Chelating resins VII: studies on chelating resins of formaldehyde and furfuraldehyde-condensed phenolic Schiff base derived from 4,4′-diaminodiphenylsulphone and o-hydroxyacetophenone

Reactive & Functional Polymers 42 (1999) 37–52 www.elsevier.com / locate / react

Chelating resins VII: studies on chelating resins of formaldehyde and furfuraldehyde-condensed phenolic Schiff base derived from 4,49-diaminodiphenylsulphone and o-hydroxyacetophenone S. Samal*, N.K. Mohapatra, S. Acharya, R.K. Dey Department of Chemistry, Ravenshaw College, Cuttack 753003, India Received 15 November 1997; received in revised form 6 April 1998; accepted 29 June 1998

Abstract The synthesis, characterization and capacity studies of two chelating resins having multiple functional groups capable of coordinating to several metal ions are reported. The resins were synthesized by condensing phenolic Schiff bases derived from 4,49-diaminodiphenylsulphone and o-hydroxyacetophenone with formaldehyde / furfuraldehyde. The polymeric Schiff bases were found to form complexes readily with several transition metal ions. The resins were completely soluble in dimethyl sulphoxide, tetrahydrofuran, partially soluble in CHCl 3 , CCl 4 , and insoluble in water. On formation of the polychelate with transition metal ions such as Cu(II) and Ni(II), the solubility sharply decreased. The Schiff bases, resins and the polychelates were characterized by FTIR, FT 1 H-NMR, 13 C-NMR and XRD studies, and thermal analyses like TGA and DSC. From FTIR studies the phenolic oxygen and the imine nitrogen of the resins were found to be the coordination sites. The 1 H-NMR data indicated the presence of bridging methylene and terminal methylol functions in the formaldehydecondensed Schiff base. The thermal stability of the resins and the polychelates was compared by analysing TG data which provided the various kinetic parameters like activation energy, frequency factor and entropy changes associated with the thermal decomposition. The DSC and XRD data indicated that the incorporation of the metal ions significantly enhanced the degree of crystallinity. The adsorption characteristics of the resins towards Cu(II) and Ni(II) in dilute aqueous solutions were followed spectrophotometrically. Cu(II) was seen to undergo preferential adsorption in a mixture of Cu(II) and Ni(II). The effects of contact time, pH, temperature, the size of the sorbents and the concentration of the metal ions in solution on the metal uptake behavior of the resins were studied.  1999 Elsevier Science B.V. All rights reserved. Keywords: Chelating resins; Phenolic Schiff bases; Formaldehyde; Furfuraldehyde

1. Introduction The chelate-forming polymeric ligands have been extensively studied by several authors *Corresponding author. Tel.: 191-671-602-335; fax: 191-671619-102. E-mail address: [email protected] (S. Samal)

[1,2]. Such ligands are characterized by the presence of reactive functional groups of O, N, S, and P in the polymer matrix capable of coordinating to different metal ions. The materials most often show preferential selectivity towards certain metal ions facilitating their use for preconcentration and separation of trace

1381-5148 / 99 / $ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S1381-5148( 98 )00055-8

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S. Samal et al. / Reactive & Functional Polymers 42 (1999) 37 – 52

metal ions from saline and non-saline water. The selective behavior is primarily based on the stability of the polychelates at appropriate pH values. Their analytical use in conjunction with atomic absorption spectroscopy (AAS) and inductively-coupled plasma optical emission spectrometry (ICP-OES) studies has been well established [3,4]. Moyers and Fritz [5] condensed m-phenylenediamine tetraacetic acid with resorcinol and formaldehyde to get a resin containing two iminodiacetic acid functional groups anchored to benzene ring and used the resin to separate Co(II) from Ni(II) in a gravity flow column. Several authors [6,7] studied resorcinol-formaldehyde oxime polymers and found the resins to be very selective for heavy metal ions. Condensation of phenol-formaldehyde and piperazine resulted in a resin selective for Cu(II) [8,9]. Schiff bases having multidentate coordination sites are known to form complexes with transition metal ions readily [10–15]. Present in a polymeric matrix, they are expected to show affinity and selectivity towards these metal ions at an appropriate pH. This led us to synthesize a number of phenolic Schiff bases by condensing several aliphatic and aromatic diamines with phenolic carbonyls, or conversely, dicarbonyls with aminophenols. These Schiff bases were found to undergo condensation polymerization readily with formaldehyde and furfuraldehyde. In the reaction conditions set for polycondensation, the C=N bond of the Schiff bases did not undergo hydrolytic cleavage. The present communication deals with the resins synthesized by condensing phenolic Schiff base derived from 4,49-diaminodiphenylsulphone (DDS) and o-hydroxyacetophenone (o-HAP) with formaldehyde (HCHO) / furfuraldehyde (FFD). The resulting polymers o-HAP-DDS-HCHO and o-HAP-DDS-FFD were found to form polychelates with a number of transition metal ions. The resins and the polychelates were characterized by spectral studies and thermal analyses. The adsorption

