활성탄소의 산-염기 특성에 관한 연구

김 기동, 박 수진, 이 재락, 송 해영*

한국화학연구소 화학소재연구단, *충남대학교 고분자공학과

A study of Activated Carbons on the Acid-Base Character

Ki-Dong KIM, Soo-Jin PARK, Jae-Rock LEE, Hae-Young SONG*

Advanced Material Division Korea Research Institute of Chemical Technology,

*Dept. of Polymer Sci. & Eng. Chungnam National University

Introduction

For a long time, the activated carbons have been widely used for the removal of organic pollutants from waste waters and drinking water, due to its large surface area and porosity. However, these activated carbons cannot be applied to advanced industries, such as environmental protection and medicines for lack of their selectivities. So commercially activated carbons are produced for these applications. Raw materials are carefully selected and processed to produce adsorbents having appropriate particle-size, surface area, pore size distribution, potential level, and so on[1].

It is a noted that the carbon surface can be recognized in both acidic and basic characters. In general, acidic reactions have been associated with the oxide structures which are part of the chemisorbed oxygen found on all carbon surfaces exposed to air or other oxidizing media. The existence of surface functional groups such as carboxyls, phenols, lactones and acid anhydrides has been postulated as constituting the sources of surface acidity, but the situation is by no means completely understood.. Also the basic properties of the carbon surface are even less well understood[2]. Efforts have been made to connect the basic sites of the carbon surface with chemisorbed oxygen as well, but it has not been clearly established that oxide structures are responsible for the basic reactions shown by some carbon surface.

The aim of this work is to characterize the acid-base properties of activated carbons by an adsorption isotherm and Iodine adsorption capacity.

Theory

Adsorption by microporous adsorbents is described by the theory of volume filling of micropores(TVFM) developed by Dubinin and co-workers. The foundamental relations are the Dubinin-Radushkevich(D-R)[3] and Dubinin-Astakhov(D-A)[4] equations, as:

[eqn: W=W_o`exp[-B(T/□)^2`log^2`(P_o`/P)]] ------------- (1)

[eqn: W=W_o`exp[-(A/□E_o`)^n`]]-------------------------------- (2)

Which W is the volume of the liquid-like adsorbate filling the microspores at absolute temperature T and relative pressure (P/Po), Wo the volume of the micropores, B the parameter characterizing the microporous structure, □ the affinity coefficient of the characteristic curves, Eo the characteristic energy of adsorption, n the equation parameter, and A the differential molar work of adsorption, i.e. the change of Gibbs free energy on adsorption, defined by

[eqn: A=-□G=RTln(P_o`/P)] -------------------------------- (3)

The microporous active carbons with molecular sieve properties possess a practically uniform micropore system. These carbons with a rather narrow pore-size distribution satisfy the D-R equation for a wide range of relative pressures. Whereas the adsorption isotherms obtained for active carbons with heterogeneous pore structure, i.e. with a broad pore-size distribution, can no longer follow the D-R equation.

In a study of iodine adsorption on carbon, Hill and Marsh found evidence that the KI3 formed by the reaction of KI and I2 ionizes by a small amount and that only the free I2 is adsorbed. The reaction and ionization constant are as follows[5]:

[eqn: KI_3`=KI+I_2`] ------------------------------- (4)

[eqn: {{[KI_3`]_e`}over{[KI]_e`[I_2`]_e`}}=870] --------------------- (5)

For any equilibrium condition the concentrations of the components are then:

[eqn: {{[KI_3`]_e`}over{[[KI]_i`-[KI_3`]_e`][[I_2`]_te`-[KI_3`]_e`]}}~=~870] ---- (6)

where [eqn: [KI]_i`-[KI_3`]_e`=[KI]_e`], [eqn: [I_2`]_te`-[KI_3`]_e`=[I_2`]_e`],

[KI]i = initial concentration as weighed into solution,

[I2]te = total iodine at equilbrium as determined by titration.

Whether KI3, I2, or KI, or all three are adsorbed is important to the interpretation of the results of this study.

Experimental

(1) Sample preparation

The porous carbon samples used were two types, AAC-501 (pH 2.4) and BAC-502 (pH 9.1) based on coconut shell, made from Union Carbon Co., each 8×12 mesh size were used.

