Removal of lead by an emulsion liquid membrane1

*Corresponding author.Removal of lead by an emulsion liquid membranePart IRania Sabry a , Azza Hafez a *, MaalyKhedr a , Adel El-Hassanin baChemical Engineering and Pilot-Plant Department, National Research Center, Cairo, EgyptTel. +20 (2) 3370933; Fax +20 (2) 3370931; email: hafeza@bNatural Resources Department, Institute of African Research and Studies, Cairo University, EgyptReceived 2 April 2006; accepted 12 November 2006AbstractAn extensive study on removal of lead (II) from aqueous solution using an emulsion liquid membrane (ELM)technique is presented. The study has highlighted the importance of emulsion stability for maximizing the removal of lead (II). The ELM consists of di-(2-ethylhexyl) phosphoric acid as a carrier, commercial kerosene as organic solvent, span 80 as an emulsifying agent and sulphuric acid as stripping phase. The important factors studied which affect the ELM stability and removal of lead are emulsification time (5–30 min), emulsification speed (1000–3000rpm); the concentrations of surfactant (2–12 % v/v), carrier (1–12 % v/v), internal phase (0.25–3 molar H 2SO 4) and the effect of volume ratio of the oil phase to the stripping aqueous phase (O/A) (0.3–3.5). The results showed that it is possible to remove 99–99.5% of lead after 5 min contact time by using ELM at the optimum operating conditions.Keywords : Emulsion liquid membrane; Stability; Lead removal; Surfactant; Carrier; Internal phase1. IntroductionHazardous wastes may account for approxi-mately 2–20% of all wastes in the world [1]. Some of the most important categories of wastes are liq-uid organic chemicals, solvents and heavy met-als. The continuous use of heavy metals in indus-trial applications with the production of contami-nated wastewaters is a serious environmental problem. As heavy metals are not biodegradable,they tend to accumulate in living organisms caus-ing various diseases and disorders.In particular, lead (II) is considered as highly toxic heavy metal, which is commonly used in a number of industrial applications such as storage battery manufacture, television tube, printing pig-ments, fuels, photographic material and explosives manufacturing. Lead bearing wastewater resulted from all these industries must be disposed off af-ter treatment.Desalination 212 (2007) 165–175doi:10.1016/j.desal.2006.11.0060011-9164/07/$– See front matter © 2007 Elsevier B.V . All rights reserved.166R. Sabry et al. / Desalination 212 (2007) 165–175Several attempts have been made to treat such types of effluents for facilitating easy disposal, or to recover the chemicals and recycle the process water. Methods of purification are: chemical pre-cipitation [2], adsorption [3], ionic exchange [4], and solvent extraction [5] which are still in use. However, industries are looking for competing al-ternative technologies which may overcome some of the inherent disadvantages of these methods.One of the promising methods for the separa-tion of such effluents is the emulsion liquid mem-brane (ELM) process, invented by Li et al. [6], about 30 years ago.The main advantages of the ELM system are: (a) high interfacial area for mass transfer, espe-cially at the inner oil–water interface, due to the small size of the aqueous phase droplets; (b) high diffusion rate of the metal ion through the mem-brane; (c) simultaneous performance of extrac-tion (at the outer interface) and stripping (at the inner interface) in the same system, and (d) capa-bility of treating a variety of elements and com-pounds in industrial setting at a greater speed and with and a high degree of effectiveness, with vary-ing contaminant concentrations and volume re-quirements.Many successful applications of ELMs for separation processes in general, and especially for removal of heavy metal ions from wastewaters, have been reported in the literature [7–9].The main objective of this work is to investi-gate the influence of various parameters affected the emulsion liquid membrane formation and its stability and testing the performance of the pre-pared ELM on removal of lead by using synthetic solution with different concentrations.2. Experimental and materials2.1. ChemicalsThe chemicals used are sorbitan monooleate (span 80) from Sigma Chemical Co., bis-(2-ethylhexyl) phosphoric acid [D2EHPA], Fluka,Mr 322.43, product of the United States, sulphuric acid [H2SO4] analar (BDH