characteristics of the resins towards Cu(II) and Ni(II) in their dilute aqueous solutions were followed by varying contact time, pH, temperature, size of the sorbents and concentration of the metal ions in solution spectrophotometrically. 2. Experimental The starting materials such as 2-hydroxyacetophenone, 4,4-diaminodiphenylsulphone (Merck, Germany), the sulphate and nitrate salts of Cu(II) and Ni(II), formaldehyde, fufuraldehyde (Merck / Qualigen, India, AnalaR grade) were used as received. The solvents were distilled prior to use.

2.1. Preparation of Schiff base The Schiff base monomer o-hydroxyacetophenone-4,49-diaminodiphenylsulphone (oHAP-DDS) was synthesized by reacting 2.48 g (0.01 mol) of 4,49-diaminodiphenylsulphone dissolved in 10 ml of methanol with 2.72 g (0.02 mol) of o-hydroxyacetophenone in the presence of 0.5 g of anhydrous sodium acetate. The mixture was refluxed for 1 h at 708C, and allowed to stand. The solid crystals were filtered off, washed repeatedly in demineralized water and recrystallized from ethanol. The Schiff base was isolated as a light yellow crystalline solid.

2.2. Preparation of the resin Schiff base monomer (1 g, 0.002 mol) suspended in 20 ml water at 408C was dissolved by adding a few drops of 1 M NaOH. Formaldehyde / furfuraldehyde in 1:2 molar ratio was added and the mixture was refluxed in an oil bath at 120–1308C for 2 h. The insoluble resin was filtered off, washed repeatedly with distilled water and dried at 708C. The yield of both the resins o-HAP-DDS-HCHO and o-HAP-DDSFFD was found to exceed 80%. The dry resin was powdered, sieved (100 mesh, ASTM) and suspended over water at pH 4 overnight. It was

S. Samal et al. / Reactive & Functional Polymers 42 (1999) 37 – 52

filtered off, washed in a large excess of water followed by methanol and dried in vacuum at 708C. o-Hydroxyacetophenone was also condensed with formaldehyde and furfuraldehyde yielding o-HAP-HCHO and o-HAP-FFD resins. The spectral features of these resins were compared with the Schiff base resins.

2.3. Preparation of the polychelate To 100 mg of the dry resin (100 mesh, ASTM) suspended over methanol, 10 ml of metal salt (0.15 M) in water was added. The mixture was stirred for 2 h at 408C. It was filtered off, washed in distilled water followed by petroleum ether and dried in vacuum at 708C.

2.4. Metal uptake The resins were treated with aqueous solutions of Cu(II) and Ni(II) of known concentration. The pH of the solutions was adjusted to the desired value using either 0.1 M HCl or 0.1 M NaOH. A suspension of the resin on the metal solution of known volume and concentration were agitated for a definite time period over a hot plate / magnetic stirrer. The resins were filtered off and were washed thoroughly with demineralized water. The filtrate along with the washings were collected and quantitative determination of metal ion concentration was done spectrophotometrically following the neocuproin method for Cu(II) and dimethyl glyoxime method for Ni(II) [16]. From the data the percentage of metal loading and capacity were calculated using the following equations: Wi 2 Wf Metal uptake (%) 5 ]]] 3 100 Wi

(1)

where Wi 5mg of metal ion in solution initially present; Wf 5mg of metal ion left in the solution after adsorption.

39

Capacity (mmol / g of resin) mg of metal ion adsorbed by the resin 5 ]]]]]]]]]]]] atomic mass of the metal 3 g of the resin (2)

2.5. Measurements The elemental analysis was carried out in a Carlo Erba 1108 elemental analyzer. The FTIR spectra were recorded in a Perkin Elmer spectrometer model 1800 in the range 4000 to 400 cm 21 in KBr phase. The 1 H-NMR spectra were recorded in DMSO d 6 solvent in a 300-MHz FT NMR (Bruker DRX-300) instrument. The proton decoupled 13 C FT NMR spectra of the Schiff base and the resins was run in a VXR 300-S-Varian Supercon NMR spectrometer operating at 75 MHz. The TG and DSC of the materials were recorded in a DuPont 9900 thermogravimetry analyzer at a heating rate of 108C / min in nitrogen atmosphere. The XRD study was performed in a PW 1820 diffractometer using a Cu-X-ray tube operating at 40 kV and 30 mA in the 2u range of 4 to 358. The estimation of the metal ion concentration in the dilute aqueous solutions was made using a Systronics Digital Spectrophotometer model 116 and the pH of the solutions were measured in a Systronics Digital pH Meter model 335.

