Effect of ZnO loading to activated carbon on Pb(II) adsorption

E?ect of ZnO loading to activated carbon on Pb(II)adsorption

from aqueous solution

Yosuke Kikuchi

a,1

,Qingrong Qian a ,Motoi Machida b,*

,Hideki

Tatsumoto

Graduate School of Science and Technology,Chiba University,Yayoi-cho 1-33,Inage-ku,Chiba 263-8522,Japan

Faculty of Engineering,Chiba University,Yayoi-cho 1-33,Inage-ku,Chiba 263-8522,Japan

Received 4March 2005;accepted 30July 2005

Available online 13September 2005

Abstract

The e?ect of zinc oxide loading to granular activated carbon on Pb(II)adsorption from aqueous solution was studied in com-parison with zinc oxide particles and oxidized activated carbon.Cu(I I ),Cd(I I )and nitrobenzene were used as reference adsorbates to investigate the adsorption.The BET surface area and point of zero charge (pH PZC )in the aqueous solution were measured for the adsorbents.The adsorption isotherms were examined to characterize the adsorption of heavy metals and organic molecules.The heavy metal adsorption was improved by both the zinc oxide loading and the oxidation of activated carbon.I n contrast,the adsorp-tion of nitrobenzene was considerably reduced by the oxidation,and slightly decreased by the zinc oxide loading.The zinc oxide loading to the activated carbon was found to be e?ectively used for the Pb(II)adsorption whereas only a part of surface functional groups was used for the zinc oxide particles and the oxidized activated carbon.From the experimental results,the surface functional groups responsible for the Pb(II)adsorption on the zinc oxide loaded activated carbon were considered to be hydroxyl groups that formed on the oxide,while those on the oxidized activated carbon were considered to be carboxylic groups.ó2005Elsevier Ltd.All rights reserved.

Keywords:Activated carbon;Impregnation;Adsorption;Adsorption properties

1.Introduction

Zinc oxide is widely used in daily life and industry,for example,additives in cosmetics for reducing UV penetration,catalysts for synthesis of methanol,vulcani-zation accelerator for rubber,sensors for oxygen gas [1–3],antistatic agents in paint,an antibacterial agent in ion-exchange resin [4],and absorbents for gases such as NO x ,SO x ,CH 4,CO 2,CO,O 2,H 2[5,6].In particular,zinc oxide is able to dissociate hydrogen in a heteroge-neous manner [7–10]and water to create hydroxyl groups [11,12].On the other hand,the pollution of water

environments and ecosystems by toxic heavy metals such as Hg(I I ),Cr(V I ),Cd(I I )and Pb(I I

)has been attracting much attention in recent years.Adsorption of these heavy metals is an attractive option to reduce their levels in aqueous solution,since in general adsor-bents are easy to handle,and can be used for various sit-uations without large apparatus [13].Carbonaceous materials such as activated carbon are the most com-monly used adsorbent for water treatment.Activated carbon has a large speci?c surface area and a well-devel-oped porous structure,resulting in an attractive force toward organic molecules [14–16].However,speci?c functional groups such as carboxyl,hydroxyl and amine on the activated carbon are essential for the adsorption of heavy metals [17,18].Minerals in ash contained in charcoal also play an important role in the adsorption of heavy metals [19].I

n the present work,zinc oxide

Corresponding author.Tel./fax:+81432903129.

E-mail address:machida@faculty.chiba-u.jp (M.Machida).1

Present address:Miura Co.,Ltd.,864-1Hojyotsuji,Matsuyama,Ehime

799-2430,Japan.

