Effect of dissolved organic matter from Guangzhou land?ll leachate on sorption of phenanthrene by Montmorillonite
Pingxiao Wu a ,b ,c ,?,Yini Tang a ,b ,Wanmu Wang a ,b ,Nengwu Zhu a ,b ,c ,Ping Li a ,b ,Jinhua Wu a ,b ,Zhi Dang a ,b ,c ,Xiangde Wang a ,b
a
College of Environmental Science and Engineering,South China University of Technology,Guangzhou 510006,PR China
b
The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters,Ministry of Education,Guangzhou 510006,PR China c
The Key Laboratory of Environmental Protection and Eco-Remediation of Guangdong Regular Higher Education Institutions,PR China
a r t i c l e i n f o Article history:
Received 29March 2011Accepted 5June 2011
Available online 13June 2011Keywords:Desorption Kinetics
Surface properties Complex Clay liner Model
a b s t r a c t
To investigate the effect of dissolved organic matter (DOM)on the adsorption of phenanthrene (PHE)by montmorillonite (MMT),organic clay complex was prepared by associating montmorillonite with DOM extracted from land?ll leachate.Both the raw MMT,DOM,and MMT complex (DOM–MMT)were charac-terized by X-ray diffraction (XRD),Fourier transform infrared (FTIR),X-ray photo-emission spectroscopy (XPS),and scanning electron microscope (SEM).Batch adsorption studies were carried out on the adsorp-tion of PHE as a function of contact time,temperature,and adsorbent dose.The sorption of PHE on complex was rapid,and the kinetics could be described well by the Pseudo-?rst-order model (R 2>0.99),with an equilibrium time of 120min.The adsorption isotherm was in good agreement with the Henry equation and Freundlich equation.Also,thermodynamic studies showed that the adsorption process was exothermic and spontaneous in https://www.doczj.com/doc/144670581.html,pared with MMT,the adsorption capacity of DOM–MMT complex for PHE was greatly enhanced.The effects of DOM on PHE sorption by MMT may be attributed to the changes in the surface structure,the speci?c surface area,the hydrophobic property,and the average pore size of MMT.A series of atomistic simulations were performed to capture the struc-tural and functional qualities observed experimentally.
ó2011Elsevier Inc.All rights reserved.
1.Introduction
Polycyclic aromatic hydrocarbons (PAHs)are formed during the incomplete combustion of fossil fuels and other organic matter.They are classi?ed as persistent toxic substances (PTS)by United Nations Environmental Program (UNEP)because of their persis-tence in the environment,tendency to bioaccumulate,and impact on public health [1,2].Therefore,it is essential to investigate the fate and transport of PAHs and explore the possible in?uential factors.
Comparing with biodegradation [3],adsorption of PAHs on min-eral phases and mineral soil is an ef?cient remediation process that has decisive effects on their transport,bioavailability [4],and fate in natural environments [5,6].Clay minerals have a high speci?c surface area and carry a charge,enabling them to bind and stabilize PAHs.Moreover,the surface properties and reactivity of clay min-erals may be modi?ed by adsorption and intercalation of small and polymeric organic species [7].Thus,PAHs adsorption process can be greatly affected by dissolved organic matter (DOM),which is largely composed of humic substances such as fulvic acid (FA)and humic acid (HA).A number of functional groups in DOM,such as carboxylic,phenolic,and carbonyl allow them to interact with PAHs through hydrophobic binding and form humic-solute com-plexes in the aqueous phase.Various terms have been used to describe the resultant products of DOM and clay minerals,such as clay–organic complexes [8],clay–humic complexes [9],or mineral–HA complexes [10].
Currently,much research interest for the in?uence of DOM on PAHs adsorption by soils has been directed toward the interactions between PAHs and clay–humic complexes [11–14].Their results suggested that the in?uence of DOM on phenanthrene sorption could be primarily described as the net effect of the ‘cumulative sorption’and the association of phenanthrene with DOM in solu-tion [15–18].Some studies revealed that humic acid (HA)fraction-ated from DOM promoted sorption of PAHs on clay minerals,while the others indicated that hydrophilic fractions in DOM impeded the distribution of PAHs into soil solids.These recent studies col-lectively suggest that DOM can affect the sorption of PAHs on clay minerals,and the impact will be dependent on the intrinsic nature
0021-9797/$-see front matter ó2011Elsevier Inc.All rights reserved.doi:10.1016/j.jcis.2011.06.019
?Corresponding author at:College of Environmental Science and Engineering,South China University of Technology,Guangzhou 510006,PR China.Fax:+862039383725.
