1866
The Journal of Physical Chemistry, Vol. 83, No. 14, 1979
Hachiya et
al.
Study of the Kinetics of Adsorption-Desorption of Pb2+ on a y-Al,O3 Surface by Means of RelaxatIon Techniques Kazuakl Hachiya, Minoru Ashida, Minoru Sasaki, Haruki Kan, Tadaomi Inoue, and Tatsuya Yasunaga"
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Department of Chemistty,Faculty of Science, Hiroshima University, Higashisenda-machi,Hiroshima 730, Japan (Received October 25, 1978; Revised Manuscript Received March 27, 1979)
Double relaxations on the order of micro- and milliseconds were found in aqueous y A l z 0 3suspension containing Pb2+by means of the electric field pulse and pressure-jump methods, respectively, with electric conductivity detection. The reciprocal fast relaxation time decreases with increasing the Pb2+concentration, but the slow one increases. The fast relaxation measured is attributed to the adsorption-desorption process of Pb2+on the hydrous oxide surface group A1-OH, and the slow one to the deprotonation-protonation process induced by adsorbed Pb2+. The values of the forward and backward rate constants of the former process were determined as kl = (1.4 f 0.4) X lo8 mol-' dm3s-l and kl = (1.0 f 0.4) x IO4 s-l, and those for the latter process k2 = (1.3 f 0.6) X 10 s-l and k-z = (1.5 f 0.1) X 106 mol-1 dm3 s-l, respectively, at 20 "C.
Introduction Catalytic reactions at solid-liquid interfaces of metal oxides have been of great interest to colloid chemists. The adsorption-desorption phenomena of various metal ions on oxides such as yA1203have been extensively investigated since adsorption-desorption is a fundamental step in heterogeneous catalytic reactions. Several mechanisms have been proposed by many investigator~.l-~~ Kinetic studies, however, have scarcely been carried out because the reaction is too fast to be measured by ordinary methods. Only a kinetic study of proton adsorptiondesorption at the Ti02-water interface has been reported by use of the pressure-jump technique by the present authors.28 The purpose of the present study is to clarify the dynamics of adsorption-desorption in the y-A1203-Pb2+ system, where the Pb2+ion adsorbs strongly on the 7-Al2O3 surface. Theory Let us consider the following scheme for the adsorption-desorption reaction of Pb2+ion on a 7-Al,03 surface, where step 1is the adsorption-desorption of Pb2+on the hydrous oxide surface group A1-OH, and step 2 the deprotonation-protonation induced by adsorbed Pb2+:
rt = rl + rz
(34 where the square brackets denote the bulk concentration, C, the concentration of particle, K the equilibrium constant, t the time, r, rm, and rt are the equilibrium, saturated, and total amount of adsorbed Pb2+,respectively, and the subscripts 1and 2 refer the states A1-O(H)-Pb2+ and AI-OPb+, respectively. The bulk and surface concentrations of each species are changed by perturbation of the equilibrium as follows: ___ [Pb2+]= [Pb2+] A[Pb2+] (44
+ [H+] = [PI+ A[H+] Fl = f , + Ar, (i = 1, 2)
(4b)
A[H+] = CPAr2
(5b)
(44 where the bar over the concentration symbols denotes the equilibrium values. According to the law of mass conservation, we obtain the following equations: A[Pb2+]+ C,(Ar, + AI',) = 0 (54 With eq 3-5, eq 2 is rewritten as: -dA[Pb2+]/dt = allA[Pb2+]+ a12A[H+]
+
-dA[H+]/dt = a21A[Pb2+] a22A[H+]
(6a) (6b)
with all = kl(C,(rt" - FJ Pbzt
step 1 TI
H+ step 2
where A1-O(H)-Pb2+ is the encounter complex formed by adsorption of Pb2+,A1-OPb+ the surface complex, T~ and T~ are the relaxation times of steps 1 and 2, respectively, and ki are the rate constants. At constant ionic strength the following rate equations are given for these steps: -d[PbZf]/dt = klC,(rt" - rt)[Pb2+]- k-lCpI'l
(2a)
a12
= k-1
a21
= k2
a22= k-2(CpT'2+
7.2
+ [pb2+])+ k-1 [F]) + k2
Thus, the reciprocal relaxation times can be expressed as q2-1
=
+ a2z) f [(all + azd2 - 4(allazz - a12az1)11'2) (7)
y2((a11
Equation 7 is simplified under special conditions (i) T ~
~ - ~72-1 (all
1867
>> az2)
+ [Pb2+]}+ kl 7
rl-I
72-1
=
= all = kl{C,(I'tm - I?,)
a22 -
(9a)
-= k-2(CpI?2+ [H+]) + k2 a1_ 1_ (1312a21
k-1122
+ [Pb2+]j+ k-1 ~
kl{Cp(I'tm- I?,)
.
