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Structure, Formation, And Reactivity of Hydrous Oxide Particles: Insights from X-Ray Absorption Spectroscopy
Published in Jacques Buffle, Herman P. van Leeuwen, Environmental Particles, 2018
Laurent Charlet, Alain Manceau
Coprecipitated Fe(III) and Cr(III) hydrous oxide gels are used in water treatment plants and in analytical chemistry to remove chromium from industrial effluent waters and other dilute solutions. The solubility of these gels is at least an order of magnitude lower than the solubility of chromium hydrous oxides obtained from homogeneous precipitation.62 The Cr- and Fe-RDF of the coprecipitated gel are very similar to each other (Figure 11a).12 Both display two metal-metal peaks beyond the first metal-oxygen one. The identity of their R position and of their relative intensity indicates that Cr(III) and Fe(III) possess a similar structure, that is, Cr(III) substitutes for Fe(III) in the αFeOOH-like framework. This isomorphic substitution is accounted for by the similarities of the electric charges and ionic radii of Cr3+ and Fe3+ and by the existence of isostructural Fe(III) and Cr(III) oxide polymorphs. On the other hand, the Cr hydrous oxide obtained from homogeneous or heterogeneous precipitation have a γCrOOH local structure (Figure 11b and 11c). The difference in efficiency of chromium extraction can be therefore attributed to the formation of different chromium phases. The solubility of the chromium coprecipitate (α(Fe,Cr)OOH) is clearly less than in the pure chromium oxyhydroxide (γCrOOH). The efficiency of the coprecipitation phenomenon explains, in turn, why in soils Cr mostly substitutes for Fe in iron oxyhydroxides, as evidenced by sequential chemical extraction.63
Physical Treatment Techniques
Published in Thomas E. Carleson, Nathan A. Chipman, Chien M. Wai, Separation Techniques in Nuclear Waste Management, 2017
The removal of uranium from Magnox sludges with HGMS has been studied in small-scale tests.23,24 Magnox sludge results from the mechanical breakdown of Magnox fuel. During the process, uranium-contaminated magnesium alloy cladding debris is generated. After storage under water, a sludge of magnesium hydroxide and uranium hydrous oxide is formed. The majority of the activity is located in the <100-μm fraction. The results indicated that with a well-dispersed system of uranium dioxide and magnesium hydroxide, separation efficiencies of more than 75% can be achieved for surrogate systems. For actual Magnox sludges, the efficiencies were lowered to 50%.
1 Interconversions
Published in Saeed Sahebdelfar, Maryam Takht Ravanchi, Ashok Kumar Nadda, 1 Chemistry, 2022
Saeed Sahebdelfar, Maryam Takht Ravanchi, Ashok Kumar Nadda
The use of aluminum hydrous oxide sol prepared by peptization of aluminum hydroxide for preparation of the catalyst resulted in good performance in the synthesis of methanol form CO2 -rich syngas (Bahmani et al., 2016). The catalyst showed stronger inter-dispersion between Cu and ZnO and well-dispersed Cu nanoparticles with improved stability and activity in methanol synthesis from syngas compared to those prepared from true solutions.
Conceptual Process Development for the Separation of Thorium, Uranium, and Rare Earths from Coarse Coal Refuse
Published in Mineral Processing and Extractive Metallurgy Review, 2023
Deniz Talan, Qingqing Huang, Liang Liang, Xueyan Song
Thorium’s speciation diagrams in the hydrochloric acid medium at 25°C are generated and provided in Figure 2. Due to the low concentrations of some species, separate diagrams were drawn to increase the readability. As seen in Figure 2) the thorium species in the solution are Th4+, ThCl3+, Th(OH)3+, and Th(OH)22+. Initially, ThCl3+ is formed due to the reaction between Cl− and Th4+ ions. In Figure 2), ThCl3+ and ThO2 display opposite behaviors. As the concentration of ThCl3+ decreases, the formation of ThO2 becomes apparent. However, when an alkaline reagent (i.e. NaOH) is introduced to the solution, the reaction is reversed, releasing Cl− ions back and freeing Th4+ ions to react with OH−. It explains the increasing concentration of Th4+ between pH 0 and 2 (Figure 2)). The decrease observed later in Th4+ concentration is due to the complexes formed with the OH− ions liberated from NaOH, which also initiates the precipitation. Meanwhile, a substantial increase of thorium hydroxide (Th(OH)4) is seen (Figure 2)) with an elevation in the solution pH. However, due to the unstable nature of thorium hydroxide, it slowly transforms into thorium dioxide (ThO2), a partially microcrystalline hydrous oxide, as shown in reaction 8 (Brookins 1988; Neck and Kim 2001). The concentration of Th(OH)4 reaches a steady level at a pH value of 4. Similarly, ThO2 formation becomes stable at a pH value slightly higher than 4, the pH region in which thorium precipitation is assumed to be completed. A further increase in the pH does not change the concentrations of any species.
A Review on the Application of Starch as Depressant in Iron Ore Flotation
Published in Mineral Processing and Extractive Metallurgy Review, 2022
Swagat S. Rath, Hrushikesh Sahoo
Balajee and Iwasaki (1969) suggested that the adsorption of starch onto hematite is mainly due to the presence of a large number of hydroxyl groups in starch molecules and on the hematite surface, which facilitates nonselective hydrogen bonding and electrostatic force of attraction. Later on, Subramanian and Natarajan (1988) concluded in similar lines while providing further insights into the adsorption studies. They confirmed the partially irreversible adsorption of starch on hematite by applying hot water in an autoclave for several minutes, where they noticed only 18% desorption. The partial irreversibility suggested the involvement of non-electrostatic forces in the adsorption process. The authors also studied the effect of pH on the adsorption of starch on hematite. The starch adsorption density was found to be higher between pH 4 and 9, which indicated that the adsorption mechanism could be attributed to reasons other than Columbic forces. The hydrous oxide precipitation was less at lower pH (less than 4), and with pH more than 4, the hydroxyl coating on the surface increased favoring the increase in starch adsorption. Similarly, at higher pH (more than 9), the complete precipitation of hydrous oxide occurred, resulting in a thicker coating allowing more cross-linking of hydroxide ions. Due to a thick coating, the lowering of surface area-to-volume ratio occurred, thereby lowering the starch adsorption density. Above pH 9.5, the drastic decrease in the adsorption density could be attributed to the increased electrostatic repulsion between the increasingly negative hematite surface and the polymeric starch chain. The authors suggested that the Columbic force of attraction was not very prominent. Instead, the adsorption was mostly governed by non-electrostatic forces. The temperature was also reported to have a role in determining the adsorption density. At a constant pH, the starch adsorption increased with an increase in the temperature from 30°C to 50°C. The increase in adsorption density due to temperature, confirmed the possibility of the chemical interaction of starch with hematite. In general, the physisorption process is characterized by an exothermic heat of adsorption, and as a result, adsorption is expected to decrease with a rise in temperature. In contrast, a higher temperature favors the chemisorption due to faster reaction kinetics. In this study, the temperature effect confirmed the chemisorption of starch on hematite.