Zinc separation of aqueous solutions using emulsion liquid membranes. the ph influence

Transcription

1 Zinc separation of aqueous solutions using emulsion liquid membranes. the ph influence H. MEDINA, J. BULLÓN, F. ONTÍVEROS, T. CHACÓN, A. CÁRDENAS* Laboratorio de Mezclado, Separación y Síntesis Industrial Escuela de Ingeniería Química. Universidad de los Andes *Corresponding author: antonioc@ula.ve ABSTRACT Zinc ions can be separated from aqueous streams by multiple emulsion membranes. This method can be applied in industrial effluent treatment. In this study, multiple emulsions were prepared by the two step process and used to extract zinc ions from the continuous external aqueous phase. For the extraction, kerosene was the membrane phase, DEHPA the carrier, SPAN 20 the surfactant and a solution of sulphuric acid the inner phase that received the zinc ions. A study of the effect of the protons (ph) in the extraction of zinc was made. The results show that below a ph of 3, there is no zinc extraction and that the extraction can be enhanced by neutralizing continuously with NaOH. A model of the kinetics of the extraction was proposed. It shows that the extraction is proportional to the concentration of zinc ions and inversely proportional to the concentration of protons. Keywords: zinc, multiple emulsion, effluent treatment, liquid membranes. SEPARACIÓN DEL ZINC DE SOLUCIONES ACUOSAS UTILIZANDO MEMBRANAS LÍQUIDAS EMULSIONADAS. LA INFLUENCIA DEL ph RESUMEN Las emulsiones múltiples se pueden utilizar para separar iones de zinc de efluentes acuosos, por ello, este método se puede usar en el tratamiento de efluentes industriales. En este estudio se prepararon emulsiones múltiples por el método de dos pasos y se usaron para extraer iones de zinc de la fase continua externa de la emulsión. En la extracción se utilizó querosén como fase membrana, DEHPA como transportador del zinc, SPAN 20 como surfactante y una solución de ácido sulfúrico en la fase interna de la emulsión. Se estudió el

2 efecto de los protones (ph) en la extracción del zinc. Los resultados muestran que a ph inferior a 3, la extracción no es posible y que se puede mejorar significativamente la extracción si se neutraliza continuamente la fase externa con NaOH. También se propuso un modelo de extracción que indica que la velocidad de extracción es proporcional a la concentración de iones zinc e inversamente proporcional a la concentración de protones. Palabras clave: zinc, emulsión múltiple, tratamiento de efluentes, membranas líquidas. Recibido: mayo de 2005 Recibido en forma final revisado: diciembre de 2005 INTRODUCTION Effluent treatment is one of the most important activities of many industries. Societies are more and more concerned about the environment and stricter laws to protect it are constantly being approved. Those laws limit the quantity of certain compounds that cannot be disposed freely into the environment because of their deleterious effects. Important compounds, because of their toxic characteristics, are heavy metals such as zinc, nickel, cobalt, chromium, cadmium and copper. Industries that use heavy metals, must treat their effluents and minimize (or eliminate) the negative impact of these metals on the environment. Zinc is a heavy metal which is used in many industries and is hazardous to the environment and its recuperation from effluents is desirable. Industries such as the viscose textile and the plating industry use zinc and their effluents must be treated to avoid contamination. One way to treat these effluents is with emulsion liquid membranes. Emulsion liquid membranes are multiple emulsions characterized because the drop that forms the dispersed phase has smaller inner drops inside. The smaller drops inside the big drop are usually miscible with the continuous external phase (CÁRDENAS AND CASTRO, 2003). The non miscible phase acts as a liquid membrane, since some compounds can pass through it selectively, separating them from other compounds in the continuous external phase. One important advantage of this method is that the compounds can be separated and at the same time concentrated in the internal droplets. In many cases, the concentrated compound can be recuperated and re-used. Multiple emulsions have been used to treat effluents that contain heavy metals, such as the recuperation of zinc from wastewaters of the viscose fiber industry, which is one of the commercial processes that uses emulsion liquid membranes (DRAXLER AND MARR, 1986, DRAXLER et al., 1988). Studies have been made to remove heavy metals from aqueous solutions by means of emulsion liquid membranes, such as the removal of zinc, cadmium, copper and

