Solubility and sorption of petroleum hydrocarbons in water and cosolvent systems

Available online at www.sciencedirect.comJOURNAL OPENVIRONMENTALSCIENCES口厦ScienceDirectISSN 101-07402LELJourmal of Eriromeatal Sciences 20(2008) 17-1182www.jesc.sc.cnSolubility and sorption of petroleum hydrocarbons in waterand cosolvent systemsCHEN Hong, CHEN Shuo, QUAN Xie*, ZHAO Yazhi, ZHAO HuiminKey Laboratory of Idusrial Ecology and Enwironmental Engineering, Ministry of Education of China, School of Environmental and BiologicalScience and Technolog>, Dalian University of Technologs, Dalian 116024, China. E-mail: hongchen82@yahoo.cnReceived 25 November 2007; revised 27 Deccember 2007; acepted 12 February 2008AbstractThe solubility and sorption of oil by uncontaminated clay loam and silt loam soils were studied from water and cosolvent/watersolutions using batch techniques. The data obtained from the dissolution and sorption experiments were used to evaluate theapplicability of the cosolvent theory to oil as a complex mixture. Aqueous solubility and soil-water distribution coefficients (Kxw，LIkg) were estimated by extrapolating from cosolvent data, with a log-linear cosolvency model, to the volume fraction of cosolvent(&) 0, and were compared with direct aqueous measurements. The extrapolated water solubility was 3.16 mg/L, in good agreementwith the directly measured value of 3.83 mg/L. Extrapolated values of Kd.w for the two soils were close to each other but consistentyhigher than the values from direct aqueous measurements, because of the presence of dissolved organic carbon (DOC). The partitionceficient (Kpoc) between the DOC and the freely dissolved phase and the OC-normalized sorption coeficient (Koc) were determined.The average values of logKpoc and logKoc were estimated as 4.34 and 3.32, respectively, giving insight into the possibility of oilbecoming mobilized and/or of the soil being remedied. This study revealed that the cosolvency model can be applied to a broader rangeof hydrophobic organic chemicals (HOCs) than has been previously thought. The results aided in a reliable determination of watersolubility and sorption coficients and provide information about the fate of oil in solvent-contaminated eavironment.Key words: cosolvent; solubilit; sorption; dissolved organic catbon; oilIntroductionvariety of HOCs (Caron et al, 1985; Frankki et al, 2006;Karickhoff et al, 1979; Servos and Muir, 1989; TrainaAnthropogenic sources and natural seepage contributeet al, 1996) using various methods (Allen-King et al,millions of gallons of oil to the environment. Oil pollution 1995; Garbarini and Lion, 1985; Harkey et al, 1994;from several large-scale oil fields in the northeast of Hegeman et al, 1995; Poerschmann et al, 1997; ShimizuChina remains one of the major environmental problems and Liljestrand, 1991). However, the knowledge aboutin these areas. The amount and composition of spilled sorption of oil by POC and DOC is scarce (Shen and Jaffe,oil water-soluble components determine the subsequent 2000) owing to the unique properties of oil, including itsaquatic toxic risk and response (Stanford et al, 2007). high hydrophobicity, low aqueous solubility, and an afinityAlthough the volume loss of siled oil by dissolution for glass surfaces. Log-linear cosolvency model proposedcan be negligible from a practical perspective, it can be by Rao et al. in 1985 has been sucessfully and extensivelysignificant from an ecotoxicological standpoint.used to determine the solubility and sorption of this classSorption, directly infuenced by the soil organic carbonof contaminants (Bouchard, 2002; Iraqi and Iraqi, 2000).present, is a central process in the bioavailability, behavior, However, it was limited to pure chemicals. Therefore,and fate of hydrophobic organic chemicals (HOCs) in cosolvent and log-linear cosolvency model were employedaquatic ecosystems. Soil organic carbon is present as to evaluate the applicability of the cosolvent theory to oil asparticulate organic carbon (POC) and dissolved organic a complex mixture, and to estimate its aqueous solubility,carbon (DOC). The POC fraction can be regarded as soil-water distribution coefficients (Kdw), and further thea potential pool for DOC containing nanoparticles and partition coeficients between the DOC and the freelycolloids (Kalbitz et al. 2000). The DOC in the subsurface dissolved nhases (Km) hv comnaring the extrapolatedmay behave as potentially mobile carriers of HOCs inand dil中国煤化工t of our