Studies on the simultaneous removal of dissolved DBP and TBP as well as uranyl ions

Studies on the simultaneous removal of dissolved DBP and TBP as well as uranyl ions from aqueous solutions by using Micellar-Enhanced Ultra filtration TechniqueS.K.Misra ⁎,A.K.Mahatele,S.C.Tripathi,A.DakshinamoorthyFuel Reprocessing Division,Bhabha Atomic Research Centre,Trombay,Mumbai-400094,Indiaa b s t r a c ta r t i c l e i n f o Article history:Received 26March 2008Received in revised form 16July 2008Accepted 28July 2008Available online 5August 2008Keywords:Di-butyl phosphate Tri-butyl phosphate UraniumMicellar-Enhanced Ultra filtration and Sodium Dodecyl SulphateStudies were carried out for the simultaneous removal of dissolved organic namely di-butyl phosphate (DBP)and tri-butyl phosphate (TBP)as well as uranyl ions from aqueous solutions by using Micellar-Enhanced Ultra filtration Technique (MEUF).Ultra filtration (UF),a comparatively low pressure driven membrane separation technique,offers great potential for the separation/recovery/removal of dissolved organic and metal contaminants at low concentration and from large volume of aqueous waste solutions.Throughout the study Sodium Dodecyl Sulphate (SDS),an anionic surfactant was used.The 47mm diameter Millipore ultra filtration membranes of 3000,5000and 10,000MWCO pore sizes were used and the pressure of the ultra filtration cell was kept at 1.7bar for all the experiments.To optimise the parameters,the effects of membrane pore size,DBP concentration in feed and feed acidity on the recovery/separation of DBP were studied.More than 90%removal of DBP from aqueous solution was achieved while keeping the feed acidity at 0.5M HNO 3,the DBP concentration in feed solution was kept at 720ppm and by using 3000MWCO pore size membrane.The interferences from other dissolved organic such as TBP and metal ions like uranium were also studied.The results showed that the technique can be effectively employed for the simultaneous removal of dissolved organic (DBP and TBP)and metal ions (uranium)from the aqueous ef fluent streams.©2008Elsevier B.V.All rights reserved.1.IntroductionPUREX and THOREX processes aim at complete recovery of uranium,plutonium and thorium from fission products present in the spent nuclear fuel of the reactor.In these processes,a mixture of 5%to 40%TBP-n -dodecane (hydrocarbon diluent)is used for coextraction of uranium,thorium and plutonium leaving behind a number of radioactive fission products in the aqueous raf finate.Tri-butyl phosphate (TBP),though very stable,is prone to hydrolysis or de-alkylation leading to the formation of di-butyl phosphate (DBP),monobutyl phosphate (MBP)and phosphoric acid (Blake,1968;Nowak,1973;Tripathi et al.,2001).TBP also reacts with nitric acid to give DBP and butyl nitrate.Presence of these organic,in aqueous streams,leads to the formation of heavy organic due to the concentration by evaporation in the evaporator leading to red oil formation which is explosive in nature.Hence the process waste stream that contains the dissolved organic like TBP and DBP along with minor quantities of metal ions like uranium and plutonium should be removed from these aqueous waste stream prior to their disposal.For the simultaneous recovery of dissolved organic and metal ions,no single technique or procedure is available.Membrane filtration has shown promise as a separation technique which is competitive to the conventional separation techniques such as distilla-tion,adsorption,absorption,extraction etc.The major component in membrane separation processes is the membrane.Membrane is a thin barrier between two bulk phases that permits transport of components of interest retaining others.A mass transport across the membrane requires a driving force.Ultra filtration (UF),a comparatively low pressure driven membrane separation technique,offers great potential for the separation/recovery/removal of