Aluminum-Substituted Gyrolite as Cation Exchanger

A successful Al-substituted gyrolite has been hydrothermally synthesized at 220°C under saturated steam pressure, and its cation exchange characteristics for K⁺ and Cs⁺ ions have been investigated. Al-substituted gyrolite took up K⁺ and Cs⁺ ions of 14 and 20 mequiv/100 g, respectively, and these removal amounts are smaller than those by Al-substituted tobermorites (ca. 80 mequiv/100 g). It was found that Al-substituted gyrolite exhibited selectivity for univalent cations in the following order: Na⁺ < K⁺ < Cs⁺ in the low-concentration regions of K⁺ and Cs⁺ ions. Al-substituted gyrolite did not lose its crystallinity and morphology, and its basal spacing did not change in the cation exchange. These results, therefore, suggest that Al-substituted gyrolite may be useful for separation and waste disposal.     

Sequestration of Cesium and Strontium by Tobermorite Synthesized from Fly Ashes

Tobermorites were synthesized from oxides and from fly ashes under mild hydrothermal conditions. The bulk compositions of the ash-based formulations were adjusted to give the same [Ca]/[Si + Al] ratio as for the oxides. Selective Cs and Sr exchange with these tobermorites was measured. Compared to those prepared from the oxides and from class C fly ash, the tobermorites obtained from class F fly ash exhibited superior Cs and Sr selective properties. This is apparently a result of more extensive Al incorporation with class F fly ash. For example, tobermorite synthesized from class F fly ash exhibited a cesium K _d (mL/g) of 3826 while those prepared from oxides and from class C fly ash showed K _d values of 1112 and 796, respectively, under the same conditions. These results suggest that fly ash waste materials can be converted into tobermorite, which can serve as a resource in the separation, immobilization, and disposal of radioactive species such as Cs and Sr.     

Selective Cation Exchange in Substituted Tobermorites

Selective cation exchange in tobermorites with three levels of Al³⁺ and Na⁺ substitution for Si⁴⁺ has been investigated. The cation exchange selectivity for a tobermorite with 2 mol% Al³⁺ and 0 mol% Na⁺ substitution increased as follows: Mg²⁺ > Ba²⁺ > Sr²⁺. Cation exchange in the above tobermorite is postulated to take place mainly from edge and planar surface sites and apparently from interlayer Ca²⁺ sites. Tobermorites with 10 and 15 mol% Al³⁺ (and Na⁺) substitution have additional Na⁺ exchange sites in the interlayers and show the following selectivity: Ba²⁺ > Sr²⁺ > Mg²⁺. The cation exchange selectivity in the substituted tobermorites can be explained by the hydrated radii of cations and the steric limitations of the ion exchange cavity in tobermorite; the latter was determined by ²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy. These basic selective cation exchange studies of substituted tobermorites are of relevance in nuclear waste treatment and disposal.     

Structural Degradation of Tobermorite during Vibratory Milling

Mechanochemistry of hydrothermally prepared Al-free and Al-substituted 1.1 nm tobermorite was studied using ²⁹Si NMR, XRD, and TGA-DTA. Tobermorite has a double-chain structure and Al/Si substitution occurs preferentially at bridging tetrahedra. By using the vibration mill, both tobermorites were observed to decompose as a function of milling time to form amorphous C-S-H-like phases. The decomposition rate was higher for the Al-substituted tobermorite. The decomposition process occurs mainly at the bridge portion of the silicate double chain. In the Al-substituted tobermorite, the breakage seems to occur preferentially at the points where Al has replaced Si. On heating the amorphous C-S-H-like phase, wollastonite is formed. However, no decrease was observed at the wollastonite formation temperature.     

Anomalous tobermorite in autoclaved aerated concrete

Three samples of commercial products were examined by X-ray diffraction, electron diffraction, chemical analysis and d.t.a. - t.g.a. The cementing materials were highly crystalline Al-substituted 11 Å tobermorite with Ca/(Al+Si) = 0.81 and Al/(Al+Si) = 0.10 for specimen A, 0.85 and 0.10 for specimen B, and 0.84 and 0.07 for specimen C. All of these tobermorites showed anomalous thermal behaviour, i.e., the basal spacing did not shrink after heating at 300°C. The content of alkalis in all the tobermorites was insufficient to balance replacement of Si by Al. All the crystals gave SED patterns characteristic of crystals with (001) cleavage, similar to those given by normal specimens.     

