Norasikin Othman*,Norul Fatiha Mohamed Noah,Lim Yin Shu,Zing-Yi Ooi,Norela Jusoh,Mariani Idroas,Masahiro Goto

1Centre of Lipids Engineering and Applied Research(CLEAR),Ibnu Sina Institute for Scientific and Industrial Research(Ibnu Sina ISIR),Universiti Teknologi Malaysia,81310 UTM Johor Bahru,Johor,Malaysia

2Faculty of Chemical and Energy Engineering,Universiti Teknologi Malaysia,81310 UTM Johor Bahru,Johor,Malaysia

3Department of Applied Chemistry,Graduate School of Engineering,Kyushu University,744 Motooka,Nishi-ku,Fukuoka-shi,Fukuoka 819-0395,Japan

1.Introduction

Phenol is a major pollutant in wastewater due to its presence in the effluent of major processing and refining plant.It will cause severe effects on human being[1].According to Mohammadiet al.[2],the concentration of phenols present in the wastewater from the industries is 6–500 mg·L-1for re fineries,28–3900 mg·L-1for coal processing and 2.8–1220 mg·L-1for petrochemical plants.Pharmaceuticals,plastics,wood products,paints,pulp and paper industries contain 0.1–1600 mg·L-1phenols.The Environment Protection Agency(EPA)has included phenol as one of primary pollutants that abide to specific regulations in order to protect the environment and human being as their toxicity is high[3].It is essential to remove phenol from wastewater as much as possible so that the environment and human being are not exposed to it.On the other hand,the recovery of phenol from effluent can benefit to many industries,such as in the application as precursors to plastics and in production of adhesive,dyes,germicides,and chemical intermediate[4].

There are various methods of phenol removal and the methods can be classified into two main groups,which are traditional and advanced techniques.The traditional technologies include steam distillation,liquid–liquid extraction(LLE),adsorption,chemical oxidation and biodegradation,while advanced technologies include photo oxidation processes and membrane separation technologies[2].A comparison had been made among the membrane separation technologies,ultra filtration,supported liquid membrane(SLM)and emulsion liquid membrane(ELM)which are categorized in Table 1.Among them,the ELM shows very promising in removal of phenol.Although the instability of ELM such as swelling and leakage are the major problems,the stability of ELM can be minimized by optimizing the experimental conditions.

Previously,most of the emulsion liquid membrane technique used petroleum based diluents as an organic liquid membrane[16,17].However,this petroleum based diluents such as kerosene,n-heptane and dicholoro-ethane have the properties of toxicity,non-renewable,non-biodegradable, flammable and volatile in nature.This will givehigh impact to the environment if they are discharged to the environment.Thus,it is a must to replace the petroleum-based diluents with properties of renewable sources and non-toxic to become a green liquid membrane.The suggested environmental and green materials are vegetable oils such as coconut oil and palm oil[18,19].Therefore,the use of vegetable palm oil based for emulsion liquid membrane is studied in order to prevent the production of secondary pollutant to the environment.

Table 1Comparison of phenol removal methods

Basically,ELM comprises of three phases that are organic phase,internal phase and external phase.The organic liquid phase consists of diluents,surfactants and carriers.External phase is the water that carries the metal or element of interest,while the internal phase is the liquid phase that will trap the recover solutes[16].

There are two types of mass transport in ELM that are Type 1 facilitation and Type 2 facilitation[20].Removal of phenol was considered as Type 1 facilitation,where the mass transfer rate is increased by incorporating a stripping agent in the internal phase and the stripping agent will react with the solutes to form membrane insoluble product[11,13,21–23].However,based on the previous study[12,14],the used of the carrier were also performed in the extraction of phenol.Hence,in this research,palm oil will be utilized as a novel,naturally occurring and green liquid membrane for the recovery of phenol from the aqueous solution.The feasibility of palm oil without the existing of carrier in the emulsion liquid membrane for phenol extraction and its stability were investigated.Furthermore,the effects of parameters which affect the extraction and recovery performance of phenol in ELM process are also presented in this paper.

