철이온의 활성탄 흡착에 대한 팽윤조작의 영향

황경진, 김동수

이화여자대학교 환경공학과

Effect of Swelling of Activated Carbon on Adsorption of Fe(III)

Kyoung-Jin Hwang, Dong-Su Kim

Dept. of Environmental Science & Engineering, Ewha Womans University,

Seoul 120-750, Korea

Introduction

Iron has been considered as the most useful metal to many fields of industries and to human body.  In the processes of producing irons, lots of water is usually needed and discharged to the environment.  Though discharging amount of iron doesn't affect to human bodies, it can cause undesirable problems to the industrial processes or ecosystems if iron is not managed properly.  For controlling the content of iron in the wastewater, there have been applied many types of technologies.  Compared with others, adsorption is evaluated as cost-effective and simple to operate.

The purpose of this study is to inspect the adsorption characteristics of Fe(III) ion onto the granular activated carbon.  Adsorption process can be modified by changing properties of adsorbent and adsorbate.  Here, it was investigated the influence of swelling treatment which changes the surface property of activated carbon.  Aspects of this study was focused on the removal capacity, adsorption kinetics and thermodynamics both on swelling and non-swelling activated carbon.

Experimental

Granular activated carbon from Merck Co. was sieved to 710∼1000□ and dried for 3 hrs.  The Fe(III) solution was prepared with iron(III) sulfate hexahydrate(Showa Chemicals Inc.) and distilled water.  The specific surface area of activated carbon was measured by weighing the equilibrium weight of EGME(ethylene glycol monoethyl ether, Junsei Chemical Co.) by drying up the EGME wetting activated carbon with P2O5 and CaCl2(Aldrich Co.).  Swelling of activated carbon was carried out by acetic acid vapor at 60℃.  Activated carbon was put into the desiccator and measured the weight as a function of time until equilibrium, and dried up at 100℃ for 24 hrs.  Adsorption experiments were conducted by a shaking of 1g of activated carbon with 50ml of each concentration of Fe(III) solution at 200rpm.  Those were carried out by changing concentration, temperature, and initial pH of solution.  The basic experimental condition was controlled to 2.0mM, 20℃, and pH 2.  The equilibrium concentration after adsorption was determined by AAS(atomic absorption spectrophotomety, Perkin Elmer, AAnalyst100) method at the wavelength of 258nm.

Results and Discussion

1. Effect of concentration

Considering the effect of Fe(III) concentration, adsorption time were determined as 30 minutes as shown in figure 1.  Here, X/M is the amount of adsorption, namely amount of adsorbate per unit mass of adsorbent.  Removal percentages of each concentration were increased from 68.83 to 84.76% with increasing concentrations.  It was observed that adsorption trends were steepened with higher concentrations.  It means that the higher the concentration is, the faster the adsorption reaction takes place.  It can be expalined by the rate of adsorption represented in table 1 computed by the Van't Hoff equation.  Adsorption reaction is proportioned to the reaction rate constant, so it can be regarded as the same order with the rate of adsorption.

Table 1. Adsorption kinetics according to concentration of Fe(III) and temperature

	concentration (mM)				temperature (K)
	0.5	1.0	1.5	2.0	293	303	313	323
reaction order, n	0.451	0.507	0.568	0.451	0.451	0.777	0.798	0.787
rate constant k(min-1)	0.0261	0.0369	0.0522	0.0596	0.0596	0.0702	0.0775	0.0794

2. Temperature effect

Removal capacity and the rate of adsorption is influenced by temperature as shown in table 1 and figure 2.  It is observed that the amount of adsorption was merely grown as temperature increased from 20 to 50℃, and adsorption trends were similar with each temperature.  The maximum amount of Fe(III) ion, 445.31 mg/g, at 50℃ is 14.71mg larger than that of 20℃, which is increased as 3.3%.  On the other hand, the adsorption time of high temperature to reach equilibrium was shorter than low temperature.  As shown in table 1, the reaction rate constant of adsorption is increased when the temperature rises.

fig.1. Adsorption equilibrium of Fe(III)                      fig.2. Effect of temperature on adsorption

according to concentration                                   of Fe(III)    (pH2, 2.0mM)

