2378
Langmuir 1998, 14, 2378-2384
Ligands Sorption Studies on Transition Metal Ion Loaded Amberlite IRC-50 S. Mustafa,* L. H. Nadia, N. Rehana, A. Naeem, and H. Y. Samad National Center of Excellence in Physical Chemistry, University of Peshawar, Peshawar, Pakistan Received June 13, 1997. In Final Form: October 30, 1997 Ligands sorption of ammonia, diethanolamine, and triethanolamine on the cation exchanger Amberlite IRC-50 having Cu2+, Ni2+, and Zn2+ ions was studied as a function of temperature 290-318 K and different initial concentrations 1-500 mmol‚L-1. Sorption was found to follow the order Cu2+ > Zn2+ > Ni2+ and ammonia > diethanolamine > triethanolamine. Desorption of metal cations from the exchanger to the aqueous phase was also observed when the temperature of the system was increased. The results were explained in terms of ligand sorption and ion exchange, which were found to be dependent upon the stabilities of the complexes inside the resin and in the aqueous solution and the basicity and hydrophobicity of the ligand involved. The ratio of metal released/amine sorbed was determined which indicated two possible mechanisms of ligand sorption. The data were explained with the help of mass action law. IR studies also confirmed the presence of metal complexes and metal-amine complexes inside the resin.
Introduction The organic ion-exchange resins have attracted the attention of researchers 1-4 due to their chemical and mechanical stability, high exchange capacity, and ionexchange rate. A great variety of such resins with different properties have been described in the literature. Generally, cation-exchange resins prefer sorption of more highly charged metal ions from the solution,5,6 which is attributed to the stronger electrostatic interaction between ionexchange groups and metal ions with a high charge. Ion exchangers containing highly charged metal cations (like Cu2+, Ni2+, Fe3+, Ag+, etc.) can be used as highly selective sorbents for the uptake of ligands such as ammonia and organic amines. These cations inside the exchangers readily shed their solvation shell to form stable amine complexes.1,2 The ligands taken up from the external solution replace solvent molecules in the solvation shell of metal ions or replace other ligands in previously formed complexes.7 Typical “ligand-exchange” reactions are
(R-)2[Cu(H2O)4]2+ + 4NH3 h (R-)2[Cu(NH3)4]2+ + 4H2O (1) (R-)2[Cu(NH3)4]2+ + 2C4H11NO2 h (R-)2[Cu(C4H11NO2)2]2+ + 4NH3 (2) where a bar over the symbols refers to the interior of the resin and no bar represents the species in the aqueous phase. Although it is possible to achieve both high capacity and selectivity through ligand sorption and ligand ex(1) Helfferich, F. Nature 1961, 189, 1001. (2) Stokes, R. H.; Walton, H. F. J. Am. Chem. Soc. 1954, 76, 3327. (3) Mustafa, S.; Hussain, S. Y.; Alam, Z. Solvent Extr. Ion Exch. 1989, 7, 705. (4) Mustafa, S.; Hussain, S. Y.; Rashid, A. Solvent Extr. Ion Exch. 1990, 8, 325. (5) Dorfner, K. Ion Exchangers; Ann Arbor Science Publishers: Ann Arbor, MI, 1972; Vol. 44. (6) Kazantsev, E. I.; Denisov, A. N. Zh. Neorg. Khim. 1963, 8, 2198. (7) Helfferich, F. J. Am. Chem. Soc. 1962, 84, 3237, 3242.