2.Materials and Methods

2.1.Materials and equipment

Kerosene,sorbitan monooleate(Span 80)(more than 60%oleic acid composition)and phenol crystals(＞99%assay)were obtained from Sigma-Aldrich,Buruh edible oil from Lam Soon Edible Oils Sdn.Bhd.Malaysia,and sodium hydroxide(NaOH)(98%assay)from J.T.Baker.The equipment used in this research includes a homogenizer Heidolph Silent Crusher M,a compact digital mixer system Cole-Parmer EW50006-00,quartz,an UV Spectrophotometer Jenway 7305,a magnetic stirrer with temperature controller IKA RCT basic Safety Control,a microscope Olympus CX31RTSF and a rotational Programmable Viscometer Brook field Model LDDV-II with LV spindle.

2.2.Liquid–liquid extraction

Phenol extraction experiment was designed to identify the type of diluent required to remove phenol from aqueous phase.The diluents such as palm oil and kerosene will be tested.The viscosity of each organic diluent was measured using Viscometer Brook field Model LDDV-II with LV spindle.An equal volume(50 ml)of phenol(300×10-6)was mixed with organic solution in a 100 ml conical flask.Then,it was shaken using a mechanical shaker(IKA KS 130 BASIC)with rotation of 320 r·min-1for duration of 1 h.The bottom layer,which was the feed aqueous phase was taken and carefully separated from organic phase for absorbance measurement at various time intervals.The experiments were repeated with different diluents.The experiments were conducted at room temperature,(25 ±1)°C.

2.3.Stability test of vegetable palm oil-based W/O emulsion

The main purpose of investigating the stability of emulsion was to evaluate the possibility of using palm oil as organic diluents for phenol removal.The stability of emulsion was tested on the different ratio of palm oil to kerosene composition(0:100,30:70,50:50,70:30,100:0).12 ml of organic phase with 3%(w/v)of Span 80 and 4 ml of internal phase,0.1 mol·L-1NaOH(membrane:internal phase=3:1)were mixed together at lower emulsification speed(1000 r·min-1)for 5 min.The stability of the white W/O emulsion was determined by transferring the emulsion into a measuring cylinder and left for phase separation.The time was recorded when the phase separation of water started to appear at the bottom of measuring cylinder where the emulsion start to break.

2.4.W/O emulsion preparation

The emulsion composition needed for W/O emulsion in the ELM system is organic phase(solvent/diluent,surfactants)and stripping agent.The solubility of the components in the organic phase was checked first and then the proper amount of diluent composition(kerosene to palm oil)that selected from Section 2.3 was prepared.12 ml of solvent was then mixed with 3%of Span 80 using magnetic stirrer.The internal phase,NaOH solution at 0.1 mol·L-1of concentration with 4 ml of volume(ratio solvent to internal phase;3:1)was slowly added into the membrane phase during the emulsification process.It was continuously stirred at 1000 r·min-1for 5 min.The white emulsion was then ready for the extraction study.The emulsion prepared must be fresh each time before the extraction process.

2.5.ELM extraction of phenol

The prepared W/O emulsion was then dispersed into the agitated vessel with 80 ml of the external solution and stirred with 400r·min-1for extraction time of 5 min.Several para meters are investigated such as emulsification time,homogenizer speed,agitation speed,surfactant concentration,feed pH,extraction time,strip phase concentration and treat ratio.All of the parameter range is presented in Table 2.One-factor-at-a-time method was applied.The samples were then introduced in to a separating funnel and left for phase separation.The volume of emulsion was measured and recorded for membrane swelling and breakage calculation.The feed phase at the bottomseparating funnel was then filtered and the concentration of phenol was analyzed while the remaining emulsion while the remaining emulsion will undergo the demulsification process. The demulsification process was performed by applying a pulsed AC field with electric potential 20 kV at 300 Hz frequency across the outer and inner electrode probe. After a high voltage demulsifier was applied to the emulsion, it was separated into oil phase and aqueous recovery phase. The recovered concentrated phenol in the aqueous phase was analyzed. Fig. 1 shows the schematic diagram of the experiment process..

