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reparation of poly(aniline-1,8-diaminonaphthalene) and its application asdsorbent for selective removal of Cr(VI) ions
in Li, Yan Qian, Hao Cui, Qiu Zhang, Rong Tang, Jianping Zhai ∗
tate Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210093, PR China
r t i c l e i n f o
rticle history:eceived 26 June 2011eceived in revised form 11 August 2011ccepted 13 August 2011
eywords:oly(aniline-1,8-diaminonaphthalene)PANDAN)hromium(VI)dsorptionreakthrough studiesaste water treatment
a b s t r a c t
In this paper, poly(aniline-1,8-diaminonaphthalene) (PANDAN) was synthesized by chemical oxidationand characterized by Fourier transform infrared spectroscopy (FTIR), electrophoretic mobility test andX-ray photoelectron spectroscopy (XPS). The adsorption behavior of Cr(VI) onto PANDAN was then exam-ined using batch and fixed-bed column sorption techniques. Cr(VI) adsorption onto PANDAN follows theFreundlish model and the pseudosecond-order kinetic model well. The pH value has considerable impacton the adsorption capacity of the adsorbent, and the maximum adsorption capacity reached 150 mg g−1
at pH 4. PANDAN displays more preferable adsorption toward Cr(VI) in the presence of K+, Ca2+, Mg2+,H2PO4
− at greater levels in solution, while increasing SO42− and CO3
2− to 5 times of the Cr(VI) concen-tration could slightly decrease the adsorption. It was proposed that Cr(VI) was removed from aqueoussolutions through an adsorption-coupled reduction process, in which the PANDAN of reduction stateconverts to oxidation state and provides protons for the reduction of Cr(VI) to Cr(III). The addition of 1,8-diaminonaphthalene (1,8-DAN) in the copolymer greatly enhanced the amount of amine and secondary
amino groups on polymer chains, offering more protons for Cr(VI) reduction and binding sites for resul-tant Cr(III), thus improving the uptake properties of the copolymer. It was also proved that the adsorbedchromium ions could be effectively desorbed by 1.0 mol L−1 NaOH. Column breakthrough studies wereconducted at different flow rate and Cr(VI) concentrations to investigate the applicability of adsorbent.The results suggest that PANDAN is a promising adsorbent for selective removal of Cr(VI) from waste effluents.
. Introduction
Many industries such as electrodeposition, leather manufactur-ng, steel production and wood preservative industries generate
huge amount of toxic metals. The discharge of such effluentsause increased adverse effects on human beings and environ-ent. Chromium is one of the most important toxic metals which
ould damage upper respiratory tract and has chronic toxicity [1].hromium exists in different oxidation states in aqueous solutionsII, III, VI). Cr(VI) is highly poisonous because it is a relatively stronghemical oxidant and could react with the enzymes of the bodyr biological systems [2]. Cr(III) is about 300 times less toxic thanr(VI). The maximum Cr(VI) concentration admitted in effluent is
mg L−1 before discharge.In order to solve heavy metal pollution, various technologies
ave been used. These include ion exchange [3], adsorption [4],embrane separation [5], biological treatment [6], solid phase
xtraction (SPE) [7], and electrochemical methods [8]. Among these
technologies, adsorption is a particularly attractive option becauseof its outstanding simplicity, high efficiency, low investment, andpotential recovery and reuse of metals [9,10]. In recent years, inves-tigations have been carried out for low cost, non-conventionaladsorbents which provide high removal efficiency as well as spe-cific interactions with the targeted toxic metals in the presence ofother coexisting environmental friendly ions such as Ca2+, Mg2+
and Al3+ [11].It is observed from previous studies that conducting polymers
