Langmuir 1992,8, 1974-1979
1974
Radiotracer Investigation on Sorption Behavior of Chromium(V1) on Antimony Trioxide: Kinetic and Infrared Study M. M. Bhutani,' P. N. Reddy, A. K. Mitra, and Ramesh Kumari Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110 016, India Received July 26,1991. In Final Form: March 18, 1992
The sorption of Cr(V1) ions on antimony trioxide has been investigated over awide range of pH (0.5-101, concentration (10+-10-* M),and temperature (303-333 K)by using the radiotracer technique. The effect of pH on sorption of Cr(V1) on Sb& is explained in the light of the deprotonation/hydroxyl ion association reaction on the oxide surface and ita subsequent interaction with the tracer. The rate of sorption of Cr(V1) is faster in the beginning, becomes slower with a lapse of time, and finally approaches a plateau within ca. 15 min. Sorption is found to incrsase with an increase in concentration and decrease in temperature. The kinetics of the process essentially followa the firsborder rate law and obeys the Freundlich isotherm. The value of the activation energy for the desorption process is found to be greater than that for the sorption process. The calculated thermodynamic parameters confirm the spontaneity and exothermic nature of sorption process and suggest some structural changes in the surfacemorphology. IR spectroscopy corroborates the existence of interaction between Cr(V1) species and the surface of antimony trioxide.
Introduction During the last few decades a large number of metal oxides have been explored for their use in adsorption and various separations of analytical and radiochemical importance.'"' The interest in these materials has grown mainly due to their extra stability toward ionizing radiations and high temperature in comparison to their organic counterparts." Moreover, some metal oxidea show high selectivity for certain ions, offering a convenient means for many difficult separations. Antimony trioxide, which is mostly used as a catalyst and as an opacifier in glass, ceramics, and vitreous enamels, has not been explored adequately for studying the sorption properties of various ions and hence is considered worthwhile perming. Chromium(V1)ion is widely known as a major pollutant from various industrial effluents8 and gives rise to various hydrolyzable species depending on the pH of the solution!JO The sorption of Cr(V1) ions has been undertaken with a view to determine the (a) pH dependence of the sorption of Cr(VI), (b)time-dependent sorption of different concentrations of Cr(V1) species, (c) temperature depend-
* Author for correspondence.
(1) Ambe, S. Langmuir 1987, 3, 489. (2) Bhattacharya,D. K.; Dutta, N. C.; De, A. J.Radioanal.Nucl. Chem. 1990,140, 121. (3) Mikhail, E. M.; Miriak, N. 2.Znt. J. Appl. Radiat. h o t . 1988,39, 1121. (4) Tewari, P. H. Adsorption from Aqueowr Solutions; Plenum: New York, 1981. (5) Gill, J. S.; Tandon, S. N. Radiochem. Radioanal. Lett. 1973, 14, 379. (6)Zsinka, L.; Szirtea, L.; Mink, J.; Kalman,A. J. Inorg. Nucl. Chem. 1974,36, 1147. (7) Mathew, J.; Tandon, S.N.; Gill, J. 5.Radiochem. Radioanal. Lett. 1977, 30, 381. (8)Huang, C . P.; Wp,M. H. J. Water Pollut. Control Fed. 1975,47, 2437. (9) Music, S. J. Radioanal. Nucl. Chem. 1985, 91, 337. (10) Bhutani, M. M.; Santosh; Purohit, H. K.Radiochim Acta 1983, 33, 153.
ence on sorption as well as on desorption, (d) existence of chemical interaction between Cr(V1) species and oxide surface by IR spectra, and (e) possible mechanism of the sorption of Cr(V1) on antimony trioxide.