change, very little is reported in the literature8 about the ligand sorption on carboxylic cation exchangers. These exchangers have increased selectivity for the sorption of heavy-metal cations in both neutral and weak alkaline media.9-12 The purpose of the present study is, therefore, to investigate the sorption of ligands such as ammonia, diethanolamine, and triethanolamine on the Cu2+, Ni2+, and Zn2+ forms of the carboxylic cation exchanger Amberlite IRC-50 under different experimental conditions of concentration, counterions, pH, and temperature. Further, IR studies of the resin before and after sorption of ligands on the metal-loaded exchanger have also been accomplished. Experimental Section Regeneration of the Resin. Amberlite IRC-50 is available in the form of spherical beads. The maximum temperature which it can tolerate is 120 °C. The working pH range is 5-14. The particle size varies from 0.3 to 1.18 mm. The total exchange capacity of the resin is 9.5 mequiv/g of the dry resin bed.8 Amberlite IRC-50, a weak acid cation-exchange resin, was washed several times with doubly distilled water. The resin was taken in a beaker and was allowed to stand for 24 h for swelling purposes. Then the deionized water was removed, to the swelled resin beads was added a solution of 1 M HCl, and the resulting solution was allowed to stand for 24 h. Further, the resin was removed from the acidic solution and was washed with deionized water in order to remove the excess hydrogen ions from the resin. The presence of acidity was checked by Universal indicator paper pH 1-11, until the pH of the filtrate became neutral. The resin was exposed to free atmosphere in order for it to dry, and then the resin was converted to the Na+ form by treating it with NaOH and afterward was stored in a glass-stoppered bottle. Conversion of Amberlite IRC-50 (Na+) into Cu2+, Ni2+, and Zn2+ Forms. Three samples of 10 g of Amberlite IRC-50 were taken in separate burets provided with glass wool plugs. (8) Dorfner, K. Ion Exchangers; Walter de Gruyter: Berlin, 1991. (9) Shiralkar, V. P.; Kulkarni, S. B. J. Colloid Interface Sci. 1986, 109, 115. (10) Limina, I. F.; Kaputskii, V. E. Colloid J. 1994, 56, 430. (11) Mustafa, S.; Bashir, H. Y.; Naeem, A. React. Funct. Polym. 1997, in press. (12) Guaus, F.; Sanz, F.; Sluytersrehbach, M.; Sluyters, J. H. J. Electroanal. Chem. 1995, 385, 121.
S0743-7463(97)00632-X CCC: $15.00 © 1998 American Chemical Society Published on Web 04/04/1998
Transition Metal Ion Loaded Amberlite IRC-50
Figure 1. Ammonia sorption isotherms (- - -) at (b) 290, (2) 298, (9) 308, and (×) 318 K. Cu2+ desorption isotherms (s) at (.) 290, (4) 298, (0) 308, and (O) 318 K on Amberlite IRC-50 (Cu-form).
Langmuir, Vol. 14, No. 9, 1998 2379
Figure 2. Ammonia sorption isotherms (- - -) at (b) 290, (.) 298, (2) 308, and (9) 318 K. Zn2+ desorption isotherms (s) at (O) 298, (4) 308, and (0) 318 K on Amberlite IRC-50 (Zn-form).
The resin was converted to the Cu, Ni, and Zn forms by treating it with 0.1 M Cu(NO3)2, Ni(NO3)2, and Zn(NO3)2, respectively. The treatment continued until the concentration of the effluent also became equal to 0.1 M Cu(NO3)2, Ni(NO3)2, and Zn(NO3)2, respectively. The samples were allowed to stand for 24 h and then dried in air and stored in glass-stoppered bottles for further studies. Sorption Behavior of Ligands and Desorption Behavior of Metals by Loaded Amberlite IRC-50. Weighed out accurately, 0.1 g of Amberlite IRC-50 (Cu2+, Ni2+, and Zn2+ forms) and 30 mL of ammonia, diethanolamine, and triethanolamine solutions with a concentration range of 1- 500 mmol‚L-1 were pipetted into the flasks, and the initial pHs of the solutions were noted. The resin and solutions were equilibrated for 24 h at temperatures of 17, 25, 35, and 45 °C for ammonia and 25, 35, and 45 °C for diethanolamine and triethanolamine. After this, the equilibrium pH of each sample, the concentrations of metal ions released, and ligands left in the solutions in each sample were determined. The concentrations of Cu2+, Ni2+, and Zn2+ (released from resin) were determined with a PerkinElmer model 3100 atomic absorption spectrophotometer. The equilibrium concentrations of ammonia, diethanolamine, and triethanolamine were determined by titrating the aqueous solutions against standard HCl using methyl orange as an indicator. Infrared Spectroscopy of the Sorbed Metals and Ligands Amberlite IRC-50. Air-dried, 5-mg samples of Amberlite IRC50 in H+, Na+, Cu2+, Ni2+, Zn2+, copper-ammonia, copperdiethanolamine, copper-triethanolamine, nickel-ammonia, nickeldiethanolamine, nickel-triethanolamine, zinc-ammonia, zincdiethanolamine, and zinc-triethanolamine forms were mixed with 1.0 g of KBr crystals and were ground into fine powder using an agate mortar. The resultant powder was pressed to a pellet and was analyzed by a PYE Unicam model sp 3-100 infrared spectrophotometer.
