Synthesis and chelation ion exchange properties of 2, 4-dihydroxyacetophenone- biuret-formaldehyde terpolymer resin

The terpolymer resin 2, 4-dihydroxyacetophenone-biuretformaldehyde has been synthesized by the condensation of 2, 4-dihydroxyacetophenone and biuret with formaldehyde in 2:1:3 molar ratios in presence of 2M hydrochloric acid as catalyst. UVvisible, IR and proton NMR spectral studies have been carried out to elucidate the structure of the resin. The terpolymer has proved to be a selective chelating ion exchange polymer for some metals. Chelating ion exchange properties of this polymer have been studied for Fe, Cu, Ni, Co, Zn, Cd and Pb ions. Batch equilibrium method has been employed to study the selectivity of metal ion uptake and distribution coefficient over a wide pH range and in media of varying ionic strengths. The polymer shows higher selectivity for Fe and Cu ions than for Ni, Co, Zn, Cd and Pb ions. Study of distribution ratio as a function of pH indicates that the amount of metal ion uptake by resin increases with the increasing pH of the medium.

Ion exchange is the reversible exchange of ions between the substrate and surrounding medium.Ion exchange technique can remove traces of ionic impurities for water and process liquors and gives a product of ultra pure quality in a simple efficient and technoeconomically viable manner.Ion exchangers are widely used in analytical chemistry 1 , hydrometallurgy 2 , antibiotic purification and separation of radio isotopes 3 and find large application in water treatment and pollution control 4,5 .
The basic requirements for any polymeric material to be useful as ion exchange resin are: (a) It must be sufficiently hydrophilic to permit diffusion of ions through the structure at a finite and usable rate; (b) It must contain sufficient number of accessible ion exchangeable groups which do not undergo degradation during use; and, (c) The swollen material must be denser than water.
Jadhao and coworker 6 have synthesized a terpolymer resin by condensation of 2, 2'-dihydroxybiphenyl and formaldehyde in the presence of acid catalyst.They studied chelating ion exchange properties of this polymer for Fe 3+ , Cu 2+ , Ni 2+ , Zn 2+ , Cd 2+ , Pb 2+ ions.A batch equilibrium method was employed in the study of the selectivity of metal ion uptake involving the distribution of a given metal ion between polymer sample and solution containing metal ions.The study was carried out over a wide pH range and the polymer was found to show higher selectivity for Fe 3+ , Cu 2+ , Ni 2+ ions than for Zn 2+ , Cd 2+ , Pb 2+ ions.Salih Bekir 7 has prepared modified 1, 4, 8, 11-tetraazocyclotridecane (cyclam) and with an AIBN initiator polymerized the modified cyclam.Cyclam containing polymer in bulk structure was removed from the suspension by filtration after washing and drying.The polymeric materials were used for transition metal ion adsorption and desorption of selected ions, Cu(II), Ni(II), Co(II), Cd(II), and Pb II), from aqueous media containing different amounts of these metal ions at different pH values.The adsorption rates were high and the adsorption equilibrium was reached in about 30 min.The affinity order of the transition metal ions was: Cu(II) > Ni(II) > Co(II) > Cd(II) > Pb(II) for competitive adsorption.Copolymers have also been synthesized by condensation of a mixture of phenol or hydroxybenzoic acid, various amine and formaldehyde [8][9][10][11] .Their ion exchange properties and semiconducting properties have been studied.However, a literature survey revealed that no terpolymers have been synthesized from 2, 4-dihydroxyacetophenone, biuret and formaldehyde.Herein, we report the synthesis and characterization of these terpolymers.
The terpolymer resins were subjected to micro analysis for carbon, hydrogen and nitrogen on Elemental Vario ELIII Carlo Erba 1108 elemental analyzer.The number average molecular weight (M n ) was determined by conductometric titration in DMF using ethanolic KOH as the titrant.The molecular weight (M n ) of the terpolymer resin was determined by non-aqueous conductometric titration in DMF against ethanolic KOH by using 25 mg of the sample.Electronic absorption spectra of the terpolymers in DMSO were recorded on Perkin-Elmer Lambda 15 spectrophotometer.Infrared spectra of 2, 4-HABF terpolymers were recorded on Perkin-Elmer 983 spectrophotometer in KBr pellets in the wave number region of 4000-400 cm -1 . 1 H-NMR spectra were recorded on Bruker Avance II 400 MHz proton NMR spectrometer with DMSO-d 6 as the solvent.
The 2, 4-HABF-II terpolymer was prepared by condensing 2, 4-dihydroxyacetophenone (3.042 g, 0.02 mol) and biuret (1.3 g, 0.01 mol) with formaldehyde (11.25 ml, 0.03 mol) in the presence of 2M HCl as a catalyst at 118 ± 2 °C in an oil bath for 4 hrs [12][13][14][15][16] (Scheme 1).The cream colored solid product was washed with hot water and ether to remove unreacted starting materials and acid monomers.The properly washed resin was dried, powdered and extracted first with diethyl ether and then with petroleum ether to remove 2, 4dihydroxyacetophenone-formaldehyde copolymer which may be present along with the 2, 4-HABF-II terpolymer.For further purification, it was dissolved in 10 % NaOH and then filtered.The terpolymer was then reprecipitated by dropwise addition of 1:1 (v/v) conc.HCl / water with constant stirring and filtered.The process was repeated thrice.The resulting polymer sample was washed with boiling water and dried in vacuum at room temperature.The purified terpolymer was washed, finely ground to pass through 300 mesh sieve and kept in vacuum over silica gel.The yield of the terpolymer was found to be about 80%.
