Water solubility enhancements of PAHs by sodium castor oil sulfonate microemulsions

5001-022Journal of Enmaionmental SciencesVdl.IS,No.5.pp.583- -589 ,2003C011-5689XArticle ID: 101-0742(2003)05-0583-07CLC number: XI31Document code: AWater solubility enhancements of PAHs by sodium castor oilsulfonate microemulsionsZHU Li-zhong', ZHAO Bao-wei, LI Zong-lai(Dpartment o Enionental Science. Zhejiang Univenity, Hanghou 310028. Chin. E-mal: lzhu@ mil.h2. zj.cn)microemulsions were evaluated. The apparent slubities of PAHs are linearly proportional to the concentations of sCOS micremulsion, andthe enhancement etent by scOS slutions is greater than that by ordiny sufecants on the basis of weight slbilizainin ratio( WSR). TheogK. values of Naph, Phen, and Py are 3.13, 4.44 and 5.01 repecivedy, which are sbout the same 斯the logKw_ volues. AI 5000 Lof scos cncentation, the aparent slubities are 8.80, 121, and 674 times as the itinsic soubiti for Naph, Phen, and Py. Theefects of inorganic ioon and temperaure on the slbiliation of solutes are also ivestigted. The soubilizain is improved wih a moderaleaditionoCe2+.Na' . NH: and the mitureofNa* , K* ,Ca*, Mg* and NHL: . WSR values are cnhanced by 22.0% for Napb.23.4% for Phen, and 24.6% for Py with lempernture increasing by 5C . The results idicated that SCOS microemulsions improve thepefornmance of the sufoctant enhanced rediation(SER) of soil, by inereasing slubilitis of organie plulants and reducing the level afsurfactant pollution and remediation expenses.Keywords: sodium castor oil slfonate; microemulsion; slubilizationi PAHsIntroductionContamination of groundwater and soils by organic pollutants has become an issue of increasingworldwide concerm, which arouses environmental scientists to explore different approaches for the clean-upof the contaminated soils and groundwater. The chemical , biological ，and chemical-biological remediationshave received considerable atention due to high eficiency in contrast to pump-and-treat systems andsurfactants play key roles in these techniques( Zhu，1999). Surfactant enhanced remediation( SER) hasbeen employed to remove nonaqueous phase liquids( NAPLs) fom conlaminated soils and aquifers( Abdul,1992; Pinto， 2000; Lagade, 2000; Taylor, 2001). The key points for SER are (1) to increase theaqueous solubility of NAPLs through the formation of surfactant micelles, and (2) to mobilize theentrapped NAPIs through decrease in NAPL- water interfacial tension. To conform with these requirements,different surface active agents, such as anionic surfactants ( Divarakanath, 1999)， nonionic surfactants(Dianne, 2000; Cowell, 2000; Zhou, 2000) ，cyclodextrins( Ko, 2000), humic acids( Van Stempoot,002) and biosurfactants( McCray， 2001)， were used to improve the solubilzation, mobilization andbioavailability of organic pollutants. However, microemulsion may be a promising approach for theremediation of soils and groundwater, because microemulsion is capable of solubilizing much more oil thanthat of micellar solution and arising ultralow interfacial tension between the aqueous phase and oil phase(Cui, 1999).Since J. H. Schulman introduced the term“microemulsion" in 1959 , microemulsions have atainedincreasing significance both in basic research and industry ( Schwuger, 1995). Recently, much attentionhas been paid to its application in SER( Fiorenza, 2000). Richard Martel et al . (Martel, 1998a; 1998b)optimized different surfactant-alcohol-solvent ( oil- in- water ) microemulsions to recover NAPLs from sandcolumns. Ouyang et al .(Ouyang, 1995; 1996) developed a surfactant- enhanced microemulsion( gasoline-in- water) approach for the remediation of NAPLs from contam中国煤化工high recoverywas obtained from these studies , the main drawback of this:MYHC N M H GconcentationFoundalion ite: The Nationl Exellent Young Seientint Fund d Chin(No. 20125719); The National Natuni Science Foundeion odf China(No20077025); * Corespnding suthor584ZHU Lizhong et al .Vol. 15of ingrediens necessary for NAPL disolution, which made the process expensive. The solvents used(e.g.toluene, xyenes, ethybenzene, D-limonene, gasoline, etc. ) would be potential contaninants to soils andgroundwater. A temary microemulsion of water-rapeseed oil-polyglycoside ( APG) significantly extractedpyrene(Py) from a real soil sample and its ingredients (APG and