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Kinetics of zirconium ions adsorption on activated charcoal from aqueous solutions
Carbon, Vol. 32, No. 8, pp. 1433-1439, 1994 Copyright 0 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 000%6223/94 $6.00 + .OO
Pergamon 0008-6223(94)00090-S
KINETICS OF ZIRCONIUM IONS ADSORPTION ON ACTIVATED CHARCOAL FROM AQUEOUS SOLUTIONS Pakistan
Institute
of Nuclear
RIAZ QADEER and JAVED HANIF Science and Technology, P.O. Box No. 1356, Islamabad,
Pakistan
(Received 2 February 1994; accepted in revised form 23 May 1994) Abstract-The batch kinetics of adsorption of the zirconium ions from aqueous solutions on activated charcoal has been investigated over a wide range of concentration of zirconium ions (1.0-5.0 g/l) and temperatures (lo-50°C). The adsorption process of zirconium ions proceeds via two stages; the first stage is rather fast, followed by a much slower one. The Bangham equation was used to study the kinetics of the zirconium ions’ adsorption on activated charcoal. It is observed that the diffusion of zirconium ions into the pores of the activated charcoal controls the kinetics of the adsorption process. Moreover, zirconium ion adsorption obeys the Freundlich and Langmuir isotherms in the concentration range studied. The adsorption equilibrium constant (k,) values for zirconium ions adsorption on activated charcoal have also been calculated at different temperatures. Various thermodynamic quantities, AC, AH, and AS were computed from k, values. The results showed that the adsorption of zirconium ions on activated charcoal is
an endothermic process. Key Words-Zirconium,
adsorption,
activated charcoal, kinetics, aqueous solution.
1. INTRODUCTION
Zirconium is an important material in chemical and nuclear engineering, and in metallurgy and marine technology, because of its desirable mechanical strength, low thermal neutron cross section (0.185 b), and corrosive resistance to acids, alkalis, and saline water. Zirconium metal occurs in meteorites, sediments, ter-
restrial materials, igneous and volcanic rocks[l], and is a major component of various precious stones and gels[2]. Hafnium is invariably found in nature along with zirconium. Generally, commercial grade zirconium contains 1 to 5 parts by weight of hafnium per 100 of zirconium. The properties of zirconium are affected by its hafnium content. For example, the presence of l-2% hafnium in zirconium could raise its thermal neutron cross section from 0.185 b to 1.0 b because of a high thermal neutron cross section of hafnium (102 b)[3]. Therefore, it is essential that zirconium be free or nearly free from hafnium for its utilization in nuclear technology. For purification purposes many processes are being used, such as precipitation, ion-exchange, fractional crystallization, and adsorption on solids. The adsorption process under certain conditions has a definite edge over other methods used for metal ions’ purification/recovery because of its simplicity, selectivity, and efficiency[4]. The primary requirement for an economic adsorption process is an adsorbent with sufficient selectivity, and high adsorption capacity and life. Owing to its large surface area, microporous structure, high adsorption capacity, radiation stability, and high purity, activated charcoal has been widely used to remove metal ions from solutions[S-81. We have previously used the activated charcoal for theadsorptionofuranium[9], thorium[lO], gadolinium[ll], samarium[l2], europium[l3], and strontium[l4] ions from solutions. The present com1433
munication reports results of our kinetics investigation of zirconium ion adsorption on activated charcoal from aqueous solutions. The results are important in relation to the recovery of zirconium ions from solutions. Some workers have studied the adsorption of zirconium ions on ion-exchange resin[lS-161, oxycellulose[l7], filter paper[l8], niobium tip[l9], tungsten (100) surface[20], and manganese dioxide[21], but no data is available of its adsorption on the activated charcoal. 2. EXPERIMENTAL
2.1 Chemicals The chemicals used in this study are zirconium oxychloride (M/s JMC; item No. 618149) and a commercial activated charcoal (M/s BDH; item No. 33032) having B.E.T.-N2 surface area 980 m2/g, porosity 75.74%, pore volume 1.43 cm3/g, and average particle diameter 3.7 k 0.2 micron. All the solutions were made in distilled water.
2.2 Instruments Siemen’s wavelength dispersive X-ray fluorescence (WDXRF) spectrometer (SRS-200) was used for measuring the zirconium ion concentration in solutions with variation <2.0%. The Hetofrig shaker (M/s Heto Birkerod-Denmark) was used for temperaturecontrolled studies. The fluctuation in the measurement of temperature is within +O. 1“C.
2.3 Procedure The adsorption of zirconium ions on the activated charcoal from aqueous solutions was carried out via a batch technique. Accordingly, 10 ml solutions of known concentration were added to 250 ml glass
1434
R. QADEERand J. HANIF
Table 1. XRF instrumental conditions for measuring zirconium ion concentrations in aqueous solutions Spectrometer Tube Detector Crystal Collimator Tube Current Tube Voltage Radiation path Analyte line LBG 219value qeak 20 value HBG 20 value Counting time
Siemen’s SRS-200 Cr Target Scintillation, NaI(T1) LiF-100 Fine 50 mA 50 kV Air Zr KLY 22.30 22.60 22.90 40 seconds at all 20 values 0
10
20
Shaking
bottles and shaken with about 0.1 g of dry activated charcoal in a thermostat shaker, the temperature of which was preadjusted to a desired value. After a predetermined time, the solution was filtered through Whatman filter paper No. 40 (circular, 14.0 cm). The first 2-3 ml portion of the filtrate was rejected because of the adsorption of zirconium ions by filter paper. The concentration of zirconium ions in the filtrate was measured using a WDXRF spectrometer at conditions given in Table 1. The concentration of zirconium ions was corrected for the loss of zirconium ions through adsorption on the walls of the glass bottles by running blank experiments (i.e., without activated charcoal added) at each time, t. The fraction of zirconium ions adsorbed at any time, F(t), was calculated using the following relation:
30
40
50
60
time> t (min)
reagent
where F( t ) = fraction of zirconium ions adsorbed at time t on activated charcoal, Ci = initial concentration of zirconium ions in solutions (g/l), and C, = concentration of zirconium ions in solution at time t (g/l).
