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Sorption Studies of Divalent Metal Ions on ZnO S. Mustafa,* P. Shahida, A. Naeem, B. Dilara, and N. Rehana National Centre for Excellence in Physical Chemistry, University of Peshawar, Pakistan Received October 6, 2000. In Final Form: October 30, 2001 The sorption of Zn2+, Ni2+, and Co2+ on ZnO was studied as a function of pH, temperature, and concentration of the metal cations. The extent of sorption was found to increase with the increase in pH and concentration and decrease with the increase in temperature. The potentiometric titrations data showed that the sorption mechanism of the transition metal cations changed from adsorption to precipitation with the increase in pH. The sorption and precipitation of the metal cations were explained with the help of a new equation derived from the law of mass action. FTIR studies revealed the formation of a double hydroxide of Zn and metal ions concerned on the surface of the solid. The dissolution studies of the ZnO were also employed to evaluate the sorption mechanism of the metal cations on the ZnO. The decrease in the extent of adsorption of the metal cations with temperature was due to an increase in dissolution of the ZnO.
Introduction Metal oxides and hydroxides are commonly found in natural aqueous systems as a discrete mineral phase and as surface coatings on particulate matter.1-17 When immersed in aqueous solution, they develop surface electrical charges by the protonation and deprotonation of the surface hydroxyl groups. They are, thus, considered important scavengers of the metal ions and play an important role in regulating their concentrations in the natural aquatic systems. While a number of studies have been reported about the adsorption of metal cations on iron oxide/hydroxide, aluminum oxide, silica titania, etc., very little is reported about the metal ion adsorption on the other important divalent metal oxides/hydroxides such as copper oxide, nickel oxide, cobalt oxide, and zinc oxide.11-17 ZnO employed as an adsorbent in the present study is also of considerable practical interest, as a common electrode material in semiconductor electrochemistry and photoelectron chemistry. In addition, it is widely used in * To whom correspondence may be addressed. E.mail:
[email protected]. (1) Schwarlz, S.; Lunkwitz, K.; Kessler, B.; Spiegler, U.; Killman, E.; Jaeger, W. Colloids Surf., A 2000, 163, 17. (2) Rodriguez, A.; Jirsak, T.; Chaturvedi, S.; Akuhn, M. J. Surf. Sci. 1999, 400, 442. (3) Marinelli, F.; Grillet, Y.; Pellenq, R. J. M. Mol. Phys. 1999, 1207, 97. (4) Mishra, S. P.; Singh, V. K. J. Radioanal. Nucl. Chem. 1999, 145, 241. (5) Yang, J. K.; Davis, A. P. J. Colloid Interface Sci. 1999, 77, 216. (6) Winiarek, P.; Kijenski, J. J. Chem Soc., Faraday Trans. 1998, 94, 167. (7) Dorfner, K. Ion Exchangers; Walter de Gruyter: Berlin, New York, 1991. (8) Sylvester, P.; Behrens, E. A.; Srziano, G. M.; Clearfied, A. Sep. Sci. Technol. 1999, 34, 1981. (9) Mustafa, S.; Shahida, P.; Aftab, A.; Dilara, B. Adsorpt. Sci. Technol. 1997, 15, 789. (10) Mustafa, S.; Dilara, B.; Neelofar, Z.; Naeem, A.; Tasleem, M. J. Colloid Interface Sci. 1998, 204, 284. (11) Leyraramos, R.; Fuentesrubio, L.; Guerrerocoronado, R. N.; Mendozabarron, J. J. Chem. Technol. Biotechnol. 1995, 62, 64. (12) Aronson, B. J. J. Phys. Chem. B 2000, 104, 449. (13) Ahmed, S. M.; Maksimov, D. J. Colloid Interface Sci. 1969, 29, 97. (14) Rodriguez, J. A.; Jirsak, T.; Dvorak, J.; Sambasivan, S.; Fischer, D. J. Phys. Chem. 2000, 104, 319. (15) Klun, U.; Zupan, J.; Solmajer, T. Macromol. Theory Simul. 1999, 8, 492. (16) Huang, C. P. In Adsorption of inorganic at Solid-liquid Interface; Anderson, M. A., Rubin, A. J., Eds.; Ann. Arbor: MI, 1981; Chapter 5. (17) Chittorati, A.; Matijevic, E. J. Colloid Surf. 1990, 48, 65.
