Aqueous SO2 Research Articles

The oxygen isotope equilibrium fractionation between sulfite species and water

Sulfite is an important sulfoxy intermediate in oxidative and reductive sulfur cycling in the marine and terrestrial environment. Different aqueous sulfite species exist, such as dissolved sulfur dioxide (SO2), bisulfite (HSO3−), pyrosulfite (S2O52−) and sulfite sensu stricto (SO32−), whereas their relative abundance in solution depends on the concentration and the pH. Conversion of one species into another is rapid and involves in many cases incorporation of oxygen from, or release of oxygen to, water (e.g. SO2+H2O↔HSO3−+H+), resulting in rapid oxygen isotope exchange between sulfite species and water. Consequently, the oxygen isotope composition of sulfite is strongly influenced by the oxygen isotope composition of water. Since sulfate does not exchange oxygen isotopes with water under most earth surface conditions, it can preserve the sulfite oxygen isotope signature that it inherits via oxidative and reductive sulfur cycling. Therefore, interpretation of δ18OSO42- values strongly hinges on the oxygen isotope equilibrium fractionation between sulfite and water which is poorly constrained. This is in large part due to technical difficulties in extraction of sulfite from solution for oxygen isotope analysis.To overcome these challenges, anoxic isotope equilibration experiments were performed with dissolved sodium sulfite in solutions with distinct oxygen isotope signatures. Sulfite was precipitated using two different agents, barium chloride and silver nitrate. The experiments were performed at 22°C and varying pH of 1.5, 6.3, 6.6, and 9.7 to investigate how changes in sulfite speciation affect the oxygen isotope equilibrium fractionation between sulfite and water.From the experiments at pH 1.5 where SO2 is the dominant sulfite species, a rough estimate of 37.0‰ was determined for the oxygen isotope equilibrium fractionation factor between aqueous SO2 and water (εSO2↔H2OEQ). The oxygen isotope equilibrium fractionation between the aqueous phases is much larger than the known oxygen isotope equilibrium fractionation between gaseous SO2 and water vapor, probably because of a stronger association with water molecules. At pH values of 6.3–9.7 a more firm estimate for the oxygen isotope equilibrium fractionation between HSO3−, SO32− and water (εSO32-↔H2OEQ) of 15.2±0.7‰ was obtained.Our results provide new insights into the oxygen isotope fractionation during reductive and oxidative sulfur cycling. They demonstrate that isotope exchange between sulfite and water during dissimilatory sulfate reduction (DSR) alone is too small to be responsible for the apparent oxygen isotope equilibrium fractionation between sulfate and water mediated by DSR. Our estimates also provide a basis for tracing and quantifying the transformation of sulfoxy intermediates during the oxidation of reduced sulfur compounds to sulfate.

Sulfur isotope fractionation during heterogeneous oxidation of SO<sub>2</sub> on mineral dust

Abstract. Mineral dust is a major fraction of global atmospheric aerosol, and the oxidation of SO2 on mineral dust has implications for cloud formation, climate and the sulfur cycle. Stable sulfur isotopes can be used to understand the different oxidation processes occurring on mineral dust. This study presents measurements of the 34S/32S fractionation factor α34 for oxidation of SO2 on mineral dust surfaces and in the aqueous phase in mineral dust leachate. Sahara dust, which accounts for ~60% of global dust emissions and loading, was used for the experiments. The fractionation factor for aqueous oxidation in dust leachate is αleachate = 0.9917±0.0046, which is in agreement with previous measurements of aqueous SO2 oxidation by iron solutions. This fractionation factor is representative of a radical chain reaction oxidation pathway initiated by transition metal ions. Oxidation on the dust surface at subsaturated relative humidity (RH) had an overall fractionation factor of αhet = 1.0096±0.0036 and was found to be almost an order of magnitude faster when the dust was simultaneously exposed to ozone, light and RH of ~40%. However, the presence of ozone, light and humidity did not influence isotope fractionation during oxidation on dust surfaces at subsaturated relative humidity. All the investigated reactions showed mass-dependent fractionation of 33S relative to 34S. A positive matrix factorization model was used to investigate surface oxidation on the different components of dust. Ilmenite, rutile and iron oxide were found to be the most reactive components, accounting for 85% of sulfate production with a fractionation factor of α34 = 1.012±0.010. This overlaps within the analytical uncertainty with the fractionation of other major atmospheric oxidation pathways such as the oxidation of SO2 by H2O2 and O3 in the aqueous phase and OH in the gas phase. Clay minerals accounted for roughly 12% of the sulfate production, and oxidation on clay minerals resulted in a very distinct fractionation factor of α34 = 1.085±0.013. The fractionation factors measured in this study will be particularly useful in combination with field and modelling studies to understand the role of surface oxidation on clay minerals and aqueous oxidation by mineral dust and its leachate in global and regional sulfur cycles.