E-mail address:pppxwu@https://www.doczj.com/doc/144670581.html, (P.Wu).
of solute,clay,and DOM compositions[15–19].However,surpris-ingly few systematic researches have been carried out to relate interfacial reaction of PAHs on clay and DOM complexes during the sorption process.Besides,in the present study,the experi-mented DOMs in literatures are generally deriving from organic composts,sediments,sewage sludges,and water from waste dis-posal sites[19].Knowledge of the stabilization of DOM from land-?ll leachate on clay minerals is inadequate.And it is dif?cult to decide how DOM from land?ll exerts an in?uence on sorption of PAHs by clay minerals.So,we draw attention to the effect of differ-ent DOM compositions on sorption of PAHs by clay minerals and these interfacial reaction mechanisms,using obtained structural and energetic information at a molecular level by application of molecular modeling methods.An effective model can capture the structural and functional qualities observed experimentally and provide insight into interaction mechanisms of interest[20].
The migration of DOM from land?lls is of our concern due to their harmful effects at very low concentrations.Today,a signi?-cant environmental problem in Guangzhou is the municipal and industrial land?lls,which can release toxic compounds,such as all kinds of organic pollutants,into the https://www.doczj.com/doc/144670581.html,nd?ll leach-ate contains four main groups of contaminants such as heavy met-als,natural dissolved organic matter(DOM),and xenobiotic organic micropollutants(XOMs).The DOM may act as a carrier of both organic and inorganic pollutants[21,22].
Many kinds of adsorbents have been developed for the removal of DOM from leachate.Recently,the usage of natural mineral sor-bents for wastewater treatment is increasing because of their abundance and low price.One type of clay mineral is bentonite, which is primarily composed of montmorillonite(MMT).More-over,several studies have shown that clay liner materials have important geochemical properties,which can increase the attenu-ation of DOM in leachate.Consequently,montmorillonite(MMT) can be used as clay liner materials to provide a reactive as well as passive barrier in land?ll containment systems[21].On the other hand,Clay–humic complexes are commonly formed in clay liner cap when the leachate permeates clay liner materials[23].They play very important roles in regulating the transport and retention of PAHs in soils and sediments[24].However,the in?uence of those natural clay–organic complexes on environmental behavior of PAHs in soils still largely unclear.Thus,understanding the vari-ous factors affecting sorption–desorption processes of natural complexes in land?ll and their quantitative mathematical model-ing is essential for rational planning and operation of site remedi-ation schemes.And the aims of this paper are the following:(i)to ascertain the effects of DOM on phenanthrene sorption by clay minerals and to provide evidence for the attenuation of pollutants in leachate by mineral liners:(ii)to compare the difference be-tween DOM–MMT complexes and the raw MMT on sorption behavior to PAHs and to model the sorption processes of natural complexes in land?ll.
2.Materials and methods
2.1.Materials
HPLC grade methanol and analytical grade phenanthrene (C14H10)were purchased from Aldrich Chemical Co.with a pur-ity>98%.Phenanthrene(PHE)is a three-ring polycyclic aromatic hydrocarbon.The molecular weights,solubility in water at25°C, log K ow of phenanthrene were178.23g/mol,1.10mg/L,and4.57, respectively[25].Raw calcium montmorillonite(MMT)was ob-tained from Nanhai,Guangdong Province;it had a cation exchange capacity(CEC)of0.78meq/g,pH of6.7,and a basal spacing(d001) of1.55nm.The BET surface area,average pore width,and particle size of montmorillonite(MMT)were measured as76.9m2/g, 78.1nm,and15.52l m.And the colloid composition of MMT was 61.1%,and the chemical composition(wt.%)of MMT was SiO2: 65.56%,Al2O3:17.97%,and SiO2/Al2O3:3.65(quality ratio).All chem-icals used in this study,e.g.,NaCl,Na2CO3,CaCl2,HCl,and NaOH, were of analytical reagent grade,purchased from Guangzhou chemical reagent factory.