Table 2Parameters investigated in the phenol extraction studies

2.6.Analysis method

The concentration of phenol was analyzed using UV SpectrophotometerJenway 7305 at 270 nm wavelength.Programmable Rheometer Brook field Model LDDV-II with LV spindle was used to determine the kinematics viscosity,v.Cyber scan 100 pH meter model was used to measure the pH measurements.Also,microscope Olympus CX31 RTSF was used to analyze W/O droplet size.The general equations of phenol removal,membrane swelling and final product enrichment are as stated in Eqs.(1),(2)and(3)respectively:

where,Ciis the initial concentration of phenol ion in feed phase before extraction,Cfrepresents the final concentration of phenol ion in feed phase after extraction process.

where,Viis the initial volume of emulsion before extraction,Vfis the final volume of emulsion after extraction.If theS(%)is negative,it indicates the breakage of emulsion occurs.

where,Ciis the initial concentration of phenol ion in feed phase before extraction,Cint,frepresents the final concentration of phenol ion in the internal after the demulsification process.

3.Results and Discussion

3.1.Liquid–liquid extraction of phenol

Fig.2 shows that the solvent extraction of phenol using different types of solvent such as kerosene and palm oil.The results show that 97%and 82%of phenol was extracted using kerosene and palm oil respectively during 60 min of extraction times.Among these diluents,kerosene has achieved highest percentage of extraction due to its nonpolar with lower specific gravity which can enhance the dispersion and coalescence in the extraction process which lead to the diffusion of phenol into the organic phase.This indicates that kerosene has an ability to improve the extraction performance of phenol by palm oil solvent.The result showed instability performance of palm oil in extracting the phenol in the earlier stage.This is because some of the phenols still diffuse back to the external phase.However,after30min,the extraction performance became plateau indicate that the equilibrium achieved.The extraction mechanism can be explained by the diffusion of phenol in aqueous solution from bulk to the interface where the reaction of phenol takes place with triglycerides present in the organic to the formation of phenol-triglyceride complex by hydrogen bonding or intermolecular interactions between them.In addition triglycerides(vegetable oils)have the ability to dissolve phenolic substances.Therefore,the extent of transport of phenols depends upon its solubility in the oil or organic phase[18].

Fig.2.The effect of Diluent/solvent on phenol removal efficiency[experiment conditions:[phenol]=300 × 10-6;extraction speed=320 r·min-1;ratio of feed phase:diluent/solvent=1:1].

Fig.1.Schematic diagram of experimental process.

3.2.Stability of primary emulsion—effect of diluents based composition

Table 3 shows the effect of diluents composition on the emulsion stability.Observation on the emulsion breakage was done at interval of 5 min.The volume of the water appeared difficult to be measured accurately because the emulsion started breaking and separating at the middle of measuring cylinder.For 30:70 compositions,the water broke at the middle and never settled down to the bottom of measuring cylinder,while water appeared at the top of the emulsion for 50:50 compositions.For 70:30 compositions,it took about 1 and half hours to settle down to the bottom of measuring cylinder.Meanwhile,very high viscosity of the solution was formed for 100:0 compositions,which was not suitable for further ELM extraction.Although 100%kerosene as liquid membrane achieved the highest stability,the main purpose in this research is to replace the kerosene with palm oil as the liquid membrane.Since 70:30 compositions achieved the most stable emulsion among the others except 100%volume of kerosene,thus it was selected as the diluent composition for this research.

Table 3The effect of diluent composition on emulsion stability

Viscosity was measured for different palm oil to kerosene ratio,which is shown in Table 4.The results showed that 100%volume of kerosene as the liquid membrane has the lowest viscosity of 2.346 × 10-3Pa·s and the increasing in viscosity along with the increased in palm oil composition,where the fully palm oil has viscosity of 51.425 × 10-3Pa·s.Based on the result of the primary stability test,the 70:30 ratio of palm oil to kerosene achieved the most stable emulsion over the other emulsions that contain palm oil.This indicates that higher viscosity of diluent formed more stable emulsion[24,25].

Table 4Viscosity measured at different palm oil to kerosene ratio

3.3.Mechanism of phenol removal in ELM process

The diffusionof phenol through themembrane is a simple permeation in the mixture of kerosene and palm oil.For the kerosene,the transport of phenol molecule occurred by diffusion in the liquid membrane phase and stripped out to the strip phase due to different concentration gradient between two phases and its solubility in kerosene.In the recovery stage,the extracted phenol recovered from the organic solvent by the aqueous NaOH solution according to the following reaction(Eq.(4)),which is an irreversible and instantaneous reaction[26].The sodium phenolate,C6H5ONa formed from the reaction cannot diffuse back to the feed phase because it was insoluble in the membrane phase.

