Removal of VOCs from Sudden Polluted Raw Water by Air Stripping

456Jourma/ of Donghua University(Eng. Ed.) Vol. 28, No. 5 (2011)Removal of VOCs from Sudden Polluted Raw Water by Air StrippingLIN Ming li(林明利)'，YIN Xiao-tao(殷晓桃)*, CUI Fu-yi(崔福义)' , ZHAO Zhi-wei(赵志伟) , JIANGQi(姜琦)State Key Laboratory of Urban Water Resource and Environment, School of Municipal and Environmental Engineering, Harbin .Institute of Technology, Harbin 150090, China2 Pan-China Construction Group, Beijing 100070, China3 Environmental Development Center of Ministry Environmental Protection, Bejing 100029, ChinaAbstract: Volatile organic compounds ( VoCs) are widely used inoften suffers from the membrane stability. Air stipping, anvarious industrial processes and generate water pollutions. VOCsefctive method in ontrolling VOCs in drinking water treatment,removal from raw water is an important task for waterworks tocan be used as an urgent treatment technology for waterworks dueguarantee drinking water security. The removal of VOCs such aschlorobenzene (CB) and ethylbenzene (EB) from raw water by airto its high flexibility, low cost，and relatively little effect on thestripping was investigated under various conditions ，including thefollowing treatment process. And it can be operated in thevariation of temperature (5-30C), pH (3. 5-10.5), and air/ waterditributing well, which is applicable to most waterworks in China.ratio ( 10-60). The air stripping removal efficiency of VOCsThe VOCs stipped can be adsorbed by adsorbers, such as activateddecreased with VOCs concentration delining in water. And Henry'scarbon, membrane , and polymeric resin[13, 14] .law constant was demonstrated as an indicator of the estimation ofVOCs removal eficiency for air stripping. The efects ofThe successful design of air stripping systems in emergencytemperature and the ratio of air and water were found to play aconditions requires an understanding of the physicochemical andgreat role in VOCs removal, but the effect of pH seemed to bemass transfer mechanisms that affect the removal efficiency. Innegligible. This study demonstrates that air stripping provides agencral, ar-stripping is a process where VOCs adsorb to thepromising opportunity in removing VOCs in drinking watersurfacc from the liquid, and then the adsorbed solute moleculestreatment, especially for the relatively high concentration of VOCs.find ways to evaporate into the air. The bulk concentration ofKey words: volaile organic compounds' ( VoCs); air sripping ;emergency water treatment; Henry's law constant ( H) ; removalthe pollutant is gradually reduced. The rate of this purificationCLC number:TU991.2Document code:Aprocess is governed by the rate at which mass transfer occursArticle ID: 1672-5220( 2011 )05-0456-04from the liquid to the liquid-gas interface and/or the solutemolecules evaporate from the interface to the air[1s]. Thediffusional mass transfer of VOCs from the liquid to the liquid-Introductiongas interface has been recognized as a key factor in determiningthe overall rate of VOCs removal[16]. However， possibleVolatile organic compounds ( VoCs ) are organic chemicalparameters of air-stripping process for emergency drinking watercompounds, in the boiling point range of 50 260 C according totreatment have been largely ignored in the consideration.the World Health Organization. And some VOCs are widely usedIn this study, experiments were conducted to establish a; chemical materials， such as trihalomethanes， chlorinatedbasic understanding of air stripping efciency for VOCs withsolvents, and other volatile constituents'11. The improper disposalrespect to temperature, pH, and the ratio of air and water and toof VOCs leads to serious water pollution and the necessity ofobtain data for system designs. Chlorobenzene ( CB ) andfinding effective treatment technologies for its removal. This isethylbenzene ( EB) were selected as the target organicparticularly true when the water is utilized as drinking watercompounds, for they had been detected frequently in lots ofbecause VOCs have a potential risk to human health.surface water(17].The removal of VOCs is an intractable problem forwaterworks, especially during a sudden pollution episode suc1 Experimental; VOCs spill in raw water. The cleanup of the suddencontaminated waters is often difficult and expensive, and it1.1 Materialsusually involves treating dilute VOCs solution. Some waterAnalytical grade chlorobenzene ( CB，minimum 99. 