Reductive Dissolution Reaction Research Articles

Phosphorus Release and Speciation in Manganese(IV) Oxide and Zeolite-Amended Flooded Soils.

Phosphorus (P) losses from flooded soils and subsequent transport to waterways contribute to eutrophication of surface waters. This study evaluated the effectiveness of MnO2 and a zeolite Y amendment in reducing P release from flooded soils and explored the underlying mechanisms controlling P release. Unamended and amended (MnO2 or zeolite, surface-amended at 5 Mg ha-1) soil monoliths from four clayey-alkaline soils were flooded at 22 ± 2 °C for 56 days. Soil redox potential and dissolved reactive P (DRP), pH, and concentrations of major cations and anions in porewater and floodwater were analyzed periodically. Soil P speciation was simulated using Visual MINTEQ at 1, 28, and 56 days after flooding (DAF) and P K-edge X-ray absorption near-edge structure spectroscopy and sequential fractionation at 56 DAF. Porewater DRP increased with DAF and correlated negatively with pe+pH and positively with dissolved Fe. Reductive dissolution of Fe-associated P was the dominant mechanism of flooding-induced P release. The MnO2 amendment reduced porewater DRP by 30%-50% by favoring calcium phosphates (Ca-P) precipitation and delaying the reductive dissolution reactions. In three soils, the zeolite amendment at some DAF increased porewater and/or floodwater DRP through dissolution of Ca-P and thus was not effective in reducing P release from flooded soils.

Temperature and freezing effects on phosphorus release from soils to overlying floodwater under flooded-anaerobic conditions.

Increased phosphorus (P) availability under flooded, anaerobic conditions may accelerate P loss from soils to water bodies. Existing knowledge on P release to floodwater from flooded soils is limited to summer conditions and/or room temperatures. Spring snowmelt runoff, which occurs under cold temperatures with frequent freeze-thaw events, is the dominant mode of P loss from agricultural lands to water bodies in the Canadian Prairies. This research examined the effects of temperature on P dynamics under flooded conditions in a laboratory study using five agricultural soils from Manitoba, Canada. The treatments were (a) freezing for 1wk at -20°C, thawing and flooding at 4±1°C (frozen, cold); (b) flooding unfrozen soil at 4±1°C (unfrozen, cold); and (c) flooding unfrozen soil at 20±2°C (warm). Pore water and surface water were collected weekly over 8 wk and analyzed for dissolved reactive phosphorus (DRP), pH, calcium, magnesium, iron (Fe), and manganese (Mn). Soils under warm flooding showed enhanced P release with significantly higher DRP concentrations in pore and surface floodwater compared with cold flooding of frozen and unfrozen soils. The development of anaerobic conditions was slow under cold flooding with only a slight decrease in Eh, whereas under warm flooding Eh declined sharply, favoring reductive dissolution reactions releasing P, Fe, and Mn. Pore water and floodwater DRP concentrations were similar between frozen and unfrozen soil under cold flooding, suggesting that one freeze-thaw event prior to flooding had minimal effect on P release under simulated snowmelt conditions.

Effect on human health of the arsenic pollution and hydrogeochemistry of the Yazır Lake wetland (Çavdır-Burdur/Turkey).

In this study, the physicochemical parameters, major ions and arsenic (As) contents of water resources in the Yazır lake wetland, were evaluated. In addition, water resources in this region were investigated from the point of water quality and health risk assessment. Thirty water samples were collected from the area in dry and wet seasons. Ca-Mg-HCO3 and Ca-HCO3 were the dominant water types. The Gibbs diagram suggests that most of the samples fall in rock-dominance zone, which indicates the groundwater interaction between rock chemistry. When compared to drinking water guidelines established by World Health Organization and Turkey, much greater attention should be paid to As, Fe, and Mn through varied chemicals above the critical values. According to the pH-ORP diagram, the predominant species is arsenate (H2AsO4-2). The high concentrations of As in the surface water and groundwater are related to oxidative and reductive dissolution reaction of Fe and Mn hydroxides within the Kızılcadağ ophiolite and melange. In addition, the seasonal changes in As concentrations depend on the increase in pH of water samples. The major toxic and carcinogenic chemical within water samples is As for groundwater and surface water. From the results of hazard index, it is verified that As which is taken by ingestion of water was the main contaminant, and toxic human risk in the study area. The obtained results will help define strategies for As problems in the water resources in future.

