Eunice F.S. Vieira

An alternative Avrami equation to evaluate kinetic parameters of the interaction of Hg(II) with thin chitosan membranes

In this work, thin chitosan membranes were utilized as an adsorbent for the removal of Hg(II) from aqueous solutions. A rise of temperature accelerates the mass transfer of Hg(II) to the membranes, surfaces. The kinetic data did not present a good fitting to the traditional Lagergren adsorption kinetic equations. An alternative Avrami kinetic equation was employed and successfully fitted to the kinetic adsorption quantities. From this new equation, two regions presenting distinct kinectic parameters were found, and the use of the parameter n was also relationed to the determination of the kinetic order. Variations of the adsorption kinetic rate in relation to the time, the initial Hg(II) concentration, and the temperature were also calculated and are discussed.

Interaction of cations with SH-modified silica gel: thermochemical study through calorimetric titration and direct extent of reaction determination

The reaction of silica gel with 3-(trimethoxysilyl)propane-1-thiol, MPTS, produced a thiol-modified silica gel, Sil–SH. This material removes silver, mercury, copper, zinc and nickel ions from aqueous solutions. Enthalpy changes that occur in the interactions between Sil–SH and these cations were obtained by a calorimetric titration technique, in which thermal effects and the quantities of cations that interact at each titration point were determined simultaneously. From this procedure, values of the maximum capacity of interaction (Ns), enthalpies of interaction at different coverages of the surface [Δint(i)Hm ] and integral enthalpy of interaction for a monolayer (ΔmonHm) have been established. It is shown that this method permits charting the basic sites created upon modification. From Δint(i)Hm values, the sequence of the interaction strength between the basic sites of the silica-attached MPTS and the cations is Hg2+>Ag+>Cu2+>Ni2+>Zn2+ , in agreement with the concept of hard and soft acids and bases. The influence of some parameters concerning the thermodynamics of solvation of the indicated transition metals has been also taken into account in the investigation of these interaction processes.

Polysaccharide-based hydrogels: preparation, characterization, and drug interaction behaviour.

Oxidized alginate (ADA) and oxidized alginate blended with chitosan (ADA-Chit) were prepared in the presence of borax and CaCl 2, and their interactions with an antifolate drug, pyrimethamine (PYR), have been investigated. Tablets with a mean diameter of 1.2 +/- 0.06 cm were produced and drug interactions were performed in dimethyl sulfoxide (DMSO) using isothermal titration calorimetry (ITC). From ITC responses, the enthalpy changes of interaction PYR/materials, Delta int H, have been determined and were found to be -11.73 +/- 0.517 kJ mol (-1) for ADA and -4.86 +/- 0.156 kJ mol (-1) for ADA-Chit. The PYR encapsulation of approximately 75% was achieved for both materials, as measured by UV spectrometer.

The removal of an anionic red dye from aqueous solutions using chitosan beads—The role of experimental factors on adsorption using a full factorial design

A factorial design was employed to evaluate the quantitative removal of an anionic red dye from aqueous solutions on epichlorohydrin-cross-linked chitosan. The experimental factors and their respective levels studied were the initial dye concentration in solution (25 or 600mgL(-1)), the absence or the presence of the anionic surfactant sodium dodecylbenzenesulfonate (DBS) and the adsorption temperature (25 or 55 degrees C). The adsorption parameters were analyzed statistically using modeling polynomial equations. The results indicated that increasing the dye concentration from 25 to 600mgL(-1) increases the dye adsorption whereas the presence of DBS increases it. The principal effect of temperature did not show a high statistical significance. The factorial results also demonstrate the existence of statistically significant binary interactions of the experimental factors. The adsorption thermodynamic parameters, namely Delta(ads)H, Delta(ads)G and Delta(ads)S, were determined for all the factorial design results. Exothermic and endothermic values were found in relation to the Delta(ads)H. The positive Delta(ads)S values indicate that entropy is a driving force for adsorption. The Delta(ads)G values are significantly affected by an important synergistic effect of the factors and not by the temperature changes alone.

The removal of reactive dyes from aqueous solutions using chemically modified mesoporous silica in the presence of anionic surfactant-the temperature dependence and a thermodynamic multivariate analysis.

The three-parameter Sips adsorption model was successfully employed to modeled equilibrium adsorption data of a yellow and a red dye onto a mesoporous aminopropyl-silica, in the presence of the surfactant sodium dodecylbenzenesulfonate (DBS) from 25 to 55 degrees C. The results were evaluated in relation to the previously reported surface tension measurements. The presence of curvatures of the van()t Hoff plots suggested the presence of non-zero heat capacities terms (Delta(ads)C(p)). For the yellow dye, it is observed that the values of Delta(ads)H are almost all positive and they decrease in endothermicity, in the absence and in the presence of DBS, from 25 to 55 degrees C. For the red dye, there is an increase in endothermicity in relation to the temperature increase. The negative Delta(ads)G values indicate spontaneous adsorption processes. Almost all adsorption entropy values (Delta(ads)S) were positive. This suggests that entropy is a driving force of adsorption. The adsorption thermodynamic parameters were also evaluated using a new 2(3) full factorial design analysis. The multivariate polynomial modelings indicated that the thermodynamic parameters are also affected by important interactive effects of the experimental factors and not by the temperature changes alone.