3. RESULTS AND DISCUSSION
Figure 1 represents the variation of zirconium ion adsorption on activated charcoal with shaking time at different temperatures. This figure indicates that initially the fraction of zirconium ions adsorbed increases rapidly, but then the process slows down and subsequently attains a constant value after about 30 minutes (i.e., when the adsorption equilibrium is established). Similar observation of adsorption equilibrium attainment by zirconium ions on manganese dioxide has been reported earlier[21]. As an approximation, the adsorption of zirconium ions can be said to take place in two distinct steps, a relatively fast one
Fig. 1. Zirconium ion adsorption on activated charcoal against shaking time at different temperatures (solution concentration 2 g/l, pH 1.9).
followed by a slower one. The slow adsorption is explained by the diffusion of zirconium ions into the pores of the activated charcoal. Figure 1 also shows that the general time dependence of the zirconium ion adsorption process essentially independent of temperature. However, the temperature variations influence the amount of zirconium ion adsorption, which increases with a rise in adsorption temperature. This is because at higher temperatures, the diffusion of zirconium ions through charcoal pores is faster and can proceed to a larger extent. Zirconium oxychloride behaves as a Lewis acid dissolving in polar solvents to give rise to hydrolysis[22] and form the tetrameric species [Zr,(OH)z(H20)r6]8+ by freeing HC1[23]. The mechanism of its hydrolysis has been described by Devia and Sykes[24]. The release of HCl results in the lowering of pH in the solution. The pH determined in the present studies is 1.9, which confirms this argument. There is no simple explanation of the adsorption of this tetrameric species on the surface of the activated charcoal. Since the surface of the activated charcoal does possess acidic surface functional groups[25], the species [Zr,(OH)z(H20),6]8+ may form a surface complex with this acidic group. The Bangham equation[26] was employed to study the kinetics of zirconium ion adsorption on activated charcoal, and is given below:
-da = k&-l, dt
where qr is the amount of zirconium ions adsorbed per g of activated charcoal at time t; LYis a constant (less than 1); k is a factor that depends on the concentration of solution and is given as: k = k,(&
- qtW/V
(3)
1435
Zirconium ion adsorption
where 4, is the original amount of zirconium ions in solutions; k0 is the proportionality constant; W is the weight of the activated charcoal, and Vis the volume of zirconium ion solution taken. Q~has been defined earlier. On substituting k values from eqn 3 into 2 and on integration, eqn 2 becomes: 40
In
koW
$0 - q,w
=
-F-
(50-&W
10 20 30 40
(4)
kow
40
Temperature (“C)
ta*
To apply eqn 4 to the experimen~l data of the adsorption of zirconium ions on activated charcoal given in Fig. 1, eqn 4 is rearranged as follows:
log log
Table 2. Determined values of Bangham equation’s parameters for zirconium ion adsorption on activated charcoal at different temperatures (Zr solution concentration = 2.0 g/l)
= log--+ iYlog t. 2.303 V
(3
Straight lines were obtained by plotting log log [ &,I( cjoqrW)J against log I, as shown in Fig. 2, indicating that the system acts in accordance with the Bangham equation. The values of LYand k,, calculated from the slopes and intercepts of plots shown in Fig. 2, are given in Table 2. The applicabifity of the Bangham equation to the zirconium ion adsorption data shows that the diffusion of zirconium into the pores of the activated charcoal controls the adsorption process1271. The rate constants of two-stage process of zirconium ion adsorption on activated charcoa1 are determined using the following equation[28]: (1 - F) = A I eekir + A,eFkZ’,
-0.1
0-L
loo t -1.2
0.6=
0.094 0.117 0.162 0.192
0.064 0.070 0.073 0.076
t (min) 0 0
(6)
1.6
‘%
A2 can be found from the intercept of the extrapolation of the linear part of the second stage, whose slope is k2. Subtraction of of A,e-kz’ from each of the experimental data will then produce new straight lines {Fig. 3), of slope kl and intercept A 1. Their determined values are given in TabIe 3. The activation energies of the first and second stages of zirconium ion adsorption on activated charcoal have been calculated from the variation of the corresponding rate constants with temperature in accordance with the Arrhenius equation. Plots of log~ithms of rate constants k, and k2 vs reciprocal of temperature, for the first and second stages of the adsorption process, are shown in Fig. 4. From the slopes of straight lines, the values of activation energies of the first and second stages of the
where kr and k2 are the rate constants for the first and second stages of adsorption, respectively, and A, and AZ are constants. Plots of ln(l - F) vs t for zirconium ion adsorption on activated charcoal at different temperatures are given in Fig. 3. The values of
0
cx
-0.5
10
20
30
40
50
I
I
I
I
I
0
lo”c
60
1 _
-1.0
-1 .?I
2.0 Lt.
-0.2 Y d
_,
-2-o
C
-0.3
\4 $J
-04
,” ;
-0.5
-2.5
0 2o”c a 3o”c A 4o”c
-3.0
-3.5 -4-o
Fig. 2. Plots of log log 40/($o - q,W) vs log t for zirconium ion adsorption on activated charcoal at different temperature.
