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1. Intr oduction

Scarcity of fresh water is a global problem, whichis continuously growing and may eventuallytrigger wars among nations. In the Middle East,plenty of fresh water is needed for land reclama-tion, irrigation, industrial and potable use, accord-ingly simple desalination technologies are desirable.Present technologies for desalination of salinewater may be generally classified into thermal andmembrane processes. The former suffer fromnumerous shortcomings that include high-energyconsumption and capital cost, stage-wise operationand other problems, such as corrosion and scaleformation. The latter suffer from low waterrecoveries, stage-wise operation and membranefouling, which is a serious problem that is currentlyundergoing tremendous investigation.

Membrane separation processes have beengaining increased attention due to their ambienttemperature operation, relatively low capital cost,high separation efficiencies and modular con-struction. The membranes are most commonlymade of a polymeric material with differentproperties, such as porosity, asymmetry or bio-compatibility, depending on the process for whichthey are intended. LMs, however, have gainedwide interest since their discovery by Li [ 11,sincethey offer great potential and merits compared tosolid membranes, such as higher permeability,simplicity in separation, higher selectivity, lowenergy consumption and absence of pores to beblocked or fouled as in solid membranes. Theyhave been shown to have great potential forwastewater treatment [2-91, separation of hydro-carbons [1O-l 21, hydrometallurgy [13-l 81 and inbiotechnological and in biomedical applications[ 19,201. LMs, however, cannot be fouled whetherby pore blocking or adsorption of foulant, andare cost-effective since they consume little energywhile permitting higher rates of mass transfer thansolid membranes. They can be mainly divided intoemulsion, supported and bulk LMs. The formersuffer from emulsion instability, swelling of the

internal phase and difficulty of breaking theemulsion following extraction operation.Instability has been remedied in some ways,e.g. by the addition of polymer to the membranephase. However, ELMS offer much larger surfaceareas than the supported type, whereas the latterare less complicated, but some configurations sufferfrom the inevitable washing out of membraneliquid from the pores of the porous support [2 13,while bulk LMs are limited only to lab investi-gations.In general, no literature cites desalination ofseawater by LMs of the three types, except byNaim [22] who conducted desalination of simulatedseawater by the ELM technique. However, numer-ous works on the transport ability of various cationsthrough bulk LM systems containing mobile carriershave been done. Izatt et al. [23] investigated thecation fluxes of binary cation mixtures in waterchloroform-water bulk LM systems usingdifferent macrocyclic ligands as carriers for theseparation of silver ions. The stabilities of the cation-carrier complexes were found to significantlyinfluence the flux of single cations through theLM, and a very stable complex results in theextraction of the cation from the source phase butlittle, if any, release of it to the receiving phase,and therefore the movement of the cation throughthe membrane may be blocked. A certain rangeexisted for the values of the equilibrium constantswithin which maximum transport occurs.

Frensdorff [24] provided a quantitative measureof the strength of complexing in solution as afunction of cyclic polyether (MC) structure, cationsize and type, and solvent. He proved that thereis an optimum ring size for different ionsdepending on their size. Lamb et al. [25] studiedthe effects of salt concentrations, in the diluteranges, and type of anion, on the rate of carrier-facilitated transport of metal cations through bulkLMs containing crown ethers. They stated thatsince macrocyclic ligands are neutral, the cationcarries its co-anion with it across the membraneto maintain electrical neutrality in the system,

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which was also confirmed by Igawa et al. [26]and Dernini et al. [27]. The rate of cation transportthrough the membranes by macrocyclic ligandshas been shown to be influenced by the nature ofthe salt anion [25,28]. It is worth mentioning thatchloroform was the LM of choice in many labinvestigations [23,25,28]. As Pederson has ob-served [29], complexing is expected to be weakwhen the polymer ring is either too small for thecation or too large compared to the cation size.Noble and Way [30] reviewed the commercialand laboratory applications of LM technology,including gas transport, sensor development, metalion recovery, waste treatment, biotechnology, andbiomedical engineering, as well as SLMs, ELMSand membrane reactors. Economic data from theliterature for LM processes were presented andcompared with existing processes such as solventextraction and cryogenic distillation of air.

Yurtov and Koroleva [31] studied the pro-perties and peculiarities for LM extraction. Therequirements for extraction in ELMS were dis-cussed. Effects of surfactant concentration, carrierconcentration, external phase composition, internalphase composition, nanodispersion in the extractingemulsion, water transfer in the extracting emulsion,and rheological properties of extracting emulsionswere investigated.

A mathematical model was proposed by Moket al. [32] to describe the behavior of ELMS forthe extraction of penicillin in a continuouscounter-current mixing column. A polyamine-typesurfactant acts not only as a carrier but also as asurface-stabilizing agent, thus the influence ofsurfactant on extraction should be considered inmathematical modeling when its effect issignificant. The proposed model, therefore, takesinto account the influence of surfactant on masstransfer. The advancing front model wasemployed for deriving the overall mass transfercoefficient in the emulsion globule, and the axialdispersion model was applied to the external feedphase.

