Zidan Yuan

Incorporation of arsenic into gypsum: Relevant to arsenic removal and immobilization process in hydrometallurgical industry.

Gypsum precipitates as a major secondary mineral during the iron-arsenate coprecipitation process for the removal of arsenic from hydrometallurgical effluents. However, its role in the fixation of arsenic is still unknown. This work investigated the incorporation of arsenic into gypsum quantitatively during the crystallization process at various pHs and the initial arsenic concentrations. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray absorption near edge spectroscopy (XANES) and scanning electron microscopy (SEM) were employed to characterize the coprecipitated solids. The results showed that arsenate was measurably removed from solution during gypsum crystallization and the removal increased with increasing pH. At lower pH where the system was undersaturated with respect to calcium arsenate, arsenate ions were incorporated into gypsum structure, whereas at higher pH, calcium arsenate was formed and constituted the major arsenate bearing species in the precipitated solids. The findings may have important implications for arsenic speciation and stability of the hydrometallurgical solid wastes.

Manufacture, microstructure and mechanical properties of Mo-W-N nanostructured hard films

Mo1−xWxNy (x = 0–0.67) hard films were fabricated on wafers of silicon and high speed steel by dc magnetron sputtering technique. The effect of tungsten concentration on the phase composition, microstructure, surface morphology, hardness, adhesion, and corrosion resistance of the films was studied by X-ray diffraction, scanning electron microscopy, nano-indentation, and scratch test. It was found that if the W concentration (x) in the film is in the range of 0–0.52, the films exhibit fcc (Mo,W)Ny single phase where larger W atoms substituted Mo atoms in fcc MoNy. At higher x values (x > 0.52) the films exhibit a two-phase structure consisting of fcc (Mo,W)Ny and pure bcc tungsten phase. The hardness of the Mo1−xWxNy films increases at first with increasing x, and then decreases after passing a maximum. The maximum hardness of 47 GPa is obtained at x = 0.37 corresponding to an adhesion strength of 60 N. The Mo–W–N coated high speed steel has a lower corrosion current density and higher corrosion potential than the bare high speed steel substrates.

Speciation change and redistribution of arsenic in soil under anaerobic microbial activities

Arsenic speciation and behavior in soil are strongly affected by redox conditions. This work investigated speciation transformation and redistribution of arsenic in soil under anaerobic conditions. The effect of microbial sulfidogenesis on these processes was examined by addition of sulfate to the incubation systems. As(III) was found to be the dominant arsenic species in solution during the process of anaerobic incubation. The change of dissolved As concentration with incubation time showed M shaped profiles, e.g. the curves displaying two peaks at approximately 24 h and 240 h for the system with added sulfate. Arsenic was released and reduced to As(III) in the early stage of the incubation, and then resequestered into the solid phase. After excess sulfide was generated, the resequestered arsenic was released again (probably due to the dissolution of arsenic sulfide by dissolved sulfide ions) via the formation of thioarsenite. At the end of the incubation process, most of the dissolved arsenic was removed again from solution. The findings may have important implications to the fate of arsenic in flooded sulfur-rich soils.

Effects of nutrient and sulfate additions on As mobility in contaminated soils: A laboratory column study

The effect of nutrient and sulfate additions on As mobility in contaminated soils was investigated under advective-flow anoxic columns in this study. The mobility of As in surface contaminated soils was investigated by the leaching of de-ionized water (DI-water), artificial ground water (AG-water) and AG-water+sulfate (Sulfate). After 144 d of experiments, compared to the DI-water column, the total As exported from the columns AG-water and Sulfate was enhanced by seven and eightfold, respectively. The results indicated that the nutrient and sulfate addition significantly enhanced the As mobility in contaminated soils. In low-sulfate soils (DI-water and AG-water systems), As mobilization was primarily attributed to As reduction and to the transformation of amorphous Fe(III) (oxy)hydroxides. In soil with sulfate addition (Sulfate system), besides As reduction and Fe(III) (oxy)hydroxides transformation, the dissolution of As sulfides and the formation of thioarsenic species under sulfidogenic condition were possibly important processes accelerating As release. In conclusion, the addition of the nutrient solution and sulfates may increase the mobility of As in contaminated soils, posing a potential threat to groundwater.

Manufacture, microstructure and mechanical properties of WTaN nano-structured hard films

Abstract W 1− x Ta x N y ( x xa0=xa00–0.95) hard films were deposited on Si substrates using reactive direct current magnetron sputtering. The effect of tantalum concentration on phase composition, microstructure, surface morphology, adhesion strength, and hardness of the films has been studied by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, energy dispersive spectroscopy, atomic force microscopy, nano-indenter, and scratch tester. It was found that regardless of tantalum concentration all the W 1− x Ta x N y films show face centered cubic structure, and form W Ta N solid solution. The hardness and Youngs’ modulus of W 1− x Ta x N y films initially increase and then decrease with increasing tantalum concentration, after passing the maximum value of 38xa0GPa and 360xa0GPa at x xa0=xa00.31, respectively. The adhesion strength of coating to silicon substrate is in the range of 27–35xa0N, no obvious variation trend of adhesion strength with tantalum concentration was observed.

Effect of iron reduction by enolic hydroxyl groups on the stability of scorodite in hydrometallurgical industries and arsenic mobilization

Scorodite (FeAsO4·2H2O) is an important arsenic-bearing solid waste in hydrometallurgical industries, but its stability in reducing environments is not well understood. This study investigated the effect of Fe(III) reduction by enolic hydroxyl groups on the stability of scorodite and arsenic mobilization at various pH values and ascorbic acid/scorodite molar ratios (AH2/Sc). The results showed that 47–89% Fe(III) reduction by ascorbic acid caused approximately 10−69% (~xa037−260xa0mgxa0L−1) arsenic release and 4.5−63% (~xa013−176xa0mgxa0L−1) Fe(II) release at pH 5–8. The releases of arsenic and Fe(II) increased with increasing AH2/Sc, whereas they decreased as pH increased. The results of the solid characterization and chemical analysis indicated that the mixture of poorly crystalline parasymplesite and probably amorphous FeHAsO4⋅xH2O was the new arsenic sink. The high solubility of this ferrous arsenate with the Fe(II)/As(V) molar ratio >xa01 was deemed to be a major contributor to the relatively high arsenic release. This work differed from our previous finding that almost all arsenic was retained in the solid phase after similar Fe(III) reduction in scorodite with hydroquinone. Phenolic hydroxyl groups complexed with aqueous Fe(II), unlike enolic hydroxyl groups, was possibly the dominant reason for the formation of different secondary minerals, which strongly influenced arsenic redistribution between aqueous and solid phases.

A qualitative and quantitative investigation of partitioning and local structure of arsenate in barite lattice during coprecipitation of barium, sulfate, and arsenate

Abstract Arsenic (As), barium (Ba), and sulfate ( SO42−

Adsorption of monothioarsenate on amorphous aluminum hydroxide under anaerobic conditions

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Characterization and properties of quaternary Mo-Si-C-N coatings synthesized by magnetron sputtering technique

), coexisting in natural and mining impacted environments, possibly lead to As-barite coprecipitation. This work investigated the coprecipitation of Ba2+, SO42−

Preparation and characterization of the Mo(C)N/Mo(C) multilayer coating

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