Indian Journal of Chemical Technology Vol. 11, January 2004, pp. 51-58 Removal of heavy metal ions using polydithiocarbamate resin supported on polystyrene Prasun K Roya, Ashok S Rawata, Veena Choudharyb & Pramod K Raia* a Centre for Environment and Explosive Safety, Metcalfe House, Delhi 110 054, India b Centre for Polymer Science and Engineering, IIT, Delhi 110 016, India Received 18 December 2002; revised received 30 July 2003; accepted 21 August 2003 Polydithiocarbamate chelating resin supported on XAD-2 type polystyrene was synthesised by emulsion polymerisation of styrene and its subsequent reaction with carbondisulphide in alkaline medium. The polydithiocarbamate resin was characterised by elemental analysis, thermal studies and IR studies. The sorbent was evaluated for its analytical characteristics and the optimum sorption conditions for metals like Cu, Ni, Pb, Fe, As and Mn were determined in order to assess the efficiency of the resin. The sorption capacity was considerably higher when compared to other conventional chelating polymers. The sorption kinetics was fairly rapid as apparent from the loading t1/2 values, indicating a better accessibility of the chelating sites. IPC Code: (C02F1/28, 101:10) C02F103:02 Chelating resins have found widespread applications for the enrichment of trace metal ions from natural waters owing to their selectivity and stability. Different approaches have been used for the immobilisation of chelating functional groups on natural and synthetic solid support for the separation and preconcentration of trace metal ions1,2. The development of highly selective and high affinity chelating phases is based on the selection of donor atoms employed in selective metal ion binding. For example, chelating resins containing bis(2-aminophenyl)disulphide3, quinoline-8-ol4, o-vanillinthiosemicarbazone5, dithizone6, bicine7, 1-nitroso,2-naphthol8, phthalocyanine9, etc. supported on poly(styrene-divinylbenzene) copolymer matrix have been applied for selective separation of specific metal ions. Crown ethers10, thioanilin11, formyl salicylic acid12 immobilised on silica gel have been used for the selective extraction and preconcentration of alkali and alkali earth metals, Pd(II) and Fe(III) respectively. The sulphur containing chemical groups act as selective ligands with high bond stability for transition metal ions13-18. Therefore, dithiocarbamate groups can be very effective towards the removal of several trace and heavy metals. Resins containing dithiocarbamate —————— *For correspondence (E mail: pramrai@rediffmail.com, pk_roy@hotmail.com) groups derived from crosslinked chloromethylated polystyrene and benzylamine have been described in the literature19,20. However, no such work has been carried out on the XAD-2 type polystyrene. In the present paper the synthesis, analytical applications and evaluation of polydithiocarbamates supported on XAD-2 polystyrene have been described. This matrix has been studied for its use in the preconcentration of Cu, Ni, Pb, Fe, As and Mn. Experimental Procedure Materials and Methods All chemical used in the present work were of 'AR' grade of E.Merck or Qualigens (Glaxo). Ethanol, tin(II)chloride, pyridine and carbondisulphide were used as such. Styrene was washed with 10% aqueous sodium hydroxide to remove inhibitor, followed by washing with water till neutral. It was further purified by vacuum distillation at 60°C. Acetic anhydride was purified by distillation and the fraction boiling at 138 -140°C was used for the study. Standard stock solutions of 1000ppm of copper(II), lead(II), iron(III), nickel(II), arsenic(III) and manganese(II) were prepared by dissolving copper nitrate, lead nitrate, ferric nitrate, nickel nitrate, sodium arsenite and manganous chloride in 0.1 M of the corresponding acid21. The following buffered solutions were prepared and used: hydrochloric acid-glycine (pH, 1-3); acetic acid- 52 INDIAN J. CHEM. TECHNOL., JANUARY 2004 sodium acetate (pH 3-5) and disodium hydrogen phosphate-potassium dihydrogen phosphate (pH, 5-8). Milli Q ultrapure water was used throughout the course of this work. Synthesis of sorbent The chelating resin was synthesised according to Scheme 1. Polystyrene(PS) was prepared by emulsion polymerisation22. A monomer premix containing water (4g), sodium lauryl sulphate (2g) and styrene (20g) was prepared with a high-speed stirrer. Aqueous solutions (0.05% w/v) of redox initiators (potassium persulphate and sodium metabisulphite) were prepared. The reaction was carried out under nitrogen atmosphere. 