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Chemical Engineering Journal 175 (2011) 24– 32

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

jo u r n al hom epage: www.elsev ier .com/ locate /ce j

ltrasonic-assisted sodium hypochlorite oxidation of activated carbons fornhanced removal of Co(II) from aqueous solutions

eifeng Liu, Jian Zhang ∗, Cheng Cheng, Guipeng Tian, Chenglu Zhanghandong 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 5 August 2011eceived in revised form 28 August 2011ccepted 2 September 2011

eywords:odium hypochloriteltrasonicctivated carbonobalt

a b s t r a c t

Ultrasonic irradiation was employed to assist the modification of activated carbons (AC) by sodiumhypochlorite, and tested for their ability to remove Co(II) from aqueous solutions. Modification of the acti-vated carbon significantly enhanced their Co(II) adsorption capacity, which increased with the increasingNaOCl impregnating concentration. Ultrasonic-assisted impregnation can enhance the oxidation abilityof the NaOCl solution, thus gave a higher adsorption capacity of Co(II) compared with the use of impreg-nation only. The Co(II) removal was a combination of adsorption and chemical precipitation. The optimaladsorption pH range for the native and modified AC was pH 7.0–8.5 and 4.5–8.5 respectively, above whichprecipitation of copper hydroxide became the main contributor to the cobalt removal. Cation exchange,electrostatic interaction, and surface complexation were demonstrated as the major mechanisms for

dsorption the adsorption. Based on the XPS and FTIR analyses, Co(II) irons were considered to form monoden-tate charged complexes with the oxygen functionalities on the native AC, while they primarily formedmultidentate coordination complexes within the modified carbon. The enhanced Co(II) removal by theultrasonic-assisted NaOCl modified carbons was probably derived from their improved cation exchangecapacity and higher content of adjacent aliphatic functional groups, which provided more complexationsites for cobalt.

. Introduction

The presence of heavy metal ions in aqueous environment is anmportant environmental issue due to their non-degradability andoxicity. Cobalt is widely present in the waste streams from min-ng, electronic, metallurgical, electroplating and paint industries1]. When their concentration exceeds the tolerance levels, cobaltan be seriously toxic to humans causing genotoxicity, carcino-enicity, cardiomyopathy and bronchial asthma [2]. Hence, theres great need to devise available technologies to remove Co ironsrom nature water and industrial wastewaters. Adsorption by acti-ated carbons has proven to be a reliable and effective method forhis purpose. A number of research work focusing on the appli-ability of activated carbons developed from various materials forobalt(II) removal have been reported [3,4].

Surface modification of activated carbon is an attractive routeowards its novel application. Among the various modificationechniques, treatment with oxidizing solutions, such as nitric acid,

ypochlorite, and hydrogen peroxide are the best known [5]. Gen-rally, these oxidative treatments will lead a more hydrophilicurface of the carbon and a higher content of surface oxygen groups.

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

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.09.004

© 2011 Elsevier B.V. All rights reserved.

Hypochlorite is a milder oxidant than nitric acid, thus exhibits amuch smaller effect on the surface chemistry and porous structureof activated carbons than HNO3 [5]. We have tested the adsorptioncapacity of cobalt(II) by native activated carbon and those modifiedwith HNO3, H2O2 and NaClO. Results revealed that NaClO oxidationgreatly improved the cobalt(II) sorption on the carbon, whereasHNO3 and H2O2 treatments showed negative effect on the sorp-tion. This result is quite interesting, leading to an attractive studyon the sorption behaviors and mechanisms of Co(II) by NaClO mod-ified activated carbons. As far as we know, few attempts have beenmade to investigate the effect of NaClO oxidation on the removalof heavy metals by activated carbons.

Recently, the effects of ultrasonic irradiation on the mechanical,physical, and chemical changes of materials have received broadattention. Effects induced by ultrasound in aqueous solution havebeen attributed to the collapse of cavitation bubbles, which gen-erate extreme temperatures and pressures around the solid–liquidinterfaces [6]. These local effects produce a variety of radicals andhighly active intermediates, which may then initiate or inducematerial modifications [7]. A number of workers have reportedthe combination of ultrasonic technique with NaOCl in wastewater

treatment [8–10]. Ultrasound can improve the biocidal efficiencyof NaOCl during water disinfection through the dispersal of bac-terial clumps and the temporary weakening of bacteria cell walls[9]. Combination with ultrasonic can accelerate the bleaching rate

neering Journal 175 (2011) 24– 32 25

apHiwotmp

Ntefctocdb

2

2

her52putfvp1t

cte

2

sffciipfXeceap

2

b

AC ACM-40 ACM-40-without ultrasonic40

50

60

70

80

90

100

Co(II)removal%

Fig. 1. Co(II) removal efficiency on the native and NaOCl modified activated carbons