metal contaminants at low concentration and large volume of aqueous waste solutions (Scamehorn et al.,1989;Chaufer et al.,1988;Geckeler et al.,1996).The separation is based on the size fractionation,using UF membranes as sieve and the driving force is transmembrane pressure gradient.Because of low pore size,the technique poses the problem of fouling of membrane.To overcome this problem micelle enhanced ultra filtration technique is a better alternative (Klepac et al.,1991;Volchek et al.,1993;Akita et al.,1997).Micellar-Enhanced Ultra filtration involves the addition of a surfactant above its critical micellar concentration (CMC)in order to entrap the ionic solute in solution (Gzara et al.,2001;Tangvijitsri et al.,2002;Kryvoruchko et al.,2002).The increased hydrodynamic size of the solutes enables their rejection by polymeric ultra filtration membranes.The surfactant forms micelles,(surfactant aggregates that contain about 50–100surfactant molecules),above its CMC.The micelles are having cationic or anionic heads which bind the metal anions/cations whereas the tails of the micelles will absorb the dissolved organic or non-polar organic matter present in the aqueous solutions.This solution is then passed through an ultra filtrationHydrometallurgy 96(2009)47–51⁎Corresponding author.Tel.:+912225591203;fax:+912225505151.E-mail address:skmisra@.in (S.K.Misra).0304-386X/$–see front matter ©2008Elsevier B.V.All rights reserved.doi:10.1016/j.hydromet.2008.07.013Contents lists available at ScienceDirectHydrometallurgyj o u r n a l h o me pa g e :w w w.e l s ev i e r.c o m/l o c a t e /hyd ro m e tmembrane with pore sizes small enough to block the passage of micelles.This technique has several advantages over other membrane process due to their low operation cost and simplicity.In this study,simultaneous removal of dissolved TBP,DBP and uranium from the aqueous medium under different experimental conditions was investigated by Micellar-Enhanced Ultrafiltration using Sodium Dodecyl Sulphate(SDS),as an anionic surfactant and the results are reported.2.Experimental2.1.MaterialsAll the reagents used in this study were of analytical grade.SDS,an anionic surfactant,was obtained from M/s Lancaster Synthetic Ltd., England.TBP and DBP were procured from MERCK—Schuchardt, Germany and Fluka AG,Switzerland respectively.Uranium solution of different concentrations were prepared by dissolving weighed amounts of UO2(NO3)2.6H2O(B.D.H.)in nitric acid.UF membranes (MWCO of3000to10,000and47mm in diameter)supplied by M/s Millipore were used.The uranyl concentrations in the feed and permeate were determined by spectrophotometric method described elsewhere(Das et al.,1991).All the experiments were done in duplicate and the material balance in these studies was found to be within±5%.2.2.Ultrafiltration cellExperiments were performed at room temperature using an indigenously developed cell that was stirred constantly by a magnetic motor.The cell capacity was80ml and having50ml processed capacity.The cell had the membrane of diameter47mm.The liquid phase in the cell was stirred at a speed of approximately200rpm under controlled pressure.2.3.Analytical methodsThe Shimadzu GC-9A,gas chromatograph using thermal conduc-tivity detector was used for the measurement of TBP and DBP concentration in the feed and permeate.10%XE-60column (1.5m×0.32cm was used with heating rate as specified below along with other operating parameters.Initial column temperature of170°C was maintained for1min.The heating rate of10°C/min was given to the column till thefinal temperature of column was reached to230°C which was maintained for10min.Throughout the study,the injection port temperature and carrier gas(He)flow were maintained at240°C and40ml/min respectively.The concentration of DBP in30%TBP-n-dodecane mixture was estimated by esterifying the DBP with ether solution of diazomethane into volatile methyldibutyl