An improved advancing front model for type IIfacilitated transport in an ELM was developed

by Han et al. [33]. The proposed model not onlytakes into account the mass transfer both insideand outside the emulsion globules, and the inter-facial chemical reaction, but also the influenceof the permeation swelling and the breakage.Computer simulation was performed by Crank-Nicolson implicit method. The results indicate thatthe model prediction agrees well with the experi-mental data.Kemperman et al. [34] presented a new methodfor stabilizing SLMs based on the application ofpolymeric top layers to the surface of micro-filtration (MF) membranes, preventing loss of theLM phase out of the support pores. The modifiedMF membranes were used as SLMs and tested inselective nitrate transport and stability. Screeningexperiments revealed that most applied top layersdid not hinder the transport of nitrate ions.However, a few top layers were able to improvethe stability of LMs. Best results were obtainedwhen piperazine and trimesoylchloride were usedas monomers. SEM investigations revealed aparticular, rippled surface texture of layers pre-pared with these monomers.Stabilization of SLMs by gelation with PVCwas developed by Kemperman et al. [35]. Thegelation by PVC of SLMs for nitrate removalusing a quaternary ammonium salt as carrier, wasdescribed. Untreated SLMs with this carrier arevery unstable. The flux decrease is the result of adecrease of the diffusion rate of carrier complexas a result of the presence of the gel network andthe thickness of the applied gel layer. The absenceof any stability improvement might indicate thatthe loss of LM phase from these membranes isdue to dissolution of carrier or ML and not a resultof emulsion formation only.

It has been shown by Naim [22] that in orderthat NaCl diffuses through the LM, a suitable MChas to be added to the LM in order to facilitate itstransport, under favorable conditions through themembrane, then the transported NaCl has to besequestered in the receptor phase by a suitabletrapping agent. The selected combinations ofcarrier and trapping agent in conjunction with

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optimal concentrations of polymer and emulsifier,in her study, have proven to be very effective indesalinating simulated seawater up to potablelevels. Under optimum conditions, extraction of>98% of the salt in simulated seawater has beenachieved in only one stage.

2. Experimental2. I. Materials

Reagent grade sodium chloride was used inthe experiments and a polysaccharide functionedas complexation agent in the acceptor phase.2.2. Apparatus

A desalination cell, that operates batch-wise,was constructed for the purpose. Fig. 1 presents aschematic diagram of the designed cell. It iscomposed of a cylindrical 600 ml beaker, in whicha glass cylinder having a diameter of 6.5 cm andopen from both ends is suspended, using aplexiglass sheet to grip onto it firmly. The lowerend of the inner cylinder is veiled with acellophane membrane, which is stretched and heldfirmly in position using rubber bands. One liquidis added to the beaker, whereas the other liquid isadded to the inner notched cylinder. The cell iscovered with a watch glass. Agitation ofthe lowerphase inside the beaker is induced by magneticstirring.2.3. Procedure

A known volume of saline water (donor phase)is added to the beaker. The cellophane membraneis stretched onto the lower notch of the cylinderusing rubber bands. The cylinder is then placedin position inside the beaker such that it issuspended at its center. A known volume of theorganic ML, which functions as LM, is added tothe cylinder, followed by a known volume of thedistilled water (acceptor phase), after which thecylinder is covered with the watch glass. Thelevels of both saline and distilled water (donor

Fig.1.Apparatus used in SLM experiments. , ube holder;2, tube; 3, cellophane; 4, LM; 5, magnet; 6, magnetic stirrer;7, watch glass; 8, AP; 9, rubber band; 10, beaker; 11, DP.

phase (DP) and acceptor phase (AP) respectively)are marked on the glass. Magnetic stirring is thenstarted. 1 ml sample is pipetted from the top liquidat hourly intervals or otherwise, for the analysisby Mohrs method. The level is again noted andmarked at the termination of agitation. The wholeset-up is then left as such overnight or as desired.The concentration ofNaC1 is then plotted vs. timeof experimentation.

2.4. Variables investigatedThe rate of diffusion of Na and Cl- throughthe SLM from the DP to the AP is expected to beaffected by several variables, which were inves-tigated in the present work. The variables are:l Type and thickness of LM.l Presence of MC in the LM.l Quantity of MC in the LM.l Additives to AP.

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3. Results and discussionsThe present work dealt with desalination using

