Liquid-liquid extraction of Cd(II) by Cyanex 923 and its application to a solid-supported liquid membrane system
Ana María Rodrígueza,b, Dulce Gómez-Límónb and Francisco José Alguacila
aCentro Nacional de Investigaciones Metalúrgicas (CSIC), Avda. Gregorio del Amo 8, Ciudad Universitaria, 28040 Madrid, Spain. E-Mail: email@example.com
bMaterials Engineering Department, School of Mines, Polytechnical University of Madrid, c/Rios Rosas 21, 28003 Madrid, Spain.
Abstract: The extraction of cadmium (II) by Cyanex 923 (phosphine oxides mixture) in Solvesso 100 from hydrochloric acid solution has been investigated. The extraction reaction is exothermic. The numerical analysis of metal distribution data suggests the formation of CdCl2·2L, HCdCl3·2L and H2CdCl4·2L (L=ligand) in the organic phase. The results obtained for cadmium (II) distribution have been implemented in a solid-supported liquid membrane system. The influence of feed phase stirring speed (400-1400 min-1), membrane composition (carrier concentration: 0.06-1 mol dm-3) and metal concentration (0.01-0.08 g dm-3) on cadmium transport have been investigated.
Keywords: Cyanex 923; extraction; membrane transport; cadmium; chloride media
In spite of its toxicity, cadmium is used in different industries such as pigments, electroplating, metallurgical products, etc. Most often, cadmium could enter the water system through industrial discharge, thus, its removal from the various effluents which contained it had attracted much attention either from the scientific and technological point of view.
During last years, liquid-liquid extraction of cadmium from various aqueous media has received particular interest and different reagents have been investigated, including acidic, basic and neutral extractants and some of their mixtures.1-7
These extraction procedures can also be implemented in a solid-supported liquid membrane configuration, where the performance of the separation is enhanced by the combination of extraction and stripping processes in one step. Moreover, liquid membrane processes have been proposed as cleaner ones due to their characteristics, ie high specifity, low energy utilisation and low organic phase inventory, etc. Thus, liquid membrane technologies have also been used for separation of cadmium from aqueous solutions.8-30
From the above, it is apparent that little data is available in the literature about the use of Cyanex 923 in this role: extractant and carrier for cadmium from hydrochloric acid media, thus, the present investigation was undertaken to obtain a quantitative characterization of the extraction reactions between cadmium (II) chloride solutions and Cyanex 923 in Solvesso 100. Furthermore, a liquid membrane system has been studied by using the extraction process mentioned above, and the parameters affecting the liquid membrane, ie stirring speed of feed solution, composition of the organic phase and cadmium concentration, have been studied.
2 MATERIALS AND METHODS
A 1 g dm-3 stock solution of cadmium (II) was obtained by dissolving CdCl2 (Fluka) in HCl (Fluka). The aqueous solutions, containing 0.05-0.5 gdm-3 (liquid-liquid experiments) or 0.01-0.08 g dm-3 (supported liquid membrane experiments), were prepared by dilution of the stock cadmium solution. HCl concentration was kept constant at 5 mol dm-3 in all the experiments, as previous tests demonstrated that maximum cadmium extraction was obtained using this acid concentration.
The extractant Cyanex 923 was used as supplied by the manufacturer (CYTEC Ind.). It contains different phosphine oxides,31 and was diluted in Solvesso 100 (ExxonMobil Chem. Iberia, Spain) having >99% aromatic content. The total extractant concentration was varied within the range 0.4-40% (v/v) (0.01-1.0 mol dm-3).
2.2.1 Liquid-liquid extraction experiments
Distribution ratio experiments were performed at 20ºC (unless otherwise stated) by shaking (700 min-1) equal volumes (20 cm3) of the organic and aqueous phases in separatory funnels for the required time. After phase separation, the metal remaining in the aqueous solution was analysed by AAS using a Perkin Elmer 1100B spectrophotometer. The amount of cadmium extracted was obtained by difference with the initial concentration in the aqueous phase. From these data, the distribution ratio, DCd, was calculated as:
where [Cd(II)]org and [Cd(II)]aq are the total equilibrium cadmium concentrations in the organic and aqueous phases, respectively.
