Em u lsion liquid mem brane for selective extraction of Bi(III)

Bah ram Mokhtari,Kobra Pourabdollah*

Faculty of Fundamental Physical and Chemical Engineering,Lomonosov Moscow State University,Moscow,Russia

Keyw ords:Em u lsion liquid mem brane Bism uth Surfactant Di(2-ethylhexyl)phosphoric

ABSTRACT Di(2-ethylhexyl)phosphoric acid was used as extractant of bism uth ions from nitrate medium by em ulsion liquid mem brane,with Triton X-100 as the biodegradable surfactant in n-pentanol bulk mem brane.The novelties andinnovative pointsof thiswork are the application ofem ulsion liquidm em brane for selective and efficientextraction of bism u th ions as w ell as the relevan t op tim ization procedures.The extraction of bism uth ions was evaluated by the yie ld of ex traction.The experimental parameters were evaluated and op tim ized,in cluding the ratio of di(2-ethy lhexyl)phosphoric acid mass concentration to Triton X-100(1.0%:0.5%),nature of diluen t(n-pen tanol),nature and concentration of stripping solu tion(su lfu ric acid,0.5 mol·L-1),stirring speed(1800 r·m in-1)and equilibrium time of extraction(20 min),initial feed solu tion of bism u th(350 mg·L-1)and the volum e ratio of internal stripping phase to mem brane phase(14).The experimental parameters of kinetic extraction reveal that the bism u th ions can be extracted by 100%.

1.Introduction

Bism uth had found several applications.It is usedin pharm aceutical industries such as bism u th nitrate for treating in testinal disorders,in preparation of cream s and hair dyes,in plastic manu factures,in syn thesisofmethanolasa catalyst,andin themetallurgicalp rocess for production of low melting alloys,as protective coatings for w ood patterns[1-3].Bism u th is usually obtained as a sub-p roduct in lead,copper,tin and go ld ores.In the industrial metallurgical process of these ores,leaching stageswith H2SO4,HCland HNO3are involved,and high ly acidic solutions with base metals and bism uth are obtained[4].In many industries,bism u th is not considered as a toxic metal.Otherwise,it is a curious metal and can be toxic in an unsuitable form[5,6].

Currently,the recovery of valuable metals is a challenge for both the scientific andindustrial comm unities.The technologies of metal extraction from their ores are pyrometallurgical or hyd rometallurgical processes.Solven t extraction technique is widely usedin the hydrometallurgical process.Other techniques such as precipitation,ion exchange,adsorption and electrochemicalrecovery arealso used[7-9],but theym ay be ineffective because they sometimes fail to meet regu lation levels for technical,econom ical and/or environm ental reasons[10,11].

Em u lsion liquid mem brane(ELM)invented by Li[12-16]has been widely used forwater treatm ent[17,18]and for rem ovaloftoxicorganic andinorganic species from aqueousstream sandindustrialwastewaters[19].ELM constitutes an attractive technique com pared to the comm on liquid-liquid extraction usedin the hyd rometallurgical process[20,21],which im proves the kinetics and selectivity of species to be removed and decreases the necessary volum e ratio of organic phase to aqueous feed solution,which is goodin an environmental aspect[18].Further,it is characterized by sim plicity and high efficiency.Besides these advan tages,ELM processes allow very high mass transfer rates due to a large surface area within the em u lsion globu les andinternal droplets[22,23].

In the ELM technique,extraction and striping are carried out in one single step[22].ELM isa three-phase dispersion system,where a primary em u lsion is dispersedin a continuous effluent phase,which is the phase to be treated[24].The con tinuous phase of em u lsion drops consists of low viscosity organic diluents,a surfactant to stabilize the em u lsion and sometimes a carrier.This in term ediate phase separates the external and em u lsifiedinternal phases and behaves as a liquid membrane,allowing the perm eation of so lu tes.For the separation of low concentration metals,the carrier is a solub le organic extractan t,which selectively com bines with the solutes that diffuse from the bu lk of the effluen t aqueous phase to the outer in terface.The metal-carrier complex perm eates through the mem branes from the outer to the inner in terface.At the inner in terface,the com plex decom poses by the reversal of the equilibrium reaction,the metal ion is liberatedin to the internal phase,and the regenerated carrier goes back into the membrane phase[25,26].The comm ercial ex tractants are most frequen tly[27]neu tral or acidic organophosphorus com pounds such as di(2-ethylhexyl)phosphoric acid(D2EHPA),which can be usedin several cycles of the extraction process du ring several months without risk of decom position.In the solvent extraction processes,D2EHPA interacts with metal ions by cation exchange toform a metal com plex.Until 1982 there was only one patent dealing with the extraction of Bi(III)[28].