Simulating Porous Magnetite Layer Deposited on Alloy 690TT Steam Generator Tubes.

In nuclear power plants, the main corrosion product that is deposited on the outside of steam generator tubes is porous magnetite. The objective of this study was to simulate porous magnetite that is deposited on thermally treated (TT) Alloy 690 steam generator tubes. A magnetite layer was electrodeposited on an Alloy 690TT substrate in an Fe(III)-triethanolamine solution. After electrodeposition, the dense magnetite layer was immersed to simulate porous magnetite deposits in alkaline solution for 50 days at room temperature. The dense morphology of the magnetite layer was changed to a porous structure by reductive dissolution reaction. The simulated porous magnetite layer was compared with flakes of steam generator tubes, which were collected from the secondary water system of a real nuclear power plant during sludge lancing. Possible nuclear research applications using simulated porous magnetite specimens are also proposed.

Stability of arsenate-bearing Fe(III)/Al(III) co-precipitates in the presence of sulfide as reducing agent under anoxic conditions

Currently, the co-precipitation of arsenate with ferric iron at molar ratios Fe(III)/As(V) ≥ 3 by lime neutralization produces tailings solids that are stable under oxic conditions. However not much is known about the stability of these hazardous co-precipitates under anoxic conditions. These can develop in tailings storage sites by the action of co-discharged reactive sulfides, organic reagent residuals or bacterial activity. The ferric matrix can then undergo reductive dissolution reactions, which could release arsenic into the pore water. Co-ions like aluminum could provide a redox-immune sink to scavenge any mobilized arsenic as a result of reduction of ferric. As such, in this work Fe(III)/As(V) = 4 and aluminum substituted Fe(III)/Al(III)/As(V) = 2/2/1 co-precipitates were produced in a mini continuous co-precipitation process circuit and subjected to excess sulfide addition under inert gas to evaluate their stability. It was found that the ferric-arsenate co-precipitate could retain up to 99% (30 mg/L in solution) of its arsenic content despite the high pH (10.5) and extremely reducing (Eh < −200 mV) environment. There was no significant reduction of arsenate and only 45% of ferric iron was reduced. Partial aluminum substitution was found to cut the amount of mobilized arsenic by 50% (down to 15 mg/L) hence mixed Fe(III)/Al(III)-arsenate co-precipitates may offer better resistance to reductive destabilization over the long term than all iron co-precipitates.

Spontaneous water oxidation at hematite (α-Fe2O3) crystal faces.

Hematite (α-Fe2O3) persists as a promising candidate for photoelectrochemical water splitting, but a slow oxygen evolution reaction (OER) at its surfaces remains a limitation. Here we extend a series of studies that examine pH-dependent surface potentials and electron-transfer properties of effectively perfect low-index crystal faces of hematite in contact with simple electrolyte. Zero-resistance amperometry (ZRA) was performed in a two electrode configuration to quantify spontaneous dark current between hematite crystal face pairs (001)/(012), (001)/(113), and (012)/(113) at pH 3. Exponentially decaying currents initially of up to 200 nA were reported between faces over 4 min experiments. Fourth-order ZRA kinetics indicated rate limitation by the OER for current that flows between (001)/(012) and (001)/(113) face pairs, with the (012) and (113) faces serving as the anodes when paired with (001). The cathodic partner reaction is reductive dissolution of the (001) face, converting surface Fe(3+) to solubilized aqueous Fe(2+), at a rate maintained by the OER at the anode. In contrast, OER rate limitation does not manifest for the (012)/(113) pair. The uniqueness of the (001) face is established in terms of a faster intrinsic ability to accept the protons required for the reductive dissolution reaction. OER rate limitation inversely may thus arise from sluggish kinetics of hematite surfaces to dispense with the protons that accompany the four-electron OER. The results are explained in terms of semiquantitative energy band diagrams. The finding may be useful as a consideration for tailoring the design of polycrystalline hematite photoanodes that present multiple terminations to the interface with electrolyte.

