College of Food Science and Technology, Henan University of Technology, Zhengzhou, 450052, ChinaCollege of Bioengineering, Henan University of Technology, Zhengzhou, 450001, ChinaSherwood Design Engineers, San Francisco, CA 94111-1223, USA
r t i c l e i n f o
rticle history:eceived 19 June 2011eceived in revised form1 September 2011ccepted 21 September 2011
eywords:dsorption
a b s t r a c t
In this study, three titanium dioxide (TiO2) adsorbents doped with Ce(III) (Ce3), Ce(IV) (Ce4) and H2O2
oxidized Ce(III) (Ce3O), respectively, were prepared and the effect of cerium valence on arsenate (As(V))adsorption was investigated. The Ce3, Ce4 and Ce3O adsorbents all existed in amorphous form and Ce3
exhibited a higher micropore surface area and micropore volume than both Ce4 and Ce3O. The adsorptioncapacity of As(V) on Ce4 and Ce3O decreased significantly when the solution pH was increased, while theadsorption capacity of As(V) by Ce3 was higher and more stable at pH values ranging between 3.7 and7.0. The adsorption kinetics of As(V) on Ce3 was better fit to pseudo-second order model while both the
rsenateeriumitanium dioxide
pseudo-second order and the pseudo-first order model described the adsorption of As(V) on Ce4 andCe3O well. The equilibrium adsorption data for As(V) on Ce3 was well fit by the Langmuir model, and theadsorption of As(V) on Ce4 and Ce3O was described well by both the Langmuir and Freundlich models.FTIR analysis indicated that the hydroxyl groups on the three adsorbents’ surfaces were involved in As(V)adsorption. The results suggested that the dominant chemical state of cerium valence plays an importantrole in affecting the adsorption behavior of As(V) by cerium-doped TiO2 adsorbents.
Arsenic contamination in groundwater is quite common andoses a threat to millions of people in West Bengal of India, Viet-am, Chile and Bangladesh [1,2]. In order to combat the problemf arsenic contamination, various technologies for arsenic removalrom water sources are proposed and adopted, such as coagulation,on exchange, adsorption and electrodialysis. Adsorption processesre regarded as one of the most promising techniques for arsenicemoval from water, and many adsorbents have been reported toffectively remove arsenic [2]. Activated alumina has long been theost popular adsorbent. However, problems including aluminum
issolution, the relatively low adsorption capacity and narrow opti-um pH ranges have restricted its applications [3,4]. To overcome
he drawbacks of traditional adsorbents, many studies have beenonducted to develop adsorbents with both high arsenic adsorptionapacity and cost effectiveness. Among newly developed adsor-
ents, various hydrous oxides of rare earth elements and dopediometal oxide adsorbents have attracted much attention due toheir high affinity toward As(V) [5–7].
Although in recent years, considerable work has been reportedon the preparation of cerium oxide adsorbent for As(V) adsorptionand the application of the redox nature of coupled Ce3+/Ce4+ hasbeen studied in many areas [8,9], little is known about the effectof cerium valence on As(V) adsorption performance of syntheticadsorbents, which is vital for the development of new As(V) adsor-bents when cerium is used as one component of the adsorbent. Ithas been reported that the adsorption capacity of cerium-dopediron oxide adsorbent (Fe–Ce08) and TiO2 adsorbent (Ce–Ti) wassignificantly higher than those of the referenced materials (such asthe pure Fe oxide (Fe3O4), TiO2 and Ce oxide (CeO2)). The commonfeature of the two studies is that the dominating chemical stateof Ce in hybrid Ce–Ti and Fe–Ce08 was Ce(III), though Ce(IV) wasused as the cerium source for the preparation of Fe–Ce08 adsorbent[5,7].