The ion exchange properties of the 2, 4-HABF terpolymer resins were determined by the batch equilibrium method 17 .The ion exchange properties of all the four resins have been studied.However, data for only the 2, 4-HABF-II terpolymer resins have been presented herein.
The terpolymer sample (25 mg) was suspended in an electrolyte solution (25 ml) of known concentration.The pH of the suspension was adjusted as required by using either 0.1M HNO 3 or 0.1 M NaOH.The suspension was stirred for a period of 24 h at 25°C.To this suspension, 2 ml of a 0.1 M solution of the metal ion was added and the pH was adjusted to the required value.The mixture was again stirred at 25°C for 24 h and filtered 14,15 .The terpolymer was washed and the filtrate and washings were combined and estimated for the metal ion content by titration against standard ethylene diamine tetraacetic acid.A blank experiment was also carried out in the same manner without adding the polymer sample.The blank was also estimated for the metal ion content.The experiment was repeated in presence of the several electrolytes.
In order to estimate the time required to reach the state of equilibrium under the given experimental conditions, a series of experiments were carried out in which the metal ion taken up by the chelating resins was estimated from time to time at 25 °C (in presence of 25 ml of 1M NaNO 3 solution).It was assumed that under the given conditions, the state of equilibrium was established within 24 h.The rate of metal uptake is expressed as percentage of the amount of metal ions taken up after a certain time related to that at equilibrium.
The distribution of each one of the seven metal ions, i.e., Fe 3+ , Cu 2+ , Co 2+ , Zn 2+ , Cd +2 , Pb +2 and Ni 2+ , between the polymer phase and the aqueous phase was estimated at 25 °C and in the presence of 1 M NaNO 3 solution.The experiments were carried out as described above at different pH values.The results are presented in Table 2.The distribution ratio 'D' is defined as: D = Weight (mg) of metal ions taken up by 1 g of terpolymer / Weight (mg) of metal ions present in 1 ml of solution or

Results and discussion
The newly synthesized purified 2, 4-HABF resin was found to be cream in colour and soluble in DMF, The molecular weight (M n ) of the terpolymer resin was determined by non-aqueous conductometric titration in DMF against ethanolic KOH by using 50 mg of sample.The average degree of polymerization as given by DP = (Total meq. of base required for complete neutralization) / (meq. of base required for smallest interval) is found to be 5.20.The number average molecular weight (M n ) is 2303 as obtained by multiplying the DP by the formula weight of the repeating unit 20,21 .
The UV-visible spectra of the 2, 4-HABF terpolymer samples in pure DMF were recorded in the region 190-700 nm at a scanning rate of 100 nm min -1 and at a chart speed of 5 cm min -1 .The UV-visible spectra of terpolymers are similar showing their almost similar nature.The spectra of these terpolymers exhibit two absorption maxima in the regions 280 nm and 304 nm, indicating the presence of carbonyl group (ketonic) having a carbon-oxygen double bond which is in conjugation with the aromatic nucleus.The appearance of the former band (more intense) can be accounted for by π→π* transition while the latter band (less intense) may be due to n→π*electronic transition.The shift from the basic value, i.e., 240 nm and 320 nm respectively, may be due to conjugation effect, and presence of phenolic hydroxy group (auxochromes) responsible for hyperchromic effect, i.e., higher ε max values 15,17 .
Theε max value gradually increases in the order: 2, 4-HABF-I < 2, 4-HABF-II < 2, 4-HABF-III < 2, 4-HABF-IV.This increasing order of ε max values may be due to introduction of more and more chromophores (carbonyl group) and auxochromes (phenolic-OH groups) in the repeat unit of the terpolymer resins 14,21 .This observation is in good agreement with the proposed most probable structures of these terpolymer resins.
The IR spectral studies reveal that all the 2, 4-HABF terpolymers give rise to nearly similar pattern of spectra.A broad band appearing in the region 3415-3361 cm -1 may be assigned to the stretching vibration of phenolic-OH group 17,28,29 .The sharp band at 1627 cm -1 may be on account of the stretching vibrations of carbonyl group (>C=O) of biuret moiety 20 .Presence of >NH is indicated by the medium band at 2365-2363 cm -1 which is merged with a broad intense peak due to hydroxyl group 10 .The spectra show bands at 800.2-800, 1373-1372, 1438-1432 cm -1 which may be ascribed to methylene groups 8 .The sharp peak at 1520-1510 cm -1 may be due to breathing modes of aromatic skeletal ring. 1, 2, 3, 4, 5 -Penta substitution are indicated by the peaks 22 at 994-990, 1094-1090, 1198-1189, 1283-1279 and 1373-1330 cm -1 .
