Adsorption equilibria of di(2-ethylhexyl)phosphoric acid at the water-dodecane-interface. Importance of zinc extraction

Adsorption equilibria of di(2-ethylhexyl)phosphoric acid at the water- dodecane-interface: Part 2. Importance of zinc extraction

Keywords: Zinc extraction, cation exchanger, interfacial activity, adsorption equilibrium, pseudo-nonionic modelling, interfacial tension

Abstract

In this study the interfacial activity of the cation exchanger complex, which is fully loaded with zinc, is presented. The importance of the extraction equilibria on the equilibrium interfacial tension is explained by adding different electrolyte concentrations and by varying the concentrations of the cation exchanger di(2-ethylhexyl)phosphoric acid (D2EHPA). In part 1 of this series, a modelling strategy of the equilibrium interfacial tension is validated for the two-phase system water-electrolyte/dodecane-D2EHPA in the case of the absence of metal salt extraction. This modelling strategy is applied to the extended system with zinc extraction. For this purpose, the previously used Gibbs-Duhem equation is extended by the influences of the two further components - the zinc complex without ligands and the zinc ion. The Langmuir isotherm of the multicomponent adsorption is used to describe the adsorption of the surfactants. The adsorption of the ionic derivate of the cation exchanger induces a counterion adsorption. This caused adsorption is formulated by the Stern isotherm. A simple model describes the considerable aggregation tendency of the D2EHPA complexes, which are fully loaded with zinc. This model is analogous to the model of micelle formation of the cation exchanger anion. All in all, the presented model reproduces acceptably the measured equilibrium interfacial tension.

1 Introduction

In liquid-liquid extraction the concentration profiles of the two different liquids merge continuously into each other the interface. The interface is not a two-dimensional field but a volume with a small thickness. If a local concentration maximum of a substance exists across the interface thickness, the substance is called a surfactant. Because of the resulting concentration elevation in the interface in comparison to the two liquids these substances determine the properties of the interface. The interfacial concentration describes the accumulation of a substance in the interface and it represents an averaging of the concentration profile across the interfacial thickness. At equilibrium the interfacial concentration is correlated with the activities of the surfactants in the two liquids by an adsorption isotherm. Since the adsorbed substances change the interfacial tension between the liquids, this material parameter is used to describe the material accumulation in the interface. The interfacial tension is an essential material parameter, which determines the drop behaviour in disperse systems and influences the design and the equipment of technical extractors [1,2].

The modelling of the adsorption equilibria is especially important for formulating the nonequilibrium processes in the liquid-liquid extraction with interfacial active reactants or rather reactive surfactants. It is well known for the adsorption of surfactants that the accumulation of a substance can be done from the bulk phase into the interface with and without kinetic inhibition [3,4,5]. If the adsorption is immediately from the interface-near volume to the interface, the mass transfer into the interface is determined by the adsorption equilibrium. In the case of a kinetic inhibited transfer sorption kinetic relations are used for modelling. For the steady state of the adsorption these relations can be converted into the adsorption isotherms. That is why the sorption kinetics can be derived from the adsorption equilibria.

In the first part of this series⁶, the adsorption equilibria of di(2-ethylhexyl)phosphoric acid (D2EHPA) at the water-dodecane-interface is presented under the condition of missing metal salt extraction. In this part the developed model is upgraded to the zinc extraction. In the literature the misconception is often presented that the interfacial tension depends only on the cation exchanger concentration during the zinc extraction [7,8]. However, our own studies ⁹ show that the non-solvated zinc complex is a surfactant but not the zinc complexes, which are solvated with D2EHPA. Because of the solvation tendency of the zinc complexes, the zinc complexes are found only in the solvated form, if sufficient unloaded D2EHPA molecules exist in the organic solution. Under these conditions, the interfacial tension is determined by the unbound D2EHPA molecules in the form of monomers and their anions. The interfacial activity of the zinc complex without ligands can only be studied for an almost completely loaded cation exchanger¹⁰.

2 Experimental

Before the tensiometric measuring starts, both phases were mixed at a constant volume ratio in a shaker for 24 hours. After this procedure the equilibrium state is reached between the two phases. In principle, systematic inaccuracies can result from the different volumes of the phases in the several tensiometric experiments. Because of the equilibrium state these methodical fault can be prevented.