10% of the monomer premix along with 25mL of distilled water was placed in a reaction kettle and heated to 60°C. 1 mL of the previously prepared initiator solution was added. After formation of the seed, the rest of the monomer premix was metered in over a period of 1h. After periodic intervals the initiator solutions were added. After the complete addition of monomer premix, reaction was further carried out at 80°C for another 4 h. The polymer was coagulated by pouring the emulsion to a saturated solution of aluminium sulphate. Coagulated polymer was then subjected to soxhlet extraction with water/acetone for several hours to remove the impurities such as surfactant, unreacted monomer, etc. The polymer was then dried at 60°C under vacuum for 12h. Synthesis of polydithiocarbamate 5 g of PS was nitrated23 with 5:2 v/v mixture of sulphuric acid and nitric acid at 60°C on a controlled water bath for 1h. The resulting nitro derivative (NPS) was washed repeatedly with distilled water till free from acid. It was reduced to amino derivative (APS) with a mixture of tin(II) chloride(40g), concentrated hydrochloric acid (45mL) in 50 mL ethanol for 12h at 90°C under nitrogen atmosphere. The product was filtered, washed first with distilled water and then with 2M NaOH to recover the free amino polymer. After washing, the aminopolystyrene was treated with 100 mL of ethanol, 1M in NaOH and 1.5M in carbondisulphide with stirring at room temperature for 6 days. The resulting dithiocarbamate derivative (DTPS) was filtered, washed repeatedly with ethanol and diethyl ether. It was dried in vacuum at 70°C for 24h. Viscosity measurements The viscosity average molecular weight of PS was determined using a Ubbelhode suspension level Scheme 1—Synthesis of dithiocarbamate based chelating resin Scheme 2—Acetylation sequence for determination of amine groups viscometer in toluene at 34°C at various concentrations (0.25 to 1% w/v). The viscosity average molecular weight was calculated using the following equation24. [η] = 9.7×10-3 × Mη0.733 … (1) Structural characterisation The elemental analysis of the polymers was carried out using a Perkin-Elmer CHNS analyzer-2400. The infrared spectra were recorded in the wavelength range 400-4000cm-1 on a Perkin-Elmer IR spectrophotometer-3100. Determination of amino content The amino content in the amino-polystyrene (APS) was determined by acetylation method22. This involves the reaction of acetic anhydride with amino group in the presence of pyridine according to reaction (Scheme 2). 5 mL of freshly prepared acetylation mixture (acetic anhydride and pyridine in the ratio of 1:3) was added to known amount (~ 1g) of the dry polymer, and the contents were heated for 3h at 120°C in an oil bath. The mixture was then filtered and 10mL of distilled water was added to hydrolyse the remaining acetic anhydride and it was then titrated against pre- ROY et al.: REMOVAL OF HEAVY METAL IONS USING POLYDITHIOCARBAMATE RESIN viously standardised 0.5N methanolic NaOH using phenolphthalein as an indicator. Percent nitrogen present as amino groups was then calculated using the following equation, %N= (V 1 − V 2) × N 1 × 14 × 100 W × 1000 where V1 and V2 are the volumes (mL) of NaOH consumed by blank and polymer respectively, N1 is the normality of methanolic NaOH and W is the weight (g) of the polymer used. Thermal characterisation The thermal behaviour was investigated using a Du Pont 2100 thermal analyser having a 910 DSC module in static air atmosphere, and a 951 TG module was used