- 5.0
1
Fig. 3. Plots of In(l - F) vs t for zirconium ion adsorption on activated charcoal at different temperatures.
1436
R. QADEERand J. HANIF
Table 3. Determined values of rate constants for zirconium ion adsorption on activated charcoal at different temperatures (Zr solution concentration = 2.0 g/l) Temperature (“C)
k, (min-‘)
A,
0.134 0.155 0.175 0.183
0.167 0.163 0.116 0.109
10 20 30 40
k, (min-‘)
1.43 3.29 5.11 8.61
x x x x
lO-3 lO-3 lO-3 10-s
I
al _
A,
0.434 0.401 0.331 0.302
E
;
I
I
0.6
a, -0 =
tc
:
g
Y
Zr ions conc.(g/ll 0 1.0 0 20 n 3.0 A&O A 5.0
0.2
”
e LL
: II r(L
n : adsorption process were calculated and found to be 5.80 and 43.00 kJ/mol, respectively. Figure 5 depicts the adsorption of zirconium ions on activated charcoal in different concentration ranges (1.0-5.0 g/l) from aqueous solutions at 20°C. This shows that the zirconium ion uptake is initially fast, becoming slower later on, and ultimately reaching saturation. The time of saturation is about 30 minutes, and the extent of adsorption remains practically unchanged. These observations are similar to the temperature-dependent studies of zirconium ion adsorption on activated charcoal, as shown in Fig. 1. It is quite evident in Fig. 5 that the fraction of zirconium ions adsorbed at equilibrium, as well as prior to equilibrium, decreases with an increase in zirconium ion concentration. The Bangham equation, eqn (5), was applied to the concentration-dependent data of zirconium ion adsorption on activated charcoal. Figure 6 shows that zirconium ion adsorption on activated charcoal in the concentration range studied also obeyed the Bangham equation. The values of parameters 01and ke, calculated from the slopes and intercepts of plots shown in Fig. 6, are given in Table 4. Similarly, eqn (6) was applied to the concentrationdependent data of zirconium ion adsorption on the activated charcoal, to determined the values of the constants of eqn 6. Their determined values are given in Table 5.
-
0 0
I
I
I
I
I
J
10
20
30
40
50
60
Shaking
time,
t (min.]
Fig. 5. Zirconium ion adsorption on the activated charcoal against shaking time at different concentrations of zirconium ions.
The data concerning the dependence of the extent of adsorption on zirconium ion concentration at equilibrium, Table 6, was subjected to examination by the Freundlich and Langmuir isotherm equations. The Freundlich and the Langmuir plots were obtained, Fig. 7, using the following well known equations[29]: rzr = AC:“’
(Freundlich)
(7)
and C
1
r zr
Kadsrmax
2=-
+ 5
(Langmuir), max
(8)
where rzr is the amount of zirconium ions adsorbed per gram of activated charcoal, C, is the equilibrium
lO?T(K) Jf:‘?‘:;lfi”
Al.0 A2.0 n 3.0
-8
I
I
I
I
-2 8
Fig. 4. Arrhenius plots for the first- and second-stage adsorption of zirconium ions on activated charcoal.
Fig. 6. Plots of log log q$,/(& - qtW) vs log t for zirconium ion adsorption on activated charcoal at different concentrations of zirconium ions.
Zirconium
ion adsorption
steps[30]: bulk transport of zirconium ions in solution, film transfer involving diffusion of zirconium ions through a hypothetical film boundary layer, and the diffusion of zirconium ions within the pore volume of activated charcoal and/or along the pore wall surfaces to an active adsorption site. The actual adsorption of solute on interior surface sites is generally considered to be very rapid, and hence, it is not a rate-determining step. Therefore, film and intraparticle diffusion may be the steps controlling the rate of adsorption of zirconium ions. Because KD values decrease with increasing concentration of zirconium ion, Table 6, film diffusion does not seem to control the rate of adsorption of zirconium ions[31]. The adsorption of zirconium ions on activated charcoal can be expressed as:
Table 4. Calculated values of parameters of Bangham equation for zirconium ion adsorption on activated charcoal as a function of zirconium ion concentration at temperature 20°C Zr ions concentration
a!