rubber, vulcanization, protective paints, ceramics, textiles, electrographic copying, and pharmaceuticals.11 Further, while the effect of pH on the adsorption of metal cations is now well documented in the literature, the effect of temperature is yet to be studied in detail. The purpose of the present study was, therefore, to investigate adsorption properties of ZnO toward divalent metal ions under different experimental conditions of concentration, temperature, and pH of the aqueous solutions. The study is the first of its kind to our knowledge and has not been reported earlier. Experimental Section All the reagents used were of AR grade and were used without further purification. All the solutions were made with doubly distilled water using Pyrex glass vessels. KOH and HNO3 were used for adjusting pH of the solutions/suspensions, whenever required. KNO3 was employed as the background electrolyte. Purification of ZnO. Commercially available ZnO (BDH) contains chloride, sulfate, and phosphate ions as impurities. To remove them, 100 g of the oxide was suspended in distilled water for 6 h at 323 K with constant stirring. The suspension was dialyzed at 323 K for 7 days with daily renewal of distilled water. Afterward, ZnO was again stirred for an hour and filtered. The residue on the filter paper was washed until free of chloride, sulfates, and phosphate ions. It was then dried in an oven at 383 K for 24 h, ground to a fine powder, and stored in a stoppered bottle. The surface area of the sample ZnO determined by the BET method was found to be 3.17 m2/g, which is comparable in magnitude with the values reported in the literature.6 Further, the X-ray diffraction studies showed the sample to be amorphous in nature. The FTIR spectra of the sample were recorded with a Perkin-Elmer 16PC FTIR spectrophotometer and are discussed in detail later on. For some studies the ZnO was activated at 400 °C for 4 h using a programmable furnace controller, C 19, Nabertherm. Potentiometric Titrations of ZnO. For a potentiometric titration, 50 mL of the suspension containing 0.2 g of zinc oxide in 0.1 M KNO3 solutions and having different concentrations of metal ion solution was transferred to the titration vessel. pH of the suspension was adjusted to 7 ( 0.01 before starting each titration experiment, using 0.1 M HNO3 solution. The suspension was allowed to equilibrate for 1 h, with constant stirring, at the desired temperature. pH of the suspension was measured using a digital pH-meter, model Orion SA 520, attached with a combined glass electrode. The titration experiments were performed at 303, 313, and 323 K. At each temperature titration was carried out with standardized solution of 0.01 M KOH, using a microburet with a very fine capillary tip. After each addition of base,
10.1021/la0014149 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/09/2002
Sorption of Divalent Metal Ions on ZnO
Figure 1. Potentiometric titration curves of 30 ppm Co2+ ions at various temperatures: (b) 303 K; (2) 313 K; (9) 323 K. the suspension was equilibrated for 5 min with constant stirring, at the end of which the pH changes were less than 0.01 units/ min. For the precipitation studies, 50 mL of the divalent metal ion in 0.1 M KNO3 solution was taken in the reaction vessel and the pH of the solution was adjusted to 5 ( 0.01 before starting each experiment. The rest of the experimental procedure was the same as mentioned above for the potentiometric titrations of the ZnO. Adsorption Study of the Metal Ions. For the adsorption studies 50 mL of the suspension containing 0.2 g of ZnO in 0.1 M KNO3 solution as a background electrolyte and various amounts (5, 10, 15, 20, and 25 ppm) of divalent metal cations were added to the reaction vessel. The initial pH of the solution was adjusted to the desired value by the addition of either 0.1 M KOH or 0.1 M HNO3 solutions. The suspensions were shaken for 24 h on an end-to-end shaker at the desired temperature. The suspensions were then centrifuged, and the supernatant liquids were decanted for determination of the Zn2+ released from the ZnO and the metal ions sorbed from the aqueous electrolyte solutions. For kinetic studies 1 g of solid was added to 1 L of an aqueous solution of 30 ppm Ni2+ at the desired pH and temperature. After different intervals of time, about 1 mL of the solution was taken out and was analyzed for Ni2+ and Zn2+ ions. The concentration of Zn2+, Ni2+, and Co2+ in the solutions was determined with a Perkin-Elmer model 3100 atomic absorption spectrophotometer. The amounts of metal ions sorbed were calculated from the difference between the initial and the equilibrium metal ion concentrations.
