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Robert O. Rye – One of the best experts on this subject based on the ideXlab platform.

  • supergene destruction of a hydrothermal replacement Alunite deposit at big rock candy mountain utah mineralogy spectroscopic remote sensing stable isotope and argon age evidences
    Chemical Geology, 2005
    Co-Authors: C G Cunningham, Robert O. Rye, Barnaby W Rockwell, Michael J Kunk, Terry B Councell
    Abstract:

    Abstract Big Rock Candy Mountain is a prominent center of variegated altered volcanic rocks in west-central Utah. It consists of the eroded remnants of a hypogene Alunite deposit that, at ∼21 Ma, replaced intermediate-composition lava flows. The Alunite formed in steam-heated conditions above the upwelling limb of a convection cell that was one of at least six spaced at 3- to 4-km intervals around the margin of a monzonite stock. Big Rock Candy Mountain is horizontally zoned outward from an Alunite core to respective kaolinite, dickite, and propylite envelopes. The altered rocks are also vertically zoned from a lower pyrite–propylite assemblage upward through assemblages successively dominated by hypogene Alunite, jarosite, and hematite, to a flooded silica cap. This hydrothermal assemblage is undergoing natural destruction in a steep canyon downcut by the Sevier River in Marysvale Canyon. Integrated geological, mineralogical, spectroscopic remote sensing using AVIRIS data, Ar radiometric, and stable isotopic studies trace the hypogene origin and supergene destruction of the deposit and permit distinction of primary (hydrothermal) and secondary (weathering) processes. This destruction has led to the formation of widespread supergene gypsum in cross-cutting fractures and as surficial crusts, and to natrojarosite, that gives the mountain its buff coloration along ridges facing the canyon. A small spring, Lemonade Spring, with a pH of 2.6 and containing Ca, Mg, Si, Al, Fe, Mn, Cl, and SO 4 , also occurs near the bottom of the canyon. The 40 Ar/ 39 Ar age (21.32±0.07 Ma) of the Alunite is similar to that for other replacement Alunites at Marysvale. However, the age spectrum contains evidence of a 6.6-Ma thermal event that can be related to the tectonic activity responsible for the uplift that led to the downcutting of Big Rock Candy Mountain by the Sevier River. This ∼6.6 Ma event also is present in the age spectrum of supergene natrojarosite forming today, and probably dates the beginning of supergene alteration at Big Rock Candy Mountain. The δ 34 S value (11.9‰) of Alunite is similar to those for replacement Alunite from other deposits in the Marysvale volcanic field. The δ 34 S values of natrojarosite (0.7‰ to −1.2‰) are similar to those for aqueous sulfate in Lemonade Spring, but are larger than those in pyrite (0.4‰ to −4.7‰). The δ 34 S and δ 18 O SO 4 values of gypsum show an excellent correlation, with values ranging from 15.2‰ to −5.2‰ and 7‰ to −8.2‰, respectively. The stable-isotope data indicate that the aqueous sulfate for gypsum is a mixture derived from the dissolution of hypogene gypsum and Alunite, and from the supergene oxidation of pyrite. The aqueous sulfate for the natrojarosite, however, is derived largely from the supergene oxidation of pyrite, with a minor contribution from the dissolution of Alunite and gypsum. The exceptional detailed spectral mapping capabilities of AVIRIS led to the recognition of a small amount of jarosite that is probably the top of the steam-heated system that produced the primary hypogene alteration at Big Rock Candy Mountain.

  • paleoproterozoic high sulfidation mineralization in the tapajos gold province amazonian craton brazil geology mineralogy Alunite argon age and stable isotope constraints
    Chemical Geology, 2005
    Co-Authors: Caetano Juliani, Robert O. Rye, Carmen M D Nunes, Lawrence W Snee, Rafael Silva, Lena Virginia Soares Monteiro, Jorge Silva Bettencourt, Rainer Neumann, Arnaldo Alcover Neto
    Abstract:

    The Brazilian Tapajos gold province contains the first evidence of high-sulfidation gold mineralization in the Amazonian Craton. The mineralization appears to be in large nested calderas. The Tapajos–Parima (or Ventuari–Tapajos) geological province consists of a metamorphic, igneous, and sedimentary sequence formed during a 2.10 to 1.87 Ga ocean−continent orogeny. The high-sulfidation mineralization with magmatic-hydrothermal Alunite is related to hydrothermal breccias hosted in a rhyolitic volcanic ring complex that contains granitic stocks ranging in age from 1.89 to 1.87 Ga. Cone-shaped hydrothermal breccias, which flare upward, contain vuggy silica and have an overlying brecciated cap of massive silica; the deposits are located in the uppermost part of a ring-structure volcanic cone. Drill cores of one of the hydrothermal breccias contain Alunite, natroAlunite, pyrophyllite, andalusite, quartz, rutile, diaspore, woodhouseite–svanbergite, kaolinite, and pyrite along with inclusions of enargite–luzonite, chalcopyrite, bornite, and covellite. The siliceous core of this alteration center is surrounded by advanced argillic and argillic alteration zones that grade outward into large areas of propylitically altered rocks with sericitic alteration assemblages at depth. Several occurrences and generations of Alunite are observed. Alunite is disseminated in the advanced argillic haloes that envelop massive and vuggy silica or that underlie the brecciated silica cap. Coarse-grained Alunite also occurs in branching veins and locally is partly replaced by a later generation of fine-grained Alunite. Silicified hydrothermal breccias associated with the Alunite contain an estimated reserve of 30 tonnes of gold in rock that grades up to 4.5 g t−1 Au. Seven Alunite samples gave 40Ar/39Ar ages of 1.869 to 1.846 Ga, with various degrees of apparent minor Ar loss. Stable isotopic data require a magmatic-hydrothermal origin for the Alunite, typical for high-sulfidation mineralization. The δ34S values of most samples of Alunite range from 14.0‰ to 36.9‰. Sulfur isotopic Alunitepyrite and oxygen isotopic Alunite SO4−OH temperatures range from 130 to 420 °C. The δDH2O and δ18OH2O values for Alunite-forming hydrothermal fluids suggest a predominance of magmatic water, with a small meteoric contribution. A rare sample of supergene Alunite has a δ34S value of 4.1‰ and an 40Ar/39Ar age of 51.3±0.1 Ma. Other than local foliation in the volcanic rocks and recrystallization of Alunite near faults, the mineralization and associated alteration appears to have been remarkably undisturbed by later metamorphism and by supergene alteration. The Au mineralization was preserved because of burial by sediments and tuffs in taphrogenic basins that probably developed shortly after mineralization and were probably first exhumed at about 60 Ma. Because high-sulfidation mineralization forms at relatively shallow crustal levels, the discoveries in Tapajos province provide new perspectives for mineral exploration for the Amazonian and perhaps for other Precambrian cratons.

  • Alunite and the role of magmatic fluids in the Tambo high-sulfidation deposit, El Indio-Pascua belt, Chile
    Chemical Geology, 2005
    Co-Authors: C L Deyell, Robert O. Rye, Gary P. Landis, Thomas Bissig
    Abstract:

    The Tambo high-sulfidation deposit, located within the El Indio–Pascua belt in Chile, produced almost 25 t (0.8 M oz) of gold from altered Tertiary rhyodacitic volcanic rocks. Episodic magmatic-hydrothermal activity in the district occurred over at least 4 my and is characterized by several stages of acid-sulfate alteration, including magmatic-hydrothermal, magmatic steam, steam-heated, and apparent supergene assemblages. Two stages of AuFAg mineralization are recognized and are hosted in barite and Alunite within hydrothermal breccias and veins. Isotopic compositions of fluid in Alunite show a dominant magmatic signature, with only a variable 18 O-enriched meteoric water component throughout the entire hydrothermal process. Alunite 40 Ar/ 39 Ar ages constrain the timing of alteration and the duration of the hydrothermal system. Pre-ore alteration occurred at about 10 to 11 Ma and was contemporaneous with the volcanism of the Tambo Formation. Alunite from this stage of alteration occurs in the matrix of barren breccias and as fine intergrowths of Alunite–quartzFclays that selectively replaced feldspars and pumice fragments. The textural relationships combined with stable-isotope systematics suggest a magmatic-hydrothermal origin for the Alunite, with a local magmatic steam overprint. Early ore-stage Alunite (8.7F0.2 Ma) occurs with barite and goldFwalthierite within open spaces of the breccia matrix, and has d 34 S values (24–27x) typical of magmatic-hydrothermal Alunite, reflecting equilibrium between aqueous H2S and SO4 . Fluid-inclusion ratios of H2S/SO2 (approximately 6) are consistent with ratios determined from stable-isotope data, and indicate reduced fluid conditions during ore deposition. Vaporphase transport of Au, and deposition from condensed magmatic vapor rising from the brittle–ductile transition is inferred. Late gold coprecipitated with a third stage of Alunite (8.2F0.2 Ma) that is characterized by nearly uniform chemical compositions and d 34 S values (1x) similar to those for associated enargite, reflecting disequilibrium between H2S and SO4 . This third-stage of Alunite is isotopically and chemically similar to that of post-ore, coarse, banded AluniteFhematite-quartz veins that crosscut the breccias in the Tambo area. Analyses of fluid-inclusion gas from Alunite in these veins indicate high SO2 and disequilibrium CO2–CO–CH4–H2 species, consistent with a magmatic-steam origin. The gases are also depleted in He, and the late goldbearing Alunite was probably precipitated from rapidly ascending SO2-rich vapors that were flashed from condensed magmatic

Charles K. Shearer – One of the best experts on this subject based on the ideXlab platform.

  • Terrestrial analogs of martian sulfates: Major and minor element systematics of Alunite–jarosite from Goldfield, Nevada
    American Mineralogist, 2006
    Co-Authors: J.j. Papike, J. M. Karner, Michael N. Spilde, Charles K. Shearer
    Abstract:

    Alunite and jarosite from Goldfield, Nevada, show spectacular relationships between early Alunite and later jarosite. In some cases, jarosite overgrows Alunite with the same crystallographic orientation and sharp contacts. Electron microprobe analyses of these phases show that they fall in the Alunitejarosite quadrilateral defined by Alunite, KAl3(SO4)2(OH)6; natroAlunite, NaAl3(SO4)2(OH)6; jarosite, KFe33+(SO4)2(OH)6; and natrojarosite, NaFe33+(SO4)2(OH)6. A large compositional gap occurs between Alunite-natroAlunite and jarosite-natrojarosite. This gap has no crystal chemical basis because Al and Fe3+ can readily substitute for each other in octahedral site coordination. We believe the “on-off switch” behavior between early Alunite and later jarosite is caused by an oxidant entering the system, oxidizing Fe2+ in solution to Fe3+, raising the Eh and possibly oxidizing H2S to lower the pH, and thus stabilizing jarosite relative to Alunite. The activity of Fe (as Fe2+) increased in the solution because of prolonged Alunite crystallization but could not readily enter the crystal structure until it was oxidized to Fe3+. The jarosite overgrowths show striking oscillatory zoning of Na- and K-rich bands. This reflects up to an order of magnitude change in the fluid K/Na ratio. These textures are interpreted to represent rapid growth and kinetic control of delivery of free Na and K to the crystal-fluid interface. This could be due to some combination of Na and K diffusion rates in the solution and complex ion breakdown involving Na and K.

  • terrestrial analogs of martian sulfates major and minor element systematics of Alunite jarosite from goldfield nevada
    American Mineralogist, 2006
    Co-Authors: J.j. Papike, J. M. Karner, Michael N. Spilde, Charles K. Shearer
    Abstract:

    Alunite and jarosite from Goldfield, Nevada, show spectacular relationships between early Alunite and later jarosite. In some cases, jarosite overgrows Alunite with the same crystallographic orientation and sharp contacts. Electron microprobe analyses of these phases show that they fall in the Alunitejarosite quadrilateral defined by Alunite, KAl3(SO4)2(OH)6; natroAlunite, NaAl3(SO4)2(OH)6; jarosite, KFe33+(SO4)2(OH)6; and natrojarosite, NaFe33+(SO4)2(OH)6. A large compositional gap occurs between Alunite-natroAlunite and jarosite-natrojarosite. This gap has no crystal chemical basis because Al and Fe3+ can readily substitute for each other in octahedral site coordination. We believe the “on-off switch” behavior between early Alunite and later jarosite is caused by an oxidant entering the system, oxidizing Fe2+ in solution to Fe3+, raising the Eh and possibly oxidizing H2S to lower the pH, and thus stabilizing jarosite relative to Alunite. The activity of Fe (as Fe2+) increased in the solution because of prolonged Alunite crystallization but could not readily enter the crystal structure until it was oxidized to Fe3+. The jarosite overgrowths show striking oscillatory zoning of Na- and K-rich bands. This reflects up to an order of magnitude change in the fluid K/Na ratio. These textures are interpreted to represent rapid growth and kinetic control of delivery of free Na and K to the crystal-fluid interface. This could be due to some combination of Na and K diffusion rates in the solution and complex ion breakdown involving Na and K.

Roger E. Stoffregen – One of the best experts on this subject based on the ideXlab platform.

  • Neutron spectroscopic study of synthetic Alunite and oxonium-substituted Alunite.
    Canadian Mineralogist, 2001
    Co-Authors: George A. Lager, Gregg A Swayze, Chun-keung Loong, Frank J. Rotella, James W. Richardson, Roger E. Stoffregen
    Abstract:

    Synthetic, polycrystalline samples of Alunite [K 0.88 (H 3 O) 0.12 Al 2.64 (SO 4 ) 2 (OH) 4.92 (H 2 O) 1.08 ] and oxonium-substituted Alunite [H 3 OAl 2.87 (SO 4 ) 2 (OH) 5.61 (H 2 O) 0.39 ] have been investigated using incoherent, inelastic neutron-scattering (IINS) methods in order to determine the nature of the non-OH “H 2 O”. IINS measurements were made on non-deuterated samples at 20 K using the HRMECS chopper spectrometer with 250 and 600 meV incident energies. Alunite and oxonium-substituted Alunite exhibit similar spectral features over the energy range of 120–550 meV, where assignments for local vibrations can be made based on the presence of OH and H 2 O groups. Salient differences in the intensities and positions of the observed low-energy vibrational bands reflect the effects of chemical substitution on the structural environment of the monovalent cation site and neighboring Al and OH sites in the framework.

  • Experimental studies of Alunite: I. 18O-16O and D-H fractionation factors between Alunite and water at 250–450°C
    Geochimica et Cosmochimica Acta, 1994
    Co-Authors: Roger E. Stoffregen, Robert O. Rye, Michael D. Wasserman
    Abstract:

    Abstract We have determined oxygen and hydrogen isotope fractionation factors between Alunite and water over a temperature range of 250–450°C by reacting synthetic natroAlunite with 0.7 m K2SO4 −0.1 to 0.65 m H2SO4 solutions to produce K-rich Alunite. From 88 to 95% alkali and isotope exchange were observed in most of these experiments, and the partial equiequilibrium method was used to compute equilibrium fractionation factors. Least-squares fits of the data give 103 In α Alunite (so 4 )-H 2 O = 3.09 ( 10 6 T 2 (K)) − 2.94 and 103 In α Alunite (OH)-H 2 O = 2.28 ( 10 6 T 2 (K)) − 3.90 . The intramineral 18O- 16O fractionation factor 103 In αAlunite(so4-OHsite) is given by the expression 0.8 ( 10 6 T 2 (K)) + 0.96 . The Alunite-water D-H fractionation factor ranges from −19 at 450°C to −6 at 250°C and does not appear to be strongly dependent on temperature. Runs with alkali exchange in the opposite direction were used to obtain 18O- 16O and D-H fractionation factors between natroAlunite (mol% Na = 70–75) and water at 350–450°C. These indicate that mol% Na has negligible effect on the fractionation factors over this temperature range. Measured 18O-16O and D-H fractionation factors between Alunite and 1.0 m KCl −0.5 m H2SO4 fluids also agree within 2σ with the values obtained from the K2SO4-H2SO4 fluids. However, experiments with Alunite and distilled water at 400°C gave a value of 103 In αAlunite(SO4)-H2O of 0.0, compared with a value of 3.9 obtained at this temperature with K2SO4- and H2SO4-bearing fluids. This suggests that changes in fluid composition can affect Alunite-water 18O-16O fractionation factors. Reconnaissance experiments with fine-grained natural natroAlunite demonstrate that Alunite-water D-H exchange can occur by hydrogen diffusion, although this process is generally not significant in the experiments with coarser grained synthetic Alunites.