can be used effectively for removal of some of toxic metal ionsfrom aqueous solutions [12]. Polyaniline (PANI), an important con-ducting polymer, have attracted growing concern as heavy metaladsorbents due to its easy and inexpensive synthesis, chemical sta-bility, a high affinity for one or more metal ions, and selectivity forthe targeted metal ions. Wang et al. used PANI for Hg(II) adsorp-tion from aqueous solutions, achieving optimum removal aroundpH 4–6 [13]. Reza found that polyaniline coated sawdust (SD/PAn)showed high adsorption capacity for Cr(VI) at pH ≤2 [14]. Li et al.improved the uptake properties of chromium in pH 3.0–7.0 by
adding humic acid (HA) into the chemical polymerization processof aniline monomers [15]. It is widely accepted that nitrogen-containing functional groups act as adsorption sites for heavymetals [16–18]. Nevertheless, the amount of amine groups on PANI
s limited, consequently limiting its adsorption capacity. Copoly-erization is one of the effective approaches to achieve enhanced
mount of amine groups on polymer chains, therefore improve thedsorption capacity. Polydiaminonaphthalene (PDAN) possesseshelating properties and/or reduction properties owing to the elec-ron donating groups (amine and secondary amino groups) on theolymer chain. Li et al. [19] have attempted to remove Ag+ by PDANrepared by chemical oxidation of 1,8-diaminonaphthalene (1,8-AN). The PDAN exhibited high adsorbability due to complexationetween Ag+ and amine/imine groups as well as the redox betweeng+ and free amine group. Therefore, the copolymer of ailine (AN),8-DAN is expected to be an effective and economical adsorbentor heavy metal removal.
The main objectives of this study were to explore the pos-ibilities of obtaining the polymer with an enhanced amount ofmine groups on polymer chains in order to achieve highly effec-ive adsorbents for selective sequestration and removal of heavy
etals from the contaminated waters. We report herein the syn-hesis of poly(aniline-1,8-diaminonaphthalene) (PANDAN) and atudy of their Cr(VI) up take behavior from the contaminated watersnder different conditions. The adsorption isotherms, kinetics and
ts mechanism are also discussed.
. Experimental methods
.1. Materials
1,8-diaminonaphthalene of analytical grade was purchasedrom Aldrich. Aniline (Merck) was purified by distillation undereduced pressure, kept in the dark, and stored in a refrigeratorefore used. Ammonium peroxodisulfate (NH4)2S2O8 (>98%) wasurchased from Fluka. A stock Cr(VI) solution (1000 mg L−1) wasrepared in ultrapure water (18.25 M� cm) using K2Cr2O7. Otherhemicals like Na2CO3, NaH2PO4·2H2O, Na2SO4, Ca(NO3)2·4H2O,g(NO3)2·6H2O, KNO3 of analytical grade were purchased fromanjing Chemical Reagent Co., Ltd (China).
.2. Chemical synthesis of PANDAN
Poly(aniline-1,8-diaminonaphthalene) was synthesized accord-ng to a procedure reported in literature with some modifications20]. Detailed processes can be represented as the followingteps: (1) 0.158 g (0.001 mol) 1,8-diaminonaphthalene and 1.86 g0.02 mol) freshly distilled aniline were dissolved in 100 mL of2SO4 (2 mol L−1). (2) 100 mL of 0.4 mol L−1 (NH4)2S2O8 solutionas slowly added to the mixture under vigorous stirring over aeriod of 5 h at room temperature. (3) After being filtered, the pre-ipitated polymer was washed thoroughly with ultrapure waternd vacuum-dried at 60 ◦C until reaching constant weight. Theesultant polymer PANDAN was powdered in a mortar and sievedhrough meshes to ensure that the particle size of the adsorbent wasetween 0.1 and 0.2 mm (70–150 screen mesh) before used. Appar-ntly, the XPS spectra (Fig. S1, Supporting Information) and surfacelemental stoichiometries (Table S1, Supporting Information) illus-rated that the addition of 1,8-DAN in the copolymer enhanced themount of amine and secondary amino groups.
