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High surface area activated carbon prepared from cassava peelby chemical activation

Y. Sudaryanto, S.B. Hartono, W. Irawaty, H. Hindarso, S. Ismadji *

Department of Chemical Engineering, Widya Mandala Surabaya Catholic University, Kalijudan 37, Surabaya 60114, Indonesia

Received 15 January 2004; received in revised form 29 March 2005; accepted 4 April 2005

Available online 15 June 2005

Abstract

Cassava is one of the most important commodities in Indonesia, an agricultural country. Cassava is one of the primary foods in

our country and usually used for traditional food, cake, etc. Cassava peel is an agricultural waste from the food and starch process-

ing industries. In this study, this solid waste was used as the precursor for activated carbon preparation. The preparation process

consisted of potassium hydroxide impregnation at different impregnation ratio followed by carbonization at 450750 C for 13 h.

The results revealed that activation time gives no significant effect on the pore structure of activated carbon produced, however, the

pore characteristic of carbon changes significantly with impregnation ratio and carbonization temperature. The maximum surface

area and pore volume were obtained at impregnation ratio 5:2 and carbonization temperature 750 C.

2005 Elsevier Ltd. All rights reserved.

Keywords: Activated carbon; Pore structure; Cassava peel; Activation

1. Introduction

Activated carbons are materials having complex por-

ous structures with associated energetic as well chemical

inhomogeneities. Their structural heterogeneity is a re-

sult of existence of micropores, mesopores and macro-

pores of different sizes and shapes. Activated carbon is

one of the most important adsorbents from an industrial

view of point. The main application of this adsorbent is

for separation and purification of gaseous and liquid

phase mixtures (Ismadji and Bhatia, 2001).A challenge in activated carbon production is to pro-

duce very specific carbons with a given pore size distri-

bution from low cost materials at low temperature.

Activated carbons with high specific surface area and

pore volumes can be prepared from a variety of carbo-

naceous materials such as coal (Hsu and Teng, 2000;

Ganan et al., 2004; Qiang et al., 2005), coconut shell

(Qiao et al., 2002; Sekar et al., 2004), wood (Tancredi

et al., 2004; Diaz-Diez et al., 2004), agricultural wastes

(Guo et al., 2004; Youssef et al., 2005; Zhang et al.,

2004), or industrial wastes (Hayashi et al., 2005; Ko

et al., 2004). In industrial practice, coal and coconut

shell are two main sources for the production of

activated carbons.

There are two processes for preparation of activated

carbon: chemical activation and physical activation.Chemical activation is known as a single step method

of preparation of activated carbon in the presence of

chemical agents. Physical activation involves carboniza-

tion of a carbonaceous materials followed by activation

of the resulting char in the presence of activating agents

such as CO2 or steam. The chemical activation usually

takes place at a temperature lower than that used in

physical activation, therefore it can improve the pore

development in the carbon structure because the effect

0960-8524/$ - see front matter 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.biortech.2005.04.029

* Corresponding author. Tel.: +62 31 3891264; fax: +62 31 3891267.

E-mail address: suryadi@mail.wima.ac.id(S. Ismadji).

Bioresource Technology 97 (2006) 734739

mailto:suryadi@mail.wima.ac.idmailto:suryadi@mail.wima.ac.id

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of chemicals. The carbon yields of chemical activation

are higher than physical one (Ahmadpour and Do,

1997).

Cassava is one of the most important commodities in

Indonesia, an agricultural country. Cassava is one of the

primary foods in our country and usually used for tradi-

tional food, cake, etc. The cassava production in Indo-nesia is very huge and only small amounts are utilized

by traditional food industries, and the rest are used as

the raw material for cassava starch industries. Cassava

starch making operation produces a large amount of

solid wastes (cassava peel), and direct discharge of this

solid wastes will cause the environmental problems.

