Studies in Surface Science and Catalysis, volume 159 Hyun-Ku Rhee, In-Sik Nam and Jong Moon Park (Editors) �9 2006 Elsevier B.V. All rights reserved

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Adsorption-desorption characteristics of modified activated carbons for volatile organic compounds

Ki-Joong Kim a, Chan-Soon Kang a, Young-Jae You a, Min-Chul Chung a, Seung Won Jeong b,

Woon-Jo Jeong c, Myung-Wu Woo a, Ho-Geun Ahn a*

aDept, of Chemical Engineering and bjeonnam Techno Park, Sunchon National University,

315 Maegok-dong, Suncheon-si, Jeonnam, 540-742 Korea. CDept. of Inform. & Comm., Chosun College Sci. & Techn., Seosuk-dong, Gwangju, Korea.

Modification techniques for activated carbon were used to increase the removal capacity by surface adsorption and to improve the selectivity to volatile organic compounds (VOCs). Modified activated carbons (MACs) were prepared by modifying the purified activated carbon with various acids or bases. The effects of adsorption capacity and modified contents

on the textural properties of the MACs were investigated. Furthermore, VOC adsorption and

desorption experiments were carried out to determine the relationship between the adsorption

capacity and the chemical properties of the adsorbents. High adsorption capacity for the

selected VOCs was obtained over l wt%-H3PO4/AC (lwt%-PA/AC). As a result, MAC was

found to be very effective for VOC removal by adsorption with the potential for repeated use

through desorption by simple heat treatment.

1. INTRODUCTION

Several techniques for VOC removal have been investigated such as thermal incineration, catalytic oxidation, condensation, absorption, bio-filtration, adsorption, and membrane separation. VOCs are present in many types of waste gases and are often removed by adsorption [ 1 ]. Activated carbon (AC) is commonly used as an adsorbent of gases and vapors because of its developed surface area and large pore volumes [2]. Modification techniques for AC have been used to increase surface adsorption and hence removal capacity, as well as to improve selectivity to organic compounds [3].

In this study, the surface properties of modified AC (MAC) and its adsorption capacity for

VOCs were investigated. The effect of modified contents on adsorption performance was studied. Furthermore, adsorption and desorption of VOCs was carried out to evaluate the

adsorption capacity and desorption characteristics after saturated adsorption.

2. EXPERIMENTAL

This subject is supported by the Ministry of Environment as "'The Eco-technopia 21 project". t corresponding author Tel: +82-61-750-3583. E-mail: hgahn@sunchon.ac.kr

458

2.1. Preparation of MAC The AC used in this study was a granular type (30-35 mesh) prepared from coconut shell.

The purified AC (PAC) was prepared by boiling the AC for 5 hr in a water bath. The acidic

and alkaline solutions for preparation of MACs were made with HNO3 (NA), H2SO4 (SA),

HC1 (HA), H3PO4 (PA), CH3COOH (AA), KOH (PH), and NaOH (SH). The AC was

modified into each solution according to the conventional wet process. The specific surface

area of the adsorbents was measured by BET method (ASAP 2020, Micrometrics, USA).

2.2. Measurement of VOC adsorption The variation of concentration in the course of adsorption was continuously obtained as a

thermal conductivity detector (TCD) signal. Model gases were BTX (benzene, toluene and o-,

m-, p-xylene), alcohols (methanol, ethanol, iso-propanol), and methylethylketone (MEK). The

concentration of VOCs for adsorption was controlled by vaporizing VOCs in the saturator

with helium, which was maintained in the range of 10,000ppm - 15,000ppm. The temperature

in the saturator was maintained with a constant temperature vessel. Total VOC flow rate was

40ml/min as it passed through a U-type adsorbent column. Before adsorption experiment, the

MAC was pretreated for 1 hr in an adsorbent column of 250 ~ and 0.2g of the adsorbent was

used for each experiment. The VOC concentrations were monitored with TCD of gas

chromatograph. In addition, the MAC desorption characteristics were investigated by

temperature programmed desorption (TPD) technique.

3. RESULTS AND DISCUSSION

3.1. Modified effect of activated carbon The prepared MAC adsorbents were tested for benzene, toluene, o-, m-, p-xylene, methanol,

ethanol, iso-propanol, and MEK. The modified content of all MACs was 5wt% with respect

to AC. The specific surface areas and amounts of VOC adsorbed of MACs prepared in this

study are shown in Table 1. The amounts of VOC adsorbed on 5wt%-MAC with acids and

alkali show a similar tendency. However, the amount of VOC adsorbed on 5wt%-PA/AC was

relatively large in spite of the decrease of specific surface area excepting in case of o-xylene,

m-xylene, and MEK. This suggests that the adsorption of relatively large molecules such as o-

xylene, m-xylene, and MEK was suppressed, while that of small molecules was enhanced. It

can be therefore speculated that the phosphoric acid narrowed the micropores but changed the

chemical nature of surface to adsorb the organic materials strongly.

The variation of amount of VOC adsorbed and the variation of BET surface area with

modified contents were shown in Fig. 1. The optimum modified content was l wt% for

benzene, toluene, p-xylene, methanol, ethanol and iso-propanol, but the amount of o-xylene,

m-xylene, and MEK adsorbed were decreased with increasing modified contents.

Interestingly, the amount of benzene, p-xylene, and ethanol adsorbed on l wt%-PA/AC was

1.5 to 2 times that on purified AC. The BET surface area of l wt%-PA/AC (1109m2/g) took

the maximum value.