phosphate(MDBP)prior to its gas chromatographic assay.The quantification of DBP was made by using a calibration plot(Tripathi et al.,1989).The diazomethane was prepared by treating p-tolylsulphonyl methylnitrosamide with alco-holic potassium hydroxide(Kibkey et al.,1968).The feed solutions(50ml)were prepared by dissolving the appropriate quantity of TBP and DBP.Acidity of the feed solution was adjusted using dilute HNO3/NaOH.Then the surfactant was added in the feed and the solution was stirred for30min followed byfiltration. The permeate solutions(filtrate)were then collected atregularFig.1.Simultaneous removal of dissolved organic(DBP and TBP)and divalent heavy metal cation(uranyl ion)from aqueous solution using Micellar-Enhanced Ultrafiltration(MEUF) and schematic diagram of anionic surfactant(SDS)micelle.48S.K.Misra et al./Hydrometallurgy96(2009)47–51intervals to calculate the rejection (Sancho et al.,2006)of uranium,TBP and DBP using the formula:R ¼1−C ½ P =C ½ F ÀÁÂ100where [C ]P and [C ]F denote the uranium/TBP/DBP concentration in the permeate and feed respectively.Commonly used pore models assume that the pores of the membranes are of the same size.If we consider the membrane pores as parallel capillaries,the permeate flux can be calculated from the following equation (Katarina et al.,2006)J ¼V =t ÃAwhere V is the volume passed through the membrane at time t and A is the area of the membrane.2.4.MechanismFig.1shows the schematic diagram of MEUF technique which can be used to remove dissolved organic and metal ions simultaneously.SDS,an anionic surfactant,was added to the solution at a concentra-tion well above its critical micelle concentration so that most of the surfactant remains in micellar form.The dissolved organic solutes get solubilised within the tail of the micelles and metal cation forms a bond with the head of the micelles surface which is oppositely charged as shown in Fig.1.The solution containing the surfactant was then passed through the ultra filtration membrane of pore size small enough to reject or retain the micelles.Unsolubilised organic,unadsorbed metal cations and surfactant monomers will pass through the membrane.The following reaction can be proposed for the cationadsorption,3.Results and discussion3.1.Effect of acidity on the rejection of DBPTo find out the effect of feed acidity on the recovery of DBP by Micellar-Enhanced Ultra filtration Technique,using SDS as surfactant,acidity of the feed solution was varied from pH 8to1.0M HNO 3.In the feed,DBP concentration was kept at 2600ppm along with 100mM ofSDS in all the experiments.UF membranes,MWCO of 3000and 47mm in diameter supplied by M/s Millipore were used as filter.During all the experiments,the pressure of 1.7bar was applied to the cell.Table 1shows the result of the experiments carried out for pure DBP separation from solutions of different acidity.The results indicate that more than 90%DBP could be recovered from all the acidity of the solutions studied.The near quantitative recovery of DBP indicated that DBP in both the conditions i.e.acidic and alkaline is attached or solubilised in the tail of the SDS surfactant and SDS makes suf ficiently bigger micelles which could easily be retained by 3000MWCO membrane.3.2.Effect of surfactant concentration on rejection of DBPTo find out the optimum concentration of SDS required for the quantitative recovery of DBP,the concentration of SDS in the feed was varied from 8mM to 100mM while keeping the DBP concentration constant at 720ppm in the feed.The effect of varying the SDS concentration at constant retentate DBP concentration can be seen from Table 2.At less than its CMC value most of the surfactant should be present in monomers and practically no retention of the metal ions should take place.To see the effect of CMC on rejection experiments were performed at lower concentration (i.e.8mM SDS)that is below the CMC of SDS.At concentration