Modeling the influence of ethanol on the adsorption and desorption of selected BTEX compounds on ben

Availableonlineatwww.sciencedirect.comSciencedirectENVIRIcN 10-25294Joumal of Environmental Sciences 2011, 23(11)1865-1872Modeling the influence of ethanol on the adsorption and desorption of selectedBTEX compounds on bentonite and kaolinTemesgen Aroma*, Lacy SkidmoreDepartment of Civil Construction, and Emvironmental Engineering. San Diego State University San Diego, CA 92182, USAE-mail: tgaroma@@ mail sdsu.eduReceived 24 December 2010; revised 21 March 2011: accepted 05 May 2011AbstractThe influence of ethanol on the adsorption capacity and desorption kinetics of benzene and toluene on bentonite and kaolin throughmodeling and experimental study was investigated. The results showed that the adsorption capacity of both soils for the targetompounds decreased as ethanol content increased. As ethanol content increased from0 to 50% the adsorption capacity for benzenebenzene and toluene adsorption on kaolin, the adsorption capacity decreased by 86. 5%(from 0.26 to 0.04 ug+/( g)and 98.2%(from0. 13 to 0.002 ug"*/(Ln.g), respectively, as ethanol content increased from 0 to 50% In addition, the desorption rate of benzene andtoluene from bentonite decreased by about one order of magnitude as the ethanol increased from o to 25% and 0 to 50%o, respectively. Itan be inferred that ethanol could affect the effectiveness of natural attenuation processes that rely on adsorption to soils as a containmenttechnique for benzene and toluene by retarding the adsorption to soils and remobilizing compounds that had already been adsorbedKey words: benzene; toluene; ethanol; soil; adsorption; desorptionDoI:I0.1016S1001-0742(10)60653-5Citation: Aroma T, Skidmore L, 2011. Modeling the influence of ethanol on the adsorption and desorption of selected BTEXcompounds on bentonite and kaolin Joumal of Environmental Sciences, 23(11): 1865-1872Introductionpart per million exposures(Lan et al., 2004).To protect the public, these compounds must be removedPetroleum products are a major source of groundwater from groundwater before it can be used as a drinking watercontamination(USEPA, 2010). The main source of this source. At many contaminated sites, natural attenuationcontamination is from leaking underground storage tanks processes which rely on adsorption of the contaminants(USTS)(USEPA, 2010; Nadim et al, 2001), which are to soils and biodegradation of the contaminants by mi-commonly used to store petroleum products. There are croorganisms are employed as containment- and treatmentabout 607,000 confirmed leaking UST sites in the U.s. techniques. Research has shown that the migration of(USEPA, 2010). Other causes for groundwater contami- hydrophobic gasoline components in the subsurface is re-nation by petroleum products are from improper disposal, tarded by their adsorption to organic matter in soils( Brownpipe breaks, and spills at extractions wells, refineries, and Burris, 1996; Zhuet al., 2004). The effectiveness of ad-distribution terminals and during transportation(Nadim et sorption as a natural attenuation processes depends, amongal., 2001). Gasoline, a major petroleum product, consists other things, on gasoline composition or formulation, soilof a mixture of hydrocarbons that are toxic to humans. organic matter, and presence of organic solvents( Chen etBenzene, toluene, ethylbenzene and xylene(bteX)are al., 2000: Delle, 2001). It is very important to accuratelymonocyclic aromatic hydrocarbons that are found in gaso- estimate the extent of contaminant removal or containmentline and are highly toxic(USGS, 2010). Human exposure by natural attenuation processes to make site decisionsto these compounds can cause kidney and liver dam- that are protective of the public and the environment. Inage, nervous disorders, and reproductive harm (USDHHS, particular, this is critical in light of the projected sharp2004; Caprino and Togna, 1998). Benzene is also classified increase in the*ha naming years dueas a human carcinogen( Caprino and Togna, 1998; Smith, to its use as ful中国煤化工 placing methyl1996). Research has shown that benzene is known to cause tert-butyl etherCNMHGleukemia and can alter blood cell counts in people below 1 Ethanol, an organIc solvent, is completely miscible inwater and could affect the physicochemical propertiesCorresponding author. E-mail: tgaroma@ mailsdsu. edu1866Joumal of Environmental Sciences 2011, 23(11)1865-1872/Temesgen Aroma et al.Vol. 2of groundwater, soil surface chemistry, and the fate and through modeling and experimental