8%treatment processes have been developed and put into pilot topurity, Tianjin Fuchen Chemicals Inc. ，China) and ethylbenzenesolve this problem. The treatment methods for removing VOCs( EB , minimum 99. 8% purity, Shanghai Xingao Chemicals Inc. ,in water include air stripping, adsorption, advanced oxidation,China) were chosen as the model contaminants. The water used inditillation , anaerobic/ acrobic biological treatment, bioreactor,the experiment was raw water, and the water quality parameters areand supported liquid membranel2 -4These methods haveshown in Table 1. A magnetic strrer was used to ensure fullsome shortcomings and limitations.dissolution of CB or EB. Freshly prepared solutions oAdsorption is economical only at low VOCs levels due to thecontaminants were used for each experiment.high cost[5,6] of the adsorbents and their regeneration ofTable1_ Raw water characteristicsreusel7.8l. On the other hand, dillation is economic only ;Raw waterUVss/ DOCSUVAvTurbidity/higher VOCs levelsBiological treatment is a clean method, but itcm-1(mg:L-')(L.m-1.mg~) NTUis time consuming and effective only at low VOCs levels. Therebybiological treatment can not meet the demand of water pollutionMopanshan7.7 0.186.12.950.98emergency rapid response. Advanced oxidation is efficient forreservoirspecific compound, but it may form new products that are moreharmful than the original one[9]. Although supported liquidNote: DOC中国煤化工”DOCmembrane could possess high flux using hollow fibers[7,10-12], it .TYHCNMHGReceived date: 2011-04-19Foundation iems: National High Technology Rescarch and Development Program of China(863 program) (No. 2008AA06A414) ; Major Scienceand Technology Proyran for Water Pollution Control and Treatment, China ( No.2008ZX07421 -003 )* Correspondence should be adressed to LIN Ming-li , E-mail: hhdxlml@ 163. comJourmal of Donghua University(Eng. Ed. ) Vol. 28, No. 5 (2011)4571.2 Analysisfrom the interface to the bulk of the air phase. A mathematicCB or EB in water sample was extracted by an automaticmodel was established to describe the air stripping process inPurge and Trap Sample Concentrator 4551A system (O. I.static mode[16]. In this mode， the experimental systemAnalytical TX, USA) on-line coupled with the GC-MS system andconsisted of a known volume of water through which a constantequipped with a Tenax 4660 adsorbent trap (O. I. Analytical).flow of air was sparged under conditions such that the soluteThe samples placed in the vial were purged under theconcentration in the exit gas was essentially in equilibrium withfollowing conditions: 25 mL purge vessel ( VOCARB 4000the aqueous concentration. A mass balance for the solute givesTrap) and 40 mL●min -1 ultra pure helium purge flow; purge-the transfer rate as[16]:ready temperature: 30C; purge time: l1 min; desorb time:- VdC/dt =HGC/RT4 min; desorb temperature: 180C; bake time: 10 min; bakewhere G is the gas flow rate (m3●h-1); V is the volume of thetemperature: 230 C.The following GC-MS ( thermo, Trace _DSQ ) conditionliquid (m2); H is Henry's law constant (Pa. m3●mol-1); Cwas used for the analysis of all water samples: ultra pure heliumis the solute concentration in the liquid phase (mg. L-1); Risflow_ l. 2 mL/min， split ratio 20: 1，injector temperaturethe gas constant(J●mol-1● K-'); Tis the system temperature180 C, column (DB-5, 30 m x0.25 μmx0. 25 mm) 50 C(K); and 1 is time ( min). This equation can be integrated fromholding 5 min,MS condition-ionization mode was electroninitial conditions wheni =0 andC=Co to give:ionizationEI)interface temperature 220 C，ion sourceC/C。=e -(HC/6OVRT)1.(2)temperature 200 C，detector scan (3 scan ●s-1 ) modeFrom a plot of the solute aqueous concentration vs. time35 -300u, EI 70 eV.