Enthalpy of formation of sulfate green rusts

As a contribution to the systematic study of iron oxide thermodynamics, this work reports enthalpies of formation of green rust, a double layered (Fe^II, Fe^III) hydroxide with the ideal stoichiometry , with sulfate as the anion in the interlayer. Samples were characterized by X-ray powder diffraction, thermogravimetric analysis, infrared spectroscopy, and Mössbauer spectroscopy. Full chemical analysis was performed. Contents of Fe^II, Fe^III, water, and sulfate were obtained. We report standard enthalpies of formation for green rust with different Fe^II/Fe^III ratios. Enthalpies of formation from single cation compounds, namely, Fe(OH)₂, Fe(OH)₃, FeSO₄ and H₂O show reasonable agreement with Gibbs free energies of formation from single cation compounds recalculated from the reported literature values. These values show that green rust has little stabilization over a mechanical mixture of these single cation compounds and there is no thermodynamic preference for any particular Fe^II/Fe^III ratio.     

Thermodynamic characterization of boltwoodite and uranophane: Enthalpy of formation and aqueous solubility

Boltwoodite and uranophane are uranyl silicates common in oxidized zones of uranium ore deposits. An understanding of processes that impact uranium transport in the environment, especially pertaining to the distribution of uranium between solid phases and aqueous solutions, ultimately requires determination of thermodynamic parameters for such crystalline materials. We measured formation enthalpies of synthetic boltwoodites, K(UO₂)(HSiO₄)·H₂O and Na(UO₂)(HSiO₄)·H₂O, and uranophane, Ca(UO₂)₂(HSiO₄)₂·5H₂O, by high temperature oxide melt solution calorimetry. We also studied the aqueous solubility of these phases from both saturated and undersaturated conditions at a variety of pH. The combined data permit the determination of standard enthalpies, entropies and Gibbs free energies of formation for each phase and analysis of its potential geological impact from a thermodynamic point of view.     

Enthalpy of formation of CaSi2O5, a quenched high-pressure phase with pentacoordinate silicon

 The monoclinic titanite-like high-pressure form of calcium disilicate has been synthesized and quenched to ambient conditions to form the triclinic low-pressure phase containing silicon in four-, five- and sixfold coordination. The enthalpy of formation of the quench product has been measured by high-temperature oxide melt calorimetry. The value obtained from samples from a series of several synthesis experiments is ΔH _f = (−26.32 ± 4.27) kJ mol⁻¹ for the formation from the component oxides, or ΔH _f  = (−2482.81 ± 4.59) kJ mol⁻¹ for the formation from the elements. The result is identical within experimental error to available estimates, although the previously predicted energy difference between the monoclinic and triclinic phases could not be verified. Received: 16 February 2000 / Accepted: 14 July 2000     

Enthalpy of diaplectic labradorite glass

Enthalpies of solution in molten 2PbO·B₂O₃ at ～988 K have been measured for diaplectic labradorite glass from the Manicouagan impact crater and a fused glass formed from the same material. The enthalpies of solution of the diaplectic and fusion-formed glasses are 4,347 and 2,023 cal mol^?1, respectively. The more endothermic enthalpy of solution of the diaplectic glass indicates a greater relative energetic stability of about 2.3 kcal mol^?1. The data are consistent with Diemann and Arndt's (1984) structural model that suggests the diaplectic glass is more ordered than fusion-formed glass and with the presence of crystallites. Comparison of data to enthalpies of solution of crystalline labradorite (Carpenter et al. 1985) indicates a maximum percentage of crystalline relics of ～15–18%, also consistent with Diemann and Arndt's (1984) estimate of <17%. Thus the diaplectic glass is intermediate in thermochemical properties between normal glass and crystal (much closer to glass) and does not represent any state more unstable than normal fusion-formed glass.     