Experimental Section All chemicals including the sorbent used were of A.R. grade. Antimony trioxide (A.R. grade) was procured from BDH Chemicals. The oxide was washed and heated up to 300 O C for 12 h and cooled slowly to room temperature. The powder thus obtained was fiially sieved to obtain particles of 100-160 mesh sieves. The X-ray diffractionpattern showsthat the sample was amorphous in nature. The specific surface area of antimony trioxide measured by BET method using purified Ns gas as adsorbate has been found to be 9.2 m2/gby a Pulse Chemisorb2700,Micromeritics Corp. Chromium-51 in the chemical form as Na261Cr0dwas used as the radioactive indicator and haa been procured from Bhabha Atomic Research Centre,Bombay, India. The count rates were measured with a well type NaI(T1) scintillation detector and SR-5 d e r ratemeter from Nuclear Enterprises,Ltd.,U.K. The pH values were measured by using Elico digital pH meter (LI-120)and pH was adjusted with dilute solutions of HNOa or NIGOH. The sorption percentage was calculated by comparing the radioactivity introduced into the sorption system with the radioactivity of the separated liquid phase after a given time of sorption. Radiometric corrections which are typical of sorption measurementswere also made. The sorption measurements were performed at room temperature except for the temperature-dependentstudies. The sorption kinetics was studied by measuring the radioactivity of the solution at various time intervals. Sorption of chromate ions was studied by taking 0.2 g of sorbent in 10 mL sorbate solution labeled with W r and shaking the system in a thermostat maintained at the desired temperature. The above procedure was repeated throughout the study and kinetic measurements were performed at room temperature for different concentrations(10+-10-2 M) and at different temperatures (303333 K)for sorption aa well as for the desorption processes. The
0743-7463/92/2408-1974$03.00/0Q 1992 American Chemical Society
Sorption of Cr( Vr) on Antimony Trioxide
Langmuir, Vol. 8, No. 8, 1992 1975
Table I. Experimental Conditions for the Preparation of Samples for IR Spectroscopic Analysis sample 1was prepared by immersing antimony trioxide in 0.1 M sodium chromate solution for 6 h at pH 3.2, filtering, and drying slowly at 80 OC sample 2 was prepared as above but it was washed twice with double distilled water sample 3 was prepared by immersing antimony trioxide in lo-' M sodium chromate solution for 6 h at pH 3.2, filtering, and drying slowly at 80 OC sample 4 was prepared in the same way as sample 3 and it was washed twice with double distilled water sample 5 was prepared by coprecipitation from SbCla(s) + NanCrO&) by the addition of H2O at pH 3.2 and was washed twice with double distilled water amount sorbed (at) at any time t was estimated from the expression at =
IO0
Ao-At C -v -
(1)
A0 m The amount desorbed (a,) is represented by the relationship A2 A,-A,
a, = -C-
c
v m
where A0 and At are radioactivities of the sorptive solutions at time zero and time t, respectively, C is the initial concentration of the sorptive solution (mol L-,),Vis the total volume (L),and m is the mass of sorbent (g). A:,denotes the radioactivity of the desorptive solution at time t whereas A1 is the activity of the sorptive solution at equilibrium. The samples used for IR spectroscopic analysis were prepared in accordance with the conditions shown in Table I. The solid samples after sorption were filtered, washed with double distilled water, and then dried slowly a t 80 O C . The infrared spectra were recorded by using a FTIR Nicolet spectrophotometer (Model 5-DX).The specimens were pressed in KBr disks.
[N q51cpb
I
\
: Traces
Results and Discussion The experimental results of sorption of Cr(V1) species are summarized in Figures 1-8 and Tables I-IV. Figure 1shows the sorption of Cr(V1) species at tracer concentration on antimony trioxide at different pH values. The amount of Cr(V1) ions sorbed from solution reaches a maximum value (-95%) in the pH range 4.0-8.0. A decrease in Cr(V1)species sorption is observed at pH M.2, and a relatively less sorption of -40% is observed at pH 9.80. The pH dependence of sorption of Cr(V1) species may be explained on the basis of the distribution of Cr(V1) species in the solution and the surface properties of the oxide sorbent at different pH values. The distribution of sorbate as HCr04-, CrOd2-, H2Cr04, and C r ~ 0 7 ~ species in dependenceon the pH of the solution has been calculated from the following equilibria by Griffin" and Tandon12 and their co-workers: H+ + Cr0;2HCrOL
* HCrO; Cr2072-+ H,O
At pH 2, the H2Cr04 is the main species, between pH 2 and 6 HCr04- species predominate, and at pH >7, Cr042is the principal species. Upon hydration the oxide undergo dissociation/deprotonation reaction as follows MOH * MO- + H+ MOH,' = MOH + H', or hydroxyl ion association reaction MOH2++ OH- * MOH + H,O The isoelectric point (IEP)13of Sbz03 is reported to be in ~~
(11) Griffin, R. A.;Au, A. K.; Frost, R. R.J . Enuiron. Sci. Health 1977, A12, 431. (12)Tandon, K.R.;Crisp, T. P.;Ellis, J . Talanta 1984, 31, 227. (13)Parke, G. A. Chem. Rev. 1965,39, 177.
