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Chemical Engineering Journal 189– 190 (2012) 295– 302

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

j ourna l ho mepage: www.elsev ier .com/ locate /ce j

reparation and evaluation of activated carbon-based iron-containing adsorbentsor enhanced Cr(VI) removal: Mechanism study

eifeng Liu, Jian Zhang ∗, Chenglu Zhang, Liang Renhandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan 250100, China

r t i c l e i n f o

rticle history:eceived 14 December 2011eceived in revised form 27 February 2012ccepted 27 February 2012

eywords:dsorptionechanism

r(VI)

a b s t r a c t

Activated carbon-based, iron-containing adsorbents (TAC/Fe) were developed through a simpleimpregnating-air drying protocol for enhanced removal of Cr(VI) from aqueous solutions. The prepara-tion conditions have been optimized using Taguchi method. Cr(VI) adsorption behaviors on TAC/Fe andnative TAC under different Cr(VI) concentration, ionic strength and solution pH conditions were investi-gated. Both the Langmuir and Freundlich models correlated the sorption isotherms on the sorbents quitewell. The adsorption showed strong dependence on solution pH. XPS and FTIR analyses were employed toinvestigate the sorption mechanisms. Results demonstrated that Cr(VI) adsorption onto TAC/Fe consistsof four reaction steps: (1) fast protonation or deprotonation of carbon surface in the presence of solution

+ −

ctivated carbon

ron

H or OH ; (2) conjunction of Cr(VI) with the surface functionalities and formation of inner-sphere com-plexes of Cr(VI) with the Fe (hydr)oxides; (3) reduction of Cr(VI) into Cr(III), accompanied by oxidation ofthe carbon surface to form new oxygen-containing functional groups; and (4) ion exchange between thereduced Cr(III) and the created carboxyl and/or hydroxyl groups. The enhanced Cr(VI) removal efficiencyon TAC/Fe was derived from its iron component, higher content of acidic functional groups and more

negative surface.

. Introduction

Chromium, a redox active metal element, is widely distributedn the environment and exists in two common oxidation states,r(III) and Cr(VI). Usually Cr(III) occurs naturally and is consideredontoxic, while Cr(VI) compounds are highly toxic agents becausef their marked carcinogenic, teratogenic and mutagenic effects touman and other living organisms. Cr(VI) is widely present in theffluents of electroplating, metal finishing, leather tanning and pig-ents industries, which may pose a severe threat to public health

nd the environment if discharged without adequate treatment [1].hus, there is great need to devise available technologies to preventurther chromium discharge and remediate Cr(VI) contamination.

variety of techniques, such as coagulation/precipitation, mem-rane filtration, ion exchange, adsorption, and biological processes2–5] have been developed for this purpose. Among these meth-ds, adsorption is a widely used and promising technique becausef its high efficiency and advantages in simple operation and cost-ffective.

Because of their high affinity and selectivity toward Cr(VI) andther oxyanions such as arsenate and arsenite [6], iron (hydr)oxidesave been extensively used as adsorbents for the removal of these

∗ Corresponding author. Tel.: +86 531 88363015; fax: +86 531 88364513.E-mail address: zhangjian00@sdu.edu.cn (J. Zhang).

385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2012.02.082

© 2012 Elsevier B.V. All rights reserved.

contaminants from aqueous environment. In most cases, however,iron oxides have disadvantages of low mechanical strength, narrowapplicable pH range and slow adsorption kinetics, which limit theirwider industrial-scale application. Recently, iron-bearing activatedcarbons have been investigated and applied for arsenic removal[6,7]. A key attribution for activated carbon is its well-developedporosity and high specific surface area ranging from several hun-dred to around 2000 m2/g. Impregnating iron (hydr)oxides intoactivated carbon can take advantage of both the high selectivityof ferric oxides for arsenic and the structure advantages of acti-vated carbon. The porous carbon structure and the high surfacearea could offer sufficient active sites for iron loading [8]. SinceCr(VI) and arsenic species both present as oxyanions in aqueoussolution and have similar affinities toward iron (hydr)oxides, mod-ification of activated carbon with iron (hydr)oxides is expected tobe an effective way to enhance the Cr(VI) removal. However, littleinformation is available in the literature on the removal of Cr(VI)using Fe-doped activated carbons.

