Experimental and molecular mechanics and ab initio investigation of activated adsorption and desorption of trichloroethylene in mineral micropores

This research investigated activated adsorption of a hydrophobic organic contaminant(HOC) in mineral micropores using experimental and molecular modeling techniques. Adsorption of trichloroethylene (TCE) on a silica gel adsorbent was measured using a frontal analysis chromatography technique at atmospheric and elevated fluid pressures. Increasing the fluid pressure yielded increased TCE uptake that was not released upon lowering the pressure back to atmospheric conditions. This showed that the increase in pressure was able to rapidly induce the formation of a desorption-resistant fraction that previous investigations have shown requires months to develop at atmospheric pressure. Grand Canonical Monte Carlo (GCMC) modeling was then used to elucidate the nature of water and TCE behavior within silica micropores. The GCMC modeling showed that molecular scale packing restrictions resulted in pore fluid densities that ranged from 0.28 to 0.78 of those in the bulk solution. The modeling also showed that TCE was able to displace water from hydrophilic mineral pores due to molecular scale packing restrictions. Exothermic isosteric heats for TCE adsorption up to -27 kJ/mol were observed and were greatest in pores of 7 and 8 A. This indicated that TCE adsorption was energetically most favorable in pores that were minimally large enough to accommodate a TCE molecule. The pressure-induced uptake appeared to result primarily from an increase in the packing density in the smallest pores. Ab initio calculations showed that small distortions of a TCE molecule from its low energy conformation require high activation energies. Results from this study indicate that activated adsorption requiring bond angle distortions in the adsorbate may be responsible forthe slow attainment of adsorptive equilibrium of HOCs on microporous solids. Likewise, activated desorption from molecular-sized adsorption sites may contribute to the slow release of HOCs from aquifer sediments.     

Heat of sorption of water in dried fruits

The heats of moisture adsorption and desorption in four dried fruits (sultana raisins, figs, prunes and apricots) were estimated from equilibrium sorption data, using the Clausius-Clapeyron equation in the temperature range 15–60°C. The net isosteric heat of sorption ( q st) decreased sharply from about 20 kJ mol⁻¹ water to near zero when the moisture content was increased from 0.05 to 0.50kg water kg⁻¹ dry matter. An exponential function was fitted to the experimental q st values at various moisture contents (X) , yielding two characteristic constants ( q 0 and X 0) for each fruit. Mean and total heats of sorption were calculated from the proposed empirical equation, which are useful for enthalpy prediction in food dehydration. The heats of desorption were higher than the heats of adsorption, indicating significant hysteresis in the sorption of water, especially in dried apricots.     

Limited multilayer desorption of brown, parboiled rice

Two mathematical models, the BET-equation and the Caurie-equation, are used to analyse experimental desorption isotherms of two long-grain varieties of brown, parboiled rice at different temperatures. Monolayer moisture contents and maximum number of sorbed water layers, estimated by the Caurie-model, decrease with increasing temperature. From the temperature dependence of the desorption isotherms, isosteric heats of desorption are calculated. For both varieties, the differential heats of desorption are already very small (1.4 kJ/mol) at a moisture content of 20%. Thus, additional moisture behaves almost like pure water.     

Electrochemical investigation of the rate-limiting mechanisms for trichloroethylene and carbon tetrachloride reduction at iron surfaces

The mechanisms involved in reductive dechlorination of carbon tetrachloride (CT) and trichloroethylene (TCE) at iron surfaces were studied to determine if their reaction rates were limited by rates of electron transfer. Chronoamperometry and chronopotentiometry analyses were used to determine the kinetics of CT and TCE reduction by a rotating disk electrode in solutions of constant halocarbon concentration. Rate constants for CT and TCE dechlorination were measured as a function of the electrode potential over a temperature range from 2 to 42 degrees C. Changes in dechlorination rate constants with electrode potential were used to determine the apparent electron-transfer coefficients at each temperature. The transfer coefficient for CT dechlorination was 0.22 +/- 0.02 and was independent of temperature. The temperature independence of the CT transfer coefficient is consistent with a rate-limiting mechanism involving an outer-sphere electron-transfer step. Conversely, the transfer coefficient for TCE was temperature dependent and ranged from 0.06 +/- 0.01 at 2 degrees C to 0.21 +/- 0.02 at 42 degrees C. The temperature-dependent TCE transfer coefficient indicated that its reduction rate was limited by chemical dependent factors and not exclusively by the rate of electron transfer. In accord with a rate-limiting mechanism involving an electron-transfer step, the apparent activation energy (Ea) for CT reduction decreased with decreasing electrode potential and ranged from 33.0 +/- 1.6 to 47.8 +/- 2.0 kJ/mol. In contrast, the Ea for TCE reduction did not decline with decreasing electrode potential and ranged from 29.4 +/- 3.4 to 40.3 +/- 3.9. The absence of a potential dependence for the TCE Ea supports the conclusion that its reaction rate was not limited by an electron-transfer step. The small potential dependence of TCE reaction rates can be explained by a reaction mechanism in which TCE reacts with atomic hydrogen produced from reduction of water.     