Carbon 44(2006)

195–202

https://www.360docs.net/doc/a34054414.html,/locate/carbon

was loaded to granular activated carbon to increase its capacity to absorb heavy metals,principally Pb(I I)ions, by creating hydroxyl groups on zinc oxide in the aque-ous solutions as mentioned above.Preparation of zinc oxide particles and oxidation of activated carbon were also carried out to compare their adsorption capability with that of the zinc oxide loaded activated carbon. 2.Experimental

2.1.Preparation of adsorbents

The base adsorbent used in the study was commer-cially available granular activated carbon made from coconut shell(Calgon Mitsubishi Chemical Corpora-tion,Diasorb W10-30).The activated carbon was repeatedly boiled with de-ionized water until the solu-tion pH no longer changed,then it was dried overnight at110°C followed by calcination at350°C in air for 4h,and allowed to cool in a desiccator;this was referred to as GAC in the study.The zinc oxide were loaded to the110°C dried granular activated carbon by the equi-librium adsorption of Zn(II)ions from aqueous solution followed by calcination in air at350°C for2h.The cycle was repeated four times to increase the amount of zinc oxide loading.The Zn(II)solution was prepared by dissolving zinc nitrate hexahydrate in de-ionized water at15.3mmol/L.Five-hundred millilitre of Zn(II) solution including7.0g of the dried activated carbon was agitated at100rpm for7days to reach the equilib-rium state.The Zn(II)adsorbed activated carbon was ?ltered from the solution,and then dried in oven at 110°C before calcination at350°C.The zinc oxide loaded activated carbon was referred to as ZnO-GAC. In order to elucidate the in?uence of zinc oxide loading on textual properties of the activated carbon,the ZnO-GAC was washed with1M hydrochloric acid to remove zinc oxide loading to the activated carbon followed by repeated washing with de-ionized water until a constant solution pH was obtained.This was referred to as ZnO-GAC(HCl aq.).Changes in structure of the activated carbon by ZnO loading were inspected comparing the GAC and ZnO-GAC with and without HCl(aq.)wash-ing.The ZnO particles were prepared by precipitation of zinc nitrate solution with ammonia solution.The precip-itated Zn(OH)2was repeatedly washed with de-ionized water to remove ammonia,dried in an oven,calcined in air at350°C for2h to remove nitrate ions,and cooled in the desiccator.The oxidized activated carbon was prepared by oxidation with nitric acid[20–26]. Eight-hundred millilitre of36%nitric acid solution including20g of the activated carbon was heat treated for6h with the temperature controlled to around 90°C to avoid boiling the acid.Brown vapor of NO2 was continuously generated during the oxidation exper-iment.The oxidized activated carbon was cooled down to room temperature,?ltered from nitric acid,washed with de-ionized water until the pH exhibited a constant value,dried overnight in the oven at110°C,and then calcined in air at350°C for4h to completely remove ni-trate ions[27].The oxidized granular activated carbon was referred to as Oxi-GAC.

2.2.Properties of adsorbents

The BET surface area and pore distribution for the adsorbents were measured using a surface analyzer (Beckman Coulter Model SA3100).The surface areas for micropores and mesopores were separately calcu-lated using the t-plot method based on N2adsorption isotherms[28,29].The point of zero charge(pH PZC) was measured to investigate the total surface charge for the adsorbents according to the method of Smicˇiklas et al.[30].The mixture of100mg of adsorbent and 50mL of0.1M potassium nitrate solution was agitated at25°C for24h to allow it to reach the equilibrium state.The initial pH of potassium nitrate solution was changed from 1.5to10using0.1M nitric acid and 0.1M potassium hydroxide.When the equilibrium solu-tion pH did not change with increasing the initial pH, the constant pH value was considered to be the point of zero charge(pH PZC)for the adsorbent.At a solution pH lower than pH PZC,the total surface charge will be positive on average,whereas at the higher solution pH it will be negative[14,31].The equilibrium solution pH was measured by a portable pH meter(HORIBA Model D-21).

Surface functional groups of the adsorbents were determined by the method of Boehm[32–34].On the Boehm titration the following assumptions were made to distinguish the surface carbon–oxygen complexes according to their acidity:NaOC2H5neutralizes carbox-ylic,lactonic,phenolic,and carbonyl groups;NaOH neutralizes carboxylic,lactonic,and phenolic groups; Na2CO3neutralizes carboxylic and lactonic groups; NaHCO3only neutralizes carboxylic groups[24,35,36]. One gram of the adsorbent was separately mixed with 15mL of the above-mentioned basic solutions,and agi-tated at100rpm for4days to complete the neutraliza-tion.Five millilitre of remaining basic solution was separated from the adsorbent,and back titration was carried out by0.1M hydrochloric acid using methyl red as a color change pH indicator.All chemicals used in the study were of reagent grade.