  • experimental studies of Alunite i 18o 16o and d h fractionation factors between Alunite and water at 250 450 c
    Geochimica et Cosmochimica Acta, 1994
    Co-Authors: Roger E. Stoffregen, Robert O. Rye, Michael D. Wasserman
    Abstract:

    Abstract We have determined oxygen and hydrogen isotope fractionation factors between Alunite and water over a temperature range of 250–450°C by reacting synthetic natroAlunite with 0.7 m K2SO4 −0.1 to 0.65 m H2SO4 solutions to produce K-rich Alunite. From 88 to 95% alkali and isotope exchange were observed in most of these experiments, and the partial equiequilibrium method was used to compute equilibrium fractionation factors. Least-squares fits of the data give 103 In α Alunite (so 4 )-H 2 O = 3.09 ( 10 6 T 2 (K)) − 2.94 and 103 In α Alunite (OH)-H 2 O = 2.28 ( 10 6 T 2 (K)) − 3.90 . The intramineral 18O- 16O fractionation factor 103 In αAlunite(so4-OHsite) is given by the expression 0.8 ( 10 6 T 2 (K)) + 0.96 . The Alunite-water D-H fractionation factor ranges from −19 at 450°C to −6 at 250°C and does not appear to be strongly dependent on temperature. Runs with alkali exchange in the opposite direction were used to obtain 18O- 16O and D-H fractionation factors between natroAlunite (mol% Na = 70–75) and water at 350–450°C. These indicate that mol% Na has negligible effect on the fractionation factors over this temperature range. Measured 18O-16O and D-H fractionation factors between Alunite and 1.0 m KCl −0.5 m H2SO4 fluids also agree within 2σ with the values obtained from the K2SO4-H2SO4 fluids. However, experiments with Alunite and distilled water at 400°C gave a value of 103 In αAlunite(SO4)-H2O of 0.0, compared with a value of 3.9 obtained at this temperature with K2SO4- and H2SO4-bearing fluids. This suggests that changes in fluid composition can affect Alunite-water 18O-16O fractionation factors. Reconnaissance experiments with fine-grained natural natroAlunite demonstrate that Alunite-water D-H exchange can occur by hydrogen diffusion, although this process is generally not significant in the experiments with coarser grained synthetic Alunites.

Zhancheng Guo – One of the best experts on this subject based on the ideXlab platform.

  • Recovery of Soluble Potassium from Alunite by Thermal Decomposition: Effect of CaO and Phase Transformation
    Metals, 2019
    Co-Authors: Yiwei Zhong, Xinle Qiu, Jintao Gao, Long Meng, Zhancheng Guo
    Abstract:

    As mining waste, Alunite is a potential resource to produce potassium salt. The decomposition of Alunite is closely associated with the recovery of soluble potassium. In this study, the effect of CaO on phase transformation of Alunite in the desulfation stage was examined. The results showed that CaO was beneficial to the desulfation of Alunite. The decomposition temperature to obtain soluble potassium salt (K2SO4) was reduced from 800 °C to 700 °C by adding CaO. When the mass ratio of CaO/Alunite was 0.1, 81% of soluble potassium was extracted by water leaching after calcination at 700 °C for 2 h. The mechanism of CaO to promote the disintegration of Alunite was proposed through analyzing the phase transformation sequences. Alkaline Ca ion was inclined to bond with acidic [SO4] groups, and thus the breakage of S–O linkages between [AlO6] octahedron and [SO4] tetrahedron were improved. Monomer [SO4] tetrahedrons were released to form K2SO4 at a lower decomposition temperature. With the increase of the amount of CaO, the excess CaO bonded with neutral Al. [AlO6] tetrahedrons in Alunite transformed into [AlO4] octahedrons due to the breakage of the Al–O network. Al3+ was dissociated and bonded with [SO4] tetrahedron to form soluble Al salts.