Therefore, the present work synthesized titanium dioxideadsorbents doped with Ce(III) (Ce3), Ce(IV) (Ce4) and H2O2 oxi-dized Ce(III) (Ce3O), respectively, and investigated the effectof cerium valence on As(V) adsorption in aqueous solutions.The three adsorbents were characterized using XRD, nitro-
gen adsorption–desorption, FTIR and XPS analysis. The effectsof solution pH on As(V) adsorption, the adsorption kinet-ics and isotherms on the three adsorbents were studied andcompared.
characterize crystal material in the adsorbent. Fig. 2 presents theXRD patterns for the Ce3, Ce4 and Ce3O adsorbents. The XRDpattern of the Ce3, Ce4 composite oxides particles exhibited an
(c)
(b)
62.555
.1
28.7
47.8
37.9
Inte
nsity
25.3
(a)
08 Z. Li et al. / Chemical Engine
. Materials and methods
.1. Materials and chemicals
All chemicals used were of analytical grade and all stock solu-ions were prepared with deionized water. A 1000 mg/L As(V) stockolution was prepared by dissolving 4.1653 g Na2HAsO4·7H2O in 1 Lf deionized water. The concentration of arsenic species is given ashe concentration of elemental arsenic.
.2. Adsorbents preparation
Three titanium dioxide adsorbents doped with cerium wererepared using different synthesis methods. In the first method,reparation procedure for Ce(III)-doped titanium dioxide adsor-ent was carried out as previously reported in the literature [7].n brief, the TiO2 was first produced by the hydrolysis of 0.2 mol/Li(SO4)2 solution in the presence of 0.16% polyvinyl alcohol (PVA) at0 ◦C in a thermostatic water bath for 2 h, and then Ce(NO3)3 solu-ion was added to reach 0.02 mol/L, followed by pH adjustment tobout 8 by the addition of NaOH solution. The precipitates wereltrated and washed with deionized water, and finally heated in anven at 80 ◦C until constant weight was reached. The sample wasesignated “Ce3”.
In the second method, the detailed procedures for the prepa-ation of Ce(IV)-doped titanium dioxide adsorbent was the sames that described in the first method except that Ce(NO3)3 waseplaced by Ce(SO4)2. The sample was designated “Ce4”.
In the third method, when Ce(NO3)3 was added to theydrolysate of Ti(SO4)2 solution described in the first method,ydrogen peroxide in the molar ratio (H2O2:Ce(III)) of 1:2 wasdded simultaneously and stirred for 5 min under heat (80 ◦C) on
magnetic hotplate to convert Ce(III) to Ce(IV) [10]. Then NaOHolution was added drop-wise until a pH of 8.0 was achieved. Theample was designated “Ce3O”.
All dried adsorbent was crushed and sieved, and granules in theize range of 0.075–0.16 mm were used for adsorption.
.3. Batch adsorption experiments
Batch adsorption experiments were conducted to examine theffects of solution pH on the adsorption behaviors, the adsorptioninetics and the adsorption isotherm, respectively. The adsorptionxperiments were carried out in 100 ml of 5 mg/L As(V) solution atH 6.5 and after the addition of 0.01 g of the adsorbent, the flaskas shaken at 150 rpm in a thermostatic shaker at 25 ◦C for 20 hnless otherwise indicated.
The effects of equilibrium pH on As(V) adsorption were inves-igated at the initial solution pH varying from 3 to 11 with thenitial As(V) concentration of 5 mg/L. The initial solution pH wasdjusted by adding dilute NaOH and HCl solutions. The kinetics ofs(V) adsorption was carried out at different time intervals rang-
ng from 12 min to 24 h. The adsorption isotherms were studied atnitial arsenic concentrations varying from 1 to 40 mg/L, and solu-ion pH was adjusted to 6.5 and kept constant during the adsorptionxperiments. After the adsorption experiment, the solution was fil-ered with a 0.22 �m membrane filter and filtrates were analyzedor arsenic using ICP/AES spectrometer (IRIS Interpid II XSP, Thermolemental).