knowledge,soil and aquifer systems (Schumacher et al, 2005). To this isMHC N M H G volume fraction of= onship between theunderstand and quantify the importance of POC and DOC,solubimany researchers have studied the sorption of a wide cosolvent has been considered for a mixture.The pore water in porous sediment materials near●Corresponding author. B-mail: quanxie@dlut.cdu.cn.1178CHEN Hong eral.Vol. 20hazardous waste disposal sites and accidental spill sitesand to enhance the solubilization of pharmaceutical drugsoften contain significant concentrations of water-miscible(Yalkowsky, 1999).solvents that can affect aqueous solubility, and thus theBased on the inverse relationship between solubility andsorption of contaminants. The net result of this cosolventsorption, a similar equation to describe the effect of aeffect is often an increase in contaminant fux through thecosolvent on the sorption of HOCs was derived as (Raounsaturated zone to groundwater (Bouchard, 1998). Theet al, 1985):present study also provides a quantitative understanding ofoil movement in an environment containing water- solublelog Kd.mix = log Ks,w - aofc(2)solvents.where, Kd,mix and Kaw are the partition cofficients in abinary cosolvent/water solution and water, respectively;1 Materials and methodsand a is an empirical factor representing the averagedeviation observed between the sorption-fe relationship1.1 Chemicalsand solubilization, and is assumed to be constant for aFuel oil No.0 was obtained commercially from Chinagiven cosolvent. Eq.(2) has been successfully employedPetroleum and Chemical Co. The soluble fraction wasto determine the sorption coefficients of several HOCsused as sorbate owing to the wide use of No.0 fuel oil.(Bouchard, 1998, 2002; Fu and Luthy, 1986; Iraqi andHexane (HPLC/Spectro grade) was purchased from theIraqi, 2000; Lee et al, 1990; Nkedi-Kizza et al, 1985,TEDIA Company Inc. (OH, USA). The cosolvent used in1989). Eqs.(1) and (2) alow the estimation of water solu-this study was acetone (analytical reagent grade), providedbility and sorption cofficient values in water-soil systemsby the Kemiou Agent Co., Tianjin, China. The otherby extrapolating from data measured in cosolvent/waterchemicals used included calcium chloride and sodiumsolutions to fc = 0, assuming that the log linear behavior isazide (analytical grade, Xilong Chemical Co., China).exhibited.1.2 Soils1.4 Determination of oil solubilityPJ is a clay loam soil that was sampled from the uncon-The solubility was determined by "slowstrring”ex-taminated agricultural top soil in Panjin, China. CBM isperiments (Tolls et al, 2002), in which a layer of neata silt loam soil with high organic matter content from thehydrocarbons floated on the water phase or the cosol-Changbai Mountain, China, with a long-established histo-vent/water mixtures; this two-phase system was gentlyry. Pj and CBM are two typical soils in the northeast of sired to minimize the formation of microdroplets, TheChina that represent a range in texture and organic carbonsolubility of No.0 fuel oil was measured at 20土1°C incontent, and are used as sorbents. The soils (0 -10 cm)water and in acetone/water solutions of 10%, 20%, 30%,were passed through a 250-μm sieve, homogenized, air-40%, and 50% volume cosolvent. The samples containingdried, and stored in closed containers at room temperature.dissolved petroleum bydrocarbons (DPH) were withdrawnTheir physicochemical properties, determnined by standardafter 72 h, since the dissolution kinetics indicated thattechniques, are summarized in Table 1.equilrium was attained within 72 h. The average con-centration (n = 10) determined the aqueous solubility. In1.3 Log-linear cosolvency modeladdition, No.0 fuel-oil-saturated water was collected forFor HOCs, the efect of organic solvent on the solubilityfurther use in the sorption experiments.has been adequately described with the following log-1.5 Sorption isotherms in water and in acetone-waterlinear relationship (Yalkowsky et al, 1972): ;systemslogS mix = logSw + ofe(1)Acetone was selected for the sorption studies becauseoil solubility exhibits a good log-linear cosolvency pro-where, Smix and Sw are the solubilties of the cosol-file in acetone/water systems. Batch-equilibrium sorptionvent/water mixture and cosolvent-free water, respectively;isotherms were obtained in water and in acetone-waterfe is the volume fraction of the cosolvent; and σ is a mea-solutions of 10%, 17%, 24%, and 30% volume acetone,sure of the solute- cosolvent interactions and an indexofthe for each of the two soils, at 20士1。