Adsorptive removal of hydrophobic organic compounds by carbonaceous adsorbents: A comparative study

Availableonlineatwww.sciencedirect.comJOURNAL OFs Science DirectENVIRONMENTALSCIENCESJoumal of Environmental Sciences 2012, 24(9)1549-1558Njesc. ac cnAdsorptive removal of hydrophobic organic compounds byy carbonaceousadsorbents: A comparative study of waste-polymer-basedcoal-based activated carbon, and carbon nanotubesFei Lian, Chun Chang, Yang Du??, Lingyan Zhu2, Baoshan Xing, Chang Liu21. Centre for Research in Ecotoxicology and Environmental Remediation, Institute of Agro-emvirorumental ProtectionMinistry of Agriculture Tianjin 300191, China2. College of Emvironmental Science and Engineering, Tianjin Key Laboratory of Ermvimnmental Remediation and PoControl, Key Laboratory ofPollution Processes and Emvironmental Criteria, Ministry of Education, Nankai University Tianjin 300071, China3. Department of Plant, Soil and Insect Sciences, University of Massachusetts, Amherst, Massachusetts 01003, USAReceived ol November 2011; revised 18 January 2012: accepted 02 March 2012AbstractAdsorption of the hydrophobic organic compounds(HOCs)trichloroethylene (TCE), 1, 3-dichlorobenzene(DCB), 1,3-dinitrobenzene(NB)and Y-hexachlorocyclohexane(hCh) on five different carbonaceous materials was compared. The adsorbents included threepolymer-based activated carbons, one coal-based activated carbon(F400)and multiwalled carbon nanotubes(MWNT). The polymerbased activated carbons were prepared using KOH activation from waste polymers: polyvinyl chloride(PVC), polyethyleneterephthalate(PET) and tire rubber (TR). Compared with F400 and MWNT, activated carbons derived from PVC and PET exhibited fast adsorptionkinetics and high adsorption capacity toward the HOCs, attributed to their extremely large hydrophobic surface area(2700 m2/g)andhighly mesoporous structures. Adsorption of small-sized TCE was stronger on the tire rubber-based carbon and F400 resulting fromthe pore-filling effect. In contrast, due to the molecular sieving effect, their adsorption on HCH was lower. MWNT exhibited the lowestdsorption capacity toward HOCs because of its low surface area and characteristic of aggregating in aqueous solutionKey words: polymer waste; activated carbon; hydrophobic organic compound; adsorption mechanismDOI:10.1016S1001-0742(11)609844Introductionthan that derived from coconut shell ( hayashi et al., 2005)AC with high microporosity and large surface area(up toWith its high chemical stability, extensive porosity and 4000 m"/g) was obtained by heat treatment of poly(2,6-variability of surface properties, activated carbon(AC) dimethyl-1, 4-phenylene oxide)(Yuan et al. 2008)has found widedespread application in many areas such Due to their unique properties(e. g, large surface areaas capacitors, catalyst supports, battery electrodes, and and uniform pore structure), polymer-based ACs have beenenvironmental remediation(Derbyshire et al., 2001). Tra- investigated for removal of organic pollutants, heavy met-ditionally, lignocellulosics or coals are the most used raw als, and Co2(arenillas et al., 2005; Hayashi et al, 2005materials for the production of ACs(Bansal et al., 1988). Manchon- Vizuete et al., 2005; Murillo et al.,2005:KartelWith the increasing amount of polymer waste around the et al., 2006; Sych et al., 2006; Alexandre-franco et alworld, there has been a growing interest in preparing ACs 2007; Almazan-Almazan et al., 2007 ) However, most offrom synthesized polymer waste such as polyacrylonitrile, these studies focused on the adsorption of organic vaporswaste tires and used polymeric resin(Park and Jung, 2002; and ionic dyes(Hayashi et al., 2005; Murillo et al, 2005Ryu et al., 2002; San Miguel et al., 2003; Skodras et al., Kartel et al., 2006: Sych et al, 2006; Almazan-Almazan2007; Long et aL, 2008). ACs derived from synthesized et al., 2007). A few studies examined the adsorptionpolymers usually exhibit superior structural characteristics mechanisms of organic compounds with polymer-basedcompared with those from traditional feedstocks (i. e, ACs in aqueous solutions. A previous study reportedbiomass and coals )mainly due to their uniform compo- that the micropores