Journal of Scientific & Industrial Research Vol. 63, January 2004, pp 74-79 Extraction of zinc(ii) using liquid membrane and performance optimization using response surface methodology D Shanthana Lakshmi, M Muthukumar and D Mohan* Membrane Laboratory, Department of Chemical Engineering, A C Tech, Anna University, Chennai 600 025 Received: 05 May 2003; accepted: 04 November 2003 Zinc(II) was transported selectively from the feed compartment to the stripping compartment through the flat sheet supported liquid membrane loaded with 2-ethylhexyl phosphonic acid mono 2-ethylhexyl ester (Ionquest801-IQT801) in kerosene. The effects of feed phase ph (1.5-3.0), carrier concentration (5-20 per cent) and stripping agent concentration (0.3-1.3M) were studied using flat sheet supported liquid membrane (FSSLM) and the enrichment factor values were calculated. Second order polynomial regression was used for analysis of the experiment. The experimental values were in good correlation with predicted values and the high correlation coefficient obtained proved the fitness of the selected model. Keywords: Extraction of Zinc (II), Liquid membrane, Response surface methodology, Performance optimization Introduction The toxicity of a metal is directly related to its reactivity with living matter. At trace levels most of the metals are essential to life (trace elements). However, heavy metal are health hazardous because they are toxic even at very low concentration, they do not degrade with time and thus tend to accumulate in living organisms and to concentrate during transfers of matter in the food chain. In recent years, membrane separation was regarded as an important technique in dealing with specific problems of pollution control and hydrometallurgy. Liquid membranes have been developed to overcome initial limitations of transport rates and selectivity with solid membranes. Selective separation and concentration of the solute of interest from dilute solutions can be achieved using liquid membranes 1. This simultaneous extraction and stripping operation is very attractive because the solute of interest can move from a low concentration solution to a high concentration solution with suitable carriers. Supported liquid membranes which use a porous polymeric membrane support impregnates with complexing carriers to separate feed solution with suitable carriers and one of the important leading * Author for correspondence E. mail : mohantarun@yahoo.com configuration of liquid membrane 2. SLM technique offers several advantages over classical solvent extraction processes. A reduction in the number of process steps, smaller quantity of expensive carriers and the ability of the process to extract all of the solute to produce a high concentration extract is economically attractive in many circumstances especially where large volumes of solutions require treatment 3. The small thickness of supported liquid membranes is convenient to minimize the time required for the ion transport inside the membrane and thus to increase the efficiency of the separation. The SLM process is one method for treating wastewater containing Zn(II). IQT801 has been shown to be an effective extractant for the separation and purification of a number of metals and organic acids due to its excellent chemical stability, high boiling point and low solubility in water and it is highly selective for the extraction of Zn(II). Reagents Commercial acidic carrier di(2-ethylhexyl) phosphoric acid (DEHPA) and 2-ethylhexyl phosphonic acid mono 2-ethylhexyl ester (Ionquest801) were received from Albright Wilson, USA. Commercial organic solvents n-hexane and butanol obtained from Merck (I) Ltd. and kerosene collected
