Effect of Surface Characteristics of Activated Carbon on the

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6 Effect of Surface Characteristics of Activated Carbon on the Adsorption of Chloroform from Aqueous Solution C H A N E L ISHIZAKI, IRIS MARTÍ, and M A G A L Y RUÍZ Instituto Venezolano de Investigaciones Cientificas, Laboratorio de Ingenieria Ambiental, Apartado 1827, Caracas 1010-A, Venezuela

Chloroform adsorption on two activated carbons of different surface characteristics was investigated. The chemical and physical characteristics of the carbon surfaces were evaluated. The equilibrium and kinetic studies indicate different adsorptive behaviors for these systems. For the equilibrium concentration range of 10-200 μg/L, the data fit the Langmuir isotherm. The observed differences in adsorption isotherms cannot be explained by differences in pore size distributions, but can be explained by the chemical nature of the carbon. The carbon with the least amount of oxygen exhibits the highest affinity for chloroform. The relative capacities of the carbons to adsorb chloroform are a function of the solution equilibrium concentrations. For equilibrium concentrations below 270 μg/L, the carbon with the least amount of oxygen exhibits the highest capacity.

T

HE ABILITY OF ACTIVATED CARBONS to adsorb various organics from

solution is recognized. However, the mechanisms by which these compounds adsorb are not clearly understood. Adsorption is governed by the chemical nature of the aqueous and solid phases, and by the chemical nature of the adsorbing compounds. Any interpretation of the adsorptive behavior of activated carbons based only on surface area is incomplete. Carbons having equal weights and equal total surface areas, when prepared by different methods, exhibit different adsorptive characteristics. Some adsorptive properties can be explained by differences in relative pore size distributions, but a more important

0065-2393/83/0202-0095$06.00/0 © 1983 American Chemical Society

In Treatment of Water by Granular Activated Carbon; McGuire, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1983.


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TREATMENT OF WATER BY GRANULAR ACTIVATED CARBON

consideration is the difference in surface chemistry of the carbons. The adsorptive properties of carbons activated by oxidation depend primarily on the chemical nature and concentration of the oxidizing agent, the temperature of the reaction, the extent to which the activation is conducted, and the amount and kind of mineral ingredients in the char. Activated carbons are structurally similar to turbostratic carbon, having microcrystallites only a few layers in thickness and less than 100 A in width. The level of structural imperfections in activated-carbon microcrystallites is very high, which results in many possibilities for reactions of the edge carbons with their surroundings (i). Oxygen combines with the carbon to form a physicochemical complex, C O , of variable composition. The oxygen functional groups suggested as being present on the surface of activated carbon include carboxyl groups, phenolic hydroxyl groups, quinone-type carbonyl groups, lactones, carboxylic acid anhydrides, and cyclic peroxides. Carboxylic, lactone, and phenolic groups are the acidic surface oxides (2). The nature of the basic surface oxides is less understood, and no fully convincing formulation has been proposed so far. Besides oxygen, hydrogen is present in most carbon surfaces. The hydrogen not only forms part of the oxygen functional groups, but it is also directly combined with carbon atoms. Hydrogen is more strongly chemisorbed than oxygen and is difficult to remove from the surface (3). It is evident that the physical and chemical properties of the carbons depend on the activation procedure employed in the manufacturing or regeneration process. Activation procedures should be controlled to optimize the adsorption of specific compounds from water and maintain maximum capacity during regeneration. Implementation of proper activation and regeneration procedures requires basic knowledge of the mechanisms by which particular adsorbates or groups of adsorbates of the same nature adsorb. The adsorption of many organics, especially phenol and its derivatives, on different carbons has been studied extensively, but the biggest difficulty in drawing conclusions about the adsorption mechanisms arises from lack of information on the surface characteristics of the carbons used in the different studies. Mullins et al. (4) clearly showed that the variation in adsorptive characteristics of different carbons for the removal of trihalomethanes is typically in the 200-300% range, indicating a strong influence of the carbon s characteristics on adsorptive capacities. The present study tries to correlate the effectiveness of different carbons for the removal of a specific adsorbate of major concern, chloroform (5), with the physical and chemical properties of the carbon surface. x

y


6.

ISHIZAKI ET AL.