Transformation of various kinds of goethite into magnetite: Effect of chemical and surface properties

FeII-induced mineralogical transformations of three goethites (G1, G2 and G3), having different crystal habits, morphologies and surface properties, into magnetite were studied. Experimental data revealed that all estimated constants including kinetic rate constant of magnetite formation, first order rate constant of goethite reductive dissolution and Langmuir sorption capacity of FeII onto goethite followed the order G1>G2~G3, which is in agreement with the calculated site density. The high intrinsic reactivity of G1 emphasizes the importance of crystal faces reactivity for the transformation process. The presence of specific crystal faces (e.g. (021) or (121)) may favor the formation of inner-sphere complexes and therefore the electron transfer. Spectroscopic investigations suggested that partial conversion of goethite to magnetite may proceed via solid-state reaction, but dissolution/re-crystallization process cannot be fully excluded. The coordination of FeII at goethite surface via the formation of bidentate or tridentate complex is a prerequisite to electron transfer, a key step to initiate solid-state transformation process or reductive dissolution reaction. The presence of strong chelating ligand (e.g. phosphate) is expected to hinder the adsorption sites and therefore affect both topotactic and reconstructive processes. Microscopic level characterizations such as crystal structure and surface site density are therefore required to describe the transformation extent of iron oxides.

Manganese and trace-metal mobility under reducing conditions following in situ oxidation of TCE by KMnO 4: A laboratory column experiment

The stability of Mn oxides, and the potential for mobilization of associated trace metals, were assessed by simulating the onset of microbially-mediated reducing conditions in a continuous-flow column experiment. The column had previously been used for an in situ chemical oxidation (ISCO) experiment in which trichloroethylene was reacted with permanganate in the presence of aqueous trace metals, which produced Mn oxyhydroxides (MnO x) that sequestered the trace metals and coated the column sand. The column influent solution represented the incursion of ambient groundwater containing dissolved organic carbon (DOC) into an ISCO treatment zone. The influx of DOC-containing groundwater initiated a series of cation-exchange, surface-complexation and reductive-dissolution reactions that controlled the release of aqueous metals from the system. Peak concentrations in the effluent occurred in the order Na, Mo, Cr, Zn, K, Mn, Fe, Pb, Mg, Ni, Cu and Ca. Manganese release from the column was controlled by a combination of cation exchange, reductive dissolution and precipitation of rhodochrosite. The trend in Fe concentrations was similar to that of Mn, and also resulted from a combination of reductive dissolution and cation exchange. Cation exchange and/or surface-complexation were the primary mechanisms controlling Cu, Ni, Mo and Pb release to solution, while Zn and Cr concentrations did not display coherent trends. Although metal release from the treatment zone was evident in the data, concentrations of trace metals remained below 0.05 mg L − 1 with the exception of Mo which reached concentrations on the order of 1 mg L − 1 . The establishment of anaerobic conditions in ISCO-treated aquifers may result in a prolonged flux of aqueous Mn II, but with the exception of MoO 4 2−, it is unlikely that trace metals sequestered with MnO x during ISCO will be released to the groundwater in elevated concentrations.

Decomposition of 2-chlorophenol employing goethite as Fenton catalyst. I. Proposal of a feasible, combined reaction scheme of heterogeneous and homogeneous reactions