rapeseed oil ) are bioderadable(Schwuger, 1995). However, 10% (wt. fraction) of APG would be a great cost for the clean-up ofcontaminated soils. Thus, these microemulsified systems are not suitable for remediation application.Meanwhile , the information about the efects of the temperature and inorganic salts co-existed with organicpollutants in soils on the performance of microemulsion are not available .The conmercial sodium castor oil sulfonate(SCOS) (Turkey red oil) is the product of natural castoroil sulfonated by concentrated sulfuric acid. SCOS is used as emulsifying and dispersing agents in variousapplications(Liu, 1995). It is a mixture of castor oil, sodium castor oil sulfonate , sodium castor oil acid,inorganic salts and water and so on. The structures of main ingredients are shown in Scheme 1. SC0S isaptitude for biodegradation due to the unsaturated bonds and hydroxyl groups in the structures of itsingredients. A stable microemulsion( oil-in-water) forms when SCOS is mixed with water. Thus, SCOSmicroemulsion would solubilize more organic pollutants than the surfactant micelle solutions arise ultralowinterfacial tension and expose litle contamination risk on the soils and groundwater; meanwhile, abundantresources of castor oil and a simple process for SCOS production would result in a relative low cost if SCOSmicroemulsion would be used in SER application. Most conventional surfactants are derived from petroleumhydrocarbans( non-reproducible resources). However, SCOS is the product of the reproducible castor oil,which has significance to resources availability .)HCH:(CH)xCHCH2CH=CH(CH)C-OCH2CH2(CH)CHCH2CH-CH(CH)C-OCHOSO)Na0CH2(CH2)CHCH2CH=CH(CH2)C-0CH2CH;(CH2)sCHCH2CH=CH(CH)C-OI(@)Castor oil() Sodium castr oil sulfonatoScheme 1 Siructures of castor oi(a) and sodium castor oil sulonate(b)Many microorganisms , including bacteria, algae and fungi, are shown to possess degradative enzymesfor the oxidative degradation of PAHs( Cermigia, 1981). However, the in-situ microbial metabolim ofPAHs is limied by low bioavailblity and removal of PAHs from contaminated soils and groundwater isgovemed by the type and concentration of enhancing agents. The objectives of this study are (1) toinvestigate the solubilization and partitin of Naph, Phen and Py in the SCOS microemulsions; (2) tocompare the extent of solubilization by SCOS microemulsions with that by the conventional surfactants; (3)to test the efcets of inorganice salts and temperature( envionmental factors) on solubiliation. This studywill be a base for the aplication of SCOS microemulsion in the remediation of contaminated soils andgroundwater, and provide a conceptual basis for assing the potential impact of microemulsions on thebehavior of organice contaminants in situations where silled or waste oils may be emulsified by sufactants.1 Experimental section.1 Materials中国煤化工The commercial sodium castor oil sulfonate was obtaineMHCNMHG_Ipany and usedas received . The content of water and inorganic sals were measucu as ..H ro alu 1.01% respecively,by the weight analysis( Zhong, 1986). A stock solution at concentation of 100.0 g/L was prepared andother solutions for batch equilibrium experiment were prepared by diluting stock solution. At ambientNo.5w ater solubility enhancenents of PAHs by sodium castor oil sulonate microemulsions585lemperalure, the solutions with concentration greater than 1000 mg/L show slight turbidity. Phenanthreneand pyrene wih puity of 98% or greater were obained from Aldich Chemical Co. and used withoutfuther purfcation. Naphthalene was obtained as analytical grade chemical from Shanghai YuanhangChemical Factory. Methanol, used for sample analysis, was obtained as an analytical grade solvent .Deionized water was used throughout the experiments.1.2 Experimental procedures and analytical methodsProcedures for sample equilibration and solubility determination were esenially the same as reportedearlier( Zhu, 2001). SCOS solutions were placed in 25 ml Corex centrifuge tubes with Telon cap liners,and PAHs were added to each tube in an amount slightly more than required to saturate the solution.Duplicate samples were prepared for each concentration of SCoS, these samples were then equilibrated on .a Model THZ-C shaker for 24 h at 25 or 20土1C. The samples were subsequently centrifuged at 5000 