Figure 3. Diethanolamine sorption isotherms (- - -) at (b) 298, (2) 308, and (9) 318 K. Cu2+ desorption isotherms (s) at (O) 298, (4) 308, and (0) 318 K on Amberlite IRC-50 (Cu-form).
Results and Discussion
Figure 4. Triethanolamine sorption isotherms (- - -) at (b) 298, (2) 308, and (9) 318 K. Ni2+ desorption isotherms (s) at (O) 298, (4) 308, and (0) 318 K on Amberlite IRC-50 (Ni-form).
Ligands Sorption on Loaded Amberlite IRC-50. The ligands sorption isotherms of ammonia, diethanolamine, and triethanolamine on Amberlite IRC-50 containing complexing cations Cu2+, Ni2+, and Zn2+ are shown in the representative Figures 1-4. Figures 1 and 2 illustrate that the ammonia is taken up by the metal-loaded Amberlite IRC-50 in appreciable quantities and uptake generally increases with an increase in concentration. Further, the sorption of ammonia is accompanied by the release of metal cations from the exchanger to the aqueous solution which is more distinguishable in the case of Cu2+ at higher temperatures. Moreover, Ni2+- and Zn2+-loaded forms of the resin exhibit a behavior different from that of the Cu2+ form. In comparison to Cu2+, both the cations
Ni2+ and Zn2+ tend to remain in the exchanger and are not easily released to the aqueous phase (Tables 1-4). They are released only when the initial concentration of ammonia increases above 50 mmol‚L-1 at all temperatures above 290 K (Tables 2 and 4). A similar release of Cu2+ from the exchanger during the sorption of ethanolamine was also observed by Hiroyuki and Takeshi.13 Figure 1 also illustrates that with an increase in temperature the sorption of ammonia decreases as a result of the increased desorption of metal cations into the (13) Hiroyuki, Y.; Takeshi, K. Solvent Extr. Ion Exch. 1986, 4, 1171.
2380 Langmuir, Vol. 14, No. 9, 1998
Mustafa et al.
Table 1. Sorption of Ammonia on Cu-Loaded Amberlite IRC-50 at 298 Ka series no.
pHi
pHe
Ci (mmol‚L-1)
Ce (mmol‚L-1)
ammonia adsorbed (X) (mmol‚g-1)
Cu2+ ion rel. (X) (mmol‚g-1)
1 2 3 4 5 6
9.95 10.32 10.44 10.65 10.70 11.03
7.72 8.59 9.36 10.12 10.57 10.79
1.00 5.00 10.00 50.00 100.00 500.00
0.96 1.58 2.42 27.50 68.30 400.00
0.07 1.03 2.27 6.75 9.51 30.00
2.75 3.00 6.25 8.25 10.00
a
pHi ) initial pH. pHe ) equilibrium pH. Ci ) initial concentration. Ce ) equilibrium concentration. Table 2. Sorption of Ammonia on Ni-Loaded Amberlite IRC-50 at 298 Ka
series no.