(g/l)
k,
0.061 0.118 0.152 0.164 0.171
1.0 2.0 3.0 4.0 5.0
0.052 0.069 0.083 0.091 0.107
Table 5. Determined values of rate constants for zirconium ion adsorption on activated charcoal as a function of its concentration at temperature 20°C Zr ions concentration (g/l)
k, (min-‘)
k, (min-‘)
A,
0.273 0.180 0.177 0.116 0.085
0.099 0.109 0.116 0.124 0.136
1.0 2.0 3.0 4.0 5.0
3.87 3.74 1.12 6.23 2.92
x x x x x
1437
s+h4+t4,
A,
1O-3 10-s 1O-3 1O-4 1O-4
2
0.059 0.384 0.518 0.613 0.676
where S is the activated charcoal; A4 is the zirconium ions, and /r; and k; are the rate constant for the adsorption and desorption processes, respectively. The equilibrium constant kc can be calculated as:
kc+-= 2
concentration of zirconium ions in solution, r,,, is the maximum surface density at monolayer coverage, and n and A are constants that can be related to the strength of the adsorptive bond and bond distribution, respectively. Figure 7 reveals that the adsorption of zirconium ions on activated charcoal obeys Freundlich and Langmuir isotherm equations over the entire range of zirconium ion concentrations studied. The values of Freundlich constants l/n and A and Langmuir constants rmax and Kads were computed from the slopes and intercepts of their respective plots shown in Fig. 7, and their respective values are 0.1365, 0.1430, 0.171, and 8.004. The adsorption of zirconium ions from aqueous solutions on activated charcoal may involve three
Table 6. Equilibrium
data of zirconium
ion adsorption
CMi F
CM.,, CM.,
cMi(l
-
where C,,,.,, is the equilibrium concentration of zirconium ions on activated charcoal and CM.,, is the equilibrium concentration of zirconium ions in solution. C,,.,; is the initial concentration of zirconium ions and F is the fractional attainment of zirconium ion adsorption at equilibrium. The kc values for the adsorption of zirconium ions on activated charcoal from aqueous solution were evaluated as a function of temperature in the lo-50°C range. For such determination, the concentration of zirconium ion solution, the shaking time, and the v/M ratio were kept constant at 1.0 g/l, 30 minutes, and 100 ml/g, respectively. The values of k, at different
on the activated
charcoal
as a function
Initial Zr ions concentration; C, (g/l)
Equil. Cont. of Zr ions in solution, C, (g/l)
Cont. of Zr ions adsorbed at equilibrium (g/l)
aAmount of Zr ions adsorbed at equil. per gram of solid, rz,, (g/g)
1.0 2.0 3.0 4.0 5.0
0.049 0.670 1.470 2.380 3.330
0.951 1.330 1.530 1.620 1.670
0. .0951 0.1330 0.1530 0.1620 0.1670
aAmount
adsorbed,
I’,, (g/g)
=
tration
Coefficient,
of zirconium
K, (ml/g)
ion in solutions,
of its concentration
bDistribution coefficient Ko (ml/g) 1941 199 104 68 50
(C, - C,) v M
bDistribution
F)
.
(C,-C,) = ___ C,
V is the volume
v x M’
where C, and C, are the initial and equilibrium
of solution
taken,
and M is the weight of the activated
concencharcoal.
1438
R.
QADEER
Table 7. Calculated values of the thermodynamic quantities of zirconium ion adsorption on activated charcoal
log Ce -1.5
-1.0
-0.5
0
0.5
and J. HANIF
1.0
Temperature W)
-“.7 I25 -0.8
10 20 30 40 50
Ls -0.9 0”
(kJ%ol)
(kJ%oJ)
(kJ/dtg? mol)
-3.73 -4.92 -6.47 -9.16 -10.81
59.50
0.2234 0.2198 0.2177 0.2193 0.2176
-1.0 and -1-1 0
2
1
3
As = (AH - AC)
4
T
Ce (g/II
Fig. 7. The Freundlich and the Langmuir isotherms for zirconium ion adsorption on activated charcoal.
temperatures were calculated using eqn 10 and are given below: Temperature k,
(“C)
10 4.88
20 8.09
30 40 15.66 49.00
50 99.00
This shows that kc values increase with an increase in adsorption temperature. The thermodynamic quantities, such as AC, AH, and AS of zirconium ion adsorption were calculated from the kc values using the following relations: AC = -RTln -AH
In kc = -
RT
k,,
(11)
+ constant,
(12)
5L 2 c
d-----J 3.0
3.1
3.2
3.3
3.4
3.5
36
’
(13)
The variation of In kc with reciprocal temperature, l/T, is given in Fig. 8. From the slope of this curve, the quantity AH is calculated and is given in Table 7, along with the values of AC and AS. The positive values of AH show that zirconium ion adsorption on activated charcoal is an endothermic process that is quite contrary to the usual observations of exothermicity. Similar observations have been reported earlier for different metal ion adsorptions on solids[9-14, 32-331. The possible explanation of endothermic heat of adsorption is given in our earlier communications[l3,34]. The values of AC are negative, as expected for a spontaneous process. The AS values are positive, and no appreciable change in AS values is observed with the increase of temperature, which means that the magnitude of AS is not affected by the temperature. The adsorption process is endothermic; it follows that under these conditions the process becomes spontaneous because of the positive entropy change. 4. CONCLUSION
High temperature favours the adsorption of zirconium ions from aqueous solution on the activated charcoal and attained equilibrium within 30 minutes. The zirconium adsorption occurs in two distinct stages, the first stage relatively fast, followed by a slower one. The magnitudes of activation energies for the firstand second-stage adsorption processes are 5.80 and 43.00 kJ/mol, respectively. The applicability of the Bangham equation to the zirconium ions adsorption data shows that the diffusion of zirconium ions into the pores of the activated charcoal controls the adsorption process. The zirconium ion adsorption obeys the Freundlich and the Langmiur isotherm equation in the concentration range studied. Positive values of AH show that the zirconium ion adsorption on the activated charcoal is an endothermic process. The data are important for the recovery of zirconium ions from solutions.
103/T(K) Fig. 8. Plot of In/r, vs l/Tfor zirconium ion adsorption on activated charcoal.