The sources and budget for dissolved sulfate in a fractured carbonate aquifer, southern Sacramento Mountains, New Mexico, USA

Abstract Climate change in the SW USA is likely to involve drier conditions and higher surface temperatures. In order to better understand the evolution of water chemistry and the sources of aqueous SO4 in these semi-arid settings, chemical and S isotope compositions were determined of springs, groundwater, and bedrock associated with a Permian fractured carbonate aquifer located in the southern Sacramento Mountains, New Mexico, USA. The results suggest that the evolution of water chemistry in the semi-arid carbonate aquifer is mainly controlled by dedolomitization of bedrock, which was magnified by increasing temperature and increasing dissolution of gypsum/anhydrite along the groundwater flow path. The δ34S of dissolved SO4 in spring and groundwater samples varied from +9.0‰ to +12.8‰, reflecting the mixing of SO4 from the dissolution of Permian gypsum/anhydrite (+12.3‰ to +13.4‰) and oxidation of sulfide minerals (−24.5‰ to −4.2‰). According to S isotope mass balance constraints, the contribution of sulfide-derived SO4 was considerable in the High Mountain recharge areas, accounting for up to ∼10% of the total SO4 load. However, sulfide weathering decreased in importance in the lower reaches of the watershed. A smaller SO4 input of ∼2–4% was contributed by atmospheric wet deposition. This study implies that the δ34S variation of SO4 in semi-arid environments can be complex, but that S isotopes can be used to distinguish among the different sources of weathering. Here it was found that H2SO4 dissolution due to sulfide oxidation contributes up to 5% of the total carbonate weathering budget, while most of the SO4 is released from bedrock sources during dedolomitization.

Geochemical evaluation of high sulfate levels in Ponca Creek, northeastern Nebraska

Most surface water and shallow groundwater occurring in northeastern Nebraska are of the calcium bicarbonate type, with minor concentrations (e.g., 10–200 mg/L) of sulfate (SO4). Examination of historical water quality data (major cations and anions) for Ponca Creek, a predominantly ephemeral stream in northeastern Nebraska, revealed that SO4 concentrations ranged from about 110 to almost 1000 mg/L and contribute to a calcium sulfate hydrochemical facies. Consequently, most SO4 concentrations were above the U.S. Environmental Protection Agency secondary maximum contaminant level in drinking water of 250 mg/L. Sulfate concentrations for the same period for a nearby stream, Verdigre Creek, range from about 20 to 120 mg/L. Research into probable sources of the elevated SO4 in Ponca Creek revealed that a Late Cretaceous shale, the Pierre Shale, occurs at or near the land surface throughout most of the creek's drainage area, whereas alluvium, other Quaternary deposits, or the Tertiary Ogallala Formation comprises the streambed in Verdigre Creek. The Pierre Shale, encompassing soils formed on this Cretaceous shale, is rich in sulfate-bearing minerals (e.g., gypsum, pyrite, jarosite) that comprise the principal source of the high sulfate in drainage basin soils, alluvium, creek discharge, and shallow groundwater of the Ponca Creek watershed. A public domain geochemical speciation software (Visual MINTEQ) was used to investigate aqueous SO4 geochemistry of Ponca Creek flow. Calculated saturation indices for Ponca Creek waters suggest that they are slightly undersaturated with respect to gypsum and anhydrite despite significant sulfate dissolution and are slightly supersaturated with respect to calcite in numerous samples.

