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inetic and thermodynamic studies on the adsorption of xylenol orange ontoIL-101(Cr)
hen Chen, Meng Zhang, Qingxin Guan, Wei Li ∗
ey Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China
r t i c l e i n f o
rticle history:eceived 26 September 2011eceived in revised form 6 December 2011ccepted 7 December 2011
eywords:
a b s t r a c t
A highly porous metal-organic framework (MOF) material based on chromium-benzenedicarboxylates(MIL-101) was applied to the adsorption of xylenol orange (XO) from aqueous solution. Adsorptionkinetics and isotherms were determined from the experimental data, and the results showed that pseudo-second-order kinetic model and Langmuir adsorption isotherm matched well for the adsorption of XOonto MIL-101. Thermodynamic parameters including free energy, enthalpy, and entropy of adsorption
were obtained, and all the results were in favor of the adsorption. It was found that the adsorbed amountsdecreased with increasing pH value of the XO solution, which indicates that the mechanism may be thecharge interactions between the dye stuffs and the adsorbents. The used MIL-101 could be regenerated bywashing with a dilute concentration of NaOH solution. Compared with other adsorbents like active carbonand MCM-41, especially in high concentrations of XO, MIL-101 demonstrated a superior dye adsorptioncapability.
In our daily life, more than 100,000 types of commercial dyes aresed with a production of over 7 × 105 tonnes annually [1,2]. Manyf them are considered to be toxic and even carcinogenic [3]. In par-icular, synthetic dyes in an effluent, even in a small amount, areighly visible and have undesired effects not only on the environ-ent, but also on living creatures. However, most toxic dyestuffs
re stable to light and oxidants, which makes them difficult toegrade [4].
There are a number of technologies available for the removal ofyestuffs, such as physical, chemical and biological methods [5–7].dsorption technology is regarded as one of the most competitiveethods because it does not need a high operation temperature
nd several coloring materials can be removed simultaneously [4].ctivated carbon, as a traditional adsorbent that has been usedxtensively in industry, is inadequate for the adsorption of dyes2,8].
Porous metal-organic framework materials (MOFs) containinganometric pores and channels, currently receive a considerable
mount of attention because of their potential applications in aumber of industrially important areas, such as gas storage, sep-ration and heterogeneous catalysis areas, which are traditionally
attributed to mineral oxides such as zeolites [9,10]. Among thenumerous MOFs reported so far, one of the most topical solids is aporous chromium-benzenedicarboxylates (Cr-BDCs) namely MIL-101 [11,12] (MIL stands for Material of Institute Lavoisier), whichis a very important material because of its mesoporous structureand huge porosity.
MIL-101 has demonstrated good performance in hydrogenstorage [10]. There derives a series application of gas stor-age/adsorption on MIL-101, such as CO2 [13], CH4 [13,14],long-chain alkanes [15]. Recently, progressively more research onMIL-101 has been applied to the field of catalysis [16,17], and itscatalytic activity is comparable to commercial catalysts. At thesame time, MIL-101 can also be applied to drug delivery wherethe drug content is larger and the delivery rate is slower comparedto similar materials with comparable cage sizes, such as MCM-41[18,19].
However, there are only a few articles about aqueous solu-tion adsorption onto MIL-101. Haque et al. [5] introduced MIL-101for the adsorption of methyl orange (MO) from aqueous solution,in which MIL-101 demonstrated excellent adsorption properties.We have found experimentally that MIL-101 is much better atadsorbing xylenol orange (XO, Fig. 1) than MO. MIL-101 alsoshows excellent adsorption of XO over a wide concentration range,
moreover, the adsorption saturation was much greater than tra-ditional adsorbents like MCM-41 and active carbon. In addition,the material can be reused after washing with lye. A series ofexperiments was carried out to understand the characteristics of
C. Chen et al. / Chemical Engineering Journal 183 (2012) 60– 67 61
Xat
2
apwaiatmsfs
dwcdct
foTpsttet
pas7aNacdt
dt 2
t
qt= 1
k2q2e
+ 1qe
t (2)
Table 1The BET surface areas and pore volumes.