0
2
6
6
8
K)
PH Figure 1. Sorption vs pH: Sb~O3,0.2g.
the range of 6.0-6.5. Below the IEP the surface is positively charged arid above it, the surface is negatively charged. The sorption mechanism involves the reaction between different species of Cr(V1) with the oxide surface at the given pH. At pH 2 the surface is positively charged, but the main species is uncharged HzCr04 and so sorption is observed to be least. As pH increases, the fraction of HCr04- ions increases in the pH range 2-6. Above IEP i.e. at pH >6.0, the sorption decreases due to repulsion between negatively charged surface and the chromate anions. N
Kinetic Study Effect of Concentration. The sorption of chromate ions on antimony trioxide has been examined as a function of chromate ion concentration (10-'3-10-2 M) in aqueous solutions at room temperature. The experimental results are shown graphically in Figure 2 as a plot of amount sorbed (mol g-l) vs time. It can be seen that the sorption of chromate ions on Sb203 increases with time and tends to attain an equilibrium value within 15 min. The amount sorbed at equilibrium increases from 3.29 X lo-* mol g1 to 0.33 X mol g-l with increase in concentration over
Bhutani et al.
1976 Langmuir, Vol. 8, No. 8,1992
-0, d
0
E
n W
m
0; I-
z
e
2
I 0.' M
"
1
" C
I
IO
1
m
I dM v
I
I
I
,
I
IO
50 40 TIME(min1
60
70
80
Figure 2. Time variation sorption of chromate ions on SbzOa at various NazCrOd concentrations: Sbz03,0.2 g; pH 2.0. the range of 10-6-10-2 M, respectively (Figure 2). The smooth and continuous nature of the curves further signifies that the process involved during the sorption is uncomplicated throughout and attains an equilibrium. Observations in the present study further indicate that the process is likely to be restricted to the formation of monolayer coverage. This is most likely due to equilibrium coverage of the sorbent surface, i.e. species already attached during the period tend to involve in bond formation, as a result of which no further change in the sorption value is observed. Also, on the basis of known ionic radii14of Cr042-species (2.401\) and specific surface area for the sorbent used (9.2 m2/g),the extent of surface coverage has been calculated in the concentration range 10-6-10-2 M. The results show that the extent of surface coverage due to sorption of Cr042- ions increases from -0.04% to -26% as the concentration increases from 104to 10-2M in the bulk phase. Further, in view of smaller ionic radii for HCrO4- species as compared to the Cr042ions and also the proportion of HCr04- species will be appreciably higher than that of CrOf- ions in the pH range 2-3, the extent of surface coverage is likely to be smaller. This also lends further support that the surface coverage appears to be restricted to monolayer only. The dependence of sorption on the sorptive concentration at equilibrium was tested through Freundlich and Langmuir adsorption isotherms. The experimental data fit only into the Freundlich adsorption isotherm which can be represented in the following form log a, = log A + l / n log C,
(3)
where a, is the amount of chromate species sorbed at equilibrium (mol/g), C, is the bulk equilibrium concentration (moUL), and A and l / n are the characteristic Freundlich constants for a particular system. The plot of log aevs log C, (Figure 3) is linear showing a fair validity of Freundlich isotherm's in the entire concentration range studied. The value of l l n , which is (14) Huheey, E. J. Inorganic Chemistry, Principle of Structure and Reactivity, 3rd ed.; Harper International Si Edition: London, 1983;p 78 (and references therein). (15) Freundlich, H. J. Phys. Chem. 1916,90,86.