and acidic O-containing groups ofsition, high carbon yield and low ash content( Hayashi et waste-polal., 2005; Vazquez-Santos et al., 2008). For example, it is the adsorpt中国煤化工-ing et al., 2009)reported that ACs derived from waste polyurethane exhibit Another stucCNMH Gof phenolic com-higher surface area and more evenly distributed pore size pounds(phenol, 3-aminophenol and 3-chlorophenol)on apolymer-based AC with a commercial sample, suggestingCorresponding author. E-mail: zhuly nankai. edu.cnJoumal of Environmental Sciences 2012, 24(9)1549-1558/ Fei Lian et althat the higher adsorption by polymer-based AC could stream at 850 C. The time of activation was optimizedbe attributed to its more basic groups(Yenisoy-Karakas obtain a 50% burn-off for all three samples for comparisonet al., 2004). However, little has been done to system- (Bota et al, 1997). It took 120 min in the case ofatically investigate the adsorption behaviors considering and 90 min for PVC and PET. The obtained samples werethe physicochemical properties of both adsorbents and washed with 1 molL HCl solution and distilled water toadsorbates. The roles of surface area, porosity and surface remove the ash and decomposed fragments and then driedchemistry of polymer-based ACs as well as molecular size at 110oC for 24 hr. The obtained solids were ground andand configuration of adsorbates are not well understood. passed through a 150 um sieve. They were labeled asOn the other hand, little research has been conducted APVC, APET and ATR, respectively. F400, supplied byto compare the adsorption of hydrophobic organic com- Calgon Carbon Co. Tianjin, China), was pulverized andpounds(HOCs)by polymer-derived ACs with other typical passed through a 150 m sieve MWNT was purchasecarbonaceous materials such as coal-based ACs and carbon from Nanotech Port Co. Shenzhen, China) the size of thenanotubesouter diameter ranged from 10 to 30 nm and the length ofIn the present study, three polymer-based ACs using carbon nanotubes ranged from 5 to 15 umwaste polyvinyl chloride(PVC), polyethyleneterephthalate1.3 Characterization of adsorbents(PET) and tire rubber (tr) were prepared with KOHactivation Surface area, porosity, and structural properties The surface chemical composition of the adsorbents wasof these ACs were characterized in detail. The adsorption determined using an X-ray photoelectron spectrometerof HOcs on these polymer-based ACs, a commercial coal-(XPS)(VersaProbe PHI 5000, USA). Ash contents werebased AC, F400 and MWNT, were compared to investigate measured by combusting 5 g of the samples in a mufflethe correlation of adsorption with the properties of both furmace at 800oC for 4 hr. Nitrogen adsorption-desorptionadsorbates and adsorbents. Four HOCs with different isotherms were measured at 77 K by an AUTOSORBphysicochemical properties, i. e, trichloroethylene (tCe), instrument(Quantachrome, USA). High-resolution trans1, 3-dichlorobenzene(DCB), 1, 3-dinitrobenzene (DNB) mission electron microscopy(HR-TEM)images wereand y-hexachlorocyclohexane(HCH), were selected to obtained using a transmission electron microscope(EMexamine the effect of molecular properties on adsorption. 2010FEF, JEOL, Japan) with optimal resolution of 2 nm atThe results would also be useful to assess whether ACs 300.derived from polymer waste are promising adsorbentscompared with conventional coal-based ACs and carbon1.4 Adsorption experimentnanotubesThe adsorption experiments were conducted at 25+ICin40 mL amber vials equipped with Teflon-lined screw caps1 Materials and methodsFor adsorption kinetics, 10 mg of adsorbent was added ina 40 mL vial, followed by background solution and solutes1.1 Raw materials and adsorbates(spiked at 10 mg/L). The background solution contained0.01 moLL CaCl2 in deionized distilled water and 200Tire rubber(TR)particles(size 0.5 mm) and PVC pellets mg/ NaN3 as a bioinhibitor. The vials were shaken onin the range of 1-3 mm were purchased from Honglian a rotary shaker(150 r/min)and two vials were taken off atRubber Industries(Tianjin, China). PET material was predetermined time periodsmixture was centrifugedobtained by cutting transparent plastic water bottles into at 4000 xg for 10 