DOM was extracted from the land?ll leachate generated from Datianshan land?ll site in Guangzhou.The leachate sample was extracted with CH2Cl2.One liter of sample was initially extracted under alkaline condition(pH=12)by adding drops of1/5(by volume)NaOH solution and then in acidic condition(pH=2)by adding some1/5(by volume)H2SO4using a separating funnel. Then,the concentrated liquid was prepared in0.02mol/LKCl to maintain constant ionic strength,and the pH was adjusted to6 by0.5mol/L NaOH.The DOM solution was shaken in the dark at 180rpm and25°C for24h.After shaking,the land?ll leachates were centrifuged at4,000rpm for20min.Then,the supernatant was immediately?ltered through a0.45-l m membrane?lter. All DOM extractions were preserved at4°C in the dark to prevent microbial degradation,photochemical decomposition,and volati-lization.
Gas chromatography–mass spectrometer method(GC–MS)was used to measure the concentration of organic pollutants in the land?ll leachate.The chemical characterization method of DOM has been previously reported in detail by Yang et al.[22,26].Table 1gave the concentrations of some target alkane compounds in the land?ll leachate samples.In this kind of land?ll leachate,at least 87kinds of organic pollutants were discovered,which included 17alkanes and ole?ns,28aromatic hydrocarbons,six acids,four esters,17alcohols and hydroxybenzenes,seven aldehydes and ke-tones,and four amides.Due to the lack of standard references,only the relative contents of DOM with a reliability of80%or above were listed in Table1.And all the compounds were identi?ed by library(WILEY)search.
2.2.Preparation of dissolved organic matter and montmorillonite complex(DOM–MMT)
To prepare the complex,MMT sample of1.000g was weighed accurately in200ml of beaker,slowly dropped into100ml diluted DOM solution to make suspension at a solid–liquid ratio of1:100 (w/v).The solution pH was adjusted to the required range[27] by titrating with either1.0M NaOH or1.0M HCl.Then,the vessel was stirred constantly in a thermostatted shaker bath(170rpm)for 15h.The?nal suspension(DOM–MMT)was centrifuged,washed three times by successive agitations with deionized water,dried at45°C,and then pulverized to pass through a200-l m mesh sieve.
2.3.Adsorption studies
A known amount of PHE was dissolved in methanol solution (HPLC grade)to prepare1000mg/L stock solution.Background solution contained5mM CaCl2to maintain a constant ionic strength and100mg/L NaN3to minimize bioactivity.The test solu-tions of PHE at various concentrations were made by spiking stock solutions to the background solution.Methanol content in the test solutions was controlled below0.1%by volume to minimize co-solute effect[28].
The adsorption behavior of PHE onto all samples was investi-gated through a batch method.A known amount of a given adsor-bent was mixed well with different concentrations of PHE in50-ml iodine?ask.All reactors were placed in a thermostatted shaker bath(170rpm).Then,the resulting suspension was separated by centrifugation at4000rpm;1.5mL of the supernatant was loaded into glass tubes and analyzed for PHE concentrations.
P.Wu et al./Journal of Colloid and Interface Science361(2011)618–627619
The isotherm experiments were carried out in two sequential steps,a sorption step followed by a desorption step.In the desorp-tion step,the sorbed solute on the solid phase was allowed to des-orb to background solution that was initially free of solute.The contents of PHE in the solution were measured,and desorbed PHE was calculated accordingly[29].
2.4.Quanti?cation of phenanthrene
Quanti?cation of aqueous PHE was performed by high-performance liquid chromatography(HPLC;L-2000,Hitachi) equipped with a UV detector(L-2420);1.5mL of sample was drawn and injected into the HPLC by an autosampler.The separation was done by the analytical reverse-phase Luna C18column with 250?4.6mm dimension,5-l m particle size,and100?pore size (Phenomenex Corp.),thermostated at30°C.Eluting reagent com-prised of90%methanol(HPLC grade,>99.9%)and10%milli-Qwater (Millipore Corp.)at a?ow rate of1.0mL/min.The detection wave-length was245nm.The losses of PHE by photochemical decomposi-tion,volatilization,and sorption to tubes were found to be negligible.
