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Determination of the reaction heat and studies of thermodynamic functions

Hydrolytic polymerization of chromium(III) at relatively high concentrations by microcalorimetry

Journal of Thermal Analysis and Calorimetry
Authors: Lu Chengxue, Yu Xiufang, and Zhang Honglin

The hydrolytic polymerization of Cr3+ at relatively high concentrations was studied by microcalorimetry. The thermal curves were determined with a 2277 thermal activity monitor. From the curves, the identified reaction heats (δrH m σ), hydrolysis constants (K) and thermodynamic functions (δrS m σrH m σ) were calculated at different temperatures.

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Owing to poor bonding between coarse fly ash particles and hydration products, gap-graded blended cements with fly ash usually show lower compressive strengths than Portland cement. Surface cementitious properties of coarse fly ash were improved by dehydration and rehydration processes in the present study. The results show that during the calcination at 750 °C, C–S–H gel is mainly transformed into a new nesosilicate, which is similar to a less crystalline C2S. The formation of melilite from hydration products is also noticed at 900 °C, however, this will not contribute to rehydration of calcined fly ash. Rehydration of new generated nesosilicate on the surface of coarse fly ash leads to a better bonding between coarse fly ash particles and hydration products. As a result, both early and late mechanical properties of gap-graded blended cements containing 25% cement clinker and 39% calcined coarse fly ash are higher than those of 100% Portland cements.

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The kinetics of the reduction of plutonium(IV) by hydroxyurea (HU), a novel salt free reductant, in nitric acid solutions has been studied. The observed reaction rate can be expressed as: -d[Pu(IV)]/dt=k 0[Pu(IV)]2[HU]/[H+]0.9, where k 0 = 5853±363 (l1.1.mol-1.1.s-1) at t = 13 °C. The activation energy is about 81.2 kJ/mol. The study also shows that uranium(VI) has no appreciable influence on the reaction rate. Compared with other organic reductants our experiments indicate that HU is a very fast reductant for plutonium(IV).

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In this work, adsorption of Ni(II) from aqueous solution onto hematite under various solution chemistry and temperature was investigated. The results indicated that the pseudo-second-order rate equation fitted the kinetic adsorption well. The adsorption of Ni(II) onto hematite was strongly dependent on pH and ionic strength. At low pH, the adsorption was dominated by outer-sphere surface complexation or ion exchange, whereas inner-sphere surface complexation was the main adsorption mechanism at high pH. A positive effect of FA on Ni(II) adsorption was found at pH < 8.0, whereas a negative effect was observed at pH > 8.0. The Langmuir, Freundlich, and D–R models were applied to simulate the adsorption isotherms at three different temperatures of 293.15, 313.15 and 333.15 K. The thermodynamic parameters were calculated from the temperature dependent adsorption, and the results indicated that the adsorption was endothermic and spontaneous.

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To optimize the hydration process of blended cement, cement clinker and supplementary cementitious materials (SCMs) were ground and classified into several fractions. Early hydration process of each cementitious materials fraction was investigated by isothermal calorimeter. The results show fine cement clinker fractions show very high hydration rate, which leads to high water requirement, while fine SCMs fractions present relatively high hydration (or pozzolanic reaction) rate. Cement clinker fractions in the range of 8–24 μm show proper hydration rate in early ages and continue to hydrate rapidly afterward. Coarse cement clinker fractions largely play “filling effect” and make little contribution to the properties of blended cement regardless of their hydration activity (or pozzolanic activity). The hydration process of blended cement can be optimized by arranging high activity SCMs, cement clinker, and low activity SCMs in fine, middle, and coarse fractions, respectively, which not only results in reduced water requirement, high packing density, and homogeneous, dense microstructure, but also in high early and late mechanical properties.

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The fate and transport of toxic metal ions and radionuclides in the environment is generally controlled by sorption reactions. The removal of 60Co(II) from wastewaters by MnO2 was studied as a function of various environmental parameters such as shaking time, pH, ionic strength, foreign ions, and humic substances under ambient conditions. The results indicated that the sorption of 60Co(II) on MnO2 was strongly dependent on pH and ionic strength. At low pH, the sorption of 60Co(II) was dominated by outer-sphere surface complexation and ion exchange with Na+/H+ on MnO2 surfaces, whereas inner-sphere surface complexation was the main sorption mechanism at high pH. The presence of HA/FA enhances 60Co(II) sorption at low pH values, whereas reduces 60Co(II) sorption at high pH values. The Langmuir and Freundlich models were used to simulate the sorption isotherms of 60Co(II) at three different temperatures of 298.15, 318.15 and 338.15 K. The thermodynamic parameters (ΔH 0, ΔS 0 and ΔG 0) calculated from the temperature dependent sorption isotherms indicated that the sorption process of 60Co(II) on MnO2 was endothermic and spontaneous.

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This article described the synthesis and mesomorphic behavior transition of a novel liquid crystalline (LC) epoxy resin 4-(2,3-epoxypropoxy)biphenyl,4″-(2,3-epoxypropoxy)phenyl-4′carboxylate (EBEPC), which combined a hydroxyl benzoic aromatic ester and biphenol rigid-rod group. EBEPC showed a clear nematic schlieren texture under curtain conditions. The reaction kinetics of EBEPC cured by 4,4′-diaminodiphenyl-methane (DDM) was studied by using an isoconversional method under isothermal conditions with differential scanning calorimetry (DSC). The isothermal DSC data can be fitted reasonably by an autocatalytic curing model. Smectic phases had been observed in the EBEPC/DDM curing system. The results of DSC showed that the formation of the LC phase had pronounced influence on the curing reaction.

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This article describes the synthesis of a liquid crystalline curing agent 4,4′-bis-(4-amine-butyloxy)-biphenyl (BABB), and its application as a curing agent for the epoxy resin (DGEBA) in comparison with normal curing agent, 4,4′-diaminobiphenyl (DABP). BABB was investigated with polarized optical microscopy, differential scanning calorimetry, and small-angle X-ray scatting, and the results showed that BABB displayed smectic liquid crystalline phase. The curing behaviors of DGEBA cured with BABB and DABP were studied by using differential scanning calorimetry (DSC), polarized optical microscopy (POM), and dynamic mechanical analysis (DMA). The results indicated that BABB showed a higher chemical reactivity than DABP. The kinetics was studied under isothermal conditions using an isoconversional method, and the isothermal DSC data can be fitted reasonably by an autocatalytic curing model. The nematic droplet texture was observed for the resulting polymer network of DGEBA/BABB system, while the DGEBA/DABP system showed an isotropic state. The storage modulus of DGEBA/BABB system was enhanced in comparison with DGEBA/DABP system because of the formation of LC phase, whereas the glass transition temperatures decreased because of the introduction of flexible spacer group.

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N,N-dialkylamides, (octan-, decan- and dodecanamides) having ethyl and butyl groups as alkyl substituents, were synthesized in order to investigate their selectivity and capability in the extraction of Th(IV) from acidic nitrate media (3 and 6M HNO3) in nuclear reprocessing. The distribution ratios of Th (IV) with the amides in kerosene decrease with the increase of the alkyl chain length (C8, C10, C12), probably due to steric hindrance.

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