3 lead from wastewater in metallurgical plants (MARR et al., 1990). Many other studies concerning the separation heavy metals and other compounds have been made and cited by HO and LI (1992). In some cases, as with the transport of ions through oily membranes, the use of carriers is necessary since the ions are insoluble in oil. These carriers facilitate the transport of the ions across the liquid membrane, giving larger fluxes of ions and selectivity, since they behave as a complexing agent to a specific compound. For zinc separation, the use of phosphorus derived complexing agents is common, since they have proved to be specific for heavy metals. One of the complexing agents most used is the bis- 2 ethyl-hexyl phosphoric acid (DEHPA) which has the advantage of being cheap. In this work, the extraction efficiency of zinc with DEHPA and the influence of ph on the feed (external aqueous phase) was studied. EXPERIMENTAL The multiple emulsions prepared were of the water in oil in water type (W1/O/W2). The oily phase (membrane) was composed of kerosene (43,19 API, 5% boiling point 156 C, 70% boiling point 200 C), which was filtered prior to use. A nonionic surfactant, SPAN-20 (ICI, sorbitan monolaureate), was used to stabilize the emulsion (the surfactant is oil soluble). As a carrier, bis-2 (ethyl-hexyl) phosphoric acid (DEHPA, BDH Chemicals, 96%) was used. Zinc chloride (Merck, PA) was used to prepare the feed solution (external phase, W2) and sulfuric acid (Riedel de Haen, 95-97%, PA) to prepare the internal water solution (W1). To neutralize the external phase, NaOH (Riedel de Haen, PA) was utilized. The multiple emulsion, or emulsion liquid membrane, was prepared by the two step procedure (CARDENAS AND CASTRO, 2003, MATSUMOTO AND KANG, 1989). A primary emulsion was prepared mixing kerosene which contained 1 % in volume of SPAN-20 and in some cases different amounts of DEHPA, with a 2 M solution of sulfuric acid (inner aqueous phase, W1). The mixing was done using a Taurus mixer during 30 seconds and the volumetric relation water to oil (W1/O) was 30/70, which is 7.5 ml of aqueous solution in 17.5 ml of oily phase. The Taurus mixer proved to be very efficient in producing emulsions. The primary emulsion (W1/O) was poured into the external continuous phase which was pure water or a solution of zinc chloride (176 or 500 ppm of Zn+2). The multiple emulsion is formed by pouring slowly the primary emulsion into the continuous external phase (W2) which is agitated by means of a magnetic stirrer or a mixer with a four blade impeller. A volume ratio (W1/O) of 30/70 v/v of internal aqueous solution to oil was used. The volumetric relation between the primary emulsion and the external phase was 1/9 and in

