Malaysian Polymer Journal, Vol. 9 No. 2, p 70-77, 2014 Available online at www.cheme.utm.my/mpj REMOVAL OF CU2+, Ni2+ AND Zn2+ METAL IONS FROM WATER BY HYDROXYL TERMINATED S-TRIAZINE BASED DENDRIMER Rinkesh Patel, Hema Patel, Dhaval Gajjar and Pravinkumar M. Patel * Industrial Chemistry Department, V.P. & R. P. T. P. Science College affiliated to Sardar Patel University, Vallabh Vidhyanagar, Gujarat, INDIA Corresponding author’s email: [email protected] ABSTRACT: Treatment of waste water containing heavy metal ions is one of the major global problems. In overview in recent techniques, adsorption technique is one the low cost methods used to remove heavy metal ions from relatively low concentrations from aqueous solutions. Dendrimer are fourth a new class of polymeric architectures. We have applied dendrimers as low cost adsorbent for metal ions. In this paper, hydroxyl terminated s-triazine based dendrimers G1, G2 and G3 terminated with 8, 32 and 128 hydroxyl groups were used as adsorbents in a series of experiment to remove Cu2+, Ni2+ and Zn2+ from aqueous solution evaluated by EDTA method. Effect of time, pH and generation on the metal ion uptake was studied. It was found that 12 h was optimum time for metal ion uptake and metal ion uptake was increased with increase in pH and generation of dendrimer. Adsorption for Cu2+ ions was maximum for all dendrimer generations whereas minimum for Zn 2+ ions. Parent G3 dendrimer and G3 dendrimer metal-complexes were further characterized by FT-IR and Thermo Gravimetric Analysis (TGA) to determine the active binding site for metal ion uptake and to confirm the presence of metal in the final metal containing dendrimer respectively. KEYWORDS: Dendrimer; Water remediation; Thermo gravimetric analysis; Divergent method; Triazine trichloride. 1.0 INTRODUCTION Dendritic macromolecules or polymers are fourth new architecturing class of polymer with structure resembles to “tree” [1]. One of the members of the above class of polymer is a dendrimer which have perfectly branched structure and monodispersity compared to other polymers in same class [2]. Since their invention in 1979 [3] dendrimer has exhibited their potential in various fields of applications such as drug delivery [4, 5], drug solubilisation [6, 7], water remediation [8], sensing application [9] and cancer therapy [10]. Contamination of heavy metal ions such as Cu, Ni, Zn, Cr, Cd, Hg, Pb in water resources has been increased due to various industrial activities [11]. Most of these metal ions are soluble in water and either toxic or carcinogenic in nature. If the concentration of these metal ions increases above specific limit, contamination of these metal ions cause hazards to humans and aquatic life [1213]. Treatment of waste water containing heavy metal ions is one of the most crucial problems [14]. Currently, waste water containing heavy metal ions can be treated by techniques such as chemical precipitation [15], electro-dialysis [16], photo-dialysis [17], membrane technique [18] and adsorption [19] etc. Amongst them, adsorption is most commonly used technique because of several limitations associated with other techniques such as high cost, production of toxic sludge, the high energy requirement can be avoided. Adsorption technique also has an advantage that it removes metal ions from lower concentrations from their aqueous solution [20]. Common adsorbents