ChemInform Abstract: The Surface-Mediated Autoxidation of Aqueous SO2 in Ceramic Powder Suspensions

ChemInform Abstract: The Surface-Mediated Autoxidation of Aqueous SO2 in Ceramic Powder Suspensions

Mechanism of Aqueous-Phase Ozonation of S(IV)

The ozonation of dissolved sulfur dioxide is an important route for sulfate formation, especially in fog and cloud droplets of high pH. However, little is known about the detailed chemical mechanism of this process. We have mapped out the fate of aqueous SO(2) in the presence of ozone by use of density functional theory (DFT) calculations in solution (via the polarized continuum model, PCM), including up to two explicit water molecules. The calculations predict that the hydrolysis of SO(2).H(2)O, although possessing a barrier, is still more energetically favorable than its ozonation. The ozonation of HOSO(2)(-) and SO(3)(2)(-) proceeds without barriers and gives S(VI) products that are more stable than the reagents by 77.1 and 88.6 kcal/mol, respectively. By comparing our calculated pH dependence of the ozonation kinetics to those determined experimentally, we conclude that, despite a high calculated energy barrier to the ozonation of sulfonate (HSO(3)(-)), it is the dominant form of S(IV) in solutions of neutral pH and is the species through which ozonation occurs.

Sulfur Dioxide and Water: Structures and Energies of the Hydrated Species SO2·nH2O, [HSO3]–·nH2O, [SO3H]–·nH2O, and H2SO3·nH2O (n = 0–8)

AbstractThe structures of a large number of hydrates of sulfur dioxide (SO2·nH2O), of the sulfonate ion ([HSO3]–·nH2O), of the tautomeric hydrogensulfite anion ([SO3H]–·nH2O), and of sulfurous acid (H2SO3·nH2O) with up to eight water molecules attached to these species have been optimized at the B3LYP/6‐31G(2df,p) level of theory (DFT). The calculated vibrational frequencies allow the definite assignment of certain characteristic modes, and in this way a convincing interpretation of published spectra of aqueous SO2 as well as of SO2 adsorbed on very cold ice crystals has been achieved for the first time. Single‐point calculations at the G3X(MP2) level of theory were used to calculate the binding energies of the water molecules in SO2·nH2O as well as the relative stabilities of the isomeric anionic species [HSO3]–·nH2O and [SO3H]–·nH2O. Generally, the water molecules tend to stick together forming clusters, whereas the particular sulfur‐containing molecule remains at the surface of the water cluster, but it is always strongly hydrogen‐bonded. Only when there are more than six water molecules are the anions more or less completely surrounded by water molecules. DFT calculations erroneously predict that the gaseous hydrated sulfonate ions are more stable than the isomeric hydrogensulfite ions, even when hydrated with six water molecules. However, if these hydrated species are calculated as being embedded in a polar continuum simulating the aqueous phase, the hydrogensulfite ions are more stable than the sulfonate ions, in agreement with various spectroscopic observations on aqueous sulfite solutions. On the other hand, at the higher G3X(MP2) level, the gaseous hydrated hydrogensulfite anions are more stable than the corresponding sulfonate ions only if the number of water molecules is larger than four, whereas for the weakly hydrated anions the order of relative energies is reversed. The possible implications of these results for the enzymatic oxidation of “sulfite ions” ([HSO3]– and [SO3H]–) by sulfite oxidase are discussed. The conversion of SO2·6H2O into its isomer H2SO3·5H2O is predicted to be exothermic (ΔH°298 = –56.1 kJ mol–1) and exergonic (ΔG°298 = –22.5 kJ mol–1). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Transition metal‐catalyzed oxidation of atmospheric sulfur: Global implications for the sulfur budget