Our team has devised an iron-tailored activated carbon pre-pared from Trapa natans husk (TAC) via a simple impregnating-airdrying protocol [9]. FeCl3 was chosen as the impregnating agent.Experimental results showed that incorporation with Fe signifi-

cantly improved the Cr(VI) adsorption capacity on activated carbon.However the Cr(VI) removal mechanisms by iron-tailored TAC wasnot clarified. Furthermore, sorption properties of iron-tailored TACmay vary a lot on different preparation conditions such as the type

2 ring Journal 189– 190 (2012) 295– 302

oTTbtua(

2

2

pmvwctadcoaT

2

Bp(taupm[(sXeAt

2

dstswU5PfcwNt

mi

Table 1Experimental variables for the modification of TAC by iron salts: factors and levels.

Level Factors

A: iron salttype

B: ironconcentrations(mol/L)

C: dryingtemperature(◦C)

1 FeCl 0.005 60

Fe loading amount. The adsorption capacity reached its maximumwhen the drying temperature was 100 ◦C. It has been reported thatdrying temperature may influence the existing form of iron (fer-rihydrite, akaganeite, etc.) on activated carbon [8]. Based on the

Table 2Design matrix and measured responses.

Run no. Factors Cr(VI) adsorptionamount (mg/g)

A B C

1 1 1 1 18.152 1 2 2 19.113 1 3 3 19.714 2 1 2 18.735 2 2 3 19.466 2 3 1 18.867 3 1 3 19.088 3 2 1 23.389 3 3 2 30.92

K1 56.97 55.96 60.39K 57.05 61.95 68.75

96 W. Liu et al. / Chemical Enginee

f iron salt, impregnating concentration and drying temperature.he roles of these factors were not discussed in the previous work.he research herein aimed to optimize the preparation conditionsy using Taguchi optimization methodology [10]. Cr(VI) adsorp-ion behaviors by the virgin and iron-doped TAC were examinednder different experimental conditions. The adsorption mech-nisms were investigated by X-ray photoelectron spectroscopyXPS) and Fourier transform infrared (FTIR) spectroscopy.

. Materials and methods

.1. Materials

In this study, T. natans husk-based activated carbon (TAC) witharticle size of 140–200 mesh was prepared according to theethod described elsewhere [9]. Fe was incorporated into the TAC

ia a simple impregnating-air drying protocol. Briefly, 1.0 g of TACas mixed with 200 mL of ferrous/ferric salt solution at a certain

oncentration (pH adjusted to 4.0–4.5) for 12 h, followed by filtra-ion and washing with sufficient water to remove any unloadednd/or weakly adsorbed iron. Then the iron-loaded carbon wasried at a certain temperature under air atmosphere for 10 h. Theompound was then allowed to cool to room temperature. Theptimal preparation condition (metal salt type, iron concentrationnd drying temperature) was decided according to the results ofaguchi optimization analyses.

.2. Characterization of the adsorbents

The structural characteristics of the sorbents, includingrunauer–Emmett–Teller (BET) specific surface area, porosity, andore size distribution, were determined by a porosimetry analyzerQuantachrome Corporation, QUADRASORB SI, USA). The Fourierransform infrared (FTIR) spectroscopes of the carbons before andfter the adsorption were recorded in the range 4000–400 cm−1,sing a FTIR spectrometer (Thermo Scientific Brand, America). TheH at the point of zero charge (pHpzc) of the sorbents was esti-ated from a batch equilibrium method described by Babic et al.

11]. X-ray diffraction (XRD) analysis using an X-ray diffractometerD/max rA model, Hitachi) was employed to determine the crystaltructure and crystallinity of the iron-loaded carbon composites.-ray photoelectron spectroscopy (XPS) was used to examine thelemental composition and chemical oxidation state of the carbons.ll binding energies were referenced to the C 1s peak at 284.6 eV

o compensate for the surface charging effects.