Thermodynamic Analysis of Water Vapor Sorption Isotherms and Mechanical Properties of Selected Paper-Based Food Packaging Materials

ABSTRACT:  Adsorption isotherms of 3 selected paper-based packaging materials, that is, vegetable parchment (VP) paper, Kraft paper, and solid-bleached-sulfate (SBS) paperboard, were determined at 3 different temperatures (25, 40, and 50 °C). The GAB isotherm model was found to fit adequately for describing experimental adsorption isotherm data for the paper samples. The monolayer moisture content of the paper samples decreased with increase in temperature, which is in the range of 0.0345 to 0.0246, 0.0301 to 0.0238, and 0.0318 to 0.0243 g water/g solid for the MG paper, the Kraft paper, and the SBS paperboard, respectively. The net isosteric heats of sorption ( q_st ) for the paper samples decreased exponentially with increase in moisture content after reaching the maximum values of 18.51, 27.39, and 26.80 kJ/mol for the VP paper, the Kraft paper, and the SBS paperboard, respectively, at low-moisture content. The differential enthalpy and entropy of 3 paper samples showed compensation phenomenon with the isokinetic temperature of 399.7 K indicating that water vapor had been adsorbed onto the paper samples with the same mechanism. Depending on the paper material, tensile strength of paper samples was affected by moisture content.     

芝麻平衡水分及吸着等热研究

采用静态称重法测定了我国4个芝麻品种的平衡水分等温线,并采用CAE、修正Chung-Pfost（MCPE）、修正Halsey（MHAE）、修正Henderson（MHE）、修正Guggenheim-Anderson-deBoer（MGAB）、修正Oswin（MOE）及StrohmanYoerger（STYE）7个水分吸着方程进行拟合,指出MOE最适合描述芝麻平衡含水率（EMC）-平衡相对湿度（ERH）之间的关系,并用于计算芝麻吸着等热。在含水率<7.5%湿基,芝麻吸着等热均随含水率增大而快速减少,同一温度下的解吸等热显著高于吸附等热。在含水率7.5%以上,芝麻吸着等热随含水率增大而变化平缓,同一温度下的解吸等热趋同于吸附等热。在含水率<7.5%湿基条件下,较低温度下的芝麻吸附等热与解吸等热均高于较高温度。在含水率10%湿基的自由水点,芝麻的吸着等热（汽化热）接近纯水的潜热,约是2450kJ/kg。在测定温度10³5℃范围,黑芝麻吸着等热数值类似白芝麻吸着等热数值。计算的25℃芝麻储运绝对安全水分是6.46%,相对安全水分是6.94%。      

Thermodynamics of the Process of Adsorption of Phosphate Starch on Cellulose

Subject of this study is the adsorption of phosphate starch (PHS) on cotton cellulose and unbleached pulp from coniferous wood at temperature of 0.20, 40 and 60°C. resp., and concentration of the PHS solutions ranging from 0,030 to 2,00 g/l. It was found that with an increase of temperature the quantity of adsorbed PHS decreases. Adsorption equilibrium is described by the adsorption isotherm of Langmuir. The equilibrium constants, isosteric heats, standard affinity and entropy of the process are defined. It was established that the desorption of PHS of sulphate unbleached pulp of coniferous wood was 46.5%.     

Influence of Roasting on the Water Sorption Isotherms of Argentinean Algarroba (Prosopis alba Griseb) Pods

Prosopis alba is an arboreal legume that occurs naturally in Argentine. Its fruits (algarroba or Prosopis pods) are dried or roasted, ground and then used in foods and feeds. The influence of roasting process on the water sorption isotherms of Prosopis pod flour at three storage temperatures was studied. The equilibrium data for each sorption isotherm were determined by the standard static gravimetric method. Experimental values were fitted to the BET and GAB sorption models. Type II isotherms were obtained according to Brunauer classification. The calculated isosteric heats of sorption (Qs) near the monolayer moisture content were 7.30 and 7.68 kJ/mol for raw and roasted flour, respectively. The results showed that roasting did not significantly change the behavior of the products with regard to water adsorption, although a slight reduction of the tendency to humectation was observed, this being somewhat less spontaneous. In this aspect, the stability of Prosopis flour is similar in the raw and roasted states.     