2.2.2 Supported liquid membrane experiments
The batch transport experiments were carried out in a permeation cell consisting of two compartments made of methacrylate, and separated by a microporous membrane. An aqueous solution (200 cm3) containing (0.01-0.08 g dm-3) cadmium (II) and 5 mol dm-3 HCl was used as source solution. For the receiving phase, the same volume of water was used. The support for the liquid membrane was a polydifluoroethylene film (Millipore GVHP 4700) with a thickness of 125 μm, 75% porosity and 0.22 μm average pore size. The liquid membrane was prepared by impregnating the laminar microporous film with a solution of Cyanex 923 2.5-40% (v/v) in Solvesso 100. In each experiment, the stirring rate in both the source and the receiving phase was kept constant at 1200 and 800 min-1, respectively, unless otherwise stated. The cadmium content of aqueous phases was periodically determined by AAS. The permeability coefficient, PCd, was calculated as:
where [Cd(II)] is the metal concentration in the source solution at time t, A is the effective membrane area (11.3 cm2) and V is the volume of the source solution. Integration of eqn (2) gives:
Thus, permeabilities values are obtained from the slope of the linear representation of ln ([Cd(II)]t/[Cd(II)]0) versus t.
3 RESULTS AND DISCUSSION
3.1 Liquid-liquid system
A preliminary test was carried out to determine the time needed to achieve equilibrium. From the results shown in Fig 1, at 0.1 g dm-3 Cd(II) and 5 mol dm-3 HCl and Cyanex 923 4% (v/v) (0.1 mol dm-3) in the organic solution, equilibrium is reached after 15 min of contact. All subsequent experiments were carried out using 15 min contact time.
The relationship between cadmium extraction and the temperature was also studied using the same organic and aqueous solutions as described above. Figure 2 shows the variation of log DCd vs 1000/T, over the range of temperatures used. There is a decrease of cadmium extraction with the increase of temperature. One explanation of these results, described elsewhere,32 is to consider the nature of the species with the temperature as predicted by the Bjerrum equation.33 The extraction process is therefore exothermic.
The metal distribution ratio, DCd, at 5 mol dm-3 HCl, was determined for different Cyanex 923 and cadmium (II) concentrations. The results are plotted in Fig 3. It can be seen that log DCd does not depend on the cadmium concentration. This behaviour indicates that no polynuclear complexes are apparently formed in the organic phase. Taking into consideration that Cyanex 923 acts as a solvating reagent, it can be assumed that cadmium (II) is extracted by the reagent according to the general following equilibrium:
where n=0,1,2 and L represents the extractant.
Assuming ideal behaviour in the organic phase and constant activity coefficient in the aqueous phase, the equilibrium constant for the above reaction can be written as:
Taking into account the definition of DCd, the following expression is obtained:
Thus, a plot of log DCd against log [Cyanex 923]TOT should give a straight line with slope of q. Since the cadmium concentration is low, the concentration of Cyanex 923 bound in the complexes can be neglected compared with the total initial extractant concentration. As can be seen in Fig 3, a straight line of slope near 2 is obtained, therefore, experimental data could be explained assuming the extraction of species with q=2.
To determine the composition of the extracted species and their extraction equilibrium constants, extraction data were numerically treated using the LETAGROP-DISTR program,34 which is based in the minimization of the error-square sum defined by:
where Dexp is the experimental value of the distribution ratio and Dcal is the corresponding value calculated from the relevant mass balance equations for the proposed model. According to the results of the graphical analysis and taking into consideration other possible reactions, ie the extraction of HCl by Cyanex 923,35 several metal-Cyanex 923 species are introduced in the various models tested. The results of computer analysis are given in Table 1, which includes the description of the model, the corresponding values of the formation constants and the values of statistical parameters that quantify the goodness of the model proposed to fit the experimental data.
Thus, the extraction of cadmium (II) can be explained by formation of CdCl2·2L, HCdCl3·2L and H2CdCl4·2L species in the organic phase.
3.2 Supported liquid membrane system
The extraction of cadmium (II) by Cyanex 923 has also been studied in a solid-supported liquid membrane, where the transport of the metal species across the liquid membrane also depends on kinetic parameters.
To achieve effective permeation of cadmium (II) in a solid-supported liquid membrane system, is necessary to explore the effect of the stirring speed on the permeability coefficient. Diffusional resistances encountered during the transport of a solute across a supported liquid membrane are of two types: (i) the resistance due to the liquid boundary layer, and (ii) that due to the membrane.