Thermochemical study of natural montmorillonite

The paper reports results of an experimental thermochemical study (in a heat-flux Tian-Calvet microcalorimeter) of montmorillonite from (I) the Taganskoe and (II) Askanskoe deposits and (III) from the caldera of Uzon volcano, Kamchatka. The enthalpy of formation Δ _f H _el ⁰ (298.15 K) of dehydrated hydroxyl-bearing montmorillonite was determined by melt solution calorimetry: ?5677.6 ± 7.6 kJ/mol for Na_0.3Ca_0.1(Mg_0.4Al_1.6)[Si_3.9Al_0.1O₁₀](OH)₂ (I), ?5614.3 ± 7.0 kJ/mol for Na_0.4K_0.1(Ca_0.1Mg_0.3Al_1.5Fe _0.1 ³⁺ )[Si_3.9Al_0.1O₁₀](OH)₂ (II), ?5719 ± 11 kJ/mol for K_0.1Ca_0.2Mg_0.2(Mg_0.6Al_1.3Fe _0.1 ³⁺ ) [Si_3.7Al_0.3O10](OH)₂ (III), and ?6454 ± 11 kJ/mol for water-bearing montmorillonite (I) Na_0.3Ca_0.1(Mg_0.4Al_1.6)[Si_3.9Al_0.1O₁₀](OH)₂ · 2.6H₂O. The paper reports estimated enthalpy of formation for the smectite end members of the theoretical composition of K-, Na-, Mg-, and Ca-montmorillonite and experimental data on the enthalpy of dehydration (14 ± 2 kJ per mole of H₂O) and dehydroxylation (166 ± 10 kJ per mole of H₂O) for Na-montmorillonite.     

Enthalpy of formation of forsterite,enstatite, akermanite,monticellite and merwinite at 1073 K determined by alkali borate solution calorimetry

Enthalpies of solution of synthetic enstatite (Mg₂Si₂O₆), forsterite (Mg₂SiO₄), akermanite (Ca₂MgSi₂O₇), monticellite (CaMgSiO₄), and merwinite (Ca₃MgSi₂O₈) and their component oxides were determined in eutectic (Li, Na)BO₂ at 1073 K. Resulting enthalpies of formation at 1073 are enstatite: $\text{?8.10 ± 0.42}\text{kcal}$; forsterite: $\text{?14.23 ± 0.45}\text{kcal}$; akermanite: $\text{?42.60 ± 0.39}\text{kcal}$; monticellite: $\text{?25.05 ± 0.41}\text{kcal}$; and merwinite: $\text{?51.10 ± 0.49}\text{kcal}$. The value for the synthetic monticellite of composition Mo_.965Fo_.035 was corrected slightly for non-stoichiometry based on experimental monticellite-forsterite phase equilibrium relations.The enthalpies of formation of enstatite and forsterite are somewhat less negative than yielded by several other solution calorimetric studies but are in good agreement with the recent Pb₂B₂O₅ solution calorimetry of Kiselevaet al. (1979), and are in good agreement with values to be derived from reliable phase equilibrium data in the system MgO-Al₂O₃-SiO₂. The enthalpies of formation of akermanite, monticellite and merwinite are all much less negative than values tabulated by robieet al. (1978) and helgesonet al. (1978) but are shown to be compatible with reliable phase equilibrium data for the system CaO-MgO-SiO₂, whereas the tabulated values are not. Several methods of analysis yield an entropy of monticellite at 1000 K of $\text{69.9 ± 0.2 cal/K}$.     

Oxytetracycline sorption onto Iraqi montmorillonite

This paper assesses the use of certified Iraqi montmorillonite clay as a potential sorbent for the removal of oxytetracycline (OTC) from aqueous solutions. The clay is characterized by a cation exchange capacity of 0.756 meq g^?1 and a zero point charge at pH 8.7. Aqueous solutions of OTC were equilibrated with montmorillonite under various experimental conditions, such as OTC concentration, pH and clay content, for 24 h at fixed ionic strength. Two forms of montmorillonite were evaluated: regular and iron-modified form. The effect of pH was minor on OTC adsorption. Kinetic study revealed that the sorption follows a pseudo-second-order model. Sorption isotherm showed a good fit with the Freundlich model. OTC sorption onto Fe-saturated montmorillonite was analyzed statistically using a response surface design to study the effects of experimental conditions. The introduction of iron improved the adsorption characteristics of the clay due to the ability of ferric ions to make stable complexes with OTC. The most favorable operating conditions for the treatment were deemed as follows: clay content, 6.85 g L^?1, oxytetracycline concentration, 1.0 mmol L^?1 and pH, 5.5 for the iron-modified form.     