Hydrothermal reaction between C6S6H and kaolinite for the formation of tobermorite at 180°C

11 Å tobermorites were made from CSH (Ca/Si = 1.14) and kaolinite with Ca/(Si+Al) = 0.8 and Al/(Si+Al)= 0.15 at 180°C. The CSH was prepared from colloidal silica and lime at 130°C and 180°C for 2 h. Reaction gives in succession CSH, poorly crystalline Al-substituted tobermorite, and highly crystalline Al-substituted tobermorite. The addition of kaolinite markedly accelerates the formation of tobermorite within 4 h, more effectively with CSH prepared at 130°C than with that prepared at 180°C. X-ray fluorescence diffractometry shows that the Al coordination number is a mixture of 4 and 6 in the initial products and shifts to 4 with an increase in processing time. This agrees with the results for the degree of reaction of the kaolinite.     

Hydrothermal reaction between CSH and kaolinite for the formation of tobermorite at 180°C

11 Å tobermorites were made from C---S---H (Ca/Si = 1.14) and kaolinite with Ca/(Si+Al) = 0.8 and Al/(Si+Al)= 0.15 at 180°C. The C---S---H was prepared from colloidal silica and lime at 130°C and 180°C for 2 h. Reaction gives in succession C---S---H, poorly crystalline Al-substituted tobermorite, and highly crystalline Al-substituted tobermorite. The addition of kaolinite markedly accelerates the formation of tobermorite within 4 h, more effectively with C---S---H prepared at 130°C than with that prepared at 180°C. X-ray fluorescence diffractometry shows that the Al coordination number is a mixture of 4 and 6 in the initial products and shifts to 4 with an increase in processing time. This agrees with the results for the degree of reaction of the kaolinite.     

Hydrothermal alkaline treatment of oil shale ash for synthesis of tobermorites

The hydrothermal alkaline activation of the oil shale fly ash was studied using SEM/EDX, XRD and ²⁹Si and ²⁷Al high-resolution MAS-NMR spectra. The silicon in the original fly ashes was completely converted into calcium-alumino-silicate hydrates, mainly into 1.1 nm tobermorite structure during 24 h treatment under hydrothermal conditions at 160 °C in the presence of NaOH. The local structure of synthesized tobermorite samples implies long silicate chains with small number of bridging sites. The results obtained in the study prove that the oil shale fly ash can be used for production of Al-substituted tobermorites.     

Synthesis of 11 Å Al-substituted tobermorite from trachyte rock by hydrothermal treatment

The alkaline hydrothermal activation of trachyte rock led to synthesis of technologically important 11 Å tobermorite. Tobermorite synthesis was studied by X-ray diffraction, scanning electron microscopy and ²⁹Si and ²⁷Al high resolution magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. The influence of the reaction conditions such as different temperatures (150–170 °C), times (5–20 h) as well as different Ca/Si ratios of 0.6, 0.9 and 1.3 on tobermorite formation were investigated. The results showed that the main rock constituents were completely converted into a well crystallized Al-substituted 11 Å tobermorite when hydrothermally activated with 3.0 M NaOH under the optimum hydrothermal conditions of 170 °C for 20 h and using Ca/Si and Al/Al + Si ratios of 0.9 and 0.17, respectively. The local structure of the synthesized tobermorites as determined by MAS-NMR spectroscopy implied an alumino-silicate mean chain length of 5.9 units with 79% of the interlayer cross-links which are of Si–O–Al configuration. The present results show that trachyte rock could be considered as a new economic resource for synthesizing Al-substituted 11 Å tobermorites.     

Substituted Tobermorites: 27Al and 29Si MASNMR, Cation Exchange, and Water Sorption Studies

²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy revealed that the maximum amount of Al that can be substituted for Si in the tobermorite structure is 15 to 20 mol%. Powder X-ray diffraction and cation exchange studies corroborate the above finding. Anomalous tobermorite structures resulted in all cases, and hydrogarnet appeared beyond the 15 to 20 mol% Al substitution limit for Si. The sorption of water molecules by synthetic [Al³⁺+ Na⁺]-substituted tobermorites and their cation-exchanged forms heated at 200°C under vacuum (≥10⁻³ torr) was measured at 25°C in order to probe the nature of the ion-exchange cavity. Samples with ≤30 mol% of Al substitution for Si showed isotherms of Brunauer, Demming, Demming, Teller (BDDT) type I and gave better linearity with the Langmuir plot than with the BET plot. Samples with ≥35 mol% of Al substitution for Si showed BDDT type II isotherms and gave better linearity with the BET than with the Langmuir equation. The Langmuir monolayer capacity was found to depend on the Al content and on the cationic form. The Li⁺- and Na⁺-exchanged tobermorites with about 20 mol% of Al substitution for Si showed the highest Langmuir monolayer water sorption capacity. The Cs⁺-exchanged tobermorites showed a smaller capacity than the Li⁺- or Na⁺-exchanged samples, which can be ascribed to clogging of the ion-exchange cavity by the large Cs⁺ ions.     

