Synergistic Dissolution Research Articles

Hydrophobic deep eutectic solvents as the green media for highly efficient extraction of Cr(VI) over a broad pH range and low oil-water ratio

The occurrence of Cr(VI) ions in aquatic ecosystems poses serious public health risks, making the development of effective removal technology for Cr(VI) ions urgent. This study presents a green and effective method for extracting Cr(VI) ions using hydrophobic deep eutectic solvents (HDESs) composed of tetran-octyl ammonium bromide and carboxylic acids. The results show that the extraction efficiency of [N8888][Br]-C6OOH(1:2) for Cr(VI) is as high as 98.2 % for high concentration of Cr(VI) (500 mg⋅g−1) with a low oil–water ratio of 1:66. Surprisingly, these HDESs demonstrate remarkable efficacy in extracting Cr(VI) ions over a broad pH range and at a low oil–water ratio, even in the presence of multiple competing ions. A serious of kinetic, thermodynamic, and spectroscopic experiments suggested that the extraction mechanism of Cr(VI) ions is a synergistic dissolution diffusion and ion exchange. As a proof of concept, we have successfully demonstrated the application of prepared HDESs for extracting trace amounts of Cr(VI) ions from H3PO4 solutions with varying mass fractions, and the extraction process resulted in a Cr(VI) removal rate that achieved 99.7 %. Furthermore, the extraction performance HDESs for Cr(VI) ions remained nearly unchanged after eight cycles of continuous extraction and regeneration. Impressively, a precipitation method was also employed to convert the isolated Cr(VI) ions into valuable BaCrO4. The research findings provide valuable insights for the development of highly efficient Cr(VI) extractants.

Effect of chromium and silicon additions on the hot corrosion resistance of nickel aluminide coatings

The effects of chromium and silicon modification on the microstructure and hot corrosion resistance of slurry aluminide coatings applied on pure nickel was studied by in-depth microscopic analyses. The simple and the Cr-modified aluminide coatings provided limited protection in Type II hot corrosion conditions but the Si-modified coating exhibited outstanding resistance with no sign of attack after 300 h of isothermal exposure at 700 °C. The synergistic dissolution between NiO and Al2O3/Cr2O3 appears responsible for the onset of the corrosion attack for the simple and Cr-modified coatings while silicon is shown to extend the incubation period.

Invited Review Paper in Commemoration of Over 50 Years of Oxidation of Metals: Current Aspects of Deposit-Induced Corrosion

Alloys and coatings used in high temperature applications are often subject to surface degradation influenced by the presence of deposits. Typical examples are fireside corrosion in coal-fired boilers and hot corrosion of blades and vanes in the hot sections of gas turbines. Depending on the source, deposit compositions may occur in a wide range from primarily sulfate to primarily oxide and various combinations of the two. There does not seem to be evidence of severe corrosion caused by gaseous Na2SO4. Generally, severe corrosion occurs when the deposits are liquid. However, significant corrosion has been observed in some cases with deposits, which are nominally solid. Corrosion is often caused by liquid deposits, in which negative oxide solubility gradients for alloy components are established across the deposits by rapid interface reactions. Hot corrosion can occur at temperatures near 700 °C by a variety of mechanisms if a phase which allows rapid transport is formed. This includes compounds such as Na2MoO4 (even in atmospheres without SO3), MSO4–Na2SO4 solutions or metastable nanostructured phases. Calling all corrosion in this temperature regime “Type II” can be misleading with regard to mechanism. There are similarities in the underlying mechanisms of some forms of hot corrosion in the 700 °C range and fireside corrosion in that they involve synergistic fluxing. The corrosive species responsible for fireside corrosion of ferrous alloys is a liquid (Na,K)2SO4–Fe2(SO4)3 solution and not alkali iron trisulfate. The propagation mechanism involves a synergistic dissolution process of protective Cr2O3 and Fe2O3. Terms such as “Type I,” “Type II” and “gas-phase-induced acidic fluxing” should be used with care in describing mechanisms of deposit-induced corrosion.

Sulfone-assisted-NH4I as electrolyte additive with synergistic dissolution and catalysis effects on reducing the activation voltage of Li2S cathode

Li2S is highly desired for high energy-density lithium sulfur batteries since it allows to be matched with Li metal-free anodes and alleviates the volume expansion of S cathode. However, Li+ extraction from Li2S usually suffers from a huge voltage barrier due to its poor electron conductivity and sluggish Li+ ion diffusion during activation. In this work, trace sulfone-assisted-NH4I is adopted as an electrolyte additive with synergistic dissolution and catalysis effects to alleviate the voltage barrier of Li2S. On one hand, the intercalation between sulfone-NH4+ and Li2S turns solid Li2S into liquid phase, which significantly speeding up the transport of electrons and Li+ ions during activation. On the other hand, I− works as a redox mediator, which further facilitates the oxidation of Li2S to polysulfides. By taking the advantages of synergistic dissolution and catalysis effects from sulfone-NH4+ and I−, the Li2S cathode can be activated at ~3.0 V with a very flat plateau. Finally, the Li2S cathode cycled in the electrolyte with optimized 0.5% sulfone-assisted-NH4I additive shows a high initial discharge capacity of 1160 mAh g−1 at 0.05C, along with enhanced cycling performances and rate capabilities.