3. Results and discussion

3.1. Solubility The powdered resin (10 mg) was suspended over 5 ml of the chosen solvent and the solubility was checked after 24 h at room temperature. Both the resins were found to be insoluble in water, partially soluble in methanol, ethanol, CHCl 3 , and CCl 4 but completely soluble in DMSO and tetrahydrofuran (THF) (Table 1). The polychelates were found to be insoluble in most of these solvents. The decrease in solubility of the polychelates could be due to alteration in polymer polarity and the in-

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S. Samal et al. / Reactive & Functional Polymers 42 (1999) 37 – 52

Table 1 Physical data Compound

o-HAP-DDS o-HAP-DDS-HCHO o-HAP-DDS-HCHO-Cu(II) o-HAP-HCHO o-HAP-DDS-FFD o-HAP-DDS-FFD-Cu(II) o-HAP-FFD a

Colour

Yellowish-white Yellow Blue Yellow Black Grey Black

Solubility in different solvents a

N (%) Found

Calc.

H2O

CH 3 OH

C 2 H 5 OH

CHCl 3

CCl 4

THF

DMSO

5.75 1.816 0.096 8.9 2.085 1.856 9.32

5.78 – – – – – –

2 2 2 2 2 2 2

1 6 2 2 2 2 2

1 6 2 2 2 2 2

1 6 2 6 6 2 6

1 6 2 6 6 2 6

1 1 6 1 1 6 1

1 1 6 1 1 6 1

(1) Soluble, (6) partially soluble, (2) insoluble.

trapolymer cross-linking [9] as well as increased crystallinity [17]. The solubility of the resins studied over a period of time was seen to decrease steadily (Fig. 1). This could be ascribed to curing of the resins resulting in crosslinking and the consequent increase in molecular weight. The curing is known to be a characteristic of the phenol-formaldehyde resins [18].

3.2. Spectral studies 3.2.1. FTIR spectra The FTIR spectra of the Schiff base, the resins and the polychelates are shown in Fig. 2. In case of the resin o-HAP-DDS-HCHO, an absorption at 2879.5 cm 21 was assigned to symmetrical methylene C–H stretch which was

Fig. 1. Solubilty of the resin dissolved in DMSO as a function of time. Sorbent size — 100 mesh, resin quantity — 10 mg, solvent — 5 ml, (m) o-HAP-DDS-FFD and (♦) o-HAP-DDS-HCHO.

not observed in the Schiff base o-HAP-DDS confirming the polycondensation reaction of the Schiff base with formaldehyde. The C=N and Ph–O absorptions of this resin seen at 1635.7 and 1213 cm 21 , respectively, did not shift on complexation with Cu(II) but their intensity sharply decreased. The C=C stretch at 1485 cm 21 in the resin appeared at 1471.6 cm 21 in the polychelate indicating a decrease in the p-electron density in the aromatic ring. These observations could be ascribed to the coordination of phenolic oxygen and imine nitrogen to the metal ion. This observation is in accordance with the findings of Oriel et al. [19]. These authors observed that incorporation of metal ion into the resin matrix did not result in a shift in the absorptions of the coordinating groups. The S(=O) 2 asymmetric and symmetric stretches of the DDS moiety observed at 1294.1 and 1160 cm 21 in the polychelate did not shift from their respective positions in the resin. Sulphate ion (from CuSO 4 ) was seen to be present as a bridging group. The absorptions at 1109, 987.5, 630, 607.5 cm 21 are characteristics of a bridging sulphate. Nakamato et al. [20] studied bridged sulphato-complexes of Co(II) and reported similar observations. In addition to the Cu(II)–O absorption at 560 cm 21 , the Cu(II)–N absorption was observed at 488 cm 21 as a sharp singlet. Ueno and Martell [10] have made extensive band assignments for the metal chelate compounds of bis-acetylacetone-ethylenediimine and bis-salicylaldehyde-ethylenediimine and have assigned the metal–ligand