min. Theaqueous solution wassmall pieces(<1 cm2)using stainless steel scissors. All the extracted with hexane to measure the concentrations oftested HOCs were chromatographic grade and purchased the solutes by gas chromatography(GC)coupled withfrom ANPEL Chemical Corp. (Shanghai, China). Their electron-capture detection(ECD). A capillary column(HPselected properties are presented in Table 15, 30m x0.32 mm, J&w Scientific Columns from Agilent1.2 Preparation of adsorbentsTechnologies) was used for the separation. The mass ofadsorbed chemicals was calculated from the measured conKOH activation was used to prepare ACs from waste PVC, centration in the solution based on mass balance ControlPET and TR. Briefly, the raw material was carbonized at experiments with no adsorbents in the vials were carried600C in a nitrogen atmosphere for 1 hr. The carbonized out simultaneously. No measurable change in the solutesample was then activated by KOH (1: 2 m/m)in a nitrogen concentrations was observed in the controlsTable 1 Selected physical-chemical properties of trichloroethylene (TCe), 1, 3-dichlorobenzene(DCB), 1, 3-dinitrobenzene(DNB)andY-hexachlorocyclohexane (HCH)Organic compoundsMw(g/mol)Sw(mmolL)Kow①LL)p(g/cm)131.48.32263TH中国煤化工66×62×3674x6.7×0.2DNBCNMHG8.7×7×1.5HCH908002579x7.0×49MW: molecular weight; Sw: aqueous solubility; Kow: n-octanol-water partition coefficient; p: density; Vs: molar volume; MD: molecular dimensions.calculated from the Chem'D Program Values of MW, Sw, Kow and p were from Schwarzenbach et al., 2003.No 9Adsorptive removal of hydrophobic organic compounds by carbonaceous adsorbents:1551For the isotherm experiments, a given amount of ad- and n in the raw materials were removed, resulting insorbents(1-10 mg), 40 mL of the background solution, a significant increase in the C content of the polymerand a stock solution of an adsorbate (in methanol)were derived adsorbents(APVC, APEt and atR). Among theadded into vials to leave minimal headspace. The vials five adsorbents, APET had the lowest surface C contentwere tumbled at 3 r/min for 7 days to reach apparent (77.6%)and highest O content (21.7%), thus the highestequilibration based on the kinetics study above. Then, polarity MWNT was dominated by graphitized C(96.7%)after 24 hr to allow complete settlement of adsorbents, an and had a lowO content(2.79%). A substantial mesoporealiquot of the upper solution was subjected to GC/ECD structure was developed in APET and APVC(over 73%analysis to determine the aqueous concentrations Calibra- mesoporous), while ATR and F400 were dominated bytion curves were obtained from control samples receiving micropores and mesopores, and MWNT showed morethe same treatment as the adsorption samples but without heterogeneous characteristics. Figure 1 illustrates the poresize distribution of the polymer-based adsorbents. APVCdisplayed a more narrow distribution(2-3 nm)than APET2 Resultsand ATR. The BET surface area(SBeT) of APVC andAPET was as high as 2666 and 2831 m"/g, respectively,2.1 Characterization of adsorbentsmuch larger than that of ATR, F400 and MWNT. The HRTEM images(Fig. 2)illustrate that just like F400, theThe results of the surface elemental composition and polymer based acs mainly consisted of short stacks oftextural parameters of the adsorbents are summarized small-sized graphite sheets arranged in a highly disorderedin Table 2. The majority of heteroatoms such as O, HshionTable 2 Surface elemental composition and textural parameters of the adsorbentsAdsorbentDFT cumulative pore volume(cm /g)(dry weight based)content (m/g)(cm/g)C(%)H(%)0(%)N(%)O+NC(%)<7A<10A<20A20500A>500A9541.313.700.50004453726661.44APET7761.7621.70520.286ATR10.30.430.120.120.16F40079.13.3016.90.60.25MwNT96.70.002790.310.0320011620030.00.08Determined by X-ray photoelectron spectroscopy(XPS); b determined by N2 adsorption using the Brunauer-Emmett- Teller(BET)method;Vottotal pore volume, determined at P/P=0.97: d cumulative pore volume, derived from the Density Functional Theory(DFT)1425ATRR0.4E2↑83Pore width(nm)Pore width(nm)Pore width(nmF400会0.15MWNT2.0中国煤化工CNMHGPore width(nm)Pore width(nm)Fig. 1 Pore size distribution of the five carbon adsorbents using density