Laboratory Suppliers, BH15 Ltd., England), hexamine [hexamethylene-tetramine], (CH2)6N4ADWIC, product of El Nasr Pharmaceutical Chemicals Co., MW 140.19, min. assay 99%, n-heptane CH3(CH2)5CH3, Lab-Scan, Analytical Science, Analytical Reagent AR, as-say (GC) 95%, lead nitrate Pb(NO3)2Laboratory Rasayan. Purified LR, MW 331.21, min. assay 99%, Prolabo, code: 26086.08 and xylenol orange tetra sodium salt indicator.2.2. ProcedureThe experimental scheme has been classified into two major parts. The first part is the forma-tion of the ELM and the determination of its sta-bility by using different operating parameters. The second part is concerned with the investigation of the performance of the ELM on the removal of lead from prepared synthetic solutions.In 250 ml beaker, a 25 ml portion of di-(2-ethylhexyl) phosphoric acid (D2EHPA) [1–12% v/v] and sorbitan monooleat (span 80) [2–12% v/v] in organic solvent (kerosene) are emulsified at stirring speed [1000–3000 rpm] by means of a motor-driven emulsifier. 25 ml of H2SO4[0.25–3molar] is added drop wise to the stirred organic phase until 1:1 volume ratio of organic membrane solution to stripping solution. The solution is stirred continuously for different time (5–30 min) to obtain a white ELM.In 250 ml beaker, 10 ml of the prepared ELM (organic phase and internal phase) is added to 100 ml of external aqueous solution (pure water in the case of measuring the emulsion stability or lead nitrate in the case of removal of lead). The con-tents are stirred by means of jar tester stirrers at variable speeds (100–400 rpm) for a different transfer time.The double emulsion (W/O/W) is allowed to be spontaneously separated by gravity in separat-ing funnel and the feed phase (external aqueous phase) is filtered through a whatman filter paper (No. 42) to be analyzed.R. Sabry et al. / Desalination 212 (2007) 165–175167Lead ions are measured by titremetric method as described by V ogel [10].2.3. The investigated parametersTable 1 represents the experimental investi-gated parameters for emulsion stability and lead removal and their ranges.2.4. Measurement of the breaking percent of the emulsion stabilityIn the ELM process, it is important to obtain stable emulsion, because membrane breaking up causes a decrease in the separation efficiency.Measuring of the breaking rate of the emulsion is carried out by contacting the emulsion with an external phase composed of pure water (distilled water). The variation of the pH of this phase with time is an indication of the emulsion breaking rate (BR) as referred by Bourenane et al. [8]. The breaking rate defined by Eq. (1), which represents the fraction in % of the expulsed internal phase volume to external phase by the breaking volume (V r ) to the initial volume of the internal phase (V int ).Table 1Investigated parameters for emulsion stability and lead removal and their rangesav/v % = volume of surfactant with respect to the total volume of membrane phase bTotal volume 50 ml in all experiments2. Emulsification time, min3. Internal phase concentration, molar4. Surfactant concentration, % v/v a5. Carrier concentration, % v/v6. O/A ratio [ratio of the oil phase to the aqueous (stripping) phase]b7. Organic solvent8. Lead concentration, mg/l 9. Contact time, min5–30 0.25–3 2–12 1–12 0.3–3.5Commercial kerosene 400–1000 0–8intBR(%)100rV V =×(1)The volume V r is calculated by Eq.(2), the material balance from the external phase measure before and after contact.o pH pHext pH+101010[H ]r iV V −−−−=−(2)where V ext = initial external phase volume; pH o =pH of the initial external phase; pH = pH of the external phase being in contact with the emulsion after a certain time of agitation; [H +]i = the pro-tons initial concentration in the internal phase.3. Results and discussionThe results and discussion of the effect of emulsification speed 1000–3000 rpm; emulsifi-cation time 5–30 min, internal phase concentra-tion 0.25–3 molar H 2SO 4, surfactant concentra-tion 2–12% v/v, carrier concentration 1–12% v/v,O/A ratio 0.3–3.5 and contact time 0–10 on the stability of the prepared ELM and removal of lead from synthetic solutions are presented as follows:3.1. Effect of emulsification time on the emulsion stability and the removal efficiency of lead at dif-ferent emulsification rotating speedThe influence of emulsification time on the emulsion stability at different emulsification ro-tating speeds (1000–3000 