Kinetic Modeling and Mechanisms of Manganese Removal from Alkaline Mine Water Using a Pilot Scale Column Reactor

Manganese (Mn) is a major element in various aqueous and soil environments that is sometimes highly concentrated in mine water and other mineral processing wastewater. In this study, we investigated Mn removal from alkaline mine water (pH > 9) with an Mn-coated silica sand packed into a pilot-scale column reactor and examined the specific reaction mechanism using X-ray absorption near-edge structure (XANES) analysis and geochemical kinetic modeling. The kinetic effect of dissolved Mn(II) removal by birnessite (δ-Mn(IV)O2) at pH 6 and 8 was evaluated at different Mn(II)/Mn(IV) molar ratios of 0.1–10. Our results confirmed the positive effect of the presence of δ-MnO2 on the short-term removal (60 min) of dissolved Mn. XANES analysis results revealed that δ-MnO2 was more abundant than Mn(III)OOH in the reactor, which may have accumulated during a long-term reaction (4 months) after the reactor was turned on. A gradual decrease in dissolved Mn(II) concentration with depth was observed in the reactor, and comparison with the kinetic modeling result confirmed that δ-MnO2 interaction was the dominant Mn removal mechanism. Our results show that δ-MnO2 contents could play a significant role in controlling Mn removability from mine water in the reactor.

Comparative analysis of XANES and EXAFS for local structural characterization of disordered metal oxides

In functional materials, the local environment around active species that may contain just a few nearest-neighboring atomic shells often changes in response to external conditions. Strong disorder in the local environment poses a challenge to commonly used extended X-ray absorption fine structure (EXAFS) analysis. Furthermore, the dilute concentrations of absorbing atoms, small sample size and the constraints of the experimental setup often limit the utility of EXAFS for structural analysis. X-ray absorption near-edge structure (XANES) has been established as a good alternative method to provide local electronic and geometric information of materials. The pre-edge region in the XANES spectra of metal compounds is a useful but relatively under-utilized resource of information of the chemical composition and structural disorder in nano-materials. This study explores two examples of materials in which the transition metal environment is either relatively symmetric or strongly asymmetric. In the former case, EXAFS results agree with those obtained from the pre-edge XANES analysis, whereas in the latter case they are in a seeming contradiction. The two observations are reconciled by revisiting the limitations of EXAFS in the case of a strong, asymmetric bond length disorder, expected for mixed-valence oxides, and emphasize the utility of the pre-edge XANES analysis for detecting local heterogeneities in structural and compositional motifs.

Oxidation State analysis of LiFeSixP1-xO4/C (x = 0.06) with X-ray Absorption Near Edge Structure (XANES) in Fe K-edge and Si K-edge

The development of LiFePO₄ as a cathode materials on lithium-ion battery was increased with the use of additional techniques such as atomic doping and coating. The material used in this report was LiFeSi_0.06P_0.94O₄/C (LFP Si-6%), synthesized with doping silicon 6% and 11wt% carbon coating by a solid state method. X-ray Absorption Spectroscopy (XAS) characterization was used to investigate the effect on electronic and atomic structure of LFP Si-6%, especially in X-ray Absorption Near Edge Strucuture (XANES) region. XANES data measured on Fe K-edge and Si K-edge. Fe foil, FeO, Fe₂O₃, FePO₄, Si powder, SiO, SiO₂ were used as a standard sample for comparison with the result of LFP Si-6%. XANES analysis showed that the energy absorption of Fe K-edge and Si K-edge in LFP Si-6% was 7124.94 eV and 1846.16 eV, respectively. The oxidation state of Fe was Fe^2.576+ between Fe²⁺ and Fe³⁺, while Si was close to the estimation of Si⁴⁺. In addition, the linear combination fitting (LCF) in XANES Fe K-edge was performed to show the ratio of Fe²⁺/Fe³⁺ (FeO/Fe₂O₃).

Ethoxylated Amine Friction Modifiers and ZDDP

The influence of a series of Ethomeens (ethoxylated alkylamine organic friction modifiers) on the durability and friction of tribofilms formed by a commercial blend of primary and secondary ZDDP in sliding/rolling contact has been studied. When pre-formed ZDDP tribofilms are rubbed in Ethomeen solution, boundary friction is reduced and some of the ZDDP film is removed. Ethomeens having just two ethoxy groups give lower boundary friction on ZDDP than those with 15 ethoxy groups, but result in much greater removal of the tribofilm itself. Based on XANES analysis, the film removed by both types of Ethomeen consists primarily of nanocrystalline orthophosphate. The level of boundary friction and its dependence on sliding speed, coupled with the dimensions of the molecules, suggests that the Ethomeens with two ethoxy groups may form quite closely packed vertical monolayers on ZDDP tribofilm surfaces, but that those with fifteen ethoxy groups cannot be close packed; yet they still reduce boundary friction significantly. The study shows that selection of an appropriate aminic friction modifier for use with ZDDP is a balance between its ability to reduce friction and its potentially harmful effect on a ZDDP tribofilm.

Residence Time Effects on Molybdenum Adsorption on Soils: Elucidation by Multi-Reaction Modeling and XANES Analysis

To investigate the influence of residence time on molybdenum [Mo(VI)] adsorption behavior in soil environments, kinetic batch experiments coupled with X-ray near-edge structure (XANES) spectroscopy were performed for a neutral-pH soil (Webster loam) and two acidic soils (Mahan sand and Windsor sand) at different time scales (1 day–1 year). Batch-type experiments indicated that retention of Mo(VI) was rate limited and typical biphasic for soils. Initial rapid retention was followed by a continued slow retention with increasing aging time for Mahan and Windsor soils. In contrast, the reaction for Webster soil was nearly complete after 8 h, reflecting difference in soil properties. XANES analysis for Webster soil confirmed that most of Mo was bound to montmorillonite during long-term reaction time, whereas kaolinite constitutes a very important host phase for Mahan and Windsor soils. Sequential extraction results indicated that the percentages of Fe/Al oxide and residual fractions increased at the advanced time periods for Mahan and Windsor soils. The goodness-of-fit of numerical modeling results indicated that a simple version of multi-reaction model (MRM) with equilibrium and kinetic sorption sites was capable of describing Mo(VI) retention data for Webster loam. However, for Windsor and Mahan soils, an additional irreversible sorption site was required to simulate Mo(VI) retention over time. Although each site from MRM model cannot be unequivocally clarified from each other by either XANES analysis or sequential extraction results, their finding provided evidence of surface irreversible reactions at long residence times.