.4. Adsorbent characterization
The adsorbents before and after adsorption were characterizedy Fourier transform infrared spectroscopy (Perkin-Elmer Spec-rum One) using the potassium bromide (KBr) pellet techniquerom 400 to 4000 cm−1. Crystallization features of the adsorbent
Binding en ergy (eV)
Fig. 1. XPS spectra for Ce 3d of (a) Ce3, (b) Ce4 and (c) Ce3O adsorbents.
were determined by X-ray diffraction analysis (D8-ADVANCE,Brucker, German). The specific surface area, total pore volumes andthe mean pore diameter of the solids were determined from the N2adsorption–desorption isotherms at the liquid-nitrogen tempera-ture using the Brunauer–Emmett–Teller (BET), Saito–Foley (SF) andBarrett–Joyner–Halenda (BJH) methods (Quantachrome Quadra-Sob SI). Micropore surface area was estimated by the t-plot method.The X-ray photoelectron spectra (XPS) analyses were carried out ona PHI Quantera SXM system. Al K� radiation was used as the sourceand the XPS spectra were referenced with respect to the 284.8 eVC1s level.
3. Results and discussion
3.1. Adsorbent characterization
The Ce 3d spectra of the Ce3, Ce4 and Ce3O adsorbents wereshown in Fig. 1. The peak at 916.8 eV corresponding to Ce(IV) wasobserved in the spectra of the Ce4 and Ce3O but it did not appearedin the spectrum of the Ce3 adsorbent. It suggested that the domi-nant chemical state of Ce in the Ce4 and Ce3O adsorbents was Ce(IV)and little Ce(III) was oxidized to Ce(IV) in the Ce3 adsorbent[7].
The X-ray diffraction (XRD) analysis has often been used to
70605040302010
2θ (ο)
Fig. 2. XRD patterns of (a) Ce3, (b) Ce4 and (c) Ce3O adsorbents.
Z. Li et al. / Chemical Engineering Journal 175 (2011) 207– 212 209
Table 1Surface areas and pore volumes of the Ce3, Ce4 and Ce3O adsorbents.a
a SBET: BET surface area; Smi: micropore surface area; Se: external surface area; Volume.
morphous diffraction peak with a weak, broad peak at approx-mately 2� = 20–30◦ which was centered at 25.3◦. The resultsndicate that the Ce3 and Ce4 adsorbents exist in both amorphousnd smaller crystallite size of anatase phase. No cerium oxide peaksppeared in the XRD spectra of the two adsorbents, which waserhaps due to its amorphous form in the adsorbents. When Ce3Oarticles were prepared in presence of H2O2, the XRD spectra werelso generally amorphous with weak crystalline peaks at 25.3◦,7.9◦, 47.8◦, 55.1◦ and 62.9◦. These peaks correspond to the (1 0 1),0 0 4), (2 0 0), (1 0 5) and (2 0 4) reflections of the crystalline anatasehase of titanium dioxide materials (JCPDS No. 83-2243)[11,12],
ndicating the presence of crystalline TiO2 in the Ce3O adsorbent.he presence of smaller crystalline CeO2 also appears, as evidencedy the very weak diffraction peak from the (1 1 1) plane at 28.7◦
5,10]. It should be pointed out that the crystalline phase of TiO2nd CeO2, which appeared in the XRD spectra of the Ce3O adsor-ent may be attributed to the degradation of PVA in the presence ofydrogen peroxide during the preparation of the adsorbent [13]. Itas reported that the amorphous phase of Ce–Ti hybrid adsorbentas related to the interference of PVA in the adsorbent [12].
The surface area and pore volumes of Ce3, Ce4 and Ce3O adsor-ents determined from the N2 adsorption–desorption isothermere shown in Table 1. The results showed that Ce3 had a slightlyigher BET surface area (SBET) (125.5 m2/g) compared with thether two adsorbents. The total pore volume (Vt) of Ce3 was lowerhan that of Ce4 and only a little higher than that of Ce3O. Theurface areas were not proportional to the total pore volumes,ecause the pore size distributions were different among the threedsorbents. As shown in Table 1, the micropore surface area andicropore volume of Ce3 were higher than Ce4 and Ce3O and the
xternal surface area (Se) was similar to each other.
