organic C (DOC). The humic fraction (i.e. humic substances; HS) was extracted and its characteristics were compared to commercial humic materials using various spectroscopic methods such as Fourier transform infrared (FT-IR), proton nuclear magnetic resonance (¹H-NMR) and fluorescence spectroscopy. The humic fraction as organic C was 47.0% on the average, however, a rainfall event brought a higher humic fraction into Han River water. The molar ratios of H/C and O/C in the HS from Han River water (HRHS) were 1.40 and 0.76, respectively, and the ratio of aliphatic to aromatic protons in the HS (P_Al/P_Ar ratio) was 5.8. Aromaticity and humification degree (i.e., degree of condensation) of HRHS were relatively lower than those from other humic materials, while the portion of oxygenated functional groups was relatively higher. FT-IR, ¹H-NMR and fluorescence spectroscopy showed distinct differences between HRHS and the commercial humic materials. Commercial humic materials are not representative of HS extracted from Han River water. The fluorescence spectra, relatively simple measurements, were found to be most useful as fingerprints for humic materials from particular sources.     

The adsorption of aquatic humic substances by two oxides of manganese

A study was made of the adsorption of humic substances (HS) by Mn₃O₄ and by oxide B, a preparation with the β-MnOOH diffraction pattern but having a manganese oxidation state of 3.4. The interactions follow trends found for other oxides. Thus in 0.01 mol dm^?3 NaCl adsorption decreases with increasing pH, while it is enhanced by Ca²⁺. The HS adsorb more strongly to the oxide with the higher zero point of charge (Mn₃O₄), while the effect of Ca²⁺ is greater for oxide B.Microelectrophoretic measurements show that the oxide particles take on the electrokinetic characteristics of the adsorbed HS. However it was found that the magnitude of the mobility depends on the underlying oxide surface and on the source of the HS. The electrokinetic properties of the two oxides dispersed in surface water samples of Esthwaite Water, Cumbria, England, can be accounted for by the adsorbed HS together with coadsorbed Ca²⁺.     

Iron(III) reduction and phosphorous solubilization in humid tropical forest soils

Phosphorus is one of the nutrients most commonly limiting net primary production in soils of humid tropical forests, mainly because insoluble Al and Fe phosphates and strong sorption to Fe(III) (hydr)oxides remove P from the bioavailable pool. Recent field studies have suggested, however, that this loss may be balanced by organic P accumulation under a wet moisture regime (>3350 mm annual precipitation). It has been hypothesized that, as the moisture regime changes from dry to mesic to wet, periods of anoxic soil conditions increase in intensity and duration, depleting Fe(III) (hydr)oxides and releasing sorbed P, but also slowing organic matter turnover, thus shifting the repository of soil P from minerals to humus. Almost no quantitative information is available concerning the coupled biogeochemical behavior of Fe and P in highly weathered forest soils that would allow examination of this hypothesis. In this paper, we report a laboratory incubation study of the effects of biotic Fe(III) (hydr)oxide reduction on P solubilization in a humid tropical forest soil (Ultisol) under a wet moisture regime (3000-4000 mm annual rainfall). The objectives of our study were: (1) to quantify Fe(III) reduction and P solubilization processes in a highly weathered forest soil expected to typify the hypothesized mineral dissolution-organic matter accumulation balance; (2) to examine the influence of electron shuttling on these processes using anthraquinone-2,6-disulfonate (AQDS), a well-known surrogate for the semiquinone electron shuttles in humic substances, as an experimental probe; and (3) to characterize the chemical forms of Fe(II) and P produced under anoxic conditions, both with and without AQDS. Two series of short-term incubation experiments were carried out, one without AQDS and another with an initial AQDS concentration of 150 μM. We measured pH, pE, and the production of Fe(II), total Fe [Fe(II) + Fe(III)], inorganic P, total P (inorganic P + organic P), and biogenic gases (CO₂, H₂ and CH₄). The same positive correlation was found between soluble P release and soluble Fe(II) production throughout incubation, implying that reduction of Fe(III) solubilized P. The Fe(II) produced was mainly particulate, evidently due to the formation of Fe(II) solid phases. Thermodynamic calculations indicated that precipitation of siderite and, in the presence of AQDS, vivianite was favored under the anoxic conditions that developed rapidly in the soil suspensions. Inorganic soluble P released during incubation was very small, indicating that the soluble P produced was mainly in organic form, which is consistent with the hypothesis that P accumulates in soil humus. Our net CO₂ production, H₂ consumption, and Fe(II) production data all suggested that reductive dissolution of Fe(III) (hydr)oxides was a terminal electron-accepting process coupled both to H₂ consumption and organic C oxidation by the native population of microorganisms in the soil. Addition of AQDS accelerated the production of Fe(II) and the release of soluble P, while hastening the decline in H₂ gas levels and suppressing CH₄ production. However, throughout incubation, the same quantitative relationships between soluble Fe(II) and P, and between pE and pH, were found, irrespective of AQDS addition. Thus we conclude that, in our soil incubation experiments, added AQDS functioned with the native microbial population solely as an electron shuttle catalyzing Fe(III) reduction. Whether humic substances in the soil also can act as electron shuttles in this way is a matter for future investigation.     