functional theory.1552Journal of Environmental Sciences 2012, 24(9)1549-1558/ Fei Lian et al.ATRPVCAPET5nmF-400MWNTFig 2 Representative TEM images of the five carbon adsorbents.2.2 Adsorption kineticswhere, ge (mg/g) and qt (mg/g) are the amounts ofsolutes adsorbed on adsorbents at equilibrium and timeFigure 3 compares the adsorption kinetics of the solutesrespectively; kI(min")and k2 (g/(mg- hr)are theon the five adsorbents. For a given adsorbate a faster equilibrium rate constants of the pseudo first-and secondadsorption was observed on the highly mesoporous aPvc order models; kip(mg/(g- hro.5) is the intraparticle diffu-and APET. Taking DCB as an example, approximately68% and 62% of the adsorption was accomplished within sion rate constant and c (mg/g)is a constant that givesinformation about the thickness of the boundary layer.I hr on APVC and aPeT, while only about 38%, 41%andOnly the pseudo second-order model provided the best44%was adsorbed on ATR, F400 and MWNT, respective. fitting for all the experiment data with r >0.99(Tablely. APVC and APET have a mesoporous structure withnarrow pore size distribution (Table 2), thus facilitatingSI). For APVC and APeT, the calculated rate constant(k?)follows an order of TCE >DCB>DNB > HCH,the transport of solute molecules into the pores. In con-which inversely correlates with the molecular size of thetrast, the pore sizes of ATR and F400 are heterogeneous, adsorbate (Table 1). Apparently, the results are largelywhile carbon nanotubes tend to entangle and aggregate correlated with the porous structure of the adsorbents andinto bundles in aqueous solution, restricting the access adsorbate properties, indicating that the pore diffusionresult, diffusion of solutes to the pores of ATR, 400 and mechanism may play a dominant role in the adsorptionMWNT becomes much difficult. The results indicate that kinetics. A similar observation of HOC adsorption bythe high mesoporosity of APET and APVC makes them zeolite-templated carbon was reported in a previous studymore efficient for the uptake of HOCs in aqueous solutions Ji et al., 2009)than atr f400 and mwnt.2.3 Adsorption isothermsTo quantitatively compare the apparent adsorption ki-netics between different adsorbents, the pseudo first-order Figure 4 illustrates the adsorption isotherms of the solutes(Eq.(I)), pseudo second-order(Eq. (2)and intraparticle on the five adsorbents. The data were fitted with thediffusion(Eq (3)(Weber and Morris, 1962)models wereFreundlich model (q=KrCe ) where q(mmolkg)and ceused to fit the kinetic data by linear regression(mmolL)are the solid-and liquid-phase concentrations atadsorption equilibrium; Kr(mmol-L /kg) is the affinitFreundlich linearity index. All the(qe -)=In(qe)-kit(1) isotherms中国煤化工 reundlich model in1the tested dCNMHGfitting parameters(2) are listed in laDle sz. ror all une adsorbent/adsorbateqr kipt2+C(3) Combinations, nonlinear isotherms are displayed withNo 9Adsorptive removal of hydrophobic organic compounds by carbonaceous adsorbents1553口APVC●APET△ ATR VF400ATR> MWNT, correlating well with the ad- slightly larger-sized pores of APVC becomes relativelysorbent surface area(Table 2). Their large surface area slower as evidenced by the lower rate constant(k2)(Tableand well-developed mesoporous structure make APET and SI). In contrast, the fast adsorption kinetics of HCHAPVc promising adsorbents for removal of HOCs from observed on ATR, F400 and MWNT could be due to thecontaminated water. Even though APET exhibits much size-exclusion effect (Ji et al., 2010). Owing to the highhigher surface polarity than APVC, it displays similar microporosity of aTR and F400, HCH molecules cannotor slightly higher adsorption capacity toward the HOCs enter the micropores and the adsorption is expected toirrespective of their properties. This is inconsistent with the occur mainly on the external surface, thus shortening theobservation that surface chemistry may overwhelm pore adsorption process. It is worth noting that APVC shares astructure effects in HOC adsorption by ACs as reported similar BEt surface area and pore structure with APET,by Karanfil and Kilduff(1999). The BET surface areas however, the rate constant(k2)is much