lower than the CMC value,no micelles are present in the bulk solution and hence no rejection of the metal ions should take place.However,24%of the DBP rejection was observed,not 0%as expected.The probable cause for this rejection is the built up of the gel layer at the membrane/bulk solution interface due to the concentration polarisation effects.The concentration of SDS in this gel layer,may well exceed the CMC value whereas the rest of the bulk solution is free from micelles and thus was able to reject some of the DBP.The rejection of DBP was observed 55%while using 25mM SDS in feed.Though this concentration was far more than its CMC value but the micelles formed with 25mM SDS may not be suf ficiently bigger to be cut off by 3000MWCO membrane.When the SDS concentration was kept at 50mM or higher it was observed that more than 90%of DBP was retained.Hence for quantitative recovery of DBP,SDS concentration in the feed should be 50mM or higher.3.3.Effect of membrane pore size on percentage rejection and flux of DBP In order to see the effect of pore size,UF membranes of 3000MWCO,5000MWCO and 10,000MWCO were used.The feed solution was adjusted to 0.5M HNO 3and the concentration of SDS was fixed at 50mM.The concentration of DBP was kept around 710ppm in the feed.The results are summarised in Table 3.For the membranes of 3000and 5000MWCO the rejection of DBP was around 93%but the rejection of DBP decreased when membrane pore size was increased above 5000MWCO.Hence,it was clear from the experiments that the size of micelles formed in the system was such that they can be rejected by UF membranes having pore size 5000MWCO or less.Table 1Effect of feed acidity on the retention of DBP Feed acidity DBP in feed (ppm)DBP in raf finate (ppm)DBP rejection (%)pH 8261016693.6pH 2234014094.00.5M HNO 3298025291.51.0M HNO 3253022291.2Concentration of SDS in feed:100mM.Pore size of membrane:3000MWCO.Initial feed volume:50ml.Final volume of feed after filtration:10ml.Applied pressure:1.7bar.Table 2Effect of the surfactant (SDS)concentration in feed on the recovery of DBP SDS in feed (mM)DBP in feed (ppm)DBP in raf finate (ppm)DBP rejection (%)1007205093.1507205692.12572031755.6872054823.9Pore size of membrane:3000MWCO.Feed acidity:0.5M HNO 3.Initial feed volume:50ml.Final volume of feed after filtration:10ml.Applied pressure:1.7bar.49S.K.Misra et al./Hydrometallurgy 96(2009)47–51Table 3shows that the flux increased almost three times from 11.65dm 3m −2h −1to 36.13dm 3m −2h −1while increasing the pore size from 3000MWCO to 10,000MWCO.The flux of the system increases as the pore size of the membrane increases.It can be explained by the Hagen –Poiseuille equation (Mulder,1991)J ¼n πr 4ΔP ÀÁ=8ητΔx ðÞ(assuming a capillary pore shape)where J =volumetric flux,n =no.of pores/unit area,r =pore size,ΔP =transmembrane pressure,η=liquid viscosity,τ=tortuosity of membrane and Δx =thickness of the membrane.From the above equation,it is clear that the flux is proportional to the fourth power of nominal pore size of the membrane.Hence increasing the pore size of the membrane had an increasing effect on the flux.3.4.Effect of DBP concentration in the feedTable 4shows the effect of DBP concentration in feed on the rejection.The feed solution was adjusted to 0.5M (nitric acid)and throughout the experiments the concentration of SDS was kept constant at 50mM.During all the experiments,pore size of the membranes was fixed at 3000MWCO and the pressure of 1.7bar was applied to the cell.These experiments were carried out by varying DBP concentration in the feed from 720ppm to 3880ppm.The result of these experiments showed that the rejection of DBP decreased with its increasing concentration of DBP in the feed.It decreased from 93%at 720ppm to 85%at 3880ppm.The results indicated that at higher concentration of DBP beyond 3000ppm in feed,DBP molecules are not fully solubilised in the micelles.The decrease