pHi
pHe
Ci (mmol‚L-1)
Ce (mmol‚L-1)
ammonia adsorbed (X) (mmol‚g-1)
Ni2+ ion rel. (X) (mmol‚g-1)
1 2 3 4 5 6
9.77 10.09 10.28 10.54 10.65 11.13
8.80 9.50 9.83 10.20 10.38 11.16
1.00 5.00 10.00 50.00 100.00 500.00
0.58 1.63 3.50 20.80 70.00 383.00
0.13 1.01 1.95 8.75 9.00 35.00
0.02 0.04 0.06
a
See Table 1, footnote a. Table 3. Sorption of Ammonia on Zn-Loaded Amberlite IRC-50 at 290 Ka
series no.
pHi
pHe
Ci (mmol‚L-1)
Ce (mmol‚L-1)
ammonia adsorbed (X) (mmol‚g-1)
Ni2+ ion rel. (X) (mmol‚g-1)
1 2 3 4 5 6
9.87 10.11 10.39 10.60 10.80 11.23
8.14 9.43 10.07 10.60 10.75 11.35
1.00 5.00 10.00 50.00 100.00 500.00
0.59 1.18 2.25 25.00 60.80 340.00
0.13 1.10 2.50 7.50 11.76 48.00
-
a
See Table 1, footnote a. Table 4. Sorption of Ammonia on Zn-Loaded Amberlite IRC-50 at 318 Ka
series no.
pHi
pHe
Ci (mmol‚L-1)
Ce (mmol‚L-1)
ammonia adsorbed (X) (mmol‚g-1)
Ni2+ ion rel. (X) (mmol‚g-1)
1 2 3 4 5 6
10.25 10.42 10.86 11.31 11.43 11.72
8.88 9.30 9.69 10.30 10.72 11.32
1.00 5.00 10.00 50.00 100.00 500.00
0.92 2.17 2.50 27.50 64.00 416.00
0.02 0.85 2.25 6.75 10.80 25.00
0.04 0.05 0.08
a
See Table 1, footnote a.
solution. This effect of the temperature is even visible in the case of the Ni2+- and Zn2+-loaded exchanger at 318 K where desorption of both the cations Zn2+ and Ni2+ can be observed (Figure 2). The difference in the behavior of the Cu2+ and Zn2+/Ni2+ ions loaded exchanger shows that two different mechanisms are responsible for the uptake of ammonia by the metal ions loaded exchanger. The two possible mechanisms are the ligand sorption and ion exchange, which are given by reactions 3 and 4, respectively.
RnM2+ + xNH4+ h RnM2+(NH3)x + xH+
(3)
RnCu2+ + 2NH4+ h Rn(NH4+)2 + Cu2+
(4)
While in case of ligand sorption (reaction 3) no desorption of metal ions would take place, a complete desorption would occur when the process is that of ion exchange (reaction 4). In the case of Zn2+- and Ni2+-loaded exchanger even at higher concentrations of ammonia and temperatures, the desorption of the metal ions into the aqueous phase is very small, showing that the main process responsible for the uptake of ammonia is ligand sorption.
However, in the case of the Cu2+ form of the exchanger, an increase in the temperature and concentration of ammonia results in a greater release of the Cu2+ ions to the aqueous phase, indicating a change in the mechanism from ligand sorption to ion exchange. The decrease in the difference between the initial and equilibrium pH values as shown in Tables 1-6 may also be due to the change in mechanism from ligand sorption to ion exchange, as no hydrogen ions would be released to the aqueous solution according to reaction 4. Similar results were obtained by Libinson14 even for the uptake of ammonia at high pH values by the carboxylic cation exchanger. The uptake of both diethanolamine and triethanolamine on the metal-loaded exchanger generally follows the same pattern as is observed in the case of ammonia (Figures 3 and 4). The sorption of diethanolamine and triethanolamine is more favorable from the dilute solutions at low temperatures. However, at higher concentrations of diethanolamine and triethanolamine in solution and at higher temperature of the system, the uptake of both diethanolamine and triethanolamine decreases because of the increased desorption of the metal cations from the exchanger, showing the change in the mechanism as (14) Libinson, G. C. Zh. Fiz. Khim. 1965, 39, 1509.
