Langmuir 1991, 7, 2654-2658
2654
Sulfate Adsorption on Zirconium Dioxide J. Randon, A. Larbot, and L. Cot Laboratoire de Physicochimie des Matkriaux (U.R.A. 1312 CNRS), Ecole Nationale Supkrieure de Chimie de Montpellier, 8, rue de l%cole Normale, 34053 Montpellier Ckdex 01, France
M. Lindheimer and S. Partyka' Laboratoire de Physico-Chimie des Syst2mes Polyphasks (U.R.A. 330 CNRS), US.T.L., Place E. Bataillon, 34095 Montpellier Ckdex 05, France Received December 18, 1990. I n Final Form: May 28, 1991 The complexation properties of a new zirconia powder prepared for ultrafiltration membranes with sulfate ions have been investigated by various techniques. Electrophoretic mobility, Fourier transform infrared spectroscopy, adsorption isotherm, and alkalimetric titration give a quantitative measurement of the adsorption. The complexation constant is determined (log Kso = -2.5) and the binding mode is defined as monodentate coordination: 1:l exchange between 5 0 4 2 - anh OH groups on the surface. The results are in good agreement with the modern double layer theory. The values of enthalpic exchanges at different degrees of coverage show that adsorption of sulfate on zirconia is determined by at least three different contributions: direct interaction between S02- and ZrOH2+ at the surface; C1- displacement from the surface by S042-;the behavior of water (hydration and displacement) during the adsorption process.
Introduction Inorganic compounds are now used in membrane manufacture. They are stable a t high temperature, and their good chemical resistance makes these compounds suitable for work in reactive media. Zirconia (ZrO2) is one of the most useful compounds for the preparation of ceramic membranes. The sol-gel process can provide thin layers of ultrafine pore diameter1 which form the separative layer of filtration. The development of mineral membranesduring the last years requires a fundamental knowledge of physical and electrochemical properties of the interface between the membrane and the feed solution. In our own experiments, the performance of zirconia membrane is different according to the nature and concentration of salt dissolved in water (Randon et al., paper in preparation). The electrochemical characteristics data of zirconia powder are essential for a better understanding of the working conditions of membranes. It is well-known that oxides, such as A1203, TiO2, Si02, and ZrO2, have some amphoteric behavior. The equilibria responsible for the development of surface charges are as follows:
Complexation with the charged sites occurs when ions of opposite charge are in the solution. The following ion pairs can then be formed: MOH + X " + o MOX'""+
+ H+
Various methods have been applied to investigate the behavior of the material, especially in sulfate electrolyte solution. A good understanding of the interfacial properties will influence the conditions of membrane use. The sol-gel process was used for powder preparation, as the surface properties are very dependent on sample synthesis?
Experimental Section Materials. Zirconia was prepared by hydrolysisof zirconium propoxide. The precipitatewas filtered and washed with distilled water. The membrane synthesis conditions have been adjusted using the results of DLVO theory:'*8 HCl was used as peptizing agent and organic binders were added to adjust stability and sol viscosity suitable for deposition on an asymmetric support. The sample, dried at 100 OC, was heat treated at 150 O C . P X-ray diffractometry (INEL CPS120 and MCA CAT0 4K) shows that zirconia has the monoclinic structure up to 1200 O C (changing from monoclinic to tetragonal). The specific surface area (66 m2/g) was determined by nitrogen adsorption using the Analsorb 9011 apparatus; it is relatively high for crystallized zirconia. Intergranular spaces in the powder have a medium pore diameter about 10 nm, whereas the membrane layer shows a diameter distributioncentered on 8 nm (pressureeffect applied during deposition on support). In a scanning electronmicrograph, we observed sphere diameters varying from 15 to 30 nm. These particles appear sintered as in the membrane morphology. All chemicals were used as received, and high resistivity water was used in all cases. Electrophoresis. The measurements were performed with a Rank Brother Mark I1 apparatus, with a microelectrophoretic (2) Yatea,D. E.;Levine, S.;Healy, T. W. J. Chem.Soc.,Faraday Trans.