r/min(2987 g) for 1 h to separate the undissolved solutes . The particles adhering to the glass walls werecarefully removed with a cotton swab, and the meniscus was subsequently aspirated to remove any particlessuspended on the surface . This centrifugation/ cleaning procedure was repeated. An appropriale aliquot ofthe supermalant was then carefully withdrawn with a pipet and diluted to 25 ml with SCoS- water-methanolsolution. Naph, Phen, and Py in solutions were analyzed at 275.8 nm, 250.7 nm, and 334.5 nm onModel UV2401 PC spectrophotometer respectively. In order to eliminate the efct of surfactant on analysis ,SCOS concentration was kept constant in the standard and experimental solutions; the methanolconcentration in the solution was also kepl constant at 10%(VI/V).Surface tension was measured by using a Model JZHY- 180 tensionmeter on the DuNouy principle .2 Results and discussion752.1 Characterization of sCoS microemulsionsFig.1 shows a plot of the surface tension as a function og6宿65logCscos at room lemperature. No distinct inflection is detected in the豆splot with concentration extended to 5000 mg/L，which indicales 器5(SCOS solutions are different from the micelle solutions. A water-oil-员35 tsufactant dispersing system， with transparent or seni-transparent 401appearance and high mobility, is a microemulsion if no phaselogCseparation occurs after it is centrifuged at 100 g of the relativeFig.1 Plot of the suface tension versus thecentifuge force( RCF) for 5 min( Prince, 1977). All SCOS solutionslog centration of scos sltionwere centrifuged al 2987 g of RCF for 1 h. No phase separation wasobserved, which ilustrates that SCOS systems are microemulsions formed by castor oil( oil)，sodium castoroil sulfonates( surfactant)，other surface active ingredients and water.2.2 Solubilization and partition of PAHs in sCos microemulsionsThe relations of the apparent solubilities of Naph, Phen, and Py as a function of SCOS concentationsat25士1C are showm in Fig. 2. The expressed SCOS concentrations have not been corrected for theamount of water and inorganic salt in each sample. A linear relation is observed between the apparentsolubilities and SCOS concentrations, throughout the range of SCOS concentrations studied. Based on theseresults, the microemulsified SCOS system is much like a separate bulk phase at all concentrations. This isin contrast to behavior of a conventional sufactant that shows a distinct break in the solubility enhancernent(or suface tension) plot in the vicinity of the measured CMC中国煤化工.ments of PAHsby SCOS microemulsions can be accounted for by peYHCN M H G between themicroemulsified phase and the aqueous phase, which can be expressed as( Kile, 1990):S: = S.(1 + XK.).(1)586ZHU Lizhong et al .Vol. 15Where Si is the apparent solubility in water, S. is the intrinsic water solubility of the solute, X is theconcentration of microemulsion, and K. is the partition cofficient of solute between the microemulsifiedphase and the aqueous phase. With the quantity of X expresed in g/ml in the solution and soluteconcentration in both water and microemulsified phase expressed in the same weight-to- weight basis, theresulting Km is a dimensionless quantity. Thus, the Ke value can be calculated from the slope and theintercept of the plot.300▲25C160[ ▲25Cb 100▲25飞250-■200.! 120■20个、8(■20公0020060150-40身1004202000 40000 60002000 40000 6000 0 2000 40000 6000sCos, mg/LFig.2 Water slubiliy enhancments of Naph(a) , Phen(b) and Py(c) by scOs micnemulsionsThe calculated K. values (on the water and inorganic salts free basis) and comparison ofslbilization exent for Naph, Phen, and Py are shown in Table 1. The logK. values for Naph, Phen,and Py are3.13, 4.44, and 5.01 repectively . Tis dference is caracerisie of equlbium prtioningof solules between organic phase and waler, in which the equilibrium partion constant is inversely relatedto lhe solute soubility in waler(S、). So the soubilizaion extent for Naph, Phen, and Py by scosmiremulsionsis in the order Py > Phen > Naph, which is also obained from the ratios of solutesolubility at 500 mg/L of SsCOS concentatio( S.s) to the inuinsic water slubiy( s.). Comparedwith octanol- walter partion cficient, logKm for Naph is slighly lower than logKw，while for Phenequivalent t0o and for Py greater than tbe cresponding logK , which shows that SsCOS microemulsionsposses the ability to solubilize nonpolar organic compound to a relative large degree. A linear relation isoblained blwen logK. values and logKw values, wih a ressesn cflicient square equal to 09983, asshown in Fig.3.2.3 Comparison of solubilizationPyat25+1Cby SCOS microemulsions withPAHs s., mg/L Sis mg/LRio0ooS logK_ logKmsurfactant micellar solutionsNaph32.05288.803.30 3.13In viewof the uniquePhen 1.18214124.464.440. 