for recording TG/DTG traces in nitrogen atmosphere. The relative thermal stability of the resin was assessed by finding the initial decomposition temperature (Ti), integral procedural decomposition temperature(IPDT), temperature of maximum rate of weight loss (Tmax), final decomposition temperature (Tf) and char yield in nitrogen atmosphere at 550°C. IPDT, which sums up the shape of thermogravimetric curve was calculated according to the method developed by Doyle25. The area under the thermogravimetric trace, from the initial temperature of 50°C to the final temperature (Tf) of 600°C was determined. The ratio of this area to the total area of the rectangular plot bounded by the curve (A*), from which the IPDT was obtained by employing the following relationship, IPDT = A* (Tf − Ti ) + Ti The activation energy (E*) was evaluated from the TG curve by employing the following equation26, 53 Flame Atomic Absorption Spectroscopy (FAAS) while As(III) was determined using hydride generation technique. The pH measurements were made with Orion pH meter (Model 106) calibrated with titrisol buffers. Batch experiment Batch equilibration technique was used to determine the optimum sorption conditions like pH, adsorption time and the capacity of the sorbent. The chelating resin was equilibrated with a suitable amount of metal ions. The resin was filtered and the metal concentration in the filtrate was determined by AAS. The amount of metal ions adsorbed on the solid phase was determined by the equation. Nf = (X −Y) Z where X = initial amount of metal ion, Y = amount of metal ions in the supernatant. Nf = amount of metal ion adsorbed and Z = amount of chelating resin. Elution was carried out using 1N HCl/HNO3 and the metal concentration in the eluent was determined by AAS. All adsorption experiments were carried out in triplicate to determine the precision of the method. Optimum pH of metal ion uptake Optimum pH of metal ion uptake was determined by batch equilibration technique. Excess metal ion (50mL, 50 μg/mL) was shaken with 100mg of resin for 2h. The pH of metal ion solution was adjusted prior to equilibration over a range of 2-9 with buffer solution. The resin was filtered off and the amount of metal ion remaining in the filtrate was determined using AAS. Adsorption experiments were carried out in triplicate to determine the precision of the method. Adsorption isotherms 100 E *θ ln ln(1 − α ) −1 = +C RTi 2 (Tf − Ti ) where α is the fraction reacted at a particular temperature, θ = T-Tmax, Ti is the temperature of inception of reaction, Tf is the temperature of completion of reaction and Tmax is the peak temperature. Analytical characterisation A GBC atomic absorption spectrophotometer (Model 932AA) was used for the determination of metal ions in solution. Metals like Ni(II), Cu(II), Pb(II), Fe(III) and Mn(II) were determined using The adsorption isotherm studies were carried out by shaking 100mg of the resin with different concentrations of metal ion solution of Fe, Ni, Cu, Pb at 30°C for 24h. The solution was then filtered and the concentration of the metal in the filtrate determined by AAS. Sorption kinetics The rate of loading of metal ions on the resin was determined under the following conditions: 50mL of metal ion solution (100μg/mL) was stirred with 100mg of the resin at room temperature (30°C) in a mechanical shaker. An aliquot of 5 mL solution was 54 INDIAN J. CHEM. TECHNOL., JANUARY 2004 removed at predetermined intervals for analysis by AAS and the amount of metal ions loaded on the resin phase was calculated. The loading half time t1/2 i.e. the time required to reach 50% of the resins total loading capacity was estimated from the resulting isotherm. of desired metal solution was passed through the resin at varying flow rates, and at 2mL/min to evaluate the effect of flow rate and preconcentration test respectively. The stripping of the metal was carried out with suitable eluting agents like 1 N HCl/HNO3. Total sorption capacity Results and Discussion Intrinsic viscosity [η] was obtained