Acknowledgement-Technical services of Mohammad Khan during this work are highly acknowledged. Thanks are also due to Gul Muhammad for typing the manuscript.
Zirconium
icm adsorption
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Pergamon 0008-6223(94)00090-S
KINETICS OF ZIRCONIUM IONS ADSORPTION ON ACTIVATED CHARCOAL FROM AQUEOUS SOLUTIONS Pakistan
Institute
of Nuclear
RIAZ QADEER and JAVED HANIF Science and Technology, P.O. Box No. 1356, Islamabad,
Pakistan
(Received 2 February 1994; accepted in revised form 23 May 1994) Abstract-The batch kinetics of adsorption of the zirconium ions from aqueous solutions on activated charcoal has been investigated over a wide range of concentration of zirconium ions (1.0-5.0 g/l) and temperatures (lo-50°C). The adsorption process of zirconium ions proceeds via two stages; the first stage is rather fast, followed by a much slower one. The Bangham equation was used to study the kinetics of the zirconium ions’ adsorption on activated charcoal. It is observed that the diffusion of zirconium ions into the pores of the activated charcoal controls the kinetics of the adsorption process. Moreover, zirconium ion adsorption obeys the Freundlich and Langmuir isotherms in the concentration range studied. The adsorption equilibrium constant (k,) values for zirconium ions adsorption on activated charcoal have also been calculated at different temperatures. Various thermodynamic quantities, AC, AH, and AS were computed from k, values. The results showed that the adsorption of zirconium ions on activated charcoal is
an endothermic process. Key Words-Zirconium,
adsorption,
activated charcoal, kinetics, aqueous solution.
1. INTRODUCTION
Zirconium is an important material in chemical and nuclear engineering, and in metallurgy and marine technology, because of its desirable mechanical strength, low thermal neutron cross section (0.185 b), and corrosive resistance to acids, alkalis, and saline water. Zirconium metal occurs in meteorites, sediments, ter-
restrial materials, igneous and volcanic rocks[l], and is a major component of various precious stones and gels[2]. Hafnium is invariably found in nature along with zirconium. Generally, commercial grade zirconium contains 1 to 5 parts by weight of hafnium per 100 of zirconium. The properties of zirconium are affected by its hafnium content. For example, the presence of l-2% hafnium in zirconium could raise its thermal neutron cross section from 0.185 b to 1.0 b because of a high thermal neutron cross section of hafnium (102 b)[3]. Therefore, it is essential that zirconium be free or nearly free from hafnium for its utilization in nuclear technology. For purification purposes many processes are being used, such as precipitation, ion-exchange, fractional crystallization, and adsorption on solids. The adsorption process under certain conditions has a definite edge over other methods used for metal ions’ purification/recovery because of its simplicity, selectivity, and efficiency[4]. The primary requirement for an economic adsorption process is an adsorbent with sufficient selectivity, and high adsorption capacity and life. Owing to its large surface area, microporous structure, high adsorption capacity, radiation stability, and high purity, activated charcoal has been widely used to remove metal ions from solutions[S-81. We have previously used the activated charcoal for theadsorptionofuranium[9], thorium[lO], gadolinium[ll], samarium[l2], europium[l3], and strontium[l4] ions from solutions. The present com1433
munication reports results of our kinetics investigation of zirconium ion adsorption on activated charcoal from aqueous solutions. The results are important in relation to the recovery of zirconium ions from solutions. Some workers have studied the adsorption of zirconium ions on ion-exchange resin[lS-161, oxycellulose[l7], filter paper[l8], niobium tip[l9], tungsten (100) surface[20], and manganese dioxide[21], but no data is available of its adsorption on the activated charcoal. 2. EXPERIMENTAL
2.1 Chemicals The chemicals used in this study are zirconium oxychloride (M/s JMC; item No. 618149) and a commercial activated charcoal (M/s BDH; item No. 33032) having B.E.T.-N2 surface area 980 m2/g, porosity 75.74%, pore volume 1.43 cm3/g, and average particle diameter 3.7 k 0.2 micron. All the solutions were made in distilled water.
2.2 Instruments Siemen’s wavelength dispersive X-ray fluorescence (WDXRF) spectrometer (SRS-200) was used for measuring the zirconium ion concentration in solutions with variation <2.0%. The Hetofrig shaker (M/s Heto Birkerod-Denmark) was used for temperaturecontrolled studies. The fluctuation in the measurement of temperature is within +O. 1“C.
2.3 Procedure The adsorption of zirconium ions on the activated charcoal from aqueous solutions was carried out via a batch technique. Accordingly, 10 ml solutions of known concentration were added to 250 ml glass
1434
R. QADEERand J. HANIF
Table 1. XRF instrumental conditions for measuring zirconium ion concentrations in aqueous solutions Spectrometer Tube Detector Crystal Collimator Tube Current Tube Voltage Radiation path Analyte line LBG 219value qeak 20 value HBG 20 value Counting time
Siemen’s SRS-200 Cr Target Scintillation, NaI(T1) LiF-100 Fine 50 mA 50 kV Air Zr KLY 22.30 22.60 22.90 40 seconds at all 20 values 0
10
20
Shaking
bottles and shaken with about 0.1 g of dry activated charcoal in a thermostat shaker, the temperature of which was preadjusted to a desired value. After a predetermined time, the solution was filtered through Whatman filter paper No. 40 (circular, 14.0 cm). The first 2-3 ml portion of the filtrate was rejected because of the adsorption of zirconium ions by filter paper. The concentration of zirconium ions in the filtrate was measured using a WDXRF spectrometer at conditions given in Table 1. The concentration of zirconium ions was corrected for the loss of zirconium ions through adsorption on the walls of the glass bottles by running blank experiments (i.e., without activated charcoal added) at each time, t. The fraction of zirconium ions adsorbed at any time, F(t), was calculated using the following relation:
30
40
50
60
time> t (min)
reagent
where F( t ) = fraction of zirconium ions adsorbed at time t on activated charcoal, Ci = initial concentration of zirconium ions in solutions (g/l), and C, = concentration of zirconium ions in solution at time t (g/l).