On the hand,for the part of palm oil as a liquid membrane,the reaction of phenol took place with triglycerides that present in the membrane,leading to the formation of phenol-triglyceride complex by hydrogen bonding or intermolecular interactions between them(Fig.3).The phenol-triglyceride complex reacted with the aqueous sodium hydroxide solution leading to the formation of sodium phenolate,and the triglyceride molecules returned back to the liquid membrane to form complex with incoming phenol[9].Therefore,both of the liquid membrane played important roles in the transportation of phenol and provided an excellent mass transfer.

3.4.Stability performance of emulsion liquid membrane

Four parameters were investigated to study the stability of ELM process such as effect of emulsification speed,emulsification time,and surfactant concentration in preparation of W/O emulsion and agitation speed on W/O/W formation.

3.4.1.Effect of emulsification speed in W/O preparation

Fig.4 shows the effect of emulsification speed on emulsion stability and phenol extraction.From the observation,the emulsification speed of 5000 r·min-1will lead to the formation of mayonnaise-like solution when the emulsion was dispersed into the aqueous phase within2 min.Higher speed of emulsification will lead to the more viscous emulsion and the formation of mayonnaise-like solution might be due to the incorporated air inside the emulsion phase.It is due to the reaction that occurred between the palm oil and air.According to Tecnico[27],an autoxidation occurs in the present of isolenic unsaturated double bonds.The autoxidation rate can be affected by fatty acid composition,light,transition metal ions,oxygen pressure,and the presence of antioxidants,prooxidants,temperature,moisture content and distribution.Hence,a lower emulsification speed with a speed range of 1000 r·min-1to 1500r·min-1was used.

In addition,the results also showed that the difference of the extraction percentage among the chosen emulsification speed was less than 10%.It was found that the extraction percentage increased from 77%for1000r·min-1until83%for1300r·min-1,respectively,and then decreased in the extraction efficiency until 80%for 1500 r·min-1.The increasing in emulsification speed will increase the extraction efficiency due to the formation of smaller size of internal phase droplets.Therefore, finer droplets were formed and contributed to larger surface area.Through a larger surface area provided for mass transfer,a good stability for the emulsion can be achieved.This result agreed with the finding by Gasseret al.[28]who attempted the extraction of Co(II).

The swelling percentage obtained for 1000 r·min-1emulsification speed was 3%and decreased to 0%swelling for1100 r·min-1.An increment of swelling percentage is shown from 0%to 9%for 1100 r·min-1and 1300 r·min-1emulsification speed respectively.There was no swelling occurred for the emulsification speed of 1400 r·min-1and 1500 r·min-1.This is due to the trade-off between swelling and breakage in the process.The result showed that the emulsification speed of 1300 r·min-1obtained the highest extraction efficiency and the highest swelling percentage.This result was contradicted with the theoretical statement by Yan and Pal[29]that emulsion swelling caused a decrease in the extraction efficiency due to the diluted internal phase concentration which reduces the solute extraction driving force.Since Wan and Zhang[30]stated that emulsion swelling of 10%is considered manageable,it means that the swelling effect at 1300 r·min-1less significantly influences the extraction efficiency.Therefore 1300 r·min-1of emulsification speed was chosen to proceed for the following parameters.

3.4.2.Effect of emulsification time in W/O preparation

Fig.3.Mechanism of mass transfer of phenol through supported liquid membrane using vegetable oils[9].

The effect of emulsification time of W/O emulsion on phenol removal was investigated in the range of 1 to 13 min and the result is shown in Fig.5.Less than 3 min of the emulsification time,the emulsion tends to immediately break during the extraction process.It is supported by the statement of Gasseret al.[28]that the insufficient of emulsification time will lead to higher breakage of the emulsion.Then,the swelling will occur and percentage of swelling increased as the emulsification time increased until an optimum condition achieved.The swelling phenomena will lower the extraction efficiency as reported by Yan and Pal[29]and Sulaimanet al.[31].