Results and Discussion Titration Studies of ZnO. The surface chemistry of an oxide in contact with an aqueous solution of metal cation is determined to a large extent by the dissociation of the surface hydroxyl groups. Both the hydrolysis of the metal cations and surface chemistry of the oxide being pH dependent, it is essential to determine its effect on both the phenomena of adsorption and hydrolysis/precipitation of the metal cation. To determine the effect of pH on metal ion hydrolysis and precipitation, the blank titration curves of Co2+ in the presence of 0.1 M KNO3 are presented in Figure 1. Similar curves were obtained for Zn2+ and Ni2+ ions. As can be seen from this figure, in the case of the metal ions titration a sharp rise in pH in the beginning is observed, which then leads to a limiting value. These limiting values coincide with the appearance of turbidity in solution, showing the beginning of precipitate formation in the system. Further, as can be seen from Figure 2 that the plateau formation signifying the onset of the precipitation follows the trend Zn2+ > Ni2+ > Co2+, which is in agreement with the values of the solubility products of the corresponding hydroxides in aqueous solution reported in the
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Figure 2. Potentiometric titration curves of 30 ppm M2+ ions: (b) Zn2+; (2) Ni2+; (9) Co2+.
Figure 3. Potentiometric titration curves of ZnO in the presence of 30 ppm Ni2+ ions at various temperatures: (b) 303 K; (2) 313 K; (9) 323 K.
literature.18 In the presence of ZnO as shown in Figure 3, the titration curves show a significant increase in the amount of base used to reach a given pH value below the plateau. Similar results were observed for Co2+ and Zn2+ ion sorption during the present investigation. The increase in the amount of the base used is due to both the release of the protons from the surface of the solid and the hydrolysis of metal cations according to reactions 1 and 2, respectively.
M2+ + nRH S RnM2-n + nH+
(1)
M2+ + zH2O S M(OH)2-z + zH+
(2)
Reactions 1 and 2 are generalized reactions and take care of all the adsorption and hydrolysis reactions, respectively. The amount of base consumed (Cb) at a given pH is equal to
Cb ) RnM2-n + M(OH)2-z The following equilibrium constants can be written for reactions 1 and 2
K1 ) [Cb1][Hn+]/[M2+]
(3)
K2 ) [Cb2] [Hz+]/[M2+]
(4)
where Cb1 and Cb2 are the amounts of base consumed in (18) Lang’s Hand Book of Chemistry, 12th ed.; McGraw-Hill Book Company: New York, 1979.
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Figure 4. Plots of log Cb1/Cb2 vs pH for ZnO in the presence of 30 ppm Co2+ ions at various temperatures: (b) 303 K; (2) 313 K; (9) 323 K.