Specific surface(m2/g)
Total pore volume(cm3/g)
Mean porediameter (nm)
Fig. 1. The structure of reactive dye.
O adsorption onto MIL-101 and the possibility of using MOFss adsorbents for the removal of dye materials from wastewa-er.
. Experimental
The MIL-101(Cr) was synthesized by a hydrothermal methods described previously [11]. Powder X-ray diffraction (XRD)atterns were collected on a Bruker D8 focus diffractometer,ith Cu K� at 40 kV and 40 mA between 2◦ and 40◦ (2�) with
scanning speed of 12◦/min. Nitrogen adsorption–desorptionsotherms of samples at 77 K were measured with a BEL-MINIdsorption analyzer. The surface area was calculated using a mul-ipoint Brunauer–Emmett–Teller (BET) model. Zeta potential was
easured with Malvern Zetasizer (Nano series). Before the mea-urement, MIL-101 was dissolved to water of different pH valuerom 2 to 12 with a certain concentration (0.05 wt%) under ultra-ound.
An aqueous stock solution of XO (2000 ppm) was prepared byissolving XO (molecular formula: C31H28N2Na4O13S) in deionizedater. The aqueous XO solution was diluted to different con-
entrations from 100 to 400 ppm. The XO concentrations wereetermined using ultraviolet spectrophotometer at 273 nm. Thealibration curve was obtained from the spectra of standard solu-ions (1–100 ppm).
Before adsorption, MIL-101 was dried in a vacuum oven at 100 ◦Cor 3 h. The powder (0.0500 g) was added to the aqueous solutionf XO (50 mL) with different concentrations from 100 to 400 ppm.he admixture was mixed well under magnetic stirring at a fixedower for a fixed time (1–180 min) at 298 K. After adsorption, theolution was separated from MIL-101 by a sintered filter (G6), andhe concentration was determined by UV spectroscopy. To obtainhe thermodynamic parameters of adsorption such as �G (freenergy change), �H (enthalpy change) and �S (entropy change),he adsorption was repeated at 308 and 318 K.
To determine the adsorption capacity at various pH values, theH value of the XO solution was adjusted with 1 M NaOH or 1 M HClqueous solution. To compare with active carbon and MCM-41, theame dose of active carbon (coconut-made charcoal, BET surface:79 m2/g) and MCM-41 (BET surface: 945 m2/g) were added to thequeous of XO (50 mL). The used MIL-101 was mixed with 0.01 MaOH aqueous solution (about 0.05 g in 50 mL aqueous solution)nd stirred by magnetic stirring for 1 h. After filtration, the filter
ake of MIL-101 was washed with 0.01 M HCl aqueous solution andeionized water until the pH value of the filtrate was near 7. Thenhe powder was dried and reused for the next adsorption run.
Powder XRD patterns of MIL-101 are shown in Fig. 2. The diffrac-tion peaks matched well with previous papers [11,12]. The peakintensity decreased with the number of times the material wasreused, and from Fig. 2, we can see that the peak that at 2◦ dis-appeared, when we washed MIL-101 with NaOH solution, but themain peak at 8–10◦ remained the same. In our opinion, the natureof the structure was destroyed and the crystallinity decreased afterthe NaOH solution wash.
The BET surface areas and pore volumes of the materials mea-sured by N2 adsorption at 77 K are shown in Table 1. The values arein good agreement with most of those reported in previous papers[14,20,21], but much smaller than the results of Ferey et al. [11,12].The values of BET surface areas steadily declined with number ofthe times MIL-101 was reused, while the pore volume and porediameter increased. The results corresponded with the analysis ofthe powder XRD patterns.
3.2. Adsorption kinetics
Adsorption was carried out at different temperature(298–318 K). The results are shown in Fig. 3. As can be seen,the adsorption curves kept the same trend, and all of the adsorp-tion achieved equilibrium in 30 min, showing a rapid adsorptionof XO onto MIL-101.