Fig.4.Effect of emulsification speed on emulsion stability and extraction performance.(Experimental conditions:[NaOH]=0.1 mol·L-1;[phenol]=300 × 10-6;TR=1:3;membrane:internal phase=3:1;emulsification time=5 min;agitation speed=500 r·min-1;extraction time=5 min).

Fig.5.Effect of emulsification time on emulsion stability and extraction performance.(Experimental conditions:[NaOH]=0.1 mol·L-1;[phenol]=300 × 10-6;TR=1:3;membrane:internal phase=3:1;emulsification speed=1300 r·min-1;agitation speed=500 r·min-1;extraction time=5 min).

However,the results showed that around 76%–80%of phenol was extracted during the process.Further increased the emulsifying time resulted with no significant effect on the extraction performance.This indicates that 70%:30%compositions of palm oil to kerosene with its viscosity of 13.722 × 10-3Pa·s provide more stable emulsion;instead of fully palm oil based.This indicates that the time required for emulsification should be shorter than fully palm oil.The results were verified that the emulsion breakage at minutes 11 to 13 of emulsification time as shown in Fig.5.

The breakage at 11 and 13 min of emulsification time might be due to the ruptured membrane.The increase in the volume of the internal phase means that the emulsion was swell and might lead to the leakage of the globules and sharp increase in emulsion viscosity[29].Also,it was observed that the longer the emulsification time,the more viscous the emulsion,where the highly viscous emulsion will lead to the lower rate of extraction[25].

The emulsification time of 5 min with 3%swelling was chosen,although 7 min provided less swelling.It is because shorter time of emulsification is more preferred.Therefore,5 min was chosen for the emulsification time to proceed to the following parameters.

3.4.3.Effect of agitation speed in ELM extraction

Fig.6.Effect of agitation speed on ELM stability and extraction performance(Experimental conditions:[NaOH]=0.1 mol·L-1;[phenol]=300 × 10-6;TR=1:3;membrane:internal phase =3:1;emulsification time = 5 min;emulsification speed=1300 r·min-1;extraction time=5 min).

The effect of agitation speed on the phenol extraction and membrane stability was investigated and the results are shown in Fig.6.At 400 r·min-1of agitation speed,the formation of mayonnaise-like solution was formed and through the observation,the globules were formed initially when the emulsion was added slowly into the continuous phase.However,there was no fully dispersion of the emulsion in the aqueous phase and the globules started to join and swell rapidly at about 2 min of contact time,when the mayonnaise-like solution was formed.The globules started to join rapidly might be due to the attractive force between the globules was stronger than the repulsive force and caused clustering of globules that lead to coalescence[27].

By increasing the agitation speed to 500 r·min-1,the globules were formed perfectly and were fully dispersed in the aqueous phase.It achieved the highest percentage of extraction(65%)as shown in Fig.6.However,it shows a decreasing trend of extraction from 65%to 60%at 800 r·min-1.The increasing of agitation speed enhanced the dispersion and the formation of smaller size of emulsion globules.This will increase the surface area for the mass transfer and thus increased the extraction efficiency.However,the continuation of increasing agitation speed,will lead to unstable globules and leakage of internal phase into the external phase due to the membrane rupture[15,20,24,32,33].It can be observed from the graph that the breakage of emulsion occurred at 600 r·min-1and 800 r·min-1of the agitation speed.The unstable emulsion globules at higher agitation speed led to the decrease in extraction efficiency.Therefore,the agitation speed of 500 r·min-1was chosen to proceed to the following parameters as it gave the highest phenol removal percentage and there was no breakage occurred.

3.4.4.Effect of surfactant concentration in ELM extraction

Surfactant plays the most important role in maintaining emulsion stability and permeation rate of phenol.Fig.7 presents the percentages of phenol extraction and emulsion swelling at various surfactant concentrations.The concentrations were varied from 1%to 5%.At 1%of Span 80,the phenol extraction efficiency was lower.This is due to the formation of larger emulsion droplet that gave lower mass transfer area.Furthermore,it was observed that the phenol extraction had increased from 46%to 99%when the surfactant concentration was increased from 1%to 5%respectively.When the Span 80 concentration was increased,the surface tension of the membrane phase will decrease,resulting in the formation of smaller emulsion droplets.Therefore,the contact area and mass trans fer area will increase between the donor and the internal phase,hence increasing the extraction efficiency.This finding seems to be similar with that of Praiprukeet al.[34]who reported that significant increases in the extraction rate due to a low surface tension of the membrane phase,resulting in small-sized emulsion droplets,allow a faster mass transfer for phenol extraction.However,further addition of Span 80 up to 7%,the mayonnaise-like ELM was observed.This increment of surfactant concentration can increase the swelling possibility due to the large osmotic pressure difference and mass transfer resistance,and higher emulsion viscosity when the experiment is carried out.