.3. Adsorption methods

Batch Cr(VI) adsorption studies were performed by mixing pre-etermined amount of virgin or iron-loaded TAC with 100 mL ofynthetic chromium(VI) solution. The mixture was shaken in ahermo stated shaker at 200 rpm. After equilibrated for 48 h, theolid material was filtered and the residual Cr(VI) concentrationas determined according to the standard methods [12], using aV–vis spectrophotometer (UV-754, Shanghai) at wavelength of40 nm. Total chromium was determined by an Inductivity Coupledlasma Spectrometer. The concentration of Cr(III) was calculatedrom the difference between the total and hexavalent chromiumoncentrations. Standard solutions of 0.1 M HCl and 0.1 M NaOHere used for pH adjustment. Ionic strength was controlled withaCl solutions. All the experiments were carried out at a room

emperature of 22 ± 2 ◦C and were performed in duplicate.The amount of Cr(VI) adsorbed, Q (mg/g), was determined by a

ass balance relationship: Q = (C0 − Ce)V/W, where C0 and Ce are thenitial and equilibrium Cr(VI) concentration (mg/L) respectively; V

3

2 Fe(NO3)3 0.02 1003 FeSO4 0.1 150

represents the solution volume (L) and W is the mass of adsorbent(g).

3. Results and discussion

3.1. Optimization of the modification

The type of iron salt, impregnating concentration and dryingtemperature were considered as crucial factors in the modifica-tion process. Taguchi method [10] was employed to determine theoptimal design parameters. Each variable (each factor) was testedat three levels which covered a broad range (Table 1). Fractionalfactorial design (L9(3)3 orthogonal array) led to a total of 9 experi-mental runs. Data analysis results by using Microsoft Excel softwarewere listed in Table 2. Response (R) was used to represent the effec-tiveness of factor level, i.e. the optimal level of a design parameteris the level with the greatest R. The response of level j in factori, Rij, was computed as Kij/3, where Kij was the total Cr(VI) sorp-tion amounts of level j in the column of factor i. R values for the 9experiments were shown in Fig. 1.

As seen from Fig. 1, the type of iron salt was an important factorthat determines the Cr(VI) adsorption efficiency (R value). The car-bon modified by FeSO4 showed much higher Cr(VI) removal thanthat modified by FeCl3 and Fe(NO3)3, while FeCl3 and Fe(NO3)3presented similar modification results. TAC was positively chargedduring impregnating due to its higher pHpzc value (6.20) than solu-tion pH (4.0–4.5). So the better FeSO4 modification result mightbe caused by the fact that Fe2+ ions experienced less electrostaticrepulsion from the positive TAC than Fe3+. The adsorption capac-ity increased as the impregnating Fe concentration increased from0.005 mol/L to 0.1 mol/L, which may be attributed to the increased

2

K3 73.38 69.49 58.24R1 18.99 18.65 20.13R2 19.02 20.65 22.92R3 24.46 23.16 19.41

W. Liu et al. / Chemical Engineering Journal 189– 190 (2012) 295– 302 297

Table 3Langmuir and Freundlich isotherm parameters for the adsorption of Cr(VI) by TAC/Fe and TAC at different ionic strengths.

Adsorbent Ionic strength (mol/L) Langmuir isotherm Freundlich isotherm

Qm (mg/g) KL (L/mg) R2 Kf (mg/g(L/mg)1/n) n R2

TAC/Fe 0 68.4931 0.4256 0.9945 43.4018 10.277 0.98840.1 65.7895 0.6786 0.9980 38.6366 7.974 0.99681.0 64.5161 0.5827 0.9984 37.0104 6.720 0.9909

at0uf

3

411tFm

Q

Q

wriL(

amTLam[cacb

Fr

may be due to the dissolution of iron from TAC/Fe at extremelyacidic conditions.

