Effects of metal cation substitution on hexavalent chromium reduction by green rust

Chromium contamination is a serious environmental issue in areas affected by leather tanning and metal plating, and green rust sulfate has been tested extensively as a potential material for in situ chemical reduction of hexavalent chromium in groundwater. Reported products and mechanisms for the reaction have varied, most likely because of green rust’s layered structure, as reduction at outer and interlayer surfaces might produce different reaction products with variable stabilities. Based on studies of Cr(III) oxidation by biogenic Mn (IV) oxides, Cr mobility in oxic soils is controlled by the solubility of the Cr(III)-bearing phase. Therefore, careful engineering of green rust properties, i.e., crystal/particle size, morphology, structure, and electron availability, is essential for its optimization as a remediation reagent. In the present study, pure green rust sulfate and green rust sulfate with Al, Mg and Zn substitutions were synthesized and reacted with identical chromate (CrO42−) solutions. The reaction products were characterized by X-ray diffraction, pair distribution function analysis, X-ray absorption spectroscopy and transmission electron microscopy and treated with synthetic δ-MnO2 to assess how easily Cr(III) in the products could be oxidized. It was found that Mg substitution had the most beneficial effect on Cr lability in the product. Less than 2.5% of the Cr(III) present in the reacted Mg-GR was reoxidized by δ-MnO2 within 14 days, and the particle structure and Cr speciation observed during X-ray scattering and absorption analyses of this product suggested that Cr(VI) was reduced in its interlayer. Reduction in the interlayer lead to the linkage of newly-formed Cr(III) to hydroxyl groups in the adjacent octahedral layers, which resulted in increased structural coherency between these layers, distinctive rim domains, sequestration of Cr(III) in insoluble Fe oxide bonding environments resistant to reoxidation and partial transformation to Cr(III)-substituted feroxyhyte. Based on the results of this study of hexavalent chromium reduction by green rust sulfate and other studies, further improvements can also be made to this remediation technique by reacting chromate with a large excess of green rust sulfate, which provides excess Fe(II) that can catalyze transformation to more crystalline iron oxides, and synthesis of the reactant under alkaline conditions, which has been shown to favor chromium reduction in the interlayer of Fe(II)-bearing phyllosilicates

Order and Disorder in Layered Double Hydroxides: Lessons Learned from the Green Rust Sulphate - Nikischerite Series.

Layered double hydroxides (LDHs) occur naturally and are synthesised for catalysis, drugdelivery and contaminant remediation. They consist of Me(II)-Me(III) hydroxide sheetsseparated by hydrated interlayers and weakly held anions. Often, LDHs are nanocrystalline andsheet stacking and Me(II)-Me(III) arrangement can be disordered, which influence reactivity andcomplicate structural characterisation. We have used pair distribution function (PDF) analysis, toprovide detailed information about local and medium range order (structure of synthetic Fe(II)-Fe(III)/Al(III) LDH. The data are consistent with ordered Me(II)and Me(III) in hydroxide sheets, where structural coherence along the c axis decreases with increasing Al content. The PDF for Fe(II)-Al(III) LDH (nikischerite) is best matched by apattern for a single metal hydroxide sheet. Parallel to decreased structural coherence betweenlayers, coherence within layers decreased to ~6 nm for synthetic nikischerite. Thus, disorderdeveloped within and between the sheets, resulting in mosaic crystals with coherent scatteringdomains decreasing in all directions. The high density of grain boundary terminations wouldaffect reactivity. Based on classical nucleation theory and the Kossel crystal growth model, wepropose that loss of structural coherence stems from increased supersaturation and the presenceof Al-hydroxides during formation of the Al-rich LDH</div

In situ photoreduction of Ag+-ions on the surface of titania nanotubes deposited on cotton and cotton/PET fabrics

This study discusses the possibility of in situ photoreduction of Ag+-ions on the surface of titania nanotubes (TNTs) deposited on the cotton and cotton/PET fabrics in the presence of amino acid alanine and methyl alcohol. TNTs were synthetized by hydrothermal method. The proposed interaction between titania, alanine and Ag+-ions was based on the results obtained by FTIR measurements. In order to enhance the binding efficiency between TNTs and fibers, the fabrics were previously impregnated with polyethyleneimine. The presence of TNT/Ag nanocrystals on the surface of fibers was proved by SEM, AAS, XRD and XPS. Larger amount of silver was detected on the cotton fabric. Fabricated TiO2/Ag nanocrystals provided maximum reduction of bacteria E. coli which was preserved after five washing cycles despite significant release of silver. The perspiration fastness tests indicated that silver release did not depend on pH. The presence of TNT/Ag nanocrystals imparted maximum UV protection to fabrics