(Fig.2), H can be deduced by fiting the experimental data1.3 Experimental proceduresaccording to Eq. (2). Table 2 gives the fitted results and theirThe VOCs removal experiment from contaminated raw waterstandard deviations together with the valucs of adj-R2. Theusing air stripping was carried out in a glass column with 1.4 Lpreviously reported values obtained in distilled water by aivolume as shown in Fig. 1. Air was introduced into the bottomstripping are also tabulated for comparison in Table 218,19].of the stripping vessel through a microporous diffuser and thenThe agreement between experimental data with the ftted resultsdiffused into micro air bubbles. The system was maintained awas satisfactory with the standard deviation values of less than20 C. The VOCs stripped were adsorbed by activated carbon.5.8% and adj-R2 values of more than 0. 99. In the otherThe air stripping experiments were performed in static anddynamic modes, respectively. The static mode of operation wassystems, an acceptable level of precision corresponding to achosen to investigate the air stripping kinetics and the effects ofstandard deviation of less than 6% was achievedl16]. The fittedresults of H were 346 and770 Pa. m3●mol-1 for CB and EB,temperature and pH on the removal of VOCs. In this mode, therespectively. However, these estimatcs were not in excellentcontaminated raw water with a constant volumes was withoutagreement with the reported literature values. Departure frominflow or outflow in the column. The volumes of CB and EBcontaminated water were 1.3 L and 1. 52 L, respectively, andprevious studies may be caused by (1) different water quality,(2) adsorption and absorption of VOCs onto impurities in thethe air flowrate was 0. 158 m3/h. The dynamic mode waswater, (3) different design of air stripping system, and (4)experimentally implemented to consider the removal of VOCs bydifferent CB and EB concentrations used in the experiments.air stripping at different ratio of air and water, and operatedwith the hydraulic retention time of4 -5 min.↑Gas exitCCB+ Water outleto2-0o。00 F/min[. EB」Air inletWater inletFlow meterFig.1Schematic process for VOCs removal from contamninatedraw water by air stripping in glass colum with diameter.2 tof 54 mn and height of 630 mm2 Results and Discussion00中国煤化工2.1 Air stripping kineticsYHCNM H G*Fig.2 Air stripping kinetics for CB (a) and EB (b) from rawThe air stripping process is generally accepted as consistingwith the initial concentrons of6. 8 a.4mg/of the solute diffusion from the bulk of water to the interface,WwaterwththeuntuaL, respectively ( air flowrate 0. 158 m'/h, solutionfollowed by transfer across the interface, and finally diffusiontemperature 20C, pH 7.7)458Joumal of Donghua University(Eng. Ed.) Vol. 28, No. 5 (2011)Table2 The fited results of Heny's law consats for cB and EBstripped from raw waler at 293. 15K and comparison with10SSS3EBo)]litera. resCompoundsHa/Adj-R2(Pa. m3●mol-1)Pa.m3●mol-')C389士130.9949346ER790士460.9903Figure 2 shows that as the solution concentrations of CBnd EB decrease, the removal eficiency dereaces. Thiexperimental result confirms air strpping process is applicable to02031remove rlatively high concentration of VOCs in considerationTemperaur/cof economy. Given that adsorption by powdered activatedFig.3 Removal of VoCs at dfene tempernture by aircarbon (PAC) is assumed as an emergencstripping in static mode ( air stripping time 4 min ,treatment technology used at low contaminated levels[20,21], itair flowrate 0.158 m*/h, pH7.7)is for this reason that air sripping fllowed by PAC adsopion2.3 Effect of pHnay be an eficiency and economic emergency treatmentFigure 4 ilustrates the effect of solution pH on CB and EBtechnology for waterworks to cope with sudden pollution ofremovals from raw water by air strpping. The removal of aboutVOCs. Figures 2(a) and (b) provide a trend of increasing H80% is observed under different solution pH, which indicateswith decreasing the stipping time of approach to equilibrium forthat pH has negligible effet on the removals of both CB andEB, from pH3.7 to 10. 