Although there are many studies in the literature relat-

ing to the preparation and characterization of activated

carbon from agricultural wastes as mentioned before,

there is no information for the preparation of the acti-

vated carbon using Indonesian cassava peel as the precur-

sor with KOH as chemical activatingagent. In the present

study, we obtained cassava peel from cassava starch

industry located near Surabaya. The main objective in

this work is to study the effect of different preparation

variables on the pore characteristic of activated products.

2. Experimental

2.1. Materials

The proximate and ultimate analysis of the cassava

peel used in this study, are given in Table 1. The con-

tents of C, H, N, O and S elements in the ultimate anal-ysis were determined by an elemental analyzer (Heraeus,

CHN-O-RAPID). The analytical results shown in Table

1reveal that cassava peel has high carbon and low ash

content indicating that this precursor is suitable for

the preparation of activated carbon. Prior to the use,

the cassava peel was repeatedly washed with distilled

water in order to remove dust and other inorganic impu-

rities, then oven-dried for 24 h at 120 C to reduce the

moisture content.

2.2. Activated carbon preparation

Chemical activation of cassava peel was performed

using KOH. Different carbonization time, temperatures,

and impregnation ratios were studied in order to estab-

lish the optimal conditions for producing high surface

area activated carbons from cassava peel. Chemical acti-vation was performed with method as follow: 10 g of

dried cassava peel was mixed with KOH solution with

different impregnation ratio for about 3 h at 50C.

The concentration of KOH solution was adjusted to

give a mass ratio of chemical activating agent to cassava

peel range from 1:2 to 5:2 (mass basis). The resulting

homogeneous slurry was dried at 110C for at least

24 h. The resulting sample was place in horizontal tubu-

lar furnace and then carbonized at desired temperatures

(450 C, 550 C, 650 C, and 750 C). The carbonization

and activation was performed under nitrogen flow of

150 cm3/min. The carbonization and activation process

was initiated by heating the sample at heating rate of

10 C/min from room temperature (around 30 C) until

the desired temperature was reached. Samples were

held at desired temperature for different carbonization

times of 1, 2, or 3 h before cooling down under nitrogen

flow. The activated carbon products were washed

sequentially with a 0.5 N HCl solution. Subsequently,

the samples were repeatedly washed with hot dis-

tilled water until the pH of solution reach 6.5 and

finally washed with cold distilled water. After that, the

samples were dried at 110 C for 24 h and stored in

desiccator.

2.3. Pore structure characterization

The pore structure characteristics of the resulting

carbons were determined by nitrogen adsorption at

196 C using an automatic Micromeritics ASAP-2010volumetric sorption analyzer. Prior to gas adsorption

measurements, the carbon was degassed at 300 C in a

vacuum condition for a period of at least 24 h. Nitrogen

adsorption isotherms were measured over a relative

pressure (P/P0) range from approximately 105 to 0.995.

The BET surface area, micropore volume and micropore

surface area of the activated carbons were determined

by application of the BrunauerEmmettTeller (BET)

and DubininAsthakov (DA) analysis software avail-

able with the instrument, respectively. The BET surface

area was determined by means of the standard BET

equation applied in the relative pressure range from

0.06 to 0.3. The pore size distribution of carbons were

determined from argon adsorption isotherm data using

the Micromeritics density functional theory (DFT) soft-

ware, with medium regularization. The argon adsorp-

tion experiments were carried out at 186 C using anautomatic Micromeritics ASAP-2010 volumetric sorp-

tion analyzer. Here, the argon adsorption with DFT

Table 1

Proximate and ultimate analysis of cassava peel

Analysis wt% Variation (%)

Proximate

Moisture 11.4 1.2

Volatile matter 59.4 0.8

Fixed carbon 28.9 0.9

Ash 0.3 1.0

Ultimate

Carbon 59.31 1.1

Nitrogen 2.06 1.1

Hydrogen 9.78 1.1

Oxygen 28.74 1.1

Sulphur 0.11 1.1

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analysis of the data is used to provide independent pore

size distribution. We used the argon adsorption data

instead of the nitrogen isotherm because when this

method (DFT) applied to nitrogen isotherm suffer from

the drawback that interactions of surface chemical

heterogeneities with quadrupolar nitrogen can affect

the pore size distribution determined.