W. Liu et al. / Chemical Engi

nd efficiency of NaOCl for aqueous dyestuffs, primarily throughromoting the production of highly oxidising hydroxyl radicals [8].owever, the combination of ultrasonic with NaOCl for the mod-

fication of solid materials was still few in the literature. In thisork, we attempt to employ ultrasonic irradiation to assist the

xidization of activated carbons with NaClO. It is supposed thathis combination could enhance the oxidation ability of NaClO, and

ay cause other unexpected effects on the textural and chemicalroperties of the carbon, which would benefit the removal of Co(II).

The present work aimed to study the adsorption of Co(II) onaClO oxidized activated carbons prepared with ultrasonic assis-

ance. The main objective of this study were (1) to investigate theffect of ultrasonic-assisted NaClO oxidation on the sorption per-ormance of Co(II) by activated carbons; (2) to understand the wayobalt ions interact with the native and modified carbons. Towardshis aim, the effect of various parameters, including the presencef ultrasonic, NaClO impregnating concentration, solution pH andontacting time, on the adsorption process has been discussed inetail. XPS and FTIR spectra of the virgin and Co(II)-adsorbed car-ons were obtained to understand the sorption mechanisms.

. Materials and methods

.1. Sorbents preparation

An activated carbon, denoted AC, was prepared from aydrophytes residue, lotus stalk according to the method describedlsewhere [11]. Carbon AC was oxidized with sodium hypochlo-ite as follows: 2 g of AC was dispersed into a flask containing0 mL of sodium hypochlorite with a certain concentration (10%,0%, 40% and 60%), and shaken in a water bath at room tem-erature for 30 min. Afterwards, the mixture was irradiated withltrasound for 30 min (frequency = 40 kHz, power = 200 W, andemperature = 25 ◦C), followed by heating at 90 ◦C in a water bathor 3 h. After cooling, the solid was separated form the solvent by aacuum pump and washed with distilled water until a near neutralH was observed. Finally, the rinsed sample was dried at 105 ◦C for0 h. The modified samples are referred to as ACM-x, where x referso the NaOCl impregnating concentration.

To investigate the effect of ultrasonic irradiation on the modifi-ation, sodium hypochlorite modified samples without ultrasoundreatment was also prepared according to the above procedure,xcept that the shaken time for the mixture was 90 min.

.2. Sorbents characterization

The structural characteristics of the samples, including specificurface area, porosity, and pore size distribution, were determinedrom nitrogen adsorption/desorption isotherms at 77 K using a sur-ace area analyzer (Quantachrome Corporation, USA). The surfacehemistries of the carbons were detected using a Fourier transformnfrared radiation (FTIR) spectrometer (Fourier-380 FT-IR, Amer-ca), where the spectra were recorded from 400 to 4000 cm−1. TheH at the point of zero charge (PZC) of the samples was estimatedrom a batch equilibrium method described by Babic et al. [12].-ray photoelectron spectroscopy (XPS) was used to examine thelemental composition and their chemical oxidation state near thearbon surface. The measurements were performed by a spectrom-ter (ESCALAB 250) with Mg K� irradiation (1486.71 eV of photons)s X-ray source. All binding energies were referenced to the C 1seak at 284.6 eV to compensate for the surface charging effects.

.3. Adsorption methods

The cobalt adsorption experiments were carried out by aatch equilibrium method at room temperature (20 ± 2 ◦C). Stock

impregnated with and without ultrasonic (C0 = 30 mg/L, contact time = 12 h, pH ∼6.1,sorbent dose = 0.5 g/L).

solution of the test reagent (500 mg/L) was prepared by dissolvingcobalt nitrate (analytical grade) in distilled water. The pH of thesolutions was adjusted with various concentrations of HCl andNaOH. Experimentally, a predetermined amount of the virgin ormodified AC was mixed with 50 mL of Co(II) solution at desiredinitial concentration and pH. The samples were equilibrated byshaking in a water bath at 200 rpm for 12 h. Then the adsorbentparticles were separated by filtration, and the residual Co(II) con-centration was determined using an Inductively Coupled Plasma(ICP) spectrophotometer. The blank experiments without theadsorbent were simultaneously carried out. All the experimentswere performed in duplicate and the average value was reported.