To obtain the adsorption kinetics, the changes of adsorptionamount with time were treated with pseudo-second-order kineticmodel [22,23]:
62 C. Chen et al. / Chemical Engineering Journal 183 (2012) 60– 67
Fo
wa(
atttMc
ig. 3. Effect of contact time and initial XO concentration on the adsorption of MOnto MIL-101: (a) T = 298 K; (b) T = 308 K; (c) T = 318 K.
here qt: amount adsorbed at time (t) (mg/g); qe: amount adsorbedt equilibrium (mg/g); k2: pseudo-second-order kinetic constantg/(mg min)).
Eq. (2) is derived from Eq. (1) by integrating for the bound-ry conditions t = 0 to t = t and qt = 0 to qt = qe. There is no needo know any parameter beforehand and the equilibrium adsorp-
ion density, qe, can be calculated from Eq. (3) [23,24]. Fig. 4 showshe plots of pseudo-second-order kinetics of XO adsorption onto
IL-101 at different temperatures. From the plots, k2 and qe werealculated with the intercepts and slopes. The results are shown
Fig. 4. Plots of pseudo-second-order kinetics of XO adsorption onto MIL-101: (a)T = 298 K; (b) T = 308 K; (c) T = 318 K.
in Table 2. As can be seen in Fig. 4 and Table 2, the data fit quitewell for the adsorption of XO onto MIL-101 under the experimen-tal conditions employed (correlation coefficient, R2 > 0.999). Withtemperature increasing, k2 increased, that is, the initial adsorptionrate increased with temperature. However, at a constant tempera-ture, k2 decreased with the initial concentration of XO.
The data of adsorption amount can also be treated with pseudo-first-order kinetic model [23,25]:
dq
dt= k1(qe − qt) (3)
ln(qe − qt) = ln qe − k1t (4)
where k1: pseudo-first-order kinetic constant (min−1).Eq. (4) is derived from Eq. (3) by integrating for the bound-
ary conditions t = 0 to t = t and qt = 0 to qt = qe. The equilibrium
C. Chen et al. / Chemical Engineering Journal 183 (2012) 60– 67 63
Table 2Pseudo-first-order and Pseudo-second-order kinetics constants of XO adsorption onto MIL-101.
T (K) Pseudo-first-order kinetics Pseudo-second-order kinetics qe,exp (mg/g)
The Langmuir equation was developed originally to describeindividual chemical adsorbents, and is applicable to physicaladsorption (monolayer) within a low concentrations range. TheFreundlich equation is an empirical approach for adsorbents with
Table 3Adsorption isotherms of XO adsorption onto MIL-101.
T (K) Langmuir adsorption isotherm Freundlich adsorption isotherm
400298 159.3 ± 16.5 6.61 × 10 0.986
308 123.9 ± 17.1 5.00 × 10−2 0.958
318 113.4 ± 19.5 3.74 × 10−2 0.893
dsorption density qe is required to fit the data, but in many cases qe
emains unknown due to slow adsorption processes. The first-orderquation of Lagergren does not fit well to the whole range of contactime and is generally applicable over the initial stage of the adsorp-ion process [24]. From the plots, k1 and qe can be calculated withntercepts and slopes. The results are shown in Table 2. This modelid not fit the experimental data, as the correlation coefficient wasoor. Under most conditions, k1 decreased with temperature, that
s, the initial adsorption rate decreased with temperature, whichas inconsistent with theory or experimental observation.
Comparing the correlation coefficient (R2), pseudo-second-rder kinetic model matched well with this work. Additionally,rom Table 2, the qes value calculated with this model is inood agreement with experimental data. On the contrary, thees value calculated with pseudo-first-order kinetic model dif-ered significantly from the results of experiment. The study inef. [26] indicated that the sorption process obeys pseudo-first-rder kinetics at high initial concentrations of solute, while it obeysseudo-second-order kinetics model at lower initial concentra-ions of solute. Moreover, that study [26] determined that rateonstants are not only dependent on the temperature, but also onhe initial concentration of the solution.