LAKSHMI et al.: EXTRACTION OF ZINC(II) 75 at 170-220 C (aliphatic in nature) fraction was used as a diluent for Zn(II) separation studies. Metal salt ZnSO 4.7H 2 O (AR grade) and H 2 SO 4 (AR grade) procured from Merck (I) Ltd, were used as received for the preparation of feed and stripping phase. Deionized water was used throughout the investigation. Membrane Supports The porous membrane made of PTFE was a product of Sartorius. Flat sheet polytetrafluoroethylene (PTFE) membrane was used as the polymeric support.the organic solutions (liquid membrane) was prepared by dissolving appropriate quantity of carrier IQT801 in kerosene to get carrier solution of varying concentrations. Flat Sheet Supported Liquid Membrane Cell Experimental design of supported liquid membrane (SLM) usually consists of a cell with two compartments and the schematic description is illustrated in Figure 1. The feed solution were prepared by dissolving appropriate quantity (25 ppm) of metal salt (ZnSO 4.7H 2 O) in deionized water to the desired concentrations and the stripping solutions were H 2 SO 4. Preparation of Supported Liquid Membrane (SLM) Supports Membrane support in general is made by adsorbing a solution of a specific carrier in a diluent medium in a microporous polymeric membrane hydrophobic. The diluent is required to be immiscible with the solutions on the two sides of the membrane and should have low dielectric constant. A microporous PTFE (Polytetrafluoroethylene) membrane was used as a support to hold the various carriers. Before impregnation, the membrane support was presaturated with water for more than 2 days 4. Circular pieces of these dry PTFE membranes were soaked in the carrier solution of different concentrations for 6 h before use to have uniform adsorption of carrier solution. The membrane was then removed from the organic solution, wiped with filter paper and rinsed with water to ensure the removal of excess carrier by which the impregnated polymeric membrane support was made suitable for separation process. Supported Liquid Membrane (SLM)-Experiments In the beginning of each run, the membrane impregnated with carrier was first clamped and the apparatus was assembled. Feed solution ph was adjusted to the desired value by dilute sulphuric acid (0.1 N). Feed phase containing metal ions and strip phase in aqueous medium were then introduced into the feed and strip chambers respectively. The aqueous solutions were presaturated with the diluent (kerosene) solution of carrier in advance, to prevent the migration of the liquid in membrane to aqueous phase. The mixing was done to avoid concentration polarization between membrane interfaces and bulk of solution 5. When the steady state (about 30 min) was reached, a sample was taken from the feed and strip phases at regular time intervals. Further, more original strip solution was added immediately to the strip phase to maintain a constant volume. A digital ph meter with glass and calomel electrodes was used to measure the ph of feed phase. Determination of Metal Ion Concentration Varian SpectrAA-200 model atomic absorption spectroscopy (AAS) was used to measure metal ion concentration in feed and strip phases and corrections due to volume replacement were made. Metal ion concentrations Zn(II) were measured in AAS at appropriate wavelength of 213.9 nm. Figure1 Supported Liquid Membrane (SLM) cell Enrichment Factor The application of SLM is often aimed at the enrichment of a solute as opposed to selective separation. The flux through the SLM and the attainable enrichment are closely related. Enrichment factor is one of the most common criteria used to evaluate a particular SLM system. In the liquid membrane system the feed phase is usually composed of a more concentrated solution than the stripping
76 J SCI IND RES VOL 63 JANUARY 2004 phase 6. The enrichment factor (Y), which is defined as the ratio of the concentration of the species in the stripping solution [S s ] to its initial concentration in the feed solution [S f ] 7. Y = [S s ]/[S f ]. Experimental Procedure Experimental Design and Response Surface Modeling The variables that affect the separation efficiency of metal ions are feed phase ph, carrier concentration and stripping agent concentration designated as X 1, X 2 and X 3 respectively. For optimizing the above factors influencing the metal ion separation, a Box-Behnken 8 design was used. The levels of variables chosen for the study for zinc and their corresponding coded values are given in Table 1. Regression Analysis A total of 15 statistically designed combinations were necessary to estimate the coefficients of the model. Second order polynomial regression containing 3 linear, 3 quadratic, 3 interaction and 1 block term were employed for estimating the separation of zinc. The different combinations for zinc in their coded form are given in Table 2. Response Surface Methodology After