The amount of Co(II) adsorbed, Q (mg/g), was determined by amass balance relationship: Q = (C0 − C)V/W, where C0 is the initialcobalt concentration (mg/L); C is the cobalt concentration at time tor at equilibrium (mg/L); V represents the solution volume (L) andW is the mass of the adsorbent (g).

3. Results and discussion

3.1. Effect of ultrasonic irradiation on the modification

Fig. 1 illustrates the comparison of Co(II) removal efficiency bythe native and NaOCl modified carbons treated with or withoutultrasonic. As seen, the Co(II) adsorption capacity of the carbonwas greatly enhanced after NaOCl oxidation. Ultrasonic-assistedmodification resulted in a greater extent of Co(II) removal (93.6%)than the case without ultrasonic (77.5%). This enhancement phe-nomenon might be explained as follows: (1) Sonolysis of H2O in thepresence of ultrasound can produce highly oxidising hydroxyl rad-icals. Meanwhile, the OCl− ions can hydrolyze into HOCl in aqueoussolutions, which could also undergo sonolysis to produce OH• rad-icals, resulting in higher concentrations of these highly oxidisingspecies. Thus the oxidation effect of the NaOCl solution to the car-bon surface was accelerated. This may also benefit the formation ofmore functional groups [8]; (2) the collapse of cavitation bubblescreated by ultrasonic results in the generation of extreme tempera-tures and pressures. These local effects may result in the cleavage ofthe aromatic structures of the carbon, favoring the access of metal

ions into the deeper inside of the carbon structure. Detailed evi-dence for the role of ultrasonic irradiation in the NaOCl oxidationprocess is still needed in further studies.

26 W. Liu et al. / Chemical Engineerin

0 5 10 15 20 25 30 35 4025

30

35

40

45

50

55

60

65 ACM-60

ACM-40

Qe(mg/g)

Ce (mg/L)

AC

ACM-10

ACM-20

Fc(

3

fiec[

Q

Q

wCmctn

mFhfia

TL

TS

ig. 2. Sorption isotherms of Co(II) onto the native and NaOCl modified activatedarbons fit by the Freundlich model (solid lines) and Langmuir model (dashed lines)contact time = 12 h, pH ∼6.1, sorbent dose = 0.5 g/L).

.2. Effect of sodium hypochlorite modification on the sorption

The sorption isotherms of Co(II) onto the native and modi-ed ACs with different NaOCl impregnating concentration werexamined (Fig. 2). The experimental data were correlated by twoommonly used models, the Langmuir and Freundlich equations13] which can be expressed respectively as

e = QmKLCe

(1 + KLCe)(1)

e = KfCe1/n (2)

here Qe (mg/g) is the amount of Co(II) adsorbed at equilibrium;e (mg/L) is the equilibrium Co(II) concentration; Qm (mg/g) is theaximum Co(II) adsorption capacity; KL (L/mg) is the Langmuir

onstant; Kf (mg/g(L/mg)1/n) is the Freundlich constant indicative ofhe relative adsorption capacity of the adsorbent, and the constant

represents the adsorption intensity.As shown in Table 1 and Fig. 2, both the Langmuir and Freundlich

odel correlated the sorption isotherm of Co(II) onto AC quite well.

or the ACM-x series, although both models exhibited relativelyigh R2 values (>0.99), the difference between the Langmuir-modeltting data and the experimental ones was significant. This devi-tion is particularly evident at low Co(II) concentration levels,

able 1angmuir and Freundlich isotherm parameters for the adsorption of Co(II) by the native a

Adsorbent Langmuir isotherm

Qm (mg/g) KL (L/mg) R2

AC 32.79 0.4155 0.9999

ACM-10 49.50 0.5372 0.9995

ACM-20 58.48 0.6333 0.9988

ACM-40 63.29 0.7281 0.9977

ACM-60 65.79 1.1515 0.9983

able 2urface areas, porosities and point of zero charge (PZC) of the native and sodium hypochl

Sample PZC SBET (m2/g) Sext (m2/g) �Sext Smic (m2/g)