.3. Adsorption isotherms
To describe the adsorption isotherm more scientifically, theangmuir and Freundlich model equations were selected for usen this study. The Langmuir adsorption isotherm has been success-ully applied to many pollutant adsorption processes from aqueousolution. The equation is expressed as:
e = Q0KLCe
1 + KLCe(5)
here Qe: the equilibrium adsorption capacity of XO on the adsor-ent (mg/g); Ce: the equilibrium XO concentration in solutionmg/L); Q0: the maximum monolayer capacity of adsorbent (mg/g);L: the Langmuir adsorption constant (L/mg), related to the freenergy of adsorption.
A linear plot of (Ce/Qe) versus Ce is obtained from the models shown in Fig. 5. KL and Q0 were calculated from the slopend intercept of the different straight lines representing the dif-erent temperature. Table 3 lists the calculated results. The datat quite well for the adsorption of XO onto MIL-101 under the
xperimental conditions (correlation coefficient, R2 > 0.999). Theseesults indicated that the adsorption of XO onto MIL-101 was aypical monomolecular-layer adsorption, and the maximum mono-ayer capacity Q0 was stable (Q0 = 322–326 mg/g). In addition, the
Fig. 5. Langmuir plots of the isotherms for XO adsorption onto MIL-101.
Langmuir constant KL demonstrated an opposite trend with tem-perature.
The Freundlich isotherm used for isothermal adsorption is a spe-cial case for heterogeneous surface energy in which the energy termin the Langmuir equation varies as a function of surface coveragestrictly due to variation of the sorption. The Freundlich equation isgiven as:
Qe = KFC1/ne (6)
where KF (mg/g(L/mg)1/n) and 1/n represents the Freundlichconstants corresponding to adsorption capacity and adsorptionintensity, respectively [3]. KF and 1/n can be determined from thelinear plot of ln (Qe) versus ln (Ce). Table 3 lists the calculatedresults. The magnitude of the exponent 1/n gives an indicationof the favorability of adsorption. Values, n > 1 represented favor-able adsorption condition [23], while the correlation coefficientR2 < 0.90 shows a poor agreement with the experimental data.
64 C. Chen et al. / Chemical Engineering Journal 183 (2012) 60– 67
Table 4Adsorption thermodynamics constants of XO adsorption onto MIL-101.
�G (kJ/mol) �H (kJ/mol) �S (J/mol/K) R2
298 K −28.89 −11.0 ± 0.6 60 ± 2 0.994
vettb
3
iitsfapf
�
w
stn
H
l
scr(r1ite
F
308 K −29.52318 K −30.08
ery uneven adsorbing surfaces [27,28]. It can be seen that the lin-ar correlation coefficient (R2) using the Langmuir model are higherhan those for the Freundlich isotherm for XO, which suggested thathe Langmuir model describes the adsorption of XO onto MIL-101etter.
.4. Adsorption thermodynamics
The adsorption thermodynamics function obtained from thesotherms will further reveal the adsorption mechanism. As shownn Table 3 and Fig. 5, the experiment was carried out over theemperature range of 298–318 K, and the Langmuir equation waselected to fit the adsorption isotherm. In this work, there wereew differences between the results of Q0, which can be consideredpproximately equal, at the same time, KL decreased with the tem-erature.The Gibbs free energy change �G can be calculated by theollowing equation:
G = −RT ln KL (7)
here KL: the Langmuir adsorption constant (L/mol).The Langmuir constant KL can be obtained from the
lope/intercept of the Langmuir plot of Fig. 5. The adsorption underhe experiment condition is a spontaneous process because of theegative free energy change (�G) shown in Table 4.