.2. Effect of pH on As(V) adsorption
The effects of equilibrium pH on As(V) removal by Ce3, Ce4 ande3O adsorbents were examined at various pH values ranging from.2 to 10.3 and presented in Fig. 3. The Ce3 adsorbent effectivelydsorbed As(V) at pH values ranging from 3.8 to 7.0 and only a
1110987654320
10
20
30
40
50Ce3
Ce4
Ce3O
As(
V) a
dsor
bed
(mg/
g)
pH
Fig. 3. Effect of pH on As(V) removal by Ce3, Ce4 and Ce3O adsorbents.
48.0 66.9 114.9
l pore volume for pores with diameter less than132 nm; Vmi: t-method micropore
slight decrease was observed as solution pH was increased, whilethe adsorption capacity decreased steeply from 40 to 4 mg/g whenthe solution pH was increased from 7.0 to 10.2. This behavior isconsistent with previously reported Ce–Ti oxide adsorbents pre-pared in the same way [12]. Similarly, As(V) removal by previouslyreported Ce–Fe adsorbents basically maintains a constant value ina pH range between 3.8 and 7.0 [4]. In the case of the Ce4 and Ce3Oadsorbents, As(V) adsorption was found to be more efficient in theacidic pH range and no evidence for maximum As(V) adsorptionwas achieved. As the pH of the test As(V) solution increased from3.2 to 10.1, As(V) adsorption amounts decreased significantly forboth adsorbents. Solution pH is therefore a strong factor in adsorp-tion of As(V) by the Ce4 and Ce3O adsorbent. It has been reportedthat the adsorption tendency of As(V) onto TiO2 suspensions underacidic conditions is stronger than that one at higher pH [14].
As reported in previous works, chemical interaction and electro-static forces play a key role in adsorption processes [15]. Accordingto electrostatic considerations, the adsorption process depends onthe predominant protonation state of adsorbent surface group andthe As(V) speciation in solution. In the pH range studied, H2AsO4
−
and HAsO42− forms of As(V) were predominant and may have been
attracted by the protonated active sites on the adsorbent [16]. As pHwas increased, the negative charge on the adsorbent increased andthe positive charge decreased. Therefore, the electrostatic repul-sion between the negatively charged surface and the oxyanions ofAs(V) caused the reduction of adsorption performance. This couldexplain the decreased As(V) adsorption capacity on Ce4 and Ce3Oadsorbents with increase of pH. However, As(V) removal by Ce3was maintained at a constant plateau below pH 6. It is thereforereasonable to assume that, in addition to electrostatic interaction,chemical interaction exists. Hence strong attraction between theAs(V) anions and the Ce3 adsorbent along with chemical interactiongive higher adsorption capacity in a broad pH range [12]. Similarresults were observed in As(V) removal from an aqueous mediumby calcined refractory grade bauxite [17].
3.3. Adsorption kinetics
Fig. 4 shows the adsorption kinetics of As(V) adsorption on theCe3, Ce4 and Ce3O adsorbents versus adsorption times. Adsorptioncapacity of the Ce3 adsorbent increased with increasing adsorp-tion time and the adsorption took place in two steps. In the firststep, the adsorption took place rapidly, after which, in the secondstep, it slowed and reached equilibrium after approximately 12 h.The As(V) adsorption amount increased gradually by Ce4 and Ce3Oadsorbents with increasing shaking time and reached equilibriumafter approximately 16 h. Although the adsorption of As(V) by Ce4and Ce3O adsorbents did not show obvious two-step kinetics, morethan 50% of As(V) was adsorbed during the first 5 h of the process.The initial adsorption process of As(V) on Ce4 and Ce3O adsorbentswas slower than that of Ce3. The faster initial adsorption of As(V)by Ce3 may be due to the availability of more active sites on theadsorbent and higher affinity of the adsorption sites to As(V) than
those on of the Ce4 and Ce3O adsorbents.