used for the said purpose are activated carbon [21], minerals [22], polymer resins [23], agricultural waste [24] etc. In previous work, we have reported synthesis of hydroxyl terminated s-triazine based dendrimer up to generation three using N,N’-bis(4,6-dichloro-1,3,5-triazin-2yl)hexane-1,6-diamine [6] as core. Dendrimer was fully characterized by spectral techniques and potential of synthesized dendrimer as solubility enhancers of drug was evaluated [6]. We have found that recent reports on the removal of heavy metal ions from water by dendrimer [25, 26] which encouraged us to evaluate candidature of synthesized dendrimer for removal of heavy metal ions from water. In this paper, we have reported sorption behavior of different generation of triazine based dendrimer (G1, G2 and G3) for removing heavy metal ions such as Cu2+, Ni2+ and Zn2+ from aqueous solutions by EDTA titration. Study of parameters such as time, pH and generation on the metal ion uptake was also reported. Pure generation three dendrimer (G3) and metal ion containing dendrimer (G3-Cu, G3-Ni and G3-Zn) were further characterized by FT-IR to evaluate binding site in dendrimer for metal ion adsorption and by Thermo gravimetric analysis (TGA) to confirm the presence of metals in final metal ion containing dendrimer. 2.0 EXPERIMENTAL 2.1. Materials Cyanuric Chloride (Triazine trichloride), hexane1,6diamine, acetone, dichloromethane and methanol were purchased from Sigma-Aldrich (India) Ltd. Nickel nitrate Patel, R. et al. Malaysian Polymer Journal, Vol. 9, No. 2, p 70-77, 2014 (Ni(NO3)2.6H2O), copper nitrate (Cu(NO3)2.6H2O), zinc nitrate (Zn(NO3)2.6H2O), sodium hydroxide and hydrochloric acid were purchased from Merck (India) Ltd. All the reagents and solvents for the synthesis and analysis were used as received. 2.2. Analytical Methods FTIR studies were carried out in the range of 250–4000 cm−1 using Perkin Elmer-Spectrum RX-FTIR spectrometer instrument through KBr disc and pellet method or nujol mull method. 1H-NMR and 13C-NMR spectra were recorded at 400 MHz in Brucker Avance II 400 (Germany) using TMS as internal standard. Thermo gravimetric analysis was performed on Perkin Elmer Pyris-1 TGA instrument with heating rate of 10˚C/min and in nitrogen atmosphere. Mass spectra were recorded on Waters Micromass Q-ToF Micro (USA) instrument equipped with ESI. 2.3. Synthesis of Dendrimer [6] 2.3.1. Synthesis of N,N’-bis(4,6-dichloro-1,3,5-triazin-2yl)hexane-1,6-diamine (Core) Cyanuric Chloride (0.02mmol) was dissolved in dichloromethane and kept in an ice bath. A solution of hexane 1,6-diamine (0.01mmol) containing sodium hydroxide (0.02 mmol) in water was added drop wise in the solution of cyanuric chloride at 0-5 °C with stirring. The solution was stirred at 0-5 °C for 2 hrs. Then the solution was filtered, washed with methanol and acetone and dried under vacuum: A white colored solid was formed. Yield, 83%; FT-IR (KBr, cm-1) ν: 3281(N-H), 2884, 2779(aliphatic C-H), 1722, 1624(C=N of triazine), 844, 796(C-Cl); 1H-NMR (400MHz, DMSO-d6) δ ppm: 1.28981.3558 (m, 4H, N-CH2-CH2-CH2-CH2-CH2-CH2-N), 1.49411.5383 (m,4H,N-CH2-CH2-CH2-CH2-CH2-CH2-N) 3.24453.2881(m, 4H, N-CH2-CH2-CH2-CH2-CH2-CH2-N); 13C-NMR (75MHz, DMSO-d6) δ ppm: 25.20 (N-CH2-CH2-CH2-CH2CH2-CH2-N), 30.52 (N-CH2-CH2-CH2-CH2-CH2-CH2-N), 40.52(N-CH2-CH2-CH2-CH2-CH2-CH2-N), 168.34 (Triazine CN), 169.40(Triazine C-Cl); Anal. Calcd. for C12H14Cl4N8: C, 34.97; H, 3.42; N, 27.19 found C, 35.02; H, 3.49; N, 27.26. 