rpm) are shown in Fig.1.ELM solutions were performed at internal phase concentration 0.5 M H 2SO 4, surfactant concen-tration 2% v/v, carrier concentration 2% v/v and organic solvent —commercial kerosene, at O/A ratio 1, mixing speed 250 rpm and contact time 5min.It was observed that the emulsion stability in-creases when the emulsification rotating speed increases from 1000 rpm to 2000 rpm, because an efficient emulsification gives a good disper-168R. Sabry et al. / Desalination 212 (2007) 165–175sion of internal phase drops into the membrane.It is remarkable that when these drops become smaller they will take much more time to coa-lesce as reported by Bourenane et al. [8]. Also it was observed that increases in emulsification speed from 2500 to 3000 rpm decreases the emul-sion stability, this is in good agreement with Bart et al. [11] and Hirato et al. [12]. They found that the emulsion has a higher tendency to breakdown when the agitation intensity is high. In addition increasing of emulsification time for all rotating speed has no effect on the emulsion stability, so 5min was sufficient to form stable emulsion.The prepared ELM was tested for determina-tion of the effect of the emulsification time on the removal efficiency of lead at different emulsifi-cation rotating speeds. The conditions used here are the same as the conditions used previously.The initial concentration of lead was 400 mg/l. It is observed from Fig. 2 that the removal efficiency of lead increases by increasing the emulsification rotating speed up to 2000 rpm, then there is no significant difference as the emulsification rotat-ing speed increases. As a result 96.6% of lead is removed at 2000 rpm emulsification speed and 5min emulsification time.Fig. 1. Effect of emulsification time on the emulsion stability at different emulsification rotating speeds at internal phase concentration 0.5 molar, surfactant concentration 2% v/v, carrier concentration 2% v/v and commercial kerosene asorganic solvent.3.2. Effect of sulphuric acid concentration in the internal phase on the emulsion stability and the removal efficiency of leadThe influence of sulphuric acid concentrations (0.25–3 molar) on the emulsion stability at opti-mum emulsification speed (2000 rpm) and emul-sification time 5 min at constant surfactant con-centration 2% v/v, carrier concentration 2% v/v and organic solvent (commercial kerosene) at a mixing speed 250 rpm, O/A ratio 1, contact time 5 min have been studies. It can be noted from Fig.3 that when the sulphuric acid concentration increases from 0.25 to 1 M, the stability of the emulsion increases, but decreases when the acid concentration is gradually increased from 1.0 to 3 M. This may be due to the reaction of the acid with span 80 which results in a partial loss of its surfactant properties as illustrated by Bourenane et al. [8].On the other hand, Fig. 4 presents the effect of sulphuric acid concentration on the removal effi-ciency of lead at the same conditions as mentioned above and at lead concentration 400 mg/l. It was observed that the removal efficiency of lead is increased by increasing the acidity in the strip-R. Sabry et al. / Desalination 212 (2007) 165–175169Fig. 2. Effect of emulsification time on the removal efficiency of lead at different emulsification rotating speeds at internal phase concentration 0.5 molar, surfactant concentration 2% v/v, carrier concentration 2% v/v and commercialkerosene as organic solvent.Fig. 3. Effect of sulphuric acid concentration in the internal phase on the emulsion stability at surfactant concentration 2%v/v, carrier concentration 2% v/v and commercial kerosene as organic solvent.ping phase. The differences of hydrogen ion chemical potentials between the two aqueous phases are the main driving force in the emulsion liquid membrane process. Thus the removal effi-ciency increases sharply with increasing the con-centration of H 2SO 4 in the internal phase from 0.25to 0.5 M, then it increases gradually from 0.5 to 1M, this owing to the increase of the capacity of the receiving phase. However, for a concentra-tion of sulphuric acid greater than 1 M, the emul-sion swells up due to osmosis, which leads to the dilution of the internal phase, causing a less ef-fective stripping. The previous phenomena are supported by Reis et al. [13] who found that the170R. Sabry et al. / Desalination 212 (2007) 165–175increase of H 2SO 4 concentration