The enthalpy change �H and �S can be obtained from the van’toff equation:
n KL = �S
R− �H
RT(8)
A linear plot of ln KL versus 1/T is obtained from the model ashown in Fig. 6.The enthalpy change (�H) and entropy change (�S)an be calculated from the slope and intercept of the van’t Hoff plot,espectively. As shown in Table 4, the negative enthalpy change�H) suggests that the adsorption of this work is an exothermiceaction. However, in literature [5], the adsorption of MO onto MIL-
01 obtained a positive value; the reason may be due to a stronger
nteraction between preadsorbed water and the adsorbent thanhe interaction between MO and the adsorbent. In addition, thentropy change (�S) is positive, suggesting the process results in an
ig. 6. van’t Hoff plots to get the �H and �S of the XO adsorption onto MIL-101.
Fig. 7. (a) The pH effect on the adsorption of XO onto MIL-101 (initial concentration:400 ppm, T = 298 K); (b) the pH effect on the Zeta potential of MIL-101.
increase in entropy. In the solid–liquid adsorption system, adsorp-tion of solute onto the adsorbent and desorption of solvent fromthe adsorbent both exist; the former one is an entropy reductionprocess, and the latter is a contrary process. The entropy change ofthe adsorption is the sum of the two processes. In this system of theadsorption of XO onto MIL-101, probably, desorbed water moleculeis larger than that of the adsorbed XO molecule as XO molecule isgiant compared with water molecule; therefore several water maybe desorbed by adsorption of XO molecule. Therefore, the drivingforce for XO adsorption (negative �G) onto MIL-101 is due to bothenthalpy effect and entropy effect.
3.5. Effect of pH value and reuse
The adsorption of a dye usually depends highly on the pH valueof the dye solution [29]. In this work, the pH value effect on theadsorption was carried out at 298 K. Selecting the concentrationof 400 ppm as an example, the pH value (2–12) of the solutionwas adjusted with NaOH or HCl solution. As shown in Fig. 7a, theadsorbed amounts decreased with increasing pH value of the XOsolution, which is quite similar to previously reported results ofMO adsorbed onto various adsorbents [5]. When the pH valueswere 4–10, the adsorbed amounts were much closer. As the acidityenhanced to pH value was 2, the percentage removal of XO reachedalmost 90%; however, in comparison, when the alkalinity increasedto pH value was 12, the amount of adsorption was zero. As shown
in Fig. 7b, the Zeta potential increased with the increase of the pHvalue in the acidic area, however, in the alkalinity area the Zetapotential decreased with the increase of the pH value. That mightbe explained by the diffuse double layer theory; when MIL-101 was
C. Chen et al. / Chemical Engineering Journal 183 (2012) 60– 67 65
on of
alttremactdsfmtt
a0satttt
F
Active carbon shows excellent capacity for dye adsorption, dueto its large specific surface and pore volume [30,31]. The meso-porous molecular sieve MCM-41 is a traditional adsorbent material
Fig. 8. The possible mechanism of the adsorpti
dded to the solution, the particle surface was surrounded by aayer of positive charge, therefore, it showed a positive Zeta poten-ial. However, as the concentration of H+ increased in the solution,he Zeta potential decreased. As to the alkalinity area, there is aeaction between OH− and carboxyl, and the oxygen anions arexposed. As the concentration of OH− increased this trend wasore obvious. When the pH value increased to 12, it demonstrated
sharp negative Zeta potential, and the structure was destroyed asan be seen from the XRD patterns, maybe that is why the adsorp-ion amount was zero at this pH value. The mechanism might beescribed by Fig. 8, in which a microstructure is stripped from thetructure of MIL-101, and the R-SO3
− group of XO are responsibleor the pH value effect on the adsorption of XO onto MIL-101. This
ight explain the behavior of all dye stuffs with a –SO3− group
hat adsorbed onto MIL-101. More experimental data are neededo verify specific mechanism.
Regeneration of an adsorbent is very important for industrialpplications. In this work, the used adsorbent was regenerated with.01 M NaOH solution, because a simple physical method like ultra-ound [5] could not remove the XO from the adsorbent. The resultsre shown in Fig. 8 with an initial concentration of 200 ppm. Fromhe patterns of powder XRD, we know that NaOH solution destroys
he nature of the structure and decreases the specific surface, buthe reused MIL-101 still had the capacity to adsorb XO, moreover,he performance was kept above 90% even when reused for the
ig. 9. The reuse of MIL-101 on adsorption of XO (concentration 200 ppm, T = 298 K).