Use of calorimetric titration to determine thermochemical data for interaction of cations with mercapto-modified silica gel

Abstract Interactions occurring between mercapto-modified silica gel and silver, mercury, copper and zinc ions in ethanolic solutions have been investigated by using calorimetric titrations with direct determination of the reaction extension. A calorimetric technique is described for the simultaneous determination of thermal effects and quantities of cations that interact at each titration point. From this procedure, values of maximum capacity of interaction ( N s ), differential interaction enthalpies at different coverages of the surface (Δ int(i) H m ), integral energy of interaction ( Q mon ) and molar integral enthalpy of interaction for a monolayer (Δ mon H m ) have been obtained. The maximum capacity of interaction values are compared with those previously published for analogous interaction processes in aqueous solutions. Thermochemical quantities are discussed in terms of evaluation criterion of the acid–base interactions, and generally support the conclusion that the Pearsons hypothesis of hard and soft acids and bases is appropriated to describe the behavior of the interaction of the above-mentioned cations.

A novel and efficient epoxy/chitosan cement slurry for use in severe acidic environments of oil wells-Structural characterization and kinetic modeling.

In this study, the biopolymer chitosan was used to synthesize a new epoxy/chitosan cement slurry. The features of the new slurry were evaluated in relation to a standard cement slurry (w/c=0.5). A kinetic study of the interaction epoxy/chitosan slurry/HCl was performed to simulate the use of the new slurry in environmental-friendly acidizing procedures of oil wells. The experimental data were well fitted to a three-parameter kinetic model. The analysis of the kinetic modeling suggests that surface reactions constitute the main interactions at the interface epoxy/chitosan-modified cement slurry/HCl. The characterization of the slurries was performed by FTIR, XRD, TG/DTG and solid-state reflectance spectroscopy. The results have pointed out that the main features of the new cement slurry were preserved, even after long-term contact with HCl in aqueous solution. The results of this study underline the excellent features of the new epoxy/chitosan-modified cement slurry for using in environmental-friendly acidizing procedures of oil wells.

Energetics of the interaction of ethylamine and acidic sites of immobilized complexes on SH-modified silica gel

Abstract Interactions of ethylamine from aqueous solutions on SilSM [thiol-modified silica gel containing complexed cations, where M=Ag(I), Hg(II), Zn(II), Cu(II) and Ni(II)] have been investigated. Using an isoperibolic calorimetric titration technique, the enthalpies evolved at different surface coverages were obtained with the corresponding interacting amount of ethylamine ( n int(i) ) determined by using an isothermal calorimetric titration technique, specially developed for this purpose. The maximum capacities of interaction ( N s ), molar enthalpies of interactions at different surface coverages (Δ int(i) H m ) and molar integral enthalpies of interaction for a monolayer (Δ mon H m ) have been determined. The Δ mon H m values, taking into account the investigation of the interaction strengths between ethylamine and SilSM, suggest that the Pearson concept is the predominant mechanism in the behavior of the SilSM/amine adsorption processes. It is shown that the Δ int(i) H m values are not constant with the surface coverage, thus the general tendency found earlier has been confirmed.

Cement–epoxy/water interfaces – Energetic, thermodynamic, and kinetic parameters by means of heat-conduction microcalorimetry

Hydration phenomena at cement/water interfaces drive the characteristics of oil-well cement slurries. In this study, new epoxy-modified cement slurries were synthesized. The slurries characterization has suggested the presence of low amounts of portlandite in the slurry with the higher content of polymerized epoxy resin. The hydration processes of the cement slurries were studied by heat-conduction microcalorimetry. The energetic and enthalpic hydration data were all exothermic in nature. The cumulative heat release curves have shown the presence of multilinearity of the kinetic processes. The hydration microcalorimetric data were well fitted to the multistep Avrami kinetic model. It was found that the epoxy-modified cement slurries present a good potential to be used in environmental-friendly oil-well operations.

The influence of the counter ion competition and nature of solvent on the adsorption of mercury halides on SH-modified silica gel

Abstract The adsorption behavior of thiol-modified silica gel, SiSH, towards mercury (II) chloride and bromide in water, ethanol, methanol and water–methanol solutions has been studied by a batch technique. The influence of the counter-ion competition and mixture of solvents were investigated. The Langmuir expression for adsorption isotherm was applied in order to determine the maximum adsorption capacity to form a monolayer, N mon , and the constant related to the adsorption intensity, b. In aqueous solutions there was a significant adsorption increase with the temperature and pronounced synergistic effects were observed. Adsorption measurements made in water(w)/methanol(MeOH) mixtures indicated that the synergism effect of mercury adsorption followed the order 75% (w)+25% (MeOH)>50% (w)+50% (MeOH)>25% (w)+75% (MeOH). From temperature effect on adsorption, exothermic enthalpy changes for a monolayer of anchored cations per gram of surface, Δ mon H m , are shown to be consistent with our previous calorimetric results for similar systems. The antagonistic effects for Δ mon H m results showed that the counter-ion competition is not favorable to the mercury/surface interaction strengths.