2.3.Adsorption measurements

The Pb(II)stock solution was prepared by dissolv-ing lead(II)chloride in de-ionized water at0.1wt.%of Pb(II).The stock solution was diluted with de-ion-ized water at a desired concentration.The adsorption

196Y.Kikuchi et al./Carbon44(2006)195–202

equilibrium of Pb(II)onto the adsorbents was investi-gated with batch systems.Fifty mL of Pb(II)solution bearing100mg of the adsorbents in a100mL stoppered conical?ask was agitated at100rpm in a water bath whose temperature was controlled at25°C.In our pre-liminary study,3days was found to be su?cient to at-tain the adsorption equilibrium state[18].The solution was separated from the adsorbent with the decantation technique,and diluted with0.1M hydrochloric acid to convert all Pb(II)species to Pb2+.The Pb(II)concentra-tion of the solution was determined by atomic absorp-

tion spectrometry(AAS,H

I TACH

Model180-30).

The amount of Pb(II)on the adsorbent at equilibrium was calculated from the di?erence between the initial and?nal concentrations of the Pb(II)solution.Adsorp-tion of nitrobenzene as an organic molecule,Cu(I I),and Cd(II)was also investigated to compare with the Pb(II) adsorption.The concentration of nitrobenzene was determined by UV spectrometry(Shimadzu UV-210).

3.Results and discussion

3.1.Amount of ZnO loading

Fig.1shows the amount of ZnO loading on activated carbon as a function of the number of zinc oxide loading cycles.The amount of Zn(II)gradually increased with increase in the number of cycles with the equilibrium solution pH kept constant.As shown in Fig.2,the Pb(II)removal was proportionally promoted by intro-ducing zinc oxide to the activated carbon.No signi?cant changes in equilibrium solution pH were observed for the Pb(II)adsorption on the zinc oxide loaded activated carbon in each loading cycle.Zn(II)cations are gener-ally known to be adsorbed to surface C–O complexes on activated carbon[37,38]as well as Cu(

I I

),Pb(

I I

) and Cd(II)[18,39,40].Therefore,during the?rst cycle the zinc loading will also principally take place,binding with acidic functional groups on the activated carbon followed by binding with the rest of them and via zinc oxide from the second through fourth steps.

3.2.Properties of adsorbents

Table1shows the pH of the mixture of de-ionized water and the adsorbents,point of zero charge(pH PZC), surface area and pore distribution of the adsorbents. Total surface area was decreased by oxidation[26,41] with a decrease in micropore surface area and an in-crease in mesopore surface area.These results indicate that the micropores were converted to mesopores on oxidation,thus decreasing the total surface area.I n con-trast,the total surface area was increased by zinc oxide loading,accompanied by an increase in mesopore area and no signi?cant decrease in micropore area.The total surface area and the pore distribution were preserved after removing zinc oxide by washing with hydrochloric

Table1

Electrostatic properties,surface area and pore distribution of the

adsorbents

Adsorbent GAC Oxi-

GAC

ZnO

particles

ZnO-

GAC

ZnO-GAC

(HCl aq.)

pH 5.6 3.1 6.6 5.9 5.6

pH PZC8.9 3.3– 6.8 4.7

Surface area,m2/g10008781712561258

Mesopore,m2/g18648211481499

Micropore,m2/g8113950773757

pH;measured for100mL de-ionized water containing100mg of

adsorbents at25°C.

