Results 1 - 10 of 26
Results 1 - 10 of 26. Search took: 0.016 seconds
|Sort by: date | relevance|
[en] Highlights: • Arsenic inertization/stabilization through alunite phases. • Arsenate incorporation in alunite structure: c cell expansion with arsenate incorporation. • Low arsenic solubility from arsenical alunite in short-term stability tests a neutral pH. • Long-term good stability of arsenical alunite. -- Abstract: AsO4-for-SO4 substitution in alunite (KAl3(SO4)2(OH)6) and hydronium alunite ((H3O)Al3(SO4)2(OH)6) has been investigated by hydrothermal precipitation at 200 °C. Arsenical alunite presented a good precipitation yield and a significant AsO4 substitution (up to 15% molar). The degree of arsenate substitution depends on the solution composition. It increased as (AsO4/(AsO4 + SO4))alunite ≅ 0.5 (AsO4/(AsO4 + SO4))L. For (AsO4/(AsO4 + SO4))L < 0.26, arsenical alunite was the unique phase and, above this ratio, mansfieldite (AlAsO4·2H2O) co-precipitated. The a unit cell parameter is practically independent of the AsO4 substitution, but the c unit cell parameter increased consistently with the differences between the As-O1 and S-O1 distances in tetrahedral sites of the structure. The maximum stability of arsenical alunite in short-term tests is between pH 5 and 8, with an As-solubilization of 0.01–0.03 mg/L in 24 h. Long-term tests were performed at some synthesized samples at its natural pH. Arsenical alunite was stabilized at 0.3 mg/L released As in 2.5 weeks. These values were similar to those obtained in pure and largely crystalline natural scorodite (0.4 mg/L released As), but lower than the obtained for synthetic scorodite (1.3 mg/L released As). Thus, arsenical alunite could be effective for arsenic immobilization, especially for effluents or wastes containing large SO4/AsO4 ratio. Hydronium alunite presents a low precipitation yield and a very low arsenate incorporation (up to 1% molar). This may be related by the difficulty of substituting protonated H2O-for-OH− groups, due to the location of the H-bridges of the H3O in the structure. These characteristics make hydronium alunite unsuitable for arsenic immobilization
[en] The jarosite group of minerals have the general formula AB3(SO4)2(OH)6 and belong to the alunite supergroup. For jarosites the B site is iron (F3+) and for alunites the B site is aluminium (Al3+) Hydronium and ammoniojarosites have the hydronium (H30+) and ammonium (NH4+) cations at the A site. The crystal structure of most jarosites is space group R-3m (166), Z = 3 with the A site possessing D3d symmetry. However, H30+ and NH4++, are C3V and Td ions respectively and as such, have no inversion centre, yet the oxygen and nitrogen atoms are commonly located at an inversion centre, the D3d site. Wills and Harrison refined the hydronium oxygen at a lower symmetry C3V site, but no justification was given. To date, the NH4+ and H3Oh hydrogen atoms remain elusive and the geometry of these ions has not been determined. In addition, the possibility of a lower symmetry space group has not been investigated. A recent neutron powder diffraction study of hydronium and ammoniojarosites and also the corresponding alunites at room and low temperatures aims to resolve the H30+. and NH4+ structure and geometry.
[en] In this paper, we consider the kinetics of the process of dehydration calcination of alunite raw materials, which is necessary for the selection equipment and effective management of complex processing of alunite ores from the Tokmak Deposit of Tajikistan. The areas of flow of dehydration calcination of alunite and the activation energy of the process have been defined. (author)
[en] The technology elaboration of complex processing of alunite ores of Tokmak Deposit of Tajikistan has been considered in present article. The optimal physicochemical and technological modes of raw processing have been defined. The flowsheet of processing of alunite ores has been proposed.
[en] In this paper, we consider the kinetics of the process of sulphuric acid decomposition of alunite raw materials, which is necessary for selection of equipment and effective management of the complex processing of alunite ores from the Tokmak Deposit of Tajikistan. The areas of flow of sulphuric acid decomposition of alunite and the activation energy of the process are defined. (author)
[en] OAK B262 Research and Education Activities We are working on developing calorimetric techniques for sulfide minerals. We have completed calorimetric studies of (Na, K, H3O) jarosites, of Na and K jarosite -alunite solid solutions, and of Cr6+ - containing jarosites. We are now working on phases containing As and Pb. These studies are important to issues of heavy metal pollution in the environment. A number of postdocs, graduate students, and undergrads have participated in the research. We have active collaboration with Dirk Baron, faculty at California State University, Bakersfield. In a collaboration with Peter Burns, Notre Dame University, we are working on thermochemistry of U6+ minerals. Navrotsky has participated in a number of national workshops that are helping to define the interfaces between nanotechnology and earth/environmental science. Major Findings Our first finding on uranyl minerals shows that studtite, a phase containing structural peroxide ion, is thermodynamically unstable in the absence of a source of aqueous peroxide ion but is thermodynamically stable in contact with a solution containing peroxide concentrations expected for the radiolysis of water in contact with spent nuclear fuel. This work is in press in Science. We have a consistent thermodynamic data set for the (Na, K, H3O) (Al, Fe) jarosite, alunite minerals and for Cr6+ substituting for S6+ in jarosite. The latter phases represent one of the few solid sinks for trapping toxic Cr6+ in groundwater. Contributions within Discipline Better understanding of thermodynamic driving for and constraints on geochemical and environmental processes
[en] Concentration of lanthanides (Lns) in alunite group mineral samples from the Kusatsu-Shirane volcano area, Gunma, Japan, were determined by neutron activation analysis. Their Ln abundance patterns showed enrichment of light Lns relative to their original rocks and GSJ geochemical reference samples of feldspars. It was found that the concentration of light Lns increased with increasing concentration of K and P. The positive correlation between the light Lns and P concentration suggested the formation of florencite, whereas the positive correlation between the concentration of light Lns and K may reflect the difference between the solubility of double salts of K and Ln sulfates of light Lns and heavy Lns, respectively. (author)
[en] The thermal decompositions of both non-activated and mechanically activated alunite ore have been studied by thermogravimetry (TG). The ore was activated mechanically in an attritor for 15 min and amorphisation in the structure was studied by X-ray diffraction analysis. It can be verified that alunite decomposes in two steps, which are dehydration and desulphation. It was also established that the mechanical activation affected especially on the temperature range of dehydration reaction. The activation energies of dehydration and desulphation reactions have been calculated from the thermogravimetric data at heating rates of 5, 10, 15 and 20 K min-1 involving isoconversional methods of Ozawa and Kissenger-Akahira-Sunose (KAS)