.3. Batch adsorption experiments
Batch adsorption experiments were carried out in 10 mLolyethylene bottles. 10 mL of Cr(VI) with an initial concen-ration of 50 mg L−1 was treated with 0.005 g of PANDAN. For
etermination of the adsorption isotherm, the adsorbent wasdded to solutions with various initial concentrations of Cr(VI)10–500 mg L−1) at fixed NaCl concentration. Other competing ionsncluding K+, Mg2+, Ca2+, CO3
2−, H2PO4− and SO4
2− were added
ournal 173 (2011) 715– 721
when necessary. The solution pH was adjusted by addition of HCland NaOH. The flasks were then transferred to an incubator shakerand vibrated at 150 rpm for 24 h to ensure the equilibrium adsorp-tion. Preliminary experiments showed that adsorption equilibriumof Cr(VI) on PANDAN was completely achieved within approxi-mately 24 h under the experimental conditions. The amount of totalchromium (including Cr(VI) and Cr(III) ions) trapped by the adsor-bent particles was calculated based on mass balance before andafter the test.
2.4. Desorption experiments
Cr(VI)-laden PANDAN were placed in 25 mL of (A) a NaOH solu-tion; (B) a NH4OH solution and (C) a HCl solution for 24 h. Eachkind of eluant was tested for three concentrations (0.2 mol L−1,1.0 mol L−1 and 2.0 mol L−1). After shaking, the suspension was fil-tered and the Cr(VI) concentration was analyzed as described.
2.5. Column experiments
The column breakthrough experiments were carried out in astainless steel column having dimensions of 50 mm height and5 mm diameter. An amount of 2 g PANDAN particles were packedwithin the column between two filterable membranes at the topand bottom end to prevent the absorbent from floating. The feed-ing solution containing Cr(VI) was pumped continuously throughthe column at desired volumetric flow rate using a peristaltic pump(Baoding, China). Up-flow column experiments were conducted inthis study, and an automated fraction collector was used to collectthe effluent samples at different intervals. The process was opti-mized for the flow rate (5.0 and 10.0 mL min−1) and feed Cr(VI)concentration (10, 50 and 100 mg L−1). After adsorption, a solu-tion of NaCl (2 mol L−1) was used as the eluting agent for columndesorption.
2.6. Characterization and analyses
Total chromium content (including both Cr(VI) and Cr(III) ions)in solution was determined by atomic absorption spectrophotome-ter (AAS-990, China). FT-IR spectra of the samples were obtained byusing a FT-IR spectrophotometer (NEXUS 870, U.S.). Electrophoreticmobilities of all the samples were measured as a function of solu-tion pH. A VG ESCALB MK-II Instrument (UK) was used for obtainingXPS data of the PANDAN. Both survey and high-resolution spectraof N1s were collected and calibrated to the binding energy of C1s at284.6 eV, and the calibrated high-resolution spectra were analyzedby XPS Peak 4.1 software. The surface elemental stoichiometrieswere determined from the peak-area ratios corrected by sensitiv-ity factor. Distribution of Cr(VI) species as a function of pH wasobtained using Visual MINTEQ version 3.0.
3. Results and discussion
3.1. FT-IR studies
Fig. 1 shows the FT-IR spectra of the resulting hybrid adsor-bent PANDAN as compared to pure poly(1,8-diaminonaphthalene)(PDAN) and polyaniline (PANI). The FT-IR spectrum of the poly(1,8-diaminonaphthalene) (PDAN) (curve 1) resembles that the onereported earlier in literature [21]. The band positions in the spectraof the polyaniline (PANI) (curve 3) were also largely in agreementwith those previously reported in the literature [22,23]. It can be
seen that the spectrum of the PDAN exhibits very broad bands whilePANI yields narrow, well-resolved bands. In the case of PANDAN(curve 2), slight decreases in the band intensities were observed at1524 and 1190 cm−1 when compared with PANI, corresponding to
Q. Li et al. / Chemical Engineering Journal 173 (2011) 715– 721 717
3500 300 0 250 0 200 0 150 0 100 0
1
2
wavenumber (cm-1
)
3
2158
1614
1505
1347
1178
832
3661
1733
1524
1357
1190
893
3671
1792
1536
1357
902
F1
t[ti[m
3
bptP
FaN
10008006004002000
c
b
Cr 2p
O 1s
N 1s
C 1s
Bind ing Energy, eV
Rela
tive i
nte
ncit
y, c/s
a
ig. 1. FT-IR spectra of (1) poly(1,8-diaminonaphthalene) (PDAN), (2) poly(aniline-,8-diaminonaphthalene) (PANDAN) and (3) polyaniline (PANI).
