Effect of Thermal Activaton on the Exchange Behavior of AlPO4

Ind. Eng. Chem. Res. 2008, 47, 5427–5432

5427

MATERIALS AND INTERFACES Effect of Thermal Activaton on the Exchange Behavior of AlPO4 Toward Cu2+ Ions Syed Mustafa,* Muhammad Javid, and Muhammad Iqbal Zaman National Centre of Excellence in Physical Chemistry, Peshawar UniVersity, Peshawar- 25120, NWFP, Pakistan

Cu2+ ion adsorption and potentiometric titration studies were carried out on AlPO4 as a function of pH, temperature, and activation. Two different samples thermally treated at 105 and 400 °C were employed for the sorption and potentiometric titration studies. The dissociation constant values (pKa) showed an increase with thermal activation, indicating a decrease in the surface acidity of AlPO4. Cu2+ ion adsorption was found to increase with pH, temperature, and thermal activation of the solid. The Freundlich equation was found to be applicable to the sorption data. Introduction In recent years, because of limited water resources, the idea of water reuse has become very important. Several techniques such as ion exchange, adsorption on activated carbon, biological and chemical precipitation, and reverse osmosis have been used for the removal of toxic substances from municipal wastewaters. However, the ion-exchange and adsorption methods have proven to be more effective because of their low cost, effective treatment of dilute solutions, high uptake capacities, faster regeneration kinetics, and greater selectivities. Heavy metals such as copper, cadmium, nickel, lead, and cobalt exist widely in industrial effluents such as those of the metallurgy, tannery, and mining fields. Such waters need to be purified before consumption by using different physicochemical processes such as filtration, precipitation, crystallization, reverse osmosis, and sorption/ion exchange. Among these different physicochemical processes, sorption/ion exchange offers the best prospects for the overall treatment of wastewaters, especially for effluents that contain moderate and low concentrations of metals cations. Until recently, sorption behavior was thought to be confined to a very limited number of inorganic compounds, namely, clays and zeolites, both natural and synthetic.1 However, as work in this area has progressed, it has been recognized that ionexchange properties are exhibited by many different classes of compounds such as metal oxides/hydroxides, metal phosphates, and metal arsenates.2 Apart from sorption properties, metal phosphates have a variety of other uses such as in nuclear medicine,3 proton conductors,4 catalysts,5 reactor cooling water systems, and protective coatings for steel automotive parts prior to painting.6,7 Boron phosphate has been widely used as a catalyst in isomerization reactions8 and for the dehydration of various organic products.5 Zirconium phosphate (ZrP) is used as a catalyst for the synthesis of various organic products.5 Cation-substituted ZrP catalyzes a wide variety of hydrogenation, hydrolysis, polymerization, and oxidation reactions.9 In recent years, aluminophosphate molecular sieves and their metalsubstituted analogues (denoted MeAlPO4) have been the focus of growing interest from researchers because of their potential use in the fields of catalysis and ion exchange.10–12 * To whom correspondence should be addressed. Tel.: 091-9216766. Fax: 091-9216671. E-mail: [email protected].

Recently, metal(III) phosphates have also been shown to act as ion exchangers.13,14 These metal phosphates exist in various amorphous, crystalline, and intermediate states of crystallinity and exhibit pH-dependent surface properties, so they can be considered as important sinks for trace metal ions in soils.15,16 Aluminum, chromium, and iron phosphates have the ability to take up divalent metal ions such as Cu2+, Ca2+, Ni2+, and Zn2+ ions.13 However, the sorption properties of aluminum(III) phosphate16 are the least studied, and specifically, no data have yet been reported on the effects of heat treatment on the sorption of metal ions by aluminum(III) phosphate. The aim of the present study is, therefore, to investigate the sorption properties of two different AlPO4 samples treated at 105 and 400 °C, toward Cu2+ ion sorption as a function of concentration, pH, and temperature. Materials and Methods Materials. The AlPO4 used in this study was synthesized by mixing solutions of aluminum nitrate and trisodium phosphate, each 0.5 M in concentration, according to the reaction Al(NO3)3 + Na3PO4 S AlPO4 V + 3NaNO3

(1)