We use observations of the oxygen‐17 excess (Δ17O) of sulfate in the Arctic to quantify the sulfate source from aqueous SO2 (S(IV)) oxidation by O2 catalyzed by transition metals. Due to the lack of photochemically produced OH and H2O2 in high latitudes during winter, combined with high anthropogenic SO2 emissions in the Northern Hemisphere, oxidation by O3 is predicted to dominate sulfate formation during winter in this region. However, Δ17O measurements of sulfate aerosol collected in Alert, Canada, are not consistent with O3 as the dominant oxidant and indicate that a S(IV) oxidant with near‐zero Δ17O values (O2) is important during winter. We use a global chemical transport model to interpret quantitatively the Alert observations and assess the global importance of sulfate production by Fe(III)‐ and Mn(II)‐catalyzed oxidation of S(IV) by O2. We scale anthropogenic and natural atmospheric metal concentrations to primary anthropogenic sulfate and dust concentrations, respectively. The solubility and oxidation state of these metals is determined by cloud liquid water content, source, and sunlight. By including metal‐catalyzed S(IV) oxidation, the model is consistent with the Δ17O magnitudes in the Alert data during winter. Globally, we find that this mechanism contributes 9–17% to sulfate production. The inclusion of metal‐catalyzed oxidation does not resolve model discrepancies with surface SO2 and sulfate observations in Europe. Oxygen isotope measurements of sulfate aerosols collected near anthropogenic and dust sources of metals would help to verify the importance of this sulfur oxidation pathway.

Kinetics of ammonia and ammonium ion inhibition of the atmospheric oxidation of aqueous sulfur dioxide by oxygen

This is the first study, which shows both NH3 and NH4+ to inhibit the autoxidation of aqueous SO2 in the pH range 7.0–8.5. The rate of the autoxidation, Raut, in both buffered and unbuffered media at a fixed pH is in conformity with the rate law: $$ R_{aut} ={{R_0 } \mathord{\left/ {\vphantom {{R_0 } {\left( {{\text{1 + }}B\left[ {\text{Inh}} \right]_{\text{T}} } \right)}}} \right. } {\left( {{\text{1 + }}B\left[ {\text{Inh}} \right]_{\text{T}} } \right)}} $$ where R0 is rate in the absence of the inhibitors, B is a pH dependent empirical constant and [Inh]T is the analytical concentration of NH3 or NH4+. Both ammonia and ammonium ions appear to inhibit the autoxidation either by scavenging SO4− radicals or by forming less-reactive /unreactive Co(II)-NH3 complexes or both. The atmospheric relevance of the inhibition by ammonia and ammonium ions is discussed.

Groundwater characterisation and modelling of water–rock interaction in an argillaceous formation (Tournemire, France)

Abstract Chemical (Na, K, Mg, Ca, Cl, SO4, HCO3, Si, Al, Fe, Mn, TOC) and isotopic (18O[H2O], 2H[H2O], 3H[H2O], 14C[DIC], 13C[DIC], 34S[SO4], 18O[SO4]) data were obtained on groundwater collected from fractures inside the consolidated argillaceous formation of Tournemire (Aveyron) and from the aquifers surrounding this formation. Because of the very low transmissivity of such fractures (10−10 m2 s−1), specific devices were developed, for limiting out-gassing and air-contamination of groundwater during the sampling. Two modelling approaches are proposed to account for the chemical evolution of fracture groundwater from Na–Cl–HCO3 to Na–SO4 type. The first one is based on mineral equilibria only and the second one also takes into account the ion-exchange reactions. The two modelling approaches are able to reproduce K+ concentrations and they led to an almost constant pCO2 (10−2.6 atm), which only depends on equilibrium with the calcite–dolomite–kaolinite–quartz assemblage. The modelling of Na+, Ca2+ and HCO 3 + behaviours is improved by taking the ion-exchange reactions into account. In some boreholes, the time evolution of the chemical composition of fluids was greatly influenced by SO4 reduction combined with oxidation of dissolved organic C. The isotopic composition of aqueous SO4 in fracture groundwater confirms that SO4 reduction processes have occurred since the drilling of these boreholes, but also previously, during the natural evolution of groundwater. These redox reactions constitute a source of aqueous inorganic C that has to be considered in the computation of groundwater “ages” with 14C. For the studied fracture groundwaters, the maximum age estimates range from 17 to 29 ka. This shows that fractures may induce relatively fast groundwater flow, compared to the very long migration times that are calculated by only considering diffusive transport in the rock microporosity.