Solubility and migration ability of rhodium in natural conditions: model experimental data

The contents of dissolved rhodium species in the near-neutral environments have been studied for the first time and data on the interaction of Rh with organic matters of natural waters and its sorption behavior during contact with the components of geochemical barriers were obtained. The solubility method was used to analyze the behavior of rhodium hydroxide in the Rh(OH)_x–H₂O and Rh(OH)_x–H₂O–FA (fulvic acids) systems. The possible contents of inorganic species of rhodium and its compounds with humic organic ligands were determined within the pH range typical of surface waters. The solubility of rhodium shows a twoorder- of magnitude increase in the presence of humic matters (FA). The sorption interaction of the soluble rhodium species with the main components of geochemical barriers such as iron oxyhydroxides (III), (including fulvic-acid modified ones), alumosilicates, and precipitates of humic acids in contact with natural waters was studied. It was revealed that rhodium has the high affinity to all studied materials; its species are sorbed by ferrihydrite within several hours. It is suggested that rhodium is mainly transferred as colloid with suspended particulate matters of waters and then is accumulated in bottom sediments. The differences revealed in the sorption behavior of Pt(IV), Pd(II) and Rh(III) may be used to predict the distribution of the considered platinum group elements between the components of ecosystems.     

Experimental determination of the stability and stoichiometry of sulphide complexes of silver(I) in hydrothermal solutions to 400°C

The solubility of silver sulphide (acanthite/argentite) has been measured in aqueous sulphide solutions between 25 and 400°C at saturated water vapour pressure and 500 bar to determine the stability and stoichiometry of sulphide complexes of silver(I) in hydrothermal solutions. The experiments were carried out in a flow-through autoclave, connected to a high-performance liquid chromatographic pump, titanium sampling loop, and a back-pressure regulator on line. Samples for silver determination were collected via the titanium sampling loop at experimental temperatures and pressures. The solubilities, measured as total dissolved silver, were in the range 1.0 × 10⁻⁷ to 1.30 × 10⁻⁴ mol kg⁻¹ (0.01 to 14.0 ppm), in solutions of total reduced sulphur between 0.007 and 0.176 mol kg⁻¹ and pH_T,p of 3.7 to 12.7. A nonlinear least squares treatment of the data demonstrates that the solubility of silver sulphide in aqueous sulphide solutions of acidic to alkaline pH is accurately described by the reactions0.5Ag₂S(s) + 0.5H₂S(aq) = AgHS(aq) K_s,1110.5Ag₂S(s) + 0.5H₂S(aq) + HS⁻ = Ag(HS)2− K_s,122Ag₂S(s) + 2HS⁻ = Ag₂S(HS)22− K_s,232where AgHS(aq) is the dominant species in acidic solutions, Ag(HS)2− under neutral pH conditions and Ag₂S(HS)22− in alkaline solutions. With increasing temperature the stability field of Ag(HS)2− increases and shifts to more alkaline pH in accordance with the change in the first ionisation constant of H₂S(aq). Consequently, Ag₂S(HS)22− is not an important species above 200°C. The solubility constant for the first reaction is independent of temperature to 300°C, with values in the range logK_s,111 = −5.79 (±0.07) to −5.59 (±0.09), and decreases to −5.92 (±0.16) at 400°C. The solubility constant for the second reaction increases almost linearly with inverse temperature from logK_s,122 = −3.97 (±0.04) at 25°C to −1.89 (±0.03) at 400°C. The solubility constant for the third reaction increases with temperature from logK_s,232 = −4.78 (±0.04) at 25°C to −4.57 (±0.18) at 200°C. All solubility constants were found to be independent of pressure within experimental uncertainties. The interaction between Ag⁺ and HS⁻ at 25°C and 1 bar to form AgHS(aq) has appreciable covalent character, as reflected in the exothermic enthalpy and small entropy of formation. With increasing temperature, the stepwise formation reactions become progressively more endothermic and are accompanied by large positive entropies, indicating greater electrostatic interaction. The aqueous speciation of silver is very sensitive to fluid composition and temperature. Below 100°C silver(I) sulphide complexes predominate in reduced sulphide solutions, whereas Ag⁺ and AgClOH⁻ are the dominant species in oxidised waters. In high-temperature hydrothermal solutions of seawater salinity, chloride complexes of silver(I) are most important, whereas in dilute hydrothermal fluids of meteoric origin typically found in active geothermal systems, sulphide complexes predominate. Adiabatic boiling of dilute and saline geothermal waters leads to precipitation of silver sulphide and removal of silver from solution. Conductive cooling has insignificant effects on silver mobility in dilute fluids, whereas it leads to quantitative loss of silver for geothermal fluids of seawater salinity.     