he N–H bending of the amine salts and the dopant, respectively23]. Meanwhile, the band observed in PANI at 1614 cm−1 shiftso 1733 cm−1 in the spectrum of PANDAN and even to 1792 cm−1
n PDAN, which corresponds possibly to the C N stretching mode21]. These changes provide evidence for the occurrence of copoly-
erization.
.2. Electrophoretic mobility
Fig. 2 shows the electrophoretic mobility versus pH of samplesefore and after Cr(VI) sorption. As shown in Fig. 2, PANDAN had a
HPZC about 5.72. At low pHs amine groups are protonated, causinghe material to carry positive charges. With the increase of pH, theANDAN became negatively charged due to competitive adsorption
4 6 8 10 12-6
-4
-2
0
2
4
6
PANDAN-Cr, pHpzc
= 4.73
PANDAN, pHpzc
= 5.72
pH
Mo
bil
ity
, 1
0-8
m2v
-1s
-1
PANDAN
PANDAN-Cr
ig. 2. Electrophoretic mobility of samples in 0.1 mol L−1 NaCl solution before andfter Cr(VI) sorption, (adsorption condition: pH 4.0; ionic strength = 0.1 mol L−1
aCl; C0 of Cr(VI) = 50 mg L−1).
Fig. 3. XPS spectra of PANDAN. Curve a: PANDAN before adsorption; curve b: PAN-DAN after adsorption; and curve c: PANDAN after desorption.
of OH− anions. A similar interpretation was proposed by Wang et al.[13] in their study of PANI, where negative potentials at elevatedsolution pHs were attributed to the specific binding of OH− to bothimine and amine functional groups on the polymer chain.
After Cr(VI) sorption, pHPZC of PANDAN-Cr shifted to a lowerpH at 4.73, possibly due to electrostatic adherence between Cr(VI)anions and the imine (–NH•+–) group on the PANDAN molecularchain, changing its surface charge.
3.3. XPS spectra
XPS spectra of both survey and high resolution scans for the keyelements on PANDAN surfaces before and after adsorption werestudied. As shown in the survey spectra in Fig. 3, all the PAN-DAN films are composed of C, N and O. The presence of Cr2p inCr-laden PANDAN evidently confirmed the adsorption of Cr(VI) bythe adsorbent. The oxidation state of the Cr bound to the biomasswas characterized. Fig. 4A shows high-resolution spectra, collectedfrom the Cr2p core region, of PANDAN, Cr-laden PANDAN, andCr-unloaded PANDAN. It is obvious that in the spectra of the Cr-laden PANDAN, two significant bands appeared at binding energiesof 577–578 and 587–588 eV respectively; the former correspondsto Cr2p3/2 orbital, the latter to Cr2p1/2 orbital. This provides firmevidence for the existence of Cr(III) [24]. The Cr(VI) forms are char-acterized by higher binding energies such as 578.1 eV (CrO3) or579.2 eV (K2Cr2O7) [25]. Therefore, the chromium bound on thesurface of PANDAN was the trivalent form, which indicates that theCr(VI) was completely reduced to Cr(III) when brought into contactwith PANDAN.