AC 5.28 1084.90 572.32 – 512.58

ACM-10 6.49 869.43 399.59 172.73 469.84

ACM-20 6.68 590.43 247.57 324.75 342.86

ACM-40 6.83 397.04 159.98 412.34 237.06

ACM-60 7.01 340.26 150.64 421.68 189.62

g Journal 175 (2011) 24– 32

and for the carbons prepared with high NaOCl impregnating con-centrations. In contrast, the Freundlich model can describe thesorption isotherms of all the modified carbons reasonably well. TheLangmuir equation is derived from the assumption of monolayeradsorption on specific homogenous sites, while the Freundlichmodel represents multilayer adsorption on heterogeneous surfaces[14]. The fitting results imply a more heterogeneous surface of themodified carbons. Freundlich constant, 1/n, were less than 1 in allcases, indicating a favorable sorption process. The Co(II) sorptioncapacity of the carbon increased with increasing NaOCl impreg-nating concentration, which may be attributed to the strongeroxidization ability of NaOCl at higher concentrations. On the otherhand, the yield of the carbon mass decreased slightly from 100%to 81% as the NaOCl concentration increased from 0% to 40%, thendecreased sharply to 39% in the ACM-60 sample. To obtain a rela-tively high sorption capacity and mass yield, ACM-40 was chosenfor the following studies.

As seen from the FTIR spectra in Fig. 3, the native AC exhibiteda broad peak at 3425 cm−1 due to the O–H stretching vibration,which originated from phenolic groups and chemisorbed water.The weak band centered at 1865 cm−1 can be attributed to the C Ostretching vibrations from anhydride groups. The band centered at1707 cm−1 can be assigned to the stretching vibrations of C O moi-eties in ketone, ester and/or aromatic carboxyl groups. The peak at1580 cm−l was related to aromatic ring stretching or highly con-jugated C O group. Peaks around 1213 cm−1 and 1075 cm−1 canbe assigned to the C–O bonds, such as those in phenol, ester andether groups [15]. The modified samples showed similar character-istic bands with each other, but somewhat different from the nativeAC. A new peak at 1353 cm−1 appeared, and the stretching inten-sity of the bands at around 1865 cm−1, 1707 cm−1 and 1353 cm−1

became stronger as the NaOCl concentration increased from 10% to60%. It has been demonstrated that the peak at 1610–1550 cm−1

companied by a peak at 1380–1340 cm−1 was an indication ofcarboxylate group [16]. The results suggested that the NaOCl treat-ment leads to an increase of carboxylate, anhydride, ketone, and/orester groups on the carbon surface, the increment of which wasproportional to the NaOCl concentration.

As seen from the textural properties of the carbons in Table 2, theAC exhibited a typical micro-mesoporous structure with an aver-age pore diameter of 2.94 nm. NaOCl treatment caused a dramaticdecrease in the surface area and pore volume in all pore size range.

The decrease in the external surface area and pore volume wasparticularly obvious than the micro ones. It appears that ultrasonic-assistant oxidization made the pore walls thinner, thus parts ofthe micropores developed into larger ones, and the meso- and

nd sodium hypochlorite modified carbon samples.

Freundlich isotherm

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

20.6022 0.1129 0.991530.2350 0.1313 0.992535.3430 0.1414 0.995838.8225 0.1408 0.996844.9073 0.1154 0.9974

orite modified carbon samples.

�Smic Vt (cm3/g) Vmic (cm3/g) Vmes (cm3/g) Dp (nm)

– 0.798 0.204 0.594 2.9442.74 0.554 0.184 0.370 2.55

169.72 0.353 0.134 0.219 2.39275.52 0.225 0.092 0.133 2.27322.96 0.198 0.073 0.125 2.33

W. Liu et al. / Chemical Engineering Journal 175 (2011) 24– 32 27

4000 3500 3000 2500 2000 1500 1000 500

ACM-60

ACM-40

ACM-20

Wavenumbers (cm-1 )

AC

ACM-10

3424.9

1865.2

1567.6

1707.5

1352.9

1205.3

1070.2

FN

msaw

3

w8wa6t(

l

wpt

st

0 2 4 6 8 10 120

20

40

60

Qt(mg/g)

t (h)

ACM-40

AC

0 20 40 60 80 1005.0

5.2

5.4

5.6

5.8

6.0

6.2

6.4

SolutionpH

t (min)

ACM-40

AC

a

b

Fig. 4. (a) Sorption kinetics of Co(II) onto AC and ACM-40, and modeling using thepseudo-first-order (dashed lines) and pseudo-second-order (solid lines) equations;

number of active sites on the sorbent [17]. The V0 value for ACM-40 is about two times higher than that for AC, indicating a fastersorption of Co(II) onto ACM-40.