A study has been conducted on the decomposition of 2-chlorophenol (2-CP) applying a heterogeneous Fenton reaction using goethite as catalyst at pH 3. The research was aimed at obtaining a workable kinetic expression apt for developing a kinetic model for scaling up purposes. Several aspects of the reaction have been described in the available literature but, for the moment, without a reasonable representation of the entire reaction behavior. In order to provide a more comprehensive and probable explanation of the whole observed performance, a set of experiments was carried out varying systematically all the significant variables. The proposal considers that the reaction is essentially a combination of four heterogeneous processes associated with one typical homogeneous Fenton reaction. Three of the surface reactions explain a very small iron leaching to the medium by a proton induced solubilization, a reductive dissolution reaction and a non-reductive iron release produced by detected 2-CP chemical decomposition byproducts, particularly, chlorohydroquinone (ClHQ) and oxalic acid (OxA). Iron concentration in the solution may be further increased in the final stages of the reaction after most of the 2-CP has been degraded, by the appearance of OxA that takes part in a third surface reaction. The fourth heterogeneous reaction rationalizes the unusual hydrogen peroxide consumption at high catalyst loadings. During the homogeneous reaction, the presence of ClHQ and ClBQ produces a homogeneous autocatalytic beneficial enhancement of the Fe3+→Fe2+ transformation. Consequently, the existence of phenolic derivatives either in the mixture or as reaction byproducts produces a beneficial enhancement of the reaction rate. Very low iron leaching is required to produce the onset of the homogeneous Fenton reaction, which was shown to be strongly dependent upon the reaction temperature. All the experimental findings were satisfactorily described by a set of 19-step feasible reaction scheme. The process could be useful for the treatment of wastewaters containing pollutants with phenolic derivatives, as long as iron leaching remains within tolerable limits.

A detailed field-based evaluation of naphthenic acid mobility in groundwater

An anaerobic plume of process-affected groundwater was characterized in a shallow sand aquifer adjacent to an oil sands tailings impoundment. Based on biological oxygen demand measurements, the reductive capacity of the plume is considered minimal. Major dissolved components associated with the plume include HCO 3, Na, Cl, SO 4, and naphthenic acids (NAs). Quantitative and qualitative NA analyses were performed on groundwater samples to investigate NA fate and transport in the subsurface. Despite subsurface residence times exceeding 20 years, significant attenuation of NAs by biodegradation was not observed based on screening techniques developed at the time of the investigation. Relative to conservative tracers (i.e., Cl), overall NA attenuation in the subsurface is limited, which is consistent with batch sorption and microcosm studies performed by other authors. Insignificant biological oxygen demand and low concentrations of dissolved As (< 10 µg L − 1 ) in the plume suggest that the potential for secondary trace metal release, specifically As, via reductive dissolution reactions driven by ingress of process-affected water is minimal. It is also possible that readily leachable As is not present in significant quantities within the sediments of the study area. Thus, for similar plumes of process-affected groundwater in shallow sand aquifers which may occur as oil sands mining expands, a reasonable expectation is for NA persistence, but minimal trace metal mobilization.

Synthesis and characterization of acrylamide/N-vinylpyrrolidone copolymer with pendent thiol groups for ophthalmic applications

Injectable acrylamide/ N-vinylpyrrolidinone copolymers with pendent thiol groups were prepared by a radical polymerization and reductive dissolution reaction. The solution of copolymers was re-gelled through oxidation in air or the thiol–disulfide exchange reaction. The re-gelation time could be adjusted from several minutes to several hours by changing the amount of the disulfide exchange reagent. The re-gelled hydrogels possessed high transmittance in the visible region but could block out some of the ultraviolet radiation. Their refractive indexes ranged from 1.34 to 1.35, and their equilibrium water contents were over 95.0%. The morphologies of the hydrogels were analyzed and the porous structure, with pore sizes of 50–300 μm, was noted. The cytotoxicities of the hydrogels were clearly reduced compared with previous results. The experimental results indicated that the injectable copolymers could be used as an artificial vitreous substance or as a scaffold for lens regeneration.

Reaction of Mn III,IV (hydr)oxides with oxalic acid, glyoxylic acid, phosphonoformic acid, and structurally-related organic compounds