S. Samal et al. / Reactive & Functional Polymers 42 (1999) 37 – 52

41

Fig. 2. (A) FTIR spectra of (a) o-HAP-DDS, (b) o-HAP-DDS-HCHO, (c) o-HAP-DDS-HCHO-Ni(II) and (d) o-HAP-DDS-HCHO-Cu(II). (B) FTIR spectra of (a) o-HAP-DDS-FFD, (b) o-HAPDDS-HCHO-Cu(II) and (c) o-HAP-DDS-HCHO-Ni(II).

stretching vibrations in these Schiff base complexes to the range 640–500 cm 21 for the M– O, and 580–430 cm 21 for the M–N bonds. For the polychelate o-HAP-DDS-HCHONi(II), similarly, no significant change was noticed for C=N absorption. The Ph–O absorption was seen to shift from 1213.1 to 1217 cm 21 . Thamizharasi et al. [21] have observed a shift of Ph–O absorptions to higher wavenumber on complexation. The asymmetric S(=O) 2 stretch at 1296 cm 21 merged with the phenolic O–H bond at 1307 cm 21 , the symmetric S(=O) 2 stretch remaining unaffected at 1141.8 cm 21 . A sharp singlet at 1384.8 cm 21 was due to the NO 2 3 (from Ni(NO 3 ) 2 ) present as a non-coordinating counter anion [22]. The Ni(II)–O and Ni(II)–N absorptions were recorded at 559.3 and 478.3 cm 21 , respectively. In case of the furfuraldehyde condensed resin

o-HAP-DDS-FFD, C=N stretch was noticed at 1668.3 cm 21 and the Ph–O stretch at 1213.1 cm 21 . On complexation with Cu(II), both the absorptions disappeared. The S=O absorptions 22 of the sulphone and the SO 4 ion overlapped. Characteristic absorptions at 1118.6, 1014, and 21 613.3 cm were assigned to a bridging sulphate as was seen in the case of the Cu(II)polychelate of formaldehyde-condensed resin. 21 The absorptions at 613.3 and 524.6 cm were assigned to Cu(II)–O and Cu(II)–N bonds, respectively. The polychelate o-HAP-DDSFFD-Ni(II), on the other hand, registered a shift in the C=N absorption from 1668.3 cm 21 in the resin to 1645 cm 21 in the polychelate. A sharp 21 peak at 1384.8 cm was assigned to nitrate anion present in the resin matrix. The Ni(II)–O absorption was observed at 513 cm 21 and Ni(II)–N absorption was not prominent.

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S. Samal et al. / Reactive & Functional Polymers 42 (1999) 37 – 52

3.2.2. 1 H-NMR spectra The proton NMR spectra of the Schiff base, resins and the polychelates are shown in Fig. 3. In the Schiff base o-HAP-DDS, the methyl protons of hydroxyacetophenone moiety were observed at 2.6 ppm. The aromatic protons in the range 6.5–7.8 ppm were assigned as shown in the diagram. The phenolic proton was noticed as a sharp singlet at 5.96 ppm. The formaldehyde-condensed resins o-HAP-HCHO and oHAP-DDS-HCHO had a number of additional peaks in the form of complex multiplets in the range 3.5 to 5 ppm. These were ascribed to the bridging methylene (f–CH 2 –f) and the methylene groups of the terminal methylol (f– CH 2 OH) functions. The furfuraldehyde-condensed resins o-HAPFFD and o-HAP-DDS-FFD did not register the bridging methyne proton. Such a proton in the triphenylmethane type structure is assigned in the range 5.5 to 6 ppm [23] and Dimitrov et al.

[24] also noticed the methyne proton in this range. The appearance of this proton in the relatively weak field was attributed to the higher descreening effect associated with the triphenylmethane type structure. A comparison of the area of the aromatic proton peaks of the resins and that of the Schiff base was made which led to an empirical calculation of the molecular weight of the resins assuming that all the ortho and para positions of the phenolic moiety have undergone substitution reaction and the results are |6000 for o-HAP-DDS-HCHO and |8000 for o-HAPDDS-FFD.

3.2.3. 13 C-NMR spectra The 13 C-NMR spectra of o-HAP-DDS and its resins are given in Fig. 4. Condensation with formaldehyde generated a number of additional peaks between 50 to 70 ppm. These peaks could be assigned to the methylene groups appearing

Fig. 3. (A) 1 H-NMR (300 MHz) spectra of (a) o-HAP-DDS and (b) its expanded aromatic region; (B) 1 H-NMR (300 MHz) spectra of (a) o-HAP-HCHO, (b) o-HAP-DDS-HCHO, (c) o-HAP-FFD and (d) o-HAP-DDS-FFD.