7
00
2
3
L
5
6
7
r') Figure 3. Freundlich adsorption isotherm for chromate ion sorption on Sbz03. -LOG C e (mol
usually related to the nature and strength of sorption forces, is 0.791 and, being less than 1, signifies the exponential distribution of active sites. It is observed that the total amount of the sorbate chromate species is not in equilibrium with the sorbate concentration at saturation point, but it certainlyestablishes an equilibrium with Cql/n only. The observed compliance with Freundlich adsorption isotherm and divergencefrom Lmgmuir type isotherm is thus likely to be due to heterogeneity of the sorbent surface as well as to the limited thermodynamic reversibility. The observed value of the Freundlich constant A as 4.46 X 10-3 further confirms a significant affinity of chromate ions for antimony trioxide. The linearity of the Freundlich plot shows that the sorption of Cr(V1) species on Sbz03is valid for physical adsorptionon a heterogeneous surface. Temperature Dependence Study. The temperature dependence of chromate ion sorption was examined over the entire temperature range (303-333K), and the results are shown in Figure 4. The general nature of temperature variation curves obtained from the plot of amount sorbed against time remains essentially similar to that shown in Figure 2, and time required to attain the equilibrium is approximately 15min. However,the amounts sorbed both at equilibrium and prior to equilibrium are considerably affected with the rise in temperature from 303 to 333 K. Usually with a rise in temperature, solubility of the sorbate is affected.'s The solubility of the sorbate is related with its chemical potential which is the controlling factor for the adsorption process. Since the solubility of the sorbate species increases with an increase in temperature, the chemical potential decreases and both the solubility and normal temperature effectswork in the same direction. This causes a decrease in the sorptive which is in agreement with the results reported by Bartell et al.17 The order of reaction for sorption of chromate ions by antimony trioxide has been investigated and the results (16) Kipling, J. J. Adsorption from solutions of non-electrolytes; Academic Press: London, 1965; p 101. (17) Bartell, F. E.; Thomas, T. L.; Ying, F. J. Phys. Chem. 1961,55, 1456.
Sorption of Cr(VI) on Antimony Trioxide
Langmuir, Vol. 8, No. 8,1992 1977
- -1.0 - -0.8 c
i
x
(3
--0.6 0 -1
-4.4
-OsZ
t 3
01 0
I
10
I
I
20
30
1
50
333K 323K 318K 4 . 313K 5. 308K 6 . 303K
Figure 6. Arrhenius plots for sorption and desorption processes. Table 11. Calculation of Rate Constants and Activation Energy for Sorption and Desorption Processes
1.
2. 3.
temp,
K 303 308 313 318 323 333
c
'm d
0
E
3.3
3.2
IT x1 0 3 ( ~ - 1 )
60
nME (min) Figure 4. Temperature variation sorption of chromate ions on SbzO3: SbZO3, 0.2 g; [NazCrO4] = 1.0 X M;pH 2.0.
-
3.1
1
I
40
rate constant, min-1 sorption kl desorption k-1 0.157 0.092 0.207 0.125 0.268 0.143 0.296 0.169 0.318 0.194 0.332 0.212
activation energy, kJ E. Ed
23.74
28.72
ID
0.6
Table 111. Calculation of Thermodynamic Parameters
X
Q
temp
3 4
w
s! 0.4 0 vl
(K) 303 308 313 318 323 333
5 6
W
0
z 0.2 t-
=I
d
0
r a
0
I
I
I
I
I
I
10
20
30
40
50
60
TIME (min)
Figure 6. Temperature variation desorption of chromate ions on Sbz03: Sb203, 0.2 g; [Na~CrOrl= 1.0 x 10-4M;pH 2.0.
of the study follow Lagergre@ equation in the form
kl
log (a, - at) = log a, - 2.303
(4)
where aeis amount sorbed at equilibrium, at is the amount sorbed at time t, and kl denotes the rate constant of the sorption process. The plot of log (a, - at) vs t results in a straight line at all the temperatures studied, indicating that the process is of first order with respect to sorptive concentration. The rate constants of sorption, calculated from the slope of the linear plot of log (a, - at) vs t , are recorded in Table 11. The rate constant values obtained in the present study of various temperatures are within (18) Lagerpen, S.Bil.K. Srenaka Vatenakapsakad Handl. 1898,24, 4.