In aqueous solutions, Cr(VI) can exist in different ionic formssuch as chromate (CrO4

2−), dichromate (Cr2O72−) and hydrogen

TAC 0 49.0196 0.04720.1 50.2512 0.03511.0 63.2911 0.0461

bove results, appropriate parameters for the modification pro-ocol were established as: FeSO4, impregnating concentrations of.1 mol/L Fe and drying temperature of 100 ◦C. The sample preparednder this condition was employed as the iron-doped TAC (TAC/Fe)or further studies, unless otherwise stated.

.2. Adsorption capacity

Sorption isotherm experiments were conducted at solution pH.0 ± 0.1 with the initial Cr(VI) concentration varied from 30 to10 mg/L. Three background electrolyte concentrations (0, 0.1 and.0 M NaCl) were studied to identify the effect of ionic strength onhe adsorption. Two commonly used models, the Langmuir [13] andreundlich [14] equations were employed to correlate the experi-ental data, which can be expressed respectively as

e = QmKLCe

1 + KLCe(1)

e = Kf Ce1/n (2)

here Qe (mg/g) is the amount of Cr(VI) adsorbed at equilib-ium; Ce (mg/L) is the equilibrium Cr(VI) concentration; Qm (mg/g)s the maximum Cr(VI) adsorption capacity; KL (L/mg) is theangmuir constant related to the apparent adsorption energy; Kf

mg/g(L/mg)1/n) and n are the Freundlich constants.As-fitted Langmuir and Freundlich parameters for the Cr(VI)

dsorption on TAC/Fe and TAC are given in Table 3. Both the Lang-uir and Freundlich models correlated the sorption isotherms on

AC/Fe and TAC quite well, with all R2 values higher than 0.97. Theangmuir equation is derived from the assumption of monolayerdsorption on specific homogenous sites, while the Freundlichodel represents physical adsorption on heterogeneous surfaces

15]. The good fitting results of both models implied that both

hemisorption and physisorption mechanisms took place in thedsorption systems. Freundlich constant, n, were higher than 1 in allases, indicating that hexavalent chromium is favorably adsorbedy TAC/Fe and TAC [16].

ig. 1. Factors that control the modification of TAC by iron salts for enhanced Cr(VI)emoval.

0.9763 5.9015 2.355 0.99450.9889 8.4858 2.855 0.98240.9798 9.4546 2.585 0.9814

As shown in Fig. 2, the isotherm plots of TAC/Fe at the three back-ground electrolyte concentrations almost overlapped, indicatingthat the sorption of Cr(VI) by TAC/Fe was ionic strength indepen-dent. This result suggests that electrostatic interaction is not amajor factor in the process [17]. Lützenkirchen [17] and Goldberg[18] proposed that insensitivity to ionic strength can be taken asan indication for inner-sphere surface complexation, in which theadsorbed molecules or ions and the sorbent surface establish cova-lent bonds. The adsorption of Cr(VI) by TAC was slightly promotedas the ionic strength increased. It is probably that the Cr(VI)–Cr(VI)intermolecular electrostatic repulsion force is significant in thesorption process by TAC, hence the enhanced charge screening athigh ionic strength leads to reduced electrostatic repulsion and thuspromoted the adsorption.

3.3. Effect of solution pH on the adsorption

The effect of pH on the removal of Cr(VI) by TAC/Fe and TAC wasexamined within pH range of 2.0–11.5 at an initial Cr(VI) concentra-tion of 50 mg/L and adsorbent dose of 0.6 g/L. As shown in Fig. 3, forboth samples, the adsorption of Cr(VI) showed strong dependenceon solution pH. The sorption was most favored (93.9–99.6%) atacidic conditions (pH 2.0). As the pH increased from 2.0 to 11.5, theCr(VI) removal efficiency declined sharply to less than 20%. Otherworkers have reported similar phenomena for the uptake of Cr(VI)by commercially activated carbons [19,20]. In most cases, the sorp-tion capacity of TAC/Fe was higher than the virgin carbon. HoweverTAC showed a slightly higher sorption extent at pH 2.0–3.0. This

Fig. 2. Cr(VI) adsorption isotherms for TAC/Fe and TAC at different ionic strengths.