study. In addition, thetransport of gasoline components in the subsurface. When effect of ethanol on adsorption capacity of soils for thethe gasoline-ethanol mixture comes in contact with water, target BTEX compounds will be investigatedthe ethanol will partition to the water phase( Corseuilet al., 2004; USEPA, 2009). The partitioning into the 1 Experimental approachwater phase decreases the polarity of the water and canincrease the solubility of bTEX compounds( Capiro etA number of adsorption and desorption experimentsal., 2007; Heermann and Powers, 1998; Williams et al., were planned, designed, and conducted During a typical2003), a process known as a cosolvent effect. Additionally, adsorption experiment: (1)a measured mass of soil(0.03ethanol is completely miscible with nonaqueous phase g bentonite or 0.1 g kaolin)was added to 40 mL glassliquid (NAPL) such as gasoline (Lee, 2008). When a vials, ( 2)a 30 mL water-ethanol solution spiked with acosolvent, such as ethanol, is introduced into the two-phase target BTEX compound at concentrations of 25, 50, 100,NAPL-water system, it could enhance the solubility of 250, and 500 ug/L was added to the 40 ml glass vials,ethanol-free NAPL (Lee, 2008: Kiven, 2005)()the vials were sealed tightly and tumbled in a shakerResearch has been conducted on the effect of ethanol at 200 r/min and kept at constant temperature of 25oC,and BTEX compounds in the subsurface. Ruiz-Aguilar et (4)after 24 hr, the vials were removed and the contentsal. (2003)reported that at sites contaminated with ethanol- were filtered using disposable syringes with 0.45 um poregasoline mixture of 10% ethanol by volume, the plume size, and (5)the samples were then analyzed using alengths of benzene were 69% longer when compared to Gas Chromatography(GC). The pH of the solution wasa site contaminated with gasoline not containing ethanol. measured at the beginning and end of the experiments ForThe same study also found that toluene plumes were 39% experiments with bentonite, the ph values were measuredlonger in the presence of the ethanol-gasoline mixture. The as 8.8+ 0.6 at the beginning of the experiments andretardation of biodegradation of BTEX compounds has 95+0.3 at the end of the experiments. ph values ofbeen attributed to the preferential degradation of ethanol 6.4+ 0.7 and 6.0+0.4 were recorded for experimentsover BTEX compounds in aquifers. Ethanol is degraded with kaolin at the beginning and end of experiments,first and depletes the oxygen and electron acceptors, leav- respectively. Step 1 to 5 was repeated for water-ethanoling BTEX compounds without the nutrients to be degraded solutions containing 0%,5%, 10%, 25%, and 50%ethanolby natural attenuation processes, or greatly slowing the by volume. All experiments were conducted in duplicatedegradation process(Da Silva and Alvarez, 2002; Chen et These concentrations of ethanol were chosen to bracketal., 2008; Lawrence et al., 2009: Mackay et al., 2006)ethanol concentrations anticipated in the environment. TheAlong with the retardation of BTEX biodegradation, the 50% ethanol concentration represents a spill in which purecosolvency effects of ethanol on BTEX compounds also ethanol escapes in a localized area, while the 5% ethanolplay an important role in the subsurface. Corseuil et al. concentration represents the lower concentration ofethanol(2004)reported that for a water-ethanol solution of 20% that may be seen as the spill advances further through theethanol content, the solubility of benzene, toluene, and subsurfaceo-xylene increased by 29%0, 34%, and 80%, respectivelyDuring a typical desorption experiment: (1)a measuredDa Silva and Alvarez(2002)measured the effect that mass of soil (0.03 g bentonite)was added to 40 mLdifferent concentrations of ethanol had on the adsorption glass vials, (2)30 mL of the target BTEX compoundof BTEX compounds to soil. The findings showed that at 500 ug/L was added to each vial, (3)the vials wereat 1% ethanol content, there were no significant signs of then placed in a shaker at 200 r/min to equilibrate, (4)decreased adsorption retardation. However, it did show that after 24 hr, the aqueous solution was separated from theat 50% ethanol, there was a significant decrease in BTEx soil by centrifuging for 30 min at 1875 r/min, (5)theretardationaqueous solution was used to determine the equilibriumA significant amount of work has been done to under- concentration and the soil remaining was used for thestand the influence of ethanol on aqueous solubility of desorption studies, (6)a water-ethanol solution was addedBTEX