1 1974, 70, 1807.
K,
(3)
The oxide surface charge changes in both cases. Mathematical calculations have been developed for the evaluation of the constants of the above eq~ilibria.~-~ (1) Larbot, A.; Julbe, A.; Randon, J.; Guizard, C.; Cot, L. ICZM 89 Proc. 1989,31, F-34053.
(3) Davis, J. A.; James, R. 0.; Leckie, J. 0. J. Colloid Interface Sci. 1978,63,480. (4) Davis, J. A.; Leckie, J. 0. J. Colloid Interface Sci. 1978, 67, 90. (5) James, R. 0.; Davis, J. A.; Leckie, J. 0.J. Colloid Interface Sci. 1978,65,331. (6) Pditt, G. D. h o g . Surf. Membr. Sei. 1976, 11, 181. (7) Derjaguin, B. V.; Landau,L. D. Acta Physicochim. 1941,14,633. (8) Verwey, E.; Overbeek, J. Th. G.Theory of Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. (9) Larbot, A,; Fabre, J. P.; Guizard, C.; Cot, L. J. Am. Ceram. SOC. 1989, 72, 257. (10) Koumeri, B. Thesis, Universit4 de Montpellier-F, 1990.
0143-1463/91/2401-2654$02.50/00 1991 American Chemical Society
Langmuir, Vol. 7, No. 11, 1991 2655
Sulfate Adsorption on Zirconium Dioxide cell of rectangular cross section. The zirconium oxide sample = 0.1 g) was suspended in 50 cms of salt solution at a suitable concentration. The pH value was adjusted with acid or basic solution containing the same ions at the same concentration as the salt solution to keep the ionic strength constant. The suspensions were stirred for 24 h. Electrophoretic mobility was determined at 25 f 0.1 "C by a rotating prism system and confirmed by several values measured manually in the null electroosmotic plane. The accuracy of the measurements was better than 3%. FTIB The evolution of vibration modes of sulfate ions was followed with a Nicolet 5ZDX spectrometer with Fourier transformation. The sample of powder (0.1 g) was suspended in Na2SO4solution (50 cms) with a sulfate concentration varying between 10-4and 10-l M. After equilibrium, the sample was filtered, dried at room temperature, and analyzed. Adsorption Isotherm. A typical adsorption experiment was carried out in a poly(viny1chloride) vessel to which 1g of ZrOz was added in NazSO4 solution (50 cm9 with a concentration varying between 5 x 10-4and 10-1M. The suspension was mixed by gentle rotation for 24 h at 20 OC. The sulfate ion concentration was determined after centrifugation and separation. The Sodzconcentration in the supernatant was measured by ionic chromatography (Dionex 20001) with a conductometric detector. Bromide ion was used as an internal standard. The reproducibility between two concentration determinations was witbin 2 % . Chloride ions released from the surface could also be detected in the same titration. Calorimetric Procedure. Measurements of adsorption enthalpies were made by adsorption from solution in a Montcal calorimeter.I1 A small volume of stock solution of sulfate ions was injected into a suspension in the calorimetric cell at thermal equilibrium (25 "C). Adsorption of S O P produces a thermal effect. Each consecutive injection increases the surface coverage. A blank experiment was carried out by injection of stock solution into the calorimetric cell filled with pure water. The heat effects due to dilution were thus determined and used to obtain the net heats of adsorption. However it was impossible to estimate experimentally the terms relating to the displacement of some water molecules. Differential molar enthalpies of adsorption were measured versus the degree of coverage. Alkalimetric titration. In a titration vessel, 0.675 g of ZrOz was suspended in an electrolyte solution of NazSO4 at 20 OC. This suspension was titrated against 0.1 M NaOH. To correct the residual acidity remaining from the synthesis, we have also titrated the supernatant solution. NaCl was,used to fix the ionic strength. An automatic titrator (Tacussel Tl'P2) and computer were used to register equilibrium pH. The speed of titration was determined by a criterion of reversibility of titration carried out at first with NaOH and subsequently with HCl.