In the NMR spectra, except for 2, 4-HABF-I, the remaining three terpolymers, viz., 2, 4-HABF-II, 2, 4-HABF-III and 2, 4-HABF-IV exhibit singlet signal in the region δ 2.3-2.7 ppm due to methylene protons of the Ar-CH 2 -Ar bridges 10,11,23 .The weak multiplet signals (unsymmetrical proton) in the region δ 7.4-7.7 ppm are due to aromatic protons.The methylenic proton of the Ar-CH 2 -N moiety appears in the δ 3.3-3.4ppm region 10,11,23 .The signals in the region δ 5.1-5.4ppm are attributed to the protons of -NH-bridges.The methyl proton of the Ar-CO-CH 3 moiety may be identified by the intense peak at δ 4.3-4.5 ppm.The signal in the range of δ 8.9-9.0 ppm is attributed to phenolic hydroxy proton.This significant downfield chemical shift of the protons of the phenolic OH group clearly indicates intramolecular hydrogen bonding of OH with the carbonyl group present at the adjacent ortho position 10,11,23 .
The influence of ClO 4 ¯, NO 3 ¯, Cl ¯ and SO 4 2¯ was examined at varying concentrations at equilibrium of metal-resin interactions.Examination of the data given in Table 1 reveals that the amount of Fe 3+ , Cu 2+ and Ni 2+ ions taken up by the 2, 4-HABF terpolymer sample increases with increasing concentrations of ClO 4 ¯, NO 3 ¯ and Cl¯ , and decreases with increasing concentrations of SO 4 2 ¯, whereas the uptake of Co 2+ , Zn 2+ , Pb 2+ and Cd 2+ ions by the terpolymer increases with decreasing concentrations of ClO 4 ¯, NO 3 ¯, Cl¯ and SO 4 2 ¯.This may be explained in terms of the stability constants of the complexes which Cu 2+ , Ni 2+ , Co 2+ , Cd 2+ , Zn 2+ , Fe 3+ and Pb 2+ ions form with these anions 24 .SO 4 2 ¯ form strong complexes with Fe 3+ , Cu 2+ and Ni 2+ ions, while ClO 4 ¯, NO 3 ¯, and Cl¯ may form weak complexes.SO 4 2¯ also forms strong chelates with Co 2+ , Zn 2+ , Pb 2+ and Cd 2+ .This type of trend has also been observed by other investigators 8,10,25,26,27 .
The rate of metal adsorption was determined to find the shortest period required for attaining equilibrium while operating at as close to equilibrium conditions as possible.Data show that Fe 3+ ion required almost 3 h for the establishment of equilibrium, while Cu 2+ , Ni 2+ , Co 2+ and Zn 2+ ions required about 5 h.Cd 2+ and Pb 2+ ions required almost 6 h for equilibrium.The rate of metal uptake follows the order: Fe 3+ > Cu 2+ , Ni 2+ , Co 2+ , Zn 2+ > Cd 2+ and Pb 2+ .
The effect of pH on the amount of metal ions distributed between two phase was studied (Table 2).The results indicate that the amount of metal ions taken up by the terpolymer increases with increasing pH of the medium.The study was carried out up to a definite pH value for the particular metal ion to prevent hydrolysis of the metal ions at higher pH.The Fe 3+ ion is taken up more selectively than any other metal ions under study.Zn 2+ and Pb 2+ ions have a low distribution ratio in the range of pH 4-6.This may be attributed to the low stability constant, i.e., weak ligand stabilization energy, of the metal complexes 26 .In the present study the observed order of distribution ratios of divalent ions was found to be: Cu 2+ > Ni 2+ > Co 2+ > Zn 2+ > Pb 2+ > Cd 2+ , which agrees well with earlier reported studies 27 .The results of this study are helpful in selecting the optimum pH for the selective uptake of a metal ion from a mixture of different ions.For example, for the separation of Cu 2+ and Fe 3+ ions, the optimum pH is 2.5, at which the distribution ratio D for Cu 2+ is 73.2 and that for Fe 3+ is 528.2.
In order to assess the potential for separation of metal ions such as Fe 3+ from other metal ions the following combinations of metal solutions were prepared: (1) Fe 3+ and Cu 2+ , (2) Fe 3+ and Ni 2+ , (3) Fe 3+ and Co 2+ and (4) Fe 3+ and Zn 2+ .The solutions for separation were prepared by mixing 1 ml of 1.0 M solution of Fe 3+ with 1 ml of 1.0 M solution of Cu 2+ , Ni 2+ , Co 2+ or Zn 2+ .Selective uptake of the metal ions was studied by adjusting the optimum pH at 2.5.The lowering of the distribution ratios of Fe 3+ is found to be small and, hence, efficient separation could be achieved.A similar trend has also been observed by earlier investigators 28,31,32 .

Table 2 
Distribution ratio (D) of different metal ions as a function of the pH.[React.cond.: Same as inTable 1]