Interaction mechanism between marmatite and chalcocite in acidic (microbial) environments

Sulphide minerals commonly existing in the form of polymetallic ores are the essential objects in the fields of acid mine drainage and biohydrometallurgy. Although interactions between different minerals have been studied extensively, there are no research reports on the interactions between marmatite and chalcocite. Hence, the interactions between marmatite and chalcocite in biotic/abiotic leaching and electrochemical systems were investigated. Electrochemical tests show that the open circuit potential of marmatite is 39 mV lower than that of chalcocite. Theoretically, marmatite should react as an anode in this galvanic couple, thus dissolving faster. However, leaching results suggest that the introduction of marmatite promotes chalcocite dissolution but its own dissolution was impeded, despite a slight acceleration observed during the early stage in abiotic systems. The results of this study reveal that the solution factors including oxidation-reduction potential, iron concentrations, and microbial concentrations might have significant effects that are more potent than the galvanic effect. The unpredictable inhibition of marmatite dissolution might be attributable to the existence of excess Cu2+ liberated from the chalcocite. A further deduction proposed that a synergistic dissolution of marmatite and chalcocite can be achieved by adjusting their mass ratios to attain a suitable Cu2+ concentration.

Low Temperature Hot Corrosion Screening of Single Crystal Superalloys.

Single crystal superalloys were screened in Type II molten (Na,K)-sulfate hot corrosion re-coat tests in air +300 ppm SO2 at 700 °C. They exhibited large 20–40 mg/cm2 weight changes, repeated spallation, and non-protective, 25–50 μm thick corrosion layers after 300 h of testing. Scale cross sections revealed dual outer Ni(Co)O and inner Al(Cr)S-rich corrosion layers. This chemical differentiation was partially consistent with previous models of oxide fluxing, alloy sulfidation, NiO micro-channel diffusion, and synergistic dissolution mechanisms. Broad shallow pits or uniform attack morphologies were consistent with prior studies performed in high >100 ppm pSO2 environments. Higher Mo experimental alloys trended toward more degradation, producing 100 μm thick scales with distinct Al(Cr)S-rich inner layers or 500 μm thick NiO. The aggressive behavior in these environments supports the need for LTHC-resistant coatings for single crystal superalloys.

Determination of the Initiation and Propagation Mechanism of Fireside Corrosion

A variety of deposit compositions were examined in short-term laboratory tests with the aim of determining the corrosion mechanisms of fireside corrosion for a range of chromia-forming alloys in various combustion systems. The deposits formed in boilers are complex, and despite decades of study, the propagation mechanism of fireside corrosion is not well understood. Alkali iron trisulfates, which are stabilized by SO3 in the gas atmosphere, have been cited to be the major corrosive species for many years. The propagation mechanism for fireside corrosion was investigated using T92 (a typical ferritic boiler steel) and a model austenitic Fe–Ni–Cr alloy in contact with synthetic coal ash deposits. The metal loss, corrosion product morphologies, and compositions were carefully characterized to define a propagation mechanism. The corrosive species responsible for degradation was a (Na,K)2SO4–Fe2(SO4)3 solution and not alkali iron trisulfates. The formation of the liquid deposit is similar to Type II hot corrosion of components in gas turbine engines. The mechanism is a synergistic dissolution process, where simultaneous basic and acidic dissolution of protective Cr2O3 and Fe2O3 disrupts protective oxide formation and locally produces negative solubility gradients at the oxide/salt interface. The dissolved Fe2O3 and Cr2O3 precipitate where there is lower solubility, creating the observed corrosion products. The effect of the deposit composition was examined with respect to the proposed fireside corrosion mechanism. These measurements were found to be consistent with the proposed mechanism based on synergistic fluxing.

Synergistic Dissolution of Manganese Oxides as Promoted by Siderophores and Small Organic Acids

Recent studies have revealed that siderophores, biogenic chelating agents that strongly complex Fe(III) and other hard metals, may function in concert with low‐molecular‐mass organic acids (LMMOAs) to facilitate Fe (hydr)oxide dissolution via synergistic reactions. The siderophore desferrioxamine B (DFOB) may also participate in a number of chemical reactions at Mn (hydr)oxide surfaces. The goal of this study was to determine the rates and products of δ‐MnO2, a layer type Mn(IV) oxide, dissolution as promoted by LMMOA–DFOB mixtures. As with Fe (hydr)oxides, the rate of DFOB‐promoted dissolution of δ‐MnO2 is strongly influenced by the presence of the LMMOAs citrate and oxalate. In the presence of DFOB, citrate increases the dissolution rate relative to the sum of dissolution rates from corresponding single‐ligand systems by promoting both Mn(III)HDFOB+ complex formation and Mn(II) production in a pH‐dependent mechanism. In contrast, oxalate–DFOB mixtures produce predominantly Mn(II), with rates enhanced up to threefold from the sum of dissolution rates in single‐ligand systems at acidic pH values. We investigated possible mechanisms to describe this synergistic dissolution processes by building on observations of single‐ligand systems. These results suggest that biological exudation of LMMOAs in conjunction with siderophores may allow the selection of dissolution products, including regulation of the formation of potentially reactive aqueous Mn(III) complexes.

ChemInform Abstract: Synergistic Dissolution of Oxides in Molten Sodium Sulfate.

AbstractThe dissolution kinetics of pure Cr2O3 and pure Fe2O3 powders in molten Na2SO4 are compared with those of the dissolution of a mixture of these oxides.

Synergistic Dissolution of Oxides in Molten Sodium Sulfate

The dissolution kinetics of pure and pure powders, respectively, into , were each measured and compared with those for the same oxides during simultaneous dissolution of the mixed oxide powders in the same environment. For dissolution of mixed oxides, accelerated kinetics for each oxide resulted from an acid‐base coupling reaction. Measurements of the changes in the and oxygen activities in the melt during oxide dissolution suggest that the dissolution reaction for the single oxides is controlled by the diffusion of (as ). Therefore, the enhanced dissolution kinetics for mixed oxides is interpreted in terms of a synergistic reaction to promote this diffusion step.

Hot corrosion of materials

Abstract