Sometimes, the magnitude of the boundary layer resistance is comparable to or even greater than the membrane resistance.36 In the present investigation, stirring of the feed phase was carried out from 400 to 1400 min-1 (Fig 4). The permeability coefficient increased from 400 to 1200 min-1, and beyond that no increase in cadmium (II) permeability was observed. Consequently, the thickness of the aqueous diffusion layer and the aqueous resistance to mass transfer were minimized and the diffusion contribution of the aqueous species to the mass transfer process is assumed to be constant.
Studying the effect of the initial concentration of cadmium (II) (0.01-0.08 g dm-3) in the feed solution it was revealed that the metal flux, defined as:
increased with the increase of the metal concentration in the feed phase (Fig 5), being this in accordance with the expected trend, since the flux of a solute varies in direct proportion with solute concentration,37-38 and hence, there should be an increase in flux with an increase in cadmium concentration.
Also, the effect of carrier concentration on cadmium permeation was studied. Figure 6 shows permeability values for the transport of cadmium through a supported liquid membrane impregnated with various solutions of Cyanex 923 in Solvesso 100. It can be seen that the permeability increased with Cyanex 923 concentration until a maximum value in PCd was obtained at near 0.6 M carrier concentration. This limiting permeability value, Plim, can be explained by assuming that diffusion in the organic membrane is negligible compared with that for aqueous diffusion and the permeation process is controlled by the diffusion in the stagnant film of the aqueous feed phase. Thus,
where Daq is the average aqueous diffusion coefficient of the metal-containing species and daq is the thickness of the aqueous source boundary layer; assuming a value of Daq 6.5x10-6 cm2 s-1,27 and Plim equal to 3.9x10-3 cm s-1, the value of daq is 1.7x10-3 cm, this value being the minimum thickness of the stagnant aqueous diffusion layer within the present experimental conditions. On the other hand, the decrease in the permeability value at higher carrier concentration can be attributable to an increase in the organic phase viscosity and thus increasing the membrane resistance to the transport.
The commercially available phosphine oxide Cyanex 923 can extract cadmium (II) from 5 mol dm-3 HCl media. There is a decrease in cadmium extraction with increasing temperature, so that the extraction is exothermic. Metal extraction is dependent on the extractant concentration but is independent of the initial metal concentration. On the basis of a computer simulation model, cadmium (II) extraction by Cyanex 923 can be explained by the formation of CdCl2·2L, HCdCl3·2L and H2CdCl4·2L species in the organic solution (L=Cyanex 923).
The extraction system has been implemented in a supported liquid membrane system in which, under the investigated experimental conditions, the metal flux is dependent on the initial metal concentration. The metal transport is dependent on carrier concentration but for a Cyanex 923 concentration of 25% (v/v) (0.6 mol dm-3) in Solvesso 100, a limiting value for permeability is obtained and the transport process is controlled by the diffusion in the aqueous stagnant film. The mass transfer coefficient in the aqueous phase is found to be 3.9x10-3 cm s-1.
This work has been carried out under CAM (Comunidad de Madrid, Spain) project 07M/0065/2002. Authors also thank to Mr.Bascones and Mr.López for technical assistance.
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Table 1. Results of numerical calculation for Cd(II) distribution
1.9 max 5.1
1.4 max 3.9
0.9 max 3.2
GK denotes the achievement of a minimum of the function U at which K values are calculated.
Figure 1. Extraction of cadmium (II) with Cyanex 923 versus contact time.
Figure 2. Arrhenius plot for cadmium (II) extraction with Cyanex 923.
Figure 3. Experimental distribution data, log DCd vs log [Cyanex 923]TOT at various metal concentrations.
Figure 4. Influence of stirring speed on permeability of cadmium (II). Feed phase: 0.01 g dm-3 cadmium in 5 mol dm-3 HCl. Extractant concentration: Cyanex 923 20% (v/v) (0.5 mol dm-3) in Solvesso 100. Receiving solution: water.
Figure 5. Influence of initial concentration of cadmium (II) on permeability flux (J) of cadmium. Feed phase: various cadmium concentrations in 5 mol dm-3 HCl. Extractant concentration: Cyanex 923 20% (v/v) (0.5 mol dm-3) in Solvesso 100. Receiving solution: water.
Figure 6. Cadmium permeability vs carrier concentration. Feed phase: 0.01 g dm-3 cadmium in 5 mol dm-3 HCl. Membrane phase: various Cyanex 923 concentrations in Solvesso 100. Receiving solution: water.