(pH2, temp. 20℃)

3. Influence of initial pH of solution

It is thought that the influence of pH is the most critical in adsorption process.  The removal capacity of Fe(III) was dependent on the pH of solution as illustrated in figure 3.  Amount of adsorption was jumped in pH 4 to 10, because the formation of ferric-hydroxide may affect to adsorption in pH 4 to 10.  It can be sure that ferric-hydroxide is stable in this pH in the pC-pH diagram.  In other words, it is also described that adsorption of hydroxide is predominant than that of Fe(III) free ion.  It is probably related to surface charge of activated carbon and Fe(III) or hydroxide.

4. Effect of swelling on adsorption of Fe(III)

Swelling method was applied to activated carbon to make surface area enlarge.  Swelling ratio and specific surface area depicted in figure 4 and 5 have in common with increasing tendency.  Adsorption experiment was carried out with 24 and 48-hr swelling adsorbent because they have median and maximum swelling ratio .  It was inspected that adsorption onto swelled adsorbent was more efficient than non-swelled one, and 48-hr swelling was the most efficient adsorption case as illustrated in figure 6.  Though their adsorption time were same, adsorption capacity and the rate of adsorption was larger in 48-hr swelling.  It is considered that adsorption capacity was increased because of the modification and enlargement of surface of activated carbon.

fig.3. Effect of initial pH of solution on                          fig.4. Swellin Ratio of activated carbon

adsorption of Fe(III)                                              (after 60℃ swelling)

(20℃, 2.0mM)

fig.5. Specific surface area of activated carbon              fig.6. Adsorption of Fe(III) onto swelling and non-

according to swellin time                                    swelling activated carbon (pH2, 20℃, 2.0mM)

5. Thermodynamic description

In general, it is considered that adsorption reaction is due to the dispersion force by concentration profile between adsorbate and adsorbent.  Dispersion force is characterized by London force whoes expression is for the potential energy, which can be represented by thermodynamics.  Thermodynamic data for the present experiment was arranged in table 2.  Adsorption reactions of all experiments were gradually increased with increasing temperatures.  In addition, adsorptions onto the swelled activated carbons have still higher enthalpies and entrypies, so their adsorption capacity were very larger than onto the non-swelled activated carbon.  For any given position, adsorption energy would be equal at the minimum of the potential curve, which of course represents the equilibrium position.  Until then, adsorption would be happened continuously, therefore adsorption onto the swelled activated carbon having high energy could occur highly.

Table 2. Thermodynamics on adsorption of Fe(III) with different temperatures

(K:adsorption equilibrium constant)

temperature (K)		293	303	313	323
Onto non-swelling activated carbon	K	5.56	5.97	6.38	7.10
	□Go	-4,176	-4,496	-4,817	-5,137
	□Ho	5,215
	□So	32.050
Onto 24hr-swelling activated carbon	K	10.24	11.42	12.79	13.21
	□Go	-5,655	-6,136	-6,617	-7,028
	□Ho	8,430
	□So	48.072
Onto 48hr-swelling activated carbon	K	17.69	20.74	26.03	30.15
	□Go	-6,955	-7,691	-8,426	-8,909
	□Ho	14,591
	□So	73.537

References

1. Onganer, Y., Temur, C., "Adsorption Dynamics of Fe(III) from Aqueous Solutions onto Activated Carbon," J. of Colloid and Interface Science, 205, 241-244 (1998).

2. Sparks, D., "Environmental Soil Chemistry," Academic Press, 42-43(1995).

3. Sasaoka, E., Sada, N., Uddin, Md., □Preparation of Macroporous Lime from Natural Lime by Swellin Method with Acetic Acid for High-Temperature Desulfurization," Ind. & Eng. Chem. Res., 37, 3943-3949 (1998).

4. Otake, Y., Suuberg, E., □Solvent Swelling Rates of Low Rank Coals and             Implications Regarding Their Structure," Short Comm., Fuel, 77, 901-904 (1991).

5. Huang, C., Cheng, W., "Thermodynamic Parameters of Iron-Cyanide Adsorption onto Al2O3," J. of Colloid and Interface Science, 188, 270-274 (1997).