obtaining the responses for the experimental combinations response surface methodology was used to describe the behavior of the variables in the factorial space considered. Multiple regression analysis was performed on the experimental data to obtain a suitable planar model that will describe the variation of the response with the change in independent variables 9. It was observed that linear models were insufficient to explain the behavior of the response and hence, a second order polynomial equation including any possible Variables Table 1 Variables and their coded values Lower level (Coded value) Middle level (Coded value) Upper level (Coded value) Feed phase ph 1.0(-1) 2.0(0) 3.0(+1) Carrier concentration 5.0(-1) 10.0(0) 15.0(+1) Stripping agent concentration 0.3(-1) 0.8(0) 1.3(+1) Table 2 The Box-Behnken design for the three independent variables Trial no. Feed phase ph Carrier concentration per cent Stripping phase concentration M 1 +1-1 0 2 +1 0 +1 3-1 -1 0 4 0 0 0 5 0-1 -1 6-1 0-1 7 0 0 0 8-1 +1 0 9 +1 +1 0 10 +1 0-1 11 0 0 0 12 0-1 +1 13 0 +1-1 14-1 0 +1 15 0 +1 +1 interaction terms was used. The polynomial used is of the form as given in Eq. (1). Y = A 0 +A 1 X 1 +A 2 X 2 +A 3 X 3 +A 11 X 11 +A 22 X 22 + A 33 X 33 +A 12 X 1 X 2 +A 13 X 1 X 3 +A 23 X 2 X 3,... (1) where Y = predicted yield or response, A 0 = constant, A 1, A 2 and A 3 = linear coefficient, A 12, A 13 and A 23 = cross product coefficients, A 11, A 22 and A 33 = quadratic coefficients, X 1 =feed phase ph, X 2 = carrier concentration (percent) and X 3 = stripping agent concentration (M). Equation (1) was maximized by an iteration procedure to obtain the optimum combination of the variables. The software package MATLAB was used for this purpose. Another software package, design expert version 2.05 (Stat-Ease Inc., Minneapolis, MN, USA) was used to generate the experimental design as well as to obtain the coefficients in the Eq. (1). The same package was used for the generation of response surface contour plots for all the three variables considered. The regression equation obtained after analysis of variance gives the level of separation of zinc as a function of feed phase ph, carrier and stripping agent
LAKSHMI et al.: EXTRACTION OF ZINC(II) 77 concentration (Tables3-5). All terms regardless of their significance are included in Eq. (2). Zinc The separation of zinc in terms of regression equation expressed as a function of feed phase ph, carrier and stripping agent concentration can be expressed as, Y Zn = 2.812 +1.62075X 1 + 0.266875X 2 + 0.013875X 3-1.1195X 11-0.00925X 22-0.13425X 33 + 0.13275X 1 X 2 + 0.05075X 1 X 3 + 0.0655X 2 X,... (2) where Y Zn is the predicted response for zinc. Results and Discussion The synthetic feed solution contains zinc metal ion (25 ppm) was fed into the feed compartment as shown in Figure 1. Zinc(II) was transported through flat sheet supported liquid membrane loaded with Ionquest-801 (IQT801) in kerosene. The effects of feed phase ph, concentrations of carrier and stripping agent, on metal ion separation in terms of enrichment factor (EF) for the metal ion through flat sheet supported liquid membrane were studied and the results are discussed in the following sections. Effect of Feed Phase ph Feed phase ph plays a vital role in the distribution of metal ion species in aqueous phase, and it acts as the main variable in the extraction of zinc(ii). The effect of feed phase ph on the transport of Zn(II) was examined by varying the ph from 0.5 to 3.0 and the results are illustrated in the Figure 2. It is evident from Figure 2, that there was no transport of Zn(II) below ph 1.5, showing zero enrichment factor Figure 2 Response of enrichment factor to feed phase ph at various intervals of time at ph 0.5. Since the carrier IQT801 forms complex with Zn(II) only at ph >1.5, zinc could not be extracted at lower ph due to lack of complex formation. However, when feed phase ph increased beyond 1.5, the enrichment factor also increased. Thus maximum enrichment factors of 2.452, 2.776, 3.423 and 3.632 for ph 1.5, 2.0, 2.5 and 3.0 respectively were observed. At higher ph values the ionized group present in carrier IQT801 can exert attractive force for the Zn(II) ions thus leading to complex formation which in turn enhances transport of zinc 10. From this