CURRENT RESEARCH Color Removal by Clays Kinetic Study of Adsorption of Cationic and Anionic Dyes V. Venkatraman Sethuraman' and Bikash C. Raymahashay" Department of Civil Engineering, Indian Institute of Technology, Kanpur, Kanpur 20801 6, UP., India
w Experiments were designed to study the kinetics of adsorption of two industrial dyes-methylene blue (cationic) and sulfur blue (anionic)-by kaolinite and montmorillonite clays. The cationic dye was removed from aqueous solution a t a continuously decreasing rate from 10 mg/g min to 0.07 mg/g min by montmorillonite whereas kaolinite adsorbed the dye a t a faster and uniform rate of 16 mg/g min. The anionic dye was removed a t a uniform rate of 2.3 mg/g min by kaolinite and 2.6 mg/g min by montmorillonite. The apparent control of ion-exchange capacities and nature of exchange sites of the clays on the rates of color removal is pointed out.
1ine)phenol. I t is used in the water soluble form by alkaline reduction with sodium sulfide. Weighed amounts of clay were added to aqueous solutions of methylene blue and sulfur blue and the suspension was shaken for varying lengths of time. The temperature was maintained a t 23 f 1 O C . At the end of a run, an aliquot was centrifuged a t 5000 rpm for 10 min and the dye concentration in the clear supernatant was determined colorimetrically using Bausch and Lomb Spectronic 20 Spectrophotometer a t 580 mw for methylene blue and 620 mw for sulfur blue. Results and Discussion Figure 1 is an adsorption vs. time plot for varying concentration of kaolinite in contact with 100 mg/l. methylene blue solution. Equilibrium is reached in a very short time
In this paper we report on the kinetics of adsorption of two industrial dyes, methylene blue (cationic) and sulfur blue (anionic), from aqueous solutions by the clay minerals kaolinite and montmorillonite. This investigation is directly applicable to problems concerning discharge of textile dye wastes into the Ganges river near Kanpur as well as to disposal into groundwater seepage through soil zones. The mechanism of adsorption of cationic dyes, particularly methylene blue, by clay minerals has been studied by several workers (1-6). Very little is reported, however, on the adsorption of anionic dyes and the rates of adsorption of dyes by clay minerals. Some related studies include adsorption of Orange I 'and Orange I1 on silica (7) and adsorption of pesticides on clay minerals (8). Experimental Procedure Commercial kaolinite and montmorillonite samples supplied by Industrial Minerals and Chemicals Co., Bombay, were used as adsorbents for dyes. Unlike many earlier investigations, the clays were used without any pretreatment like hydrogen saturation, so that natural conditions were simulated as far as possible. Distilled water suspensions were prepared in tall jars and the middle fraction, after oven drying, was used in the experiments. Some properties of the clay minerals are listed in Table I. Two industrial dyes were used: BDH grade methylene blue, molecular weight, 319.87, and C-I sulfur blue 11, 53235, molecular weight, 305, manufactured by Imperical Chemical Industries (India) Private Limited. The second dye was obtained from the Elgin Mills, Kanpur, where it is widely used for dyeing cotton. According to a personal communication from the manufacturers, this dye is prepared by aqueous sulfurization of p-(2,4-dinitroaniPresent address, Graduate Engineer (Civil), Metallurgical and Engineering Consultants India Ltd., Bangalore-2.