Table 3Pseudo-first-order and pseudo-second-order kinetic parameters for the adsorptionof Co(II) by AC and ACM-40.

Kinetic models Parameters AC ACM-40

Pseudo-first-order k1 (1/h) 1.0742 11.1462Qcal (mg/g) 7.2572 1.4530R2 0.8217 0.7977

Pseudo-second-order k2 (g/(mg h)) 0.37185 0.2238

ig. 3. FTIR spectra for the native and modified activated carbons with deferentaOCl impregnating concentration.

acroporous network was greatly destroyed. The basic graphitictructure of the carbon may partially fracture, resulting into moreliphatic functional groups on surface of the modified samples,hich are direct products of the oxidation.

.3. Sorption kinetics

The kinetics for the adsorption of Co(II) onto AC and ACM-40ithout pH adjustment (pH ∼6.1) are illustrated in Fig. 4(a). About

6% and 90% of total (ultimate) Co(II) sorption occurred rapidlyithin 0.5 h on AC and ACM-40 respectively, followed by a gradu-

lly slow process. The sorption equilibrium can be achieved within h for both samples. To further understand the sorption kinetics,he pseudo-first-order (Eq. (3)) and pseudo-second-order modelEq. (4)) [17] were employed to correlate the experimental data.

n(Qe − Qt) = ln Qe − k1t (3)

t

Qt= 1

k2Q 2e

+ t

Qe= 1

V0+ t

Qe(4)

here k1 (1/h) and k2 (g/(mg h)) are the pseudo-first-order andseudo-second-order rate constants; V0 represents the initial sorp-

ion rate (mg/(g h)).

As fitted model parameters were summarized in Table 3. Aseen, unlike the poor fitting results of the pseudo-first-order equa-ion, the pseudo-second-order model correlated the kinetic data

(b) changes of solution pH as a function of contacting time in the sorption systems(C0 = 30 mg/L, pH ∼6.1, sorbent dose = 0.5 g/L).

quite well for both AC and ACM-40, according to the high correla-tion coefficients (R2 > 0.99) (Fig. 4(a)). The good fitting result of thismodel implies that, for both adsorbents, chemisorption controls theadsorption process, and the sorption capacity is proportional to the

Qcal (mg/g) 29.9401 56.4972V0 (mg/(g h)) 333.333 714.286R2 0.9999 0.9999

Qexp (mg/g) 29.6560 55.9760

28 W. Liu et al. / Chemical Engineerin

2 4 6 8 100

10

20

30

40

50

60

70

Qe(mg/g)

pH

total uptakeadsorptionprecipitation

2 4 6 8 100

10

20

30

40

50

60

70

Qe(mg/g)

pH

total uptakeadsorptionprecipitation

a

b

Fa

s1oawpsts

3

ni(td

C

C

p

ig. 5. Co(II) removal extent (adsorption + precipitation) on AC (a) and ACM-40 (b)s a function of solution pH (C0 = 30 mg/L, contact time = 12 h, sorbent dose = 0.4 g/L).

As shown in Fig. 4(b), the solution pH in the AC and ACM-40ystems decreased and increased, respectively sharply in the first5 min, followed by a gradual but continuous decrease. The PZCf AC and ACM-40 is 5.28 and 6.83 respectively, which is blow andbove the initial solution pH. The rapid pH change in the first 15 minas, therefore, considered as a fast deprotonation and protonationrocess, in which the carbon would release/consume H+ from theolution. The next slow pH-decreasing stage might arise from cer-ain adsorption reactions, which will be discussed in the followingection.

.4. Effect of solution pH on adsorption

Knowledge of the pH study is very important since pH affectsot only the surface charge of adsorbent, but also the degree of

onization and speciation of adsorbate. The pH dependence of CoII) uptake onto AC and ACM-40 is illustrated in Fig. 5. It is knownhat Co (II) ions can undergo hydrolysis reactions in aqueous phaseepending on the solution pH [18].

o2+ + OH− ↔ Co(OH)+ logK1 = 4.3 (5)

o2+ + 2OH− ↔ Co(OH)2 logK2 = 8.4 (6)

The equilibrium species distribution of Co(II) as a function ofH is depicted in Fig. S1. It can be seen that Co2+ remains as the

g Journal 175 (2011) 24– 32

prevailing species up to a pH value of 8.0 thereafter Co(OH)+ andCo(OH)2 starts to form. Chemical precipitation of copper hydroxidebecomes remarkable with increasing pH. Therefore, the variationof adsorption capacity with pH is plotted in terms of total uptake,adsorption, and precipitation. The real Co(II) adsorption is definedas the difference between the total uptake of Co(II) species and theprecipitated amount of Co(II) complexes [19].