The high-resolution spectrum of N1s (Fig. 4B) provides an insightinto the adsorption mechanism of Cr(VI) removal by this novelmaterial. N1s could be fitted into three species with varied frac-tions, namely nitride (–N ), amine (–NH–) and imine (–NH•+–) atbinding energies of about 398.6, 399.5 and >400.0 eV, respectively.The contents of the major species including N1s and Cr2p werelisted in Table 1. PANDAN shows two apparent peaks correspond-ing to nitrogen atoms in amine (–NH–) and doped imine (–NH•+–),indicating that this PANDAN is in its emeraldine salt state. AfterCr(VI) adsorption, only a trace amount of amine (–NH–) (0.01%)
•+
was detected and doped imine (–NH –) also decreased dramati-cally. Whereas the amount of nitride (–N ) slightly increased from5.07% to 5.12%. The [(–N )]/[(–NH–) + (–NH•+–)] ratio for the freshPANDAN film is 0.80, which increased to 1.11 for the Cr-laden
718 Q. Li et al. / Chemical Engineering Journal 173 (2011) 715– 721
594591588585582579576573570
(A)Cr 2p
1/2
Cr 2p3/2
Rela
tive in
ten
cit
y, c/s
Binding Energy, eV
c
b
a
410408406404402400398396394392
Rela
tive in
ten
cit
y, c/s
Binding Energy, eV
(B) -NH+
--NH --N=
c
b
a
Fbd
PttaifoNC
TD
2 4 6 8 10 120
10
20
30
40
50
60
70
80
90
0
20
40
60
80
100
HCrO4
-
Cr2O
7
2-
Qe, m
g/g
pH
CrO4
2-
%
Fig. 5. Effect of solution pH on Cr(VI) removal by PANDAN, C0 of Cr(VI) = 50 mg L−1;−1 −1
ig. 4. XPS spectra of high resolution scan of (A) Cr2p and (B) N1s. Curve a: PANDANefore adsorption; curve b: PANDAN after adsorption; and curve c: PANDAN afteresorption.
ANDAN, showing that PANI is less protonated [26]. This indicateshat the PANDAN of reduction state converts to oxidation state andhe protons in PANDAN may be taken up by the reduction of Cr(VI)dsorbed on the surface of PANDAN. The resultant cationic Cr(III)ons then binds to PANDAN because of the interaction between theree electron pairs of the nitrogen in the amine group and the void
rbitals of the metal, which is proved by the decreasing trend of1s binding energy. It can also be found that Cr-laden PANDAN andr-unloaded PANDAN contains 3.03% and 1.45% Cr, respectively,
able 1istribution of major species on different composites.
ionic strength = 0.1 mol L NaCl; S/L ratio = 0.5 g L ; pH adjusted by HCl and NaOHsolutions; and the pH value is the solution pH at equilibrium. Distribution of Cr(VI)species as a function of pH was obtained using Visual MINTEQ version 3.0.
which suggests that a large amount of Cr is removed by desorptionand the PANDAN can be reused.
3.4. Effect of pH on Cr(VI) removal
Effect of solution pH on Cr(VI) uptake by PANDAN wasexamined, and the results are presented in Fig. 5. A species dis-tribution diagram is drawn for a total Cr(VI) concentration of9.62 × 10−4 mol L−1 using Visual MINTEQ version 3.0. Removal oftotal chromium using PANDAN is greatly affected by pH of the solu-tion on the whole, but efficient removal of Cr(VI) was observed ina relative wide pH range of 3-5. The adsorption capacity decreasedgradually with pH increasing from 6 to 9, and dropped substantiallyfrom 10 to 12. Cr(VI) normally exists in the anionic form, as Cr2O7
2−,HCrO4
− and CrO42− forms in aqueous solutions. A low pH makes
the adsorbent surface positively charged as amino groups becomeprotonated, enhancing the removal rate of Cr(VI) in the aqueousphase and accelerates the redox reactions. The resultant cationicCr(III) ions were captured by adjacent amine groups through a spe-cific metal coordination, thus increasing acidity is unfavorable forCr(III) coordination.