Results and Discussion Electrophoresis. Figure 1 represents the electrophoretic mobility of zirconia in an electrolyte solution of NaCl w equilibrium pH. These curves are characteristic of an indifferent electrolyte; the isoelectric point is about 6.7 for the two concentrations. Conversely, Sod2-is a specifically adsorbed ion. When sulfate ions are added (Figure21,their concentration under the slipping plane increases. Adsorbed sulfate ions move with the oxide particle, so that electrophoretic mobility in the acidic region having positive values decreases to become a negative region when the adsorption of sulfate ions increases. FTIR. In NazSO4 solutions,the freesulfate ions belong to the high symmetry point group Td (Figure 3a). Only two vibration modes of the four fundamentals are infrared active: these are u3 and u4, determined respectively at 1127 and 616cm-l. Zirconiapowder (Figure 3b) shows vibration (11) Partyka,S.;Keh,E.; Lindheimer,M.;Croazek,A. J.ColloideSurf. 1989,37,309.
3)
-3:
I
3
4
5
6
7
0
9
10
11
1 :
PH
Figure 1. Electrophoretic mobility of zirconia in NaCl electrolyte M and 1WM) at 25 O C against the equilibrium pH. solution (le2
* M'01
-42
3
4
5
6
7
8
9
10
11
12
PH
Figure 2. Electrophoretic mobility of zirconia in NaB04 electrolyte solution 10-9, and lo-' M) at 25 OC against the equilibrium pH.
bands at 3417 and 1637 cm-l corresponding to hydroxyl groups and water adsorbed on the surface.12 The band at 745cm-l is due to Zr-0-Zr deformation. The other bands below 745 cm-' agree with various Zr-0 vibrations attributable to the coordination of oxygen with adsorbed water molecules.13 After the salt adsorption on zirconium oxide (Figure 3c), there are three new vibration modes at 1129,1061, and 992cm-l. The first two appear when the ion symmetry is lowered by complex formation. This lowering is confirmed by the appearance of the symmetry vibration valency v1 at 992 cm-l.14 Td symmetry decreases to c,. This point group corresponds to an unidentate binding mode. The change in the selection rules caused by the lowering of symmetry is different for unidentate and bidentate complexes. If S042-had reacted with two ZrOHz+ groups to form a 1:2 complex with the surface, the symmetry mode would be C% with three bands resulting from v3. We have never observed these bands at any concentration of sulfate ions. In conclusion, the FTIR results show the existence of a k l complex between sulfate and surface groups for a coverage ratio close to 1. (18)Vivien, D.; Livage, J.; Mazierea, C. J. Chim. Phys. 1970,67,199. (13) Clarfield, A. Reu. Pure Appl Chem. 1964, 14,91. (14) Jaulmea, S. Reu. Chim. Min6ral. 1965,2, 147. (16)h n ,M. J. Surfactants and Interfacial Phenomena;J. Why: New York, 1978; p 63.
Randon et al.
2656 Langmuir, Vol. 7, No. 11,1991
,o
Figure 5. pH variation during adsorption of sulfate ions on zirconia against degree of coverage 8 = I'/I', at 20 O C . 120.
100-
cmo
m
Roo
1Mo
la m'
80.
wavelength
Figure 3. Infrared spectra of (a) Na&Od, (b) zirconia powder, and (c) zirconia powder after adsorption of 5042-ions. 0
°
1
7 I ' 0
v
sulfate adsorbed (pmol/g)
D
:: 0
Figure 6. Displacement of the chloride ions released from the zirconia surface by sulfate ions at 20 O C .