an SLM in which cellophane was used to supportthe organic ML separating the DP and AP. Theratio of DP to AP was kept constant at 4:l allthrough the experiments. Magnetic stirring wasused during all the experiments at daytime. Preli-minary work with chloroform as ML and noadditives in either the ML or the AP did not affectany desalination.Addition of sorbitol to the AP did not showany improvement even after 3 d of desalination.Addition of other polyelectrolytes (PEs) was notany better. However, preliminary work with 1,2dichloroethane (DCE) as ML together with PE inthe AP resulted in little desalination. A gradualtransfer ofNaC1 took place to the AP so that 0.93 gNaCl were transferred to the AP, in 25 d, whilethe concentration of the DP dropped from 14.4 to10.99 g. However, this result showed that aconsiderable amount ofNaCl (obtained by differ-ence) accumulated in the LM phase due to theabsence of a MC, and that the presence of a carrierwas necessary to transport the insoluble NaClmolecules through the LM phase.From Fig. 2a, it is seen that Ci decreases veryslowly by time and the mass of NaCl transportedwas negligible. Accordingly, addition of emulsifierto the LM did not prove effective all by itselfwithout a carrier. In addition, cellophane supportruptured after 8 d and the experiment wasterminated. Fig 2b shows the NaCl trapped withinthe LM was almost zero. The resistance to masstransfer seems therefore to be in the boundarylayers of the LM due to absence of stirring, especi-ally in the AP.The importance of a MC in the LM in effectingmass transfer is emphasized by studying Fig. 3a,in which Ci increases with time while Co decreases.The two curves coincide after 24 d, indicating thatboth DP and AP became identical in concen-tration. Fig 3b indicates that NaCl fluctuatesaround zero, indicating that no accumulation inthe LM takes place, and that mass transport is

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rapid in the LM due to the presence of a carrier.Accordingly, this result proves the importance ofthe presence of a carrier in effecting desalination.However, due to prolonged experimentation time(24 d), the PE used suffered from fimgal growthand the experiment had to be terminated.Figs 4a and 4b show the effect of changingthe LM thickness. Doubling the thickness resultedin a lower rate of mass transfer through the LM(compare slope of Ci curves in both Figs. 3a and

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4a). Also, after almost the same number of daysthe two curves of Ci and Co in Fig. 4a did notintersect, whereas they did in Fig. 3a. Neverthe-less, although the LM thickness is doubled, yetthe result is altogether acceptable and points outthe importance of the presence of a suitable MCfollowed in importance by the optimum thicknessof LM, which allows complete separation of thetwo aqueous phases, at the same time being thinenough to minimize resistance to mass transfer.Figs 5a and 5b point out the importance of the

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presence of an optimum quantity of crown ether.Addition of only 0.1 instead of 0.3 g MC improvedthe result (compare Fig. 5a with Fig. 4a) probablydue to a less viscous LM but it should be notedthat the thickness of the LM is less in this case(see Fig. 5a). In addition, it is realized that for thefirst time the two curves of Ci and Co intersect,meaning that Ci exceeded Co but only after about36 d of experimentation.Figs. 6a and 6b present the results of using 1,2dichlorobenzene (DCB) instead of DCE as LM.

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The results are rather identical to the experimentspresented by Figs. 2a and 2b. However, it mustbe noticed that in this case no emulsifier was used,and therefore it may be concluded that DCB ispreferred as a LM.

It must be added that from Fig. 6b it is clear thatthe amount of NaCl trapped in the LM is appre-ciable, due to the absence of a MC, which signifiesits importance in transporting NaCl across theLM.

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4. ConclusionsSeveral conclusions have been deduced fromthe present work. First it has been shown that theSLM technique is a very time-consuming methodby which desalination can take place, compared

with the very rapid ELM technique [22]. How-ever, it is a suitable method for further lab investi-gations with other complexation agents in the DP,other MCs, other membrane liquids, etc. It hasalso been shown that the emulsifier alone did not

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affect mass transfer, but that the presence of anoptimum quantity of MC is of primary importancein affecting desalination. The presence of a PE inthe AP is essential in sequestering the NaCl, al-though its effect is not pronounced as in the ELMtechnique applied to desalination [22]. Stirringshould have assisted in minimizing the resistanceat the AP-LM interface and should have promotedmass transfer towards the AP. On the other hand,continuous magnetic stirring of the DP could haveproduced even better results, only this was hardto fi_tltil or various reasons. As to the LM thick-ness, its effect is less significant than the MC, butcomes second in importance. Lastly, it is noticedthat different organic LMs lead to differentpermeabilities. To this end, it could be added thattesting other supporting membranes too could leadto minimizing the membrane resistance todiffusion of NaCl.

Further work remains to be carried out to opti-mize the extent of desalination and make it morerapid, and this is going on at present in our lab.Furthermore, a flowing LM technique is currentlyinvestigated for desalination in our lab as well,with the DP and AP flowing con-currently to eachother and counter-currently to the LM.ReferencesVI121

[31

[41

[51

[61

N.N. Li, Separating hydrocarbons with liquid mem-branes, US Patent 3,410, 794, 1968.J.A. Marinsky and Y. Marcus, Ion exchange andsolvent extraction: a series advances, Liquid Mem-branes, lO(2) (1988) 63-103.W. Halwachs and K. Schugerl, The liquid membranetechnique - a promising extraction process, Inter.Chem. Eng., 20(4) (1980) 519-528.J. Draxler, W. Hurst and R. Man; Separation of metalspecies by emulsion liquid membranes, J. Membr. Sci.,38 (1988) 281-293.J. Draxler and R. Marr, Emulsion liquid membranesfor waste water treatment, in: T. Sckine (Ed.), SolventExtraction 1990, Elsevier, 1992, pp. 37-48.M.M. Naim and A.A. Monir, Application of theemulsion liquid membrane technique to the extractionof alkali from aqueous solutions. 1. Extraction of

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