2.3.2. Synthesis of generation 1 dendrimer (G1) N,N’-bis(4,6-dichloro-1,3,5-triazin-2-yl)hexane-1,6-diamine (0.01mmol) was dissolved in an excess of diethanolamine (0.04mmol) which was used as both solvent and reactant. The resulting mixture was refluxed for 2 hrs. After cooling, it was dispersed and washed by acetone repeatedly to give generation 1 dendrimer which was light brown colored. Yield.75%; FTIR (KBr, cm-1) ν: 3364 (O-H), 2941 (aliphatic C-H), 1668 (C=N of triazine), 1063 (C-O); 1H-NMR (400MHz, D2O) δ ppm: 1.3138-1.3358 (m, 4H, N-CH2-CH2CH2-CH2-CH2-CH2-N), 1.4945- 1.5145 (m, 4H, N-CH2-CH2CH2-CH2-CH2-CH2-N), 3.4191- 3.4381 (m, 4H, N-CH2-CH2CH2-CH2-CH2-CH2-N) 3.5785-3.6151 (m, 16H, N-CH2-CH2OH), 3.7801-3.8145 (m, 16H, N-CH2-CH2-OH); 13C-NMR (75MHz, D2O) δ ppm: 25.20 (N-CH2-CH2-CH2-CH2-CH2-CH2N), 30.52 (N-CH2-CH2-CH2-CH2-CH2-CH2-N), 40.52 (N-CH2CH2-CH2-CH2-CH2-CH2-N), 58.04 (N-CH2-CH2-OH), 61.04 (N-CH2-CH2-OH), 168.04(Triazine C-N), 169.90 (triazine C- N-(CH2-CH2-OH)2); Anal. Calcd. for C28H54N12O8: C, 48.97; H, 7.92; N, 24.47; Found C, 49.02; H, 7.98; N, 24.50. 2.3.2. Synthesis of generation 1.5 dendrimer (G1.5) Cyanuric Chloride (0.08mmol) was dissolved in dichloromethane and kept in an ice bath. A solution of G1 dendrimer (0.01mmol) containing sodium hydroxide (0.08 mmol) in water was added drop wise in the solution of cyanuric chloride at 0-5 °C with stirring. The solution was stirred at 0-5 °C for 2 hrs and refluxed for 6 hrs. Then the solution was filtered, washed with methanol and acetone and dried under vacuum: A white colored solid was formed. Yield 84%; FT-IR (KBr, cm-1) ν: 3214 (N-H), 2834, 2832, 2780 (aliphatic C-H), 1779, 1753, 1717(C=N of triazine), 1061(C-O), 786 (C-Cl); 1H-NMR (400MHz, D2O) δ ppm: 1.3138-1.3358 (m, 4H, N-CH2-CH2-CH2-CH2-CH2-CH2-N), 1.4945- 1.5145 (m, 4H, N-CH2-CH2-CH2-CH2-CH2-CH2-N), 3.4191-3.4381 (m, 4H, CH2-NH); 3.8585-3.9350 (m, 16H, N-CH2-CH2-O-Tri), 4.0301-4.0832 (m, 16H, N-CH2-CH2-OTri); 13C-NMR (75MHz, D2O) δ ppm: 25.20 (N-CH2-CH2-CH2CH2-CH2-CH2-N), 30.52 (N-CH2-CH2-CH2-CH2-CH2-CH2-N), 41.47 (N-CH2-CH2-O-tria), 61.04 (N-CH2-CH2-O-tria), 163.01 (Triazine C-N), 166.66 (Triazine C-N-(CH2-CH2-O)2), 170.18 (Triazine C-O), 172.28 (Triazine C-Cl); Anal. Calcd. for C52H46Cl16N36O8: C, 33.39; H, 2.48; N, 26.96; Found: C, 34.02; H, 2.50; N, 27.01. 2.3.4. Synthesis of generation 2 dendrimer (G2) Generation 1.5 dendrimer (0.01mmol) was dissolved in an excess of diethanolamine (0.16 mmol) which was used as both solvent and reactant. The resulting mixture was refluxed for 2 hrs. After cooling, it was dispersed and washed by acetone repeatedly to give generation 2 dendrimer which was light brown colored. Yield 75%; FTIR (KBr) ν: 3389(O-H), 2939, 2950(aliphatic C-H), 1671, 1619(C=N of triazine), 1068 (C-O).; 1H-NMR (400MHz, D2O) δ ppm: 1.3158-1.3355 (m, 4H, N-CH2-CH2CH2-CH2-CH2-CH2-N), 1.4984-1.5138 (m, 4H, N-CH2-CH2CH2-CH2-CH2-CH2-N), 3.4191-3.4381 (m, 4H, CH2-NH), 3.6743-3.7837 (m, 16H, N-CH2-CH2-OH), 3.8126-3.8970 (m, 16H, N-CH2-CH2-OH), 3.9803-4.0433 (m, 64H, N-CH2CH2-O-tri), 4.0917-4.1672 (m, 64H, N-CH2-CH2-O-tri); 13CNMR (75MHz, D2O) δ ppm: 25.70 (N-CH2-CH2-CH2-CH2-CH2CH2-N), 30.31 (N-CH2-CH2-CH2-CH2-CH2-CH2-N), 40.81 (NCH2-CH2-CH2-CH2-CH2-CH2-N), 60.81 (N-CH2-CH2-O-tria), 61.28 (N-CH2-CH2-OH), 63.78 (N-CH2-CH2-O-tria), 65.80 (NCH2-CH2-OH), 163.66(Triazine C-N), 168.66(Triazine C-N(CH2-CH2-O-)2), 170.18(Triazine C-O), 172.21(Triazine CN(CH2-CH2-OH)2); Anal. Calcd. for C116H206N52O40: C, 46.92; H, 6.99; N, 24.53; Found C, 47.01; H, 7.02; N, 24.59. 