The sorption capacity of phenanthrene on solid phases was cal-culated using the equations below:
q
e
?
V0ec0àc eT
s
e1T
where q e is the amount of PHE sorbed on solid phases at equilib-rium(l g/g),c0,c e(l g/L)are the initial and the equilibrium concen-tration of PHE respectively,V0is the volume of the solution used (mL),and W s is the initial amount of adsorbent(g).
3.Results and discussions
3.1.Characteristics of the adsorbent
3.1.1.Powder X-ray diffraction(XRD)
The XRD results of MMT and DOM–MMT complex are shown at Fig.1.The d001re?ection for basal spacing was found to shift from 1.55(original clay)to1.58nm.This proved that DOM molecules did not signi?cantly intercalate the Al–Si layers of MMT and was bound primarily on the edges and outer planar surfaces of MMT. This binding was probably by H-bonding and electrostatic interac-tions between the positively charged edges of the clays and the negative charges on DOM[30].In addition,the slight increase of 0.03nm may also be attributed to ion-exchange reaction between MMT and DOM.Because of small ionic hydrated radius[31],some primary hydrolyzed cations of DOM can replace Ca2+of interlayer of MMT easily and then caused the increase in basal spacing.And the decrease in the peak intensity of DOM–MMT complex sug-gested the formation of a much more disordered crystalline struc-ture.Therefore,DOM–MMT complex showed a delaminated
Table1
GC–MS analysis result of dissolved organic matter from land?ll leachate.
Organic pollytant Relative content(%)Reliability(%)Organic pollutant Relative content(%)Reliability(%)
Dacane 1.4983Pentanoic acid,2- 1.6582 Tetradecane0.6887methy-,anhydride
Hexadecane 1.4595Hexadecanoic0.1387 Octadecane 2.3680Octadecanoic 1.2689 Eicosane 1.9586Naphthalene0.2697 Indene 1.38841,3-bimethyl
Docosane 2.7893Coprostenol 4.8780 Tetracosane 2.8696Nanphthalene0.3291 Pyrene0.2086,2-bi-
Eucalyptene 2.7390methyl
Benzoylamide,N,N-bi-methyl-3-methyl0.3088Camphor 2.7398 1,2,4-trimethylbenzene0.2891Cedrol 1.4980 Phenol0.5197Cyclohexanol17.1392 Phenol,4-proply 2.9682,3,3,5-trimethyl87 Carboline0.5693Glycol 1.2683 Heptacosane 3.3180Benzenemethanol 3.3181 Naphthalene 1.3692Benzophenone0.37
Octadecanoic 2.1190Valeric acid 2.1081 Nonacosane 5.0483Succinic acid2,3-diehyl- 1.6980 Triacontane 4.14911-.alpha.-terpineol 3.3190 Cholest-4-en-3-one0.2899phenol0.5187 2,6,10,14,18,22-tetra-cosahexaene,
2,6,10,15,19,23-hexamethyl-
3.09981,4-benzenediol,2-(1,1dimethylethyl0.1493
Cholestane,3-ethoxy-,(3.beta.,5.alpha) 1.0483Dihydrocholestenol 1.9195 Phenol,4,40-(1-methyl-thylidene)bis- 1.5794ethanol,2-cholro-,phosphate(3:1) 1.2183 Naphthalene,2-vinyl-0.1590Menthone0.6096 Valeric acid,4-phenyl- 1.8086Decalone0.8185 Ethanone,2,2-dimethoxy-1,2-diphenyl0.6191Pentanoic acid0.7187
Phenol,4-methyl- 1.9590
1,2-benzenedicarboxylic acid,dibutyl ester 1.3590
620P.Wu et al./Journal of Colloid and Interface Science361(2011)618–627
structure,which can be further proved by the change in the FTIR spectra and SEM analysis.Furthermore,from200ppm to 2000ppm,the change of DOM–MMT complex did not obviously occur in peak intensity and basal spacing.This may due to the sta-ble structure of DOM–MMT,which could not vary with the initial concentration of DOM.PHE adsorption occurred only on the exter-nal surface of DOM–MMT complex,as the complex only swelled to
a d001spacing of1.59nm.