compound and their adsorption on soil. There is to each vial, (7)the vials were then tumbled in a shaker atlimited data on the influence of ethanol on desorption of 200 r/min and temperature of 25 C, and( 8)samples wereBTEX compounds from soil, where both modeling and taken at different time intervals, between 1 hr and 8 daysexperimental work is conducted simultaneously, although Each sample was centrifuged for 30 min at a speed of 1875the such data are very important at UST sites that had r/min to separate the liquid from the soil. The samplesalready been contaminated with gasoline components. were then analyzed using a GC Step 6 through 8 wasThis is because as ethanol replaces MTBE as a fuel- repeated for water-ethanol solutions containing varyingoxygenate andyor ethanol becomes a major component of concentrations of ethanol, 0%, 5%,10%, 25%, and 50%gasoline, ethanol-gasoline mixture releases at these sites by volume. All experiments were conducted in duplicatecould remobilize the BTEX compounds that had alreadybeen adsorbed to soils. Considering the lack of such data,1 Analyses V凵中国煤化工the main objective of this research is to investigate the Analyses ofCN MH Grformed with ainfluence of ethanol on the desorption kinetics of selected GC(6890, Agilent USA)equipped with a flame ionizationBTEX compounds, namely benzene and toluene, from soil detector using EPA method 502.2 Revision 2.1. The GCNo 11 Modeling the influence of ethanol on the adsorption and desorption of selected BTEx compounds on bentonite and kaolin1867is coupled to an autosampler(CTC Combi-Pal, CTc for kaolin. In addition, bentonite has a smaller particleAnalytics AG, Switzerland). The CTC Combi-Pal is a mul- size distribution than that of kaolin. The D50 and D90tifunctional auto sampler for headspace, liquid injection, for bentonite were 4.44 and 21.08 um, respectively. Whileand solid phase micro-extraction systems. The samples the D50 and D90 for kaolin were 6.86 and 35.52 um,were vigorously shaken for 15 min at 99 C in a separate respectively. D50 is the diameter in which 50% of thecompartment. From there, the auto sampler injected 1 mL particles have a larger equivalent. Similarly, D90 is definedof headspace sample into the GC. Nitrogen was used as a as the diameter in which 10% of the particles have a largermakeup gas at 23.2 mL/min, while hydrogen and air were equivalent diameterused as carrier gases at 40.0 and 400 mL/min, respectivelyThe initial oven temperature was 40oC, which was kept 2 Results and discussionconstant for 5 min. After 5 min, the oven temperature wasramped at a rate of 10"C/min, holding for 3 min at each 2. 1 Influence of ethanol on adsorption capacity10C increment. The ramping continued until 250oC wasreachedThe isotherm data were fitted to Langmuir and Fre-In order to determine the total organic carbon(Toc) undlich models. The R values for the Freundlichpresent in soils used in this study, bentonite and kaolinisotherms were consistently higher than the values fora TOC analyzer (5000A, Shimadzu, Japan) with a solidthe Langmuir isotherms, and therefore, the Freundlichsample module was used. The analyzer was set at 900oC, model is used to analyze the data. The adsorption capacitya temperature at which all the organic matter was oxidized kF, and Freundlich constant, n, determined by fitting theand measured. Prior to analysis of the sample, the instru. experimental data to Freundlich model given in Eq.(I)ment was calibrated using 0, 1, 2.5 and 5 mg of glucose are presented in Table 1. The results indicate that higher(40% carbon)and 0, 3.53, 8.83, 17.66 mg of Na2 CO3 thatethanol concentrations resulted in lower adsorption capaccontains about 0, 0. 4, 1 and 2 mg of carbonity of benzene and toluene for bentonite and kaolin used inThe particle size of bentonite and kaolin was analyzed this study. As ethanol content increased from o to 50%, thesing a Particle Size analyzer(PSA LS13320, Beckman adsorption capacity for benzene and toluene on bentoniteCoulter,USA). The pump speed and run time of the decreased by 85% and 99.5%, respectively. For benzeneanalyzer for bentonite and kaolin were 35% and 93 sec and toluene adsorption on kaolin, the adsorption capacityrespectively. The obscuration was around 9% for bentonite decreased by 86.5% and 98.2%, respectively, as ethanoland 11% for kaolin. The particle diameter of bentonite and content increased from 0 to 50%kaolin was calculated by taking the mean median and mode ge kFecllof the results obtainedwhere, ge(ug/g)is the mass of adsorbate(target BTEX1.2 Materialscompound)per mass of