3.5. Effect of competing anions on Cr(VI) removal
Coexisting ions such as K+, Ca2+, Mg2+, SO42−, H2PO4
−, andCO3
2− are commonly present in natural waters and industrial efflu-ents. They could compete with the toxic metals for active sitesof a given adsorbent. The influence of these coexisting ions onthe adsorption of Cr(VI) by PANDAN was investigated separatelywith varying initial concentrations of these ions (0–50 mmol L−1)by keeping Cr(VI) concentration constants (1 mmol L−1). The resultsare illustrated in Fig. 6. It can be seen that cations including K+, Ca2+
and Mg2+ does not pose any significant effect on Cr(VI) removalin the test concentration ranges. In the case of anions, H2PO4
−
had no obvious effect on Cr(VI) adsorption, whereas the divalentcompeting ions SO4
2− and CO32− result in a decrease in Cr(VI)
adsorption onto PANDAN. When SO42− and CO3
2− increased from0 to 5 times of the Cr(VI) concentration, the adsorption capacityslightly reduced from 58 to 35–45 mg g−1. Further increasing these
competing ions to 50 times of Cr(VI) concentration leads to an obvi-ous drop of its capacity to below 10 mg g−1. This behavior showsthat PANDAN can effectively remove Cr(VI) in the presence of othercommon ions within a wide concentration range.
Q. Li et al. / Chemical Engineering Journal 173 (2011) 715– 721 719
50403020100
0
10
20
30
40
50
60
CO3
2-
H2PO
4
-
SO4
2-
Ca2+
Mg2+
K+
Qe,
mg
/g
Concen tration of coe xisting i ons , mm ol/ L
FC
3
urmbtb
Q
wlara
aww
Q
F(
Table 2Evaluation of adsorption isotherm by the Langmuir and Freundlich models.
I, mol L−1 Langmuir model Freundlich model
KL (L mg−1) R2 KF n R2
0.01 0.0433 0.9043 3.09 1.38 0.9732
ig. 6. Effect of coexisting ions on Cr(VI) removal by PANDAN, C0 ofr(VI) = 50 mg L−1; S/L ratio = 0.5 g L−1; pH 4.0.
.6. Adsorption isotherms
Cr(VI) adsorption isotherms onto PANDAN were determinednder different ionic strength (0.01, 0.1, 1.0 mol L−1 NaCl), and theesults are shown in Fig. 7. The Langmuir and Freundlich isothermodels were employed to evaluate the adsorption process of Cr(VI)
y PANDAN [27]. The Langmuir equation, based on monolayer sorp-ion on a surface with a finite number of identical sites, is giveny
e = QmKLCe
1 + KLCe(1)
here Qe (mg g−1) is the total chromium amount adsorbed at equi-ibrium, Qm (mg g−1) is the maximum total chromium adsorptionmount, Ce (mg L−1) is total chromium concentration at equilib-ium, and KL is a constant related to the energy or net enthalpy ofdsorption.
The Freundlich isotherm model was employed to evaluate thedsorption process of Cr(VI) by PANDAN. The Freundlich isotherm,
hich considers non-ideal sorption on heterogeneous surfaces asell as to a multilayer sorption, is given as
e = KFC1/ne (2)
0 10 0 20 0 30 0 40 00
30
60
90
120
150
180
0.01 M
Qe,
mg
/g
Ce, mg/L
0.10M
1.00M
Freundli sh Model
ig. 7. Cr(VI) adsorption isotherms onto PANDAN at three levels of ionic strength0.01, 0.10, and 1.00 mol L−1 NaCl), S/L ratio = 0.5 g L−1; T = 298 K; pH 4.0.
where Qe (mg g−1) is the total chromium amount adsorbed atequilibrium, Ce (mg L−1) is total chromium concentration at equi-librium, KF is the equilibrium constant indicative of adsorptioncapacity and n is an empirical constant. The calculated data fromFreundlich isotherm model were listed in Table 2.
A good fit of the data was obtained for PANDAN regardless ofionic strength as shown in Fig. 7. The regression analysis resulted inhigh correlation co-efficients, indicating a strong positive relation-ship. The magnitude of KF and n shows easily separation of heavymetal ion from wastewater and high adsorption capacity.