Figure 5. Plots of slope vs pH for ZnO in the presence of 30 ppm Co2+ ions at various temperatures: (b) 303 K; (2) 313 K; (9) 323 K.
reactions 1 and 2, respectively, and were determined from the amount of base consumed from the titration of the metal cations alone (reaction 2) and in the presence of the solid (reaction 1). Dividing eq 3 by eq 4, the following relationship is obtained
K1/K2 ) Cb1[H+]n-z/Cb2
(5)
After further simplification and taking the logarithm, eq 5 changes into
log Cb1/Cb2 ) log K + (n - z) pH
(6)
where K) K1/K2. Equation 6 implies that in the beginning in the region of low pH as z will be smaller compared to n (adsorption predominates), the function log Cb1/Cb2 would have a positive slope and after passing through a maximum when n ) z (adsorption ) precipitation), the function would decrease having a negative slope when z > n (precipitation predominates). The plots of log Cb1/Cb2 vs pH for Co2+ are shown in Figure 4. Similar curves were obtained for Zn2+ and Ni2+. As predicated, the curve possesses initially a positive slope, passes through a maximum, and then decreases, having a negative slope. This behavior of the curves indicates that the uptake mechanism of Zn2+, Ni2+, and Co2+ changes from adsorption into precipitation. The values of the slopes (n - z) of the curves of Figure 4, which would help in deciding the mechanism of metal ions uptake, are plotted against pH at different temperatures for Co2+ ions in Figure 5. It is clear from these curves that in almost all the cases, the values of the slopes of the curves decrease from + 2 to - 2, showing the change in the mechanism from ideal adsorption to perfect precipitation. Similar results were obtained for Ni2+ and Zn2+. To confirm the process of precipitation in the system, the values of solubility products are calculated at the pH values where the curves in Figure 4 pass through the maximum. The values thus calculated are given in Table 1.These values if compared with the values of the solubility product given in the literature18 are found to be much lower, probably due to the formation of an active form of the hydroxide on the zinc oxide surface which has a lower solubility product as compared to that of the final polynuclear hydroxide complex. Thus, adsorption being the more favorable process at low pH gradually changes into precipitation with the increase in the pH of the aqueous solution. At higher pH
Figure 6. Adsorption isotherms of Ni2+ ions on ZnO at pH 7: (b) 303 K; (2) 313 K; (9) 323 K; also (+) represents Ni2+ isotherm at 303 K of the ZnO heated at 400 °C. Table 1. Solubility Product Constants of Metal Cations in the Presence of 0.1 M KNO3 metal ions 1. 2. 3.
303 K 10-19
313 K
323 K
10-19
lit. values
9.05 × 2.41 × 4.08 × 1.20 × 10-17 Co2+ 7.25 × 10-19 1.29 × 10-19 7.78 × 10-19 1.60 × 10-15 Ni2+ 7.48 × 10-19 2.03 × 10-19 1.64 × 10-19 2.00 × 10-15 Zn2+
10-20
Table 2. Sorption Data for Ni2+ on ZnO at 303 K
1. 2. 3. 4. 5.
pHi
pHeq
initial Ni2+ (Ci) × 104 (mol/L)
Ni2+ sorbed (X) × 106 (mol/g)
6.96 7.00 7.00 6.98 7.02
6.34 6.36 6.32 6.15 6.05
0.85 1.76 2.56 3.41 4.26
5.60 7.20 8.00 8.32 9.04
values an “active” form of the hydroxide, that is, probably, a very fine crystalline precipitate with a disorder lattice, is formed, which may convert slowly into a more stable “inactive” form of the correspondingly hydroxides with time. Adsorption Studies on ZnO. Adsorption of Zn2+, Ni2+, and Co2+ on zinc oxide was studied at different concentrations, temperatures, and pH values of the systems. The sorption isotherms containing different concentrations of Ni2+ are shown in Figure 6 and Tables 2 and 3. As can be seen, adsorption of metal ions increases with increasing concentration of the metal cations and pH and decreases with the increase in temperature. Similar behavior was observed for Co2+ and Zn2+ ion sorption on the ZnO during the present investigation.