2.3.5. Synthesis of generation 2.5 dendrimer (G2.5) Cyanuric Chloride (0.32mmol) was dissolved in dichloromethane and kept in an ice bath. A solution of G2 dendrimer (0.01mmol) containing sodium hydroxide (0.32 mmol) in water was added dropwise in the solution of cyanuric chloride at 0-5 °C with stirring. The solution was stirred at 0-5 °C for 2 hrs and refluxed for 6 hrs. Then the solution was filtered, washed with methanol and acetone and dried under vacuum: A white colored solid was formed. MPJ 71 Patel, R. et al. Malaysian Polymer Journal, Vol. 9, No. 2, p 70-77, 2014 Yield: 70%; FT-IR (KBr, cm-1) ν: 3280 (N-H), 2852, 2819 (aliphatic C-H), 1750, (C=N of triazine), 1045(C-O), 787 (C-Cl); 1H-NMR (400MHz, DMSO-d6) δ ppm: 1.3118-1.3351 (m, 4H, m, 4H, N-CH2-CH2-CH2-CH2-CH2-CH2-N), 1.49841.5150 (m, 4H, N-CH2-CH2-CH2-CH2-CH2-CH2-N), 3.41513.4389 (m, 4H, CH2-NH), 4.0153- 4.1380 (m, 16H, N-CH2CH2-O-tri), 4.2168-4.2929 (m, 16H, N-CH2-CH2-O-tri); 13CNMR (75MHz, DMSO-d6) δ ppm: 25.20 (N-CH2-CH2-CH2CH2-CH2-CH2-N), 30.05 (N-CH2-CH2-CH2-CH2-CH2-CH2-N), 40.45 (N-CH2-CH2-CH2-CH2-CH2-CH2-N), 60.06 (N-CH2-CH2O-tria), 61.23 (N-CH2-CH2-OH), 63.30 (N-CH2-CH2-O-tria) , 65.53 (N-CH2-CH2-OH), 164.11 (Triazine C-N), 166.41 (inner C-N-(CH2-CH2-O-)2), 168.81 (outer Triazine C-O), 171.74 (outer C-N-(CH2-CH2-O-)2), 172.57(outer C-Cl), 174.94 (inner triazine C-O); Anal Calcd. for C212H174Cl64N148O40 : C, 33.05; H, 2.28; N, 26.91; Found C, 33.12; H, 2.30; N, 26.99. 2.3.6. Synthesis of generation 3 dendrimer (G3) Generation 2.5 dendrimer (0.01mmol) was dissolved in an excess of diethanolamine (0.64 mmol) which was used as both solvent and reactant. The resulting mixture was refluxed for 2 hrs. After cooling, it was dispersed and washed by acetone repeatedly to give generation 1 dendrimer which was light brown colored. Yield: 71 %; FT-IR (KBr, cm-1) ν: 3368 (O-H), 2947, 2872 (aliphatic C-H), 1639 (C=N of triazine), 1033 (C-O); 1HNMR (400MHz, D2O) δ ppm: 1.3138-1.3353 (m, 4H, m, 4H, N-CH2-CH2-CH2-CH2-CH2-CH2-N), 1.4954-1.5181 (m, 4H, NCH2-CH2-CH2-CH2-CH2-CH2-N), 3.4144-3.4333 (m, 4H, CH2NH), 3.6833-3.7574 (m, 264H, N-CH2-CH2-OH), 3.88293.9243 (m, 264H, N-CH2-CH2-OH) 4.0953- 4.1880 (m, 80H, N-CH2-CH2-O-tri), 4.2432-4.3168 (m, 80H, N-CH2-CH2-Otri); 13C-NMR (75MHz, D2O): 25.50 (N-CH2-CH2-CH2-CH2CH2-CH2-N), 30.14 (N-CH2-CH2-CH2-CH2-CH2-CH2-N), 40.44 (N-CH2-CH2-CH2-CH2-CH2-CH2-N), 59.92 (N-CH2-CH2-O-tria), 60.60 (N-CH2-CH2-OH), 63.39 (N-CH2-CH2-O-tria) , 66.69 (NCH2-CH2-OH), 168.11(Triazine C-N), 169.99(inner C-N(CH2-CH2-O-)2), 171.55(outer Triazine C-O), 175.81(outer C-N-(CH2-CH2-O-)2), 177.28 (outer C-N-(CH2-CH2-O-)2, 179.11(Triazine C-O); Anal Calcd. for C468H814N212O168: C, 46.46; H, 6.78; N, 24.54; Found C, 46.51; H, 6.80; N, 24.59; ESI-Mass: Molecular Wt. 12086 found (M+1): 12087. 