3.7. Adsorption kinetics
Fig. 8 presents the plots of Cr(VI) uptake versus contact time forPANDAN under different initial concentrations. It can be seen thatCr(VI) adsorption approaches to equilibrium within 4 h and 8 h for10 mg L−1 and 50 mg L−1 Cr(VI), respectively. Kinetic data were thenrepresented by the pseudo-second-order model [28]:
t
Qt= 1
K2Qe2
+ t
Qe(3)
where Qe (mg g−1) is the amount of Cr(VI) adsorbed on the adsor-bent at equilibrium, Qt (mg g−1) is the amount of Cr(VI) adsorbedon the adsorbent at time t (min) and K2 (g mg−1 min) is the equilib-rium rate constant of the second-order adsorption. High correlationcoefficients larger than 0.99 indicate that chromium uptake ontoboth adsorbents can be approximated favorably by the pseudo-second-order model. This suggested the chemical adsorption of thePANDAN to the Cr(VI) ions were the rate limiting step. This resultalso offered the evidence for covalent bonds formation betweenCr(III) and amide groups.
Table 3 compares the adsorption capacities of the PANDAN forCr(VI) with that of several adsorbents reported in the literatures.
It can be seen that the PANDAN exhibits higher uptake propertiesof chromium than that of many other sorbents, especially in smalldosage.
0 50 0 10 00 150 0 200 0 250 0 300 00
10
20
30
40
50
60
70
Qe,
mg
/g
Time, min
C0=10 mg/L
pseudosecond-order model
R2=0.99
k2=5.89(10
-3g mg
-1 min
-1)
C0=50 mg/L
k2=4.65 (10
-4g mg
-1 min
-1)
R2=0.99
Fig. 8. Cr(VI) adsorption kinetics onto PANDAN at various initial concentrations(50 mg L−1 and 10 mg L−1), T = 298 K; S/L ratio = 0.5 g L−1; pH 4.0.
720 Q. Li et al. / Chemical Engineering Journal 173 (2011) 715– 721
Table 3Comparison of maximum adsorption capacities of various adsorbents for Cr (VI).
the process, and hence highest breakthrough time.Effect of initial Cr(VI) concentration on the percentage removal
of Cr(VI) was also studied. Fig. 11A shows the effect of sample initial
0.0
0.2
0.4
0.6
0.8
1.0
Ce/C
o
10mg/L
50mg/L
100mg/L
(A)
Ch-Fe 295 4.7 [4]PANDAN 154.7 3.5 This study
.8. Desorption
The type and concentration of eluent were important factorsn desorption of the analytes. A suitable eluent should effectivelylute the analytes with small volume. In this study, the exhaustedANDAN was subjected to regeneration by using a series of HCl,aOH and HCl,solutions containing 5% sulfocarbonide with vari-us concentrations. The results in Fig. 9 indicate that the preloadedhromium can be well extracted by both 1.0 mol L−1 and 2.0 mol L−1
Cl solutions with 5% sulfocarbonide, with the correspondingesorption efficiency higher than 95%. Therefore, the optimum con-entration of HCl was determined to be 1.0 mol L−1 for economiceasons. However, the desorption of Cr was incomplete, and it willhorten the lifetime of the adsorbent. Further studies in pilot-plantcale and cost evaluation are still necessary.
.9. Column adsorption
Cr(VI) retention in a fixed-bed column was investigated with dif-erent flow rate and initial Cr(VI) concentrations. Fig. 10 illustratesn effluent history of a fixed-bed column packed with PANDANor a feeding solution containing Cr(VI) at the flow rate range of–10 mL min−1. As shown in Fig. 10, the values of Ct/C0 were very
ow and the breakthrough curves (BTC) were smooth in the begin-ing. In this period, the total chromium concentration of the outow was below 1.0 mg L−1. With more and more Cr(VI) solutionassed through the absorption layer, the BTC became sharper, and
uickly got breakthrough point. As seen in Fig. 10, the effectivereatment volume decreases along with the increase of sampleow rate from 5.0 to 10.0 mL min−1, and the breakthrough capac-
ty decreases from 36.4 to 17.5 mg g−1. This is due to the shorter
0.2 1.0 2.00
20
40
60
80
100
Eluant con cen tration, mol/ L
HCl and 5% sulfocarbon ide NaOH HCl
De
so
rpti
on
ra
te,
%
ig. 9. Desorption ratio of Cr(VI) using a series of HCl, NaOH and HCl solutionsontaining 5% sulfocarbonide with various concentrations.