Transition Metal Ion Loaded Amberlite IRC-50
Langmuir, Vol. 14, No. 9, 1998 2381
Table 5. Sorption of Diethanolamine on Cu-Loaded Amberlite IRC-50 at 298 Ka
a
series no.
pHi
pHe
Ci (mmol‚L-1)
Ce (mmol‚L-1)
1 2 3 4 5 6
8.96 10.38 10.55 11.14 11.33 11.76
5.92 9.47 10.01 10.66 10.89 10.96
1.00 5.00 10.00 50.00 100.00 500.00
0.83 2.42 6.67 41.66 95.00 500.00
diethanolamine adsorbed (X) (mmol‚g-1) 0.05 0.78 1.00 2.50 1.50
Cu2+ ion rel. (X) (mmol‚g-1) 1.92 2.04 2.39 6.00 15.00 37.50
See Table 1, footnote a. Table 6. Sorption of Triethanolamine on Cu-Loaded Amberlite IRC-50 at 298 Ka
a
series no.
pHi
pHe
Ci (mmol‚L-1)
Ce (mmol‚L-1)
1 2 3 4 5 6
9.48 9.81 9.99 10.42 10.62 11.02
7.32 9.17 9.65 9.93 10.48 10.94
1.00 5.00 10.00 50.00 100.00 500.00
0.25 2.08 6.83 44.16 95.00 500.00
triethanolamine adsorbed (X) (mmol‚g-1) 0.23 0.88 0.95 1.75 1.50
Cu2+ ion rel. (X) (mmol‚g-1) 0.46 2.10 5.62 25.00
See Table 1, footnote a.
discussed in the case of ammonia. As such, the sorption of diethanolamine and triethanolamine essentially remains the ligand sorption at low temperatures and concentrations which may change into the ion exchange when both the concentration and temperature are increased. The only difference of the ethanolamines system from ammonia is that their sorption is smaller and extent of metal desorption is more intense (Tables 5 and 6). By comparing the maximum sorption of diethanolamine and triethanolamine, it is observed that the sorption of triethanolamine is less compared to diethanolamine, while the extent of desorption of the metal cations from the exchanger is greater in diethanolamine than in the triethanolamine. Thus, it is, indeed, the two opposing effects of the ethanolamine sorption and metal-cation desorption which give rise to the maxima in the adsorption isotherms of triethanolamine (Figure 4). From the foregoing discussion, it can be concluded that uptake of ligands such as NH3 and ethanolamines by the metal ions loaded weak acid exchangers follows two different mechanisms, ligand sorption as a result of complexation of the amines with the metal ion in the exchanger and ion exchange with the protonated amines resulting in the desorption of the metal cations from the exchanger. The mechanism preferred by the system would depend on several factors, such as the stability of the complexes formed inside the resin and in the aqueous solution, acidity/basicity and hydrophobic/hydrophilic characters of the ligands involved, and strength of the resin-metal bond. As far as the influence of the resinmetal bond on the uptake mechanism is concerned, although no data regarding the strength of the metalresin bond are available, one can get an indication from the strength of the metal-acetate bond. The strength of the metal-acetate bond follows the order15 Cu2+ > Zn2+ > Ni2+, showing that the metal-resin bond strength has very little effect on the mechanism of the amine sorption as compared to the influences of the other factors mentioned above. The stability of the amine complexes16 in aqueous solution follows the order Cu2+ > Ni2+ g Zn2+ and ammonia (15) Dean, J. A. Lange’s Handbook of Chemistry, 13th ed.; McGrawHill Book Company: New York, 1987. (16) Dean, J. A. Lange’s Handbook of Chemistry, 12th ed.; McGrawHill Book Company: New York, 1979.
Figure 5. Cu2+ desorption isotherms on Amberlite IRC-50 (Cu-form) in the presence of (4) ammonia, (0) diethanolamine, and (b) triethanolamine at 298 K.
Figure 6. Mass action law plots for zinc-ammonia at (b) 290, (4) 298, (O) 308, and (0) 318 K.