Effect of freezing temperature on rehydration and water vapor adsorption characteristics of freeze-dried rice porridge

Effect of freezing temperature of rice porridge on quality properties of freeze-dried rice porridge was investigated at various temperatures. Slow freezing produced porous freeze-dried rice porridges with large pores and more brittle structure than rapid freezing. All the quality properties tested were greatly influenced by both freezing temperature and temperature for the property evaluation. Both the initial rehydration ratio and the dissolution time of the freeze-dried rice porridge decreased with increase in water temperature. Results on the effect of freezing temperature on rehydration ratio and dissolution time indicated slow freezing is more preferable for the freeze-dried rice porridge. The Peleg model was fitted better than the Singh and Kulshrestha model to explain the water vapor adsorption kinetics. The activation energy for the water vapor adsorption of the freeze-dried rice porridge determined by using the Peleg model ranged 16.2-23.1 kJ/mol depending on the freezing temperature.     

Moisture Sorption Characteristics of Freeze Dried Blueberries

Moisture sorption isotherms of freeze dried lowbush blueberries at 4, 16, 25, 35, and 45°C were determined using indirect gravimetric method. Moisture sorption kinetics at these temperatures and 75% RH were also investigated. The isotherms followed a type III shape with a temperature inversion effect at 0.9 water activity. They were best described by the GAB equation. Net isosteric heat of moisture sorption in freeze dried blueberries, determined using the Clausius Clapeyron equation, varied from 5.5 to 0.25 kJ/mol as moisture content changed from 8 to 36% (dry basis). Moisture adsorption kinetics in freeze dried blueberries were well described by the first order kinetics equation. The rate constant followed the Arrhenius relationship with an activation energy of 38.6 kJ/mol.     

Moisture adsorption isotherms for mushroom

The moisture adsorption isotherms of Agaricus bisporus and Pleurotus florida mushrooms were determined at temperatures (30-70°C) typically found in drying and storage. The samples were equilibrated above saturated salt solutions. Equilibrium moisture content of mushroom decreased with an increase in temperature at constant water activity. The data was adjusted to 11 sorption models to ascertain the best fit. Comparisons were based on mean relative error, standard deviation and coefficient of determination. Of the models tested, the Chung and Pfost model showed the best fit. The net isosteric heat of sorption was determined using the Clausius-Clapeyron equation. The net isosteric heat of sorption decreased from 13 to about 1 kJ/mol when the moisture content increased from 5 to 50 g /100 g dry matter.     

Equilibrium moisture content and heat of sorption of Gelidium sesquipedale

Moisture equilibrium data for adsorption and desorption of water from Gelidium sesquipedale were investigated at temperatures in the range of 30-50°C and water activity ranging from 0.05 to 0.9. The experimental procedure used was based on the gravimetric static method. The sorption curves of Gelidium sesquipedale decreased with increase in temperature at constant relative humidity. The hysteresis effect was observed. The experimental data of sorption were described by six models. The GAB and modified BET models were found to be the most suitable for describing the sorption curves. The isosteric heats of desorption and adsorption of water were determined from the equilibrium data at different temperatures.     

不同流体界面上大豆11S球蛋白吸附膜膨胀黏弹性的比较研究

采用轴对称滴形分析法(ADSA)检测了空气/水、正十二烷/水和大豆油/水界面上不同浓度(0.001%~0.1%)的大豆11S球蛋白吸附膜的表面膨胀黏弹性随吸附时间的变化。研究表明,在体相蛋白溶液pH8.0和离子强度0.05mol/L的恒定条件下,随着吸附时间的延长,表面膨胀弹性增大,膨胀弹性明显大于膨胀黏性。从表面流变学的角度分析,空气/水和大豆油/水界面上大豆11S球蛋白吸附膜实际上是弹性的,膨胀弹性随液滴体相蛋白浓度的增加而增大,受界面类型的影响很大。空气/水界面上吸附膜的膨胀黏弹性最大,而大豆油/水界面上吸附膜的膨胀黏弹性最小。     

Study of the Kinetics of the Adsorption of Dialdehyde Starch on Cellulose

The kinetics of adsorption of dialdehyde starch water solution on sulphite unbleached and bleached coniferous cellulose were studied. The kinetics of adsorption is described by an exponential kinetic equation applied for energetically uniform heterogeneous surfaces. The energy of activation for unbleached cellulose was in the limits from 10.6–27.6, while those of bleached cellulose were from 12.6–20.0 kJ/mol and were not dependent on the initial concentration of the DAS solution but linearly grow with the increase of the adsorbed DAS amount. The studied process was effected by the entropy factors exerting a defining influence on the initial rate, while the energy ones defined the changing of the rate.     

