Adsorption of Cu2+ on various crystalline modifications of MnO2 at 300°K

Adsorption of

Various Crystalline Modifications of MnO2 at 300°K

C u 2+ o n

S. B. KANUNGO i AND K. M. PARIDA Regional Research Laboratory, Bhubaneswar 751013, Orissa, lndia Received September 8, 1982; accepted August 16, 1983 Adsorption of Cu 2+ on various crystalline modifications of MnO2 at 300°K and in buffered medium ofpH 4.4 has been studied. The adsorption isotherms follow the Langmuir equation above an equilibrium concentration of 2.0 × 10-3 mmoles/liter of Cu2+. The isotherms reveal that the 6-MnO2 group of samples show the highest sorption capacity followed by the electrolytic ~'-MnO2 and a-MnO2 group of samples. Beta-MnO2 shows the poorest sorption capacity. Similar is the behavior for release of Mn 2+ in solution during the adsorption ofCu 2+. It has been observed that there is a direct relationship between the amount of Mn 2÷ release and the fraction of lower valence manganese present in the samples. Adsorption capacity is also found to be directly related to BET surface area. INTRODUCTION

The ability of manganese dioxide, especially in its hydrated form, to adsorb metal ions from aqueous solution has been a subject of study by various investigators (1). The cation exchange property of MnO2 considerably influences its functioning as a cathode material in a Leclanche type dry cell (2). Several authors (3, 4) have emphasized the importance of ionexchange phenomenon in the measurement of the electrode potential of MnO2 at different pH values. In soils containing hydrous MnO2 the concentration of many agriculturally important metal ions is reported to be high (5). MnO2 is a well-known scavenger under sea and fresh water environments as exemplified by the presence of Cu 2÷, Ni 2+, Co 2+, etc. in both marine and lake sediments (6). Hydrated MnO2 is sometimes used for removing radionucleides in water treatment plants (7). The sequence of adsorption capacity of various metal ions on MnO2 has been studied by Murray et al. (8) and Murray (9) who have shown the general sequence 1Author to whom all correspondence should be addressed.

Li + < N a + < K + < M g 2+ < Ca 2+ < Sr 2+ < Ba 2+ < N i 2+ < Z n 2+ < M n 2+ ~< C o 2+. A nearly similar sequence has also b e e n rep o r t e d b y o t h e r a u t h o r s (10-12). It has b e e n f o u n d (9) t h a t alkali m e t a l i o n s are a d s o r b e d only electrostatically, a l k a l i n e e a r t h m e t a l s b y b o t h electrostatic a n d c h e m i c a l forces, a n d t r a n s i t i o n m e t a l ions are a d s o r b e d strongly b y c h e m i c a l forces. T h e reversibility o f e x c h a n g e r e a c t i o n with p H follows a l m o s t the s a m e seq u e n c e as above; t r a n s i t i o n m e t a l ions exhibit strong irreversibility indicating that these m e t a l ions e n t e r i n t o the lattice b y replacing m a n g a n e s e w h i c h goes i n t o t h e solution. Various a u t h o r s (6, 9, 11) have p u t f o r w a r d different e x p l a n a t i o n s for the p h e n o m e n o n o f release o f m a n g a n e s e in solution a c c o m p a n i e d b y high a d s o r p t i o n o f h e a v y m e t a l ions o n h y d r o u s 6-MnO2. But very little w o r k has been d o n e u s i n g M n O 2 o f different crystalline m o d ifications. Again, a m o n g t h e h e a v y m e t a l i o n s a d s o r p t i o n o f C u 2+ is the least studied, p r o b a b l y because o f s o m e i n h e r e n t difficulties as discussed in a later section o f the present paper. Since surface p r o p e r t i e s o f M n O 2 have b e e n f o u n d to v a r y with the m e t h o d o f p r e p a r a t i o n 245 0021-9797/84 $3.00

Journal of Colloid and Interface Science, Vol. 98, No. 1, March 1984

Copyright © 1984 by Academic Press, Inc. All righls of reproduction in any form reserved.

246

KANUNGO

and also with their crystalline structure, the purpose of the present work is to study the adsorption behavior of Cu 2+ on a number of synthetic polymorphic forms of MnO2. MATERIALS

AND

METHODS

All the chemicals used were of analytical grade. Redistilled water was used for making solutions.