钠基蒙脱土水合演化机制

为了查明钠基蒙脱土的水合演化过程,以天然钠基蒙脱土为研究对象,开展在相对湿度（ ）为0～0.98区间的水汽等温吸-脱附试验,通过吸附速率曲线、BET曲线界定钠基蒙脱土各水合阶段及相应的水合主控因素;通过测定晶层 值变化规律,从吸附水影响黏土矿物晶层厚度的角度探讨钠基蒙脱土的水合演化特征;基于傅里叶红外光谱,从水分子结构伸缩振动信息角度对钠基蒙脱土水合演化过程进行定性定量验证;通过热重/差热分析,以吸附水相变所需能量与吸附水重量变化的角度解释钠基蒙脱土的吸附水特征与其水合机制的关系。试验结果表明：在较低相对湿度下（0 0.15）,以钠基蒙脱土矿物外表面吸附为主,形成表面吸附水;0.15 0.40为钠基蒙脱土层间阳离子水合阶段;0.40 0.98,为晶层内外表面水合阶段,水分子逐步完整的包裹蒙脱土,形成多层吸附层。钠基蒙脱土的水合演化过程受控于层间钠离子与晶层基面,层间钠离子的水合能影响了钠基蒙脱土水合演化的起始顺序。     

Structural properties of reduced Upton montmorillonite

Reduction of octahedral Fe in the crystalline structure of smectites influences, possibly controls, surface-sensitive physical and chemical properties. The purpose of this study was to investigate if reduction of structural Fe by Na-dithionite or bacteria affects the chemical environment of constituent cations in montmorillonite, employing solid state multinuclear (²⁹Si and ²⁷Al) magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Reduction of structural Fe resulted in a positive (down field) chemical shift of the main Si Q³ (Q³(0Al)) site which was strongly correlated with Fe(II) content and inferred that distortions in Si-OT (T=Si, Al) bond angles and Si-O bond lengths occur with increasing layer charge. The line width (W) of the ²⁹Si Q³ signal also increased with increasing levels of reduction. No change occurred in the position of the peak maximum for the octahedral Al (²⁷Al_VI) signal; however, an increased W was observed for this peak with increasing Fe(II) content. These results are attributed to decreases in Si-O-T bond angles and Si-O bond distances, corresponding to a better fit between the tetrahedral and octahedral sheets brought about by the presence of Fe(II) in the clay structure. The increased ²⁷Al_VI signal width (W) may also be due to a lessening of the paramagnetic influence of Fe(III) nuclei and enhancement of ²⁷Al_VI signals with different quadrupole coupling constants (QCC). Multinuclear MAS NMR analyses of dithioniteand microbially-reduced montmorillonite indicate that reduction of structural Fe caused reversible changes in the smectite structure, at least as far as this method could discern.     

Sodium-cadmium and sodium-zinc exchangeability in montmorillonite

A sodium montmorillonite from Ivan?ice (Czech Republic) was treated with solutions containing various molar concentrations of Zn and Cd for the purpose of determining ion exchange equilibria. The sorption isotherms for Zn and Cd exhibit a rather similar shape, which depends on pH; the maxima for Zn and Cd sorbed on Na-montmorillonite are very close to each other (e.g., 0.419?mmol/1?g and 0.440?mmol/1?g, respectively, for pH=5). It is a characteristic of both elements that a high level of sorption is reached at low concentration in solution. The leachability of Zn and Cd in deionized water from fully saturated montmorillonites is very similar, but a different results were observed for Na leached from fully saturated Na-montmorillonite. The total amount of leached Na was 21.7% (after four consecutive leaching runs). However, only 2.1% and 1.6% were found for Cd and Zn, respectively. Different quantities of Cd and Zn exchanged in Na-montmorillonite influence significantly the shape and position of maximum of the 001 XRD profile. Experimental XRD profiles of montmorillonites, fully saturated with Na, Cd and Zn, corrected for instrumental and physical factors exhibit maxima at 12.59, 14.86, and 14.77?Å, respectively. When the d ₀₀₁ spacing is determined from a corrected profile, it varies systematically with the percentage of exchanged Zn and Cd and the relationships seem to be linear for both elements. For montmorillonites, not fully saturated with one element, the peak shift and peak broadening, characteristic for mixed layered structures with random layer sequences, has been observed.     