Sulfate formation in sea‐salt aerosols: Constraints from oxygen isotopes

We use observations of the mass‐independent oxygen isotopic composition (Δ17O) of sulfate in the marine boundary layer (MBL) to quantify the sulfate source from aqueous SO2 (S(IV)) oxidation by O3 in alkaline sea‐salt aerosols. Oxidation by O3 imparts a large Δ17O signature to the resulting sulfate (8.8‰) relative to oxidation by H2O2 (0.9‰) or by OH or O2 (0‰). Ship data from two Indian Ocean Experiment (INDOEX) cruises in the Indian Ocean indicate Δ17O values usually <1‰ in the submicron sulfate aerosol but considerable variability in the supermicron sulfate with frequent occurrences above 1‰ and up to 6.7‰. The large Δ17O values are associated with high concentrations of sea‐salt aerosols, providing evidence for the S(IV) + O3 pathway. We use a global chemical transport model (GEOS‐CHEM) to interpret quantitatively the INDOEX observations and to assess the global importance of sulfate production in sea‐salt aerosols. The model accounts for titration of sea‐salt alkalinity in the MBL by uptake of acid gases (SO2, H2SO4, and HNO3), shutting down the S(IV) + O3 pathway. We find that this titration occurs rapidly over much of the oceans except at high latitudes (strong sea‐salt emission) and is due to both the S(IV) + O3 reaction and HNO3 (g) condensation; that is, sulfate formation in sea‐salt aerosols is limited by the alkalinity flux from the ocean and by competition for this alkalinity supply from HNO3 (g). The model is consistent with the Δ17O magnitudes and patterns in the INDOEX data. Titration of alkalinity is critical for the success of the model simulation. Regeneration of sea‐salt aerosol alkalinity by OH uptake is inconsistent with the Δ17O observations in INDOEX. Model results indicate that sulfate production in sea‐salt aerosols decreases MBL SO2 concentrations and gas phase H2SO4 production rates by typically 10–30% (up to >70%) and increases MBL sulfate concentrations by typically >10% (up to 30%). Globally, this mechanism contributes 9% of atmospheric sulfate production and 1% of the sulfate burden. The impact on H2SO4 (g) formation and implications for the potential formation of new particles in the MBL warrants inclusion in models examining the radiative effects of sulfate aerosols.

Nitrone in l-lyxose series: cycloaddition way for the synthesis of new C-α-fucosides

Nitrone 3 analogue of 5-deoxy-l-lyxose was derived from d-ribose. Its reduction with aqueous SO2 gave the corresponding 4-amino-sugar 2, a potent α-l-fucosidase inhibitor. 1,3-Dipolar cycloaddition of 3 with alkenes allowed the synthesis of acetohydroxamic acid and ethane–phosphonate derivatives. Hydroxy-ethane derivative 15 is a nanomolar α-l-fucosidase inhibitor.

Synergy in the Autoxidation of S(IV) Inhibited by Phenolic Compounds

The effects of phenol (C6H5OH), catechin [C6H4(OH)2], and gallic acid [HOOCC6H2(OH)3] on the kinetics of aqueous SO2 oxidation by molecular oxygen have been investigated for dilute systems, as pertain to the chemistry of troposphere and surface waters. In experiments performed under batch (homogeneous oxidation) or semibatch (heterogeneous oxidation) conditions, we measured the decay of oxygen with a Yellow Spring Instruments probe or the increase in S(VI) concentration with an Orion conductivity cell. All of the studied phenolic compounds caused inhibition of the uncatalyzed autoxidation of S(IV), but the inhibiting effectiveness of phenol was much lower than that of catechin and gallic acid. The dual role of phenol in not only inhibiting but also promoting action was demonstrated in experiments carried out in the presence of Co or Mn catalyst when the synergy effect was evident. Another extraordinary behavior was observed in the case of gallic acid at prolonged experiments leading to oscillations in the rate of S(VI) formation. The results are discussed in terms of a radical chain reaction with termination by a step first order in SO4•- radicals and in a phenolic compound, with initiation supplemented by a step first order in phenoxyl radicals and in aqueous S(IV) species. From such a model of the inhibited autoxidation of S(IV), we determined the following rate constants for SO4• scavenging: 4.3 × 109 M-1 s-1 for catechin and 2.6 × 109 M-1 s-1 for gallic acid. The consequences of this inhibition in the environment are discussed.