4 most cases it was 25 ml of primary emulsion dispersed in 225 ml of external phase. In other cases 50 ml of primary emulsion was poured in 450 ml of external phase. When the multiple emulsion was formed, conductivity, ph and zinc concentration measurements were made. The conductivity was measured using a CDM 210 Meter Lab (Radiometer analytical, France) and the ph by a phm210 Meter Lab (Radiometer analytical, France) ph-meter. The measurements of the zinc concentration were done by tritrating with EDTA and eriochrome black T or by atomic absorption using a Varian Spectra 55B spectrophotometer. Because the ph in the external phase is reduced due to the release of protons from the internal aqueous phase (W1), experiments were carried out neutralizing the external phase with NaOH 0.1 M. Some experiments were made neutralizing the solution at the beginning and in others a ph near 7 was controlled during the whole process. RESULTS AND DISCUSSION Once the multiple emulsion was formed, it proved to be very stable, for at least 20 minutes, when breaking of the droplets was less than 1 %. Even at times of 60 minutes, breaking was below 20%, as shown in figure 1. The breaking was determined by measuring the conductivity of the external aqueous phase (which initially was pure water). The inner droplets contained sulphuric acid which was released from the internal to the external phase, and an increase in conductivity was observed. This increase in conductivity is proportional to the quantity of sulphuric acid in the external phase and indicates how much acid is released as the multiple emulsion breaks. The initial breaking of the multiple emulsion is very low (less than 1%) and it suggests that breaking due to agitation effects is not important. The fact that breaking begins after 20 minutes suggests swelling as the main mechanism of emulsion breaking, because of the time required to swell and break. In this case, the primary emulsion is 30/70, which means that the membrane is «thick» and is more difficult to break (CÁRDENAS AND CASTRO, 2003). This result is important because it shows that if the residence time in the extractor is less than 20 minutes, then the efficiency of the extraction will be high. The extraction of Zn+2 from the external continuous phase is done by a transport mechanism in which a carrier (in this case DEHPA) transports the zinc ion from the external phase to the internal aqueous phase. In this mechanism, two molecules of DEHPA form a complex with the zinc and then they release the ion in the internal phase, exchanging each zinc ion with two protons to maintain electro-neutrality. The reactions are as follows:

5 Reaction 1 occurs at the external (W2)/membrane interface, where zinc ions are complexed by the DEHPA and protons are released to the external phase. The complex moves towards the internal droplets, where reaction 2 occurs at the internal aqueous phase (W1)/membrane interface. In this case, the zinc ion is released and two protons are taken by the DEHPA to keep electro-neutrality. The complex formed, must be sufficiently strong to permit the zinc to be transported, but not so strong as to inhibit the zinc release in the internal phase. This means that the bond energy for complex formation must be in the order of -17 to -40 kj/ mol (NOBLE et al, 1989). Reactions 1 and 2 show a release and complexation of protons with DEHPA. This implies that the ph has an effect on the transport of zinc ions. In fact, the bis(2-ethyl-hexyl) phosphoric acid has the following equilibrium: DEHPA DEHPA- + H+ 3 This means that when the concentration of protons is high (ph below 3) the reaction shifts to the left and the ability of the DEHPA to complex the zinc ions is lost. When the zinc extraction starts, there is a net transfer of protons to the external phase, and the ph of this phase diminishes as shown in figure 2. When the ph reaches a value of 2.4, the transport of zinc stops because the acid equilibrium of the DEHPA shifts to the left in reaction 3. This implies that the efficiency of the extraction is diminished by this fact. In fact, the DEHPA stops its extraction capability at ph s below 3 (COPP AND MARR, 1982). The extraction changes for different cations as the ph changes, in the case of zinc, the extraction can be achieved at a relatively low ph (COX AND FLETT, 1983). Moreover, the initial concentration of zinc does not change the behavior of the ph with time, as shown in figure

6 2. In the two cases, the zinc extraction stops in a ph near 2.4. When the ph is calculated with the data of the zinc extraction, that is two protons released as one zinc ion is extracted, the ph curve obtained, matches very well the experimental ph measurements, as shown in figure 3. Moreover, this indicates that there is no other source of protons to the external phase other than the zinc/proton exchange, which indicates a very stable emulsion, since if breaking was important, the ph profile would not match the calculated curve.

7 Two kinetic models were proposed for the concentration of zinc in the external phase, as shown in table 1. The two models differ in the proton behavior, in model 1 the change in zinc concentration is inversely proportional to the proton concentration in the external phase. In model 2, the change of the concentration of zinc in the external phase is proportional to the inverse square of the proton concentration. In both models, the change in the external phase zinc concentration is directly proportional to the concentration of zinc. The proportionality factor (P) used is known as the permeation coefficient. The experimental data was adjusted using Maple 7, being model 1 the one which fits best with the experimental data. This result agrees with the result of REIS et al. (1999) for the extraction of zinc with MTPA (mono-thio phosphoric acid). The model 1 prediction and the experimental data for two initial zinc concentrations (501 and 176 ppm) are shown in figure 4, were it can be observed the very good fit of this model. The permeation coefficients (P) obtained where: 1.089*10-4 mol/dm3.min for an initial concentration of zinc ions of 501 ppm and 2.64*10-5 mol/dm3.min for an initial concentration of 176 ppm. The results obtained agree with the models of extraction of other metals such as indium and europium (KONDO AND MATSUMOTO, 1996, KONDO AND MATSUMOTO, 1998).