298 W. Liu et al. / Chemical Engineering Journal 189– 190 (2012) 295– 302

Table 4Acquisition and curve fitting parameters of C 1s and Cr 2p3/2 signal for the XPS spectra of the raw and Cr-loaded TAC/Fe and TAC.

Signal Binding energy (eV) Atom ratio % Binding energy (eV) Atom ratio %TAC/Fe TAC

CH ( CH3, CH2, CH) 284.57 45.45 284.56 64.94C O 286.76 41.36 286.12 18.83O C O 288.35 13.18 288.24 16.23

Signal Binding energy (eV) Atom ratio % Binding energy (eV) Atom ratio %TAC/Fe Cr TAC Cr

CH ( CH3, CH2, CH) 284.57 35.47 284.57 60.98C O 286.66 49.26 286.12 16.46

15.278.121.8

cm

H

H

2

sttaeC

btuEpsIigocf

H

FiC

O C O 288.13

Cr(III) 577.34

Cr(VI) 578.87

hromate (HCrO4−), depending on the solution pH and total chro-

ate concentration [21]:

2CrO4 ↔ H+ + HCrO4− (3)

CrO4− ↔ H+ + CrO4

2− (4)

HCrO4− ↔ Cr2O7

2− + H2O (5)

The calculated Cr(VI) species distribution diagram was pre-ented as dashed lines in Fig. 3. It is clear that at pH lower than 6.1,he dominant species of hexavalent chromium is HCrO4

−, which ishen gradually changed to CrO4

2− as the pH increases. Intensive Crdsorption occurred in the pH range where HCrO4

− species mainlyxisted. It is known that HCrO4

− is more favorably adsorbed thanrO4

2− due to its low adsorption free energy [20].The uptake extent of Cr(VI) as a function of solution pH can

e considered as a clear indication of the electron transfer reac-ion. The redox potential (E0) of the Cr(VI)/Cr(III) system dependspon solution pH. For instance, at pH ≈ 1, E0 ≈ 1.3 V and at pH ≈ 5,0 ≈ 0.68 V [22]. In acidic solution, Cr(VI) presents a very highositive redox potential, implying that hexavalent chromium istrongly oxidizing and unstable in the presence of electron donors.t is well accepted that the carbons bond to the oxygen functional-ties on activated carbon, such as ketone, carboxylic and hydroxylroups, can play a role as electron donors [23]. Therefore, Cr(VI)xyanion is readily reduced to Cr(III) in the presence of activated

arbon at acidic solutions, according to the following electron trans-er reactions:

CrO4− + 7H+ + 3e− ↔ Cr3+ + 4H2O (6)

ig. 3. Effect of solution pH on the Cr(VI) removal by TAC/Fe and TAC. Dashed linesndicate the calculated ratios of chemical species of hexavalent chromium (50 mg/Lr).

7 288.08 22.563 577.5 86.967 579.33 13.04

CrO42− + 8H+ + 3e− ↔ Cr3+ + 4H2O (7)

Sur-C-e− ↔ Sur-COxH (8)

where Sur-C represents the C bond on the sorbent surface, andSur-COxH indicates the newly formed oxygen containing functionalgroups caused by Cr(VI) oxidization. As can be seen in Eqs. (6) and(7), the reduction of Cr(VI) is accompanied by a large amount ofproton consumption, indicating vital importance of solution pHfor the Cr(VI) removal. Decreasing the solution pH will elevatethe redox potential of the oxidant, thereby extend the oxidationtoward more-resistant surface functionalities. The redox reactionscan cause the solution pH continuously increase until the redoxpotential of Cr(VI)/Cr(III) was too low to oxidize the carbon surface.It was observed in this study that the H+ content decreased afterCr(VI) adsorption at acidic pH ranges for both TAC/Fe and TAC. Forinstance, when the initial pH was 4.02 ([H+] = 95 × 10−6 mol/L), theequilibrium H+ concentration in the TAC/Fe and TAC systems wouldbe 0.3 × 10−6 mol/L and 15 × 10−6 mol/L, respectively (see Fig. S1).