The equilibrium time required for maximum removal of As(V)by Ce-doped iron oxide adsorbent was 24 h [4]. The adsorption ofAs(V) and As(III) by nanocrystalline TiO2 reached equilibrium in
210 Z. Li et al. / Chemical Engineering Journal 175 (2011) 207– 212
3025201510500
10
20
30
40
50 Pseudo-second order Pseudo-first order
Ce3
Ce4
Ce3O
As(
V) a
dsor
bed
(mg/
g)
arrnaw
tppt
l
wec
l
was
astppooCotwoct
201510500
10
20
30
40
50
60Ce3
Ce4
Ce3O
Langmuir fitting Freundlich fitting
As(
V) a
dsor
bed
(mg/
g)
TP
t (h)
Fig. 4. Adsorption kinetics of As(V) on the Ce3, Ce4 and Ce3O adsorbents.
pproximately 4 h, which was longer as compared to the reportedate of As(V) removal by P25 [14,18]. The longer time needed toeach adsorption equilibrium was explained by the high porosity ofanocrystalline TiO2 [18]. The slow adsorption of As(V) on Ce3, Ce4nd Ce3O therefore confirms the high porosity of the adsorbents,hich was also supported by the above pore characteristics.
In order to investigate the adsorption rates of As(V) on thehree adsorbents, the adsorption process was determined by usingseudo-first order and a pseudo-second order kinetic models. Theseudo-first order kinetic model is known as the Lagergren equa-ion and is usually written as follows [19]:
og(qe − qt) = log qe − k1t
2.303(1)
here qe and qt are the amount of As(V) adsorbed (mg/g) at thequilibrium time and time t (min), respectively, and k1 is the rateonstant of adsorption (1/min).
The pseudo-second order equation is usually expressed as fol-ows [20]:
t
qt= 1
kq2e
+ t
qe= 1
h+ t
qe(2)
here h represents the initial sorption rate (mg/g/min), qe is thedsorption capacity at equilibrium (mg/g), and k2 is the rate con-tant of adsorption (g/mg/min).
The first and second order corresponding parameters for As(V)dsorption on the Ce3, Ce4 and Ce3O were calculated and arehown in Table 2. The results of the present study indicated thathe adsorption of As(V) on Ce3 adsorbent was better fit with theseudo-second order kinetic model over the entire contact timeeriod by non-linear regression analysis. The adsorption of As(V)n both the Ce4 and Ce3O adsorbents was fit well by the pseudo-firstrder model and the pseudo-second order model. For Ce3, Ce4 ande3O adsorbents, the initial adsorption rates of the pseudo-secondrder kinetic model were 40.08, 18.80 and 26.38 mg/(g h), respec-ively. Thus, the initial adsorption rate of As(V) on to Ce3 adsorbent
as initially much faster than the others. The pseudo-second
rder kinetic model, which assumes that the rate limiting step ishemical adsorption has been successfully used in many adsorp-ion processes [20,21]. From the adsorption kinetics study, we may
able 2arameters of pseudo-first order and pseudo-second order kinetic model for adsorption o
Fig. 5. Adsorption isotherms of As(V) on the Ce3, Ce4 and Ce3O adsorbents.
assume that the initial rapid adsorption was presumably due toelectrostatic attraction between As (V) and the adsorption sites.The slow adsorption in the later stage may have represented agradual adsorption of As (V) at the inner surface by complexa-tion or ion exchange. The adsorption kinetics of As(V) by Ce-dopediron oxide adsorbent obeyed a pseudo-first order rate equation [4].The adsorption kinetics of As(V) and As(III) by nanocrystalline TiO2were described by a pseudo-second order equation [18].
