Chapter 22 Use of Emulsions, Microemulsions, and Hollow Fiber Contactors as Liquid Membranes Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0642.ch022
1
2
2
John M.Wiencek ,Shih-Yao Hu , and Bhavani Raghuraman 1
Department of Chemical & Biochemical Engineering, University of Iowa, Iowa City, IA 52242 Department of Chemical & Biochemical Engineering, Rutgers University, Piscataway, NJ 08855
2
Liquid membranes as a generic concept have primarily involved the use of U-tubes, porous solid films impregnated with a liquid carrier, or emulsified systems employed in a stirred contactor. Although such systems can display high selectivities and in some cases reasonable flux, the stability of the liquid membrane to rupture (i.e. leakage) and unwanted water transport (i.e. swell) have limited their commercial application. Our lab has focused on developing improved emulsion liquid membranes. In particular, we have investigated the possibility of employing microemulsions as liquid membranes to separate metals (especially mercury) from contaminated water. The lack of a clear advantage in such microemulsion systems suggested that a hybrid technique would provide the added stability without a loss in flux. Our current efforts utilizing emulsion liquid membranes within a hollow fiber contactor as a means to minimize swell and leakage will be discussed briefly.
This paper reviews the use of emulsions and microemulsions as liquid membranes with special emphasis placed on the separation of mercury, as Hg(N0 ) , from water using oleic acid as the extractant Although emulsion (either macro- or micro-) liquid membranes offer advantages in terms of fast rates of separation, new modes of creating a stabilized liquid membrane utilizing hollow fiber contactors offer comparable flux in a more stable format. The paper w i l start with a review of the basic types of liquid membranes as currently used in research. The discussion will then focus on the author's experience with emulsified liquid membrane systems. The last section of the paper will discuss the obvious next step in liquid membrane technology, the use of emulsion liquid membranes in hollow fiber contactors. The utility of liquid membranes has been discussed in several reviews and handbooks (1-2). Emulsion liquid membranes remain an actively researched separation technique for the removal of trace contaminants from aqueous streams. Liquid membranes are attractive because they combine extraction and stripping into a single step; thus, equilibrium constraints of partitioning are overcome by changing the chemical 3
2
0097-6156/96/0642-0319$15.00A) © 1996 American Chemical Society
In Chemical Separations with Liquid Membranes; Bartsch, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
320
CHEMICAL SEPARATIONS WITH LIQUID MEMBRANES
nature of the substance being separated (3). When liquid soluble ion exchangers or chelators are added to the liquid membrane, a variety of metals can be effectively separated from water such as: zinc and chromium ions using Aliquat 336 as a carrier (4); silver, gold and palladium ions using macrocyclic crown ethers as a carrier (5); copper utilizing LDC reagents (6-8); and mercury using dibutylbenzoylthiourea (9) or oleic acid (10-11).
Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0642.ch022
Basic Types of Liquid Membranes U-tubes. U-tubes are a very simple implementation of the liquid membrane concept. Two miscible fluids, usually aqueous phases, are placed in separate containers. One container is the feed solution and the other is the receiving solution. These two containers are connected via an inverted or normal U-shaped tube containing an immiscible solvent (e.g. hexane or chloroform). An inverted U-tube is used if the solvent is less dense than the receiving and feed solutions (e.g. hexane vs water); whereas, a normal U-tube is used if the solvent is more dense than the receiving and feed solutions (e.g. chloroform vs water). Mixing bars and sampling drawoff ports are usually placed in the feed and receiving phase containers. The U-tube is a useful device for preliminary experimental tests on new extractants because it allows for the experimental measurement of the solute concentration in all three phases (i.e. the feed phase, the membrane phase, and the receiving phase). U-tubes are not a practical contacting device if the separation is to be utilized on a large scale. The limited surface area for mass transfer at die feed phase/solvent and the solvent/receiving phase as well as the macroscopic diffusion distances in the solvent phase within the U-tube lead to very slow rates of separation. Supported Liquid Membranes and Hollow Fiber-Contained Liquid Membranes. Other configurations reported for simultaneous extraction and stripping include supported liquid membrane (SLM) and hollow fiber-contained liquid membrane (HFCLM). The SLM configuration has the organic phase liquid (containing the metal complexing agent) immobilized in the pores of a microporous membrane with aqueous feed phase flowing on one side and the aqueous receiving phase on the other side. A major disadvantage of SLMs is their instability mainly due to (a) loss of extractant and/or organic solvent into the flowing aqueous phase because of solubility and (b) short-circuiting of the two aqueous phases if the pressure differential across the membrane exceeds the capillary forces which hold the organic liquid in the pores. The operation will have to be interrupted to replenish lost extractant (12). In addition, SLMs are known to form emulsions at the membrane interface leading to contamination of the feed solution with extractant. The HFCLM configuration has two sets of microporous hollow fiber membranes, one carrying the feed phase and the other the strip phase (13,14) The organic liquid is contained between these two sets of fibers by mamtaining the aqueous phase at a pressure higher than the organic phase but lower than its breakthrough value. As with the system described later in this paper, HFCLM offers long term stability since the membrane liquid is connected to a reservoir and is continuously replenished to make up for any loss by solubility. The major disadvantage here is the difficulty in mixing the two sets of fibers to achieve a low contained liquid membrane thickness. Presently reported thicknesses are large (approximately 500 microns) and could yield low fluxes if diffusion through the membrane is rate-limiting. Emulsion Liquid Membranes. Extraction and stripping of metals are traditionally performed in separate operations. By use of emulsion liquid membranes (ELMs), the two steps can be accomplished in a single step. ELMs, first invented by Li (3), are made by forming a surfactant-stabilized emulsion between two immiscible phases. A water-in-oil emulsion, consisting of an oil phase with a metal extractant and an aqueous
In Chemical Separations with Liquid Membranes; Bartsch, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0642.ch022
22.