adsorbent(soil) at equilibrium, kFAll chemicals and reagents used in this study were (ug"/(" g)represent the Freundlich adsorption capacityof analytical grade. Reagents used in this study were at equilibrium, Ce(ug/L) represents the concentrationdistilled deionized water, and ethanol (200 proof, of the target BTEX compound at equilibrium and nHPLC/Spectrophotometric grade) from Sigma Aldrich, a Freundlich constant, which measures the affinity ofUSA. Target compounds(benzene, toluene, and ethylben- the adsorbate for the adsorbent surface. the greater thezene)used in this research were purchased from Restek adsorbate affinity for the surface, the lower the value of nCorporation, USA. The purities for benzene and toluene and the concentration required to establish high adsorbentare 99.9% and 99.8%0, respectivelyloading, resulting in a highly favorable isotherm.The soils used were bentonite, purchased from AlfaAdsorption takes place in physical, chemical or electroAesar, USA, and kaolin, purchased from Arcos Organics, static interactions( Voice and Weber, 1983 ) Since benzeneUSA. Comparison of bentonite and kaolin show that and toluene do not have a charge, electrostatic forcesbentonite has 0.75% TOC content compared with 0.032% usually do not play a role in the adsorption. The decreasein the adsorption capacity from 0 to 50% ethanol contentTable 1 Adsorption capacity for benzene and toluene onto bentonite and kaolin in the presence of varying concentrations of ethanolkF(ug+/(Lg))Freundlich constant nfraction(%)BenzeneTolueneToluene360±0.40191±0.16l.19±0.02099±001200±0.281.19±0.241.1±0.061.01±0001.79±0.49094±0.200.99±009096±0.0825143±0.53009±0.03103±0080.75±0030.54±0.131.17±009005050026±0.040.13±001m7+0.031.00±003022±0.04005±0.02中国煤化工092±0070.13±0.02004士0020.78±0.15008±0.02001±000CNMHG004±0020002±000093±0.15062±003Data represent mean s one standard diviation from duplicate data.Journal of Environmental Sciences 2011, 23(11)1865-1872/Temesgen Aroma et al.VoL 23can be explained by the cosolvency effect. In the presence compounds.of ethanol, there is a decrease in the polarity of water From Eq. (2), it can be concluded that the adsorption(Da Silva and Alvarez, 2002). Hydrophobic compounds, capacity for hydrophobic compound in water-ethanol sys-such as benzene and toluene, are highly non-polara tem, kF, decreases as the fraction of ethanol increases. Bydecrease in polarity of water can radically alter the stability plotting In(F/kF)vs. fe, the cosolvency power of ethanol,of molecules in a solution( Voice and Weber, 1983). The 0, can be determined. The graph will yield a straight linedecrease in adsorption capacity with increase in ethanol with a slope of o. Figure 1 shows the plot of In(F/kF)VS.content may also be attributed to the swelling effects that f for benzene and toluene.cosolvents have on soils(Brusseau et al., 1991). Adsorp The data show that the cosolvency power of benzenetion is dependent upon the surface and internal regions (3.99)is lower than toluene (12.36)for ethanol withof organic matter in soil(Brusseau et al., 1991). When bentonite. Similarly, the cosolvency power of ethanolan organic solvent, such as ethanol, is introduced into a with kaolin is in the order benzene (4.58)< toluenesolution with soil, such as bentonite or kaolin, it causes (9.07). These results are expected because of the directthe soils to swell. This swelling increases the thickness relationship between the log Kow and the cosolvency powerof the internal regions, which decreases the surface area (Corseuil et al, 2004). The log Kow for benzene and tolueneto volume ratio(Brusseau et al., 1991), preventing the is 2.13 and 2.69, respectively. In addition, the cosolvencyadsorption of benzene and toluene onto the organic matter power was found to be directly proportional to the logKoeof the soilsand is inversely proportional to the water solubility of the2.1.1 Cosolvency powertarget btEX compounds tested in the current study (table2)The cosolvency power represents a hypothetical par-tition coefficient of hydrophobic compounds between a 2.2 Comparison of adsorption capacity for bentonitecosolvent and water. The cosolvency power of ethanol withand kaolinrespect to the target bTEX compounds can be modeledFigure 2 represents the adsorption capacity of bentoniteby the log-linear cosolvency equation(Nkedikizza et al., and kaolin for benzene and toluene for both chemicalslarger adsorption capacity values are observed for benTable 2 Correlation between cosolvency power(o)and logKo(2)log Koe, and water solubility(Cs)for benzene and tolueneTarget BTEX Soillog Kow log Koe Cs(mg/L)owhere, kF (ug/+/(Ln. g)) is the Freundlich adsorption capacityfor target BTEX compound in a water-ethanol BenzeneBentonite 2.131.91780solution, while kF(ugt/Ln. g))is the adsorption capacityKaolinTolueneBentonite 2. 