Bi(III)was extracted with D2EHPA fixed on an Am berlite XAD-1180 solid supportby theextractant im pregnated resin technique.The increase ofimm ersion aqueousvolum e decreased the sorption ofBi(III).Moreover,the an tagonistic effect on the cation sorption was observed by adding sodium ch loride[29].The differences between this work and that reported by Belkhouche and Didi[29]are the application of em ulsion liquid mem brane for selective and efficient extraction of bism uth ions as w ell as the relevant op tim ization procedures.

Determination of trace am ounts of Bi(III)has been developed using cloud poin t extraction follow ed by flam e atom ic absorption spectrometry.This method has been applied to several matrices such as water,food and pharm aceutical sam ples[30].Bu lgariu and Bu lgariu[31]studied the Bi(III)extraction behavior in an aqueous po lyethylene glyco l(PEG)-(NH4)2SO4two-phase system using som e inorganic anions as ex tractants.The distribution coefficients of Bi(III)were calculated as a function of inorganic extractant concentration.

The objective of this work on Bi(III)is to study its kinetic extraction by the ELM technique from nitrate medium based on the op tim ization of experim ental parameters,using D2EHPA as carrier and Triton X-100 as biodegradab le surfactant.

2.Experimental

2.1.Reagents

D2EHPA con tain ing 40%m ono(2-ethy lhexy l)phospho ric acid was pu rchased from Fluka.The biodegradab le non-ionic surfactan t,isooctylphenoxypo lyethoxyethano l(Triton X-100)with the critical micellar concentration of3.0 × 10-4mol·L-1and the cloud poin t temperature of 68°C at 1%(by mass)in water,was supplied by Sigm a-Ald rich.Dich loromethane was pu rchased from M erck.Su lfu ric acid(98%)and ch loroform were provided by Pro labo.Kerosene was pu rchased from Riedel de Haen.Hep tane was supplied by Fluka and pen tano l was from Reachim.The stock of pure basic bism u th nitrate(BiONO3)was supplied by Reachim and potassium iodide was obtained from Gerhard Buchm ann.

2.2.Apparatus

A Haier mechanical agitator with standard platform was usedin ex traction experiments,and pH measurements were taken with a Consort C831 pH-meter using a com bined electrode.The w eighing was made with an electronic analytical balance,type Kern ABS.A mixer of type Vortex at 2500 r·m in-1was used for the em u lsion form ation.A UV-visib le absorption spectrophotometer,type Lam bda 800 Perkin Elm er,was used for Bi(III)analysis.

2.3.Preparation of em u lsion

An organic solu tion(7.5 ml)was prepared by mixing Triton X-100 surfactant and D2EHPA extractan t in pen tanol(as diluent).Then the acidic aqueous solution was added dropwise to the organic solution under severe agitation using the Vortex mixer.

2.4.Extraction and pre-concentration of Bi(III)

The obtained em u lsion was mixed with 62.5 ml feed solu tion(containing bism uth ions)and stirred by a Haier mechanical agitator for the reaction time.At the end of extraction reaction,10 μl of the feed solution was analyzed to determine the quan tity ofbism uth rem aining in the feed solution after extraction by UV-visibleatom icabsorption spectrometry at 460 nm with the iodide method[32-34].The analytical response was given by the yield ofextraction ofBi(III).Allexperim entswere perform ed at the temperature of 20°C.All aqueous solutions were prepared using dem ineralized water.Each extraction experim ent was repeated three times and the average value was usedin calculations.

3.Results and Discussion

3.1.Effect of extractant/surfactant ratio on Bi(III)extraction

The influence of the ratio D2EHPA/Triton X-100 on Bi(III)extraction was studied.The concentration of extractant was fixed and the concentration of surfactant was varied(see Fig.1).The liquid mem brane was based on dich loromethane as diluent.Fig.1 show s that the ex traction yield of Bi(III)increases with the ratio D2EHPA/Triton X-100 in both cases,reaching about 80%at a ratio of 2.

Previous study has show n that higher surfactan t concentration increases the viscosity of mem brane solution,m aking the chrom ium(VI)extraction w orse[25,35],whileotherwork show s that the am ountofsurfactant in the mem brane must be optim al to stabilize the em ulsion[36].

3.2.Influence of diluent on Bi(III)extraction

Polarand apolar diluen tswere testedin the preparation of theem u lsion liquid mem b rane,nam ely dich loromethane,n-hep tane,ch loroform,kerosene,and pen tano l.Fig.2 show s the ex traction yield of Bi(III)by ELM as a function of the nature of diluen t.Extraction yields of Bi(III)from nitrate medium are higher by using apolar diluents in the excep tion of ch loroform,with the maxim um extraction of Bi(III)reaching 93%using pen tano lasdiluent.These results can be in terp reted by the effect of solven t po larity and viscosity.D2EHPA ex tractant exists asm onom er form with high diffusivity,from which the com plexation of metal is higher,so the extraction will be more effective.Pentanol was chosen as diluen t for the con tinuation of our work.