MnOe Samples: Methods of preparation of different MnO2 samples and their physicochemical characteristics have been discussed in detail in an earlier paper (13). However, for the sake of ready reference, methods of preparation are summarized below: Sample 1: Oxidation of MnSO4 by K M n O 4. Sample 2: Reduction of hot KMnO4 solution by 1:1 (v/v) HC1. Sample 3: Leaching of sample 2 in 3 N HNO3 at 90°C. Sample 4: Oxidation of Mn(NO3) by NaC103 in a strong HNO3 medium. Sample 5: Oxidation of MnSO4 by (NH4)28320 8 in 2 N H2SO 4. Sample 6: Oxidation of MnSo4 by K2S208 in 2 N H2SO4. Sample 7A: Electrolytic oxidation of MnSO4 at pH 3-4 on a Pb anode. Sample 7B: Electrolytic oxidation of MnSO4 at pH 2-3 on a carbon anode. Sample 8: Thermal decomposition of MnCO3 in air at 400°C. Sample 9: Sample 8 calcined at 700°C and then leached in 3 N HNO3 at 90°C. Sample 10: Oxidation of Mn(OH)2 by (NH4)2S208 at pH 9.5. Sample 11: Leaching of MnOOH in 3 N HNO3 at 90°C. Sample 12: Thermal decomposition of Mn(NO3)2 at 150°C. Sample 13: Oxidation of MnSO4 by NaOC1 in an alkaline medium. Sample 14: Sample 13 leached in 3 NHNO3 at 90°C. Journal of Colloid and Interface Science, Vol. 98, No. 1, March 1984

AND

PARIDA

Adsorption Isotherm About 0.5 g of accurately weighed sample and 25 ml of CuC12 solution of different concentrations but prepared in acetate buffer (pH 4.4) were taken in a series of 100-ml stoppered conical flasks. The suspensions were shaken for a period of 25 hr with the help of an electric shaker bath maintained at 27 _+ 0.5°C. The suspensions were then filtered through dry sintered glass crucibles (porosity G4), washed with 15-20 ml of buffer solution, and the total volume was made up to 50 ml. In a suitable aliquot Cu 2+ was estimated iodometrically by titrating with 0.01 M Na2S203. The difference between initial and equilibrium concentration corresponds to the amount of Cu 2+ adsorbed. In another aliquot (10-15 ml) 10 ml ofconc. HNO3 and 5 ml oforthophosphoric acid were added. After a little warming about 0.3 g of KIO3 was added and the solution was allowed to boil for 3-4 rain over a sand bath, cooled, and diluted in a 50-ml volumetric flask. The intensity of the pinkish-violet color due to Mn(VII) was estimated at 550 nm. A suitable calibration curve was prepared using known amounts of Mn(II) solution. RESULTS

AND

DISCUSSION

At the outset, it is worthwhile to mention a few inherent problems involved in the study of the adsorption of Cu 2+ on hydrated MnO2 samples. First, Cu 2÷ begins to hydrolyze at pH 5.2, thus making it impossible to study adsorption phenomenon at pH higher than 5.0. At the same time, at pH lower than 3.0 some of the MnO2 samples undergo appreciable dissolution in the medium. This results not only in the loss of sample weight but also in the release of some additional Mn 2+ in solution which may bring complexity in the adsorption system. Secondly, as will be shown in our subsequent communication, the point of zero charge (PZC) of most of the samples (excepting fl-MnO2) lie at pH below 4.2. It is well known that below pH (PZC) oxides and hydroxides are positively charged, thereby complicating

ADSORPTION OF Cu2+ ON MnO2 the i n t e r p r e t a t i o n o f e x p e r i m e n t a l results. C u 2+ fornls various hydrolytic a n d polymeric species i n the whole p H range o f 2-5. However, their c o n c e n t r a t i o n s increase with increase i n p H (9, 14). C o n s i d e r i n g all these aspects it was decided to use a buffer solution of a n o p t i m u m p H value for the study of the sorption o f C u 2+ o n various M n O 2 sampies. Since the adsorption of metal ions o n MnO2 d e p e n d s u p o n the state o f purity o f the adsorbents, detailed chemical analysis o f the samples is given i n T a b l e I. T h e data show that all the samples c o n t a i n M n 2+ a n d M n 3+ which i m p a r t n o n s t o i c h i o m e t r y i n the c o m position. Since the samples were n o r m a l l y washed t h o r o u g h l y with distilled water before drying, it m a y r e a s o n a b l y be a s s u m e d that n o lower valence m a n g a n e s e ions occur i n the adsorbed state. However, in electrolytic MnO2 some M n 2÷ m i g h t escape washing f r o m their highly p o r o u s structure. It m a y also be observed from the T a b l e I that b a r r i n g sample 8 total water c o n t e n t increases with a n increase Thirdly,

