Epaminondas Voutsas

Bioconcentration of heavy metals in aquatic environments: the importance of bioavailability

The importance of heavy metal bioavailability on the bioconcentration in aquatic biota is examined. To this purpose, mono- and multivariate statistical techniques are applied to develop correlations between heavy metal bioconcentration factor and sediment characteristics, that are expected to affect bioavailability, using a database of heavy metal concentrations in biota and sediment along with the available physicochemical characteristics. The statistical analysis shows that satisfactory correlations are obtained only when factors that affect bioavailability, such as metal oxides concentration and organic carbon content in the sediment, are taken into account.

Vapor-liquid equilibria for alcoholhydrocarbon systems using the CPA Equation of state

Abstract The recently developed Cubic-Plus-Association Equation of State (CPA EoS) is extended in this study to binary systems containing one associating compound (alcohol) and an inert one (hydrocarbon). CPA combines the Soave-Redlich-Kwong (SRK) equation of state for the physical part with an association term based on perturbation theory. The classical van der Waals one-fluid mixing rules are used for the attractive and co-volume parameters, α and b, while the extension of the association term to mixtures is rigorous and does not require any mixing rules. Excellent correlation of Vapor-Liquid Equilibria (VLE) is obtained using a small value for the interaction parameter (kij) in the attractive term of the physical part of the equation of state even when it is temperature-independent. CPA yileds much better results than SRK and its performance is similar to that of other association models, like the Anderko EoS, and the more complex SAFT and Simplified SAFT EoS.

Prediction of phase equilibria in water/alcohol/alkane systems

Abstract The Cubic Plus Association Equation of State (CPA EoS) is applied to the prediction of Vapor–Liquid Equilibrium (VLE) and Liquid–Liquid Equilibrium (LLE) in ternary associating mixtures containing water, alcohols and alkanes. The appropriate set of combining rules for the cross-association energy and volume parameters of the EoS is identified by evaluating the performance of four such sets in the correlation of VLE and LLE for water/alcohol mixtures. Using only one interaction parameter per binary mixture, in the physical part of the EoS, CPA gives very satisfactory predictions of ternary VLE and LLE.

Correlation of liquid-liquid equilibria for alcoholhydrocarbon mixtures using the CPA equation of state

Abstract A recently developed equation of state, which combines the physical term of the classical SRK EoS with the association term based on the perturbation theory of Wertheim, called the Cubic Plus Association Equation of State (CPA EoS) (Kontogeorgis et al. [1]), is applied to liquid-liquid equilibrium (LLE) calculations in alcohol hydrocarbon mixtures. The CPA EoS performs very well in all cases using the conventional van der Waals one-fluid mixing rules in the parameters of the physical term and a temperature-independent interaction parameter. The performance of the CPA EoS is compared to that of the conventional SRK EoS and UNIFAC models, and with another association model, the ESD EoS of Suresh and Elliott [2]. The CPA EoS gives much better results than the two conventional models and results comparable to those obtained with the ESD model.

Prediction of the bioaccumulation of persistent organic pollutants in aquatic food webs.

Predictive correlations of the bioaccumulation factor of persistent organic pollutants in aquatic biota are presented as functions of their octanol/water partition coefficient. The correlations demonstrate the importance of differentiating among the different levels in the food web and of accounting for the pollutants bioavailability by considering the amount freely dissolved in water instead of the total concentration. They also reveal the significance of the pollutants octanol/water partition coefficient value on its biomagnification along the levels of the trophic chain. Prediction results, finally, demonstrate that the correlations provide reasonably accurate estimates of bioaccumulation, typically within an order-of-magnitude.

The performance of EoS/GE models in the prediction of Vapor-Liquid Equilibria in asymmetric systems

Abstract An extensive comparison of four EoS/GE models: LCVM, MHV2, PSRK and Wong-Sandler in the prediction of Vapor-Liquid Equilibria (VLE) in asymmetric systems is presented and demonstrates that only LCVM yields satisfactory results.