Fig.1.Effect of ratio D2EHPA/Triton X-100 on the extraction of Bi(III).Stripping solution:4.5 ml of H2SO4(0.5 mol·L-1);stirring:210 r·m in-1;[Bi3+]=500 mg·L-1(pH 1).

Fig.2.Effect of diluent on the extraction yield of Bi(III).Stripping solution:4.5 ml of H2SO4(0.5 mol·L-1);stirring:210 r·m in-1;[Bi3+]=500 mg·L-1(pH 1);ratio of D2EHPA/Triton X-100:2.

3.3.Effect of stirring speed on the em u lsion form ation

Preparation of em u lsion plays an im portan t role in the extraction of Bi(III)by the em u lsion liquid mem brane technique.The stirring speedin the em u lsion form ation affects the ELM extraction of Bi(III)(see Fig.3).The results show that increasing the stirring speed of the bu lk mem brane phase increases the efficiency of ELM ex traction of Bi(III)from nitratem edium,with the yield ofELM extraction ofBi(III)reaching 93%at1800 r·m in-1.An efficien tem u lsification givesagood dispersion of the internal phase in the mem brane phase.The droplets of the stripping phase becom e sm aller and will take much more time to coalesce.The mean size of droplets was measured by a Cou lter Counter instrum en t.At a stirring rate of 1800 r·m in-1the mean size of droplets was220μm and at2500 r·m in-1itwas202μm.This isconducive to the good stability of the em ulsion.Sm aller droplets of the internal phase at higher agitation in tensity create a larger surface area for ex traction,leading to higher recovery of ions[13,37].

Fig.3.Effect of stirring speed on the ex traction.Stripping solution:4.5 ml of H2SO4(0.5 mo l·L-1);stirring:210 r·m in-1;[Bi3+]=500m g·L-1(pH 1);ratio ofD2EHPA/Triton X-100:2.

3.4.Effect of stirring time on the em u lsion formation

W hen the stirring speed affects the form ation of droplets of the stripping phase in thep reparation ofem u lsion,the stirring timegoverns the overall maturation[21,22].Fig.4 show s the effect of em ulsification time on ELM ex traction of Bi(III).The yield of extraction of Bi(III)increaseswith em u lsification time,achieving93%with 20m in ofstirring time.Then the yield decreases,due to the instability of em u lsion.

3.5.Effect of stripping solution on the extraction

The stability of em u lsion is a key factor in the extraction by ELM[12,25].The nature of stripping solu tion depends on the physicochemical properties of D2EHPA and the mode of extraction.Fig.5 show s the influence of the nature of the stripping phase on ELM extraction of Bi(III)with various acid solutions such as nitric acid,su lfuric acid and hyd roch loride acid.The bism u th ions are pre-concentrated better with su lfuric acid solution,with the yield of ELM extraction of Bi(III)reaching 93%.

Fig.4.Bi(III)extraction versus stirring time of em ulsion.Stripping solution:4.5 ml H2SO4(0.5 mol·L-1);stirring:210 r·m in-1;[Bi3+]=500 mg·L-1(pH 1);ratio of D2EHPA/Triton X-100:2.

Fig.5.Effect of stripping solution on the Bi(III)ex traction.Stripping solution:4.5 ml(0.5 mol·L-1);stirring:210 r·m in-1,[Bi3+]=500 mg·L-1(pH 1);ratio of D2EHPA/Triton X-100:2.

3.6.Effect of stripping solution concentration on Bi(III)extraction

The capacity to trap and concentrate the solute in the stripping solution was determined by the concentration of aqueous so lution acidin the internal phase.The influence of concentration of the internal phase on Bi(III)extraction by ELM is show n in Fig.6.with the increase of su lfuric acid concentration,the yield of Bi(III)extraction increases,reach ing 93%at 0.5 mo l·L-1.Then the ex traction yield drops and becom es constan t at higher concentrations of acid.This is due to the increase in the d riving force for diffusion through the ELM owing to an in crease of meta l com plex con centration at the in terface of the mem brane-stripping solu tion[35].