from the plot of ηsp/c versus concentration of polystyrene in toluene at 34°C (Fig. 1) as an intercept and was found to be 104.96 mL/g. The viscosity average molecular weight calculated using Mark Houwink equation was found to be 3.19×105 g/mol. The results of the elemental analysis of the nitro, amino, and dithiocarbamate resin are given in Table 1. The results for nitrogen suggests that the degree of nitration is about 1.25. The nitrated and aminated polystyrene were found to contain 10.75 and 11.78% of nitrogen respectively. Determination of amine content, by non-aqueous titrimetry shows that only 8.3% of nitrogen was present as amine groups in APS. Therefore, 3.48% of nitrogen is pre- Total sorption capacity of the resin was determined by shaking an excess of metal ion solution (100mL, 50μg/mL) with 100mg resin for 24 h at optimum adsorption pH at 30°C in a mechanical shaker to ensure complete equilibrium. The resin was filtered off and the concentration of metal ion in the filtrate was determined using AAS. Resin stability test The following conditions were employed for resin stability study. 100mg of the resin was stirred with 100mL of 50 ppm solution for 6h at 30°C. The elution operation was carried out by shaking the resin with 20mL of the eluent for 4h to ensure complete equilibration. The metal content in the eluent was determined by AAS. Effect of co-ions To study the effect of co-ions, 100mg of the resin was shaken with 100mL of a complex metal ion mixture of Cu, Ni, Fe, Pb (50ppm each). The pH of the solution was maintained at 4 with buffers. After 24 h, the solution was filtered and the metal ion concentration determined by AAS. Column experiment The preconcentration test and the effect of flow rate were determined in a glass column, 5 mm in diameter. The column was packed with 100mg of the chelating resin. A suitable aliquot (20mL of 10ppm) Concentration ×103, g/mL Fig. 1—Intrinsic viscosity determination of PS at 34°C (solvent toluene) Table 1—Elemental analysis of resins Sample name NPS (mononitroproduct) ( dinitro product) APS (from mononitro product) (from dinitro product) DTPS (from mononitro product) (from dinitro product) Calcd.(%) Expt. (%) C H N S 64.62 4.69 9.39 - 49.48 3.09 14.43 - 80.67 7.56 11.76 - 71.64 7.46 20.89 - 55.67 4.12 7.21 32.98 42.40 2.47 9.89 45.96 C H N S 56.42 3.98 10.75 - 52.67 4.56 11.78 - 44.96 3.37 9.5 25.45 ROY et al.: REMOVAL OF HEAVY METAL IONS USING POLYDITHIOCARBAMATE RESIN 55 Table 2—Important IR bands for resins IR bands(cm-1) Sample designation νCH(ar) νCH(al) νNH PS 3026.5 2922.5 NPS 3078.2 2926.9 APS ~3000 DTPS 3113.5 νC-NO2 νNCS - - - - 1517.1 1345.2 - 2917.8 3300 1514.2 1346.1 - 2924.9 3403.6 - 2104.6 Table 3—Thermogravimetric analysis of resins Char at 5000C (%) IPDT (°C) 464.6 1.8 430.8 84.8 335.5 27.0 333.5 48.2 Sample designation Ti (°C) Tm (°C) Tf (°C) PS 407.4 445.2 NPS 303.7 328.9 Activation Energy (kJ/mol) APS 374.1 428.8 454.9 43.4 423.7 50.6 DTPS 378.3 458.7 496.4 57.0 383.0 46.6 sent in the form of unconverted nitro groups in the amino product. Higher amino content (8.3% amino nitrogen) during the investigation indicates higher percentage conversion of the nitro to aminopolystyrene as the reduction was carried out in the inert medium as compared to reported values27. The important IR bands of the nitro, amino and dithiocarbamate resin are presented in Table 2. Absorption bands at 1345 and 1517cm-1 in the nitro product is assigned to the C-NO2 stretching, while νNH appears at 3300-3400 cm-1 in the amino polystyrene. In dithiocarbamate chelating resin a characteristic band at 2100 cm-1 can be attributed to the NCS group showing the conversion of amino groups to the dithiocarbamate. Thermal characterisation From the DSC traces of the support polymers and their derivatives, the glass transition temperature (Tg) was observed at 101°C for PS. The functionalised polymers did not show any Tg, however, they all showed decomposition at around 250°C, as indicated by an exotherm. The TG/DTG traces clearly show single step decomposition