A combinatorial activity coefficient model for symmetric and asymmetric mixtures

Abstract A new combinatorial activity coefficient model, based on the Staverman-Guggenheim (SG) one, is developed in this study. It involves a system-dependent exponent, determined from the size ratio of its components only, and is useful for group-contribution models. The model, referred to as R-UNIFAC, is applied to the prediction of activity coefficients in the following nearly athermal mixtures: (a) alkane / alkane mixtures; (b) alkane / polymer mixtures; (c) polymer / solvent mixtures, where the activity coefficient data were obtained by constant-pressure Monte Carlo simulations. For all these mixtures the residual contribution to the activity coefficient is expected to be close to zero. The new model yields satisfactory predictions for all systems considered, independently of system asymmetry, comparable to those obtained with free-volume models such as the Entropic-FV and p-FV ones. It has the advantage, however, that it does not require pure-component liquid molar volumes and, thus, is directly applicable to systems containing compounds where such volumes are not accurately known or supercritical fluids.

A molecular simulation-based method for the estimation of activity coefficients for alkane solutions

In a recent study Sheng et al. (1995, A.I.Ch.E. J.41 (10) 2306–2313) presented activity coefficients calculated by molecular simulation (MS) for athermal model polymer-solvent systems. Both activity coefficients of the monomeric solvent in the polymer (γ1∞) and of the polymer in the solvent (γ2∞) were calculated at infinite dilution conditions. The MS data cover a broad range of system asymmetry with respect to size in the area of oligomer-solvent mixtures (up to segment ratio 601 which corresponds to e.g. a system of n-heptane with nC460). The MS results were compared with classical and recently proposed free-volume (FV) activity coefficient models in order to conclude on the suitability of the models for phase equilibrium calculations for asymmetric systems. On the basis of the work by Sheng et al., extrapolation methods for estimating the activity coefficient of a solvent and that of a polymer in real solvent-polymer systems are developed here. The so-obtained MS-based activity coefficients are compared with experimental data (in the case of solvent activities) and with the predictions of various activity coefficients models (in the case of polymer activities).

Solvent Effects on Equilibrium Position and Initial Rate of Lipase-catalyzed Esterification Reactions in Organic Solvents: Experimental Results and Prediction Capabilities

The solvent effect on the equilibrium position and the initial rate of esterification of 1-hexanol with acetic acid catalyzed by a lipase has been experimentally investigated. A variety of non-polar and polar solvents have been considered and the results obtained indicate that the solvent effect on the equilibrium conversion is very important compared to that for transesterification reactions. A theoretically sound methodology using the group-contribution UNIFAC model for the prediction of solvent effects on the equilibrium position of enzymatic reactions is presented and it is applied to the reaction of 1-hexanol with acetic acid as well as to a similar reaction from the literature. The results obtained are better than those from empirical methods proposed in the literature such as correlations with the octanol-water partition coefficient of the solvent, as well as the solubility of water in the solvent. Moreover, the proposed methodology can be used for the determination of the equilibrium constant of the reaction. For the prediction of the solvent effect on the initial rate of enzymatic reactions it is found that it is more accurately determined using the product of the activities of the reactants, which can be predicted by the UNIFAC model, than the octanol-water partition coefficient of the solvent or the solubility of water in the solvent.

Enzymatic Reactions in Non-Conventional Media: Prediction of Solvent Water Content for Optimum Water Activity

Most enzymes provide their optimum performance at a given water activity (aw), which is generally solvent independent. For a given organic liquid solvent at a specific temperature or for a supercritical solvent at a specific temperature and pressure this corresponds to a water concentration in which water has the desired activity. We present here a methodology for predicting this water concentration thus reducing substantially the amount of experimental work needed to find the optimum solvent with respect to equilibrium conversion. If the enzyme optimum water activity is known, the methodology predicts the required water content in the solvent to achieve this aw value. If, in addition, the enzyme water activity curve is available, this methodology provides the total water that must be added to the system (enzyme plus solvent) so that a specific water activity can be obtained. The same methodology can also be applied to predict the effect of the total water content of the system (initial or initial plus produced) on the water activity values. It is shown that: (a) for esterification reactions taking place in hydrophobic organic solvents, the produced water can lead to a substantial change in water activity, but not for less hydrophobic solvents; (b) introduction of dry CO2 into a system, pre-equilibrated to a certain water activity at atmospheric pressure, can lead to a substantial decrease in the water activity especially at temperatures just above the critical one of the solvent and pressures larger than that.