XO onto MIL-101 (R-SO3− representatives XO).
second time, showing an excellent capacity for regeneration.Further research is needed to find better physical methods forregeneration.
3.6. Adsorption onto different adsorbents
Fig. 10. Adsorption of XO onto different adsorbents (a) compared MIL-101 to MCM-41 and active carbon, initial concentration: 100 ppm; (b) compared MIL-101 to activecarbon for different initial concentration, T = 298 K.
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6 C. Chen et al. / Chemical Engi
s it exhibits hexagonal arrays of uniform channels, high BETurface area and large pore volume. In this work, MIL-101was com-ared with active carbon (AC, coconut-made charcoal, BET surface:79 m2/g) and MCM-41 (BET surface: 945 m2/g), for the adsorptionf XO at the same concentration (100 ppm) and at different tem-eratures for 1 h. The results are shown in Fig. 10. The same dosef active carbon was also added to different concentrations of XOo compare further with MIL-101 at 298 K.
From the data in Fig. 9, MCM-41 showed weak adsorption ofO, while the adsorption of AC was slightly higher than MIL-101. Inddition, the adsorption of XO onto the three adsorbents showedhe same decreased trend with temperature. However, when ACnd MIL-101 were both used in solution of higher dye concentra-ions, MIL-101 showed superior adsorption of XO. The saturateddsorption of AC increased a little with the increase of initial XOoncentration, whereas MIL-101 demonstrated a rapid growth. Theaturated adsorption of MIL-101 grew to nearly twice that of AChen the initial concentration was increased to 400 ppm. That is,IL-101 has a strong dye adsorption capacity over a wide concen-
ration range, while AC is only suitable for dye adsorption at loweroncentrations.
Comparing the adsorption of these three materials, we initiallyonsidered that surface area or pore volume were not the key todsorption of XO from wastewater. Contact with the effect of pH onhe adsorption of MIL-101, the key might be the charge interactionsetween dye stuffs and adsorbents.
. Conclusions
In this work, MIL-101(Cr) was introduced for the adsorp-ive removal of XO from wastewater. The results indicate thatseudo-second-order kinetic model matched much better with thedsorption of XO onto MIL-101 compared with pseudo-second-rder kinetic model. The Langmuir model fits the data betterompared with the Freundlich model in terms of regression coef-cients. The thermodynamic parameters, including free energy,nthalpy, and entropy of adsorption, were calculated from theesult of isotherms, suggesting that the adsorption of XO ontoIL-101 was a process with negative free energy change, nega-
ive enthalpy change, and positive entropy change. The adsorbedmounts decrease upon increasing the pH value of the XO solution,nd when the pH value increased to 12, the adsorption was zero.ombination of the Zeta potential data with pH value, the key fac-or of the adsorption might be the charge interactions between dyetuffs and adsorbents. MIL-101 could be regenerated with a moreilute concentration of NaOH solution, and reserved the adsorptionbility. However, the structure may be destroyed using the NaOHolution, and the specific surface decreased with the increase in theumber of the reuse cycles. Comparing the adsorption capacity ofIL-101 to MCM-41 and active carbon, the amount of adsorption
f MCM-41 was much less than the other two adsorbents; activearbon was only suitable for dye adsorption at low concentration,hereas MIL-101 showed good capacity for dye adsorption over
wide concentration range. Therefore, the material of MIL-101as a great prospect in the dye adsorption area, and is ready foreuse.
cknowledgements
The authors acknowledge financial support from the National
atural Science Foundation of China (Grant No. 21073098),
he Natural Science Foundation of Tianjin (11JCZDJC21600),nd the Research Fund for MOE (IRT-0927), the Doctoral Pro-ram of Higher Education (20090031110015), and the Program
[
[
g Journal 183 (2012) 60– 67
for New Century Excellent Talents in University (NCET-10-0481).
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