> diethanolamine > triethanolamine. The stability constants in aqueous solution of Cu(NH3), Zn(NH3), and Ni(NH3) are 4.31, 2.37, and 2.80, and those of Cu(C6H15O3N), Zn(C6H15O3N), and Ni(C6H15O3N) are 4.30, 2.00, and 2.70, respectively. Comparing the extent of the ligand sorption with their stabilities in the aqueous solution, it can be observed that, when the stability of the complex in the aqueous phase is greater,17 the metal tends to desorb into the aqueous phase. Consequently, an increased desorption of the (17) Marcus, Y.; Kertes, A. S. Ion Exchange and Solvent Extraction of Metal Complexes; Wiley-Interscience: New York, 1969.
2382 Langmuir, Vol. 14, No. 9, 1998
Mustafa et al. Table 12. Values of n and log K Calculated from Equation 10 for Zinc-Triethanolamine Complex part I
Figure 7. Mass action law plots for zinc-triethanolamine at (b) 298, (0) 308, and (4) 318 K. Table 7. Values of n and log K Calculated from Equation 10 for the Copper-Ammonia Complex part I
temp (K)
n
log K
n
log K
1 2 3
298 308 318
1.20 1.20 1.20
4.17 4.41 4.40
3.10 3.00 3.10
10.50 10.50 10.50
mine > triethanolamine. However, the reversal of the Cu2+ desorption order diethanolamine > triethanolamine > ammonia is probably due to the hydrophobic/hydrophilic interaction of the ligands in aqueous solution. The diethanolamine having two OH groups would be more hydrophobic compared to triethanolamine and ammonia and consequently will be exchanged more extensively than triethanolamine and ammonia. As discussed earlier, a probable mechanism for the sorption of ligands can be expressed as
RM + nX h RM(X)n
part II
series no.
temp (K)
n
log K
n
log K
1 2 3 4
290 298 308 318
1.20 1.20 1.21 1.26
1.96 2.08 2.26 2.30
3.52 4.21 4.50 4.00
5.70 7.43 8.33 7.40
Table 8. Values of n and log K Calculated from Equation 10 for the Zinc-Ammonia Complex part I temp (K)
n
log K
n
log K
1 2 3 4
290 298 308 318
1.18 1.20 1.21 1.30
1.82 2.30 2.40 3.40
3.62 3.60 3.73 4.30
5.70 6.85 7.52 10.49
Table 9. Values of n and log K Calculated from Equation 10 for Copper-Diethanolamine Complex part I
part II
series no.
temp (K)
n
log K
n
log K
1 2 3
298 308 318
1.20 1.29 1.42
3.52 3.75 4.30
2.90 3.30 3.00
8.57 9.71 9.91
Table 10. Values of n and log K Calculated from Equation 10 for Zinc-Diethanolamine Complex part I
K)
[RM(X)n] [RM][X]n
K)
X [θt - X - Mdes][X]n
K)
[X]
temp (K)
n
log K
n
log K
[θt - X][X]n
1 2 3
298 308 318
1.20 1.31 1.30
3.89 4.16 4.20
2.68 3.40 3.20
8.37 11.00 10.94
Taking θ ) Xθt, eq 8 takes the form
θ ) KXn 1-θ
Table 11. Values of n and log K Calculated from Equation 10 for Copper-Triethanolamine Complex series no.
temp (K)
n
log K
n
log K
1 2 3
298 308 318
1.30 1.40 1.40
4.01 4.30 4.30
2.80 2.76 2.83
8.50 8.62 8.80
metals in the order Cu2+ > Zn2+ g Ni2+ is obtained (Tables 1, 2, and 4). With the desorption of Zn2+ and Ni2+ being negligible at 298 K, it is advisable to compare the extent of desorption of Cu2+ as a function of the nature of the ligand which follows the order ammonia < triethanolamine < diethanolamine, as given in Figure 5. The pKb values18 of the ligands showing the ease of their protonation resulting in subsequent desorption of the Cu2+ ions then must follow the order ammonia > diethanola-
(7)
where X is the amount of ligand adsorbed, θt is the exchange capacity in the resin, and Mdes is the amount of metal desorbed at each ligand concentration. As was observed in the preceding paragraphs, the release of metal ions from the resin is significant only when the concentration of amines in the solution is high. Thus, neglecting the metal ions desorption in the region of low sorption density of the amine, eq 7 is obtained in the form
series no.
part II
(6)
Equation 6 can also be written in the form
part II
part I
(5)
where RM is the Amberlite IRC-50 in the metal form, X represents the ligand sorbed, and RM(X)n is the metal ligand complex formed in the resin. Applying the law of mass action to reaction 5, the equilibrium constant can be written as
part II
series no.
part II
series no.