3.7.Effect of contact time and stirring speed on extraction

Stirring speedinfluences the mass transfer in the extraction.The effect of stirring speed on Bi(III)ex traction by ELM is show n in Fig.7.At a given stirring speed,the ex traction yield of Bi(III)increases with time.In general,it becom es constan t w hen the time is longer than 30m in,indicating anegligib leeffectoffluidflow on Bi(III)m ass transfer resistance.Thus w e obtained a maxim um value of 93%of the extraction yield at 210 r·m in-1after 30 min of stirring.Further experim ents were conducted with a stirring speed of 210 r·m in-1in order to save energy and a con tact time of 30 min to avoid the concentration effects.

Fig.6.Effect of stripping solution concentration on Bi(III)extraction.Stripping solution:4.5m lH2SO4;stirring:210 r·m in-1;[Bi3+]=500m g·L-1(pH 1);ratio ofD2EHPA/Triton X-100:2.

Fig.7.Effect of stirring speed on the extraction of Bi(III).Stripping solution:4.5 ml of H2SO4(0.5 mol·L-1);[Bi3+]=500 mg·L-1(pH 1);ratio of D2EHPA/Triton X-100:2.

3.8.Effect of feed concentration on extraction

Fig.8 show s the influence of initial concentration of Bi(III)on ELM extraction with initial pH of 1.At a given initial concentration of Bi(III),the extraction yield of Bi(III)increased until 30 min of extraction time then it becam e constant.As the feed concentration of Bi(III)increased from 350 to 650 mg·L-1,the yield of Bi(III)ex traction decreased.It reached 97%at 350 mg·L-1of an initial concentration of Bi(III).Thus at lowinitial Bi(III)concentration,m ost of the solutes in the peripheral region of the em u lsion globu le were stripped by the internal aqueous phase droplets[13].How ever,at an initial concentration of 650 mg·L-1,the extraction yield of Bi(III)was only 69%,due to the saturation of internaldropletsof the stripping em u lsion.On the other hand,them ass transfer resistance in the em u lsion globule becom es im portant at higher concentration of Bi(III)com plex,decreasing the rate of stripping and the extraction yield of Bi(III)[35].

3.9.Effect of ratio VS/VM

Fig.8.Effect of feed concentration on the Bi(III)extraction.Stripping solution:4.5 ml of H2SO4(0.5 mg·L-1);stirring:210 r·m in-1;ratio of D2EHPA/Triton X-100:2.

Fig.9.Effectof ratio V S/V M on the Bi(III)ex traction.Stripping solution:4.5m lofH2SO4(0.5 mol·L-1);stirring:210 r·m in-1;[Bi3+]=350 mg·L-1(pH 1);ratio of D2EHPA/Triton X-100:2.

In the separation and pre-concentration processof metalions by the ELM technique,the volum e ratio of stripping solution(VS)and membrane so lution(VM)plays a very im portan t role[20,35].Fig.9 show s the influence of ratio VS/VMon the extraction yield ofBi(III)whilem aintaining the mem brane vo lum e(VM)constan t.The extraction of Bi(III)increases with ratio VS/VM,reaching 100%at a ratio of 14.The increase in the volum e of the stripping phase involvesan increase in them edium receiver,increasing the incarceration ofBi(III)ionsin the internalphase.

3.10.The significance test

The experimental parameters such as ex tractan t/surfactan t ratio,stirring speed,stirring time,stripping solution,stripping solution concentration,feed concentration,and con tact timeareevaluated.It reveals which parameters have more sign ifican t influence on the selective extraction of bism u th from nitrate medium.The experiments show that num erous factors influence the bism uth(III)ex traction.Based upon the M initab softw are,the rank of the above factors is examined as presentedin Tab le 1.It is obvious that“stirring speed”has the most significan t im pact and ratios“extractan t/surfactan t”and “VS/VM”have less effect on the bism u th(III)extraction.

4.Conclusions

Theextraction and recovery ofBi(III)ions from nitratem edium were carried out by the em u lsion liquid mem brane technique,using di-(2-ethylhexyl)phosphoric acid as extractant and Triton X-100 as biodegradab le comm ercial surfactan t.The study show s that Bi(III)ions can be ex tracted by 100%.The liquid mem brane is constitu ted from 1%D2EHPA with Triton X-100(0.5%)in the n-pen tanol bu lk mem brane.The em u lsion was perform ed with su lfuric acid solution(0.5 mol·L-1)under 1800 r·m in-1of stirring for 20 min.Better extraction of Bi(III)ions from nitrate medium was obtained with 350 mg·L-1feed concentration and 30 min of equilibrium time under 210 r·m in-1of stirring,and VS/VM=14.It is revealed that“stirring speed”has the most significan t im pact,and ratios“extractan t/surfactan t”and “VS/VM”have less effect on the bism uth(III)extraction.

Tab le 1 The rank of different factors on the bism u th(III)extraction