higher than APEtof APVC and APET are up to 2600 and 2800 m/g, 2- except for adsorption of HCH. This may be attributed to4 times larger than that of adsorbents used in the study the fact that APET has much higher polarity than APVCof Karanfil and Kilduff (1999). This suggests that the revealed by the surface(O+N)/C molar ratio (Table 2). Thelarger surface area of APET partly compensates for the O-containing polar groups facilitate the formation of waterdisadvantageous effect of O-containing polar groups and molecule clusters on the surface of the adsorbent, impedingenables comparable adsorption capacity relative to APVC. the diffusion of HOCs into inner pores( Chun et al., 2004)To further illustrate the correlation of the adsorption of3 DiscussionHOCS with adsorbent porosity and adsorbate propertiesadsorption data were fitted with the Polanyi-DubininThe impact of adsorbent porosity on the HOC adsorption Manes(PDM)model (Eq (4), which is applicable for bothed by surface area(Fig. 5). The normalized adsorption et al, 0 d pore-filling adsorption mechanisms(Yangcan be better illustrated by comparing isotherms normal- surface areaof TCe was much higher on ATR and F400; however,that of HCH was lower on AtR and F400, implying theimportance of structural characteristics of adsorbents andRT Inadsorbates. For ATR and F400, nitrogen adsorption results 10gq-loge+aindicate that approximate 21% of their pores are less than10 A (Table 2 ). TCE has a planar shape with molecular di- where, @(mmolkg)is the maximum adsorption capacitymensions of 6.6Ax62Ax36A(Table 1). It is suggested of the adsorbate; a and b are adsorption constants; Vs isthat planar TCE molecules can access the deep portions of the molar volume of solute; R (8.314x10-3 kJ/(mol-K)ismicropores(<7 A )in a flat form, and pores with width the universal gas constant, T(K) is absolute temperatureless than 10 A are crucial for TCE adsorption(Karanfil and and Cs stands for solubility at 20C.Dastgheib, 2004). Thus, the larger normalized adsorption The PDM model fits the adsorption data quite well inof TCE on ATR and F400 could be attributed to the the tested concentration ranges with 20.977-0999(Ta-pore-filling effect, which greatly enhances the adsorption ble S2). The surface monolayer coverage(@m, mmolkg)through overlapping adsorption potentials of opposite pore of the solutes on the adsorbents was calculated using thewalls when the adsorbate molecular dimension is close following equation: 0m=S/A-NA, where S(m2/g)is theto the pore size(Dubinin, 1975). Contrary to ATR andpecific surface area of adsorbent, A(nm")is the adsorbedF400, the pores less than 10 A in APVC and APET are cross- ection area of an adsorbate molecule (Table 1)only around 2% of the total pore volumes(Table 2)and and Na (mol-)is Avogadro's number. The maximummost micropores are much larger than the molecular size adsorption capacity(@ )was derived from the PDM modelof TCE. Thus the pore-filling effect is insignificant by (Table S2), and the ratio of g and @m(Re)is presentedcomparison On the other hand, the molecular dimensions in Fig. 6. For a given adsorbate on APVC and APET, Rof HCH are 7.9 A x 7.0A x 4.9 A(Table 1), much 1, implying that monolayer adsorption of the adsorbateslarger than TCE. Thus a large portion of micropores cannot could only partially cover the surface of the two adsorbentsbe accessed by HCH molecules. More than 73% of the in the experimental Ce ranges. As for ATR and F400pores in APVC and APET are mesopores(> 20 A), however, the ratio ro is larger than 1 for TCE, DCBwhich are available for HCH, and the molecular sieving and dNBeffect is less prominent compared with ATR and F400 adsorption中国煤化式rtmB(Fig.5). Moreover, it was suggested that the micropores of DNb by aCNMHGH, only 59% andACs mainly consist of slit pores while mesopores exhibit 80% of the surface of ATR and F400 is covered, stronglyboth slit-like and rounded/elongated morphologies, which supporting the presence of the molecular sieving effecthas been supported by scanning tunneling microscopy Similar results were observed for MWNT, indicating thatAdsorptive removal of hydrophobic organic compounds by carbonaceous adsorbents1555APVc●APET△ ATR F400