无机—有机柱撑蒙脱石对苯酚的吸附

分别用无机－有机改性柱撑蒙脱石对苯酚进行吸附试验,研究它们吸附苯酚的适宜条件圾吸附等温线（２５℃）,结果表明,用表面活性剂改性的柱撑蒙脱石,能大幅度提高对苯酚的吸附能力,柱撑蒙脱石对苯酚的吸附能力主要取决于改性粘土吸附剂的微孔结构和表面组分,而不仅仅依赖于表面积,经５００℃高温灼烧后柱撑蒙脱石柱结构及层间距（１．８３ｎｍ）稳定,因而柱撑蒙脱石可再生循环使用,是一种潜在的吸附环境污染物的物质。     

聚苯胺／蒙脱石纳米复合材料的实验制备研究

对聚苯胺 /蒙脱石纳米复合材料的制备方法与制备产物进行了研究。将新疆某地蒙脱石在钠化改型后进行了季铵盐改性处理。以苯胺作为客体物质 ,利用季铵盐 /蒙脱石插层复合物与有机物良好的相容性 ,将季铵盐 /蒙脱石插层复合物分散在苯胺单体中后 ,苯胺单体进入了蒙脱石层间域 ,通过原位聚合法制备了聚苯胺 /蒙脱石纳米复合材料。XRD、SEM、TG_DTA分析结果表明 ,苯胺单体的进入使蒙脱石的层间距大大增加 ,经聚合后聚苯胺 /蒙脱石复合物中蒙脱石及其插层复合物的X射线衍射特征、形态特征和热学性质特征完全消失 ,表明蒙脱石晶层已被剥离分散在聚苯胺中形成聚苯胺 /蒙脱石纳米复合材料。     

蒙脱石伊利石化的水热实验模拟

以蒙脱石为初始材料,通过水热实验模拟在不同温度压力条件下蒙脱石的伊利石化过程。利用X射线粉末衍射及X射线荧光光谱、红外光谱和扫描电镜等方法对样品的矿物物相、化学成分、晶体结构与微观形貌等进行分析,其结果表明:(1)蒙脱石至伊利石的转化受温度和压力条件的控制;(2)随着温度和压力的增高,蒙脱石伊利石化沿着蒙脱石-无序伊蒙混层-有序伊蒙混层-伊利石的转化序列进行转化;(3)水热实验模拟表明蒙脱石至伊利石的转化以溶解重结晶机制为主。实验结果不仅有助于了解埋藏与低级变质作用中的蒙脱石伊利石化过程,同时为研究断层活动所致断层泥中黏土矿物的转化及反演断层形成时温度压力条件提供基本参数。     

蒙脱石中Cu2+的吸附态研究

对在ＰＨ≤４．５、痕量浓度条件下吸附了Ｃu＾２＋的蒙脱石的Ｘ射线衍射（ＸＲＤ）进 行了研究。该蒙脱石的ｄ（００１）由原始蒙脱石的１．４７９ｎｍ变为１．５２３ｎｍ,说明水合铜离子置换了蒙脱石的层间阳离子;在该蒙脱石的ＥＰＲ谱中记录到了ｇ＝２．０７、ｇ ＝２．１４精细结构线和ｇ为约２．７～２．２超精细结构线三组有关Ｃu＾２＋ 的信号,说明其中的Ｃu＾２＋有三种不同的结构状态。同时,该蒙脱石热处理产物的ＥＰ