3. RESULTS AND DISCUSSION
Figure 1 represents the variation of zirconium ion adsorption on activated charcoal with shaking time at different temperatures. This figure indicates that initially the fraction of zirconium ions adsorbed increases rapidly, but then the process slows down and subsequently attains a constant value after about 30 minutes (i.e., when the adsorption equilibrium is established). Similar observation of adsorption equilibrium attainment by zirconium ions on manganese dioxide has been reported earlier[21]. As an approximation, the adsorption of zirconium ions can be said to take place in two distinct steps, a relatively fast one
Fig. 1. Zirconium ion adsorption on activated charcoal against shaking time at different temperatures (solution concentration 2 g/l, pH 1.9).
followed by a slower one. The slow adsorption is explained by the diffusion of zirconium ions into the pores of the activated charcoal. Figure 1 also shows that the general time dependence of the zirconium ion adsorption process essentially independent of temperature. However, the temperature variations influence the amount of zirconium ion adsorption, which increases with a rise in adsorption temperature. This is because at higher temperatures, the diffusion of zirconium ions through charcoal pores is faster and can proceed to a larger extent. Zirconium oxychloride behaves as a Lewis acid dissolving in polar solvents to give rise to hydrolysis[22] and form the tetrameric species [Zr,(OH)z(H20)r6]8+ by freeing HC1[23]. The mechanism of its hydrolysis has been described by Devia and Sykes[24]. The release of HCl results in the lowering of pH in the solution. The pH determined in the present studies is 1.9, which confirms this argument. There is no simple explanation of the adsorption of this tetrameric species on the surface of the activated charcoal. Since the surface of the activated charcoal does possess acidic surface functional groups[25], the species [Zr,(OH)z(H20),6]8+ may form a surface complex with this acidic group. The Bangham equation[26] was employed to study the kinetics of zirconium ion adsorption on activated charcoal, and is given below:
-da = k&-l, dt
where qr is the amount of zirconium ions adsorbed per g of activated charcoal at time t; LYis a constant (less than 1); k is a factor that depends on the concentration of solution and is given as: k = k,(&
- qtW/V
(3)
1435
Zirconium ion adsorption
where 4, is the original amount of zirconium ions in solutions; k0 is the proportionality constant; W is the weight of the activated charcoal, and Vis the volume of zirconium ion solution taken. Q~has been defined earlier. On substituting k values from eqn 3 into 2 and on integration, eqn 2 becomes: 40
In
koW
$0 - q,w
=
-F-
(50-&W
10 20 30 40
(4)
kow
40
Temperature (“C)
ta*
To apply eqn 4 to the experimen~l data of the adsorption of zirconium ions on activated charcoal given in Fig. 1, eqn 4 is rearranged as follows:
log log
Table 2. Determined values of Bangham equation’s parameters for zirconium ion adsorption on activated charcoal at different temperatures (Zr solution concentration = 2.0 g/l)
= log--+ iYlog t. 2.303 V
(3
Straight lines were obtained by plotting log log [ &,I( cjoqrW)J against log I, as shown in Fig. 2, indicating that the system acts in accordance with the Bangham equation. The values of LYand k,, calculated from the slopes and intercepts of plots shown in Fig. 2, are given in Table 2. The applicabifity of the Bangham equation to the zirconium ion adsorption data shows that the diffusion of zirconium into the pores of the activated charcoal controls the adsorption process1271. The rate constants of two-stage process of zirconium ion adsorption on activated charcoa1 are determined using the following equation[28]: (1 - F) = A I eekir + A,eFkZ’,
-0.1
0-L
loo t -1.2
0.6=
0.094 0.117 0.162 0.192
0.064 0.070 0.073 0.076
t (min) 0 0
(6)
1.6
‘%
A2 can be found from the intercept of the extrapolation of the linear part of the second stage, whose slope is k2. Subtraction of of A,e-kz’ from each of the experimental data will then produce new straight lines {Fig. 3), of slope kl and intercept A 1. Their determined values are given in TabIe 3. The activation energies of the first and second stages of zirconium ion adsorption on activated charcoal have been calculated from the variation of the corresponding rate constants with temperature in accordance with the Arrhenius equation. Plots of log~ithms of rate constants k, and k2 vs reciprocal of temperature, for the first and second stages of the adsorption process, are shown in Fig. 4. From the slopes of straight lines, the values of activation energies of the first and second stages of the
where kr and k2 are the rate constants for the first and second stages of adsorption, respectively, and A, and AZ are constants. Plots of ln(l - F) vs t for zirconium ion adsorption on activated charcoal at different temperatures are given in Fig. 3. The values of
0
cx
-0.5
10
20
30
40
50
I
I
I
I
I
0
lo”c
60
1 _
-1.0
-1 .?I
2.0 Lt.
-0.2 Y d
_,
-2-o
C
-0.3
\4 $J
-04
,” ;
-0.5
-2.5
0 2o”c a 3o”c A 4o”c
-3.0
-3.5 -4-o
Fig. 2. Plots of log log 40/($o - q,W) vs log t for zirconium ion adsorption on activated charcoal at different temperature.