Furthermore, little Cr(III) was observed to be released backinto the solution when pH > 3, suggesting that the reduction andadsorption of Cr species on the carbon surfaces was an irreversiblephenomenon. It is presumed that the reduced Cr3+ was adsorbedon the newly formed oxygen functionalities by an ion exchangemechanism [23]:

Cr3+ + H2O + Sur-COxH ↔ (Sur-COx− [Cr(OH)]2+) + 2H+ (9)

The oxygen functionalities on the surface of TAC and TAC/Feare electron-donating in nature (Lewis base), while Cr(III) ion iselectron-accepting in nature (Lewis acid). A Cr(III) ion may attachitself to three adjacent carboxylic group and/or hydroxyl group,which can donate some lone pairs of delocalized �-electrons to themetal ion for the formation of surface oxide compounds (Sur-COx

−

[Cr(OH)]2+) [24]. The pHzpc of TAC/Fe (3.37) is lower than that ofTAC (6.20), suggesting that the surface of TAC/Fe is more negativeat the same pH level. It would make the situation electrostaticallyfavorable for a higher Cr(III) uptake on TAC/Fe.

3.4. XPS and FTIR analyze

In order to fully understand the adsorption mechanisms, X-ray photoelectron spectroscopy analysis was performed for thesorbents prior to and after Cr(VI) adsorption. As shown from thesurvey spectra in Fig. 4a and curve fitting parameters in Table 4,the high resolution XPS spectrum of Cr(2p3/2) on the Cr(VI)-loadedTAC indicated that the adsorbed chromium predominantly existedin trivalent form (86.96% atomic ratio). Only a small amount of

chromium was present as Cr(VI) (13.04%). Furthermore, C 1s XPSspectra of TAC Cr (Fig. 4b and Table 4) showed a notable increase incarboxyl groups (O C O signal, from 16.23% to 22.56%), togetherwith a decrease in the CHx groups (C–H signal, from 64.94% to

W. Liu et al. / Chemical Engineering Journal 189– 190 (2012) 295– 302 299

F

6bbp

wCwhea[tbFtCb(

wso

ig. 4. XPS spectra of the virgin and Cr(VI)-adsorbed TAC: (a) Cr 2p3/2 and (b) C 1s.

0.98%) as compared to the raw TAC. This result shows that car-onic acid groups were created ( C H→ C O) on the TAC surfacey the powerful oxidation of hexavalent chromium that took placearallel to the chemical redox reaction.

In the case of TAC/Fe, the reduction of hexavalent chromiumas also obvious according to the Cr(2p3/2) XPS spectrum of ther(VI)-loaded TAC/Fe (Fig. 5a). Most of the adsorbed chromiumas reduced to Cr(III) (78.13%) with only 22.87% remaining asexavalent form. The Fe 2p peaks of TAC/Fe centered at bindingnergies of 711.89 and 725.52 eV with a usual shakeup satellitet 719.03 eV (Fig. 5c), which can be attributed to the Fe in Fe2O325]. After chromium adsorption, the Fe 2p3/2 peak shifted slightlyo 711.42 eV with the shakeup satellite to 718.84 eV, which alsoelong to the fully oxidized iron. The shifting of the BE peaks ine 2p indicated that chemical reactions took place. It could be thathe iron (hydr)oxides formed stable inner-sphere complexes withr(VI), thus contributed to the total chromium uptake. This mighte the reason why the Cr(III) atomic ratio on Cr(VI)-loaded TAC/Fe78.13%) was lower than that on TAC Cr (86.96%).

FTIR spectra for the sorbents with and without Cr(VI) adsorptionere further examined. Fig. 6a presents the comparison of FTIR

pectra of the virgin and Cr(VI)-loaded TAC. After the adsorptionf Cr(VI), correlated bands centered at 3350, 1557, and 1213 cm−1,

Fig. 5. XPS spectra of the virgin and Cr(VI)-adsorbed TAC/Fe: (a) Cr 2p3/2, (b) C 1s,and (c) Fe 2p.