3.1.2.Fourier Transform Infrared(FTIR)
The FTIR spectra for MMT,DOM–MMT complex are presented in Fig.2.The following are the major differences the absorption band of MMT at3427cmà1,corresponding to the H–O–H hydrogen bonded water,weakened and shifted to the higher wave number 3441cmà1.The results suggested that the association of MMT with DOM was a chemical bonding process instead of a physical process. Moreover,the decrease in the peak at1643cmà1(OH bending vibration)intensity and width demonstrated an decrease in inter-layer water content due to the replacement of inorganic cations [32]and the association of hydroxyl groups on MMT surface.This observation showed that the binding of hydrophobic DOM fraction to clay minerals could change the mineral surfaces form hydro-philic to hydrophobic[25,33],leading to the preferential sorption of PHE.
In addition,the FT-IR spectrum of the DOM–MMT complex showed a new vibration sign at1402cmà1,which were attributed to carboxylic acids or aliphatic compounds[32].However,the band (1402cmà1)disappears after adsorption of PHE.These shifts indi-cated that phenanthrene molecules interact stronger with the aliphatic DOM–MMT complex through the phenyl rings than the MMT[25].Thus aromatic hydrocarbons,alcohols,and hydroxy-benzenes in DOM are primarily responsible for the enhancement in adsorption of PHE by MMT.
3.1.3.X-ray photoelectron spectroscopy(XPS)
The XPS spectra of the O1s,Ca2p levels are shown in
Fig.3(parts a,b).The charge effect was corrected using the internal reference C1s line from adventitious aliphatic carbon(284.6eV). The recorded lines were?tted using the XPSPEAK4.1program after subtraction of the background(Shirley baseline).
Table2shows the relative content of C,O,Si,Al,and C deter-mined by XPS.As for Si and Al,which were included in the crystal structure,the variation in atomic concentration was small.The XPS results for DOM–MMT with a higher Si/O atomic ratio of about 0.5144suggested that Si was well dispersed in the complex and, as such,would facilitate the interaction between MMT and PHE [7,34–37].On the other hand,for a synthetic complex of MMT, the surface C/O atomic ratio(0.348)was much larger than the va-lue of raw MMT(0.212),which indicated that silica layers with three-dimensionally polymerized SiO4units covered the outer par-ticle surfaces of the complex[7].
The O1s photoelectron spectrum(Fig.3a)showed that binding energy was shifted toward lower energy side by0.3eV after adsorption.In the same way,the Si2p and Al2p binding energy varied from101.2eV to100.9eV and from72.86to72.66eV, respectively.This result suggested that adsorption sites existed on the phyllosilicate surface,and the lower binding energy also could be attributed to DOM interaction with both‘‘aluminol’’and ‘‘silanol’’edge sites on MMT[7,38].Moreover,as the electron den-sity decreased with the binding energy[39,40],the changed elec-tron density of O1s on MMT surface after associating with DOM could be attributed to its stronger interaction with O2–and OH–ions within the aluminosilicate layers.The results showed that during the combination process on MMT surface,alcohols and hydroxybenzenes fractions of DOM were preferentially sorbed by MMT,while alkanes and ole?ns fractions were left in the solution [41].And the Ca2p photoelectron spectrum(Fig.3b)showed two peaks and each can be deconvoluted into two components corre-sponding to(i)non-exchangeable Ca2+ions occupying octahedral sites within the layer structure;and(ii)exchangeable Ca2+ions occupying interlayer sites[7].Both the Ca2p binding energy
P.Wu et al./Journal of Colloid and Interface Science361(2011)618–627621
622P.Wu et al./Journal of Colloid and Interface Science361(2011)618–627
tion,which suggests that plural adsorption sites exist on the sur-
face and interlayer of MMT.The XPS results are in agreement
with the XRD and FTIR study.
3.1.
4.Scanning electron microscope(SEM)
Fig.4shows the morphology of MMT and DOM–MMT
(a?5000,b?5000)).The image of MMT shows aggregated mor-
phology,and a compact structure with non-porous surface.After
association with DOM,the clay surface was changed to a non-
aggregated morphology and coarse porous surface.And there were
a large number of massive?akes with severely crumpled struc-
tures.The morphological changes may be due to the change in
the surface charge of the particles and the ligand exchange be-
tween DOM and hydroxyl groups on MMT surface.Particle sizes
of MMT and DOM–MMT complex are shown in Fig.5.As seen,
compared with that of raw MMT(15.52l m),the average particle
size of DOM–MMT complex decreased from15.52l m to
14.69l m,with increase of the BET area from76.9to101.4m2/g
of MMT.The pore size of DOM–MMT complex increased from
78.1to102.9nm.The incorporation of DOM could form larger sur-
face area and numerous cavities,which resulted in an increase in
the absorption capacity of PHE on DOM–MMT.