Sorption of Divalent Metal Ions on ZnO
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Figure 7. Adsorption isotherms of M2+ ions on ZnO at pH 9: (b) Zn2+; (2) Ni2+; (9) Co2+. Table 3. Sorption Data for Ni2+ on ZnO at 303 K
1. 2. 3. 4. 5.
pHi
pHeq
initial Ni2+ (Ci) × 104 (mol/L)
Ni2+ sorbed (X) × 106 (mol/g)
9.01 9.09 9.04 9.03 9.10
7.58 7.53 7.51 7.49 7.41
0.85 1.76 2.56 3.41 4.26
14.82 16.30 16.90 17.52 18.00
The selectivity of the oxide can be observed from Figure 7, which is found to be in the order Zn2+ > Ni2+ > Co2+. On comparison of the extent of adsorption and pH of hydrolysis of metal cations in solutions, it is observed that the oxide prefers the cations with low pH of hydrolysis as easily hydrolyzable metal ions are considered to have high affinity for the solid surfaces.19-23 While the increase in sorption with temperature is well researched,11-17 the decrease in the extent of adsorption of divalent metal ions with temperature by oxides/ hydroxides to our knowledge is reported here for the first time. The data reported in the literature11-17 generally indicate that the sorption of cations is found to increase with the increase in temperature. The data given in Tables 2 and 3 show that the uptake of metal ions leads to decrease in equilibrium pH of the solution. This drop in pH accompanying the adsorption process may be attributed to the following reactions:
(i) adsorption reaction ROH + MOH+ S ROMOH + H+
(7)
(ii) hydrolysis of metal cation M2+ + H2O S MOH+ + H+
(8)
(iii) surface precipitation M2+ + 2H2O S M(OH)2 + 2H+
(9)
(iv) dissolution of the zinc oxide ZnO + 2H2O S Zn(OH-)3 + H+
(10)
However, as the extent of adsorption is observed to decrease with temperature as shown in Figure 6, it may be concluded from the increased consumption of base with temperature that except the reaction 7, all other reactions (8-10) are probably favored by the increase in temperature. (19) Johnson, B. B. Environ. Sci. Technol. 1990, 24, 112.
Figure 8. Plots of log IAP vs pHeq for ZnO in the presence of 15 ppm Ni2+ ions at various temperatures: (b) 303 K; (2) 313 K; (9) 323 K.
To determine the effect of precipitation, the ionic activity products [Co][OH-]2 are determined and are plotted as a function of pH in Figure 8. If the surface precipitation controls the uptake of the Co2+ ions from the aqueous solution, the ionic activity product should be equal to or exceed the solubility product of the phase formed on the surface. As can be seen from Figure 8, the initial slope of the curves is equal to the theoretical slope of 2.0 according to the equation
log(IAP) ) 2pH + log Co2+ + 2 log Kw showing that in this region, the process responsible for the Co2+ uptake is adsorption. However, with the increase in pH, the slope gradually diminishes and ultimately becomes equal to zero at all the temperatures under investigation, indicating a change in the mechanism of the metal uptake from adsorption to surface precipitation. It is also interesting to compare the solubility product of the Co(OH)2 with the value available in the literature. The value of -13.75 at 30 °C is close to the value of -14.2 for the blue form of Co(OH)2 formed in aqueous solution. The increase in value of the solubility product with the increase in temperature is probably due to the formation of more active forms of the Co(OH)2 on the surface of the zinc hydroxide. Similar conclusions were drawn by Persson et al.,24 who obtained a value of -15.2 for the solubility product of the Co2+-containing phase formed on the surface of the zinc oxide. To determine the effect of surface CO32- species, the adsorption of Ni2+ was studied on the calcined ZnO which has lost both CO32- and OH-, as revealed by the FTIR spectrum given in Figure 9d. As is obvious from the sorption isotherms in Figure 6 the adsorption capacity of ZnO is almost the same both before and after elimination of CO32- species from its surface at 400 °C. It shows that the presence of trace quantities of CO32- on the surface has no significant effect on the adsorption capacity of the metal cations on the ZnO. Similar views were put forward by Rowlands et al.25 and Balistrieri and Murray.26 (20) Mustafa, S.; Safdar, M.; Nawab, G. Phys. Chem. 1990, 9, 79. (21) Gray, M, J.; Malati, M. A. J. Chem. Technol. Biotechnol. 1979, 29, 127. (22) Mustafa, S.; Naeem, A.; Rehana, N. J. Chem. Soc., Faraday Trans. 1993, 89, 3843. (23) Forbes, E. A.; Posner, A. M.; Quirk, J. P. J. Soil Sci. 1976, 27, 154. (24) Persson, P.; Parks, G. A.; Brown, J. G. E. J. Langmuir 1995, 11, 3782. (25) Rowlands, W. N.; O’Brien, R. W.; Hunter, R. J.; Patrick, V. J. Colloid Interface Sci. 1997, 188, 325.