300 W. Liu et al. / Chemical Engineering Journal 189– 190 (2012) 295– 302

FC

wrigbtpvCt

aCbbaataht

3

ariotsoC

ig. 6. Comparison of the FTIR spectra of TAC and Cr(VI)-loaded TAC (a), TAC/Fe andr(VI)-loaded TAC/Fe (b).

hich can be attributed to the stretching of O H, C O and C Oespectively, were increased. It seems that the adsorption resultedn an increase in the content of surface carboxyl and/or lactoneroups, which is coincident with the XPS results. Generally, theands in the low-frequency 400–1000 cm−1 region are assignedo metal-oxygen and metal-hydroxyl vibrations [26]. The intenseeak at 495.3 cm−1 in TAC Cr can be attributed to the Cr O bondibration, confirming the hypothesis that the adsorbed Cr(VI) orr(III) was bonded with the oxygen-containing functionalities ofhe sorbent surface.

As illustrated in Fig. 6b, the changes in the FTIR spectra of TAC/Fefter Cr adsorption were different from that of TAC. The intensity of

O and C O stretching in TAC/Fe was enhanced with a remarkableand shifting after Cr adsorption. This finding suggested that car-oxyl and/or lactone groups were created in the presence of Cr(VI),nd the chromium species chemically bonded to these groups. It islso clear that the peak at 423.5 cm−1 characterizing Fe O vibra-ion disappeared, but a new peak due to Cr O bond at 487.3 cm−1

ppeared after adsorption. This result provides evidence for theypothesis that Fe Cr interaction is an important mechanism forhe Cr(VI) uptake by TAC/Fe.

.5. Sorption mechanisms

On the basis of the above results, we propose a possible mech-nism for the removal of Cr(VI) by TAC/Fe and TAC. The Cr(VI)emoval process by TAC/Fe consists of four reaction steps (schemat-cally illustrated in Fig. 7): (1) fast protonation or deprotonationf the carbon surface in the presence of solution H+ or OH− (see

he supporting information); (2) conjunction of Cr(VI) with theurface functionalities and formation of inner-sphere complexesf Cr(VI) with the Fe (hydr)oxides; (3) reduction of Cr(VI) intor(III), accompanied by oxidation of the carbon surface to form

Fig. 7. Mechanism for the removal of Cr(VI) by TAC/Fe.

new oxygen-containing functional groups; and (4) ion exchangebetween the reduced Cr(III) and the created carboxyl and/orhydroxyl groups. The adsorption by TAC proceeded a similar pro-cedure except that no Cr(VI) was adsorbed via combining with Fe.

In the reaction process, it is very important to supply a largeamount of proton for promoting the reduction of Cr(VI) into Cr(III).The adsorption was most favored at acidic conditions. Neverthe-less, iron may dissolve out from TAC/Fe in extreme acid solutions(pH < 3), leading to irreversible damage of the sorbent. Further-more, many contaminated waste streams exist at neutral to slightlyalkaline pH. Due to the high buffer capacities of these waters andthe high cost of pH adjusting agents, such as H2SO4, it is not oftenviable to adjust the pH to extremely low values. Considering the

adsorption efficiency and the feasibility of actual operation, ini-tial pH 4.0 was recommended for actual sorption systems. TAC/Feexhibits good stability under this pH level with no iron dissolving

W. Liu et al. / Chemical Engineering Journal 189– 190 (2012) 295– 302 301

Table 5Surface areas and porosities of TAC/Fe and TAC.

Samples SBET Sext Smic Vt Vmic Dp

(m2/g) (m2/g) (%) (m2/g) (%) (cm3/g) (cm3/g) (%) (nm)

os

3

dFitptkmencu

tgvwtsat

Flfdptcc

Ff

TAC/Fe 976 501 51.33 475

TAC 1224 708 57.84 516

ut into the solution, as detected from the kinetic study (see theupporting information).