Fig. 10. Effect of flow rate on adsorption of Cr(VI) on breakthrough curve, C0 ofCr(VI) = 50 mg L−1; ionic strength = 0.1 mol L−1 NaCl; pH 4.0.
residence time of Cr(VI) ion in column at higher flow rate. It isexpected that at a lower flow rate due to adequate interactiontime, adsorption was very efficient, at least in the initial step of
0 100 0 200 0 300 0 400 0
water yield, m L
0 50 0 100 0 150 0 200 00.0
0.1
0.2
0.3
0.4
0.5
water yield, m L
Ce,
mg
/L
10mg/L
50mg/L
100mg/L
(B)
Fig. 11. (A) Effect of initial Cr(VI) concentration on breakthrough curve, flowrate = 5.00 mL min−1, ionic strength = 0.1 mol L−1 NaCl; pH 4.0. (B) Total chromiumconcentration of out flows at different initial Cr(VI) concentrations in the beginningof the breakthrough curves.
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Q. Li et al. / Chemical Engine
oncentration on adsorption capacity. Sharper BTC was obtained athe higher feed Cr(VI) concentration, because with the increase innitial Cr(VI) concentration, the PANDAN bed would get saturatedarlier with the Cr(VI) ion. Fig. 11B illustrates the total chromiumoncentration of out flows at different initial Cr(VI) concentra-ions in the beginning of the breakthrough curves. The obtainedesults showed that Cr(VI) retention by PANDAN would result ints conspicuous decrease even to below 0.1 mg L−1. At the initialoncentration of 10, 50 and 100 mg L−1, the breakthrough capacityas 24.3, 36.8 and 49.4 mg g−1, respectively, which indicates highr(VI) removal by the PANDAN bed.
. Conclusions
In this work, PANDAN was successfully fabricated and applied asdsorbent to remove Cr(VI). The results of electrophoretic mobil-ty and XPS spectra show that Cr(VI) was first adsorbed onto theANDAN via electrostatic adherence between Cr(VI) anions and themine (–NH•+–) group on the PANDAN molecular chain. Most of ther(VI) was reduced to Cr(III) and then binds to PANDAN because ofhe complexation. The adsorption was found to be strongly depen-ent on pH, with maximum adsorption capacity obtained at pH. The presence of K+, Ca2+, Mg2+, H2PO4
− ions had no signifi-ant effect on Cr(VI) removal, while SO4
2− and CO32− result in a
light decrease in adsorption capacity. The equilibrium data wasell fitted by Freundlich isotherm. Pseudo-second-order kineticsodel was found to be the predominant. On the basis of desorp-
ion study, it is concluded that the PANDAN could be regeneratednd reused with only little loss of adsorption capacity. Continu-us operation showed that the breakthrough adsorption capacitys strongly dependent on the feed flow rate and inlet Cr(VI) concen-ration. As the flow rate increases, the breakthrough curve becomesteeper and the breakthrough time and adsorbed Cr(VI) ion concen-ration decrease. Much sharper breakthrough curves are obtainedt the higher inlet Cr(VI) concentration. The above result indicateshat PANDAN could be employed as a low-cost adsorbent for theemoval for Cr(VI) and from aqueous solution including industrialastewater.
cknowledgements
This work was supported by the Natural Science Foundationf China (grants 51008154), foundation of State Key Laboratoryf Pollution Control and Resource Reuse of China, and the Scien-ific Research Foundation of Graduate School of Nanjing Universitygrants 2010CL07).
ppendix A. Supplementary data
Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.cej.2011.08.035.
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