As shown in Fig. 5(a), the precipitation of Co(II) remained neg-ligible up to pH ∼8.0 and then rose sharply due to the formation ofCo(OH)2. The total uptake of Co(II) by AC increased relatively lin-early with the increasing pH. Removal of Co(II) via adsorption wasmost effective over the pH range 7.0–8.5, below which the sorptiongradually decreased as the pH decreased. After pH 8.5, there was asharp decrease in the amount of adsorption with further increasein pH due to precipitation. The pH sorption profiles by ACM-40(Fig. 5(b)) showed some difference from that of the virgin carbon.The total Co(II) uptake increased sharply as the pH increased from2.5 to 4.5, followed by a slow increase stage up to pH 10.5. The netadsorption onto ACM-40 was most effective in a wider pH range of4.5–8.5, below or above which the adsorption was bare minimum.

The pH where an activated carbon has an equal number of nega-tively and positively charged sites is called the “point of zero charge(PZC)” for the carbon. The surface of the activated carbon is pos-itively charged at pH lower than PZC, and negatively charged atpH higher than PZC. It should be noted that while the net surfacecharge may be predominately negative or positive, some oppositelycharged sites can still exist. The PZC of AC and ACM-40 is pH 5.28and 6.83, respectively. At low pH, Co2+ species can experience pro-nounced electrostatic repulsion from the positively charged carbonsurface, resulting in a depressed adsorption. As the pH increasedto the optimum range, the carbon surface became more nega-tive which can electrostatically attract more Co2+ and Co(OH)+

cations, thus giving a higher sorption extent. However, the surfaceof ACM-40 was more positive than that of AC duo to its higher PZCvalue. That is, ACM-40 should possess a lower Co2+ and Co(OH)+

adsorption capacity though electrostatic attraction than AC, whichis contrary to the experimental results. Therefore, besides elec-trostatic interactions, there must be other mechanisms which aremore important in the adsorption process.

Experimental results showed that the final pH of the solutionwas always less than the initial pH at the optimum pH range.For instance, when the initial pH of the reaction mixture variedbetween 5.5 and 7.0, the final pH remained at 5.0–5.6 and 5.3–5.8for the AC and ACM-40 adsorption system, respectively. This canbe taken as an indication of cation exchange reaction between theCo2+ or Co(OH)+ species and H+ ions on the carbon surface. ACM-40has more protonated sites than AC due to its higher PZC value, thusgiving a higher cobalt uptake amount and a wider effective sorp-tion range. This is in good agreement with the experimental results.Cation exchange has also been proposed as a major mechanism forthe removal of cobalt by commercial activated carbons [4,20].

As is well known, surface functional groups play an importantrole in the sorption of heavy metals by activated carbon. Accord-ing to the Hard-Soft-Acid–Base (HSAB) theory, hard acids prefer tocoordinate to hard bases and soft acids prefer to coordinate to softbases [21]. Since the oxygen containing groups, such as carboxyl,carbonyl and hydroxyl groups, present on the surface of AC andACM-40 are soft bases, one would expect the coordination of themwith Co2+ and Co(OH)+ (soft acids) in the aqueous solution.

3.5. Adsorption mechanisms

To fully understand the mechanisms by which cobalt interactedwith the surface functionalities, XPS and FTIR spectra were obtainedfor the AC and ACM-40 before and after cobalt adsorption. As shownfrom the XPS survey spectra in Fig. 6, only elements C and O existed

W. Liu et al. / Chemical Engineerin

AC

AC-Co

Bonding energy (eV)780782784786788790

C 1sO 1s

Co 2p3/2

778

ACM-40

C 1sO 1s

ACM-40-Co

776778780782784786788

Bonding energy (eV)

Co 2p 3/2

1200 1000 800 600 400 200

Binding energy (eV)

0

F

it2alAsc

TR

B

ig. 6. XPS survey spectra for AC and ACM-40 before and after cobalt adsorption.

n AC and ACM-40 prior to adsorption. Cobalt was detected onhe surface after the adsorption. High-resolution XPS spectra of Cop3/2 on the cobalt-adsorbed AC showed intensive peaks at 781.5nd 783.8 eV, a broad satellite peak at 786.1 eV and a lower satel-

ite at around 788.5 eV. XPS Co 2p3/2 spectra of the Co-adsorbedCM-40 exhibited intensive peaks at 781.1 and 783.2 eV, and aatellite peak at 786.1 eV. It has been demonstrated that cobalt(II)ompounds were distinguished from cobalt(III) compounds by the

able 4elative contents of functional groups in C 1s and O 1s XPS spectra.