Fig.7.Effect of surfactant concentration on ELM stability and extraction performance(Experimental conditions:[NaOH]=0.1 mol·L-1;[phenol]=300 × 10-6;TR=1:3;membrane:internal phase=3:1;emulsification time=5 min;emulsification speed=1300 r·min-1;agitation speed=500 r·min-1;extraction time=5 min).

At1%Span80,the emulsion breakage was observe dat20%as shown in Fig.7.This is due to the insufficient Span 80 concentration,which leads to the formation of large-size emulsion droplets.Hence,it will cause emulsion breakages.As the surfactant increased to 3%,there was no breakage and swelling observed.This indicates that 3%of Span 80 was enough to give a stable emulsion.Further increases of Span 80 concentration up to5% tend to increase the emulsion swelling.It is indicated that beyond 3%of Span 80,all additional surfactants tend to form aggregates,and they existed as reverse micelles that transport water from the external phase into the internal phase.Present findings are consistent with the previous research that found that water transport into the emulsion globules is offered by the combination of surfactants and water molecules and there exists an optimum value[35].Hence,the amount of surfactant in the membrane phase must be minimal and enough to stabilize the emulsion.Therefore,based on the extraction performance and emulsion stability,3%of Span 80 was sufficient to be used in the next parameter.

3.5.Emulsion liquid membrane extraction performance

3.5.1.Effect of feed pH

Fig.8 demonstrates the effect of external pH towards the extraction of phenol and membrane swelling percentage.It can be inferred from Fig.8 that the increased pH value from pH 2 to 10 did not give any significant effect towards the performance of phenol extraction and swelling.This indicates that the phenol extraction performance in kerosene and palm oil was not affected by the pH change.This proved that the extraction of phenol was through the diffusion mechanism as discussed in Section 3.3.Therefore,pH 8,the normal pH of phenol solution,was selected as an optimum pH value for further ELM experiments.

Fig.8.Effect of pH feed phase on ELM stability and extraction performance(Experimental conditions:[NaOH]=0.1 mol·L-1;[phenol]=300 × 10-6;TR=1:3;membrane:internal phase=3:1;[Span 80]=3%;emulsification time=5 min;emulsification speed=1300 r·min-1;agitation speed=500 r·min-1;extraction time=5 min).

3.5.2.Effect of contact/extraction time

Fig.9.Effect of contact time on ELM stability and extraction performance(Experimental conditions:[NaOH]=0.1 mol·L-1;[phenol]=300 × 10-6;TR=1:3;membrane:internal phase=3:1;[Span 80]=3%;emulsification time=5 min;emulsification speed=1300 r·min-1;agitation speed=500 r·min-1).

The effect of contact time on the phenol removal and stability at emulsion is shown in Fig.9.It can be seen that the phenol removal percentages were not significantly changed.The phenol removal was 92%for the first 4 min,5 min,9 min and 11 min.Silvaet al.[36]stated that the seconditions could be well clarified by Fick's second law of diffusion,where there will be a final equilibrium between the solute in the feed phase and in the extraction solvent.Therefore,the excessive contact time was no longer convenient to extract more phenolic compound.However,the swelling percentages were decreased for the first 5 min from 34%to 15%.This is because by increasing the contact time,the globule size became smaller;hence it will increase the stability of emulsion.Further increases up to 9 min,the swelling percentage had gradually increased and the breakage occurred at the 11th minute with 2%at emulsion breakage.This is due to the fact that by extending the extraction time,the emulsion promoted more entrainment of water into the internal phase of emulsion.Kulkarniet al.and Othman et al.[37,38]also agreed that longer extraction times will cause the membrane to swell due to more water being transferred into the internal phase and break the emulsion,where the solute was transferred from the internal to external phase.Prolonged contact would lead to a decrease in the phenolic content of crude extract as oxidation of phenolic compounds can possibly occur by prolonging the exposure to environment factors,such as light and oxygen[39–41].