WIENCEK ET AL.
Emulsions, Microemulsions, & Hollow Fiber Contactors
stripping reagent as an internal phase, is dispersed into an aqueous waste stream containing metal ions. Copper(II) extraction using ELMs in a stirred contactor (SC) is shown in Figure 1. Combining extraction and stripping removes equilibrium limitations inherent in conventional solvent extraction and metal concentrations in feed solutions are reduced to very low levels. After extraction, the emulsion can be demulsified to recover the enriched stripping phase. Demulsification (emulsion breaking) by application of high voltage electric fields has proven to be most successful (15). Extraction using ELMs have been reported for copper, zinc, mercury, cobalt, chromium, and nickel (4,5,7,9,11,16,17). The main problem associated with ELM extraction is the integrity of the emulsion. Swelling and leakage of the ELM can occur after prolonged contact with the feed phase (18). Swelling occurs when the feed stream gets into the internal phase by either osmotic pressure or physical breakage and subsequent reformation of the membranes. The water content in the emulsion can thus increase from 10-20 wt. % to 30-50 wt. %. Swelling reduces the stripping reagent concentration in the internal phase which in turn lowers its stripping efficiency. Dilution of the solute that is to be concentrated in the internal phase also results in a less efficient separation. Leakage of the internal-phase contents into the feed stream because of membrane rupture not only releases extracted metals back into feed stream but further contaminates the feed stream with stripping reagents. Leakage can be minimized by making a more stable emulsion with a higher concentration of surfactant, but this makes the downstream demulsification and product recovery steps more difficult as well as increases swell. Lower shear rates would also minimize leakage, but mass-transfer resistance could then become very significant. Experiences with Emulsions and Microemulsions: A Case Study with Mercury Separation A number of processes exist for the treatment of aqueous streams. containing heavy metals such as precipitation (19), ion exchange (20) and electrochemical methods. Unfortunately, these processes have serious disadvantages: both ion exchange and chemical precipitation result in the formation of a solid waste from which the heavy metal cannot be easily recovered. Consequently, although the aqueous stream has been reclaimed, the precipitated sludge or adsorbent must be landfilled and with it, the concomitant problems of leaching into ground water, ion-exchange with nearby soil and possible microbial conversion to a more toxic form of the metal (e.g. conversion of a mercury salt to the very toxic methyl-mercury) continue. Solvent extraction is an alternative technology to remove heavy metals from wastewater. Extractants in the organic phase complex the metal ions present in the waste stream. The metals can be recovered by stripping the loaded organic phase with a different aqueous stream containing a stripping agent The result is a more concentrated solution from which metal ions can be recovered by electrowinning or electroplating techniques (21). This technique has been widely used by the mining industry. Metal extractants are now commercially available for a variety of separation requirements. By choosing the appropriate extractant and extraction conditions, high selectivity can be achieved. In addition, extractants can usually be regenerated without losing activity, thus improving the economics of the separation. Much effort has been expended in our labs over the last few years investigating the use of emulsion liquid membranes to carry out such wastewater treatment schemes with a special focus on the removal of mercury ions from water. Both coarse or macroemulsions as well as microemulsions were studied and compared. The advantage of emulsion liquid membrane extraction is the large surface area available for mass transfer which results in fast separations. Because the volume ratio of the feed to internal receiving phase is high, the separated metal is concentrated by factors as high as
In Chemical Separations with Liquid Membranes; Bartsch, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0642.ch022
322
CHEMICAL SEPARATIONS WITH LIQUID MEMBRANES
Oil Phase
Copper
Figure 1 A schematic representation of copper ion extraction with an emulsion liquid membrane. Copper(II) is transported to the emulsion/feed phase interface and reacts with the complexing agent (RH) to form a soluble copper complex (CuR ). This complex diffuses to the interior of the emulsion droplet until it encounters a droplet of the internal phase where the metal ion is exchanged for a hydrogen ion. The net effect is a unidirectional mass transport of the cation from the original feed to the receiving phase with counter-transport of hydrogen ions. Mercury exhibits a comparable mechanism for transport in these systems. 2
In Chemical Separations with Liquid Membranes; Bartsch, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
22. WIENCEK ET AL. Emulsions, Microemulsions, & Hollow Fiber Contactors 323
Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0642.ch022
100. As mentioned earlier, the main disadvantage of coarse emulsion liquid membranes is the inherent instability which makes them sensitive to the mixing rate, the volume ratio of the feed phase to the membrane phase, and time. Because of mis instability, leakage of internal phase into the feed phase can occur. In addition, water can be transported into the internal phase, diluting the concentrate. This phenomenon is called swell. Both leakage and swell result in reduced separation efficiency. Our studies on mercury were directed at comparing microemulsions to coarse emulsions to ascertain whether microemulsions could display anticipated benefits including reduced leakage and increased rates of separation. Extraction Chemistry. The binding of mercury as Hg(N0 ) to oleic acid (HR) was thoroughly characterized as a function of pH and oleic acid concentration (10). In the absence of any surfactant the binding followed a simple acid/basetitrationequilibrium as described by: 3
2+
Hg + 2 (HR) Hg(R · HR) + 2H 2
+
2
Keq,l = 0.449
2
Thus, the reaction is essentially shifted completely to the right at low hydrogen ion concentrations (pH>2) and extraction occurs. High hydrogen ion concentrations (pH