2.4. Metal ion uptake Aqueous solution of 3 mmol metal salt [Ni(NO3)2.6H2O, Cu(NO3)2.6H2O, Zn(NO3)2.6H2O] in 20 mL water was added to 200 mg of dendrimer(G1, G2 and G3) separately. The solution pH was set at 4, 7 and 9 by using buffers. The mixture was shaken in a thermostatic water-bath shaker which operated at 25 ˚C. To study effect of time, 200 mg of dendrimer was allowed to react with 20 ml metal salt solution for fixed time intervals. The metal-dendrimer complexes were collected by filtration, washed with aqueous solution of the same pH to remove noncomplexed metal ions and dried in vacuum oven. The filtrate and washings were collected to 50 ml volumetric flask and titrated against ethylene diamine tetracetic acid disodium (EDTA-2Na) salt using Murexide indicator to determine amount of metal adsorbed by dendrimer [8]. 3. Result and Discussion 3.1 Dendrimer synthesis and Characterization Dendrimer was synthesized from N,N’-bis(4,6-dichloro1,3,5-triazin-2-yl)hexane-1,6-diamine as core [6, 27] using temperature controlled nulcleophillic substitution of triazine trichloride. N,N’-bis(4,6-dichloro-1,3,5-triazin-2yl)hexane-1,6-diamine was synthesized in first step using triazine trichloride and hexane1,6-diamine as starting materials at low temperature to ensure that only one chlorine atom of triazine trichloride was substituted by a diamine. A core was then reacted with diethanolamine which was used as both solvent and reactant to give generation 1 (G1) dendrimer. The above two steps were iterated until generation 3 dendrimer was obtained. Dendritic generations were fully characterized by spectral techniques such as FTIR, 1H-NMR, 13C-NMR, Electrospray ionization mass spectrometry and elemental analysis reported elsewhere [6, 27]. 3.2 Metal ion uptake Sorption capacities full generation dendrimers G1, G2 and G3 terminated with 8, 32,128 hydroxyl groups res for heavy metal ions such as Cu2+, Ni 2+ and Zn 2+ was studied in relation to time, pH and generation number. 3.2.1 Effect of time To investigate effect of time factor [Figure 1.], a known amount of dendrimer was added to metal ion solutions and solutions were stirred at different time periods of 6h, 12h, 18h and 24h at pH 7, then resulting solutions were filtered and amount of metal ion uptake was determined by EDTA method. It was observed that 12 h was the optimum time for equilibrium [26]. Metal ion uptake was increased with increase in time after 12 h, uptake was nearly constant. Therefore, further studies were carried out at 12 h time. 3.2.2. Effect of pH Effect of pH factor metal ion uptake by dendrimer generations were studied by executing metal ion uptake experiments as pH 4, 7 and 9. The results are furnished in Fig 2. It was observed that for all the generations metal ion uptake increased with increase in pH. As pH, increased, protonation of ligands decreased which led to higher metal uptake [26]. MPJ 72 Patel, R. et al. Malaysian Polymer Journal, Vol. 9, No. 2, p 70-77, 2014 Scheme 1: Synthetic route to s-triazine based dendrimer MPJ 73 Patel, R. et al. Malaysian Polymer Journal, Vol. 9, No. 2, p 70-77, 2014 Figure 2: Effect of pH and generation on metal ion uptake by A) G1 dendrimer B) G2 Dendrimer and C) G3 dendrimer 3.3.1. Infrared Spectroscopy IR spectrum of pure G3 dendrimer showed (Table 1) absorption band at 3368 cm-1 for O-H stretching of hydroxyl groups. After complexion with Cu ions, Ni ions and Zn ions the said absorption band shifted to 3438, 3423 and 3405 cm-1 respectively for G3-Cu, G3-Ni and G3-Zn. While other prominent bands for C-H stretching, C-O stretching and C-N stretching remained at same value [Figure 3.]. This denotes that hydroxyl group must be the active binding site for metal ion uptake [26, 8]. Figure 1: Showed plot of metal ion uptake by A) G1 dendrimer, B) G2 dendrimer & C) G3dendrimer v/s time at pH 7. 