  • Recovery of Soluble Potassium from Alunite by Thermal Decomposition: Effect of CaO and Phase Transformation
    MDPI AG, 2019
    Co-Authors: Yiwei Zhong, Xinle Qiu, Jintao Gao, Long Meng, Zhancheng Guo
    Abstract:

    As mining waste, Alunite is a potential resource to produce potassium salt. The decomposition of Alunite is closely associated with the recovery of soluble potassium. In this study, the effect of CaO on phase transformation of Alunite in the desulfation stage was examined. The results showed that CaO was beneficial to the desulfation of Alunite. The decomposition temperature to obtain soluble potassium salt (K2SO4) was reduced from 800 °C to 700 °C by adding CaO. When the mass ratio of CaO/Alunite was 0.1, 81% of soluble potassium was extracted by water leaching after calcination at 700 °C for 2 h. The mechanism of CaO to promote the disintegration of Alunite was proposed through analyzing the phase transformation sequences. Alkaline Ca ion was inclined to bond with acidic [SO4] groups, and thus the breakage of S–O linkages between [AlO6] octahedron and [SO4] tetrahedron were improved. Monomer [SO4] tetrahedrons were released to form K2SO4 at a lower decomposition temperature. With the increase of the amount of CaO, the excess CaO bonded with neutral Al. [AlO6] tetrahedrons in Alunite transformed into [AlO4] octahedrons due to the breakage of the Al–O network. Al3+ was dissociated and bonded with [SO4] tetrahedron to form soluble Al salts

  • Phase transformation and non-isothermal kinetics studies on thermal decomposition of Alunite
    Journal of Alloys and Compounds, 2017
    Co-Authors: Yiwei Zhong, Jintao Gao, Long Meng, Zhancheng Guo
    Abstract:

    Abstract Alunite was considered as a potential alternative resource for potassium and alumina production. The disintegration of Alunite was a principal step to extract valuable components and therefore was highly relevant to great and efficient utilization. In this research, the phase transformation and kinetics of Alunite during thermal decomposition were examined. The results showed that increasing the calcination temperature was beneficial to the decomposition of Alunite. The soluble potassium salt K 2 SO 4 was recovered by water leaching after calcination. The recovery ratio of K reached 83.9% after calcined at 900 °C for 2 h, and the purity of K 2 SO 4 was 83.77%. A crystal structure disintegration mechanism of KAl 3 (SO 4 ) 2 (OH) 6 was proposed on the basis of the phase transformation sequences characterized by XRD and FTIR. Dehydroxylation was attributed to the breakage of the Al OH and (Al)O H bonds. [AlO4] tetrahedrons in Alunite were transformed into [AlO6] octahedrons. The S-O linkages between [SO4] tetrahedron and [AlO6] octahedron were broken during desulphation. Then the staged kinetics of Alunite decomposition was studied by thermo-gravimetric analysis using the Kissinger-Akahira-Sunose method. The apparent activation energies of dehydroxylation and desulphation were determined.

José Viñals – One of the best experts on this subject based on the ideXlab platform.

  • arsenic immobilization as Alunite type phases the arsenate substitution in Alunite and hydronium Alunite
    Journal of Hazardous Materials, 2013
    Co-Authors: Alba Sunyer, Marta Currubi, José Viñals
    Abstract:

    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  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.

  • Arsenic immobilization as Alunite-type phases: the arsenate substitution in Alunite and hydronium Alunite.
    Journal of hazardous materials, 2013
    Co-Authors: Alba Sunyer, Marta Currubi, José Viñals
    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 AsO1 and SO1 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.