New nanoporous carbon materials with highadsorption capacity and rapid adsorption kinetics

for removing humic acids

Sangjin Han a, Sukjae Kim a, Hacgyu Lim a, Wonyong Choi b,Hyunwoong Park b, Jeyong Yoon a,*, Taeghwan Hyeon a,*

a School of Chemical Engineering and Institute of Chemical Processes, Seoul National University, Seoul 151-742, South Koreab School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, South Korea

Received 7 May 2002; received in revised form 18 October 2002; accepted 8 November 2002

Abstract

We report that new nanoporous carbon materials (SMC1), which were produced using silica sol particles as tem-

plates, show higher and faster adsorption of humic acids than two commercial activated carbons (F400 and Norit SA).

In the best result, SMC1 showed 16 times the adsorption capacity of conventional activated carbons. In addition, the

adsorption of humic acids on SMC1 proceeded very quickly, reaching the equilibrium concentration of humic acids

within 15 min.

� 2002 Elsevier Science Inc. All rights reserved.

Keywords: Porous carbon; Carbonization; N2 gas adsorption; Humic acids; Adsorption properties

1. Introduction

The natural organic matter in raw water is

troublesome not only for causing color, odor and

taste problems but also for forming halogenated

by-products such as trihalomethanes (THMs)

during disinfection processes that use chlorine [1].

Humic acids account for a significant portion (40–

90%) of the dissolved organic matter found innearly all water supplies [2] and have been shown

to produce, upon chlorination, larger amounts ofTHMs than any other natural organic matter [3,

4]. They are also known to affect the transport

and the fate of other organic and inorganic species

in natural waters through partition/adsorption

processes, and catalytic and photosensitized reac-

tions.

The removal of humic acids or precursors of

disinfection by-products in water treatment pro-cesses is of significant importance in terms of

meeting the drinking water standards. Adsorption

processes using activated carbon (AC) have been

widely used for this purpose [1–10]. Humic acids

are known to be representative substances for es-

timating the adsorption capacities of porous car-

bons. Porous carbons possess four types of pore

*Corresponding authors. Tel.: +82-2-880-7150; fax: +82-2-

880-1604.

E-mail addresses: jeyong@snu.ac.kr (J. Yoon), thyeon@

plaza.snu.ac.kr (T. Hyeon).

1387-1811/02/$ - see front matter � 2002 Elsevier Science Inc. All rights reserved.

PII: S1387-1811 (02 )00611-X

Microporous and Mesoporous Materials 58 (2003) 131–135

www.elsevier.com/locate/micromeso

structures: primary micropores (pore diameter <0.7 nm), secondary micropores (0.7 nm < pore

diameter < 2 nm), mesopores (2 nm < pore di-

ameter < 50 nm), and macropores (pore dia-

meter > 50 nm). While microporous AC is useful

for adsorbing various substances of relatively smallsize such as THMs, the use of mesoporous and

macroporous carbons for this purpose has hardly

been reported [5]. The adsorption of humic acids

on AC is largely controlled by their molecular size

[6], and the lower molecular weight fractions of

humic acids are preferentially removed by AC.

Recently, we have developed a new synthetic

method to produce nanoporous carbon materialswith a high mesopore volume using commercial

silica sol particles as templates. These carbon

materials were designated as silica sol mediated

carbon (SMC) [11,12]. In particular, some SMC1

carbons have a broad pore size distribution (10–

100 nm), which can make SMC1 carbons good

adsorbents for humic acids, because these have a

wide range of molecular weights and sizes, whichrange from a few hundred to as much as several

hundred thousand gmol�1 and from 1 to 50 nm in

size [13]. The present study reports on the superior

adsorption performance of SMC1 carbons for

humic acids compared to those of commercial

activated carbons which have been popular in

commercial water purification processes.

2. Experimental section

2.1. Synthesis of the nanoporous carbon material

The detailed synthetic procedures of SMC1

carbon have been reported in a previous paper

[12]. Briefly, the aqueous colloidal silica sol solu-tion, Ludox� HS-40 (40 wt.% silica in water, av-

erage particle size �12 nm) was purchased from

Aldrich Chemicals Co. Resorcinol (99%, A.C.S.

reagent) and formaldehyde (37 wt.% aqueous so-

lution, A.C.S. reagent) were polymerized in the

presence of silica sol particles to generate silica–

RF (resorcinol–formaldehyde) gel composites.