Sample C 1s

Peak 1 Peak 2

AC BE (eV) 284.60 286.36

Relative content % 67.34 19.73

AC–Co BE (eV) 284.60 286.21

Relative content % 60.21 13.70

ACM-40 BE (eV) 284.60 286.28

Relative content % 62.34 17.93

ACM-40-Co BE (eV) 284.60 286.25

Relative content % 66.54 16.73

E – Binding energy; – Not detected.

g Journal 175 (2011) 24– 32 29

presence of multielectron excitation satellites in the former [22].Hence, the cobalt remained divalent after adsorption on the car-bons.

High-resolution XPS C 1s spectra (Fig. 7(a)) of the samples caneach be deconvoluted into three peaks: peak 1 (284.6 eV), graphi-tized carbon; peak 2 (286.2–286.4 eV), carbon in phenolic, alcoholand/or ester groups; and peak 3 (288.1–288.6 eV), carbon in car-bonyl groups. The O 1s spectra (Fig. 7(b)) can be resolved intothe following four peaks: peak I (530.7–531.0 eV), C O oxygen inanhydrides, carboxylates, ketones and/or esters which are closeto the aromatic structure of the carbon; peak II (531.5–531.6 eV),C O oxygen atoms in aliphatic groups; peak III (532.7–533.2 eV),C–O oxygen in phenol and/or ester groups; peak IV (534.15 eV),chemisorbed oxygen and/or water [23]. The fitting results of theC 1s and O 1s peaks are summarized in Table 4. As seen, after theNaClO oxidative treatment, the relative amount of the graphiticcarbon (peak 1) and C–O groups (peak 2 and peak III) decreased,accompanied by an increase in the C O groups (peak 3, peak I andpeak II). Most of the aromatic C O groups in anhydrides, carboxy-lates, ketones or esters (peak II) were converted to aliphatic ones(peak II) due to the oxidation. The finding is in good agreement withthe FTIR spectra results.

The binding energy of an element increases when more elec-tron density is withdrawn from its electron shell and vice versa.Comparing the cobalt-adsorbed AC to the original AC, the bindingenergies of all the deconvoluted peaks of C and O showed slightshift after cobalt adsorption, indicating that no electron transferoccurred between the sorbed cobalt iron and the oxygen function-alities. On the other hand, the content of graphitic carbon and theC–O groups decreased resulting in a relative increase in the C Ogroups. It appears that cobalt(II) interacted with the C–O moietiesin phenolic and carboxylic groups during the sorption. The inter-action was not actually through chemical covalent bonding but anon-covalent combination. It is probably that Co2+ and Co(OH)+

cations replaced the H+ of the phenol and carboxyl groups, and elec-trovalently attached on the C–O−, forming monodentate chargedcomplexes. This conclusion can be confirmed by the FTIR spectra(Fig. S2), where the vibration intensity of O–H decreased after cobaltadsorption due to the dissociation of H+. The absorption intensity ofC–O groups also weakened. It may arise from the fact that the elec-trovalently attached Co cations attracted the outer shell electron ofthe O−, resulting in a decreased polarity of these groups after theadsorption. Chen and Lin [24] and Puziy et al. [25] have proposedsimilar mechanisms for the sorption of Co(II) by activated carbons.The schematic diagram for this mechanism is shown in Fig. 8(a).

In the case of ACM-40, the binding energies of the C peaks

showed negligible shift after cobalt adsorption. However, the peakof O at 530.72 eV related to the C O groups near the aromaticstructure disappeared, and a new peak at 534.15 eV correspondingto chemisorbed oxygen and/or water appeared. However, the

O 1s

Peak 3 Peak I Peak II Peak III Peak IV

288.58 530.93 – 532.71 –12.93 31.69 – 68.31 –

288.19 531.00 – 532.86 –26.09 40.74 – 59.26 –

288.29 530.72 531.53 533.14 –19.74 6.8 48.67 44.53 –

288.32 – 531.58 533.14 534.1516.73 – 65.05 24.78 10.17

30 W. Liu et al. / Chemical Engineering Journal 175 (2011) 24– 32

282284286288290292Binding energy (eV)