3. Results and discussion

A series of activated carbons were prepared from cas-

sava peel as precursor with chemical activation using

KOH as activating agent. The effects of different prepa-

ration variables on the pore characteristic and surface

chemistry (surface acidity) of activated products were

also studied. Details are discussed as follow.

3.1. Yield of activated carbon

Carbonization temperature plays an important role

on the yield of activated carbon. Fig. 1depicts the effect

of carbonization temperature on the yield of activated

carbon at different carbonization time (impregnation

ratio 1:1). As seen in the figure, carbonization time does

not have much effect on the yield of activated carbon

while the carbonization temperature has significant ef-

fect. At carbonization temperature range 450550C,

the rate of weight loss is high primarily due to the initial

large amount of volatiles that can be easily released with

increasing temperature as well as the loss of moisture to

a lesser extent. The yield of the activated carbon at car-

bonization temperatures more than 650C are less than

fixed carbon in initial precursor (seeTable 1). Since the

potassium hydroxide is a strong base, it catalyzes the

oxidation reactions.

3.2. Effect of carbonization time on the pore

characteristic

As mentioned before that the carbonization time has

no significant effect on the yield of activated carbon.

Table 2summarizes the effect of carbonization time on

the pore characteristic of chemically activated carbons

under conditions of carbonization temperature 650

Cand impregnation ratio 1:1. This table clearly shows that

carbonization time does not have much effect on the

pore characteristic of activated carbon products. In gen-

eral, the BET surface area and total pore volume,VT, re-

main constant.

3.3. Effect of temperature on the pore structure

Since the carbonization time does not have any signif-

icant effect on the activated carbon properties, we dis-

cuss the effect of temperature on the pore structure

and surface chemistry only at carbonization time 1 h.Pore evolution of the activated carbon with varying car-

bonization temperature is shown in Fig. 2. Details of

pore characteristics of activated carbon at different car-

bonization temperatures are given in Table 3. Fig. 3

shows the nitrogen adsorption isotherms at 196 C ofactivated carbons derived from cassava peel.

Temperature, Co

400 450 500 550 600 650 700 750 800

Yield,%

20

25

30

35

40

Carbonization time = 1 hCarbonization time = 2 hCarbonization time = 3 h

Fig. 1. Effect of carbonization temperature on the yield of activated

carbon (impregnation ratio 1:1).

Table 2

Effect of carbonization time on the pore characteristic of chemically

activated carbons under conditions of: carbonization temperature

650 C, impregnation ratio 1:1

Carbonization

time (h)

Pore characteristic

BET

(m2/g)

Variation

(%)

VT(cm3/g)

Variation

(%)

1.0 1154 0.6 0.519 0.6

2.0 1108 0.9 0.517 0.9

3.0 1183 0.5 0.520 0.5

Carbonization temperature, Co

400 450 500 550 600 650 700 750 800

Porevolume,cm3/g

0.35

0.40

0.45

0.50

0.55

0.60

Micropore volumeTotal pore volume

Fig. 2. Pore evolution during carbonization.

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The effect of carbonization temperature in the poredevelopment is very significant as seen inFig. 2. Increas-

ing the carbonization temperature from 450 C to

750 C increases the evolution of volatile matters from

the precursor, leading to increase of the pore develop-

ment, and creates new pores. At temperature higher

than 400 C, the reaction between potassium hydroxide

and carbon occurs (Ganan et al., 2004) according the

following reaction

6KOH+C $ 2K+3H2 + 2K2CO3 1

The presence of metallic potassium will intercalate to

the carbon matrix. This phenomenon results in widening

of the spaces between carbon atomic layers and increas-

ing the total pore volume, VT (Ahmadpour and Do,

1997). However, the micropore volume, VP, increase

until the carbonization temperature reach 650 C and

at higher temperature, the micropore volume decreases.