3.4. Adsorption isotherm
The adsorption isotherms of As(V) on the Ce3, Ce4 andCe3O adsorbents are presented in Fig. 5. The adsorption amountincreased with increasing As(V) concentration for all the threeadsorbents. Among them the Ce3 adsorbent presented a uniqueadsorption pattern and exhibited a higher adsorption capacity thanboth the Ce4 and Ce3O adsorbents over the range of concentrationsinvestigated.
Adsorption isotherms have commonly been used to evaluate themaximum adsorption capacity of an adsorbent for an adsorbate. Todetermine the maximum adsorption capacity of As(V) on the threeadsorbent, the Langmuir and Freundlich models were selected todescribe the experimental data [14]. The Langmuir isotherm modelis a theoretical model for monolayer adsorption:
q = qmaxbCe
1 + bCe(3)
where q is the amount of metal adsorbed (mg/g dry weight), b isthe Langmuir constant related to the affinity of the binding sites(L/mg), qmax is the maximum metal uptake (mg/g), and Ce is thefinal metal concentration in the solution (mg/L).
The Freundlich isotherm model is an experimental model and itis usually expressed as follows:
q = KC1/ne (4)
where K and n are the Freundlich constants.Constants for the Langmuir and Freundlich isotherms were cal-
culated by using non-linear regression for As(V) adsorption on theCe3, Ce4 and Ce3O adsorbents and are summarized in Table 3. As
hown in Table 3 and Fig. 5, the Langmuir equation successfullyescribes As(V) adsorption behavior on the Ce3 adsorbent, whichossibly exhibited monolayer adsorption of As(V) on to the adsor-ent surface. The adsorption isotherms of As(V) on Ce4 and Ce3Odsorbents were described well by both the Langmuir and Fre-ndlich models. Adsorption of As(V) at pH 4 for both HombikatV100 and Degussa P25 TiO2 surfaces fit well with the Langmuirquation, but the Freundlich equation gives a better adsorption ofs(V) at pH 9 [14].
It was clear that the maximum adsorption capacity of Ce3 adsor-ent for As(V) was much higher than those of the Ce4 and Ce3Odsorbents. It can be found that the adsorption capacity and Lang-uir constant (b) of the Ce3 adsorbent for As(V) were higher than
hose for the other two materials, which suggested a strong affinityetween the Ce3 adsorbent and As(V) (Fig. 5; Table 3). It should beoted that the dominant chemical state of Ce in the Ce3 adsorbentas Ce(III) as reported before [7]. However, the cerium valence ine4 and Ce3O adsorbents was Ce(IV). Therefore, the unique adsorp-ion behavior appears to be related to the dominant chemical statef Ce in the adsorbents prepared by a different method. It has beeneported that the cerium doped iron oxide adsorbent (Fe–Ce08)as significantly higher than those of the two referenced materi-
ls (the Fe oxide (Fe3O4) and Ce oxide (CeO2)) [5]. The results alsondicated that the dominant chemical state of Ce in Fe–Ce08 wase(III), although Ce(SO4)2 was used as the cerium source, and theominant chemical state of Ce in Ce oxide, on the other hand, wase(IV) [5].
.5. FTIR analysis
To investigate the interaction between the adsorbent and As(V),he infrared spectra of Ce3, Ce4 and Ce3O adsorbents before andfter As(V) adsorption were measured as shown in Fig. 6a–c,espectively. In the spectra of Ce3, Ce4 and Ce3O adsorbentsefore adsorption, the broad bands observed between 3700 and300 cm−1 centered around 3400 cm−1 are attributed to the O–Htretching vibration [�(O–H)]. The bands at 1635 cm−1 are assignedo H–O–H bending vibration [ı(OH)2] in molecular water. Theands at around 500 cm−1 are characteristic of metal-oxygen vibra-ions [22]. The bands appearing at 2926 and 1419 cm−1 belong tohe C–H stretching and C–H bending of CH2 [23,24], which is dueo the addition of PVA in the adsorbent.