study it is clear that Zn(II) requires higher ph for effective separation. Effect of Carrier Concentration The carrier concentration in the flat sheet supported liquid membrane (FSSLM) system is one of the important governing factor for selective transport of metal ions. In the case of zinc separation, commercial acidic carriers such as phosphonic acid (2-ethylhexyl)-mono(2-ethylhexyl)ester (Ionquest801- IQT 801) and di(2-ethylhexyl) phosphoric acid (DEHPA) were tested as carriers using conventional solvent extraction procedure. However IQT801 was found to be a suitable carrier due to higher ionization nature of IQT801 at 1.5 ph under the present experimental conditions. Hydrogen ions present in the carrier IQT801 are exchanged for metal ion complexation. The change in the acidity of these carriers is caused by the exchange of OR groups by R groups. This change reflects their ability as carriers, that can be demonstrated with the different extraction rate of phosphoric, phosphonic and phosphinic esters. Thus, Zn(II) could be extracted at a lower ph than that expected with DEHPA. Transport of Zn(II) from mixture of metal ions for varying concentrations of carrier IQT801 was studied and the results in terms of enrichment factor are presented in Figure 3. It is evident from the Figure 3 that with increasing carrier concentration from percent to 15 per cent, Zn(II) separation gradually increased from 1.922 reaching a maximum of 2.452 at about 15percent carrier concentration due to higher complexing capacity in view of favorable diffusion of zinc ion complex. A further increase in carrier concentration to 20 per cent decreased the enrichment factor to 2.287. This behavior might be correlated to the higher density of 20 per cent IQT801 carrier, when compared to the lower percentage, thereby
78 J SCI IND RES VOL 63 JANUARY 2004 Figure 3 Relation between carrier concentration and enrichment factor reducing the diffusion of metal ion complex. Further, the viscosity of the organic phase also increases at higher carrier concentration, which favors the aggregation of the complex leading to saturation of membrane pores and formation of a layer by the carrier over the membrane interface 11. This, in turn, hinders the diffusion of, the metal complex effecting the retention of separating constituent to the entry side, and hence reduces the enrichment factor at higher carrier concentration. Effect of Stripping Agent Concentration Since the SLM technique involves simultaneous extraction and stripping process for the continuous transport of metal ions, it is essential to explore the effect of concentration of stripping agent in the receiving phase. The effect of stripping agent concentration on the Zn(II) transport was investigated using strong acid (H 2 SO 4 ). The concentration of acid was varied from 0.3M to 1.3M and the results obtained are shown in the Figure 4. As the concentration of stripping agent in the receiving phase increased from 0.3M to 0.8M, the enrichment factor increased from 1.841 to 2.452, and further increase in H 2 SO 4 concentration reduced the stripping efficiency. This could be due to the ionic atmosphere and increase in ionic strength of H 2 SO 4 that creates competition between like charged ions. However at higher concentrations the 2- excess SO 4 ion tends to form an ionic atmosphere 12 thereby causing a reduction in stripping efficiency. Hence, the enrichment factor decreases after reaching a maximum at 0.8M conc H 2 SO 4. Factor Plot-Zn(II) The factor effect functions and plot were used to assess the effect of each factor graphically. The factor plot function of a certain factor is a function that describes how the response moves as the level of that factor changes when the other factors are fixed at their optimum levels. From the factor plot Figure 5, it was observed that each of the three variables used in the present study had its individual effect on Zn(II) separation by FSSLM technique. The enrichment factor (EF) increased gradually up to 0.8 and thereafter decreased in the separation of Zn(II) with increase in feed phase ph from -1 (coded value) to +1 (coded value). As the concentration of carrier increased from -1 to + 1 (coded values), the separation increased and was maximum at +1. As the stripping agent concentration increased from -1 to +1 (coded values) the separation increased and was maximum at +0.45. Based on the experimental data, for maximum separation of Zn(II) optimum values of feed phase ph, carrier concentration and stripping agent concentration, were obtained corresponding to 3.78 Figure 4 Dependency of enrichment factor on stripping agent concentration Figure 5 Factor plot representing the individual variable effect on zinc (II) separation