8 To improve zinc extraction, the protons were neutralized with NaOH. This improved the extraction of zinc ions as shown in figure 5. In figure 5 two cases are shown, in one case the NaOH is introduced at the outset of the experiment and in the other, the NaOH is added continuously. When the NaOH is added at the beginning of the experiment, it

9 neutralizes the protons initially released by the sulphuric acid. Then the aqueous solution lowers its ph, because the NaOH has been consumed, until the extraction of zinc stops. It lowers the efficiency of the process. On the other hand, when NaOH is added continuously and the ph is kept above 3 (in the order of 5.5), the protons are constantly neutralized and more zinc is extracted from the continuous external phase. This gives an extraction efficiency of 86 % in comparison of 49 % when neutralized only at the beginning of the experiment. CONCLUSIONS Zinc can be efficiently extracted by multiple emulsions using DEHPA as a carrier. The extraction is limited to ph in the external phase superior to 3, because at lower ph s, the DEHPA does not have capacity to complex the zinc ions. The kinetic behavior of the proton concentration was modeled and good agreement between experimental and predicted results was obtained. ACKNOWLEDGMENTS The authors acknowledge the financial aid of FONACIT through its program F and the CDCHT of the Universidad de los Andes, Venezuela, for financing the research projects of the Laboratorio de Mezclado,

10 Separación y Síntesis Industrial, through the projects I F and I B. We also acknowledge the help of Jorge Peña for the kerosene characterization. REFERENCES 1. CÁRDENAS, A., CASTRO, E., 2003, Breaking of multiple emulsions under osmotic pressure and the effect of the W1/O relation, Interciencia, 28 (9), COPP, A., MARR, R, 1982, Liquid membrane phenomena-a survey of phenomena, mechanisms and models, International Chemical Engineering, 22 (1), Neutralizing the external aqueous phase (feed) increases the extraction efficiency of zinc. 3. COX, M. FLETT, D. 1983, Metal Extractant Chemistry, in Handbook of Solvent Extraction, New York, Wiley- Interscience Publications. 4. DRAXLER, J., MARR, R. J., 1986, Emulsion liquid membranes: part I. phenomenon and industrial application, Chemical Engineering Process, 20, [ Links ] 5. DRAXLER, J., FURST, W., MARR, R. J., 1988, Separation of metal species by emulsion liquid membranes, J. Membrane Science, 38, HO, W. S., LI, N., 1992, Emulsion liquid membranes: definitions, Chapter 36 in Membrane Handbook, W. S. Ho and K Sirkar editors, van Nostrand Reinhold, New York, KONDO, K., MATSUMOTO, M., 1996, Solvent Extraction of Europium with Diisostearylphosphoric acid and its application to an Emulsion Liquid Membrane Technique, Sep. Sci. Technol. 31 (4), KONDO, K., MATSUMOTO, M., 1998, Separation and concentration of indium (III) by an emulsion liquid membrane containing diisostearylphosphoric acid as a mobile carrier, Sep. Purif. Technol. 13, MARR, R. J., BART, H. J., DRAXLER, J., 1990, Liquid membrane permeation, Chemical Engineering Process, 27, MATSUMOTO, S., KANG, W., 1989, Formation and applications of multiple emulsions, J. Dispersion Science and Technology, 10 (4&5), [ Links ] 11. NOBLE, R., KOVAL, C., PELLEGRINO, J., 1989, Facilitated transport membrane systems, Chemical Engineering Progress, March,

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