Carbonization followed by NaOH etching of thecomposites was used to produce SMC1 carbon. In

a typical synthesis, a mixture containing a 1:2

molar ratio of resorcinol and formaldehyde was

added to the Ludox HS-40 silica sol solution. The

reaction mixture, after adjusting the initial pH,

was aged at 85 �C for one week to get a composite

of silica and RF (resorcinol–formaldehyde gel).

For carbonization, the composite was heated in anitrogen atmosphere from room temperature to

850 �C with a heating rate of 5 �C/min and held atthat temperature for 3 h. The resulting silica–car-

bon composite was stirred in a 5 M NaOH solu-

tion for 5 h at 80 �C to remove the silica particles.

2.2. Humic acids adsorption experiments

To compare the adsorption capacity, two com-

mercial activated carbons (F400 from Calgon Co.

and Norit SA from Norit Co.), which have been

frequently applied to water purification processes,

were used. Humic acids were purchased from

Aldrich Co. and used without further purification.

Activated carbons and SMC1 carbon were crushed

and mechanically sieved to yield uniform particlesizes between 150 and 250 lm, and the powders

were washed with distilled water. Finally, these

carbon materials were dried in a vacuum oven at

120 �C for one day to remove residual water.

Two different types of adsorption experiments

were conducted. Firstly, adsorption isotherms

were measured to obtain the adsorption capacities

of the carbon materials. Secondly, kinetic studieswere performed to compare the rates of adsorp-

tion. The adsorption isotherm experiments were

conducted at 25 �C by shaking the aqueous humic

acids solution (12 mg/l) containing the carbon

sample (200–2200 mg/l) at 150 rpm for 10 h and

then filtered. The adsorption capacities for the

humic acids were measured by determining the

total organic carbon (TOC) value using an ana-lyzer (SIEVERS 820, USA). The adsorption iso-

therm experiments were conducted at three

different pH values, viz. neutral (pH ¼ 7), slightly

basic (pH ¼ 8), and acidic (pH ¼ 4) by using, re-

spectively, a phosphate buffer, a carbonate buffer,

and HCl. The addition of the carbonate buffer

increased the TOC of the sample solution by less

than 3%. For batch kinetic experiments, a 500 mlaqueous solution containing 12 mg/l of humic

acids and 250 mg of the carbon sample was kept

132 S. Han et al. / Microporous and Mesoporous Materials 58 (2003) 131–135

under stirring at pH ¼ 7, and a total of 20 ml of

the sample were collected by a 0.45 lm syringe

filter at regular intervals. The TOC values of the

samples were determined.

3. Results and discussion

Table 1 shows the pore characteristics of SMC1

carbon and the two activated carbons. SMC1

carbon possesses a slightly higher surface area

than the activated carbon F400, and a very high

pore volume of 2.6 cm3/g in the pore range of >2nm. In contrast, most of the pores in the activatedcarbons were found to be in the micropore range.

Even though some activated carbons have an ex-

tremely high surface area of >2000 m2/g, most of

the surface area comes from small micropores. The

scanning electron micrograph (SEM) of SMC1

carbon (Fig. 1) shows pores in the range of 10–100

nm. These pores were generated from the replica-

tion of the silica template.The zeta-potentials of the three carbon materi-

als were measured to determine the surface charge

because this surface property affects the adsorp-

tion capacity and kinetics. The measured zeta-

potentials were similar for these carbon materials

()1.2 mV for SMC1 and )2.0 mV for Calgon

F400), demonstrating that the adsorption charac-

teristics can be explained primarily on the basis ofthe physical pore characteristics of these carbons.