S. Samal et al. / Reactive & Functional Polymers 42 (1999) 37 – 52

Fig. 4. 13 C-NMR spectra of (a) o-HAP-DDS, (b) o-HAP-DDS-HCHO and (c) o-HAP-DDS-FFD. 43

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S. Samal et al. / Reactive & Functional Polymers 42 (1999) 37 – 52

as bridging and terminal functions. Brauman et al. [25] have studied the 13 C-NMR spectra of phenol / cresol-formaldehyde resin and assigned peaks in the range 30–42 ppm to ortho–ortho, ortho–para and para–para –CH 2 – bridging. In our case the methylene bridging was noticed in the range 45–60 ppm. The down-field shift of the methylene peak was assigned to deshielding caused by the electron drift towards –SO 2 – group. These authors also assigned the orthoand para-substituted terminal CH 2 OH carbons to peaks in the vicinity of 60 ppm. These peaks in the present case appeared in the range 68 to 69.95 ppm. The region between 111.4 to 136.5 ppm was assigned to the aromatic carbons. The phenolic C–OH carbon at 152.8 ppm in the Schiff base shifted to 160.7 ppm in the resin. The o-HAP-DDS-FFD resin showed a number of peaks in the region between 109.9 to 158.3 ppm. The peak at 158.3 was ascribed to the phenolic C–OH carbon. A sharp peak at 71.5 ppm was assigned to the –CH(OH)– terminal function consequent upon the reaction of the phenolic moiety with furfuraldehyde. Zigon et al. [26] while studying the 13 C-NMR spectra of resorcinol-cinnamaldehyde resin observed the –CH(OH)– signals between 65 and 72 ppm.

3.3. Thermogravimetric analyses The thermograms of the resins and the Cu(II) polychelates are shown in Fig. 5 and the relevant data are furnished in Table 2. Up to 2008C the resin o-HAP-DDS-HCHO lost 3.8% of its original weight whereas its Cu(II)-polychelate lost only 0.19%. Between 200 and 4008C, the materials suffered rapid weight loss. The loss rate was 0.2556% / 8C for the resin at 342.318C, the temperature of maximum rate of weight loss (T max ); for the polychelate it was 0.4121% / 8C at T max of 341.918C. Thus the polychelate was seen to lose weight at a much faster rate than the resin above 2008C. The furfuraldehyde condensed resin, on the other hand, lost 6.15% of its original weight within 2008C whereas its Cu(II) polychelate lost

9.47%. Between 200 and 4008C the rate of weight loss for the resin was 0.2861% / 8C at a T max of 229.498C, whereas the polychelate lost at the rate of 4.268% / 8C at a T max of 288.688C. In this case also the polychelate lost weight much faster than the resin. The thermograms could not provide a clear indication of relative thermal stability. Various kinetic models such as Coats–Redfern, VanKrevelen and Broido were employed fitting in 14 different mechanisms into each model: Coats–Redfern [27] ln[g(a ) /T 2 ] 5 ln[AR /b E(1 2 2RT /E)] 2 E /RT (3) Van Krevelen [28] ln[g(a )] 5 ln[A /b (0.368 /T max )E / RT max (E /RT m 1 1)21 ] 1 (E /RT max 1 1) ln T

(4)

Broido [29] 2 ln[g(a )] 5 ln[A /b 3 (R /E)T max ] 2 (E /R) 3 1 /T

(5) where a5[W0 2W] / [W0 2Wf ], W0 5initial weight, W5weight at temperature T, Wf 5final weight, b5heating rate, T max 5temperature of maximum rate of weight loss (K), E5activation energy (kcal), A5frequency factor (s 21 ) and g(a)5a function of a, the various a values depending on the mechanism of thermal decomposition. The activation energy E and the frequency factor A were computed for the different models from the slope and the intercept of the plot of ln[g(a) /T 2 ] vs. 1 /T for the Coats and Redfern, ln[g(a)] vs. ln T for the Van Krevelen and ln[g(a)] vs. 1 /T for the Broido models after linear regression analysis of the data (Fig. 6). The Coats–Redfern model does not involve T max in the expression. This model was used to evaluate the kinetic parameters in the entire range (130–6008C). On the other hand, the Van Krevelen and Broido models make use of T max in the expressions and hence were used to

S. Samal et al. / Reactive & Functional Polymers 42 (1999) 37 – 52

45

Fig. 5. TG-DTG traces of (a) o-HAP-DDS-HCHO and (b) o-HAP-DDS-HCHO-Cu(II).

calculate the kinetic parameters for different decomposition stages using the relevant T max value of each stage. The entropy of activation