Monochro rnator
(9b)
At equilibrium, the equation for the amount of Pb2+ adsorbed is obtained from eq 3a-c:
-
I'cl = (rtrn)-l + (I'trnKL)-1[Pb2t]-1
4 0 kHz
(10) Figure 1. Block diagram of the pressure-jump apparatus with electric conductivity and absorbance detections: (1) and (2) conductivity sample and reference cells; (3) BaTiO, piezoelectric element; (4) spectrophotometric sample cell.
J. Phys. Chem. 1979.83:1866-1871. Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 08/27/15. For personal use only.
with
2
m
where KLrepresents the pseudo-Langmuir constant. This equation is identical with the Langmuir isotherm if the value of [H+] is constant. The apparent and true overall equilibrium constants and KO, respectively, are related by the following equation:
with KO= KlK2
Experimental Section Apparatus. (a) Pressure-Jump Apparatus. The pressure-jump apparatus used is a modification of that described by Knoche and W i e ~ eand , ~ ~the block diagram is shown in Figure 1. An advantage of this apparatus is that chemical relaxation induced by pressure jump can be observed simultaneously by both changes in electric conductivity and optical absorbance as seen from the sectional views of the autoclave in Figure 2. For conductivity detection, the relaxational signal picked up by a wave detector is stored by a HD-50 data-memorizing device (Union Giken Co., Ltd.), and reproduced on the monitor and recorder. A spectrophotometric sample cell which is located outside the autoclave is connected optically with a monochromator (Shimazu AQV-50) and a photomultiplier (Hamamatsu TV 931A) by two flexible light guides (Olympus LG-305 and LG-10-5). The relaxational change in optical absorbance is also stored and reproduced in the same way as conductivity detection. The autoclave is closed with a burst diaphragm (0.05 mm thick phosphor bronze) which ruptures spontaneously at a pressure of about 100 atm. The pressure decreases to atmospheric pressure within about 100 ps, as can be seen from Figure 3. In this apparatus water is used as the pressure transducing medium instead of air. The operation is easily repeated by loosening the bayonet socket by a quarter turn and moving the strip of phosphor bronze. (b) Electric Field Pulse Apparatus. A description of the electric field pulse apparatus used has already been reported.30 The electric field intensity in the cell was 16 kV cm-l. The duration of the high voltage pulse applied was 25 ps, where the rise and decay times of the pulse were much faster than 0.1 ps. Materials. A commercial product, Aluminum Oxide C, supplied by Japan Aerosil Co. was produced by the hydrolysis of AlCl, in a flame process. The X-ray diffraction
8
s
Figure 2. Sectional views of the autoclave: (1) spectrophotometric cell, (2) conductivity sample and reference cells, (3) BaTiO, piezoelectric element, (4) light guides, (5) conical lenses, (6) vacuum pump, (7) bayonet socket, (8) burst diaphragm, (9) heat exchanger, and (IO)pressure pump.
pattern confirmed that the sample was predominantly yA1203 The zero point of charge was pH 8.3. The specific surface area determined by the BET adsorption measurements was 100 f 15 m2 8-I. The particle size was smaller than 1 pm and apparently uniform. Prior to use, the sample was electrodialyzed until the electric conductivity became equal to that of distilled water. The sample dried at 100 OC was redispersed by ultrasonication, and was aged for a few days. The particles in the suspension did not sediment as observed during the period of 1 h. Analytical Method. In order to measure colorimetrically the bulk concentration of lead ion in the aqueous suspension of yAl,O, containing lead nitrate, the interfering particles were filtered with a Millipore filter (Millipore Filter Corp. GSWP 04700; with pore size of 0.22 pm). The colorimetric determination of lead ion was performed at
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1-
% JOUMI Of -1
(xwm*rby. Vol.
83. No. 14. 1979
Flame 5. -1
1 .
L
.
I
I
.
.
I
Tvolcal relaxation curve In a o m u s +v-AI.O.-PMNO.h
s~pensionObieNed by thS electrlc flekl &Sa mebod: dm-' at 20 ' C , sweep, 2 psldivislon.
kp2 2