Fig. 2(a)–(c) compare the adsorption isotherms

of the three carbon materials (F400, Norit SA, and

SMC1) for humic acids at pH values of 7, 4, and 8,

respectively. As shown in Fig. 2, regardless of the

pH conditions, the adsorption capacity of the

nanoporous SMC1 carbon was much higher than

that of F400 and Norit SA carbons. In the best

case (at pH ¼ 4), SMC1 carbon exhibited about 16

times the adsorption capacity of the two otheractivated carbons. At low pH, the adsorption ca-

pacities of all three carbon materials were in-

creased. This can be attributed to the charge

reduction of the weakly acidic humic acids as the

pH was lowered [14]. On the other hand, at high

pH, the overall adsorption capacities of the acti-

vated carbons were reduced significantly. Com-

paring the two activated carbons, Norit SA carbongenerally showed a slightly higher adsorption ca-

pacity than Calgon F400 carbon. Although both

SMC1 and F400 had nearly the same BET surface

Table 1

Pore characteristics of SMC1 and activated carbons

Surface area Pore volume

Carbon SBET SBJH Smacro Smeso Vsingle VBJH Vmacro Vmeso(m2/g) (m2/g) (m2/g) (m2/g) (cm3/g) (cm3/g) (cm3/g) (cm3/g)

F400 856 170 1.30 169 0.51 0.20 0.03 0.17

Norit SA 524 200 1.60 198 0.39 0.27 0.04 0.23

SMC1 982 957 67 890 2.61 4.12 1.60 2.52

SBET is the surface area calculated by the BET method; SBJH and VBJH are the cumulative adsorption surface area and pore volume (1.7

nm < pore diameter < 300 nm) calculated by the BJH method; Smacro and Vmacro are the surface area and pore volume in the macroporerange (pore diameter > 50 nm) calculated by the BJH method; Smeso and Vmeso are the surface area and pore volume in the mesoporerange (2 nm < pore diameter < 50 nm) calculated by the BJH method; Vsingle is the single point total pore volume (pore diameter < 150

nm).

Fig. 1. SEM of SMC1 carbon. The micrograph was obtained

on a JEOL JSM 840-A electron microscope.

S. Han et al. / Microporous and Mesoporous Materials 58 (2003) 131–135 133

area, humic acids could not effectively adsorb onto

F400 because most of the pores in the activated

carbon are in the micropore range. Humic acids

can be effectively adsorbed in the large mesopores

of SMC1, whereas they cannot penetrate into the

micropores of the activated carbons. Most of the

surface of the nanoporous SMC1 carbon can be

effectively utilized as adsorption sites for humic

acids.

Fig. 3 compares the adsorption kinetics of hu-mic acids into SMC1 and F400 carbons. The rate

of adsorption of humic acids on SMC1 carbon was

much faster than on F400 carbon. In the case of

SMC1 carbon, the equilibrium concentration of

humic acids was reached in just 15 min, removing

60% of the initial humic acids. In contrast, it took

more than 120 min to reach the equilibrium con-

centration when F400 carbon was used as adsor-bent. The nearly spherical pore shape of the

nanoporous carbon materials, which facilitates the

diffusion of humic acids, could explain the rapid

adsorption of humic acids into SMC1 carbon.

4. Conclusion

SMC1 carbon exhibits adsorption characteris-

tics for humic acids which are superior to those of

commercial activated carbons in two aspects:

Firstly, SMC1 carbon shows a much higher ad-sorption capacity than the activated carbons.

Secondly, the adsorption of humic acids on SMC1

carbon proceeds rapidly indicating the possibility

of better carbon utilization in water treatment

processes.

Ce(mg/l)0.1

(a)

(b)

(c)

1 10

qe(mg/g)

10

100F400SMC1Norit SA

Ce(mg/l)1 10

qe(mg/g)

10

100F400SMC1Norit SA

Ce(mg/l)1 10

qe(mg/g)

1

10

F400SMC1Norit SA

Fig. 2. Adsorption isotherms of humic acids onto ðdÞ F400,ð�Þ SMC1 and ð.Þ Norit SA carbons. (a) pH ¼ 7, (b) pH ¼ 4,

(c) pH ¼ 8.

time(min)0 200 400 600 800

C/Co

0.0

0.2

0.4

0.6

0.8

1.0

F400SMC1

Fig. 3. Kinetics of humic acids adsorption onto ðdÞ F400, ð�ÞSMC1 in the batch reactor.

134 S. Han et al. / Microporous and Mesoporous Materials 58 (2003) 131–135

Acknowledgements

We are grateful to the Brain Korea 21 Program

supported by the Korean Ministry of Education

and the Korea Science and Engineering Founda-tion (Basic Research Program # 98-05-02-03-01-3)

for financial support.

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