C in screw. cappedsolubilizing power of the cosolvent (Bouchard, 1998). Atglass centrifuge tubes. The background solution comprisedfe= 1,σ is equal to the logarithm of the ratio of solubility0.01 mol/L CaCl2 in ultrapure water (Millipore-Waters,in pure cosolvent to that in water. This method has been Netherlands) and 200 mg/L NaN3 as a biocide. The initialsuccessfully used to estimate the aqueous solubility ofconcentrations of DPH ranged from 15% to 100% of theorganic compounds that are sparingly soluble in watersolubility. The soil (g) to solution (ml) ratios (M/N) wereTable1 Physicochemial proper中国煤化工SoilTexturepH(1:2.5 H2O)Sand (%)Organic cartbo' (OC, %)JClay loam7.220.TYHCNMHJ.91+0.02Silt loam7.0688.817.70+0.05pH at equilibrium with 0.01 mol/L CaCl2 and 200 mg/ NaN; at the soil-to-solutio ratio used for isotberms; . mean + standard deviation (n= 3).No.10Solubility and sorption of petroleum hydrocarbons in water and cosolvent systems1179varied with the solution matrices and soil types to achieve1.8 Data analysis30% -80% removal eficiency of DPH at equilibriuimn, whileThe aqueous solubility and the soil-water distibutionthey were constant for a given soil and acetone/water solu-coefficients were determined by direct measurement andtion over the range of an isotherm. The M/V ratios rangedextrapolation in cosolven/water solutions. The sorptionfrom 3:40 to 0.3:40 for water, and from 25:40 to 2.5:40isotherm data were fit to the linear sorption model (C;for acetone/water solutions. The isotherms consisted of; Ka_wCw or C; = Kd.mixCmix), where, C; (4g/g) is the6 initial concentration points and each point was run inconcentration of sorbed oil, and Cw (mg/L) and Cmixduplicate. The DPH of different concentrations in water(mg/L) are the equilibium concenrations in the aqueousor acetone/water mixture was added directly into the tubesor in the mixed cosolvent/water solution (mg/L), respec-containing the soils preweighed to no headspace; thus, thetively. For each soil, the Kd.mix values estimated at eachloss of the DPH's volatile components was efectively rfc were applied in Eq.(2), and Kdw was determined byduced. The tubes containing soil were prepared in triplicateextrapolationto fc= 0.for each soil to determine the DOC concentration in thesupemnatant after centrifugation.The tubes were immediately sealed tightly using screw2 Results and discussioncaps with Telfon liners, and were then tumbled at 102.1 Solubility of oill/min for 2d in the dark at 20土1°C, which was foundto be sufficient to reach equilibrium in a preliminaryA good log-linear cosolvency model well describedkinetic study. After the tubes were centrifuged at 3,000ne solubility of oil in binary acetone/water solutions,r/min for 30 min, the supematant was withdrawn for thealthough oil is a complex mixture (Fig.1). Rubino andDPH concentration analysis. Control tubes with DPH onlyYalkowsky (1987) showed the value of σ to be correlatedpermitted the measurement of losses of DPH to glass-with solute properties, such as the molecular surface area,wall sorption, to volatilization, and to biodegradation,Theand with solvent properties such as the dielectric constantlosses were less than 3%. Hence, the DPH retained by theand bulk surface tension. It was in close agreement withsorbents could be calculated by mass difference.the log-linear cosolvency relationship for oil, which canprobably be explained by the fact that the diference in the1.6 DPH analysiscosolvency power of acetone for each component of theAnalysis of the DPH was performed using a 2010 oil was extremely small to be observed. It also indicatedgas chromatograph (GC, Shimadzu, Japan) with a famethat Eq.(2) was applicable to describe the sorption ofionization detector (FID). Samples (100 ml) were spikedoil as a complex mixture in acetone/water solutions. Thewith n-C24 (7 ug) and extracted twice with 5 ml hexane. water solubility of No.0 fuel oil determined by directThe hexane extracts were combined, rotary-evaporated to aqueous phase measurements was 3.83土0.28 mg/L (n =approximately 0.5 ml, and then, column chromatography10). The extrapolated water solubility from acetone/waterwas performed according to Reddy and Quim (1999). Asolutions was 3.16 mg/L, in good agreement with the value2-ul sample was injected and compounds were separated measured directly from water. Futhermore, water-solubleinto a poly (dimethylsiloxane) capillary column (Rtx-1,30 No.0 