DS (cal K 21 mol 21 ) was calculated using the relation A5(kT max /h)(e DS / R ) where k is the Boltzmann constant (0.32944310 223 cal K 21 ),

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S. Samal et al. / Reactive & Functional Polymers 42 (1999) 37 – 52

Table 2 TG data Sample

o-HAP-DDS-HCHO o-HAP-DDS-HCHO-Cu(II) o-HAP-DDS-FFD o-HAP-DDS-FFD-Cu(II) a

T m (K)a

615.31 614.91 502.49 563.23

Yc (%)b

% Weight loss in the temp. range Up to 2008C

200–4008C

400–6008C

3.8 0.19 6.15 9.47

33.04 32.55 27.93 49.6

54.437 0.67 39.41 1.23

8.723 66.59 26.51 39.7

Temperature of maximum rate of decomposition. Char yield at 6008C.

b

Fig. 6. Coats–Redfern plot of thermogravimetric analysis data of o-HAP-DDS-HCHO-Cu(II).

h, the Planck’s constant (1.5836310 234 cal s), 21 21 and R, the gas constant (1.9872 cal K mol ). The data are furnished in Table 3. An analysis

of the data indicated that in case of o-HAPDDS-HCHO, metal ion incorporation into the resin matrix enhances thermal stability; whereas the o-HAP-DDS-FFD resin and its Cu(II) polychelate are nearly similar in their thermal response. Biswas and co-workers [30,31] have noted that incorporation of the metal ion increases thermal stability. Chiang and Mei [32], on the other hand, observed that complexation did not bring any improvement in thermal stability. They attributed the results to intra-polymer cross-linking, because if there were interchain crosslinking, a strengthened matrix and consequently a higher thermal stability for the polychelate over that of the resin would have been observed.

Table 3 Kinetic parameters Sample

T m (K)

Methods a

E (kcal)

A (min 21 )

DS (cal K 21 )

o-HAP-DDS-HCHO

615.31

CR VK BR

20.037 7.582 8.591

2.14310 5 5.6 1.505310 1

243.721 264.677 262.713

o-HAP-DDS-HCHO-Cu(II)

614.91

CR VK BR

19.674 17.274 18.012

3.593310 8 5.156310 5 1.361310 6

228.961 241.967 240.039

o-HAP-DDS-FFD

502.49

CR VK BR

13.344 4.01 5.333

1.333310 4 – 1.8415

248.827 – 266.483

o-HAP-DDS-FFD-Cu(II)

563.23

CR VK BR

16.093 5.257 6.8

1.198310 8 3.615 1.772310 1

230.963 254.201 262.207

a

CR, Coats–Redfern; VK, Van Krevelen; BR, Broido.

S. Samal et al. / Reactive & Functional Polymers 42 (1999) 37 – 52

3.4. DSC study The DSC trace of the resins and the polychelates are shown in Fig. 7. The data indicated that the resin o-HAP-DDS-HCHO, beyond its glass transition temperature (T g , end set 508C), had no exotherm indicating thermal curing of the resin was noticed. The material exhibited a sharp melting temperature at 183.338C and

47

continued to absorb thermal energy till about 3158C, beyond which exothermic decomposition reactions set in. The Cu(II) polychelate (T g endset 99.098C) furnished several endothermic peaks pertaining to desorption of the solvents and melting of the different components. The melting endotherm was prominent with a peak at 257.428C. The o-HAP-DDS-FFD resin on the other hand showed a small curing exotherm

Fig. 7. DSC traces of (a) o-HAP-DDS-FFD and (b) o-HAP-DDS-FFD-Cu(II).

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S. Samal et al. / Reactive & Functional Polymers 42 (1999) 37 – 52

followed by a melting at 136.98C. Incorporation of the metal ion enhanced crystallinity of the material seen from a very prominent endotherm with a melting temperature at 114.378C in the DSC trace of o-HAP-DDS-FFD-Cu(II).

3.5. XRD study The XRD pattern of the resin o-HAP-DDSHCHO and its Cu(II)-polychelate is presented

in Fig. 8. The resin exhibited two peaks at (2u) 20 and 32.788. The polychelate exhibited a number of reflection planes, the intense ones appearing at 18.82, 24.73 and 33.788. This was assignable to a significant increase in crystallinity of the polymer consequent upon coordination to the metal ion. From a comparison of the peak area it was observed that the polychelate could be nearly 24 times more crystalline than the resin. However, the observed increase in the

Fig. 8. XRD pattern of (a) o-HAP-DDS-HCHO and (b) o-HAP-DDS-HCHO-Cu(II).