in the rejection may also be due to complete loading of SDS with DBP (as only 50mM of SDS was used for all these experiments).The flux also decreased as the concentration of DBP was increased in the feed from 11.66dm 3m −2h −1for 720ppm to 7.58dm 3m −2h −1for 3880ppm.The reduction in flux for higher concentration of DBP is due to the early gel layer formation near the membrane.To understand the formation of gel layer,when we plotted DBP flux versus concentration of DBP in feed,alinear plot was observed (Fig.2).By extrapolating this graph,we found that the flux became zero when the DBP concentration in feed reached to nearly 10,000ppm.It con firms the formation of gel layer near the membrane.3.5.Simultaneous removal of organic and metal solutesInitially DBP/SDS system was studied to optimise the parameters for the separation of dissolved DBP,from aqueous solutions.The optimum parameters (feed adjusted to 0.5M nitric acid,concentration of SDS:50mM,pore size of the membrane:3000MWCO)were applied for simultaneous separation of DBP,TBP and uranium from aqueous solution.Experiments were carried out to see the effect of other dissolved organic,TBP,on the recovery/rejection of DBP.The feed containing 300ppm of TBP along with the 720ppm of DBP was subjected to filtration.No signi ficant change in the recovery of DBP is observed whereas almost 99%TBP was rejected along with DBP as shown in Table 5.In order to investigate the performance of the system for the simultaneous removal of metal ion and organic solute,the experiments were carried out to remove the UO 2++ions,TBP and DBP simultaneously.The results are shown in Table 5.The removal of metal ions depends on the electrostatic attraction between the metal ions and the charged micelles whereas removal of non-ionic organic molecules depends on the solubilisation of its molecules in the interior of the micelles.Hence both of them can be removed simultaneously.To find out theTable 4Effect of DBP concentration in feed on its rejection and flux DBP in feed (ppm)DBP in raf finate (ppm)DBP rejection (%)Average flux (dm 3m −2h −1)7105093.111.66150010093.311.06298025291.59.43388064583.47.58Concentration of SDS in feed:50mM.Pore size of membrane:3000MWCO.Feed acidity:0.5M HNO 3.Initial feed volume:50ml.Final volume of feed after filtration:20ml.Applied pressure:1.7bar.Fig.2.Plot of DBP flux versus concentration of DBP in feed.Concentration of SDS in feed:50mM.Pore size of membrane:3000MWCO.Feed acidity:0.5M HNO 3.Initial feed volume:50ml.Final volume of feed after filtration:20ml.Applied pressure:1.7bar.Table 5Effect of uranium and TBP on the rejection of DBPUraniumTBP DBP Without uraniumWith uranium Without uranium With uranium Feed (ppm)–65.83003007201340Filtrate (ppm)–13.1b 5b 543.256.5Rejection (%)–80.1999994.095.8Concentration of SDS in feed:50mM.Pore size of membrane:3000MWCO.Feed acidity:0.5M HNO 3.Initial feed volume:50ml.Final volume of feed after filtration:10ml.Applied pressure:1.7bar.Table 3Effect of the membrane pore size on DBP rejection and flux Pore size ofmembranes (MWCO)DBP in feed (ppm)DBP in raf finate (ppm)DBP rejection (%)Average flux (dm 3m −2h −1)30007105093.111.6550007105192.812.9010,00071029458.636.13Concentration of SDS in feed:50mM.Feed acidity:0.5M HNO 3.Initial feed volume:50ml.Final volume of feed after filtration:20ml.Applied pressure:1.7bar.50S.K.Misra et al./Hydrometallurgy 96(2009)47–51interference of metal ion on the separation of dissolved TBP and DBP, feed solution containing300ppm of TBP and1340ppm of DBP along with67ppm of uranium was subjected tofiltration.The results indicate that no significant effect was observed on the rejection of DBP in the presence of UO2++and TBP.More than80%uranium was also rejected along with TBP and DBP.4.ConclusionMore than90%of DBP was rejected by using3000MWCO pore size UF membrane from aqueous solution while keeping the feed acidity at 0.5M HNO3and