The details of the method for AlPO4 are given elsewhere.17 The thus-prepared samples were stored in Pyrex bottles in a desiccator for further studies including surface area, point of zero charge, and X-ray diffractometry. Point of Zero Charge (PZC). The point zero charge (PZC) was determined by the method of Kinniburgh et al.18 for both aluminum(III) phosphate samples treated at 105 and 400 °C. Dissolution Studies. Dissolution studies were performed for nonactivated and activated AlPO4 samples at 303 K in the pH range 2-9. Thirty milliliters of 0.1 M KNO3 was taken in 50mL conical flasks, and 0.2 g of the solid was added to each flask. The pH values of the suspensions were adjusted from 2 to 9 with either KOH or HNO3 and were shaken for 24 h at 303 K in an end-to-end shaker bath. The equilibrium pH values of the suspensions were then recorded, and the concentrations of aluminum and phosphate ions dissolved were determined by the methods described in the literature.19,20 The same procedure was adopted for the dissolution of the solid in the presence of divalent metal ions under investigation.

10.1021/ie070533h CCC: $40.75  2008 American Chemical Society Published on Web 07/03/2008

5428 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008

Potentiometric Titrations. Potentiometric titrations of AlPO4 in Cu2+ ions were performed in a doubled-walled Pyrex glass cell connected to a thermostatted water bath via a watercirculating pump. Thirty milliliters of Cu2+ solution with 0.1 M KNO3 as the background electrolyte was taken in the cell at a constant temperature maintained by circulating water from a thermostat. After 30 min of equilibration, 0.2 g of AlPO4 was then added to the cell solution. The initial pH of the suspensions was adjusted to pH 4 with 0.1 M HNO3. The suspensions were then allowed to equilibrate for 30 min, with constant stirring, and any changes in the pH were readjusted with standard HNO3 or KOH. The titrations were then carried out by the addition of 0.2 mL of 0.1 M KOH solution using a microburette with a very fine capillary tip, recording the pH after every 2-min interval as a function of the volume of alkali added until the final pH reached 10. Adsorption Studies. The adsorption studies of Cu2+ ions on AlPO4 were performed in an end-to-end shaker bath (Labortechnic type LE-209) provided with hooks for 50-mL conical flasks. Different concentrations of Cu2+ ions with 0.1 M KNO3 as the background electrolyte were prepared in doubly distilled water. Before the beginning of an experiment, 0.2 g of AlPO4 was taken in a 50-mL Pyrex glass flask, to which 30 mL of Cu2+ ion solution with 0.1 M in KNO3 was added. The initial pH of each suspension was recorded and adjusted to the desired pH value of 4 or 5 by the addition of standard HNO3 or KOH. The flasks were then transferred into the shaker bath for 24 h at constant temperature. After 24 h of equilibration, the equilibrium pH values were noted. The suspensions were filtered, and the filtrate was analyzed for the equilibrium concentration of Cu2+ ions using an atomic absorption spectrometer (Perkin-Elmer model 3100). The amounts of metal ions sorbed were computed from the difference between the initial and equilibrium concentrations of the metal ion.

Figure 1. Release of (a) aluminum and (b) phosphate ions as a function of pH from nonactivated and activated AlPO4 at 303 K.

Results and Discussion Characterization of the Solid AlPO4. The surface area of the powdered AlPO4 was found to increase with activation from 95 to 128 m2 g-1. The number of water molecules determined by the weight loss method were found to be 3.0 and 1.5 molecules per unit formula for nonactivated and activated AlPO4, respectively. The X-ray diffraction patterns showed that both the AlPO4 samples were amorphous in nature. Further, the point of zero charge was found to increase from 3.45 to 5.1 with heat treatment. Dissolution of AlPO4. Plots of aluminum and phosphate ions released via hydrolysis of the exchanger are presented in Figure 1. This figure shows that the extent of AlPO4 dissolution is minimum in the pH range of 4-8. The amount of Al3+ ions released is less than the amount of phosphate ions released. This behavior was similar for both nonactivated and activated AlPO4. Comparison of the dissolution of activated and nonactivated AlPO4 indicates that the activation has an essentially negligible effect on the hydrolytic stability of the AlPO4 in the pH range of 4-9. Potentiometric Titrations. The potentiometric titration curves in Figure 2 for activated AlPO4 in the presence of Cu2+ ions show a shift toward the lower pH region that increases further with increasing Cu2+ ion concentration. This increasing shift in the titration curves might be the result of the enhanced sorption and hydrolysis of Cu2+ according to the reactions Cu2+ + H2O S Cu(OH)+ + H+

(2)

Figure 2. Effect of Cu2+ concentration on the potentiometric titration curves of activated AlPO4 at 303 K.