247

i n M n 2 0 3 content. Therefore, n o n s t o i c h i o m etry a n d high water c o n t e n t are c o m m o n l y f o u n d in MnO2 particularly with samples belonging to the 6-variety. A d s o r p t i o n isotherms o f C u 2+ o n various MnO2 samples at p H 4.4 are s h o w n i n Figs. 1A a n d B. W i t h the exception o f sample 10, all other 6-MnO2 samples (Nos. 1, 2, a n d 13) exhibit high sorption capacity i n c o m p a r i s o n with other modifications o f M n O 2 . Relatively low a d s o r p t i o n o n sample 10 indicates that a part of the a d s o r p t i o n site is already covered with metal ions d u r i n g its preparation. Samples 9 a n d 12 exhibit a very slow increase i n adsorption with increase i n c o n c e n t r a t i o n o f C u 2+. A d s o r p t i o n isotherms o f all other samples lie w i t h i n this range o f high a n d low adsorption. T h e isotherms were converted to linear forms with the help o f the L a n g m u i r e q u a t i o n which m a y be represented i n its c o n v e n t i o n a l form as C e q = feq ~_ 1 M M' ---5' bM [1]

TABLE I Chemical Analysis of Different MnO2 Samples H20 (%)

Sample

MnO (%)

Mn20~ (%)

MnO2 (%)

K20 (%)

Na20 (%)

NH~ (%)

By difference°

From TGAb

1 2 3 4 5 6 7A 7B 8 9 10 11 12 13 14

1.160 0.178 0.030 0.044 0.075 0.045 0.466 0.030 0.087 0.030 0.693 0.195 0.045 0.569 0.120

14.27 14.00 6.54 5.47 3.71 7.22 7.30 5.39 23.79 4.30 17.63 3.16 3.69 18.82 6.24

72.82 70.95 86.41 89.24 89.03 81.87 86.18 88.81 69.83 91.74 70.10 94.24 95.49 69.60 87.37

3.80 7.40 4.91 --7.20 ----------

---0.22 ---------4.66 1.85

----2.06 -----6.57 -----

7.95 7.47 2.11 5.03 5.12 3.67 6.05 5.77 5.58 3.44 5.01 2.41 0.77 6.35 4.43

8.37 8.40 2.25 5.11 5.48 3.85 5.53 4.00 6.94 3.62 8.75 2.57 0.60 7.52 3.74

As balance of 100%. bDifferencebetweentotal weight loss at 700°C and calculatedloss of oxygen at 700°C (assumingcomplete conversion to

Mn203).

Journalof Colloidand InterfaceScience,Vol. 98, No.

1, March 1984

248

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249

ADSORPTION OF Cu2÷ ON MnO2 TABLE II Adsorption of Cu2+ on Various MnO2 Samples at 300°K and Consequent Release of Mn 2+ from Some of Them at pH 4.4 Sample

Crystalline modification

1 2 3 4 5 6 7A 7B 8 9 10 11 12 13 14

Hexagonal Hexagonal Tetragonal Orthorhombic Tetragonal Tetragonal Orthorhombic Orthorhombic Tetragonal Orthorhombic Amorphous Orthorhombic Tetragonal (rutile type) Amorphous (partly) Orthorhombic

Symbol

Adsorption capacity from Langmuir equation (mmole/g)

Maximum quantity of Mn 2+ released (moles/g × 104)

15.83 15.62 10.70 8.89 5.71 4.61 7.63 10.00 10.93 4.00 8.00 5.71 Trace quantity 14.29 9.09

2.667 1.905 0.238 --0.465 --2.857 -0.444 0.270 -1.820 0.323

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cerned. Other investigators (10, 1 1) have also observed similar deviation from the L a n g m u i r equation for the adsorption o f Z n 2+, Co e+, and Ni 2+, etc. on MnO2 at concentrations lower than 1.5 mmoles/liter. The adsorption capacity o f Cu e+ obtained from the slope o f the initial linear portion o f the L a n g m u i r plot varies from sample to sample, i.e., based on the m e t h o d o f preparation and subsequent treatment. D a t a in Table II show that 6-MnO2 samples have the highest sorption capacity followed by some selected 7-MnO2 samples such as 4, 7A, 7B, and 14. G a m m a - M n O e samples such as 9 a n d 11 prepared from acid leaching o f lower oxides/ oxyhydroxides o f manganese have lower adsorption capacity. A m o n g a - M n O 2 samples, sorption capacity o f samples 3 and 8 is higher than that o f 5 and 6. This is due to the larger specific surface area o f samples 3 and 8.