- 5.0
1
Fig. 3. Plots of In(l - F) vs t for zirconium ion adsorption on activated charcoal at different temperatures.
1436
R. QADEERand J. HANIF
Table 3. Determined values of rate constants for zirconium ion adsorption on activated charcoal at different temperatures (Zr solution concentration = 2.0 g/l) Temperature (“C)
k, (min-‘)
A,
0.134 0.155 0.175 0.183
0.167 0.163 0.116 0.109
10 20 30 40
k, (min-‘)
1.43 3.29 5.11 8.61
x x x x
lO-3 lO-3 lO-3 10-s
I
al _
A,
0.434 0.401 0.331 0.302
E
;
I
I
0.6
a, -0 =
tc
:
g
Y
Zr ions conc.(g/ll 0 1.0 0 20 n 3.0 A&O A 5.0
0.2
”
e LL
: II r(L
n : adsorption process were calculated and found to be 5.80 and 43.00 kJ/mol, respectively. Figure 5 depicts the adsorption of zirconium ions on activated charcoal in different concentration ranges (1.0-5.0 g/l) from aqueous solutions at 20°C. This shows that the zirconium ion uptake is initially fast, becoming slower later on, and ultimately reaching saturation. The time of saturation is about 30 minutes, and the extent of adsorption remains practically unchanged. These observations are similar to the temperature-dependent studies of zirconium ion adsorption on activated charcoal, as shown in Fig. 1. It is quite evident in Fig. 5 that the fraction of zirconium ions adsorbed at equilibrium, as well as prior to equilibrium, decreases with an increase in zirconium ion concentration. The Bangham equation, eqn (5), was applied to the concentration-dependent data of zirconium ion adsorption on activated charcoal. Figure 6 shows that zirconium ion adsorption on activated charcoal in the concentration range studied also obeyed the Bangham equation. The values of parameters 01and ke, calculated from the slopes and intercepts of plots shown in Fig. 6, are given in Table 4. Similarly, eqn (6) was applied to the concentrationdependent data of zirconium ion adsorption on the activated charcoal, to determined the values of the constants of eqn 6. Their determined values are given in Table 5.
-
0 0
I
I
I
I
I
J
10
20
30
40
50
60
Shaking
time,
t (min.]
Fig. 5. Zirconium ion adsorption on the activated charcoal against shaking time at different concentrations of zirconium ions.
The data concerning the dependence of the extent of adsorption on zirconium ion concentration at equilibrium, Table 6, was subjected to examination by the Freundlich and Langmuir isotherm equations. The Freundlich and the Langmuir plots were obtained, Fig. 7, using the following well known equations[29]: rzr = AC:“’
(Freundlich)
(7)
and C
1
r zr
Kadsrmax
2=-
+ 5
(Langmuir), max
(8)
where rzr is the amount of zirconium ions adsorbed per gram of activated charcoal, C, is the equilibrium
lO?T(K) Jf:‘?‘:;lfi”
Al.0 A2.0 n 3.0
-8
I
I
I
I
-2 8
Fig. 4. Arrhenius plots for the first- and second-stage adsorption of zirconium ions on activated charcoal.
Fig. 6. Plots of log log q$,/(& - qtW) vs log t for zirconium ion adsorption on activated charcoal at different concentrations of zirconium ions.
Zirconium
ion adsorption
steps[30]: bulk transport of zirconium ions in solution, film transfer involving diffusion of zirconium ions through a hypothetical film boundary layer, and the diffusion of zirconium ions within the pore volume of activated charcoal and/or along the pore wall surfaces to an active adsorption site. The actual adsorption of solute on interior surface sites is generally considered to be very rapid, and hence, it is not a rate-determining step. Therefore, film and intraparticle diffusion may be the steps controlling the rate of adsorption of zirconium ions. Because KD values decrease with increasing concentration of zirconium ion, Table 6, film diffusion does not seem to control the rate of adsorption of zirconium ions[31]. The adsorption of zirconium ions on activated charcoal can be expressed as:
Table 4. Calculated values of parameters of Bangham equation for zirconium ion adsorption on activated charcoal as a function of zirconium ion concentration at temperature 20°C Zr ions concentration
a!