Fig.4.SEM image of MMT(a)and DOM–MMT(b)(magni?cation20kV?5000).
absorbent structure [43].In order to further investigate the effect of temperature on the adsorption,thermodynamic parameters such as change in Gibbs free energy D G were estimated using the following equations:
D G ?àRT ln
q e c e
e2T
where D G is the molar free energy change (kJ/mol),R is the gas con-stant (8.314J/mol k),and T is the absolute temperature(K).The mo-lar free energy values of phenanthrene adsorption on MMT and DOM–MMT are summarized in Table 3.The negative values for the D G showed that the adsorption process for DOM–MMT complex was feasible and spontaneous thermodynamically.Moreover,the increase in D G values of DOM–MMT complex showed that the PHE adsorption was favorable on organic clays [25].3.3.Effect of adsorbent dose
Initial adsorbent amount was adjusted in the ranges of 0.1–1.0g for adsorption under natural pH at 25°C as shown in Fig.8.Sorp-tion of PHE on per unit mass of DOM–MMT decreased from 193.35l g/g to 21.37l g/g,with increase in the amounts of adsor-bent from 0.1to 1.0g.The observation can be explained that a large adsorbent amount DOM–MMT complex reduced the unsatu-ration of the adsorption sites.Correspondingly,the number of such sites per unit mass came down.In addition,a higher adsorbent amount created particle aggregation,resulting in a decrease in to-tal surface area [44,45].3.4.Effect of pH
The effect of the pH value of the original solution on the adsorp-tion capacity of PHE is shown in Fig.9.It can be seen that the effect of the pH on the adsorption capacity of PHE was weak.Since the log K ow is often used as a descriptor to estimate the (liquid)solubil-ity and polarity,it is a predominant parameter in the sorption of
polycyclic aromatic hydrocarbons [46].The log K ow of PHE used in our study is 4.57.In other words,its effect on the concentration of the counter ions on the functional groups of the adsorbent and the degree of ionization of the adsorbate during reaction were lim-ited [47],which suggested that pH was not controlling the adsorp-tion process onto the modi?ed MMT.Furthermore,comparatively high adsorption capacity of PHE on the adsorbent still occurred at pH 7.0due to the fact that chemical interactions between PHE and DOM–MMT taken place.
3.5.Desorption studies
The desorption of PHE from MMT and DOM–MMT complex is presented in Fig.10.As is seen from Fig.10,PHE released from the DOM–MMT was less than 9%of the adsorbed amount.The data
Table 2
Change of atomic ratios collected from MMT and DOM–MMT before and after adsorption.Samples
C (%)O (%)Si (%)Al (%)C/O Si/O MMT
11.16152.63526.318 6.4710.2120.500DOM–MMT
17.19949.31525.366 4.9250.3480.5144DOM–MMT–PHE
21.122
48.021
23.522
4.700
0.440
0.4897
7.Adsorption of phenanthrene on MMT and DOM–MMT at temperature,45°C.
Table 3
Thermodynamic parameters for PHE adsorption onto MMT and DOM–MMT.Samples
D G (kJ/mol)298K
308K 318K MMT
à7.11à7.89à8.04DOM–MMT à13.5
à14.29
à14.43
Interface Science 361(2011)618–627623
also showed that the desorption percent of MMT was higher than that of DOM–MMT.Moreover,the desorption equilibrium of com-plex was achieved after only30min oscillation,while the equilib-rium of MMT achieved slowly.This indicated that DOM modi?cation not only augmented the PHE adsorption capacity of MMT but also increased the bond strength and the stability of adsorption.The release of PHE from the MMT surfaces may be due to a weak hydrophobic interaction between the free and ad-sorbed PHE on the surfaces.In a case,DOM enhanced the salting out of the non-bound PHE molecules from the adsorbed PHE[48].