(8)
(9)
After taking the log, eq 9 is transformed into
log
θ ) log K + n log X 1-θ
(10)
Equation 10 is an equation of a straight line. By plotting log [θ/(1 - θ)] against log X, one can determine both the stability and the stoichiometry of the complexes formed inside the resin. The representative plots for metal complexation inside the resin are presented in Figures 6 and 7. The values of log K and n calculated from the figures are listed in Tables (7-12). As can be seen from (18) Riddick, J. A.; Bunger, W. B.; Sakano, T. K. Organic Solvents, Physical Properties and Methods of Purifcation, 4th ed.; John Wiley and Sons: New York, 1986; Vol. II.
Transition Metal Ion Loaded Amberlite IRC-50
Figure 8. Infrared absorption curves of the resin in metalloaded forms: (s) commercial; (-‚‚-) Na+; (‚‚‚) Cu2+; (- - -) Ni2+; (-‚-) Zn2+.
Langmuir, Vol. 14, No. 9, 1998 2383
Figure 11. Infrared spectra of the Zn form of the resin before and after sorption of ligands: (s) Zn2+; (- - -) zinc ammonia; (-‚-) zinc diethanolamine; (‚‚‚) zinc triethanolamine.
12 occurs in the region of higher sorption densities of the ligand above pH 10. Similarly, the values of n observed in the case of diethanolamine and triethanolamine ligands lie between the range of 1 and 3. As such, the uptake of diethanolamine and triethanolamine occurs by two distinct types of mechanisms depending upon the concentration of amines in the solution.
(R-)2[M(H2O)]2+ + C4H11NO2 h (R-)2[M(C4H11NO2)]2+ + H2O (13) Figure 9. Infrared spectra of the Cu form of the resin before and after sorption of ligands: (s) Cu2+; (- - -) copper ammonia; (‚‚‚) copper diethanolamine; (-‚-) copper triethanolamine.
Figure 10. Infrared spectra of the Ni form of the resin before and after sorption of ligands: (s) Ni2+; (-‚-) nickel ammonia; (- - -) nickel diethanolamine; (‚‚‚) nickel triethanolamine.
these figures, at least two distinct straight lines are obtained, showing that two different types of complexes are formed inside the resin. The values of n indicate that the sorption of ammonia from the aqueous solution takes place according to the following two possible mechanisms:
(R-)2[M(H2O)]2+ + NH3 h (R-)2[M(NH3)]2+ + H2O (11) (R-)2[M(H2O)4]2+ + 4NH3 h (R-)2[M(NH3)4]2+ + 4H2O (12) It is also interesting to note that reaction 11 takes place when the sorption of the ligands is low whereas reaction
(R-)2[M(H2O)3]2+ + 3C4H11NO2 h (R-)2[M(C4H11NO2)3]2+ + 3H2O (14) In comparing the present data (Tables 7-12) with that reported in the literature, it is interesting to observe that the metal-amine complexes in the resin phase are less stable compared to those in the aqueous phase. Similar results were reported in the literature.17 The lesser stability of the complexes inside the resin was explained due to the interaction of the organic part with the resin skeleton and steric hindrance. By comparing the stabilities of the complexes in the aqueous phase to those in the resin, it becomes evident that instead of the absolute stabilities of the complexes involved, the process of ligand sorption/ion exchange is governed by the ratios of the two stability complexes, i.e., in aqueous solution and inside the resin. The ratios of the two follow the order 3.59 > 1.89 > 1.33 for Cu(NH3), Zn(NH3) and Ni(NH3), complexes, respectively, similar to the desorption of the metal cations, which is an indication of the ion-exchange sorption of ammonia. Results similar to those of this work were obtained by Stokes and Walton.2 Infrared Spectroscopy of Loaded Amberlite IRC50. IR studies have been carried out for analysis of the complexation process inside the resin. In the present investigation the spectra of exchangers in H+, Na+, Cu2+, Ni2+, Zn2+, and metal-ligand loaded forms (copperammonia, Ni-ammonia, zinc-ammonia, copper-diethanolamine, nickel-diethanolamine, zinc-diethanolamine, copper-triethanolamine, nickel-triethanolamine, and zinctriethanolamine) are recorded. As is obvious from Figure 8, a broad O-H stretching band appears at 3600-3000 cm-1, while a weak band at 1440-1395 cm-1 refers to the O-H deformation in the plane. The carboxylic groups are evident from C-O and CdO stretching bands at 12171315 and 1710 cm-1, respectively. The absorption band
2384 Langmuir, Vol. 14, No. 9, 1998
Mustafa et al.