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Figure 10. Dissolution of ZnO in the presence of 25 ppm: (2) Ni2+; (9) Co2+; (b) blank. Table 4. Ratio of M2+ Ion Sorbed/Zn2+ Dissolved for ZnO at Different Temperatures Ni2+ sorbed/ Zn2+ dissolved 1. 2. 3.
Figure 9. FTIR spectra of ZnO: (o) ZnO pure; (b) ZnO + 100 ppm Ni2+ at pH 8; (d) ZnO calcined at 400 °C; (g) ZnO (calcined at 400 °C) + 100 ppm Ni2+ at pH 8.
FTIR Studies of ZnO. The FTIR spectrum of ZnO is shown in Figure 9o. In this spectrum IR bands are observed at 3410, 1562, 1399, 1040, 834, and 715 cm-1. The band at 3410 cm-1 may be assigned to the OH stretching vibration of surface hydroxyl groups.27 The bands observed at 715, 834, and 1040 cm-1 are assigned to the internal vibration of the ZnO matrics.28,29 Similarly the bands at 1399 and 1562 cm-1 can be assigned to the presence of carbonate species, which may be incorporated into ZnO particles via dissolution of CO2 from air into the solution during the preparation of ZnO. However, the bands at 1399,1562, and 3410 cm-1 disappear after calcinations of ZnO at 400 °C, which shows the loss of both the CO32- and OH- from the surface of ZnO (Figure 9d). The present spectra of ZnO are in agreement with the one reported in the literature.29 An additional strong support for the precipitation mechanism can be derived from the FTIR spectra of ZnO taken after sorption of the Ni2+ ion as given in Figure 9b. Comparison of the spectrum of the pure ZnO with the one obtained after sorption of Ni2+ ion at pH 8 reveals the (26) Balistrieri, L. S.; Murray, J. W. Geochim. Cosmochim. Acta 1982, 46, 1253. (27) Nyquist, R. A.; Kagel, R. O. Infrared Spectra of Inorganic Compounds; Academic Press, Inc., New York and London, 1971. (28) Nasrallah-Abbukais, N.; Boughriet, A.; Gengembre, L.; Aboukais, A. J. Chem. Soc., Faraday Trans. 1998, 94, 2399. (29) Badreddine, U.; Legrouri, A.; Barroug, A.; DE Roy, A.; Besse, J. P. Collect Czech. Chem. Commun. 1998, 63, 741.