RH + CuOH S RCu(OH) + H+ 2RH + Cu

2+

2+

S (R)2Cu

+

+ 2H

(3) (4)

where RH is the adsorbent capable of exchanging protons for the metal cations from aqueous solution. To estimate the

Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 5429

range 4-5 on AlPO4 that is subsequently used for the sorption studies is almost negligible. A comparison of the titration curves in Figure 3 shows that the curves rise early in the case of activated AlPO4. This indicates that the liberation of H+ ions from activated AlPO4 is lesser than that from nonactivated AlPO4. This shows that activated AlPO4 is a weaker acidic exchanger than nonactivated AlPO4. A similar decrease in ion exchange with heat treatment was reported21 for zirconium phosphate. Effect of Temperature. The pH titration curves of AlPO4 in Figure 3 shows the effect of temperature to be very small in the early stages of titration. However, at high pH, the effect becomes prominent, and the curves are increasingly shifted toward the lower pH region, pointing toward an increase in the dissociation of H+ ions from the surface. This behavior of the curves is similar in both the nonactivated and activated AlPO4. Further, Figure 3 shows that the shift for activated AlPO4 is lower than that for nonactivated AlPO4, indicating that the release of protons from activated AlPO4 is lower than that from nonactivated AlPO4. Thus, it is concluded that activation decreases the acidity of AlPO4, which has a low H+ cation-exchange capacity that, however, increases with increasing concentration of metal ions in solution and temperature of the system. Determination of pKa. The dissociation of the exchanger can be assumed to takes place according to the reaction RH S R- + H+

(4a)

Using the law of mass action, one can write for the equilibrium constant as [R-][H+] (4b) [RH] If the degree of dissociation R is taken equal to R-/θ, where θ is the total number of OH ionizable groups, the above equation for dissociation constant becomes Kc )

Figure 3. Potentiometric titration curves of Cu2+ ions (7.87 × 10-4 mol L-1) for activated and nonactivated AlPO4 at different temperatures: (a) 303 and (b) 323 K.

Kc )

R[H+] (1 - R)

(4c)

and pKc ) pH - log

R 1-R

(4d)

j. if it is assumed22,23 that pH ) pH As pKc depends on the degree of dissociation, then an apparent constant can be determined by integrating the expression for pKcbetween the R limits of 0 and 1 pKa )

Figure 4. Plots of pKc vs R of 7.87 × 10-4 mol L-1 Cu2+ ions for activated AlPO4 at different temperatures.

hydrolysis of Cu2+ ions, the computer program Visual Minteq was used, which showed that, for a 15.74 × 10-4 mol/L Cu2+ solution, 99.93% of the Cu2+ was present in unhydrolyzed form, i.e., as Cu2+. This shows that the effect of hydrolysis in the pH

∫

1

0

pKc dR

(5)

Plots of pKc vs R are presented in Figure 4 for activated AlPO4. Similar curves were obtained for nonactivated AlPO4. R was taken as 0 at pH 4, the pH at which the titration was started. This assumption was made as the solid AlPO4 was found to be unstable below pH 4. As such, the pKa values determined are apparent in nature and can be used only for comparative purposes. The pKa values (Table 1) for AlPO4 in the presence of Cu2+ ions at various temperatures and concentrations were determined from the area under the curves of the respective plots by using a third-order polynomial equation with a coefficient of correlation in the range of 0.98-0.99. It is interesting to note that the pKc values are independent of temperature for low values of R, particularly in the case of activated AlPO4. This suggests that the screening effect of the

5430 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 Table 1. Effects of Concentration and Temperature on the pKa Values for Nonactivated and Activated AlPO4 [Cu2+] ) 7.87 × 10-4 mol L-