Y.Kikuchi et al./Carbon44(2006)195–202197

acid,showing that some activated e?ects of developing the pore structure in the activated carbon were occurred by introducing zinc oxide.As also tabulated in Table1, the point of zero charge(pH PZC)was obtained from Fig.3according to the method of Smicˇiklas et al.[30]. The equilibrium solution pH was plotted against the ini-tial solution pH for GAC,Oxi-GAC,ZnO-GAC and ZnO-GAC(HCl aq.);the constant equilibrium solution pH exhibiting the point of zero charge(pH PZC)was at-tained by increasing the initial pH as shown in Fig.3 [42].Though both the zinc oxide loading and the oxida-tion of activated carbon decrease pH PZC,the e?ect was more pronounced for the oxidation than for the zinc oxide loading,indicating that the pH range of negative surface charge is wider for oxidized activated carbon (Oxi-GAC)than zinc oxide loaded activated carbon (ZnO-GAC)as described in the next section.The results are consistent with the changes in surface functional groups for GAC and Oxi-GAC,because the oxidation with nitric acid can selectively increase the carboxylic groups on the activated carbon surface as shown in Table2.While pH measured in de-ionized water exhi-bited similar values for GAC,ZnO-GAC and ZnO-

GAC(HCl aq.),the pH PZC of ZnO-GAC(HCl aq.) was lower than that of ZnO-GAC suggesting that ZnO loading could modify the surface of the activated car-bon.As a result of the ZnO loadings,not carboxyl groups but hydroxyl groups were increased,because the adsorption of organic compounds was not signi?-cantly reduced using ZnO-GAC as described in the pre-vious section,and in addition only some phenolic groups were increased when GAC was prepared by cal-cination at350°C from110°C dried activated carbon as also shown in Table2.Based on the results,it is consid-ered that calcination at350°C makes the Zn(II)ions ad-sorbed on the carbon bind to the activated carbon via oxygen atoms upon the formation of zinc oxide,in which the mesopore surface area also may progressively increase.

3.3.In?uence of pH on Pb(II)adsorption

It is generally recognized that the solution pH greatly in?uences the adsorption of heavy metals from aqueous solution onto adsorbents[43–45].Fig.4shows changes in Pb(II)removal as a function of equilibrium solution pH for GAC,Oxi-GAC and ZnO-GAC.A steep increase in Pb(II)removal was observed in the order of Oxi-GAC, ZnO-GAC and GAC as the equilibrium solution pH was increased,an order consistent with that of pH PZC.The surface charge of the adsorbents is positive in the pH range below pH PZC while it is negative above pH PZC. Since the Pb()species are cations,which are mostly Pb2+,and small amounts of PbeOHTt;Pb4eOHT4t

Pb3eOHT2t

in the pH range studied[19,46–48]

,an

Table2

Acidic surface functional groups of the adsorbents(Boehm titration)

110°C-dried GAC GAC Oxi-GAC Surface functional groups,meq./g

Carboxylic groups0.030.03 1.14 Lactonic groups0.040.040.04 Phenolic groups0.150.230.23 Carbonylic groups0.010.020.15

Total acidic groups0.230.32

1.56

198Y.Kikuchi et al./Carbon44(2006)195–202

electrostatic attractive force operates between the Pb(II) and the surface of the adsorbents in the pH range above pH PZC,whereas a repulsive force acts for Pb(I I)adsorp-tion below pH PZC.The Pb(I I)removal,therefore,is valid over a relatively wide range of pH for Oxi-GAC,though it is narrow for GAC.For ZnO-GAC,the pH PZC value would lie between those of Oxi-GAC and GAC,because the surface hydroxyl groups on the zinc oxide can form on activated carbon by coming into contact with water [11,12];the carboxylic groups mostly found on Oxi-GAC generally exhibit a lower p K a value than the hydro-xyl groups possibly formed on ZnO-GAC.

3.4.Adsorption isotherms

The linear form of Langmuir?s equation was em-ployed to represent the adsorption isotherms for the Pb(II)and nitrobenzene as follows:

C e Q

e ?

X m K e

C e

X m

e1T

where C e and Q e are the equilibrium concentrations in the solution and on the adsorbent,and X m and K e are the maximum number of adsorption sites and the adsorption a?nity onto the adsorption sites,respec-tively.X m and K e can be calculated from the slope and intercept of the approximated straight line when C e/Q e is plotted against C e.