LAKSHMI et al.: EXTRACTION OF ZINC(II) 79 against a maximum of 3.63 experimentally determined. Also the optimum combination arrived was not the same maximum combination experimentally studied and thus using design of experiments. Zn(II) separation is maximized using a minimum number of statistically designed combinations. The high correlation coefficient obtained confirms the fitness of the selected model in analyzing the experimental data. The lowest and highest possible ranges where the maximum separation can be achieved was also determined. The conditions for maximum Zn(II) separation was optimized. Conclusions The regression models obtained were able to explain the experimental result with high levels of significance. The analysis of variance demonstrates that the polynomial model was adequate for describing the relationship between response and the variables in both the metal ion separation studies. Thus using the response surface technique it was possible to examine the separation process with a relatively small set of experiments. It was also possible to assess the influence of individual variables on metal ion separation and study interaction among them, using the response surface model. The contour plots and the regression equations were able to predict the contribution of optimum levels of variables. The optimum points were verified experimentally and were found to be real optima as the response obtained under optimum conditions was higher than any other combination of variables. Acknowledgement One of the author Duraikkannu Shanthana Lakshmi, is grateful to Council of Scientific & Industrial Research (CSIR), New Delhi for the award of Senior Research Fellowship. References 1 Boyadzhiev L, Liquid pertraction of liquid membranes State of the art, Sep Sci Technol, 25 (1990) 187-205. 2 Li N N, Separation of hydrocarbons by liquid membrane permeation, Ind Eng Chem Process Des Dev, 10 (1971) 215-221. 3 Chakravarti A K, Chowdhury S B & Mukherjee D C, Liquid membrane multiple emulsion process of separation of Copper(II) from waste waters, Coll Surf, A: Physiochem Eng Aspects, 166 (2000) 7-25. 4 Shukla J P, Sonawane J V, Anil Kumar & Singh R K, Amine facilitated up-hill transport of plutonium(iv) cations across an immobilized liquid membrane, Indian J Chem Technol, 3 (1996) 145-148. 5 Anilkumar, Singh R K, Shukla J P, Bajpai D D & Nair M K T, Macrocyclic facilitated transport of uranyl ions across supported liquid membranes using dicyclohexano-18-crown- 6 as mobile carrier, Indian J Chem, 31A (1992) 373-375. 6 Kazushige Nishizawa, Takahito Miki, Tomonori Satoyama, Toshiyuki, Tadashi Yamamoto & Masao Nomura, Enrichment of zinc isotopes by a liquid membrane system using Crown ether, Sep Sci Technol, 33 (1998) 991-1002. 7 Daoud J A, El-Reefy S A, & Aly H A, Permeation of Cd(II) ions through a supported liquid membrane containing cyanex-302 in kerosene, Sep Sci Technol, 33 (1998) 537-549. 8 Box G E P & Wilson K B, On the experimental attainment of optimum conditions, J Roy Stat Soc, B13 (1951) 1-45. 9 Box G E P, Hunter W G & Hunter J S, Statistics for experiments An introduction to design, data analysis and model building (John Wiley and Sons, New York) 1978. 10 Yoshinobu Sato, Kazuo Kondo & Fumiyuki Nakashio, Extraction kinetics of zinc with 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester using a hollow fiber membrane extractor, J Chem Eng Jpn, 22 (1989) 686-688. 11 Shukla J P, Anil Kumar & Singh R K, Effect of solvent type on neutral macrocycle-facilitated transport of uranyl ions across supported liquid membrane using dicyclohexano-18- crown-6 as carrier, Radiochimica Acta, 57 (1992) 185-191. 12 Ritcey G M & Ashbrook A W, Introduction to solvent extraction processing- Part I, solvent extraction: principles and applications to process metallurgy (Elsevier Scientific Co, Amsterdam, The Netherlands) 1984.