T ) 303 K Cu2+ ion conc (mol L-1) 3.15 × 10-4 7.87 × 10-4 15.74 × 10-4

nonactivated

activated

temperature (K)

nonactivated

activated

7.85 7.20 7.07

7.88 7.64 7.41

293 303 313 323

7.49 7.20 7.10 6.97

7.72 7.64 7.39 7.23

Cu2+ ions on the ionized activated AlPO4 is higher because of a decrease in the surface sOH groups as a loss of surface water molecules. The pKa values thus determined also indicate the exchanger to be of weaker acid type. It is observed that the pKa values decease with increasing concentration of Cu2+ ions and temperature, which shows an increase in the ability of the protons to dissociate from the surface of the solid. It is further noted that the pKa values increase with activation, confirming that the activated AlPO4 is more weakly acidic in nature than the nonactivated AlPO4. Sorption Studies. Effects of pH and Concentration. The effects of pH and concentration on the sorption of Cu2+ ions on both nonactivated and activated AlPO4 can be seen in Figures 5 and 6, which show that the sorption of Cu2+ ions increases with the concentration of Cu2+ ions and the pH of the solution. These figures also show that the effects of both the concentration and pH are much more prominent for activated AlPO4. The Cu2+ ion sorption almost doubles upon activation, which might be due to the dehydration of the solid, i.e., the loss of interstitial and chemical bound water, which increases both the surface area and the number of sites available for sorption. These observations are similar to those reported recently for zirconium oxide.24 It is also observed from Table 2 that, in the case of activated AlPO4, the sorption of Cu2+ ions leads to an increase in the pH up to pH 4, which shows that the activated solid, being a weaker acid than the nonactivated solid, has a higher affinity for H+ ions than for Cu2+ ions. The same conclusions can be drawn from the increase of the PZC of the solid with activation. However, a decrease in the pH values of the system is noted at pH 5 (Table 2), showing that the exchanger now prefers Cu2+ ion as compared to H+ ions. Comparing the pH changes of the activated and nonactivated AlPO4 samples, it is observed that the release of H+ ions accompanying metal ion sorption is much lower for activated AlPO4. This indicates that, in addition to ion exchange, the sorption of Cu2+ ions takes place on the active sites created by activation of the solid.

Keeping in view all of these observations, the mechanism of sorption on activated and nonactivated AlPO4 can be assumed to occur according to Schemes 1 and 2, respectively. The AlOH and POH groups formed on the surface through the chemisorption of water are responsible for the H+ exchange with Cu2+ ions, whereas Scheme 2 represents the activated form of AlPO4 in which most of the physisorbed and chemisorbed waters have been removed through the treatment at high temperature.24 The lone pair of electrons present on the oxygen is probably responsible for the bonding of metal cations to the surface in the case of activated AlPO4. Some molecular sorption of the metal cations into the pores of the solid, which are formed by activation, might also accompany the uptake process. Similar models for metal phosphates have also been proposed in the literature.25,26 Effect of Temperature. Figure 6 shows the sorption to increase with temperature for both the activated and nonactivated AlPO4. As expected, an increase in the temperature in the case

Figure 5. Sorption isotherms of Cu2+ ions on activated AlPO4 at 303 K and pH 4 and 5.

Figure 6. Sorption isotherms of AlPO4 toward Cu2+ ions at pH 5 and different temperatures: (a) 303 and (b) 323 K.

Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 5431 Table 2. Effect of Temperature on pH Changes for Activated and Nonactivated AlPO4 at pHi ) 4 and 5a pHe values for pHi ) 4

Scheme 2. Mechanism of Sorption on Activated AlPO4

pHe values for pHi ) 5

+2

nonactivated activated nonactivated activated initial Cu conc × 104 (mol L-1) 293 K 323 K 293 K 323 K 293 K 323 K 293 K 323 K 1.57 3.15 4.72 6.29 7.88 9.44 11.01 a

3.967 3.986 3.981 3.952 3.975 3.958 3.942

3.641 3.531 3.501 3.472 3.462 3.463 3.481

4.654 4.412 4.428 4.484 4.475 4.385 4.480

4.669 4.532 4.460 4.345 4.268 4.234 4.179

4.589 4.573 4.435 4.458 4.435 4.358 4.430

4.010 4.010 4.021 3.960 3.790 4.160 3.920

4.905 4.898 5.054 4.957 4.825 4.873 4.842

4.831 4.822 4.837 4.778 4.651 4.582 4.541

pHi ) initial pH and pHe ) equilibrium pH.