Thio complexes of gold and the transport of gold in hydrothermal ore solutions

The solubility of gold in aqueous sulphide solutions has been determined from pH_20°C ≈ 4 to pH_20°C ≈ 9.5 in the presence of a pyrite-pyrrhotite redox buffer at temperatures from 160 to 300°C and 1000 bar pressure. Maximum solubilities were obtained in the neutral region of pH as, for example, with m_NaHS = 0.15 m, pH_20°C = 5.96, T = 309°C, P = 1000 bar where a gold solubility of 225 mg/kg was obtained. It was concluded that three thio gold complexes contributed to the solubility. The complex Au₂(HS)₂S^2? predominated in alkaline solution, the Au(HS)₂^? complex occurred in the neutral pH region, and in the acid pH region, it was concluded with less certainty that the Au(HS)° complex was present. Formation constants calculated forAu₂(HS)₂S^2? and Au (HS)₂^? emphasize their high stability. In the temperature range from 175 to 250°C, values of pβ for Au₂(HS)₂S^2? vary from ?53.0 to 47.9 (±1.6) and from ?23.1 to ?19.5 ( ± 1.5) for Au(HS)₂^?. Equilibrium constante for the dissolution reactions, $\text{Au° + H}{}_2\text{S + HS}{}^{\mathrm{?}}\text{? Au(HS)}{}_2{}^{\mathrm{?}}\text{+}\text{1}\text{2}\text{H}{}_2$ and 2Au° + H₂S + 2H8^? ? Au(HS)₂^? + H₂ vary from pK_m = +2.4 to +2.55 (±0.10) for Au₂(HS)₂S^2? and from pK_n = + 1.29 to + 1.19 (±0.10) for Au(HS)₂^? over the temperature range 175 to 250°C. Enthalpies of these dissolution reactions were calculated to be ΔH_m° = ?5.2 ±2.0 kcal/mol and ΔH_n° = +1.7 ±2.0 kcal/mol respectively. It was concluded that gold is probably transported in hydrothermal ore solutions as both thio and chloro complexes and may be deposited in response to changes in temperature, pressure, pH, oxidation potential of the system and total sulphur concentration.     

Partitioning and Hydrolysis of Nb and Ta and Their Implications with Regard to Mineralization

K₂NbOF₅ · H₂O and K₂TaF₇ were prepared through melting Nb₂O₅ and Ta₂O₅ respectively with KHF₂ · 2H₂O, followed by recrystallizing. The hydrolysis properties of K₂NbOF₅ and K₂TaF₇ were determined again by using a rapidly quench vessel. As temperature (from 250 to 550 °C) and pressure (from 500 to 1500 bars) increase, the degree of hydrolysis of both K₂NbOF₅ and K₂TaF₇ will increase. Nb- and Ta-fluorine complex compounds are instable in supercritical aqueous fluids. The degree of hydrolysis of both K₂NbOF₅ and K₂TaF₇ decreases with increasing concentration of HF, independent of the concentration of NaF. The partition coefficients of Nb and Ta between granitic melt and fluid phase are less than 0.15, i.e., most of Nb and Ta are left in granitic melt. The partition coefficient of Ta is more dependent on the concentration of HF than that of Nb. The significance of hydrolysis in Nb- and Ta- mineralization is also discussed in the present paper.     