Phosphonoformic acid, oxalic acid, glyoxylic acid, and 10 additional organic compounds that are structurally related to them have been reacted with synthetic MnO 2 (birnessite), consisting of 22% Mn III and 78% Mn IV, and synthetic MnOOH (manganite), consisting solely of Mn III. Significant concentrations of dissolved Mn III were detected in reactions of phosphonoformic acid with MnOOH, indicating that ligand-assisted dissolution took place. Reaction of phosphonoformic acid with MnO 2, and reaction of all other organic reactants with either MnOOH or MnO 2, yielded only Mn II, indicating that reductive dissolution was predominant. As far as reductive dissolution reactions are concerned, MnO 2 yields a range of reactivity that is nearly 20-times greater than that of MnOOH. Oxidation converts phosphonoformic acid into orthophosphate ion, glyoxylic acid into formic acid, pyruvic acid into acetic acid, and 2,3-butanedione into acetic acid. When differences in surface area loading are accounted for, oxalic acid, pyruvic acid, and 2,3-butanedione yield virtually the same dissolution rates for the two (hydr)oxides. At pH 5.0, glyoxylic acid reacts 14-times faster with MnO 2 than with MnOOH. MnO 2 reacts more slowly than MnOOH by a factor of 1/16th with oxamic acid, 1/20th with lactic acid, and 1/33rd with dimethyl oxalate. Adsorptive, complexant, and reductant properties of the 13 organic reactants are believed responsible for the observed reactivity differences.

The potential for metal release by reductive dissolution of weathered mine tailings

Remediation programs proposed for decommissioned sulphide tailings may include the addition of a cover layer rich in organic-carbon material such as sewage sludge or composted municipal waste. These covers are designed to consume oxygen and prevent the oxidation of underlying sulphide minerals. The aerobic and anaerobic degradation of such organic-carbon-rich waste can release soluble organic compounds to infiltrating precipitation water. In laboratory experiments, and in natural settings, biotic and abiotic interactions between similar dissolved organic compounds and ferric-bearing secondary minerals have been observed to result in the reductive dissolution of ferric (oxy)hydroxides and the release of ferrous iron to pore waters. In weathered tailings, oxidation of sulphide minerals typically results in the formation of abundant ferric-bearing secondary precipitates near the tailings surface. These secondary precipitates may contain high concentrations of potentially toxic metals, either coprecipitated with or adsorbed onto ferric (oxy)hydroxides. Reductive dissolution reactions, resulting from the addition of the organic-carbon covers, may remobilize metals previously attenuated near the tailings surface. To assess the potential for metal release to tailings pore water by reductive dissolution reactions, a laboratory study was conducted on weathered tailings collected from the Nickel Rim mine tailings impoundment near Sudbury, Ontario, Canada. This site was selected for study because it is representative of many tailings sites. Mineralogical study indicates that sulphide minerals originally present in the vadose zone at the time of tailings deposition have been replaced by a series of secondary precipitates. The most abundant secondary minerals are goethite, gypsum and jarosite. Scanning electron microscopy, coupled with elemental analyses by X-ray energy dispersion analysis, and electron microprobe analysis indicate that trace metals including Ni, Cr and Cu are associated with these secondary minerals. To assess the masses of trace metals associated with each of the dominant secondary mineral phases, a series of extraction procedures was used. The masses of metals determined in three fractions (water soluble, reducible and residual) suggest that the greatest accumulation of metals is in the reducible fraction. These measurements indicate that high concentrations of metals are potentially available for release by reductive dissolution of the ferric-bearing secondary minerals. The actual mass of metals that can be released by this mechanism will depend on a number of site-specific characteristics, particularly the intensity of the reducing conditions established near the tailings surface.

Basic studies on the kinetics and mechanism of the rapid dissolution reaction of plutonium dioxide under reducing conditions in acidic media

Dissolution of PuO 2 in aqueous solutions is a notoriously difficult task. Until the 1980s few dissolution processes existed except those based on the use of highly corrosive solutions consisting of nitric and hydrofluoric acids at boiling temperature. Thermodynamics calculations suggest that PuO 2 dissolution can be achieved under strong oxidizing or reducing conditions, however. Basic results dealing with the reductive dissolution reaction of PuO 2 with Cr 11 in sulphuric acid media are presented herein. Simple heterogeneous kinetics modelling is proposed and satisfactorily accounts for the reaction rates observed under various physicochemical conditions. The electron transfer mechanism was investigated by applying 18O-labelling techniques, which provided striking evidence of the pure heterogeneous nature of the electron transfer reaction.