3.6.Kinetics of adsorption and desorption
In order to investigate the adsorption and desorption processes of PHE on the adsorbents,Pseudo-?rst-order and Pseudo-second-order models were used.The linear forms of the two models could be expressed as:
logeq
e àq tT?log q eà
k1t
2:303
e3T
t q t ?
t
q
e
t
1
k2q2
e
e4T
where q t(l g/g)and q e(l g/g)are the amounts of PHE adsorbed at time t(min)and at equilibrium,respectively;k1and k2are the sorp-tion rate constants of the Pseudo-?rst-order equation and Pseudo-second-order equation,respectively.
Table4shows the rate constants(k)and correlation coef?cients (R2)of the two kinetic models.Pseudo-?rst-order model for DOM–MMT showed correlation coef?cient(R2)of0.994(Table4), whereas that of second-order kinetic order was0.985.The insuf?-ciency of the pseudo-second-order model to?t the kinetics data could possibly be due to the polarity of PHE in?uencing the sorp-tion process.Moreover,functional groups existing on the surface of DOM–MMT such as–COOH groups and–OH groups also contrib-uted to the chemisorption of PHE on DOM–MMT in solutions.The coef?cient of determination R2for the pseudo-?rst equation of MMT was observed to be close to1,which was higher than that of DOM–MMT.It demonstrated that the sorption of DOM–MMT was more likely to be described by cumulative adsorption mecha-nism[18],the association of PHE with DOM in solution[49],and the modi?ed surface characteristics of MMT due to DOM binding [50].
The pseudo-second-order rate constant(see Table5),k2,and q e were calculated from the slope and intercept of the plots of t/q t versus t.The experimental q e values of DOM–MMT were in agree-ment with the calculated q e values.Hence,this study suggested that the pseudo-second-order kinetic model better represented the desorption kinetics,suggesting that the chemical reaction was signi?cant in the rate controlling step of desorption.It as-sumed that the PHE were strongly held to the MMT and DOM–MMT surfaces by chemisorptive bonds,involving valence forces through sharing or exchange of electrons[37].
3.7.Adsorption isotherms
Equilibrium relationships between adsorbate and adsorbent are described by adsorption isotherms.Fig.11shows the Henry iso-therms of the adsorption of PHE onto the adsorbent.The Henry [49],Langmuir[51],and Freundlich[24]isotherm models were used to describe the equilibrium data,and their linear forms were presented as:
k d?
q
e
e
e5T
q
e
?k f c n
e
e6Tc e
q
e
?
1
ebq mT
t
1
q
m
c ee7T
where c e(l g/L)and q e(l g/g)are the equilibrium concentration of PHE in the liquid phase and in the solid phase,respectively;k d is the distribution coef?cient of solute between soil and water;b and q m are Langmuir coef?cients representing the equilibrium con-stant for the adsorbate–adsorbent equilibrium and the monolayer
Table4
Kinetics parameters for PHE adsorption on MMT and DOM–MMT.
Adsorbent Pseudo-?rst-order model Pseudo-second-order model
q e k1R2q e k2R2
MMT17.2300.5370.99717.4940.1320.985 DOM–MMT40.0600.8960.99440.0050.0580.989
Table5
Kinetics parameters for PHE desorption on MMT and DOM–MMT.
Adsorbent Pseudo-?rst-order model Pseudo-second-order model
q e k1R2q e k2R2
MMT 3.4390.8400.932 3.724 1.7800.985 DOM–MMT 3.2400.5700.975 3.352 3.2870.989 624P.Wu et al./Journal of Colloid and Interface Science361(2011)618–627
P.Wu et al./Journal of Colloid and Interface Science361(2011)618–627625
equilibrium interfacial structures for PHE on MMT and DOM–MMT surface:side(a)and top(b)view of the initial con?guration
adsorption on DOM molecule;side(d)view of PHE adsorption on the surface of DOM–MMT complex.Gray,red,white, Al atoms,respectively.Only silicon and basal oxygen atoms of the tetrahedral sheet exposed to PHE are shown in
this?gure legend,the reader is referred to the web version of this article.)