6050.
I1
%
40..
30-
1
This is much larger than the theoretical value which is about 35 A2.'5 The result confirms the existence of unoccupied surface on the ZrO2 particles. We also observe a release of chloride ions from the synthesis. Some ZrOHz+ groups are complexed by C1-. Sulfate ions can shift the chloride equilibrium and the surface reacts as an ion exchangerle ZrOH2C1+ 50:-
*=)
ZrOSO,
+ C1- + H20
(6)
The plot of chloride ions released vs sulfate ions adsorbed is a straight line relationship with a slope of 1.3 (Figure 6). A lower number of sites can be occupied by the sulfate ions than by the chloride ions, indicating that the capacity of the surface for the latter ions is relatively high. Adsorption Equilibrium Constant. In a previous paper,17 we determined pK1 and pK2 intrinsic acidity constants: pKPt = 8.3and p K P = 6.2. These data are in good agreement with those quoted in the literature in spite of the differences in the method of synthesis (for overview see ref 18). (16)Stumm, W.;Kummert, R.;Sigg, L. Croat. Chem. Acta 1980,59, 291.
(17)Perain, M.;Randon, J.; Sarrazin, J.; Larbot, A.; Guizard, C.; Cot, L. Submitted for publication in Can. J. Chem. (18)Randon, J.; Larbot, A,; Guizard, C.; Cot, L.; Lindheimer, M.;Partykn, s. Colloid8 Surf. 1991,52, 241.
Langmuir, Vol. 7, No.11, 1991 2657
Sulfate Adsorption on Zirconium Dioxide 2001
111
160
+
1 0 3 ~
1401
7-++ I a
6"
e***
*
,+,'
******* **
*** .
.
6!0 012
014
Of6 Of8 110
1i2 1j4
without 10-3~
1:s
I
lf8
V (ml) NoOH 0.1M
Figure 7. Titration curve of ZrOz suspension without (.) and M, *, 104 M)at 20 with various sulfate concentrations (+,
Figure 8. Proton adsorption vs the sulfate adsorption at 20 "C.
"C.
Figure 7 represents titration curves of a suspension of ZrOz with and without various sulfate concentrations. The method according to Huang and StummIghas been used for calculation of the complex constant. For a ZrOz suspension in sulfate electrolyte we have equilibrium 5 with [ZrOSO,] [OH], (7) Ks04= [ZrOH][SOt-], We have to omit the s index (characteristicof the surface) in the subsequent part of the text because we will determine an apparent complex constant. According to Huang, the difference between the amounts of NaOH added, at constant pH, is equal to the amount of SOrZ-associated with the surface groups. The adsorbed H+ is given by the difference between the amount of NaOH necessary to reach pH,, without sulfate ions and the amount of NaOH necessary to reach the pH value in the presence of the adsorbed Sod2- ions. The pH of the suspension increases with in~reasingS04~concentration. This change is due to the release of OHions which can react with H+ present in the bulk solution. Therefore, the number of H+ ions decreases. From the above relationships, we obtain the amount of adsorbed s04'- vs H+ adsorbed or OH- released (Figure 8). With [C]= [ZrOHz+]+ [ZrOSO3-],equaltotheamount of H+ on the surface or OH- released by the surface, we obtain the following relationships: [ZrOSO,] = [C]
P=
B[s0,2-1
+
1 /3[so,2-]
[ZrOSO,] [ZrOH,] [SO:-]
Figure 8 shows a straight line in agreement with 1:l complex. The deviation for 10-3 M concentration corresponds to the complete adsorption of sulfate ions. To determine the Kso, constant we plot log ([ZrOSO3-]/[ZrOH]) vs log ([OH-]/[S04z-]) according to eq 7. This graph is represented in Figure 9. It intercepts the Y axis at -2.6 which is the log of the equilibrium (19)Huang, C. P.;Stumm, W.J. J . Colloid InterfaceSci.976,55,281. (20)Partyh, S.;Lindheimer, M.; Zaini, S.;Keh, E.;Brun, B. Lang-
mutr 1986,2, 101.