Evaluation of Contributions of Acid and Ligand to Ni, Co, and Fe Dissolution from Nonferrous Smelter Slags in Aqueous Sulfur Dioxide

To assess the effectiveness of aqueous SO2 in extracting metal values from discarded smelter slag, the dissolution behavior of Co, Ni, and Fe using SO2(aq) and H2SO4 was investigated. Experiments were carried out in batch mode under ambient conditions, with an emphasis on the relative contribution of acid (H+) and SO2(aq) complexation to metal extraction. The results suggest that both acid and sulfur(IV) ligand attack the reactive sites on the smelter slag surface to dissolve the metals. SO2(aq) was found to be more effective in leaching slag than H2SO4 at the same pH. The effect of acid was more predominant at lower pH, while the ligand effect of SO2(aq) became stronger at higher pH. The leaching process was modeled to quantify the relative contribution of these two dissolution mechanisms using a new approach, which involves combining the shrinking-core model with an adsorption model. The model fit the leaching curves of Co, Ni, and Fe well and adequately described the experimental data.

Analysis of regional budgets of sulfur species modeled for the COSAM exercise

The COSAM intercomparison exercise (comparison of large-scale sulfur models) was organized to compare and evaluate the performance of global sulfur cycle models. Eleven models participated, and from these models the simulated surface concentrations, vertical profiles and budget terms were submitted. This study focuses on simulated budget terms for the sources and sinks of SO2 and sulfate in three polluted regions in the Northern Hemisphere, i.e., eastern North America, Europe, and Southeast Asia. Qualitatively, features of the sulfur cycle are modeled quite consistently between models, such as the relative importance of dry deposition as a removal mechanism for SO2, the importance of aqueous phase oxidation over gas phase oxidation for SO2, and the importance of wet over dry deposition for removal of sulfate aerosol. Quantitatively, however, models may show large differences, especially for cloud-related processes, i.e., aqueous phase oxidation of SO2 and sulfate wet deposition. In some cases a specific behavior can be related to the treatment of oxidants for aqueous phase SO2 oxidation, or the vertical resolution applied in models. Generally, however, the differences between models appear to be related to simulated cloud (micro-)physics and distributions, whereas differences in vertical transport efficiencies related to convection play an additional rôle. The estimated sulfur column burdens, lifetimes and export budgets vary between models by about a factor of 2 or 3. It can be expected that uncertainties in related effects which are derived from global sulfur model calculations, such as direct and indirect climate forcing estimates by sulfate aerosol, are at least of similar magnitude.

Regional simulation of anthropogenic sulfur over East Asia and its sensitivity to model parameters

We discuss a series of simulations of anthropogenic sulfur over East Asia with a SO2/SO42− chemistry-transport model driven in on-line mode by a regional climate model. Sensitivity to OH and H2O2 concentration, cloud parameters, SO2 dry deposition and emission strength is analyzed and the different components of the sulfur budget are examined. The SO2 and SO2−4 column burdens show pronounced variability at temporal scales from seasonal to synoptic and sub-daily, with SO2 and SO2−4 behaving differently due to the interplay of chemical conversion, removal and transport processes. Both SO2 and SO2−4 show marked spatial variability, with emission being the dominant term in regulating the SO2 spatial distribution. The atmospheric SO2 and SO2−4 amounts show close to a linear response to surface emission. Aqueous phase SO2→SO2−4 conversion and wet removal are the primary factors that regulate the SO2−4 amounts, with dry deposition and gas phase SO2→SO2−4 conversion being of secondary importance. Aqueous phase conversion and dry deposition are the dominant loss mechanisms for SO2 . The model shows low sensitivity to variations in OH, H2O2, and cloud parameters, while the sensitivity to prescribed dry deposition velocity is more pronounced. Overall, our results are in line with previous modeling studies and with very limited available observations.