3. Indeed, CB and EB are non-ionicequno fstar, Wanri; fyecees wih vile Hwil aand non-charged compounds in solution,hvsicalthat voCs withlower H haseither their physicala stronger interaction with water molecule. This resultand chemical characters nor mers nor mass transfer rates are influenced bysolution pFpH.Thereby the removals of both CB and EB areexperimentally confirm the importance of the Henry's lawsoluconstant as a first indicator of the estimation of VOCs removalsimilar at different solution pH under the same experimentalconditions.efficiency for air stripping22].2.2 Effect of temperatureFigure 3 represents the infuence of temperature on thexo 0σ9_→∞→89σ≈8=σ0removaalsofCBandEs by air stripping in static mode. Thepercent removals of CB and EB show similar behavior, i. e. ,-0- EB-0- CBincrease with increasing temperature within the examined range.Previous suie(22) have shown that increasing temperaturecould acelerate VOCs mass transfer from liquid phase to air.phase to air.According to Lincoff[23], increaing temperature at a givenratio of air and water caused the Heny's constant arising, andyielded higher stipping effciency. The mass transfer rate for airstripping were demonstrated to be temperature dependent(24].Comparing Figs.3 (a) and (b), it can be noticed that thepercent removal of CB is always smaller than thatof EB in theFig.4 Removal of VOCs at difrent pH by air stipping in staticerange of tentemperature under thesame experimental condition.mode (air stipping time 4 min, air flowrate 0.158 m*/h,This result demonstrates that CB is more difficult to strip thantemperature 20 C)EB, which may be that CB has a stronger molecular interaction2.4 Effect of air/ water ratiowith water molecule.Figure 5 shows the removals of CB and EB with changes inair/ water ratio. It typically represents the behavior of the extentof contaminants removal in aqucous phase for air strpping2ZCBa)systems with an initial rapid increase in VOCs removal followedby a slower change in the VOCs removal and finally with VOCsremoval remaining fairly constant.stant. These experimental valuessuggest a general trend that an increase in the percent removalsof both CB and EB with an increase in the air/water ratio. Thehighest removal percentage crresponds to the highest ait/waterratio in all cases. Other authors(. 25., 26] have reported similar出4relationships ( VOCs removal vs. air flowrate) for air srippingof VOCs from water using conventional technologies. It hasbeen demonstrated by Chao et al. (25] that VOCs mass transfercofficient values would increase because of the increase of airflowrate in the “ mass transfer zone". Furthermore, it is30observed tharn ntirnf pir and water exerts aTemperature/Cgreater efec中国煤化工val than that in Bremoval (FiFovals of CB and EBare close to9 N MHGr and waler are over50 and 30, respectively. And the removal of CB are alway'slower than that of EB at a given ratio of airir and water, whichalso indicates CB is more dfficultdifficult to strip. This result could beJoumal of Donghua University ( Eng. Ed.) Vol. 28, No. 5 (2011)459explained by the fact that an increase in Henry's law constant[ 4] Freitas dos Santos L M. Biological Treatment of VOC-Containing(TatTable2) could result in a general increase in the extent ofWastewaters: Novel. Extractive Membrane Bioreactor vs.VOCs removal, which is expected from theory and previousConventional Aerated Bioreactor [ J ]. Process Safetv anEnvironmental Protection, 1995 , 73(3): 227-234.studiesl22]. In addition, the difference in removals of CB and[ 5] JuangR S, Wu F C, Tseng R L. Characterization and Use ofEB is found to decrease as the air/ water ratio increases.Activated Carbons Prepared from Bagasses for Liquid-PhaseAdsorption[ J]. Colloids and Surfaces A: Physicochemical and(Engineering Aspects, 2002 , 201(1/2/3): 191-19922[6] Pan B C, Xiong Y, Su Q, et al. Role of Amination of aPolymeric_ Adsorbent on Phenol Adsorption from AqueousSolution[ J]. Chemosphere, 2003, 51(9): 953-962.[ 7 ] Juang RS, LinS H, Yang M C. 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