fuel oil was dominated by UCM (93.7%).m, 0.25 mm i.d., 0.25 um flm, Shimadzu, Japan) with N22.2 Sorption isotherms in water and acetone-water sys-as the carrier gas at a constant flow of 1.47 m/min. ThetemsGC oven was temperature programmed from 40°C (1 minhold) to 120 at 30°C/min, and then from 120 to 280°C atTo investigate the applicability of the cosolvent theory5°C/min (30 min hold). Owing to the lack of calibrationproposed by Rao et al. (1985) to oil sorption, No.0solution of dissolved oil, No.0 fuel oil was used as a fuel oil sorption isotherms by the two surface soils werestandard to generate the response factor. The DPH wasquantified (as the unresolved complex mixture, UCM) by.0「logSmi= logSw +ofcintegrating the total area and using response factors deter-R2=0.99 o=4.78mined from No.0 fuel-oil standards. Recoveries of No.0fuel oil spiked into Millpore-water were (96.7土8.1)%.Precision, based on the duplicate samples, as expressed bythe relative standard deviation, was less than 5%. A methoddetection limit of 0.54 mg/L was estimated according tothe procedures described by Glaser et al. (1981).置is1.7 DOC analysis.0 tThe supernatants in the direct aqueous sorption mea-surements wereanalyzed for DOC concentrations after 2中国煤化工-0.d of equilibration. After centrifuging for 30 min at 3,000CNMHGr/min, the supematants were acidified, and analyzed usinga Shimadzu TOC-VCPH analyzer (Shimadzu, Japan).Fe 1 Measured solubility of No.O fuel oil in acetone/water solutions.1180CHEN Hong et aLVol,. 20measured in binary solutions of acetone/water of 10%，of the logKd.mix- -f relationship in the cosolvent range17%, 24%, and 30% volume acetone, respectively. Thetested (Fig.3). This reverse relationship between logKd.mixsorption isotherms of oil to PJ and CBM soils along withand fc suggests that solvophobic interactions dominate oilthe linear regressions are shown in Fig.2. All sorptionsorption from aqueous and acetone/water solutions.isotherms were well ftted by the linear sorption mod-The extrapolated Kdw values were significantly higherel as indicated by the correlation coficients exceedingthan those determined by direct measurement from aque-0.95. The linear sorption coeficients estimated from theous solutions, with the difference being soil-dependent.batch equilibration studies along with the results from theHigh aqueous Kaw values estimated by extrapolationapplication of the log-linear cosolvency sorption modelfrom acetone/water solutions may be owing to cosolvent-are summarized in Tables 2 and 3. The aqueous sorptionenhanced mass transfer, cosolvent modification of soilcoefficients were well correlated with the soil OC, withsorption characteristics, or the efect of DOC from aqueousaverage logKoc values of 3.32 and 3.14 estimated bysoil suspensions in reducing sorption. Dividing the aocosolvent extrapolation and direct aqueous measurement,values of slopes 4.20 and 3.73 for PJ and CBM soils, byrespectively.σ estimated from the solubility data, the cosolvent-sorbentThe values of Kd.mix (L/kg) in acetone/water solutions,interaction terms (a) were determined to be 0.88 and 0.78,estimated from the batch-equilibration studies, were plot-respectively (Table 2). The small deviation of a from unityted as a function of fe in Fig.3. As predicted by Eq.(2),indicates that the impact of acetone on sorption appears toan inverse relationship between logKd.mix and fc wasbe dominated by acetone-induced changes in the solutionobserved. Regression of logKd.mix on fc yielded an R2phase actity of No.0 fuel oil.greater than 0.96, indicating the high degree of linearity00 r40- .s/00 F300 t20,3200F0.料00 t1220 24 28101520C. (mg/L)Flg 2 Sorption isotherms of No.0 fuel oil to PJ (a) and CBM () soil in acetone-free water and in acetone/water solutions. The solid lines are the linearsorption model fits. Cs: solid phase conceatration; Cw: eqilibrated solution phase concentration.Table 2 Linear sorption coficients at various volume fractions of acetonefcPJ soilCBM soilKamis (LAkg)logKd.mixRlogKamix10%1.010.955102.892.010.99417%5.660.7552.360.98024%000.480.9950.1616.780.986xo4.20 .3.73a0.880.78Kdumis: linear sorption coficientis in cosolven/water slutios, respectively; T: cosolvency power of the solvent for the solute; a: empirical factorrepresenting the avenge deviation.Soil0C(%)Kemix Lkg)Cpoc (mgL)ExtrapolationMceasurement中国煤化工-PJCBM0.9128.1818.70MYHCNMHG18.0117.70245.47164.3629.01logKoc (SD)3.32 (0.25)3.14 (0.24)SD: standard deviations; Koc: OC-normalized sorption cofficicntNo. 