AGO, kJ mol-'
-1.350 -1.290 -1.634 -1.480 -1.326 -1.240
AH',
ASo,
kJ mol-'
J/K/mol -35.04 -34.67 -33.02 -32.98 -32.95 -32.22
-11.97
the general range reported1+21in the literature for several oxide-adsorptive systems. The rate constants show only a small variation between 303 and 333 K, indicating that the mechanism of chromate sorption on antimony trioxide is due to a physical rather than a chemical process.21 Desorption Study on Presorbed Chromate Ions. The amounts of chromate ions that are desorbed from the antimony trioxide surface at different intervals of time and temperature are presented in Figure 5. It is seen that the rate of desorption is initially high, decreases gradually with the increase in time, and ultimately reaches an equilibrium value in 15-20 min. The amount of chromate ions desorbed increases consistently from 0.22 X lo* to 0.55 X lo* mol g-l with a rise in temperature from 303 to 333 K in the aqueous solution. However, the time required to approach the equilibrium remains almost unchanged in the entire temperature range. The desorption of (19) Mikami, N.; Saaaki, M.; Hachiya, K.; Astumian, R. D.; Ikeda, T.; Yaaunaga, T. J . Phys. Chem. 1983,87, 1454. (20)Tripathi, P. S.M.; Tripathi, R.; Praaad, B. B. R o c . Natl. Acad. Sci. U.S.A. 1975, 41, 156. (21) Mishra, S.P.; Singh, S.N. Int. J. Appl. Radiat. Zsot. 1987,38,541.
1978 Langmuir, Vol. 8, No. 8,1992
Bhutani et al.
Table IV. Characteristic Infrared Band Positions of Antimony Trioxiddhromate Interface (cm-1). characteristic peaks
NaZCrOd 960 (S) 894 (S) 400 (MI
0
samples
Sbz03 870 (S) 742 (S) 606 (S) 498 (S,B) 384 (S)
2 852 (S) 702 (S) 594 (M) 498 (S,B) 384 (S) 552 (S,B)
1 852 (S) 694 (S) 588 (M) 498 (S,M) 384 (S) 558 (S,B)
3 858 (S) 720 (S) 588 (M) 498 (S,B) 384 (S) 546 (S,B)
5 858 (S) 712 (SI 590 (MI 498 (S,B) 384 (S) 552 (S,B) 1070 (W,B)
4 852 (S) 720 (S) 588 (M) 498 (S,B) 384 (SI 552 (S,B)
Key: S, strong; M, medium; B,broad; W, weak.
t
A n
$9.496
1 815
2000
1700
1400
1100
900 750 bo0 WAVFNUMBIR 1cm-l I
450
200
Figure 7. IR spectrum of SbzOs.
62761 2000
I
I
I
1700
1400
1100
I I 900 750 W A V ~ N U M B E R(cm-71
I
I
bOO
450
I
200
Figure 8. IR spectrum of SbzO~-chromate interface (sample no. 1).
chromate ions sorbed on antimony trioxide in aqueous solution at different temperatures follows the first-order rate law. The values of rate constant for desorption process (k-1) are recorded in Table 11along with the rate constants for the sorption process. As can be seen from Table 11,the rate constants for the desorption process increase from 0.092 to 0.212 min-l with a rise in temperature from 303 to 333 K, and the values of rate constants for the sorption of chromate ions at all temperatures are greater than that for the desorption process, which further indicates that the sorption of chromate species is faster than its desorption under similar conditions. The activation energies for sorption and desorption processes have been estimated through Arrhenius plots (viz. log kl w 1/13 for the investigated system, utilizing the sorption and desorption data obtained at different temperatures (Figure 6). The high value of activation energy for the sorption process (Table 11)is indicative of physical sorption tending toward activated sorption thereby showinga tendency toward the bond formation. The forces of attraction are stronger and suggest that the process of uptake of chromate ions on antimony trioxide can even be made under ordinary conditions. Calculation of Thermodynamic Parameters. The changes in standard free energy, enthalpy and entropy have been calculated (Table 111) by using the equations AGO
-RTh K
(5)
and AGO = AHo - T A P
(7)
where the symbolshave their usual s i g d h n c e . The equilibrium constant K has been calculated by taking the ratio of rate constant of sorption process and the desorption process (Le. K = kl/k-l).