Solubilization of Manganese Dioxide-Bound Copper by Naturally Occurring Organic Compounds

Sorption of metals onto manganese oxides is an important mechanism for retarding migration in soils and sediments. Subsequent influx of natural organic compounds can bring about metal release, thereby increasing metal mobility and bioavailability. This paper examines the release of Cu from a synthetic MnO[sub 2] phase. When natural organic compounds possessing only reductant characteristics (e.g., hydroquinone and pyruvate) are introduced, Cu release is linked to the progress of the MnO[sub 2] reductive dissolution reaction: Mn[sup II] generated in the reduction step competes effectively with Cu for a diminishing number of sorption sites. Organic compounds capable of forming stable Cu complexes in solution (e.g., oxalate and citrate) cause additional Cu release that can surpass release by reduction processes alone. Additional experiments are described in which the reductant and complexant properties of natural organic matter samples are evaluated as they pertain to the release of MnO[sub 2]-bound Cu. 61 refs., 8 figs.

Kinetics and reaction stoichiometry in the reductive dissolution of manganese(IV) dioxide and co(III) oxide by hydroquinone

Oxidation of hydroquinone (QH 2) to p-benzoquinone (Q) in aqueous suspensions by manganese(IV) dioxide (MnO 1.97) and cobalt (III) oxide (CoOOH) and subsequent reduced metal ion release were examined in order to explore the kinetics and mechanisms of surface chemical redox reactions. When the ratio [Q(aq)] [Me 2+(aq)] exceeds the value predicted from reaction stoichiometry, reduced metal ions are accumulating on oxide surfaces. This behavior was observed for both oxides, and is especially pronounced early in the course of reaction and in suspensions of high pH. Sorption of added Mn 2+ onto Mn(IV) dioxide and Co 2+ onto Co(III) oxide in reductant-free suspensions resembles the retention of reduced metal ions observed during the reductive dissolution reactions. Malonate has been shown to facilitate Co 2+(aq) release during reaction of Co(III) oxide with hydroquinone. Rates of reduced metal ion adsorption/desorption relative to rates of prior surface chemical reactions are discussed using a numerical model. Comparisons are made with the reductive dissolution of Fe(III) oxides by hydroquinone.

Photochemical Dissolution of Goethite in Acid/Oxalate Solution

AbstractDuring photochemical dissolution of goethite in acid/oxalate solution, Fe3+, Fe2+, and CO2 were released and towards the end of the reaction ferrous oxalate precipitated. The dissolution process involved an initial slow stage followed by a much faster reaction. The slow stage was eliminated by addition of 20 ppm Fe2+ to the system at the start of the reaction. The presence of this Fe2+ did not accelerate the secondary dissolution process. Both protons and oxalate ions appear to have been involved in the dissolution process. Dissolution was accelerated by an increase in oxalate concentration (from 0.0025 to 0.025 M) in the system and also depended on pH, reaching a maximum rate at pH 2.6. Highly substituted (15.9 mole % Al) goethite dissolved more slowly per unit area than unsubstituted goethite. Lepidocrocite (γ-FeOOH) dissolved faster than goethite. The first stage of the dissolution process probably proceeded by slow release of Fe3+ through complexation with oxalate adsorbed on the goethite surface. The faster, secondary step appears to have been a reductive dissolution reaction involving adsorbed ferrous oxalate.

Microbial metabolites and the reductive dissolution of manganese oxides: Oxalate and pyruvate

Microorganisms may participate in the reductive dissolution of Mn oxides in sediments either directly, by using Mn oxides as a terminal electron acceptor, or indirectly, through the production and excretion of reductants and other molecules that perturb the chemical conditions in sediments. If the latter process is predominant, it should be possible to calculate rates of Mn mobilization from sediments from a knowledge of the kinetics of the reductive dissolution reaction, once the structures and concentrations of reductants found in sediments are known. Laboratory experiments show that synthetic Mn (III, IV) oxides are reduced and dissolved by the microbial metabolites oxalate and pyruvate at appreciable rates. This provides additional evidence that indirect involvement of microbes in the mobilization of Mn in sediments can occur. Rates of reductive dissolution increase as the pH is decreased; the effect of pH on dissolution rate is more pronounced for oxalate than for pyruvate. Ways in which chemical conditions in freshwater and marine sediments affect rates of Mn mobilization are discussed.