(2) Specific surface area (SBET)[6]

The 77K N2-adsorption isotherms were obtained with Quantachrome Asorb2pc equipment. Prior to each analysis samples were outgassed at 273K for 6hours to obtain a residual pressure of less than 10-3Torr. A BET plot was constructed and the apparent BET surface area was extracted from the linear region (relative pressure = 0.01∼0.10). Allocation of pore size distribution involved subdividing the adsorbed amount in the range of pressure 0∼0.01, 0.01∼0.40 and 0.40∼0.95, corresponding broadly to adsorption in primary micropores, secondary micropores and mesopores, respectively. The amount adsorbed was converted to liquid volume by assuming a molar volume of liquid nitrogen of 35cm3/mol.

(3) Iodine adsorption capacity (I.N.)[5]

This index represents the number of mg of adsorbed iodine per gram of dry activated carbon. Several variations of a basic procedure were used to measure iodine adsorption from aqueous solutions. Common to all procedures, weighed quantities of each carbon, ground to powder, were shaken with measured quantities of KI-I2 solutions in stoppered flasks. After filtering off the carbon, the filtrate was titrated with standard Na2S2O3 solution. Change in total I2 content divided by the carbon weight gave the amount adsorbed at each filtrate concentration. For the iodine adsor- ption capacity, about 0.1, 0.3, 0.5 and 1.0g each samples were accurately weighed and 50ml of 0.1N iodine solution was added. Then filtrated after 15min vibrating at room temperature, the residual iodine concentration was determined by 0.1N Na2S2O3 titration.. The iodine number is defined as the amount of iodine adsorbed in mg/g when the equilibrium concentration is 0.02N total iodine, hence the carbon weight must be selected to bring the final concentration near 0.02N.

Results and Discussions

The effect of pH on the adsorption of iodine and nitrogen gas onto two activated carbons are studied in this work. Table 1 presents the experimental weights of iodine adsorbed and complete BET-data with nitrogen adsorption.. The activated carbons, AAC-501 and BAC-502, have very different surface properties.

Table 1. Adsorption characteristics of acidic/basic activated carbons

Terms Samples	I.N. [mg/g]	Langmuir S.A. [m2/g]	Adsorption S.A.[m2/g]	Total pore volume [cc/g]	SBET [m2/g]
AAC-501 (pH 2.4)	529	1386	5.1	0.046	11
BAC-502 (pH 9.1)	968	387	12.5	0.150	136

Fig. 1 shows the isothermal iodine adsorption and Fig. 2, 3 and 4 was adsorption surface area, pore volume and isothermal nitrogen adsorption of AC-series, at pH 2.4 and pH 9.1. In both cases, the amounts of iodine adsorbed and isothermal adsorption increase with an increase of pH. Conversely, the decrease of pH leads to increases the mesopores by means of BET-classification[7]. Especially, the sample BAC-502 shows the type-1 like according to BET-classification; i.e. Langmuir model which is related to the high adsorption properties. Following the above results, the acidic surface of the carbons leads to decrease their surface areas and micropore volumes. In the adsorption of iodine and nitrogen gases, the adsorption capacity decreases with the oxidation of the carbon because its surface area and micropore volume are decreased. It may be concluded that the data presented here tend to support the view that acidic surface oxides on the carbon surface are carboxylic in nature and that the basic sites are connected with the carbon structure itself.

Fig. 4.  BET isothermal adsorption

of AC-series.

Fig. 3.  Pore volume of AC-series.

Fig. 2.  Adsorption surface area

of AC-series.

Fig. 1.  Isothermal Iodine adsorption

of AC-series.

Referrences

1. S. J. PARK. in Interfacial Forces and Fields : Theory and Applications (Ed. by J.    P. H) Marcel Dekker, New York, 1998.

2. S. S. Barton, M. J. B. Evans, E. Hallilop and J. A. F. Macdonald, Carbon, 35,     1361 (1997).

3. M. M. Dubinin and L. V. Radushkevich, Doklady. Acad. Nauk. SSSR, 55, 331      (1947)

4. M. M. Dubinin and V. A. Astakhov, Izv. AN SSSR. Ser. Khin., 5, 11 (1971)

5. A. J. Juhola, Carbon, 13, 437 (1975)

6. S. Brunauer, P. H. Emmett and T. L. Hill, J. Am. Chem. Soc., 60, 309 (1938)

7. S. Lowell and J. E. Shields, "Powder Surface Area and Porosity", 3rd ed.,

Chapman & Hall, New york, 1993.