The negative value of AGO indicates that the sorption process is spontaneous. The negative value of AHo indicates that the process is exothermic and the sorption of Cr(V1) species on Sbz03 occurs by long range attractive forces. The negative value of ASo suggests some structural changes in sorbate and sorbent system under study. Association, fixation, or immobilization of chromate ions as a result of sorption attributes to a decrease in the degree of freedom of sorbate ions and hence the value of ASo is negative. IR Studies. IR spectroscopy is a useful technique for the study of interaction between oxide surface and the adsorbed anions.22~23 In this work, IR spectroscopy has been used to confirm the possible chemical interaction of chromium(V1) species at the Sb(II1)oxide/HzO interface. Sodium chromate shows22+24 strong IR bands at 950 and 890 cm-l and a doublet at 390-400 cm-l. Antimony trioxide25 shows characteristic bands at 870 and 740 cm-l and several peaks between 400 and 300 cm-l (Figure 7). The interactions between Cr(V1) species and the oxide surface are followed by shifts of IR band positions which correspond to the Cr-0 bonds. The measured positions of IR bands, recorded for the samples described in the experimental part, are shown in Table IV. The antimony trioxide peaks at 870 and 740 cm-l are observed to be shifted to 852 and 702 cm-1, respectively, after interaction with sodium chromate species. The shift in frequencies can be explained as the bond length of Sb-OCrOsH is greater than that of SbOH. The sorption of chromate ions on the oxide surface is also confirmed by the appearance of a new peak at 558 cm-l which is likely to correspond to Cr-0 bond vibration (22) Music, S.;Ristic, M.; Tonkovic, M. 2.Wasser.Abwasser Forsch. 1986,19, 186. (23)Panday, K.K.;Prasad, G.; Singh,V. N. J . Chem. Technol. Biotechnol. 1984,34A, 367. (24) Miller, F.A.;Carlson, G. L.; Bentley, F. F.; Jones, W. H. Spectrochim. Acta 1960, 16, 135. (25) Bentley, F. F.;Smithson,L. D.;Rozek, A. L. Infrared spectra and characteristic Frequency; Interscience: New York, 1968.
Sorption of Cr(V2) on Antimony Trioxide
Langmuir, Vol. 8, No. 8, 1992 1979
(Figure 8). The results of other IR bands are given in Table IV. In sample 5, where coprecipitation reaction takes place, a weak peak of antimony chromate observed at 1070cm-l may be due to Cr-0 vibration. However, the intensity of the peak depends upon the sodium chromate concentration used. On the basis of the experimental results presented in this work and careful analysis of the resulta of other researchers on specific anion adsorption, it can be suggested that the sorption of chromium(V1) species on antimony trioxide is likely to follow the mechanism of ligand e x ~ h a n g e . ~Accordingly, ~-~~ the simplified surface reactions can be represented as follows: =WH2+
= W H
and
+ HCrO4-
-
=SbOCrO3H
+ H20 =%-OH
I
7I
I
+ car2-
=
It may be worth mentioning that in view of high adsorbability of Sb203 for Cr(V1) at low pH, -3, high concentration, and low temperature, the sorption study is likely to find some useful applications for concentration of Cr(V1) species at ultramicroconcentration level as well as for disposal of toxic Cr(V1) from industrial and radioactive wastes for pollution control. Acknowledgment. Ramesh Kumari thanks the Council of Scientific and Industrial Research, New Delhi, for financial support. Registry No. Cr,7440-47-3; Sb203, 1309-64-4.