Naturally occurring 1.4 nm tobermorite and synthetic jennite: Characterization by Al and Si MASNMR spectroscopy and cation exchange properties

A naturally occurring 1.4 nm tobermorite and a synthetic jennite were characterized by ²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance (MASNMR) spectroscopy and their cation exchange properties were measured. ²⁷Al MASNMR spectroscopy revealed that both the 1.4 nm tobermorite and synthetic jennite contained trace quantities of Al in tetrahedral coordination. The 1.4 nm tobermorite also contained octahedrally coordinated Al probably due to the presence of an aluminum compound as a trace impurity. The ²⁹Si MASNMR spectrum of 1.4 nm tobermorite exhibited a strong resonance at −85.2 ppm and a small shoulder at −80 ppm which are attributed to chain middle groups (Q²) and end groups (disilicates) respectively. ²⁹Si MASNMR spectroscopy of the synthetic jennite showed a strong resonance at −85.7 ppm and a moderately strong resonance at −81.4 ppm which correspond to single chains and end groups respectively. Jennite showed a considerably larger quantity of end groups than the 1.4 nm tobermorite. The unsubstituted 1.4 nm tobermorite and synthetic jennite exhibited only small cation exchange capacities.     

Al-substituted tobermorite—The coordination of aluminum as revealed by solid-state 27Al magic angle spinning (MAS) NMR

The coordination of A1 in several synthetic A1-substituted tobermorites was determined to be tetrahedral by solid-state ²⁷Al MASNMR. Earlier results of tetrahedral Al coordination in Al-substituted tobermorites by x-ray emission were confirmed unequivocally by the new high resolution MASNMR technique. However, two ²⁷Al chemical shifts at about 68 and 57 ppm were observed in the Al-substituted tobermorites which probably indicates two different environments for the Al atom in the structure. ²⁷Al MASNMR technique clearly distinguishes the tetrahedral and octahedral coordination of Al in solid samples.     

Synthesis and characterization of calcium–alumino-silicate hydrates from oil shale ash – Towards industrial applications

The influence of the alkaline medium on the hydrothermal activation of the oil shale fly ash with NaOH and KOH was studied using SEM/EDX, XRD, ²⁹Si and ²⁷Al high-resolution MAS-NMR spectra. In the presence of NaOH the silicon in the original fly ash was completely converted into calcium–aluminum–silicate–hydrates, mainly into 1.1 nm tobermorite structure during 24-h treatment at 160 °C. At similar reaction conditions, the activation with KOH resulted only to the formation of amorphous calcium–silica-hydrate gel on the surface of ash particles at temperature. The results obtained in this study indicate that the oil shale fly ash can be used for production of Al-substituted tobermorites when strongly alkaline media (NaOH) is applied. The synthesized product was used in a catalytic d-lactose isomerization reaction.     

亚熔盐法粉煤灰脱铝渣水热处理后碱含量的影响因素

以亚熔盐法处理粉煤灰的脱铝渣为原料,采用动态水热法分解脱碱,研究了不同A/S(Al2O3/SiO2质量比)、C/S(CaO/SiO2质量比)和不同脱铝溶出工艺对硅渣碱含量的影响. 结果表明,随脱铝渣A/S增加,碱含量先降低后升高,脱铝渣A/S为0.11,硅渣Na2O含量降至1.18%,适当的A/S有利于提高硅渣中含铝托贝莫来石的晶化程度;脱铝渣C/S为0.98,硅渣Na2O含量仅有1.31%,随脱铝渣C/S增加,硅渣碱含量增加,C/S过高会降低硅酸钠钙(NaCaHSiO4)的分解率,不利于生成含铝托贝莫来石相;溶出时间和停留时间较长的脱铝渣在脱碱过程中不易生成含铝托贝莫来石.     