Sulfur Dioxide – A Leachant for Oxidic Materials in Aqueous and Non-aqueous Media

Leaching of various oxide materials using sulfur dioxide is documented in this review. Emphasis is given to the leaching of Fe(III), Mn(IV), Mn(III) oxides/hydroxides highlighting the mechanisms and kinetics of these systems. Treatment of magnesium oxide associated with serpentinic silicate tailings, copper–zinc containing silicate slags, nickel containing waste catalysts, roasted zinc sulfide concentrate and aqueous reduction of As(V) to As(III) are included. Acetone and aqueous SO2 mixtures form α-hydroxy sulfonic acid, which is useful for leaching phosphate and uranium ores. Aspects related to leaching of some oxidic materials in organic solvents such as dimethyl sulfoxide (DMSO), dimethyl formamide (DMF) and acetonitrile in the presence of sulfur dioxide are also considered, since novel reactions in non-aqueous solvents may open up new opportunities for the treatment of complex materials.

Sulfur Dioxide – A Leachant for Oxidic Materials in Aqueous and Non-aqueous Media

Leaching of various oxide materials using sulfur dioxide is documented in this review. Emphasis is given to the leaching of Fe(III), Mn(IV), Mn(III) oxides/hydroxides highlighting the mechanisms and kinetics of these systems. Treatment of magnesium oxide associated with serpentinic silicate tailings, copper-zinc containing silicate slags, nickel containing waste catalysts, roasted zinc sulfide concentrate and aqueous reduction of As(V) to As(IH) are included. Acetone and aqueous SO2 mixtures form α-hydroxy sulf⊙nic acid, which is useful for leaching phosphate and uranium ores. Aspects related to leaching of some oxidic materials in organic solvents such as dimethyl sulfoxide (DMSO), dimethyl formamide (DMF) and acetonitrile in the presence of sulfur dioxide are also considered, since novel reactions in non-aqueous solvents may open up new opportunities for the treatment of complex materials.

Volume Changes Due to SO2−4, SeO2−4, and H2PO−4Adsorption on Amorphous Iron(III) Hydroxide in an Aqueous Suspension

Volume changes due to SO2−4, SeO2−4, and H2PO−4adsorption on amorphous iron(III) hydroxide were determined by dilatometry in a mixture comprising an aqueous solution of Na2SO4, Na2SeO4, or NaH2PO4and amorphous iron(III) hydroxide suspended in 0.1 mol dm−3NaClO4at an initial pH ranged from 4.50 to 6.50. System volumes increased during SO2−4, SeO2−4, and H2PO−4adsorption on amorphous iron(III) hydroxide, suggesting that some hydrated water around aqueous SO2−4, SeO2−4, or H2PO−4ions and amorphous iron(III) hydroxide were released to the bulk. The volume changes due to SO2−4, SeO2−4, and H2PO−4adsorption at initial pH 4.50 were +18, +18, and +14 cm3mol−1, respectively. Phosphate, which adsorbed as an inner-sphere complex, released most of its hydration water to the bulk. The volume changes due to SO2−4and SeO2−4adsorption indicated that they were dehydrated at the water/amorphous iron(III) hydroxide interface. The smaller changes in volume for H2PO−4compared to SO2−4and SeO2−4may be explained by differences of charges and adsorption mechanisms of these oxoanions.

Performance of the HPLC/fluorescence SO2 detector during the GASIE instrument intercomparison experiment

Sulfur dioxide (SO2) in synthetic air and diluted ambient air was measured as part of the Gas‐Phase Sulfur Intercomparison Experiment (GASIE) using the high performance liquid chromatography (HPLC)/fluorescence technique. SO2 was analyzed by equilibrating the gaseous sample with aqueous SO2, sulfite, and bisulfite, then converting the aqueous S(IV) to an isoindole derivative. The derivative was separated by reversed phase HPLC and detected via fluorescence. The system was calibrated with mixtures of SO2 in zero air prepared from an SO2 permeation device through a two‐stage dilution system. The instrument has a 4‐min. sample integration time and a measurement period of 9‐min. During the GASIE intercomparison the lower limit of detection averaged 3.6 parts per trillion by volume (pptv). The precision of replicate measurements over the entire intercomparison period was better than 5% at the 20 pptv level. Instrument performance was unaffected by the interferent gases included in the GASIE protocol (H2O, O3, NOx, DMS, CO, CO2, and CH4). During diluted ambient air tests, the HPLC/fluorescence technique exhibited an approximately 10% reduction in response relative to some other techniques. The cause of this apparent calibration change is not understood.