10Solubility and sorpion of petroleum hydrocarbons in water and cosolvet sytems11812.52.4 Environmental implications■PJsoil_The lg-linear relationship between the solubility or2.0▲CBM soilsorption cofficients and the volumetric fraction of water-miscible solvent was successfully applied to the complexmixture of No.0 fuel oil. This suggests that the cosolventpowers of acetone for the components of oil are similar.1.0This relationship quantitatively describes the efects ofacetone, which are found in most waste streams from0.5 .industrial waste, on the dissolution and sorption processesof No.0 fuel oil. The small deviation from the extrapo-lated average logKoc (relative standard deviation, RSD =0.05 0.10 0.15 0.20.2530 0.357.53%) indicates that hydrophobic partitioning dominatesthe sorption of oil by soils. The estimated high averagee3 Log-irer plot of the lner sopp cofocieae lRKum YeBhlogKpoc of oil demonstrates that the sorption of oil isvolume fnctio of aetone fe for sorption of No.fuel oil by PJ andCBM significantly afected by the DOC and indicates a highsoilspotential for DOC associated transport, especially, whenlevels are elevated. Furthermore, the extrapolation of the2.3 Efects of DOC on sorptionbatch-measured adsorption characteritics of oil to fieldNkedl-Kizza et al. (1985) proposed that high extrapolat- conditions should be done with caution because of theed Kdw is mainly owing to the reduced sorption by DOCvarying soil- water ratios and thus the uncertainties ofDOCin aqueous soil suspensions. Moreover, it was confirmedconcentrations.that Ks, the apparent sorption coficient in the presence ofDOC, determined with the method of batch-equilibration 3 Conclusionstechnique, was usually underestimated because of thepresence of DOC (the particle concentration efect). TheDOC fraction represents a potentially mobile pool of soil between the solubility or sorption coefficients and theorganic carbon. The partitioning to DOC suggests thevolumetric fraction of water- miscible solvent is operativelikelihood for contaminants to become mobilized and/orfor oil, and thus, a reliable aqueous solubility (Sw) andfor the soil to be remedied. Thus, the KDoc value for No.0Kd.w for No.0 fuel oil from water and cosolvent/waterfuel oil was determined according to the Eq.(3) (Liu andsolutions were obtained. For oil, Kd.mix decreased log-Lee, 2005):linearly as fc increased. This log-linear relationship is alsovery important in understanding the contaminant sorptionKs = Krwe/(1 + Kpoc x Coc)3)and transport in the presence of water-miscible cosolvents.Furthermore, the logarithm of the partition cofficientbetween the DOC and freely dissolved phases (logKpoc)where, IKrue is the true sorption cofficient on the partic-was estimated as 4.34, indicating that DOC may behaveulate organic matter (POM) in the absence of DOC, andas a potentially mobile carrier of dissolved-phase oiCpoc (mg/L) is the DOC concentration. Theretcally,itisconstituents. In addition, the sorption isotherms in aqueousassumed that the concentration of DOC is low relative toand acetone/water solutionscontribute to a better under-the percent of acetone present, and thus, the presence ofstanding of the efects of cosolvents on oil constituents,cosolvent should minimize the efect of DOC on sorption.and indicate that sorption of oil appears to be driven byIt can also be assumed that only DOC is responsible forhydrophobic partitioning.the difference between direct and cosolvent-extrapolatedAcknowledgementssorption cofficients. The extrapolated Kaw value for agiven soil can be considered equal to the Krne in Bq.(3)This work was supported by the National Basic Re-along with the DOC associated with that soil. The DOCsearch Program of China (No. 2004CB418504).concentrations of 18.01 and 29.01 mg/L were measuredfor supernatants obtained from PJ and CBM soils, respec-Referencestively, at the M/V ratios used in the sorption isotherms.According to Eq.(3), the logKDoc values were calculated toAllen-King R M, Groenevelt H, Mackay D M, 1995. Analyticalbe 4.45 and 4.23 for PJ and CBM soil, respectively. Thus,method for the sorption of hydrophobic organic pollutantsthe overall average logKpoc was 4.34土0.16. It is notedin clay-rich materials. Environ Sci Technol, 29: 148 -153.that the logKpoc value was approximately 1 order of mag-Bouchard D C, 1998. Sorption kinetics of PAHs in methanol-nitude greater than the average Koc value. 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