.6. Physical–chemical properties of adsorbent

The N2 adsorption/desorption isotherms as well as the pore sizeistributions of the virgin and modified carbons are presented inig. 8. Characterization parameters are quoted in Table 5. As seen,ron doping led to a slightly drop in the specific surface area andotal pore volume. The reduction of external surface area and meso-orous volume contributed 83.5% and 94.0% respectively to theotal decrements, while the microporous area and pore volumeept almost constant. It seemed that the impregnated iron pri-arily blocked the mesopores of the carbon and distributed in the

xternal surface. Drop of specific surface area after modification didot reduce the Cr(VI) adsorption capacity of activated carbon, indi-ating that physisorption was of minor importance for the Cr(VI)ptake.

TAC/Fe showed similar FTIR spectra as TAC (Fig. 6), except thathere was remarkable shifts in positions and shapes of C O and C Oroup peaks, and a new peak at 423.5 cm−1 characterizing Fe Oibration appeared. It is probably that the doped Fe was combinedith the C O and C O groups on the carbon surface. In addition, all

he bands appeared with higher intensities for TAC/Fe than for TAC,uggesting an increase in the content of acidic functional groupsfter iron modification [27]. This leads to a decrease in the pHpzc ofhe carbon (6.20 for TAC → 3.37 for TAC/Fe).

To summarize the above results, The enhancement effect ofe modification on the Cr(VI) removal can be explained as fol-ows: (1) the doped iron (hydr)oxides combined with Cr(VI) toorm stable inner-sphere complexes; (2) iron impregnation intro-uced more acidic functional groups such as carboxyl, lactone and

henol groups to the carbon surface, which provided more adsorp-ion sites for Cr; (3) more functionalities increased the negativeharge of the sorbent surface, which could acquire more positivelyharged Cr(III). All these three factors contributed to the higher

ig. 8. Pore size distributions and nitrogen adsorption/desorption isotherms (inset)or TAC/Fe and TAC.

48.77 0.74 0.19 25.68 3.0242.16 1.04 0.21 20.19 3.40

Cr(VI) removal efficiency of TAC/Fe. The virgin TAC did not possessany iron (hydr)oxides and the content of acidic functional groupswas low, resulting in a lower adsorption capacity.

4. Conclusions

In this work, T. natans husk-based activated carbon was incor-porated with iron via a simple impregnating-air drying protocolfor enhanced removal of Cr(VI) from wastewaters. The optimalmodification conditions were: FeSO4, impregnating concentrationsof 0.1 mol/L Fe and drying temperature of 100 ◦C. Both the Lang-muir and Freundlich models correlated the sorption isothermson TAC/Fe and TAC quite well. The adsorption on both sorbentsdecreased sharply as solution pH increased. Cr(VI) removal byTAC/Fe was found to consist of four reaction steps: fast protona-tion or deprotonation of the carbon surface; conjunction of Cr(VI)with the surface functionalities and formation of inner-sphere com-plexes of Cr(VI) with the Fe (hydr)oxides; reduction of Cr(VI) intoCr(III), accompanied by oxidation of the carbon surface; and ionexchange between the reduced Cr(III) and the created carboxyland/or hydroxyl groups. The adsorption by TAC proceeded a similarprocedure except that there was no chelation between Cr(VI) andFe. The enhanced Cr(VI) removal efficiency of TAC/Fe was derivedfrom its iron component and improved surface chemical proper-ties. The present work will provide useful information for potentialCr(VI) removal as well as its environmental risk assessment.

Supplementary data

Cr(VI) adsorption kinetics and changes of solution pH as a func-tion of contacting time are presented in the supporting information.

Acknowledgements

The work was supported by the National Key Technology R&DProgram for the 11th Five-year Plan (No. 2006BAC10B03), theNational Water Special Project (2009ZX07210-009-04), The Scien-tific Technology Research and Development Program of Shandong,China (No. 2010GZX20605) and Graduate Independent InnovationFoundation of Shandong University (2009JQ009).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.cej.2012.02.082.

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