from 2 to 2.5 M caused a significant effect on water transport. So it is concluded that 1 M H 2SO 4 achieve 99.4 % of lead removal efficiency.3.3. Effect of surfactant concentration on the emul-sion stability and the removal efficiency of lead The effect of surfactant concentration on the behavior of the emulsion stability and removal of lead were investigated. The operating parameters were: emulsification speed 2000 rpm, emulsifi-cation time 5 min, internal phase concentration 1M H 2SO 4, carrier concentration 2% v/v and or-ganic solvent (commercial kerosene) at O/A ratio 1, mixing speed 250 rpm, contact time 5 min. The concentration of the surfactant was varied from 2% to 12% v/v. The effect of surfactant on emul-sion stability is presented in Fig. 5. It was ob-served that the emulsion stability improves by increasing the surfactant concentration till 8% v/v then the emulsion stability becomes constant.On the other hand, in Fig. 6 an increase in the surfactant concentration decreases the removal efficiency of lead due to mass transfer resistance caused by the surfactant film. As reported by Bou-renane et al. [18], Juang et al. [14] and Li et al.[15], the viscosity of the organic phase increasesFig. 4. Effect of sulphuric acid concentration in the internal phase on the removal efficiency of lead at initial concentra-tion 400 mg/l, surfactant concentration 2% v/v, carrier concentration 2% v/v and commercial kerosene as organic solvent.when the surfactant concentration increases, re-sulting in lowering the emulsion liquid membrane permeation, and diffusivity of complexes in the organic phase is reduced. Also as indicated by Reis et al. [13], the amount of the surfactant in the membrane must be minimal but it must be enough to stabilize the emulsion. So we concluded that the surfactant concentration must not exceed 4%v/v to achieve 99% lead removal efficiency.3.4. Effect of carrier concentration on the emul-sion stability and the removal efficiency of lead It was observed that the carrier concentration also plays a key role in the emulsion stability and removal efficiency of lead. The experimental con-ditions were kept constant at emulsification speed 2000 rpm, emulsification time 5 min, internal phase concentration 1 M H 2SO 4, surfactant con-centration 4% v/v and organic solvent (commer-cial kerosene) at O/A ratio 1, mixing speed 250rpm, contact time 5 min and organic solvent (com-mercial kerosene). The effect of carrier concen-trations (1–12% v/v) on the emulsion stability is shown in Fig. 7. The emulsion stability decreases with increasing the carrier concentration, this may be due to the interfacial properties of the carrier,which favors o/w emulsions and this is opposedR. Sabry et al. / Desalination 212 (2007) 165–175171to the span 80 action as referred to by Bourenane et al. [8].The effect of carrier concentration (1–12% v/v)on the removal efficiency of lead concentration 400 and 1000 mg/l has been studied. The carrier removed nearly all lead for the initial lead concen-tration 400 mg as shown in Fig. 8, so another ex-periment was investigated by using 1000 mg/l concentration of lead to study the effect of the carrier concentration at a high lead concentration as illustrated in Fig. 9. It was observed that the removal efficiency increases by increasing the Fig. 5. Effect of surfactant concentration on the emulsion stability at internal phase concentration 1 molar, carrier concen-tration 2% v/v and commercial kerosene as organic solvent.Fig. 6. Effect of surfactant concentration on the removal efficiency of lead at initial concentration 400 mg/l, internalphase concentration 1 molar, carrier concentration 2% v/v and commercial kerosene as organic solvent.carrier concentration till 4% v/v, where lead re-moved from 65% to nearly 99%, then when the car-rier concentration increased from 4% to7% v/v,the removal of lead remained constant. It was illus-trated by Reis et al. [13] that increasing the con-centration of carrier promotes the permeation swelling, which dilutes the aqueous receiving phase and decreases the efficiency of the process,therefore, in order to get a better emulsion stabil-ity and overall mass transfer, it is recommended to maintain 4% v/v of carrier concentration to achieve 99% lead removal efficiency.172R. Sabry et al. / Desalination 212 (2007) 165–1753.5. Effect of volume ratios of the oil phase to the internal aqueous phase (O/A) on the