1294.87.26744.885.01composition of SCOS microemulsions ，it is of interest to compare thesouizaion extet of PAHs by SCOS micrnemulsins from this study wih.0that by the conventional surfactant solutions from the earlier work. Weightslbiliaion ratio( WSR) is a common paraneter, which qunitatively4.0expresses the solubilizing capacity of a solute by unit weight of suface-35-active agent. For solublization by microemulsions, it is defined asWSR = (S. - S、)1X.(2)30 3540k45505SWereS: (mg/L) is thelogKow中国煤化工. X(mg/L) ofmicroemulsion concentration.solubility ofFig.3 Plot of logK.m versus logKosolute. For solubilization byTYHCNMHGWSR = (S; - S)/(X- Xmn).(3)No.5Water solubility enhancements of PAHs by sodium castor oil sulonate microemulsions587Where s; (mg/L) is the apparent solubility of solute at X(mg/L) of surfactant concentation, while S.mr(mg/L) is the apparent slubility at X (mg/L)(CMC) of sufactant concentration. The value of WSRcan be calculated from the slope of solubility curves. Table 2 shows WSR values( on water and inorganicsalts free basis) for Naph, Phen, and Py from this study and the earlier work. WRS values for Naph,Phen, and Py by SCOS are greater than that for corresponding solutes by nonionic and anionie surfactanls，except that WSR values for Naph, Phen and Py by Brij 30, for Naph and Phen by Tergiol NP-10，and forNaph by Triton X- 100 are greater than that for corresponding solutes by SCoS. However, Brij 30, with 4of polyoxyelhlene number, is not suitable for remediation application due to its low solubility in water.Naph is volatile compound and its accumulation in soils is not a concem. From these points of view , SCOSmocroemulsions possess a high ability to enhance water solubility of PAHs. Because WSR by surfactantbelow CMC is largely smallr than that above CMC, correspondingly, WSR by SCOS microemulsion at lowconcentrations is much greater than that by surfactant below CMC. As mentioned above, SCOS forns stableemulsion in water and thus behave much like a bulk organic phase in concentrating organic solutes. Inaddition, a microemulsified droplet can be considered as a micelle in which some oil(e.g castor oil) issolubilized, so the former holds larger volume than the ltter to contain organic solutes. Anyhow ,microemulsified droplets exist even though the microemulsion concentrations go much down, but surfactantsexist as monomers in aqueous solution below CMC, which have no signifcant efcts on water solubilityenhacements of organic solutes. Consequently, SCOS microemulsions are superior to the conventionalsurfactant micelle solutions for the solubilization of organic pollutants .2.4 Influence of temperature on solubilizationAn increase in temperature from 20 to 25C results in WSR values increasing by 22.0%, 23.4%，and .24.6% for Naph, Phen, and Py, respectively, as shown in Fig.2. The temperature efet can be atibutedt0 (1) changes in the aqueous Tabl2 Cmprton of WSR vuhus of Naph, Phen and Py by scos wih that bysolubility of PAHs and (2) changes surfactants at 25+1Cin the properties of microemulsifiedSurfactantChemical namePAHsWSRphase . Temperature increase enlarges Brij 30°Lauryl polyoxyethene(4) etherNaph 1.12x 1I0~'the volume of microe- mulsifedhen 7.45x 10-2phase, thus can include more solutePy 3.98x 10-2Igpel CA-720* Octypheny! plyethylene( l2) etherNeph 5.63x 10-2molecules .hen 2.51x 10~22.5 Effects of inorganic cationsy 1.17x10-2on solubilizationsTegitol NP -10*Nonyphemy) poyethyle( 10.5) etherlaph 6.70x 10-2TheperformanceofPhen 4.17x 10-2micmemulsion formed by anionicPy 1.70x 10-2surfactant is strongly afeted by theTriton X-100 Oecylphenyl plyoyethylene(9.5) etheriaph 6.92x 10-2Phen 3.16x 10-2presence of electrolytes in thePy 1.14x 10-2solution ( Cui，1999 ). This is APFOENaph 1.24 x 10-particularly importantin fieldPhen 9.54x 10~5application, since in such situation ,Py 1.92x 10-5subsurface matrix solutions containSDBS*Sodium dodeeyl pheny sulfaleNaph 1.43x 10-2Phen 3.32x 10-3electrolytes such as Na* ，K'中国煤化工The scosSodiumYHCNMHG6.06x 10-?inluence of common inorganic cations3.77x 10-2in soils on the solubilization of PAHsPy 2.03x 10-2Notes:a. Edwers, 1991; b. Chun, 2002; c. An, 2002at 25+1C was investigated.58ZHU Li-zhong et al.Vol.15The solubilizations of Py by SCOS microemulsions in the presenceof 1.0, 5.0, 10, 20, 50, 70, and00 mmol/L of NaCl were tested. The quantity of inorganic salts in SCOS reagent was not included. 