3.2.3. Effect of generation number It was also evident from Fig 2 that with increase in generation number, metal ion uptake was increased. As generation number of dendrimer increased, number of terminal hydroxyl groups responsible for metal ion uptake was increased. Also with increase in generation of dendrimer, size of the dendrimer also increased. Hence, due to increase in surface group and size of dendrimer, metal ion uptake increased. The result was in accordance to previous results [8]. So, overall metal ion uptake was increased with increase in pH, generation number. It was also observed that uptake of Cu ions was maximum where as it was minimum for Zn ions. 3.3. Characterization of metal-dendrimer complexes Generation 3 dendrimer and G3 dendrimer-metal complexes were further characterized by IR and TGA to confirm presence of metal in final metal containing dendrimer. Table 1: Prominent peaks in infrared spectroscopy for pure G3 dendrimer and dendrimer metal complexes G3 dendrimer and dendrimer-metal complex G3 G3-Cu G3-Ni G3-Zn FT-IR absorption bands(cm-1) O-H C-H C-O C-N 3368 3438 3423 3405 2847 2847 2847 2847 1033 1033 1033 1033 1639 1639 1639 1639 3.3.2 Thermo gravimetric Analysis Pure generation 3 dendrimers and dendrimer containing Cu, Ni and Zn were characterized by thermo gravimetric analysis. TGA graph of G3 dendrimer (Fig. 4 A) showed two stage thermal decomposition, in first stage G3 lost 18% wt. up to 150 °C due to bound moisture. In second stage G3 dendrimer lost its remaining wt. ended up with residual wt. of 4.034 % at 600°C. Dendrimer containing metals such as G3-Cu, G3-Ni and G3-Zn showed similar thermal behavior (Fig 4. B-D) but their final residual weights were 14%, 13% and 12% respectively due to formation of metal oxides. These confirmed presence of metal in final metal containing dendrimer [26, 8] was highest, Zn ions was least. Generation 3 dendrimer had highest sorption capacity. MPJ 74 Patel, R. et al. Malaysian Polymer Journal, Vol. 9, No. 2, p 70-77, 2014 Figure 3. FT-IR Spectrum of A) pure G3 dendrimer, B) G3 dendrimer containing Cu metal, C) G3 dendrimer containing Ni metal and D) G3 dendrimer containing Zn metal Figure 4: Thermo gravimetric Analysis Graphs of A) pure G3 Dendrimer, B) G3-Cu dendrimer, C) G3-Ni dendrimer and D) G3-Zn dendrimer 4. CONCLUSION Synthesized hydroxyl terminated triazine based dendritic architectures (G1-G3) were evaluated for removal of heavy metal ions such as Cu, Ni and Zn from water. It was found that the optimum time for metal ion uptake for all dendrimer generations was found to be 12 h. With increase in pH and generation number, metal ion uptake was increased. Adsorption for Cu2+ ions was maximum for all dendrimer generations whereas minimum for Zn 2+ ions. Infrared spectroscopy of G3 dendrimer and metal containing dendrimer revealed that hydroxyl group was MPJ 75 Patel, R. et al. Malaysian Polymer Journal, Vol. 9, No. 2, p 70-77, 2014 the active binding site for metal up take. TGA study of dendrimer and metal containing dendrimer confirmed presence of metal in final metal containing dendrimer. 5. ACKNOWLEDGEMENT The authors are grateful to The Principal, V. P. & R.P.T. P. Science College for providing laboratory facility. Authors are also grateful to U.G.C., New Delhi for funding this research work. 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