S. Samal et al. / Reactive & Functional Polymers 42 (1999) 37 – 52

number of reflections and intensities could be also the result of an increase in electron density due to metal ion uptake. Yang and Chen [17] reported that the polymers having p-phenylene units exhibited one strong reflection at around 208 and weaker reflections at 278 and the authors assigned these peaks to the crystalline nature of the resins.

3.6. Structural features The information from the foregoing studies led us to propose a structure for the resins and a Cu(II)-polychelate. It was seen from the FTIR spectra of o-HAP-DDS-HCHO-Cu(II) that the metal ion is bound to the resin matrix via coordination of phenolic oxygen and the imine nitrogen. The SO 22 ion was seen to be present 4 as a bridging group. To account for these

Fig. 9. Reaction scheme showing the inter-molecular structure of the polychelate o-HAP-DDS-HCHO-Cu(II) with the sulphate group as a bridging function.

49

observations, a structure for o-HAP-DDSHCHO-Cu(II) was proposed (Fig. 9).

3.7. Metal ion uptake studies 3.7.1. Effect of contact time The saturation time for the metal uptake of the resins was obtained by plotting percentage of metal uptake against contact time keeping the initial metal ion concentration (0.08 M) constant (Fig. 10). The equilibrium time was found to be 5 min in the natural pH of the salt solutions for Cu(II) and 15 min for Ni(II). Both the resins showed almost identical capacity (4.07 mmol / g) for Cu(II) whereas the capacity of the resins towards Ni(II) was low. Several authors [33,34] have noted higher adsorption for Cu(II) over other metal ions. It is known that the insoluble chelating resins take up transition metal ions in high yields from aqueous media but they often adsorb metal ions very slowly due to the lower activity of the ligands placed inside the resin. The metal complexing capacity of the resin depends not only on the nature of the ligand groups but also the accessibility towards the metal ions. Thus steric hindrance by the polymeric matrix and the hydrophobic nature of the polymeric ligand can limit the chelating reaction [35].

Fig. 10. Effect of contact time on sorption behavior of the resins: resin quantity5100 mg, sorbent size5100 mesh, temp.5308C, pH54.1, [M 21 ]50.08 M, extent of metal loading of the resin o-HAP-DDS-HCHO for (h) Cu(II), (n) Ni(II) and resin o-HAPDDS-FFD for (m) Cu(II) and (j) Ni(II).

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S. Samal et al. / Reactive & Functional Polymers 42 (1999) 37 – 52

3.7.2. Effect of metal ion concentration The uptake of Cu(II) and Ni(II) was studied in the metal ion concentration range 0.04–0.32 M. Increasing metal ion concentration enhanced metal ion loading within the range of study (Fig. 11) beyond which a leveling effect was noticed because of a saturation of the available coordinating sites. Tikhomirova et al. [36] have observed a similar trend. From this study the optimum metal ion concentration for the determination of the capacity of the resins was ascertained to be 0.08 M. Decreasing metal ion concentration slows down the reaction considerably. The adsorption coefficients, k ad , of the resins for each metal ion adsorbed was computed from the Freundlich adsorption isotherm:

adsorption, k ad 54 s 21 and n50.6. The high k ad values indicated that the equilibrium for metal ion adsorption was attained quickly.

3.7.3. Effect of temperature The effect of temperature variation from 25 to 658C was studied for Cu(II) for both the resins at the natural pH of the metal ion solution. The capacity of both the resins remained nearly unaffected in the range of temperature. This could be because of the high reactivity of the resins and consequent saturation of the available coordination sites even under ambient conditions.

where C is the initial concentration of the metal ion in mmol, m the weight of the resin in grams, x the quantity of the metal ion adsorbed by the resin in mmol and n a constant. The results are as follows; resin: o-HAP-DDS-HCHO, for Cu(II) adsorption, k ad 55.41 s 21 , n50.49; and for Ni(II) adsorption, k ad 54.04 s 21 , and n5 0.6; resin: o-HAP-DDS-FFD, for Cu(II) adsorption, k ad 54.38 s 21 , n50.54 and for Ni(II)

3.7.4. Effect of pH of the reaction medium The effect of pH on the extent of adsorption of Cu(II) was studied for o-HAP-DDS-FFD in two different concentrations of the metal ion, 0.0008 and 0.08 M. Cu(OH) 2 was precipitated at pH 5.1 preventing capacity studies beyond pH 5. The metal uptake was seen to significantly increase with increasing pH (Fig. 12). This was ascribed to the ease of coordination of the phenoxide ion over that of the phenolic-OH group at higher pH and also the enhanced basicity of the C=N nitrogen which got protonated in the acidic conditions. Several workers

Fig. 11. Variation of metal ion concentration: resin — 100 mg, size — 100 mesh, time — 15 min, temp — 308C, pH55, adsorption of (j) Cu(II), (h) Ni(II) on o-HAP-DDS-HCHO and (♦) Cu(II) and (m) Ni(II) on o-HAP-DDS-FFD.