Effects of heat treatment on fluorescence properties of humic substances from sandy soil in arid land and their Hg(II) binding behaviors

The topsoil temperature in arid areas of Xinjiang, China can be up to about 80°C in summer. This may significantly affect the chemical properties of soil humic substances. However, the effects of high temperature on characteristics of soil humic substances and their complexation with toxic metals are still poorly known. In the present study, binding of Hg(II) to unheated soil humic substances and heated soil humic substances from sandy soils was comparatively investigated using three-dimensional excitation?Cemission matrix (EEM) fluorescence spectroscopy. Two fluorescent peaks (peak I at Ex/Em?=?365?C370/470?C474?nm; peak II at Ex/Em?=?270?C275/468?C472?nm) identified as humic-like fluorescence were observed in the EEM spectra of humic substances. Both peaks were clearly quenched by Hg(II), indicating the strong interaction of humic-like substances with Hg(II), and showed blue shifts after heat treatment. Heat treatment caused an increase of the fraction of accessible fluorophore (f _a), binding sites number (n) and effective quenching constants (logK _a), indicating that more binding sites in humic substances could bind Hg(II) and form more stable humic substances?CHg(II) complexes after heat treatment. However, a decrease of binding constants (logK _b) suggested that heat treatment would reduce the binding capacity of each binding site of humic substances to Hg(II). This study implies the transport of Hg(II) may be affected by high temperature in the arid zone due to the modification of the physicochemical properties of humic substances in soil.     

Experimental study of Pd hydrolysis in aqueous solutions at 25–70°C

The hydrolysis of the Pd²⁺ ion in HClO₄ solutions was examined at 25–70°C, and the thermodynamic constants of equilibrium K ₍₁₎⁰ and K ₍₂₎⁰were determined for the reactions Pd²⁺ + H₂O = PdOH⁺ + H⁺ and Pd²⁺ + 2H₂O = Pd(OH)₂⁰ + 2H⁺, respectively. The values of log K ₍₁₎⁰ = −1.66 ± 0.5 (25°C) and −0.65 ± 0.25 (50°C) and log K ₍₂₎⁰ = −4.34 ± 0.3 (25°C) and −3.80 ± 0.3 (50°C) were derived using the solubility technique at 0.95 confidence level. The values of log K ₍₁₎⁰ = −1.9 ± 0.6 (25°C), −1.0 ± 0.4 (50°C), and −0.5 ± 0.3 (70°C) were obtained by spectrophotometric techniques. The palladium ion is significantly hydrolyzed at elevated temperatures (50–70°C) even in strongly acidic solutions (pH 1–1.5), and its hydrolysis is enhanced with increasing temperature.     

Adsorption and surface complexation modeling of palladium,rhodium and platinum in surficial semi-arid soils and sediment

Sorption isotherms, time-dependent adsorption and surface complexation modeling studies were used to investigate the post-depositional mobility of three of the platinum group-elements (Pd, Rh, and Pt) in semi-arid soil and sediment samples with varying surface properties. The acidity constants (Log K_a1 and Log K_a2), optimized from batch titration data, ranged from 4.69 to 5.34 for Log K_a1 and from −6.51 to −7.61 for Log K_a2, suggesting the occurrence of both protonation and deprotonation reactions on the solid surfaces. Partition coefficients and removal rates of the metals had a general trend of Pd > Pt > Rh. The sediment sample, with the highest clay content and exchangeable cation concentrations, also had the highest affinity for the metals. The times required for sediment to adsorb 63% of the metals were 2.63 h, 4.08 h and 10.64 h for Pd, Pt and Rh, respectively. The FITEQL program successfully optimized the conditional binding constants of the metals on the solids from batch adsorption data. The constants decreased in the order of Pd > Rh > Pt, which was consistent with the observed high affinity of the solids for Pd. The modeling results also showed that aqueous Pd was the least sensitive to pH followed by Rh and Pt. However, metal adsorption below the points of zero net proton charges (ca. pH 6.7) is attributable to the involvement of permanent negatively charged binding sites in the adsorption process. Notably, partition coefficients, removal rates and conditional binding constants all showed a high affinity of Pd for the solids. A similarity between the model outputs and the batch adsorption data indicates the suitability of the model for describing the mobility and retention of the three metals in semi-arid soils and sediments.