The zinc was extracted from sulphate solutions. In addition to varying the initial concentrations of the cation exchanger and the zinc sulphate in the corresponding liquids, the extractions were performed with the addition of various quantities of sulphuric acid and sodium sulphate. The pH value was measured to reduce the number of mass balance equations.

The D2EHPA initial concentrations of the organic solutions were varied between 1 mmol/l and 1 mol/l. All electrolyte additives were diversified between 10-⁴ and 10-¹ mol/l in the aqueous solution before starting the mixing of the liquids.

The measurement of the interfacial tension was carried out at 20°C on an aqueous pendant drop, which was hanging in a cuvette filled with dodecane. The steady value of the measured dynamic interfacial tension profile is the equilibrium interfacial tension. The drop shape analysis is used as a physical principle. In this method, the drop shape of a rotationally symmetrical drop, which is recognized as a silhouette, is theoretically adjusted by the Gauss-Laplace equation with the interfacial tension as a fitting parameter¹¹. The principle experimental setup is illustrated in the first part of this series⁶.

With the exception of the cation exchanger (D2EHPA, 95 weight-%, technical quality, Sigma) the used chemicals were checked for possible contaminations by tensiometric measurements and in the case of the diluent (n-dodecane, 99 weight-%, for synthesis, Merck) a preparation was necessary. The diluent was cleaned by repeated washing with deionized water. The other chemicals were used in analytical quality.

For the determination of the extractive equilibrium data 10 ml organic solution was mixed with 20 ml aqueous solution for 24 hours. After phase separation the pH value was measured in the aqueous phase. Following the zinc concentration was determined in the aqueous sample by inductively coupled plasma-atomic emission spectroscopy (Perkin Elmer ICP-OES PE Optima 3000). The zinc concentration of the organic phase was calculated by the concentration change in comparison to the initial value of the zinc concentration in the aqueous solution. The D2EHPA initial concentrations were varied between 10-³ and 1 mol/l for this test series, because the extracted quantities of zinc must be greater than the error of chemical analysis in the aqueous sample. All added electrolytes - zinc sulphate, sodium sulphate, sodium hydroxide, sulphuric acid - were varied according to the concentrations of the tensiometric measurements.

3 Modelling

Unlike the first part of the series⁶ the adsorption of the zinc complex without ligands must be considered in addition to the immediate adsorption of the monomer and the cation exchanger anion. Also the consequential adsorption of the counterions must be supplemented by the influence of zinc ions in comparison with the system without zinc sulphate addition. Because of these extensions and neglecting any electrostatic influences of the electrochemical double layer it is obtained for the Gibbs adsorption equation:

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(1)

In this equation the Langmuir isotherms of the multicomponent adsorption Eq. (2) until Eq. (4) are introduced.

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(2)

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(3)

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(4)

The adsorption of the counterions is a consequence of the adsorption of the cation exchanger anion and it is described by the Stern isotherms:

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(5)

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(6)

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(7)

If these isotherms and the link between anion, proton and monomer, which reduces the variables on the independent activities⁶,

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(8)

are introduced into the Gibbs adsorption equation Eq. (1), the following relation results for the equilibrium interfacial tension after integration:

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(9)

In deriving this relation, the reactive coupling between the monomer and the zinc complex is neglected. This simplification is possible because the zinc complex occurs only at high loadings, which are associated with very low monomer concentrations¹⁰.

In the previous equations the activity of the zinc complex means the activity of the disaggregated complexes. The aggregation of the organic zinc complexes in the dodecane phase is formulated analogously to the micelle forming reaction of the D2EHPA anion in the aqueous phase⁶. The micelle formation of the D2EHP itself is irrelevant under the chosen conditions because of the zinc extraction from acid solutions.

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(10)

On the basis of the mass action law of this reaction

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(11)

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(12)

This equation assumes that the aggregate formation is without influence on the chemical reaction equilibria. That is why the activity of the zinc complex aggregate is the same like the summation of the activity of the disaggregated complexes according to the aggregated number.

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