Probing the adsorption characteristic of metal-organic framework MIL-101 for volatile organic compounds by quartz crystal microbalance

As volatile organic compounds (VOCs) are a major group of air pollutants, development of materials for efficient adsorption and removal of VOCs is of great significance in both environmental and analytical sciences. Here we report metal-organic frameworks (MOFs) MIL-101 for the effective adsorption of VOCs at atmospheric pressure. A simple device was designed for quartz crystal microbalance (QCM), and six VOCs with various functional groups and polarities, i.e., n-hexane, toluene, methanol, butanone, dichloromethane, and n-butylamine, were chosen as targets to probe the adsorption properties of MIL-101. The developed device allows measurement of the adsorption isotherms and monitoring of the dynamic process for the adsorption of VOCs on MOFs, and also provides a useful tool for characterization of MOFs. The adsorption isotherms of the VOCs on MIL-101 followed the Dubinin-Astakhov equation with the characteristic energy from 5.70 (methanol) to 9.13 kJ mol(-1) (n-butylamine), Astakhov exponent from 0.50 (n-butylamine) to 3.03 (n-hexane), and the limiting adsorption capacity from 0.08 (n-hexane) to 12.8 (n-butylamine) mmol g(-1). MIL-101 exhibited the strongest affinity to n-butylamine, but the weakest affinity to n-hexane. The determined Astakhov exponents and the isosteric heats of adsorption revealed the energetic heterogeneity of MIL-101. MIL-101 showed the most energetically homogeneous for n-hexane, but the most energetically heterogeneous for n-butylamine. The dynamic process of adsorption monitored by the QCM system demonstrated the distribution of the sorption sites within MIL-101. The metal sites within the MIL-101 were vital in adsorption process. MIL-101 gave much higher affinity and bigger adsorption capacity to VOCs than activated carbon, offering great potential for real applications in the adsorption and removal of VOCs.     

基于吸附理论分析活性炭对卷烟烟气的吸附

为了解活性炭孔隙结构及被吸附化合物的性质对吸附效率的影响,测定了纯丙酮气体在活性炭上的吸附特性及不同结构活性炭对烟气羰基物的吸附效率。分别用Langmuir模型和D-R模型对活性炭上丙酮气体的吸附数据进行拟合,从模型拟合精度及吸附热预测角度对Langmuir模型及D-R模型进行了比较。进一步分析了吸附效率与模型参数间的关系以及模型参数与活性炭结构和被吸附化合物性质间的关系。结果表明:①与Langmuir模型相比,D-R模型对活性炭上纯丙酮气体吸附数据的拟合相关系数更高,平均相对标准偏差更低,拟合结果更好。②由10-4-3型势函数计算得到活性炭上纯丙酮气体的理论吸附热为17.9 kJ/mol,吸附热较小,说明此吸附以物理吸附为主。D-R模型吸附热预测值为15.8 kJ/mol,与理论计算值较为接近;Langmuir模型吸附热预测值为40.7 kJ/mol,比理论计算值偏大较多。③实现活性炭对不同化合物吸附效率预测的关键是对化合物吸附热的预测。吸附效率主要与吸附温度,活性炭的用量、孔容,化合物的分子量,碰撞直径和能量参数有关。通过分析吸附能可以推断孔径对吸附效率及吸附选择性的影响。     

A new method to estimate adsorption energies between cations and soil particles via wien effect measurements in dilute suspensions and an approximate conductivity-activity analogy