c
l
1
20
60
40
,
120
100
80
TIME ( m i n u t e s )
Figure 1. Kinetics of
methylene blue adsorption on kaolinite
Table I. Some Physical and Chemical Properties of Clay Minerals Used in Adsorption Experiments Kaolinite
Montmorillonite
X-ray diffraction
Presence of illite, montmorillonite a n d quartz (estimated 20% of total)
Trace of kaolinite
Average particle size b y Fisher subsieve sizer C E C b y NaOH titration of Hsaturated clay Exchangeable cations by analysis of acid extract
1.6 p
2.4 p
5 meq/100 g
60 meq/100 g
analysis
-
Na:K:Ca:Mg
2.3:O.l:l.O: 0.3 meq/100 g
=
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(on the order of 5 min), and the amount of dye adsorbed a t a given time increases with increasing clay concentration. Comparison with a similar graph for the methylene bluemontmorillonite system (Figure 2) shows that the rate of adsorption is much slower, steady values being attained only after 2 hr. The unit rate of adsorption by the two clays, as calculated from the curves for 1.0 g/l. clay, also shows striking differences. Kaolinite adsorbs methylene blue a t a uniform rate of 16 mg/g min whereas adsorption by montmorillonite shows a steady decrease from 10 mg/g min in the first 5 min to 0.55 mg/g min in the next hour to 0.07 mg/g min for the final hour. The nonuniform adsorption by montmorillonite is further brought out in Figure 3 where the amount of dye adsorbed is plotted against the square root of time. The first or instantaneous adsorption, similar for both clays, may be related to ion exchange with
,2'5 g1IMONT
3
/ / L " = y"
'"'y'y
'
L2I
3
20
x
20
LO
60
80
100
120
TIME ( m i n u t e s )
Figure 5. Kinetics of sulfur blue adsorption on montmorillonite surface cations on the clays whereas the intermediate (gradual) adsorption in montmorillonite may be due to cation exchange a t the interlayer position of the clay. The unit rate for adsorption of the anionic dye, sulfur blue, is much lower (see Figures 4 and 5), the values being 2.3 mg/g min for kaolinite and 2.6 mg/g min by montmorillonite. This is explained by the comparatively lower anionexchange capacities of clays. I t is also interesting to note that anion exchange being restricted to the surface and edges of clay particles, the rates for the two clays are similar and there is no intermediate interlayer exchange stage for montmorillonite as was the case for cation exchange with methylene blue.
w
1
20
1
1
1
60
LO
100
80
120
TIME (minutes)
Figure 2. Kinetics of methylene blue adsorption on montmorillonite
. F 0
25911 MON1
Acknowledgments The authors are indebted to the authorities of the Elgin Mills, Kanpur for facilities to conduct certain on-the-spot studies.
c
Y o
7,
3
,125
9
1/27
11
TIME ( m i n u t e s )
Figure 3. Three stages of methylene blue adsorption on montmorillonite, after Huang and Liao ( 8 )
2.5911 KAOL KAOL
A1.Cgil
2
2 0 V '
2' 0
'
LO '
'
60 '
'
80 '
'
100 '
TIME ( m i n u t e s )
Figure 4. Kinetics of sulfur blue adsorption on kaolinite 1140
Conclusion This series of experiments illustrates the strong control of the charge of the dye ion in solution and the corresponding ion-exchange capacity of clay minerals on the kinetics of color removal by clays. The cationic dye, methylene blue, is adsorbed by kaolinite at a fast and uniform rate. Montmorillonite, on the other hand, adsorbs much larger amount of dye at a slower and nonuniform rate. The rate of adsorption of the anionic dye, sulfur blue, is much lower and the rates for the two clays are similar. These differences in kinetics are apparently controlled by the nature of exchange sites of the two clay minerals.
Environmental Science ti Technology
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Literature Cited (1) Plesch, P. H., Robertson, R. H. S., Nature, 161,1020 (1948). (2) Fairbairn, P. E., Robertson, R. H. S., Clay Min. Bull., 3, 129 (1957). (3) Ramachandran, V. S., Kacher, K. P., Patwardhan, N. K., N Q ture, 191,696 (1961). (4) Faruqi, F. A., Okuda, S., Williamson, W. O., Clay Miner., 7,19 (1967). (5) Hang, P. T., Brindley, G. W., Clays Clay Miner., 18, 203, (1970). (6) Brindley, G. W., Thompson, T. D., Isr. J . Chem., 8,409 (1970). (7) Allingham, H. M., Cullen, J. H., Giler, C. H., Jain, S. K., Woods, J. S., J . Appl. Chem., 8,108 (1958). (8) Huang, J., Liao, C., J . San. Eng. Diu., Proc. A m . SOC.Ciu. Eng., 96, 1057 (1970). Receiued for review December 16, 1974. Accepted J u l y 21, 1975. Work supported by the grant of Q CSZR Junior Research Fellowship t o V. V . Sethuraman for his M . Tech. Thesis project on this topic at the Department of civil Engineering, Indian Institute of Technotogy, Kanpur.