using50mM of SDS as the anionic surfactant and applying1.7bar pressure to the UF cell.No change in the rejection of DBP was found by varying the concentration of DBP in feed from 720ppm to3000ppm.Membrane pore diameter of5000MWCO or less should be used for more than90%rejection of DBP.TBP and uranium metal ion were found to have no interference for the recovery of DBP.Near quantitative amount of TBP and more than80%of uranyl ions were also recovered along with DBP.Thus the above studies clearly indicate that MEUF technique is feasible for the simultaneous separation of dissolved organic as well as metal ions from the aqueous streams.AcknowledgementThe authors wish to thank Mr.S.D.Misra,Director,Nuclear recycle group,Mr.P.K.Dey,Head Fuel reprocessing Division,Mr.S.K.Munshi Chief Superintendent Reprocessing Facilities Bhabha Atomic Research centre,Trombay,Mumbai for their keen interest and for their constant encouragement during the course of the work.The authors also wish to thank Dr.R.Kannan,Mr.S.D.Chaudhari and Mrs.M.Bindu for their kind help and support during the course of the study.ReferencesAkita,S.,Yang,L.,Takenchi,H.,1997.Micellar-enhanced ultrafiltration of gold(III)with nonionic surfactant.J.Membr.Sci.133,189.Blake Jr.,C.A.,1968.Solvent stability in nuclear fuel processing—Evaluation of the literature,calculation of radiation dose and effects of iodine and plutonium,ORNL-4212.Oak Ridge National Laboratory.Chaufer,B.,Deratami,A.,1988.Removal of metals by complexation ultrafiltration using water-soluble macromolecules:perspective of application to wastewater treatment.Nucl.Chem.Waste Manag.8,175.Das,S.K.,Rege,S.G.,Mukherjee,A.,Ramanujam,A.,Dhumwad,R.K.,1991.BARC Report, p.1139.Geckeler,K.E.,Volchek,K.,1996.Removal of hazardous substances from water using ultrafiltration in conjunction with soluble polymers.Environ.Sci.Technol.30,725. Gzara,L.,Dhahbi,M.,2001.Removal of chromate anions by micellar-enhanced ultrafiltration using cationic surfactants.Desalination137,241.Katarina,T.,Slavica,S.,2006.Removal of heavy metal ions from water by complexation-assisted ultrafiltration.Chemosphere64,486.Kibkey,A.H.,Davis Jr.,W.,AEC Report ORNL-TM-2289.Klepac,J.,Simmons,D.L.,Taylor,R.W.,Scamehorn,J.F.,Christian,S.D.,e of ligand-modified micellar-enhanced ultrafiltration in the selective removal of metal ions from water.Sep.Sci.Technol.26,165.Kryvoruchko,A.,Yurlova,A.L.,Kornilovich,B.,2002.Purification of water containing heavy metals by chelating-enhanced ultrafiltration.Desalination144,243. Mulder,M.H.V.,1991.Basic principles of membrane technology.Kluwer Academic Publishers,Dordrecht.Nowak,Z.,1973.Radiative and thermal variations on the dodecane-30%TBP-HNO3 system.Nukleonika18,447–454.Sancho,M.,Arnal,J.M.,Verdú,G.,Lora,J.,Villaescusa,J.I.,2006.Ultrafiltration and reverse osmosis performance in the treatment of radioimmunoassay liquid wastes.Desalination201,207–215.Scamehorn,J.F.,Christian,S.D.,Elington,R.T.,e of micellar-enhanced ultrafiltration to remove multivalent metal ions from aqueous streams,in surfactant-based separation processes.Surfactant Science Series,33.Dekker,New York,pp.29–51. Tangvijitsri,S.,Saiwan,C.,Soponvuttikul,C.,Scamehorn,J.F.,2002.Polyelectrolyte-enhanced ultrafiltration of chromate,sulfate,and nitrate.Sep.Sci.Technol.37,993. Tripathi,S.C.,Misra,S.K.,Ramanujam,A.,1989.Proceedings of ISAS1st National Symp.On Recent Trends in Chromatographic Techniques,Dec15–17,University of Madras. Tripathi,S.C.,Ramanujam,A.,Gupta,K.K.,Bindu,P.,2001.Studies on the identification of harmful radiolytic products of30%TBP-n-dodecane-HNO3by gas-chromatography, part II:formation of high molecular weight organophosphates.Sep.Sci.Technol.36, 2863–2883.Volchek,K.,Krentsel,E.,Zhilin,Y.u.,Shtereva,G.,Dytnersky,Y.u.,1993.Polymer binding/ ultrafiltration as a method for concentration and separation of metals.J.Membr.Sci.79,253.51S.K.Misra et al./Hydrometallurgy96(2009)47–51。