69in water in the absence of ethanol(0% ethanol fractionKaolinf). o is the cosolvency power for the target BtEX0.689y=-1236xR2=099旨-4.800040.506-8中国煤化工50LCNMHGFig. 1 Plot of In(F /kr)vs. fe (a)benzene with bentonite; (b)benzene with kaolin; (c)toluene with bentonite;( d) toluene with kaolin.Modeling the influence of ethanol on the adsorption and desorption of selected BTEX compounds on bentonite and kaolina Bentonite目 Kaolin3.020≌1.00.100.2500.0L≡0.05Fig. 2 Adsorption capacity of bentonite and kaolin for: (a) benzene and b)toluene.tonite at all ethanol-water content considered in this study, sorbed to soil can be described usingeunaliIcwhich has a higher percentage of organic carbon(0. 75%6), model given in Eq.(1). Rearranging Eq. (1)and solvingand smaller adsorption capacity values are observed in for Ce results in Eq.(4). Similarly, the expression forkaolin, which has a lower percentage of organic carbon concentration of the target BTEX compounds in a solution(0.032%). These results are expected because the organic during desorption can be given by Eq (5)content is a critical factor in the adsorption capacity of asoil. In general, soils with higher organic carbon contenthave a higher adsorption capacity for nonionic organic C= gecompounds than soils or clay with lower organic carboncontent(Bartelt-Huntet al., 2003). Another important vari-able in adsorption is the particle size. There is an inverserelationship between the particle size and the adsorption c=9capacity(Voice and Weber, 1983). A smaller particle sizekEoffers a larger surface area for adsorption of compoundsFrom an analysis on the particle size distribution, bentonitewhere, q(ug/g)is the mass of adsorbateBTEXhas smaller size particles than that of kaolin. The D50 and compounds)per mass of adsorbent(soil)attime t(sec)during desorption. Substituting the expression givenD90 for bentonite are 4.44 and 21.08 um, respectively. for Ce in Eq (4)and for C in Eq. (5)into Eq(3),resultsFor kaolin, the D50 and D90 are 6.86 and 35.52 um, in Eg (6). Multiplying Eq(6)by (F/qe)gives Eq (7)respectively.The desorption of a given constituent from soil can be Differentiation of the right hand side and re-arrangementgiven by the first-order kinetics(Eq (3)):esults in Eqka(ce-O(3)(2)where,ka(sec-1)is the desorption rate. A number of df=ka//- a(6)kEstudies have reported that desorption rate from soils andsediments is biphasic in nature and consist of an initialrapid release of chemicals from liable sites that occursover a few hours or days followed by a much slower drrelease from non-liable sites which can take months oryears( Carroll et al., 1994; Pignatello et al., 1993).Inthe current study, the desorption studies are conducted for ( aeight days, and therefore, the ka value in Eq (1)is assumed(如)(to represent the release the target BTEX compounds fromthe liable sites. C(ug/L)represents the concentration of thetarget BTEX compound in a solution at a given time t. The 2.2.1 Method of solution to the desorption equationexpression for C and Ce can be obtained from isotherm Equation( 8)is used to predict the desorption of targetmodels, such as Freundlich and Langmuir. In the current BTEX compounds, in terms of q/qe, from soil for givenstudy, the experimental data from isotherm studies were desorption rate(ka), Freundlich constant(n), and ratio offitted to both models and the r values for the Freundlich adsorption capacity at any time and equilibrium(kF/kF.model were consistently higher than the R values for the The equation is中国煤化 Tuation (ODELangmuir isotherm, and therefore, the Freundlich isotherm with known inin to the Odeis usedis obtainedCNMHG ODE SolverAt equilibrium, the relationship between the target available in MATLAB, version 7.7.04.471. Ode15s is aBTEX compound concentrations in a solution and ad- stiff and a variable-step solver based on the numericalJoumal of Environmental Sciences 2011, 23( 11)1865-1872/Temesgen Garoma et al.differentiation formula. The variable-step feature allowsode15s to control error by varying the step size duringthe simulation, reducing the step size to increase accuracywhen a model's values are changing rapidly and increasingthe step size to avoid taking unnecessary steps when themodels values are changing slowly The ka and kF values gk,=10-sec-e estimated by fitting experimental data obtained