For all adsorption experiments for the zinc oxide loading adsorbents,zinc ions in the aqueous solution were detected at less than2mg/L,mostly below1mg/ L,indicating that the in?uence of zinc ions on Pb() adsorption was negligible in this study,and that zinc can bind strongly to activated carbon.Fig.5shows the adsorption isotherms of the Pb(II)onto GAC, Oxi-GAC and ZnO-GAC,and the ZnO particles,in which the solution equilibrium pH was between pH PZC and6.0for Oxi-GAC,and ranged from5.0to6.0for the other adsorbents,revealing that adsorption to the adsorbents except GAC progressed on the negatively charged surface of the adsorbents.Furthermore,Pb2+ aqua may be the principal Pb(

)species,and a small

portion of Pb(OH)+can be present under the condi-

tions,since the equilibrium solution pH was always less

than6.0for all adsorbents.Fig.6shows the adsorption

isotherms of the nitrobenzene onto GAC,Oxi-GAC and

ZnO-GAC.It is obvious that ZnO-GAC and GAC exhi-

bit good performance compared with Oxi-GAC for the

nitrobenzene adsorption,whereas ZnO-GAC and Oxi-

GAC are superior to GAC for the Pb(II)adsorption.

The Langmuir parameters calculated from the approxi-

mated lines in the isotherms were as tabulated in Table

3.The adsorption capacity,X m,of0.37mmol/g for

ZnO-GAC is the largest among the adsorbents for the

Pb()adsorption except the ZnO particles,and it is

close to the amounts of zinc oxide loading of

0.35mmol/g,suggesting the mono-layer spreading of

Table3

Langmuir parameters obtained from Pb(II)and nitrobenzene

adsorption

Adsorbent Pb(II)adsorption Nitrobenzene adsorption

X m,mmol/g K e,L/mmol X m,mmol/g K e,L/mmol

GAC0.05212 2.023

Oxi-GAC0.24560.68 2.8

ZnO

particles

1.3 4.0––

ZnO-GAC0.3732 1.611

Y.Kikuchi et al./Carbon44(2006)195–202199

zinc oxide onto the GAC surface,while0.24mmol/g of X m for Oxi-GAC is limited to only20%of the carboxylic sites of1.14meq./g determined by the Boehm titration.

Though the adsorption a?nity to ZnO-GAC was superior compared to the ZnO particles,the adsorption capacity of the ZnO particles is3.5times larger than that of ZnO-GAC.However,the total amount of zinc in the ZnO particles is36times as great as that in ZnO-GAC. The results suggest that zinc oxide loading to activated carbon can be e?ectively used for Pb(II)adsorption, whereas a part of acidic functional groups,probably those on the external surface,will contribute to binding the Pb(II)for the ZnO particles and Oxi-GAC.The acti-vated carbon thanks to its developed porous structure and high speci?c surface area which can accommodate zinc oxide in a spreading manner,probably the zinc oxide connects to the activated carbon surface via oxy-gen atoms,for the adsorptive removal of heavy metals. The adsorption a?nities for Pb(II)adsorption decreased in the order of Oxi-GAC,ZnO-GAC and GAC.This is caused by the fact that the carboxylic groups,which are considered to be the strongest binding sites for heavy metals[49–51],are abundant for Oxi-GAC though the hydroxyl groups may be abundant in composition of surface acidic sites for GAC,ZnO-GAC and the ZnO particles.Tamura et al.examined the adsorption of hea-vy metals onto MnO2and Fe2O3,and concluded that surface hydroxyl groups contributed to the binding of heavy metals for the metal oxide[52].n addition,the adsorption and desorption of Cu(II)and Cd(II)were examined as well as those of Pb(II)as shown in Table 4.The amount of Pb(II)on the zinc oxide loaded GAC was more than that of the oxidized activated car-bon under the conditions,whereas Oxi-GAC was advantageous for Cu(II)and Cd(II)adsorption.For the desorption experiment,though a small amount of the heavy metals was released to the aqueous solution for the zinc oxide loaded activated carbon,no Pb(), Cu(II)and Cd(II)was detected for the oxidized GAC probably due to strong binding between heavy metal ions and carboxyl functional groups.The carboxylic functional groups are mostly responsible for binding heavy metal ions for Oxi-GAC,and the hydroxyl groups for the binding for ZnO-GAC.