At temperature higher than 650C, the surface metal

complex is responsible for further carbon gasification,

leading to widening of micropore to mesopore (Ganan

et al., 2004).

The adsorption isotherm of activated carbons pro-

duced from cassava peel at carbonization temperature

450650C belong to a mixture of Type I and Type

IV isotherms (seeFig. 3). According to IUPAC classifi-

cation, the Type I isotherm is associated with micropo-

rous structures and Type IV isotherm indicates mixture

of microporous and mesoporous material. The plateau

of this isotherm commences at high relative pressures

(P/P0) and toward the end of isotherm, steep gradient

is seen as the result of a limited uptake of nitrogen, indi-cating capillary condensation in the mesopores. This

characteristic indicates the development of micro and

mesoporous structure on this char during carbonization

process. The adsorption isotherm of carbon as seen in

Fig. 3 clearly shows the largely microporous nature of

the activated carbon produced from cassava peel at car-

bonization temperature 450650 C, with some meso-

pores leading to gradual increase in adsorption after

the initial filling of the micropores, followed by more

rapid increase near saturation. At carbonization temper-

ature 750 C, the activated carbon has more mesoporous

structures as indicated by adsorption isotherm of nitro-

gen at 196 C. At 750 C the development of meso-pores are more pronounced and thermal degradation

of the pore wall occurs causing the widening of

micropores.

The structural heterogeneity of porous material is

generally characterized in terms of the pore size distribu-

tion (Ismadji and Bhatia, 2001). This pore size distribu-

tion represent a model of solid internal structure, which

assumes that an equivalent set of non-interacting and

regularly shaped model pores can represent the complex

void spaces within the real solid. The pore size distribu-

tion is closely related to both kinetic and equilibrium

properties of porous material, and perhaps is the mostimportant aspect for characterizing the structural heter-

ogeneity of porous materials used in industrial applica-

tion. As mentioned before, here, the argon adsorption

with DFT analysis of the data is used to provide inde-

pendent pore size distribution. We used the argon

adsorption data instead of the nitrogen isotherm be-

cause when this method (DFT) applied to nitrogen

isotherm suffer from the drawback that interactions of

surface chemical heterogeneities with quadrupolar nitro-

gen can affect the pore size distribution determined. The

pore size distributions of activated carbon derived from

cassava peel at various carbonization temperatures are

shown inFig. 4. These figures confirm that the activated

carbons produced at carbonization temperatures 450

650 C contain micro and mesoporous structure while

at 750 C has significant mesoporous nature.

3.4. Effect of chemical ratio

The impregnation ratio has been found to be one of

the important parameter in preparation of activated car-

bon using chemical activation (Ahmadpour and Do,

1997). In order to study the effect of impregnation ratio

on the pore characteristic of the activated carbons, here

Table 3

Pore characteristics of activated carbon at different carbonization

temperatures (impregnation ratio 1:1)

Carbonization

temperature

(C)

Pore structure

BET

(m2/g)

VP(cm3/g)

VT(cm3/g)

Micropore

surface

area (m2/g)

Variation

(%)

450 972 0.383 0.421 857 0.3

550 1074 0.394 0.435 908 0.2

650 1154 0.462 0.519 938 0.6

750 1378 0.393 0.583 881 0.5

Relative pressure, p/po

0.0 0.2 0.4 0.6 0.8 1.0

Volumeadsorbed

,cm3/gSTP

0

100

200

300

400

500

600

Carbonization temperature 450 CCarbonization temperature 550 CCarbonization temperature 650 CCarbonization temperature 750 C

Fig. 3. Nitrogen adsorption isotherm on activated carbons prepared

from cassava peel.