at 875-960 cm-1 has been ascribed to the OH-O outof-plane deformation. Similar bands for carboxylic acid were reported by Hadzi and Sheppard.19 The surface-bound carboxylic acid groups can be readily transformed into the corresponding carboxylate salt when treated with metal cations such as Na+, Cu2+, Ni2+, and Zn2+. The IR spectrum of the sodium form of the resin is given in Figure 8. The CdO stretching frequency of the carboxylate carbonyl observed at 1710 cm-1 is shifted to the lower wavelength of 1560 cm-1. This is consistent with literature reported20 in case of sodium acetate salt, where the absorption occurs at 1578 cm-1. As discussed in the literature, the carboxylate ion may coordinate to a metal in one of the following modes. O
M O C R O unidentate complex
M
M O C R
O bidentate complex
C R M O bridging complex
The difference (∆) between the CdO stretching frequencies in the H+ and salt form of the resin could decide about the nature of the carboxylic acid complexes form inside the resin. The ∆ value in the case of Na is 150, which is between the ∆ values observed for the ionic and bridging complexes. However, the resin carboxylate (19) Hadzi, D.; Sheppard, N. Proc. R. Soc. London, Ser. A 1953, 216, 247. (20) Cheng, S. S.; Scherson, D. A.; Sukenik, C. N. Langmuir 1995, 11, 1190. (21) The Sadtler Standard Spectra. Spectra 1962, 93, 10636.
sodium bond should be ionic in character due to the demand of electroneutrality inside the resin. In the Cu, Ni, and Zn forms of the resin (Figure 8), the shift in the band at 1710 cm-1 occurs, and instead of a single absorption at 1560 cm-1, a series of bands appear in the region 1550-1570 cm-1. Such a behavior would predict that the nature of bonds formed by the transition-metal cations may vary from ionic to bridging. The ∆ values found in the case of Cu, Ni, and Zn forms of the resin are 150, 140, and 140, respectively, showing the bond character to be ionic rather than bridging, as is observed in the case of Na+. The desorption of the metal cations as discussed in the preceding section also points toward the same. To study the complexes of Cu, Ni, and Zn formed with ammonia, diethanolamine, and triethanolamine (IR spectra are reported in Figures 9-11), the CdO stretching band is found in the same region (1550-1570 cm-1) as in the case of the metal-loaded resin. As such, the presence of the ligands ammonia and ethanolamines has no effect on the nature of the metal-resin bond. However, the appearance of the new bands at 1100-1000 and 34002900 cm-1 due to C-N stretching and C-H or O-H stretching confirms the presence of the metal-amine complexes inside the resin. These new bands are also observed for metal-amine complexes in solution.21 This is understandable as the resin phase would behave more or less like that of the aqueous solution due to the swelling of the resin matrix. Acknowledgment. We greatly appreciate the valuable comments by the anonymous reviewers. LA9706328