Release of Mn 2+ during Adsorption of Cu 2+ It has been observed by several a u t h o r s (9, 10) that adsorption o f heavy metal ions such as Co 2+, Zn ~+, C u 2+, Ni 2+, etc. on h y d r o u s 6MnO2 is a c c o m p a n i e d by the release o f M n e+

in solution. The a m o u n t o f manganese released depends u p o n the nature o f adsorbing ion and also on the nature o f MnO2. N o work has so far been reported on the behavior o f M n 2+ release from different crystalline m o d ifications o f manganese dioxide. Figure 3 shows the corresponding isotherms

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for the release of Mn 2+ during the sorption of C u 2+ o n different MnO2 samples at p H 4.4. High release of Mn 2+ from samples 1, 2, 8, and 13 indicates that a considerable n u m b e r of adsorption sites are lower valency manganese ions in the disordered layer structure of fi-MnO2. F r o m the nature of the M n 2+ release curve for sample 7A it appears that some M n 2+ escaped washing from deep inside the deposit on anode. This is further supported by the lack of any appreciable release of m a n ganese from the other electrolytic MnO2 sample (No. 7B). For other "r-MnO2 samples such as 11 and 14 the a m o u n t of M n 2+ released is similar to those of a-MnO2 samples such as 3 and 6. Loganathan and Burau (1 l) have used the Langmuir equation for the determination of m a x i m u m a m o u n t of Mn 2+ released during sorption of Co 2+ and Zn 2+ as ~-MnO2. A similar attempt has been made in the present work. Figures 4A and B illustrate the linearized forms of manganese release isotherms from different samples. Unlike Figs. 2A and B, satisfactory linear behavior is followed from the lowest to the highest concentration of Cu 2+. The results are shown in Table II which also gives the adsorption capacity of the samples. Journal of Colloid and Interface Science, Vol. 98, No. 1, March 1984

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ADSORPTION OF Cu E+ ON MnOz

251

This suggests that the phenomenon of adsorption is limited to the surface of the individual grains of manganese dioxide. Practically no sorption takes place in the micropores, cracks, fissures, etc. which, if present at all, do not contribute a much appreciable extent to the total specific area of the samples. Deviations for some samples show that besides specific surface area other factors also play an important role. These aspects will be dealt with in a subsequent paper.

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the Langmuir plot. This is illustrated in Fig. 5 which also shows the relationship between the sorption capacity of Cu 2+ and the lower valence manganese content of the samples. It appears that most of the samples lie on two separate curves indicating that functioning of the lower valence maganese ion as an adsorption site varies with the nature of the sample. Thus negative deviation for sample 10 indicates that some of the adsorption sites are already covered with metal ions, probably during its preparation. Similarly, strong positive deviation for sample 8 may be attributed to the presence of a large amount of free Mn203 formed by the thermal decomposition of MnCO3. The sorption capacity of Mn203 is extremely poor in comparison with the hydrated manganese sesquioxide (MnOOH).

Effect of Surface Area Kozawa (15) first reported that the adsorption capacity of MnO2 for Zn 2+ is directly proportional to the BET surface area. This was subsequently confirmed by Gabano et al. (10). In the present work an almost similar behavior is observed as shown in Fig. 6. Barfing a few deviations most of the samples lie on the straight line passing through the origin.

ACKNOWLEDGMENTS The authors' sincere thanks are due to Dr. B. R. Sant for helpful discussion and constant encouragement during the course of the investigation, and to Professor P. K. Jena, Director, for his keen interest and kind permission to publish the paper. One of the authors (K.M.P.) is thankful to the Council of Scientific and Industrial Research, New Delhi, India, for the award of a Junior Research Fellowship.

REFERENCES 1. Fuller, M. J., Chromatogr. Rev. 14, 45 (1971). 2. Sasaki, K., Kurano, T., and Fuseya, Y., J. Electrochem. Soc. (Japan) 4, 67 (1936). 3. Johnson, R. S., and Vosburg, W. C., J. Electrochem. Soc. 99, 317 (1952). 4. Benson, P., Price, W. B., and Tye, F. L., Electrochem. TechnoL 5, 517 (1967). 5. Taylor, R. M., and McKenzie, R. M., Austral J. Soil Res. 4, 29 (1966); Taylor, R. M., J. Soil Sci. 19, 77 (1968). 6. Burns, R. G., Geochim. Cosmoehim. Acta 40, 95 (1976). 7. Tewari, P. H., Campbell, A. B., and Lee, W., Canad. J. Chem. 50, 1642 (1972). 8. Murray, D. J., Healy, T. W., and Fuerstenau, D. W., Advan. Chem. Ser. 79, 74 (1968). 9. Murray, J. W., Geochim. Cosmochim. Acta 39, 505 (1975). 10. Gabano, J. P., Etienne, P., and Laurent, J. F., Electrochim. Acta 10, 947 (1965). 11. Loganathan, P., and Burau, R. G., Geochim. Cosmochim. Acta 37, 1277 (1973). 12. Gray, M. J., and Malati, M. A., J. Chem. Technol. Biotechnol. 29, 129, 135 (1979). 13. Parida, K. M., Kanungo, S. B., and Sant, B. R., Electrochim. Acta 26, 435 (1982). 14. Stumm, W., and O'Melia, C., J. Amer. Water Works Assoc. 60, 514 (1958). 15. Kozawa, A., J. Eleetrochem. Soe. 106, 552 (1959). Journal of Colloid and Interface Science, Vol. 98, No. 1, March 1984