fromk,="desorption studies to model predictions using lsqcurvefit,an optimization function available in MATLABn=0.99kF/kF= 0.902.2.2 Sensitivity of the desorption modelSensitivity of the desorption model to kd, n, and kF/kFe0.85is investigated. mhe 2, n=0.99, and kF/ kFe=0.90.Thefollowing values are used as basevalues: ka= 10-5secsensitivity of the desorption model in relation to variationof selected parameters is tested while keeping constant095other parameters at base valuesThe influence of desorption rate, kd, on q/qe for ka valuesof 10-7, 10-6, 10-,10-4, and 10-3sec- is presented in sn=080n=0.90Fig 3a. The lines in the figure show successive increases indesorption of the constituent from adsorbent surface withn=10-sec-lncreasing ka, which is expected. However, increasing thekP/kFF 0.90Freundlich constant, n, from 0.8 to 1. 20 did not affect des-0.85orption of adsorbate from adsorbent(Fig. 3b). This result isin agreement with the findings of Carter et al (1995 )who 1.00evaluated the influence of n on site energy distribution forpreloaded adsorbents. They showed increase in n causeda decrease in the width of energy distribution site andincrease in site energetics, resulting in no change in overallenergies of adsorption. Figure 3c illustrates the influence gk八ke=0.85of ratio adsorption capacities during desorption and atequilibrium on desorption from the adsorbent surface. Theresults in the figure show successive decrease in the ratio080}k6=105of ke/kFe increased desorption.n=099The desorption data were fitted to the ode given inEq.( 8)and the results are presented in Fig 4 for benzeneand toluene desorption from bentonite with varying con-Fig 3 Predicted effect of ka(a), n(b), and kF/kFe(c)on the desorptioncentrations of ethanol. The results in Fig. 4a-c represent of adsorbate from adsorbent.the desorption for benzene from bentonite to a water-ethanol solution with 0, 10%, and 25% of ethanol by proportional to the logka( Fig. 5). Like cosolvency power,volume,respectively. While the desorption of toluene from the desorption rate of benzene is less than that of toluenesurface of bentonite to a water-ethanol solution with 0, On the basis of the properties of benzene and toluene, it25%, and 50% ethanol by volume is presented in Fig. can be inferred that desorption rate is directly proportional4d-f, respectively. The experimental data and the model to logKow and log Koe and is inversely proportional to thepredictions fitted with R2 values in the range of 0.71 to water solubility098In summary, this study investigated the influence ofThe results indicate that as the ethanol content increased, ethanol on the adsorption capacity and desorption kineticsthe desorption rate, kd, increased for both benzene and of benzene and toluene on bentonite and kaolin. Fortoluene For benzene, as ethanol content increases fromo both soils, the adsorption capacity for the target BtEXto 25%, the ka value increased from 1. x 10-5 to 1.7x10- compounds deceased as the ethanol content increased. Thesec-, close to one order of magnitude increase. Similarly, cosolvency power, a hypothetical partition coefficient ofas ethanol content increases from 0 to 50% ethanol, the hydrophobic compounds between a cosolvent and water,ka value for toluene increased from 4.7x10-to 40x10 for benzene and toluene with respect to ethanol is directlysec-. This is a significant finding for sites contaminated proportional to log Kow and logKoe and is inversely propowith BTEX compounds and where adsorption to soils tional to water solubility. The desorption rate of benzeneis used as a natural attenuation process. The release of and toluene fre中国煤化工 out one order ofethanol at such a site may remobilize BTEX compounds magnitude as th25% ando tothat has already been adsorbed to soils.50%, respectivaCNMHGthe desorptionIn addition, ethanol fraction, f, was found to be directly rate, ka, is directly proportional to logKow and logKoNo.11Modeling the influence of ethanol on the adsorption and desorption of selected BTEX compounds on bentonite and kaolin1871O ExpModel1.00No ethanolNo ethanol099BenzeneR2=0.95090960910951.0025% ethano09825% ethanolBenzeneToluene097R2=0.8609609609509400.9309110050% ethanol50% ethanol098BTolueneR2=0.71R2-0098097095Flg. 4 Desorption of benzene and toluene from bentonite to a solution containing varying concentration of ethanol.ReferencesBartelt-Hunt S L, Burns S E, Smith J A, 2003. Nonionic organic3.5y=387x-4.81solute sorption onto two organobentonites as a function ofR2=0.94organic-carbon content. 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