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Table 1. The prepared MACs and their specific surface area and VOC adsorption on PAC and

various 5wt%-MACs

Aromatics Alcohols BET Surface

Sample Area, ,~,tm2/o~ Ben- Tol- o- m- P- MeOH EtOH iso- MEK zene uene xylene xylene xylene propanol

PAC 892 5.1 3.6 4.7 6.3 4.2 5.3 1.9 1.6 4.4

NA/AC 894 4.7 3.1 5.2 3.9 3.9 4.4 1.9 1.6 3.1

AA/AC 867 5.7 3.6 4.5 6.2 4.2 5.4 1.6 1.9 3.5

HA/AC 718 4.0 2.3 3.9 3.1 3.5 3.7 1.5 1.3 3.0

SA/AC 840 5.1 3.2 4.4 3.9 4.4 5.1 1.9 1.7 3.5

PA/AC 719 6.4 4.1 4.9 4.1 5.2 5.7 2.2 1.9 4.0

PH/AC 668 4.5 2.9 4.1 3.6 4.4 4.3 1.8 1.4 3.2

SH/AC 636 3.8 2.7 4.0 4.5 3.9 4.7 2.0 1.5 3.1

The reason for enhancement of adsorption

performance of PA/AC was considered to be

due to combination effect of increase of

BET surface area and chemical modification

by the treatment with PA. Consequently,

l wt%-PA/AC was determined to be a best

candidate as an adsorbent for removing

benzene, toluene, p-xylene, methanol,

ethanol, and iso-propanol. Therefore, l wt%-

PA/AC was used as the adsorbent to

investigate the adsorption isotherm,

adsorption and desorption performance.

- - - ~ I~-r ~ Area 12 1 + Benzene 1200

| * I "-0- Tcluene I / \ I -'*- ~

' ~ I-"-r,,X~,~ ~000 __

E --~.- ahard 8 8 8oo g

~ 09

< ~ ~ 4 ~ o , ~ , 2oo4~176 m

0 5 10 15 20

conterts [wt~ o]

Fig. 1. Effect of modified contents on PA/AC.

3.2. Adsorption isotherms Langmuir and Freundlich adsorption isotherms for toluene and MEK are shown in Figs. 2

0.25

o.2

l Oto,uene ~ 0 i i i I

0 0.2 0.4 0.6 0.8 1

Ce[ mmol/L]

Fig. 2. Langmuir adsorption isotherm of

toluene and MEK on l wt%-PA/AC.

0.15 .I.

0.1

o 0.05

1.6

1.2 ,---.,

0 E 0.8 E ,._.,, r 0 .4 0 c m F [Oto,uene I

0 '

-2 -1.5 -1 -0.5

InCe[ mmol/L]

Fig. 3. Freundlich adsorption isotherm of

toluene and MEK on l wt%-PA/AC.

460

and 3, respectively. Langmuir and Freundlich isotherms expressed relatively well the

adsorption of toluene and MEK, indicating the dependence on both physical and chemical

adsorption. This analysis indicated that the adsorption capacity of MEK on l wt%-PA/AC was

larger than that of toluene.

3.3. Desorption performance by TPD Fig. 4 shows the TPD experimental

results for l wt%-PA/AC with toluene and

MEK. The desorption characteristics of

l wt%-PA/AC with toluene and MEK were

determined by raising the room temperature

by 5~ /min to 300~ Maximal desorption

concentrations and temperatures of both

toluene and MEK were 1.13, 1.25 and 150 ~

100~ respectively. The toluene and MEK

adsorbed on l wt%-PA/AC was completely

desorbed when the adsorbent was heated in

an electric furnace. The temperature of the

adsorbent was programmed, and

concentration of VOCs desorbed could be

controlled at some extents. The desorbed

efficiencies of toluene and MEK were

98.1% and 99.1%, respectively. Similar TPD

patterns were obtained even though the

adsorption-desorption operation was repeated. It was therefore considered that the

adsorbents will be effective for repetition of

adsorption-desorption processes. 35O

1.2

1.0

o 0.8 C.)

0.6

0.4

0.2

o.o ~ ~" ~ ~ "" ~ ~ ~ ' ~ - 0 I0 2o 3o 4o 5o

Time on strean~min]

Fig. 4. TPD spectra from the desorption of

toluene and MEK on l wt%-PA/AC.

3OO

25o

2oo

(~

E

too ~-

0

60

4. CONCLUSIONS

Adsorption capacity and desorption characteristics of modified activated carbon (MAC)

prepared with various acids and bases were investigated. Among the prepared MACs, PA/AC

showed the greatest adsorption capacity for benzene, toluene, p-xylene, methanol, ethanol and

iso-propanol due to chemical modification of its surface despite the decreased specific surface

area. Also, the maximum BET surface area with modified content of PA/AC showed l wt%-

PA/AC. The amount of VOC adsorbed on l wt%-PA/AC was larger than that on purified AC

excepting that of o-xylene, m-xylene, and MEK. The adsorbed toluene and MEK were easily

desorbed by heat treatment to 300 ~ suggested the possibility for repeated use. The findings

confirmed the potential of lwt%-PA/AC as a promising adsorbent for controlling VOC

emissions with low concentrations.

REFERENCES

[ 1 ] C.L. Chuang, P.C. Chiang, Chemosphere, 53, 17 (2003).

[2] D.J. Kim, W.G. Shim and H. Moon, Korean Jr. Chem. Eng., 18, 518 (2001 ).

[3] L. Guo, A.C. Lua, J. Porous Materials, 7,491 (2000).