3.5.3.Effect of stripping agent concentration

The effect of stripping agent concentration on phenol removal was determined as shown in Fig.10.The pH for feed phase was around 8,which was the lowest alkaline level that is close to the neutral phase.The phenol removal increased from 90%to 99%with an increase of stripping concentration from0.01mol·L-1until0.1mol·L-1.However,the phenol removal percentage decreased to92%at0.5mol·L-1.The increase of NaOH concentration increased the ability of stripping,which delayed the accumulation of the complex in the membrane phase.This result was correlated with the observation reported by Goyalet al.(2011a)that the extraction efficiency was very little affected while changing the stripping agent concentration after the sufficient amount of 0.1 mol·L-1for the stripping process.Moreover,there was a declining trend in the removal and swelling percentage when the NaOH concentration was increased beyond 0.1 mol·L-1.Excess NaOH could be responsible for hydrolyzing the number of surfactant molecules,which would transport more water into the internal phase[42].Thus,0.1 mol·L-1of NaOH was chosen.

Fig.10.Effect of stripping agent concentration on ELM stability and extraction performance(Experimental conditions:[phenol]=300×106;TR=1:3;membrane:internal phase=3:1;[Span 80]=3%;emulsification time=5 min;emulsification speed=1300 r·min-1;agitation speed=500 r·min-1;extraction time=5 min).

3.5.4.Effect of treat ratio

The effect of treat ratio(emulsion:external)on the phenol removal and emulsion stability is shown in Fig.11.The results showed that the phenol removal percentage decreased from 88%to 83%at treat ratio 1:3 to 1:7,respectively and became constant at treat ratio 1:10,which was 83%phenol removal.Basically,as the volume of external solution decreased with the fixed amount of the emulsion in the feed solution,the number of available of globules and interfacial surface area per unit volume of the feed solution also increased.Thus,it enhanced the mass transfer of stripping agent from the external phase to internal phases[42,43].Nevertheless,the swelling percentages increased and became constant when the treat ratio was increased from 1:5 to 1:10.Since the swelling percentage from 1:5 to 1:10 was around 10%,it was acceptable and did not give significant effects on the emulsion stability in this process.

3.6.ELM recovery

Table 5 shows the results of parameters of ELM recovery study.The recovery performance(enrichment)of phenol in the internal phase was analyzed.It can be seen that the percentages of recovery increased with an increase in stripping agent concentration,but decreased as it went beyond 0.1 mol·L-1.The increase of stripping agent concentration beyond certain limits will cause a significant effect on water transport[44].Excess of NaOH could be responsible for hydrolyzing a number of surfactant molecules,which form the salt of fatty acids and the release of sorbitan molecules into the aqueous pools.Thus,the hydrolyzing surfactant will transport more water into the internal phase and dilute the concentrated internal phase,which induces a large amount of the osmotic pressure difference.Moreover,the percentage of recovery increased with an increase in treat ratio,which was from 6.06 enrichment at 1:3 treat ratio to 10.77 enrichment at 1:10.Practically,the low treat ratio with maximum enrichment is preferable with respect to the feed phase and cost effectiveness of the process as long as the percentage removal of phenol was high[30].Thus,the treat ratio of 1:10 is chosen as the optimum treat ratio.

Table 5Effect of stripping agent concentration and treat ratio

4.Conclusions

This study proved the possibility of using vegetable palm oil as a solvent in extraction phenol from liquid waste solution using emulsion liquid membrane process.The 70:30 ratio of palm oil to kerosene was used as a liquid membrane in the process it was stable enough for the ELM extraction process.At optimum conditions of 5 min and 1300 r·min-1of emulsification process,500 r·min-1of agitation speed,3%an80 concentration,pH of 8 external phases,5 min of contact time,0.1 mol·L-1of NaOH as stripping agent and1:10of treat ratio,the extraction performance was around 83%and the enrichment was around 11 times.The results showed that the vegetable palm oilbased ELM is very promising in removing of phenol from wastewater.

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