for all samples. Polystyrene showed a weight loss of 98% at 500°C whereas 73% of weight loss was observed for nitropolystyrene (Table 3). The char yield of the amino and dithiocarbamate derivative was 43.42 and 57.01% respectively. Tm, Tf and char yield (at 500°C) was maximum for dithiocarbamate derivative, whereas it was lowest in the case of polystyrene. This clearly indicates that the degradation mechanism of polystyrene was altered upon functionalisation. From the thermogravimetric curves, kinetic parameters of the pyrolysis reactions have been determined. Several methods and mathematical treatment for such evaluation have been reported in the literature26,28-31. But none of these equations takes into account the variations in sample size and heating rate. Dhwarwadkar and Karkhanawala26 have modified the expression given by Horwitz and Metzger30 to eliminate these drawbacks. In the present studies the modified equation was used to calculate E*. A plot of ln ln (1-α)-1 versus θ gives a straight line. The slope 100 E * is related to the activation energy. The RTi 2 (Tf − Ti ) representative plots are shown in Fig. 2. The calculated values for the activation energies are listed in Table 3. The total activation energy is comparable with other polystyrene based chelating polymers studied previously32, although in the latter, two step decomposition was observed. Sorption of metal ions as a function of pH In a preliminary experiment, the sorption behaviour of some metals on the resins at different pH values INDIAN J. CHEM. TECHNOL., JANUARY 2004 56 has been examined by batch equilibration technique. The pH of the metal test solution were measured during the sorption process. After equilibration with the resin, a decrease in the pH of the solution was observed which can be attributed to the formation of complex due to release of protons from the resin. The adsorption of metal ions increases with increase in pH, reaching a limiting value in each instance followed by a decrease in adsorption, beyond the limiting value. It was surprising to note the negligible chelation effect of the resins towards arsenic and manganese, because dithiocarbamates have been reported to have a very strong tendency to form complexes with these metals19,33. Fig. 2—Plot of lnln (1-α)-1 against θ Adsorption isotherm The adsorption data for heavy metals (Fe, Ni, Cu, Pb) were analysed by a regression analysis to fit the Freundlich and Langmuir isotherm model. These data were plotted as a function of the amount of heavy metal sorbed on the resin at equilibrium versus the heavy metal concentration of the solution at equilibrium. The coefficients of these two models were computed using linear least square fitting. Langmuir isotherm The Langmuir model was used to explain the observed phenomenon. The equilibrium data was analysed using the following linearised equation. Fig. 3—Langmuir isotherm for adsorption of metals Ce/qe= 1/kb + Ce/b where Ce is the equilibrium concentration (mg/L), qe is the amount adsorbed at equilibrium (mg/g) and k and b are the Langmuir constants related to adsorption capacity and the energy of adsorption respectively. The linear plot (Fig. 3) for the four metal ions show that the adsorption obeys the Langmuir model. The Langmuir constants were evaluated and they are reported in Table 4. Freundlich isotherm Fig. 4—Freundlich isotherm for adsorption of metals The adsorption behaviour was also confirmed by Freundlich model 1 log qe = log kf + log ce n where ce is the equilibrium concentration (mg/L) and qe is the amount of metal adsorbed (mg/g). A linear plot (Fig. 4) of log qe versus log ce shows the applicability of the Freundlich model. The Freundlich constants kf and n were calculated and are reported in Table 4—Langmuir model Metal Langmuir model K b R2 Nickel ce/qe= 0.0275 + 0.023ce 0.84 43.2 0.98 Iron ce/qe= 0.0063 + 0.027ce 2.57 36.23 0.99 Copper ce/qe= 0.0005 + 0.025ce 7.66 39.5 0.99 Lead ce/qe= 0.0102 + 0.036ce 3.57 27.3 0.99 ROY et al.: REMOVAL OF HEAVY METAL IONS USING POLYDITHIOCARBAMATE