emulsion sta-bility and the removal efficiency of lead The effect of volume ratios (O/A) in the range of 0.3–3.5 on the emulsion stability at the opti-mum emulsification speed 2000 rpm, emulsifica-tion time 5 min, internal phase concentration 1 M H 2SO 4, surfactant concentration 4% v/v, carrier concentration 4% v/v, and organic solvent (com-Fig. 7. Effect of carrier concentration on the emulsion stability at internal phase concentration 1 molar, surfactant concen-tration 4% v/v and commercial kerosene as organic solvent.Fig. 8. Effect of carrier concentration on the removal efficiency of lead at initial concentration 400 mg/l, internal phaseconcentration 1 molar, surfactant concentration 4% v/v and commercial kerosene as organic solvent.mercial kerosene) at constant mixing speed 250rpm, contact time 5 min are presented in Fig.10. It is observed that the emulsion stability increases slightly with increasing the membrane/inner phase ratios from 0.3 to 1, because the ejec-tion of the internal phase is easier when its pro-portion in the emulsion is higher as indicated by Bourenane et al. [8].The emulsion stability will be stable from 1 to 1.5, then decreased from 1.5to 3.5, as reported by Bourenane et al. [8] andR. Sabry et al. / Desalination 212 (2007) 165–175173Fig. 9. Effect of carrier concentration on the removal efficiency of lead at initial concentration 1000 mg/l, internal phaseconcentration 1 molar, surfactant concentration 4% v/v and commercial kerosene as organic solvent.Fig. 10. Effect of O/A ratio on the emulsion stability at internal phase concentration 1 molar, surfactant concentration 4%v/v, carrier concentration 4% v/v and commercial kerosene as organic solvent.Juang et al. [14], the O/A ratio will decrease the membrane stability.As shown in Fig. 11 in the case of 400 mg/l initial concentration of lead, the removal effi-ciency is nearly 100%. So the effect of O/A ratios on the removal efficiency of lead is not indicative at this concentration of lead. So we studied the effect of O/A ratios at a concentration 1000 mg/l of lead as illustrated in Fig. 12. It is observed that the removal efficiency of lead increases from 91.6% to 99.8% as O/A ratio increases till 1, this is related to that the viscosity of the emulsion sharply increases at O/A ratio less than 1 as shown in Fig. 13, which prevents lead ions permeation through the membrane as reported by Juang et al.[14] and Li et al. [15]. After O/A ratio 1 the re-moval efficiency remains constant till O/A ratio 2, then the removal efficiency sharply decreases up to O/A ratio equal 3.5. So, the optimum O/A is 1 to achieve all lead removal.174R. Sabry et al. / Desalination 212 (2007) 165–1754. ConclusionThe influence of the effect of the emulsifica-tion speed 1000–3000 rpm, emulsification time 5–30 min, internal phase concentration 0.25–3molar H 2SO 4, surfactant concentration 2–12% v/v,carrier concentration 1–12% v/v, O/A ratio 0.3–3.5 and contact time 0–10 on the stability of the prepared ELM and removal of lead (400–1000)from synthetic solutions were investigated. The optimum conditions for preparation stable emul-Fig. 11. Effect of O/A ratio on the removal efficiency of lead at initial concentration 400 mg/l, internal phase concentra-tion 1 molar, surfactant concentration 4% v/v, carrier concentration 4% v/v and commercial kerosene as organic solvent.Fig. 12. Effect of O/A ratio on the removal efficiency of lead at initial concentration 1000 mg/l, internal phase concentra-tion 1 molar, surfactant concentration 4% v/v, carrier concentration 4% v/v and commercial kerosene as organic solvent.sion which capable of 99–99.5% lead removal were 2000 rpm emulsification speed, 5 min emul-sification time, 1 M H 2SO 4, surfactant concentra-tion 4% v/v, carrier concentration of 4% v/v and O/A ratio of 1.AcknowledgmentThe authors are indebted to the National Re-search Centre for financial support of this work.R. Sabry et al. / Desalination 212 (2007) 165–175175 Fig. 13. Effct of O/A ratio on the emulsion viscosity at internal phase concentration 1 molar, surfactant concentration 4%v/v, carrier concentration 4% v/v and commercial kerosene as organic solvent.References[1]G. Kiley, Environmental Engineering. McGraw Hill,New York, 1997.[2]J. Loredo, A. Pereira and A. Ordone, Untreated aban-doned mercury mining works in a Scenic of Asturias (Spain), Environ. 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