In the spectra of Ce3 and Ce4, the strong band at 1120 and060 cm−1, which corresponds to the bending vibration of hydroxylroup of metal oxides (M–OH) [5], was observed from the sampleefore adsorption (Fig. 6a and b). It should be pointed out that theydroxyl group in PVA exhibited a characteristic peak at 1128 cm−1
24]. Therefore, the strong band at 1120 cm−1 may be attributedo the overlap of the bending vibration of the hydroxyl group of
etal oxides (M–OH) and the hydroxyl group present on the PVA7]. After adsorption, the peak at 1120 cm−1 shifted to lower waveumbers and the weak shoulder band at 1060 cm−1 disappeared. Inhe cases of the Ce3O adsorbent, only the weak band at 1053 cm−1
as observed from the sample before adsorption (Fig. 6c). Afterdsorption, the peak disappeared. There were also two large peakst 1523 and 1346 cm−1 observed in the sample, possibly corre-ponding to unidentate carbonate [25], which was perhaps from the
Fig. 6. Fourier transform infrared spectra of (a) Ce3, (b) Ce4 and (c) Ce3O adsorbentsbefore and after As(V) adsorption.
degradation of PVA due to the presence of hydrogen peroxide dur-ing the adsorbent preparation [13]. The FTIR results indicate thatthe adsorption of As(V) affected the chemical bonds of the hydroxyl
group in the adsorbent and the hydroxyl groups on Ce3, Ce4 andCe3O adsorbents’ surfaces were involved in the As(V) adsorption.As all the three adsorbents exhibited amorphous structure andhad similar BET specific surface areas, the high adsorption capacity
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12 Z. Li et al. / Chemical Engine
f Ce3 was not due to the increase in specific surface area or itsmorphous structure. Therefore, it is speculated that the higherdsorption capacity of arsenic on the Ce3 adsorbent probably dueo its dominant Ce(III) chemical state which may influence the
icrostructure of the adsorbent and the M–OH groups state on it.ome researchers have also reported that the peak of the hydroxylroups decreased or disappeared after the adsorption As(V) on thee–Ti, Fe–Ce and Fe–Mn adsorbents [5,7,26].
. Conclusions
Three adsorbents, Ce3, Ce4 and Ce3O were prepared to studyhe effect of the dominant chemical state of Ce on As(V) removalrom aqueous solution. The Ce3, Ce4 and Ce3O adsorbents all existn amorphous form and Ce3 had higher micropore surface areand micropore volume. The adsorption capacity for As(V) on thee4 and Ce3O adsorbents decreased significantly when the solu-ion pH increased from 3.2 to 10.1, while the Ce3 adsorbent had
uch higher adsorption capacity for As(V) at pH values rangingrom 3.8 to 7.0 and decreased rapidly when the solution pH wasncreased from 7.0 to 10.2. The adsorption kinetics revealed thathe adsorption equilibrium of As(V) on the Ce4 and Ce3O adsor-ents was achieved after 16 h, which was a little longer than for Ce3.he pseudo-second order model can well describe the adsorptioninetics of As(V) on the Ce3 adsorbent and the adsorption of As(V)n the Ce4 and Ce3O adsorbents can be fit well by both pseudo-econd order model and the pseudo-first order model. Adsorptionsotherms indicated that the Ce3 adsorbent obtained from Ce(III)ad much higher adsorption capacity than the Ce4 and Ce3O pre-ared by Ce(IV) and oxidized Ce(III) respectively. As(V) adsorptionn Ce3 was better fit by the Langmuir isotherm model than the Fre-ndlich model, while the adsorption isotherms of As(V) on Ce4 ande3O adsorbents were described well by both Langmuir model andreundlich model. The FTIR analysis indicated that the hydroxylroups on the three adsorbents’ surfaces were involved in As(V)dsorption. As(V) adsorption on the three adsorbents suggestedhat the dominant chemical state of Ce played an important rolen influencing the properties and the As(V) adsorption capacities oferium-doped biometal adsorbents.
cknowledgment
This research was supported by Doctoral Foundation of Henanniversity of Technology (2010BS058).
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