AC-Co

AC

ACM-40

ACM-40-Co

C 1s

O 1s

IIII

I

III

III II

I

II1

IV

Binding energy (eV)

AC-Co

AC

ACM-40

ACM-40-Co

528530532534536

23

1

23

1

23

1

23

III

a b

tra fo

pFwg

Fig. 7. High-resolution XPS C 1s (a) and O 1s (b) spec

ossibility of chemisorbed water was excluded as seen from theTIR spectra, where the intensity of O–H stretching vibrationeakened after cobalt adsorption. The relative content of C–O

roups (peak III) decreased sharply from 44.53% to 22.57%, while

r AC and ACM-40 before and after cobalt adsorption.

the content of aliphatic C O groups (peak II) largely increased.It appears that the O moieties in C O (especially those near thegraphitic structure) and C–O groups donate their electrons tothe cobalt(II) iron, resulting in a decreased electron density of

W. Liu et al. / Chemical Engineering Journal 175 (2011) 24– 32 31

mech

tboAwkaco

sAkCce4gtlCaa

Fig. 8. Schematic diagrams for the sorption

he oxygen atoms, and thus a shift of the O 1s signal to higherinding energies (peak I → peak II and peak III → peak IV). Basedn the above analysis, we propose that when interacted withCM-40, Co(II) irons formed multidentate coordination complexesith the surface functionalities, such as adjacent carboxylates,

etones, phenols and/or esters, with Co(II) as the central atomnd oxygen as the donor atom. Referring to the formula of cobaltomplexes in the literature [26,27], one of the possible structuref the coordination complexes was illustrated in Fig. 8(b).

The above coordination would result in a higher electron inten-ity and thus a lower binding energy of the Co(II) atoms inCM-40-Co than the non-coordinated ones on AC–Co, which was ineeping with their Co 2p3/2 XPS spectra. The stretching intensity of–O and C O groups in the FTIR spectra of ACM-40 increased afterobalt adsorption (Fig. S2), which was an indication of conjugationffect. The fact that multidentate complexes can form within ACM-0 was probably duo to its higher content of adjacent functionalroups, most of which are aliphatic, thus provided a “space advan-age” for the coordination. AC cannot form such a complex due to its

imited and dispersed functionalities. It should be noted that whileo(II) primarily formed multidentate complexes with the function-lities in ACM-40, some monodentate charged complexes mightlso exist.

anisms of Co(II) by AC (a) and ACM-40 (b).

Based on the above results, the enhanced Co(II) removal onultrasonic-assisted NaOCl modified carbons can be expressed asfollows: (1) the oxidation caused a higher PZC of the modified car-bons, resulting in more positive sites for the exchange with Co2+

and Co(OH)+ species; (2) the NaOCl treatment introduced moreoxygen functional groups, especially aliphatic ones, which providedmore complexation sites for cobalt. Ultrasonic irradiation may haveenhanced the oxidation ability of NaOCl and favored the formationof aliphatic groups, thus promoted the sorption.

4. Conclusion

In this work, ultrasonic irradiation was employed to assist themodification of activated carbons (AC) by sodium hypochlorite. TheCo(II) adsorption capacity of the carbon was greatly enhanced bythe modification treatment. Ultrasonic-assisted impregnation canenhance the oxidation ability of NaOCl, thus further improve theCo(II) adsorption capacity. The modified AC exhibited a wider opti-mal pH range for Co(II) adsorption at pH 4.5–8.5 than the native AC

(optimal pH 7.0–8.5), above which precipitation of copper hydrox-ide was mainly responsible for the cobalt removal. Cation exchange,electrostatic interaction, and surface complexation were consid-ered as major mechanisms for the adsorption. Co(II) irons were

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2 W. Liu et al. / Chemical Engin

ound to form deferent type of complexes with the native andodified carbon. The higher Co(II) sorption ability of the modi-

ed carbons were probably due to their improved cation exchangeapacity and higher content of adjacent aliphatic functional groups.he present work provides a new modification protocol for acti-ated carbon, which will advance the development of adsorptionechnique for heavy metal removal.

cknowledgements

The work was supported by the National Key Technology&D Program for the 11th Five-year Plan (no. 2006BAC10B03),he National Water Special Project (no. 2009ZX07210-009-04),he Scientific Technology Research and Development Program ofhandong, China (no. 2010GZX20605) and Graduate Independentnnovation Foundation of Shandong University (no. 2009JQ009).

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.cej.2011.09.004.

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