-
'25.0
-4.5
-4.0
-3.5 -3.0 -2.5 log([OH-]/[S04=])
-2,O
-1,s
-1.0
Figure 9. Determination of complexation constant by Huang and Stumm's method at 20 O C .
constant. However, this is not an intrinsic constant because the surface potential factor exp(-e3/kT) hasbeen omitted from the calculation. This value is a good approximation to predict adsorption and stability of colloid oxides. The log Kso, value determined as [ZrOSOa-],[ZrOHz+], and [S042-] in eq 7 is 2.4. We obtain a value of pKso, = 2.5 f 0.2. Calorimetric Adsorption. The variations of the differential (Ahad) and integral molar enthalpies (Ah'i) of adsorption with the degree of coverage (0 = IY/I'-) are shown in Figure 10. The integral molar enthalpy is the experimental enthalpic effect Q measured to reach a degree of coverage of the solid by the sulfate ions, the effect being normalized per mole of adsorbed sulfate ion. The differential molar enthalpy is, for a given B value, the derivative of the experimental enthalpic effect as a function of the number of adsorbed sulfate ions ( n e )
Ahad= dQ/dna Ahai= Q/na Ahadis exothermic at low degree of coverage. At higher coverages, Ah'd becomes endothermic. When the coverage approaches unity, the enthalpy of adsorption is close to zero.20 The principal reaction occurring in the adsorption process is ZrOH + H++ s04'ZrOSO3- + HzO but ZrOH ++ZrO- + H+ is a simultaneous reaction which cor-
-
Randon et al.
2658 Langmuir, Vol. 7, No. 11, 1991
attributed to the adsorption of individual sulfates ions onto the most active zirconia sites. The AG of the sulfate adsorption is always negative. At coverages above 0.35 there is progressively further adsorption with positive Ahad. At this stage of the adsorption the entropic effect is the principal driving force for the adsorption. At the same time, ion exchange phenomena bring about modifications in water structure at the interface. Indeed, SO42- is responsible for the destruction of the organized water. Conversely, the OH- group seems to promote formation of structured water while C1- ions appear to have little effect.2l
E
3
Conclusion -8!O
0,l
0,2 0.3 0,4 0,5 0,6 0,7 0,8 0,9 1!0 0
Figure 10. Integral and differential molar enthalpies of adsorption with the coverage degree of sulfate ions performed at 25 O C .
responds to protonation constant K2. The above phenomena exist at all coverages and are dominating at different coverage ratios. However, it is still impossible to determine quantitatively the enthalpic contribution of each reaction during the adsorption process. At the same time, variations are observed in the hydration sphere of ions and in the reorganization of water near the interface. Unfortunately the effect of hydration cannot be experimentally quantified and taken into account. The initial decrease in the enthalpy is certainly due to the occupation of energetically heterogeneous surface sites. The adsorption occurs initiallyon the most energetic sites. The extrapolated Ahad value for 6 = 0 (10.3 kJ/mol) is
Adsorption of sulfate at the water-oxide interface has been investigated using a fine ZrOz powder prepared by the sol-gel process. The results are in agreement with the general properties of oxides quoted in the literature. After determination of the mode of complexation onto the surface by FTIR, the extent of adsorption has been quantified. Ion exchange capacity is clearly demonstrated by the chromatographic technique. The complexation constant was determined as log K = -2.5. However, the adsorption of sulfate ions is not as straightforward as the constant seems to imply. Surface structure appears to be an important parameter for interpreting calorimetric results. We believe that knowledge of all the possible variations in the structure of the oxide surface will be necessary to optimize membrane performance. (21) Frank, H.S.;Wen, W.Y.Discus Faraday SOC.1957,24, 133.