(g/l)
k,
0.061 0.118 0.152 0.164 0.171
1.0 2.0 3.0 4.0 5.0
0.052 0.069 0.083 0.091 0.107
Table 5. Determined values of rate constants for zirconium ion adsorption on activated charcoal as a function of its concentration at temperature 20°C Zr ions concentration (g/l)
k, (min-‘)
k, (min-‘)
A,
0.273 0.180 0.177 0.116 0.085
0.099 0.109 0.116 0.124 0.136
1.0 2.0 3.0 4.0 5.0
3.87 3.74 1.12 6.23 2.92
x x x x x
1437
s+h4+t4,
A,
1O-3 10-s 1O-3 1O-4 1O-4
2
0.059 0.384 0.518 0.613 0.676
where S is the activated charcoal; A4 is the zirconium ions, and /r; and k; are the rate constant for the adsorption and desorption processes, respectively. The equilibrium constant kc can be calculated as:
kc+-= 2
concentration of zirconium ions in solution, r,,, is the maximum surface density at monolayer coverage, and n and A are constants that can be related to the strength of the adsorptive bond and bond distribution, respectively. Figure 7 reveals that the adsorption of zirconium ions on activated charcoal obeys Freundlich and Langmuir isotherm equations over the entire range of zirconium ion concentrations studied. The values of Freundlich constants l/n and A and Langmuir constants rmax and Kads were computed from the slopes and intercepts of their respective plots shown in Fig. 7, and their respective values are 0.1365, 0.1430, 0.171, and 8.004. The adsorption of zirconium ions from aqueous solutions on activated charcoal may involve three
Table 6. Equilibrium
data of zirconium
ion adsorption
CMi F
CM.,, CM.,
cMi(l
-
where C,,,.,, is the equilibrium concentration of zirconium ions on activated charcoal and CM.,, is the equilibrium concentration of zirconium ions in solution. C,,.,; is the initial concentration of zirconium ions and F is the fractional attainment of zirconium ion adsorption at equilibrium. The kc values for the adsorption of zirconium ions on activated charcoal from aqueous solution were evaluated as a function of temperature in the lo-50°C range. For such determination, the concentration of zirconium ion solution, the shaking time, and the v/M ratio were kept constant at 1.0 g/l, 30 minutes, and 100 ml/g, respectively. The values of k, at different
on the activated
charcoal
as a function
Initial Zr ions concentration; C, (g/l)
Equil. Cont. of Zr ions in solution, C, (g/l)
Cont. of Zr ions adsorbed at equilibrium (g/l)
aAmount of Zr ions adsorbed at equil. per gram of solid, rz,, (g/g)
1.0 2.0 3.0 4.0 5.0
0.049 0.670 1.470 2.380 3.330
0.951 1.330 1.530 1.620 1.670
0. .0951 0.1330 0.1530 0.1620 0.1670
aAmount
adsorbed,
I’,, (g/g)
=
tration
Coefficient,
of zirconium
K, (ml/g)
ion in solutions,
of its concentration
bDistribution coefficient Ko (ml/g) 1941 199 104 68 50
(C, - C,) v M
bDistribution
F)
.
(C,-C,) = ___ C,
V is the volume
v x M’
where C, and C, are the initial and equilibrium
of solution
taken,
and M is the weight of the activated
concencharcoal.
1438
R.
QADEER
Table 7. Calculated values of the thermodynamic quantities of zirconium ion adsorption on activated charcoal
log Ce -1.5
-1.0
-0.5
0
0.5
and J. HANIF
1.0
Temperature W)
-“.7 I25 -0.8
10 20 30 40 50
Ls -0.9 0”
(kJ%ol)
(kJ%oJ)
(kJ/dtg? mol)
-3.73 -4.92 -6.47 -9.16 -10.81
59.50
0.2234 0.2198 0.2177 0.2193 0.2176
-1.0 and -1-1 0
2
1
3
As = (AH - AC)
4
T
Ce (g/II
Fig. 7. The Freundlich and the Langmuir isotherms for zirconium ion adsorption on activated charcoal.
temperatures were calculated using eqn 10 and are given below: Temperature k,
(“C)
10 4.88
20 8.09
30 40 15.66 49.00
50 99.00
This shows that kc values increase with an increase in adsorption temperature. The thermodynamic quantities, such as AC, AH, and AS of zirconium ion adsorption were calculated from the kc values using the following relations: AC = -RTln -AH
In kc = -
RT
k,,
(11)
+ constant,
(12)
5L 2 c
d-----J 3.0
3.1
3.2
3.3
3.4
3.5
36
’
(13)
The variation of In kc with reciprocal temperature, l/T, is given in Fig. 8. From the slope of this curve, the quantity AH is calculated and is given in Table 7, along with the values of AC and AS. The positive values of AH show that zirconium ion adsorption on activated charcoal is an endothermic process that is quite contrary to the usual observations of exothermicity. Similar observations have been reported earlier for different metal ion adsorptions on solids[9-14, 32-331. The possible explanation of endothermic heat of adsorption is given in our earlier communications[l3,34]. The values of AC are negative, as expected for a spontaneous process. The AS values are positive, and no appreciable change in AS values is observed with the increase of temperature, which means that the magnitude of AS is not affected by the temperature. The adsorption process is endothermic; it follows that under these conditions the process becomes spontaneous because of the positive entropy change. 4. CONCLUSION
High temperature favours the adsorption of zirconium ions from aqueous solution on the activated charcoal and attained equilibrium within 30 minutes. The zirconium adsorption occurs in two distinct stages, the first stage relatively fast, followed by a slower one. The magnitudes of activation energies for the firstand second-stage adsorption processes are 5.80 and 43.00 kJ/mol, respectively. The applicability of the Bangham equation to the zirconium ions adsorption data shows that the diffusion of zirconium ions into the pores of the activated charcoal controls the adsorption process. The zirconium ion adsorption obeys the Freundlich and the Langmiur isotherm equation in the concentration range studied. Positive values of AH show that the zirconium ion adsorption on the activated charcoal is an endothermic process. The data are important for the recovery of zirconium ions from solutions.
103/T(K) Fig. 8. Plot of In/r, vs l/Tfor zirconium ion adsorption on activated charcoal.
Acknowledgement-Technical services of Mohammad Khan during this work are highly acknowledged. Thanks are also due to Gul Muhammad for typing the manuscript.
Zirconium
icm adsorption
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