Natural 1.1 and 1.4 nm tobermorites from Fuka, Okayama, Japan: Chemical analysis, cell dimensions, Si NMR and thermal behavior

A natural specimen of tobermorite, which contained both 1.1 and 1.4 nm phases, was studied by optical microscopy, X-ray powder diffractometry (XRD), EPMA, analytical TEM, ²⁹Si NMR and TG-DTA, and the results were compared with those for a specimen from Crestmore, California. The sample, occurring as a vein in contact with metamorphic rocks, could be divided into three zones. In two of the zones containing only the 1.1 nm phase, about 10% of the Si were substituted by Al. The third zone was an intergrowth of 1.1 and 1.4 nm tobermorites and contained almost no Al. The basal spacings of the 1.1 and 1.4 nm tobermorites were changed mainly to 0.9 nm by heating at 300 °C for 24 h but in some parts remained at 1.1 nm. ²⁹Si NMR results showed that silicate anions in the 1.1 nm tobermorite were double chains, which changed mainly into single chains, on heating at 300 °C. XRD results indicated that natural tobermorites are more highly crystalline than the synthetic ones of calcium silicate products.     

新型托勃莫来石的结构与表征

以粉煤灰为原料采用水热合成反应合成一种新型托勃莫来石,并且运用X射线衍射、扫描电子显微镜、傅立叶变换红外光谱、热重/差示扫描量热等测试手段对新型托勃莫来石的元素组成与微观结构进行了表征。结果表明,新型托勃莫来石比表面积为867 m~2/g,总孔体积为0.52 cm~3/g,平均孔径为2.34 nm,微粒内部及表面呈蜂窝状孔隙发育,由纳米晶片堆砌成微米多孔状聚集体,是一种高孔隙率的多孔吸附材料。     

Extended X-ray Absorption Fine Structure Investigation of Calcium Silicate Hydrates

²⁹Si MAS-NMR, and Ca-EXAFS experiments have been conducted on calcium silicate hydrates (C-S-Hs) with structure derived from wollastonite. Crystalline compounds (wollastonite, xonotlite, hillebrandite, foshagite, 1.1 nm and 1.4 nm tobermorites, and jennite) and C-S-H were synthesized and characterized. ²⁹Si NMR provides information on silicate chains and EXAFS on calcium environment. The refined EXAFS values are in agreement with XRD data, except for tobermorite. The calcium order in C-S-H (C/S molar ratio from 0.7 to 1.4) is similar to that of tobermorite but different from that of jennite. Structural models of C-S-H are discussed.     

Synthesis of cation exchange resin-supported iron and magnesium oxides/hydroxides composite for nitrate removal in water

In this study, we reported on the concept and practical use of cation exchange resin (CER) for removing anions in water via pretreating the CER with metal salts. The cation exchange resin-supported iron and magnesium oxides/hydroxides composite (Fe-Mg/CER) was synthesized and introduced as a new and potential adsorbent for selective removal of nitrate ion in the water environment. Characteristics of Fe-Mg/CER were determined by techniques such as Fourier-transform infrared spectroscopy, scanning electron microscopy, and X-ray diffraction. The results showed that Fe-Mg/CER material had a high nitrate adsorption capacity of 200 mg NO3-·g-1 with a fast equilibrium adsorption time of 30 min at pH 5. In addition, it had good durability of at least 10 times of regeneration, which could be applied to practical water and wastewater treatment.     

Solid-state high-resolution 29Si NMR spectroscopy of synthetic 14 Å, 11 Å and 9 Å tobermorites

A new method for the investigation of the structure of silicates - solid-state high-resolution ²⁹Si NMR spectroscopy - was used for the investigation of 14 Å, 11 Å and 9 Å tobermorites prepared under different conditions. It is possible to determine selectively the individual SiO₄ structural units in tobermorites (end groups, chain middle groups, branching sites, etc.) and to draw conclusions about the type and structure of the silicate anions in the lattice. Synthetic 14 Å tobermorite consists predominantly of single chains, the 11 Å and 9 Å tobermorites contain, depending on the method of preparation, double chains and/or single chains.     

Synthesis of normal and anomalous tobermorites

Tobermorites were made from several starting materials at 105 – 180°C and Ca/(Si + A?) 0.8 – 1.0. Reaction gives in succession C-S-H, normal, mixed and anomalous tobermorites, and finally xonotlite. High C/S ratio (1.0), short time, low temperature, stirring, presence of A?, and if quartz is used, small particle size, all tend to stop it at normal tobermorite. In some cases, this effect is due to promotion of crystal growth of normal tobermorite. Low C/S ratio (0.8), long time, high temperature, no stirring, presence of A? plus alkali, and, if quartz is used, large particle size, all tend to give anomalous tobermorite. However, at 180° C this changes easily into xonotlite if C/S > 0.9.