Fig. 12. Effect of pH of the reaction medium: resin: o-HAP-DDSFFD, quantity — 100 mg, size — 100 mesh, time — 15 min, temp — 308C, [M 21 ]50.08 M. (m) Ni(II), (♦) Cu(II).

log(x /m) 5 logk ad 1 1 /n logC

(6)

S. Samal et al. / Reactive & Functional Polymers 42 (1999) 37 – 52

51

Table 4 Adsorption studies of Cu(II) and Ni(II) under competitive conditions: resin o-HAP-DDS-FFD, 100 mg, contact time 15 min, mesh 100, temperature 308C, pH 5 [Cu(II)], M

[Ni(II)], M

% Loading

[Ni(II)], M

[Cu(II)], M

0.16 0.8 1.12 1.44 1.6

0 0 0 0 0

13.877 60.137 68.36 74.3 75.885

0.16 0.8 1.12 1.44 1.6

0 0 0 0 0

[Cu(II)], M

[Ni(II)], M

% Loading

0.16 0.48 0.8 1.12 1.44

1.44 1.12 0.8 0.48 0.16

% Loading 2.095 38.245 47.5237 52.312 54.748

Selectivity coefficient a

Cu(II)

Ni(II)

3.3 23.35 58.661 65.35 73.753

50.903 43.8 32.92 16.5 1.8

0.0329 0.3908 2.8872 9.0246 153.345

Concentrations of the metal ions in mmol. h[Cu(II)] / [Ni(II)]j R Selectivity coefficient, D 5 ]]]]] where R represents the metal content in resin, S represents metal content in solution. h[Cu(II)] / [Ni(II)]j S

[37] have reported the enhanced adsorption of metal ions with an increase in the pH.

3.7.5. Adsorption behavior in the presence of both Cu( II) and Ni( II) The capacity of o-HAP-DDS-FFD resin towards both Cu(II) and Ni(II) in a mixture was studied to determine the behavior of the resins under competitive conditions. The results are furnished in Table 4. The concentrations of the metal ions in the mixture were varied maintaining the pH at a fixed value. At low Ni(II) concentration, the capacity of the resin towards Cu(II) remained nearly unaffected. With increasing Ni(II) concentration, the capacity of the resin towards Cu(II) progressively decreased. The extent of adsorption of Ni(II) as compared to Cu(II) was significantly low. Dev and Rao [37] employed N-hydroxyethylethylenediamine functionalized resins for effective separation of Cu(II) from Ni(II), Co(II) and Cd(II). The selectivity coefficient [38] of the resin was calculated and it was seen that the resin showed a preferential selectivity towards Cu(II) than Ni(II) at low Ni(II) concentration. Further details of the study involving preconcentration and separation of some trace

elements like UO 21 and Mo(IV) in the presence 2 of other anions and cations are being carried out and the results will be presented in our subsequent communications. Acknowledgements We are thankful to the Regional Sophisticated Instrumentation Centres of India located at Central Drug Research Institute, Lucknow, Indian Institute of Technology (IIT), Chennai, IIT-Mumbai, and Punjab University, Chan1 13 digarh for providing FTIR, FT H / C-NMR, TG-DTG, DSC and XRD results. References [1] E. Blasius, B. Brozio, in: H.A. Flaschka, A.J. Barnard Jr. (Eds.), Chelates in Analytical Chemistry, vol. I, Marcel Dekker, New York, 1967, p. 49. [2] C. Kantipuly, S. Katragadda, A. Chow, H.D. Gesser, Talanta 37 (1990) 491. [3] S. Tao, Y. Shijo, L. Wu, L. Lin, Analyst 119 (1994) 1455. [4] X. Luo, Z. Su, W. Gao, G. Zhang, X. Chang, Analyst 117 (1992) 145. [5] E.M. Moyers, J.S. Fritz, Anal. Chem. 49(3) (1977) 418. [6] H.V. Lillin, Angew. Chem. 66 (1954) 649. [7] R.C. De Geiso, L.G. Donaruma, E.A. Tomic, J. Appl. Polym. Sci. 9 (1965) 411.

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