Measurement of the Wien effect (increased electrical conductivity at strong electrical fields) in soil suspensions is proposed as the basis of a new method to characterize energy relationships between cations and soil particles. The simplified theory behind the method, the working principle of the short high-voltage pulse apparatus, and the measuring procedure are outlined briefly. The new method was used to evaluate the adsorption energies of two monovalent (Na, K) and two divalent (Ca, Cd) cations on yellow-brown soil and black soil particles, assuming an analogy between the activity of the cations and their contribution to the electrical conductivity of the suspension. Both the mean free bonding energies, deltaGbo, and the adsorption energies, deltaGad, of the cations for these two soils increased in the order: Na < K < Ca < Cd. Under the conditions of our experiments, estimated deltaGbo ranged from 4.7 to 6.4 kJ mol(-1) and from 7.0 to 8.2 kJ mol(-1) for mono- and divalent cations, respectively. The bonding energies obtained with the new method were similar to those determined previously by ionic activity measurement. The determined mean adsorption energies of cations desorbed at a field strength of 100 kV cm(-1), for example, ranged from 0.7 to 1.2 kJ mol(-1) and from 1.9 to 2.3 kJ mol(-1) for mono- and divalent cations, respectively.     

Nylon fibers coated with diclofenac: adsorption properties

Dried gauzes properly coated with anti-inflammatory drugs could be helpful in providing topical treatment in inflammatory process. Therefore, thermodynamic of adsorption of Diclofenac Sodium, over Nylon fibers, as a function of concentration, temperature, and pH of the medium, have been studied. Maximum amount adsorbed in the best conditions tested, pH 5.6, 323 K and initial concentration of 150 mg/L has been 2.6 mg/g. Isotherm adsorption data were well fitted to the Langmuir and Freundlich model; therefore, multilayer formation must be considered. Negative and low values of free-energy change, ?17.9 kJ/mol and ?15.9 kJ/mol, in addition to moderate value of activation energy 13.14 kJ/mol point to physical interactions governing the process that, besides, is exothermic with negative enthalpy-change, ?43.44 kJ/mol, and also follows pseudo-first order kinetic model that is typical of this interaction. Desorption experiments carried out under physiological conditions confirm that the proposed procedure for DCF administration could be achieved.     

Moisture sorption isotherms – Experimental and mathematical investigations of orange (Citrus sinensis) peel and leaves

The aims of this work were to determine chemical composition and sorption isotherms of Tunisian orange peel and leaves of the “Maltaise” variety. These by-products were found to be rich in fibre, soluble sugars, protein, minerals and phenols. The equilibrium moisture contents of “Maltaise” peel and leaves were measured using the static gravimetric method at three temperatures (40, 50 and 60 °C) and in wide range of water activity (0.109–0.891). Desorption and adsorption data of peel and leaves were best fitted by the Peleg model at 40, 50 and 60 °C. The net isosteric heats of desorption and adsorption were determined from sorption isotherms. For both peel and leaves, the net isosteric heat of desorption was higher than the net isosteric heat of adsorption and both decreased continuously with increasing of the equilibrium moisture content.     

Understanding trichloroethylene chemisorption to iron surfaces using density functional theory

This research investigated the thermodynamic favorability and resulting structures for chemical adsorption of trichloroethylene (TCE) to metallic iron using periodic density functional theory (DFT). Three initial TCE positions having the plane defined by HCC atoms parallel to the iron surface resulted in formation of three different chemisorption complexes between carbon atoms in TCE and the iron surface. The Cl-bridge initial configuration with the HCC plane of TCE perpendicular to the iron surface did not result in C-Fe bond formation. The most energetically favorable complex formed at the C-bridge site where the initial configuration had the C=C bond in TCE at a bridge site between adjacent iron atoms. In the C-bridge complex, one C atom formed two a bonds to different Fe atoms, while the second C atom formed a sigma bond with a second Fe atom. Surface complexation atthe C-bridge site resulted in scission of all three C-Cl bonds and also resulted in a shortening of the C==C bond to a distance intermediate between a double and a triple bond. Initial configurations with the C==C bond adsorbed at top or hollow sites on the iron surface resulted in formation of C-Fe a bonds between a single C and two adjacent Fe atoms, and the scission of only two C==Cl bonds. Bond angles and bond lengths indicated that there were no changes in bond order of the C==C bond for top and hollow adsorption. Chemisorption at the C-bridge site had an activation energy of 49 kJ/mol and an early transition state where all three C-CI bonds were activated. The early transition state and the loss of all three Cl atoms upon chemisorption are consistent with most experimental observations that TCE undergoes complete dechlorination in one interaction with the iron surface. The absence of chemisorption and scission of only two C--Cl bonds at the Cl-bridge site is consistent with experimental observations that trace amounts of chloroacetylene may also be produced from reactions of TCE with iron.