1.0mmol/L of NaCl has no distinct efect on the solubilization in contrast to the control solution( no NaCladition). 5.0, 10, 20, and 50 mmol/L of NaCl increase WSR values by 2.0%，4.9%，13.3% and9.4% respectively. However, 70 mmol/L of NaCl results in the precipitation of SCOS and decrease insolubility of Py，while 100 mnol/L of NaCl does more distincly . The infuence of NH, CI is somewhat thesame as that of NaCl. No change in WSR values occurs in the presence 1.0 or 5.0 mmol/L of NH,CI .WSR values increase by 5.9%， 14.8%， and 13.8% in the presence of 10, 20, and 50 mmol/L ofNH,CI. Preipitation of SCOS also occurs in the presence of 70 and 100 mmol/L of NH,CI. The ffelts ofCat were tested at 0.1, 0.2, 0.5, 0.8, 1.0, 2.0, and 4.0 mmol/L of CaCl. For CaCl2 concentrationsat0.5, 0.8，1.0, and 2.0 mnol/L, water solubility of Py increase as a functin of SCOS concentrationand WSR values increase by 2.5%, 7.4%，13.3% , and 3I .5% , while 0. 1 or 0.2 mmol/L of CaCl, hasno efet on solubilization of SCOS. Precipitation occurs in the preence of 4.0 mmol/L CaCl2. Theinluences of NaCI, NH,Cl and CaCl on WSR values of Py are shown in Table 3. From the results above,it is shown that cation ion has both positive and negative eflet on the solubilization of Py. A moderateaddition of inorganie salts enhances solubilization. This could be atributed to the static eletric screen of“anionic heads”of sufactants by cation ions, which decrease the repulsion among“anionic heads" andincrease the number of sufactants in the microemulsified phase, and then enlarge the volume omicroemulsified drolets. However, a large amount of sals result in precipitation of anionic sufactant anddestroy the structure of microemulsion.Table 3 Influences of Na, NH,C1 and CaCl on WSR valbes of PyNaCl, mmol/LNHLCI,mmo/LCaC2 . mmol/L5.01020s5.0020500.50.81.0 2.0WSR0.0207 0.0213 0.0230 0.02220.0203 0.0215 0.0233 0.02310.0208 0.0218 0.0230 0.0267R20.9970 0.9986 0.9995 0.99230.9980 0.9984 0.9992 0.98980.9966 0.9952 0.9990 0.9920△WSR,%2.0 4.9 13.39.40.0 5.9 14.8 13.82.5 7.4 13.3 31 sTable4 Infuence of tbe mixed silts(5.0 mmol/L of catio) onThe inluence of mixed cations on .WSR values of PAHssolubilization is also investigated, as shown inPAHsNo inorganic saltMixed inorganice salsTable 4. CaCh， NH,Cl, MgCl, NaCI, and▲WSR，% KCl were mixed together, with each cation ionNaph 0.0602 0.99690.0700 0.987416.3concentration equal to 1.0 mmol/L. 5.0 mmol/LPhen 0.0377 0.99780.0534 0.998541.6of mixed cation concentration is at levelsy 0.0203 0.99720.0233 0.996414.8representative of soil solution concentration. Thslubiltis of Naph, Phen and Py are lnealy portional to the concentration of SCos in the preene ofthe mixed inorganic sals and WSR values increase by 16.3%，41.6% and 14.8% for Naph, Phen andPy, repecively, which ilustrates the solublization extent by SCOS would be enhanced by erolytes insubsurface water in field application.3 ConclusionsSCOS microemulsion significantly enhances the solubili中国煤化工l0 its uniqueproperties. The capacity of solubility enhancement by SCOTYHCNM H G"that by the .conventional nonionic and anionic surfactants on the basisand suitablequantity of inorganic cations can enhance the soubiliating capacities of SCOS micremulsions on PAHs.The rsuls sugesed that SCOS would improve the performance of the sufacan-enhanced remediationNo.5Water solubility enhancerments of PAHs by sodium castor oil slfonate micromulsions589(SER) of soils by increasing the biavailablity and biodegradation of non-aqueous-phase organic plutantsand reducing the level of surfactant pllution and remediation expenses. The data also indicated thepotential impact of microemulsion on the transport and fate of organic contaminants in environment.References :AbdulAS, GibsonTL, Rai DN, 1992. 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