Scheme 1. Mechanism of Sorption on Nonactivated AlPO4

of nonactivated AlPO4 would lead to an increase in the dissociation of H+ ions from the weakly dissociated surface sOH groups, resulting in an increase in the uptake of Cu2+ ions from solution. However, for activated AlPO4, an increase in temperature would also favor the adsorption of the Cu2+ cations by complexation according to Scheme 2, as well as adsorption in the channels and pores created by the activation of the solid. The changes in pH with temperature can be seen from Table 2, which shows that, for both activated and nonactivated AlPO4, the pH decreases with increasing temperature, indicating an increase in the ion-exchange sorption of Cu2+. The results of comparative sorption studies of the Cu2+ at pH 5 and different temperatures are shown in Figure 6, which indicates that the extent of sorption almost doubles upon activation of the solid AlPO4 at all temperatures under investigation. The enhanced sorption of Cu2+ ions with activation as discussed earlier might be due to the dehydration of the aluminum phosphate, an increase in the surface area of the solid, and a change in the mechanism with heat treatment at 400 °C. Freundlich Plots. Various conventional isotherms such as the Langmuir, Freundlich, and Kurbatov models were applied to the data. However, only the well-known Freundlich sorption isotherm was able to explain the data. This equation can be written in the form 1 log Ce + log K (6) n where Γ (mmol g-1) is the amount of metal ions sorbed, Ce (mmol L-1) is the equilibrium concentration of metal ions in solution, K (L g-1) is the equilibrium constant, and 1/n is a constant that accounts for the heterogeneity of the surface sites. Figure 7 shows the linear variation of log Γ vs log Ce with coefficient of correlation values between 0.95 and 0.99. The values of 1/n and K calculated from the slopes and intercepts of the plots are reported in Table 3. As can be seen, both 1/n and K increase with pH, temperature, and activation. Further, the K values for activated AlPO4 are much higher than those observed for nonactivated AlPO4, in agreement with the increased Cu2+ ion sorption and complexation by the solid. log Γ )

Conclusions From the above discussion, it can be concluded that the AlPO4 · H2O has an appreciable sorption capacity for Cu2+ ions,

Figure 7. Plot of the Freundlich equation for Cu2+ ion sorption on AlPO4 at pH 5 and different temperatures: (a) nonactivated and (b) activated. Table 3. 1/n and K × 102 (L g-1) Calculated from the Freundlich Equation for Cu2+ Ion Sorption by AlPO4 K × 102 (L g-1)

1/n nonactivated

activated

nonactivated

temperature (K)

pH4

pH5

pH4 pH5

pH4

293 303 313 323

0.83 0.83 1.06 1.67

1.03 1.15 1.16 1.32

1.59 1.56 2.33 2.38

pH5

activated pH4

pH5

2.08 9.25 11.87 6.54 2.44 11.09 12.39 8.76 2.44 11.87 18.02 9.95 2.22 3.10 24.28 10.52

13.19 15.40 20.65 22.37

which is increased further by the activation of the solid. The surface acidity of the OH groups decreases with heat treatment because of a loss of water molecules. Temperature also has a positive effect on adsorption on both activated and nonactivated AlPO4. Further ion exchange is the main mechanism for Cu2+

5432 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008

ion sorption on nonactivated AlPO4. However, Cu2+ ion complexation is responsible for Cu2+ ion sorption in activated AlPO4. Literature Cited (1) Jakubov, T. S.; Mainwaring, D. E. Adsorption-Induced Dimensional Changes of Solids. Phys. Chem. Chem. Phys. 2002, 4, 5678. (2) Clearfield, A. Role of Ion Exchange in Solid State Chemistry. Chem. ReV. 1988, 88, 127. (3) Bortun, A. I.; Bortun, L. N.; Khainakov, S. A.; Clearfield, A. Ion Exchange Properties of the Sodium Phlogopite and Biotite. SolV. Extrn. Ion Exch 1998, 16, 1067. (4) Clearfield, A. Inorganic Ion Exchangers, Past, Present and Future. SolV. Extrn. Ion Exch. 2000, 18, 655. (5) Clearfield, A.; Thakurd, D. S. Zirconium and Titanium Phosphate as Catalysts: A Review. Appl. Catal. 1986, 26, 1. (6) Rausch, W. Die Phosphatierung Von Metallen, 2nd ed.; Leuze Verlag: Saulgau, Germany, 1988; Chapters 3 and 4. (7) Tegehall, P. E. The Mechanism of Chemical Activation with Titanium Phosphate Colloids in the Formation of Zinc Phosphate Conversion Coatings. Colloids Surf. 1990, 49, 373. (8) Gao, S.; Moffat, J. B. The Isomerization of 1-Butene on Stoichiometric and Nonstoichiometric Boron Phosphate: The Dependence of the Acidity on Stoichiometry. J. Catal. 1998, 180, 142. (9) Clearfield, A. Inorganic Ion Exchange Materials; CRC Press, Inc.: Boca Raton, FL, 1982. (10) Prakash, A. M.; Hartmann, M.; Kevan, L. Synthesis, Characterization, and Adsorbate Interactions of CoAPO-41 and CoAPSO-41 Molecular Sieves. J. Phys. Chem. B. 1997, 101, 6819. (11) Schulz, M.; Tiemann, M.; Froba, M.; Jager, C. NMR Characterization of Mesostructured Aluminophosphates. J. Phys. Chem. B. 2000, 104, 10473. (12) Tiemann, M.; Froba, M.; Rapp, G.; Funari, S. S. In Situ Small Angle Scattering (SAXS) Studies on the Formation of Mesostructured Aluminophosphates/Surfactant Composite Materials. Stud. Surf. Sci. Catal. 2000, 129, 559. (13) Mustafa, S.; Naeem, A.; Murtaza, S.; Rehana, N.; Samad, H. Y. Comparative Sorption Properties of Metal(III) Phosphates. J. Colloid Interface Sci. 1999, 22, 63.