While surface carboxylic and hydroxyl groups are considered to be adsorption sites for heavy metals in the study,the graphene layer of activated carbon plays a decisive role in the adsorption of organic molecules, especially those containing aromatic rings;the adsorp-tion can take place via interactions between p electrons of the graphene layer and those of the aromatic rings [14,21].Both the adsorption capacity,X m,and the adsorption a?nity,K e,for the nitrobenzene adsorption were slightly reduced by loading zinc oxide onto the activated carbon but signi?cantly reduced by the oxida-tion as clearly shown in Fig.6and Table3,because the p electrons of the graphene layer were more strongly withdrawn by the carboxyl groups generated by oxida-tion than by the hydroxyl groups probably created on the zinc oxide.From the results,therefore,it can be concluded that the adsorption of the Pb(II)from the aqueous solutions onto the activated carbon will be improved by loading zinc oxide without a signi?cant decrease in adsorption capacity or a?nity of the organic molecules as observed in the oxidized activated carbon.

4.Conclusions

Based on the experimental results for zinc oxide load-ing to granular activated carbon compared with ZnO particles and oxidation of activated carbon,the follow-ing conclusions can be deduced for the e?ect of zinc oxide loading on the Pb(II)adsorption to the activated carbon.

(1)Loading zinc oxide on the activated carbon exhib-

ited some activated e?ects of enlarging the speci?c

surface area due to the increase in mesopore sur-

face area,while a detrimental e?ect was observed

for the oxidation of activated carbon.

(2)Most of the zinc oxide loading to the activated car-

bon can be e?ectively used for the Pb(II)adsorp-

tion compared with the ZnO particles and the

oxidized activated carbon,because the number of

Pb(II)adsorption sites calculated from the iso-

therms was close to the measured number of zinc

oxide loading,but it was far less than the number

of hydroxyl groups possibly formed on the ZnO

particles and carboxyl groups introduced by the

oxidation of activated carbon.

(3)The zinc oxide loading and the oxidation of acti-

vated carbon reduce the point of zero charge

(pH PZC);the increasing order of the oxidized acti-

vated carbon(Oxi-GAC),the zinc oxide loaded

activated carbon(ZnO-GAC)and GAC in the

pH PZC observation are consistent with the order

of the steep increase in the Pb(II)removal when

the equilibrium solution pH is increased.

Table4

Adsorption and desorption of Pb(I),Cu(I)and Cd(I)

Adsorbent GAC Oxi-GAC ZnO-GAC

Pb(I)ads.,mmol/g0.0430.1840.221

Pb(I)des.,mmol/g0.019n.d.0.004

Cu(I)ads.,mmol/g0.0590.2350.080

Cu(I)des.,mmol/g0.022n.d.0.001

Cd(I)ads.,mmol/g0.0150.1240.059

Cd(I)des.,mmol/g0.009n.d.0.007

I nitial metal concentration;0.48mmol/L,agitation at25°C for3days.

200Y.Kikuchi et al./Carbon44(2006)195–202

(4)The hydroxyl groups created on the zinc oxide are

likely to be responsible for the Pb(II)adsorption for the zinc oxide loaded activated carbon (ZnO-GAC)whereas the carboxyl groups are adsorption sites for the oxidized activated carbon (Oxi-GAC),because the point of zero charge (pH PZC )for the zinc oxide loaded activated carbon was between GAC and Oxi-GAC,and some desorption of Pb(),Cu()and Cd()was observed for ZnO-GAC but no desorption was detected for Oxi-GAC,and the adsorption of nitrobenzene was slightly reduced for ZnO-GAC but it was signi?-cant for Oxi-GAC.

Acknowledgements

The authors thank Calgon Mitsubishi Chemical Cor-poration for providing the granular activated carbon.They are also grateful to Dr.Keiichi Nagao of the Safety and Health Organization of Chiba University,and Dr.Masanori Fujinami and Dr.Koichi Oguma of the Faculty of Engineering of Chiba University,for their experimental support in the study.

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