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we used carbonization time 1 h and carbonization tem-perature 750C. The pore characteristic of activated

carbons produced at various impregnation ratios are

given inTable 4.The yield of activated carbon decrease

with increase of impregnation ratio as seen in Fig. 5.

Potassium hydroxide promotes the oxidation process,

with high KOH ratio the gasification of surface carbon

atoms is the predominant reaction leading to increase

in the weight loss of carbon.

Table 4indicates that the BET surface area and pore

volume (both total and micropore volumes) are both

increasing continuously with impregnation ratio. The

pores were created due to the evolution of gaseous car-

bonization products and catalytic oxidation of carbonsurface by potassium metallic salt. The catalytic oxida-

tion causes the widening of some micropores to meso-

pores as mentioned before. From Table 4, it also can

be noted that the micropore volume of the activated

carbon products are also increase with increasing

impregnation ratio. At high ratio of KOH, the micro-

porosity development is mostly due to the intercalation

of potassium metal in the carbon structure.

The pore size distribution of activated carbons pro-

duced from cassava peel at various impregnation ratios

at carbonization temperature 750 C are given inFig. 6.

These pore size distributions are determined using DFT

software package with medium regularization. From

this figure, it is obvious that the impregnation ratio gives

Pore size, 3 6 20 30 60 200 300 60010 100

Poresizedistribution,cm3/g.

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

Carbonization temperature, 450 oCCarbonization temperature, 550 oCCarbonization temperature, 650 oCCarbonization temperature, 750 oC

Fig. 4. Pore size distribution of activated carbons prepared from

cassava peel.

Table 4

Pore characteristics of activated carbon at different impregnation ratio

(carbonization temperature 750C)

Impregnation

ratio

Pore structure

BET

(m2/g)

VP(cm3/g)

VT(cm3/g)

Micropore

surface

area (m2/g)

Variation

(%)

1:2 1027 0.390 0.434 850 0.8

1:1 1378 0.393 0.583 881 0.5

3:2 1491 0.432 0.612 1023 0.7

2:1 1562 0.487 0.668 1074 0.5

5:2 1605 0.513 0.691 1100 0.3

Impregnation ratio0.0 0.5 1.0 1.5 2.0 2.5 3.0

Yield,%

20

22

24

26

28

30

Fig. 5. Effect of impregnation ratio on the yield of activated carbon.

Relative pressure, p/po

0.0 0.2 0.4 0.6 0.8 1.0

Volumeadsorbed,cm3/gSTP

0

200

400

600

800

1000

Impregnation ratio 1:2Impregnation ratio 1:1Impregnation ratio 3:2Impregnation ratio 4:2Impregnation ratio 5:2

Fig. 7. Nitrogen adsorption isotherm on activated carbons prepared

from cassava peel under different impregnation ratio.

Pore size, 5 20 50 200 500 200010 100 1000

pore

sizedistribution,cm3/g.

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

Impregnation ratio 1:2Impregnation ratio 1:1Impregnation ratio 3:2Impregnation ratio 2:1Impregnation ratio 5:2

Fig. 6. Effect of impregnation ratio on the pore size distribution of

activated carbon.

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significant effect on the pore structure of activated

carbon produced. At low impregnation ratio (50%),

the pore structure of activated carbon produced mainly

consists of micropore, however with the increase of

impregnation ratio, the creation of micropore structure

and widening of micropores to mesopores also increase.

The adsorption isotherms of activated carbons producedunder different impregnation ratio are shown in Fig. 7.

Here, as mentioned before, potassium metallic acts as

the catalyst for oxidation reaction, therefore more

carbon atom in surface is oxidized leading to widening

of the pore structures.

4. Conclusion

High surface area activated carbons were prepared

from Indonesian cassava peel with chemical activation.

As the activating agent, potassium hydroxide was used.

From this study it was found that cassava peel is a goodprecursor for activated carbon preparation. Carboniza-

tion temperature and impregnation ratio give significant

effect in the pore characteristic of activated carbons

produced.

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