(14) Anthony, S. R. J.; Ellis, B. G. Chemical and Physical Properties of Iron and Aluminum Phosphates and Their Relation to Phosphorous Availability. Soil Sci. Soc. Am. Proc. 1968, 32, 216. (15) Campelo, J. M.; Marinas, J. M.; Mendioroz, S.; Pajares, J. A. Texture and Surface Chemistry of Aluminum Phosphates. J. Catal. 1986, 101, 484. (16) Mustafa, S.; Javid, M.; Gul, R.; Murtaza, S. The Mechanism of Co2+ Ion Sorption by AlPO4. EnViron. Technol. 2002, 23, 1173. (17) Mustafa, S.; Javid, M.; Zaman, M. I. Effect of Activation on the Sorption Properties of AlPO4. Sep. Sci. Technol. 2006, 41, 3467. (18) Kinniburgh, J. D.; Syres, J. K.; Jackson, M. L. Specific Adsorption of Trace Amount of Calcium and Strontium by Hydrous Oxides of Iron and Aluminum. Soil Sci. Soc. Am. Proc. 1975, 39, 161. (19) Clesceri, L. S.; Greenberg, A. E.; Trussell, R. R. Standard Methods for the Examination of Water and Wastewaters, 17th ed.; American Public Health Association: Washington, DC, 1989; Section 3-65. (20) Murphy, J.; Riley, J. P. A Modified Single Solution Method for the Determination of Phosphate in Natural Water. Anal. Chem. Acta 1962, 27, 31. (21) Clearfield, A.; Stynes, J. A. The Preparation of Crystalline Zirconium Phosphate and Some Observations on Its Ion Exchange Behaviour. J. Inorg. Nucl. Chem. 1964, 26, 117. (22) Gaines, G. L., Jr.; Thomas, H. C. Adsorption Studies on Clay Minerals. II. A Formulation of the Thermodynamics of Exchange Adsorption. J. Chem. Phys. 1953, 2, 714. (23) Kenta, O.; Yoshitaka, M.; Shunsaku, K.; Hiroshi, M.; Mitsuo, A. The pH Titration Study of Lithium Ion Adsorption on λ-MnO2. Bull. Chem. Soc. Jpn. 1988, 61, 407. (24) Tarnopolsky, V. A.; Aliev, A. D.; Churagulov, B. R.; Burukhin, A. A.; Novikova, S. A.; Yaroslavtsev, A. B. Influence of Thermal Treatment on the Ion Transport Properties of Hydrated Zirconia. Solid State Ionics 2003, 162-163, 225. (25) Moffat, J. B.; Neeleman, J. F. Infrared Spectroscopic Studies of Boron Phosphate and Adsorbed Species. J. Catal. 1974, 34, 376. (26) Veith, J. A.; Sposito, G. On the Use of the Langmuir Equation in the Interpretation of Adsorption Phenomena. Soil Sci. Soc. Am. J. 1977, 41, 697.

ReceiVed for reView April 16, 2007 ReVised manuscript receiVed May 14, 2008 Accepted May 19, 2008 IE070533H