In Henry isotherm model,k d values were generally greater in DOM–MMT system than in MMT system,indicated that the DOM–MMT had higher adsorption capacity.The increase in adsorption capacity may be due to three different reasons:(i)As a highly hydrophobic organic compound with a log K ow of4.46, PHE sorption is closely related to organic matter,which dominates phenanthrene sorption by soils[51].The DOM sorption to MMT may increase the organic matter content and provide new sorption sites.Therefore,‘cumulative sorption’would increase the capacity of taking up PHE and promote its sorption;(ii)When several mol-ecules come together to form bigger aggregates(SEM result),a micelle-like conformation of DOM–MMT could be formed,and the binding with PHE could be enhanced due to the molecular geometry and interactions such as p?–p interaction[25];(iii)sorp-tion of PHE to DOM–MMT could take place through hydrophobic regions of DOM and ionizable groups oriented to the aqueous solu-tion[25],which may make the MMT surface more hydrophobic resulting in preferential sorption of PHE(FTIR results).
Both linear and non-linear sorption isotherms have been reported for sorption of PHE to soils and sediments in aqueous sys-tems[18,29,34,52,53].In our study,linear isotherms are consid-ered to primarily result from partitioning(dissolution)of PHE into the three-dimensional matrix of DOM–MMT.A description of the overall process can be summarized in the following simpli-?ed steps:
(a)DOMtmineral surface()DOM residualtDOM
àmineral surfacee8T
(b)DOMàmineral surfacetPHE()PHEàDOM
àmineral surfacee9T
(c)DOM
residual
tPHE()PHEàDOM residuale10T
(d)PHEtmineral surface()PHEàmineral surfacee11T
The sorption processes is simulated in Fig.12,which compared the difference between DOM–MMT complex and the raw MMT on sorption behavior of PHE by application of molecular modeling methods[30,54].The DOM–MMT complex system featured signif-icant direct and indirect H-bonding and hydrophobic interactions between organic and mineral components,resulting in preferential sorption of PHE.The all-atomforce?eld COMPASS[Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies,Accelrys,San Diego,CA,USA]was used to describe these interactions[55].The COMPASS force?eld adequately captured major structural features experimentally observed in MMT and DOM–MMT.
4.Conclusions
Compared with those of the original MMT,the adsorption capacities of DOM–MMT samples for PHE were greatly improved and DOM–MMT complex exhibited the higher adsorption capaci-ties for PHE.The effects of the factors such as the changes in the surface structure,the hydrophobicity,the speci?c surface area, and the average pore size may be result in the higher adsorption capacity of the DOM–MMT.The XRD patterns con?rmed that DOM from land?lls was bound primarily on the edges and outer planar surfaces of MMT via direct and indirect H-bondings and cat-ion bridges.Also,the IR spectra demonstrated that the binding of hydrophobic DOM fraction to MMT could make the mineral inter-face more hydrophobic,leading to the preferential sorption of PHE.Further,the DOM–MMT complex exhibited larger speci?c surface areas,numerous cavities and possessed higher Si/O atomic ratio on silica layers as indicated by the results of SEM and XPS.
Adsorption experiments showed that the adsorption capacity of DOM–MMT for PHE decreased with adsorbent dose.The sorption of PHE on complex was rapid,and the kinetics could be described well by the Pseudo-?rst-order model(R2>0.99)with an equilib-rium time of120min.Maximum removal of PHE on DOM–MMT was at pH about7.0.The Herry and Freundlich isotherm provided the best correlation of the equilibrium data.Thermodynamic parameters demonstrated that the adsorption process of PHE on DOM–MMT was spontaneous and exothermic in nature.Therefore, the PHE sorption on DOM–MMT is partitioning and the character-istic of sorption isotherm is linear.
Acknowledgments
The authors are grateful for?nancial support from the National Science Foundation of China(Grant No.41073058,40973075, 40730741,40573064),Research Fund for the Doctoral Program of Higher Education of China(No.20100172110028),Science and Technology Plan of Guangdong Province,China(Grant Nos. 2006B36601004,2008B30302036,2009B050900005),Natural Science Foundation of Guangdong Province,China(Grant Nos. 06025666,9351064101000001),and the Fundamental Research Funds for the Central Universities,SCUT(Grant Nos.2009ZZ0048, 2009ZZ0073,2009ZM0202).
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