RESIN 57 Table 5. Values 1<n<10 show the positive sorption of metal ion34. Correlation indices (R2) were determined to compare the two models. Sorption kinetics The kinetics of resin metal interaction is of considerable importance, if the resin is to be used in a dynamic system such as a packed column and a flowing stream. To determine the rate of loading of Ni(II), Cu(II), Pb(II) & Fe(III) on the resin, batch experiments were carried out on DTPS at optimum pH for the respective metal ions at room temperature. The loading half time defined as the time required to reach 50% of the resins total loading capacity was estimated from the curves, and the results are reported in Table 6. The faster uptake of Cu(II) metal ion shows a better accessibility of this metal and strong bond formation with the ligand. Fig. 5—Effect of interfering ions on metal ion uptake Total sorption capacity The capacity of the resin is an important factor to determine the amount of resin required for complete removal of a specific metal ion from the solution. The loading capacity of the resins was calculated and the results are presented in Table 7. The observed sorption capacity during the present investigation was quite high as compared to the other resins based on dithizone6 and o-vanillinthiosemicarbazone5. Fig. 6—Effect of flow rate on metal sorption Table 5—Freundlich model Metal Langmuir model K n R2 Nickel logqe= 1.53 + 0.058logce 4.37 9.23 0.90 Resin stability tests Iron logqe= 1.33 + 0.169logce 3.8 5.89 0.79 The resin stability tests reveals that the sorbent is highly stable and can be used repeatedly. There was no decrease in sorption capacity under static conditions even after 10 cycles of operation. Copper logqe= 1.45 + 0.156logce 4.21 6.39 0.87 Lead logqe= 1.32 + 0.108logce 3.75 9.24 0.94 Table 6—Loading half time for the metal ions Effect of co-ions The competetive chelation and sorption of metal ions was studied in a complex mixture of four metals (Cu, Fe, Ni and Pb) at pH 4. The results are shown in Fig. 5. As is apparent, the resin showed a higher tendency towards chelation of Cu and Ni as compared to Pb and Fe. Effect of flow rate The dependence of uptake of the metal on the flow rate was studied for Ni(II), Cu(II), Pb(II) and Fe(III) at the pH chosen for maximum complexation, the solution flow rate being varied from 1 to 5 mL/min. The results are shown in Fig. 6. It was observed that at flow rate greater than 3, there was a decrease in percentage sorption. Metal Load half time (min) Copper 7 Nickel 8 Iron 9 Lead 12 Table 7—Sorption capacity of the chelating resins at optimum pH Metal Optimum pH Capacity (mg/g resin) Ni(II) 3-5 39 Cu(II) 5 39 Pb(II) 4-5 26 Fe(III) 4 35 INDIAN J. CHEM. TECHNOL., JANUARY 2004 58 Table 8—Preconcentration and recovery of metal ions Metal Conc of metal ion (mg/L) Vol. of eluent (mL) Recoverya (mg/L) Recovery (%) Recovery in the presence of Na, Mg, and Ca (10ppm) (%) Cu 10 10 9.85 98.5 98.5 Ni 10 10 9.66 96.6 96.3 Fe 10 10 9.77 97.7 97.5 Pb 10 10 9.86 98.6 98.6 a The relative standard deviation was in the range 0.1-2% for triplicate analysis Preconcentration and recovery Experiments on the recovery of metal ions from resin were carried out using column technique at their optimum pH. Metals were recovered with 10 mL of suitable eluting agents. The effect of interfering ions has also been studied. Metal solutions of Pb, Ni, Cu and Fe containing Na, Mg and Ca ions as interferants were analysed and Table 8 reports the recovery rates in absence as well as in excess of these foreign ions. As it is apparent, alkali and alkaline earth metals do not affect the recovery of metal ions from the solution. This suggests the use of this resin for trace concentration from natural sample. Acknowledgement The authors are thankful to the Director, Centre for Environment and Explosive Safety for taking keen interest and for providing the laboratory facilities. Thanks are also due to Dr. R.K. Sharma, Delhi University for IR spectra. 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