Surface Characteristics ofActivated-Carbon Adsorption


Experimental Carbons. The activated carbon used was Filtrasorb 200 (F200), granular carbon produced from coal by high temperature steam activation The mean particle diameter of the carbon used was 606 /xm. The amount of oxygen surface groups on the original carbon was reduced by heat treating the carbon at 1000°C for 17 h in an inert atmosphere. The carbon was then allowed to cool to room temperature under inert atmosphere and kept under these conditions until used for its characterization and adsorption studies. This carbon will be identified as OG. The F200 carbon was washed several times with distilled water and ovendried in thin layers at 105°C for 24 h. The O G carbon was used without further treatment. The physical and chemical characteristics of the surface of the two carbons were evaluated as follows. Specific surface area measurements were made by the B E T technique, and pore size distributions were made using a Monosorb surface area analyzer model MS-4 and Quantasorb adsorption system model QS-7, respectively. Direct transmission infrared spectroscopic studies of 0.25% in weight KBr tablets coupled with standard neutralization , techniques were used to investigate the nature and distribution of the surface functional groups on the different carbons. The technique used for the preparation of the KBr pellets has been described previously (6). Table I summarizes the specific area and average pore size of the different carbons. Figure 1 shows the pore size distribution for these carbons in the 10-300 A pore radius range. The neutralization capacities of these carbons for different neutralizing solutions are given in Table II, and the acidic surface group distribution according to Boehm (2) are presented in Table III. Figure 2 shows the infrared direct transmission spectra of the carbons. A thorough discussion of the structure of the surface oxides present on the different carbons has been presented elsewhere (6,7). The spectra as well as the neutralization capacities indicate that there has been a reduction in surface oxides for the O G carbon, as expected.

Table I. Specific Surface Area and Average Pore Size Specific Surface Area (m /g) 655 647

Carbon

2

F200 OG

Average Fore Size in the 10-300 A Range 23 20

Table II. Neutralization Capacities for Different Neutralizing Solutions (in mEq/g) Neutralizing Solutions (~0.25 N) Carbon F200 OG

NaHC0

Na C0

NaOH

HCl

0.07 0

0.26 0.05

0.74 0.42

0.50 0.52

3

2

3


97


0150

10

20

40

o

l00

200

400

Pore Radius (A)

Figure 1. Pore size distributions for different carbons. Key O , F200; and OG.

Adsorption Studies. A l l adsorption studies were conducted in batch systems consisting of 125-mL glass vials with "Mininert" Teflon sealing valve caps. The initial chloroform concentration was 300 /xg/L for the equilibrium and kinetic studies. The carbon dosages varied between 0.2 and 1.0 g/L. The stock chloroform solution was prepared with distilled water freed from volatile organics and gases by nitrogen gas striping. The p H of the solutions was not buffered to avoid the introduction of additional species into the water-chloroform-activated-carbon systems. A 48-h contact time was selected for the equilibrium studies based on the adsorption kinetics. A Burrell wrist-action shaker was used for agitating the samples. Controls without carbon were run in all cases. Analytical Procedure. A model 3700 Varian gas chromatograph with a CDS 111 Varian data system was used for chloroform analysis. This chromatograph was equipped with a flame-ionization detector and temperature programming A 1.8-m (6-ft) long 0.3-cm (1/8-in) diameter stainless steel column filled with 100-120-mesh Chromosorb 101 and 5% Carbowax was used. The samples were concentrated using a Teckmar model LS C-2 liquid concentrator.


6.

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Surface Characteristics of Activated-Carbon Adsorption

99

Table III. Distribution of Acidic Functional Groups According to Boehm's Selective Neutralization Capacities Carbon

Phenolic Groups f Lactone Groups Carboxylic Groups (%) (%) (%)

F200 OG

65 88

26 12

9 0


Results and Discussion Equilibrium Studies. The equilibrium isotherms for the adsorption of chloroform on the two carbons investigated are presented in Figure 3 (on a weight basis) and in Figure 4 (normalized for the corresponding surface areas). It is observed in these figures that, for the equilibrium concentration range under consideration (10-200 /xg/L), the O G carbon exhibits a stronger affinity and larger capacity than the F200 original carbon The equilibrium data are plotted according to the Langmuir equation in Figure 5. The equilibrium constants (K) and monolayer coverages (Z) obtained from these plots are shown in Table IV. Considering the relationship between the equilibrium constant (K) and the adsorption energy given by the equation: (1)

-±H°/RT

Kae

1 I l 1 l

I

l

1

1

I 1 I I I I

I

I

i

I

2500

2000

1

1

1

1

1

I

I

I

I

I

1500

WAVENUMBER

I 1000

(cm-t)

Figure 2. IR direct transmission spectra of different carbons, top trace (F200) and bottom trace (OG).


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TREATMENT OF WATER BY GRANULAR ACTIVATED CARBON


800

600

400

200-

200

Figure 3. Adsorption isotherms for chloroform on different carbons on mass basis. Key: O , F200; and #, OG. and using the experimentally obtained values of K (Table IV), it follows that: AH°

OG

= 2.6 A f / °

F 2 0 0

(2)

According to the Polanyi approach to model adsorption, the distribution of adsorption energies follows the pore size distribution, and it is limited to London force adsorption. Rozwadowski et aL (

Effect of Surface Characteristics of Activated Carbon on the

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