pHi
303 K
7.00 8.00 9.00
0.16 2.67 3.52
Co2+ sorbed/ Zn2+ dissolved
323 K
303 K
323 K
0.47 2.42
0.78 0.43 2.32
0.01 0.19 0.60
appearance of two new weak bands at 2927 and 2845 cm-1, the disappearance of the band at 834 cm-1 assigned to the internal OH- vibration of the ZnO matrix,29 the shift in the OH stretching vibration from 3410 to 3434 cm-1, and a decrease in the intensity of the bands located at 1399 and 1562 cm-1, showing the formation of a new phase formed on the surface of ZnO. The new phase formed at the surface with the OH- stretching vibration at 3434 cm-1 is indeed, a mixed hydroxide of the Ni2+ and Zn2+ ions as the OH- stretching bands for corresponding hydroxides are observed to be at 3410 and 3434 cm-1, respectively.27 The OH- stretching band at 3490 cm-1 is also observed in double hydroxides by Houri et al.30 The appearance of the new bands (Figure 9g) in the calcined ZnO at 2927 and 2845 cm-1, in addition to the OH stretching vibration at 3434 cm-1, after adsorption of the metal cations also confirm that the surface of the calcined ZnO was covered with the oxide/hydroxide of the of the Ni2+ ions such as that of the noncalcined sample. Dissolution of ZnO. Another process responsible for the decrease in the metal ion adsorption with the temperature may be an increase in the extent of dissolution of oxide. As can be seen from Figure 10 the dissolution of ZnO decreases with the increase in pH in the presence of 25 ppm metal ion at 303 K. Further, the dissolution of the ZnO also depends on the nature of metal cations present in the system Ni2+ > Co2+ (Figure 10), which is similar to the selectivity order obtained for the adsorption. To determine the effect of the oxide dissolution on the metal ion adsorption, the ratios of metal ion adsorbed/Zn2+ ion dissolved have been calculated and are given in Table 4. These ratios in all the cases are observed to increase with the increase in pH and decrease with the increase in temperature. While, the increase in ratios with pH excludes the displacement of the structural Zn2+ by the metal ions from solution as a possible mechanism of adsorption, the decrease in ratios with temperature indicates that the dissolution of the ZnO at high temperature has a very significant effect on the extent of adsorption of divalent metal cations. (30) Houri, B.; Legrouri, A.; Barreug, A.; Forano, C.; Besse, J. P. Collect. Czech. Chem. Commun. 1998, 63, 732.
Sorption of Divalent Metal Ions on ZnO
Figure 11. Kinetics of 30 ppm Ni2+ ions adsorption on ZnO at (b) 303 K, (2) 313 K, and (9) 323 K.
The kinetics of the Ni2+ sorption on ZnO was studied in the temperature range 303-323 K at pH 8. As can be seen from Figure 11, at 303 K, the kinetic curve has a normal behavior; i.e., the adsorption increases with time until equilibrium is established in the system after about 90 min. However, at the higher temperatures of 313 and 323 K, the kinetic curves passes through maxima revealing that the uptake is accompanied by another reaction, the rate of which is observed to increase with temperature. This secondary reaction, the dissolution of the oxide, increases with temperature as is shown in Figure 12. It is also interesting to note from Figures 11 and 12 that the maxima observed in the kinetic curves of the Ni2+ adsorption at 313 and 323 K almost coincide with the maxima of dissolution curves of the solid given in Figure 12. Thus, it can be suggested that two different opposing reactions occur at the oxide/solution interface, i.e., adsorption/precipitation of the metal cations and dissolution of the ZnO at the same time. At lower temperature (303 K) the adsorption reaction predominates the dissolution
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Figure 12. Kinetics of ZnO dissolution in the presence of 30 ppm Ni2+ ions at (b) 303 K, (2) 313 K, and (9) 323 K.
reaction, while the converse becomes true at higher temperatures. Conclusions From the above discussion, it may be summarized that the adsorbent ZnO has appreciable sorption capacity toward the divalent transition metal ions. The sorption is found to increase with the increase in pH and concentration of metal cations and decrease with the increase in temperature. The pH titration study reveals that with the increase in pH the sorption mechanism changes from adsorption to precipitation. The sorption of the metal cations is found to follow the order Zn2+ > Ni2+ > Co2+, which is parallel to their solubility products in aqueous solution. The FTIR spectra reveal the formation of a double hydroxide on the surface, which results in the reduction of the dissolution of the zinc oxide. Finally, the decrease in the extent of adsorption of the metal cations with the increase in temperature is due to the destruction of the surface on account of the dissolution of the ZnO. LA0014149
