Kuzovenko, V. A.

Yanovska, E. S.

Кичкирук, О. Ю.

Тертых, В. А.

Zhytomyr State University Library

 ISSN 1070-3284, Russian Journal of Coordination Chemistry, 2007, Vol. 33, No. 8, pp. 616–620. © Pleiades Publishing, Ltd., 2007.Original Russian Text © E.S. Yanovskaya, V.A. Kuzovenko, V.A. Tertykh, O.Yu. Kichkiruk, 2007, published in Koordinatsionnaya Khimiya, 2007, Vol. 33, No. 8, pp. 627–631. 616 INTRODUCTIONInorganic adsorbents with surface-immobilized het-erocyclic azo compounds, including 4-(2-pyridy-lazo)resorcinol ( PAR ), can be used for extraction,solid-phase concentration, and chromatographic sepa-ration of heavy metals [1, 2]. These adsorbents [3–5]are characterized by different degrees of washing out ofadsorbed PAR, depending on the application condi-tions. Sorbents with covalently immobilized PAR mol-ecules are much more resistant to washing out due tohydrolytically stable chemical bonds formed with thesurface groups [6, 7].The multistage and very labor-consuming process(introduction of nitro groups bound to the benzene ringinto the surface layer, their reduction followed by dini-tration, and azo coupling with PAR) has been used pre-viously to synthesize supports with a covalently immo-bilized reactant, giving a sorbent with a low content ofgrafted complex forming groups and heterogeneouscomposition of the surface [6, 7]. The method of one-step aminomethylation (Mannich reaction) usingparaformaldehyde as a methylating agent turned out tobe promising in this respect. This method has beenapplied successfully to chemical grafting of oxine,pyridylazonaphthol, and PAR to the silica surfaces [8].In this work, we present the results of studying theadsorption properties of silica gel with chemicallygrafted PAR molecules toward toxic metals Pb(II),Cd(II), and Zn(II), as well as toward other metals,namely, Cu(II), Fe(III), Co(II), and Ni(II). 4-(2-Pyridy-lazo)resorcinol manifests high affinity to these metals,and the adsorbent selectivity can decrease substantiallyin their presence.EXPERIMENTALSilica gel with chemically bound PAR was preparedaccording to a described procedure [8] using the one-step modification of the silica surface by the Mannichreaction involving 3-aminopropyltriethoxysilane andparaformaldehyde via the scheme:Si OHOHOHNN N+ (H5C2O)3SiCH2CH2CH2NH2 + (CH2O)n +OHOHNN NNHSiOOC2H5OC2H5Si Complex Formation of Heavy Metalswith 4-(2-Pyridy-lazo)resorcinol Chemically Immobilizedon Silica Gel E. S. Yanovskaya a, V. A. Kuzovenko a , V. A. Tertykh b , and O. Yu. Kichkiruk c a  Shevchenko National University, Kiev, Vladimirskaya ul. 64, Kiev, 252017 Ukraine b  Institute of Surface Chemistry, National Academy of Sciences of Ukraine, Kiev, Ukraine c  Franko State University, Zhitomir, Ukraine Received September 11, 2006 Abstract —The complexing properties of modified silica with 4-(2-pyridylazo)resorcinol covalently immobi-lized by the Mannich reaction are studied with respect to heavy metals. This adsorbent can be used for the pre-concentration of heavy metal ions due to its satisfactory kinetic characteristics. The structures of the heavymetal complexes with 4-(2-pyridylazo)resorcinol immobilized on the silica gel surface are studied by ESR anddiffuse reflectance spectroscopy. DOI:  10.1134/S107032840708012X RUSSIAN JOURNAL OF COORDINATION CHEMISTRY        Vol. 33        No. 8        2007 COMPLEX FORMATION OF HEAVY METALS 617 The silica gel with a specific surface area of256 m 2 /g and a particle diameter of 0.1–0.2 mm wasused as the support. A weighed sample of PAR (0.43 g,2 mmol) was dissolved in ethanol (40 ml) on heating( 60°ë ). Then parafomaldehyde (0.12 g, 4 mmol),3-aminopropyltriethoxysilane (0.44 ml, 2 mmol), andthe support (50 g) were added. The total volume ofalcohol in the reactor was brought to 115 ml, and themixture was kept in a thermostat for 5 h at  60°ë  withperiodical shaking. The obtained modified adsorbentwas dried for 2 h at  120°ë , washed with ethanol10 times with 70-ml portions and 10 times with 50-mlportions, and then dried at  120°ë  to a constant weight.Spectrophotometry was used to determine the concen-tration of grafted PAR molecules [9]. A weighed sam-ple of the chemically modified silica gel was dissolvedin alkali, and the absorption intensity of the obtainedsolution was measured at  λ  = 485 nm. Solutions of PARin alkali were used to plot a calibration curve. The anal-ysis showed that after thorough washing with ethanol,the modified silica surface contained molecules of thegrafted reactant in an amount of 0.2 mmol/g.The initial solutions of metals were prepared by thedissolution of exact weighed samples of the followingsalts: Pb ( NO 3 ) 2 ,  ZnCl 2 ,  Cd ( NO 3 ) 2 ,  Co ( NO 3 ) 2 ,  CuSO 4 , Fe 2 ( SO 4 ) 3 ,  and  (all analytical grade). Thenthe solutions were acidified to avoid hydrolysis. Themetal concentrations in the initial solutions wererefined titrimetrically using EDTA. The working solu-tions with the required metal concentration were pre-pared by diluting the initial solutions with twice dis-tilled water.Adsorption processes were studied in the staticregime with a sorbent weight of 0.1 g and volumes ofthe working solutions of 25 cm 3 . The pH value of theworking solutions was maintained by the addition ofbuffers prepared from fixanals of standard solutions orammonia–acetate mixtures and controlled on anI-130.2M.1 ionometer.The equilibrium concentration of the Pb(II), Cd(II),Co(II), and Cu(II) complexes with PAR was deter-mined photometrically on an SF-46 spectrophotometer(quadratic cells with  l  = 1 cm) at  λ  = 520, 500, 510, and490 nm, respectively [10–12]. A procedure with murex-ide at  λ  = 455 nm [13] was used for the photometricdetermination of the Zn(II) concentration in equilib-rium solutions, whereas the reactions with phenanthro-line in the presence of a reducing agent [14] and withdimethylglioxime [15] were used for Fe(III) and Ni(II),respectively. The amount of metal adsorbed by thechemically modified silica gel was determined from thedifference between the ion concentrations in the initialand equilibrium solutions.Diffuse reflectance ( DR ) spectra of the complexeson the modified silica gel surface were recorded on aSpecord M40 instrument. ESR spectra of the copper(II)complexes were measured on an RE-1306 radio spec-trometer at 293 K. The resonance frequency was mea-Ni NO3( )2 sured using a ChZ-54 instrument with a YaZCh-87 fre-quency converter. Diphenylpicrylhydrazyl ( g  = 2.0036)and Mn 2+  ions in the MgO matrix ( g  = 2.0015) servedas standards. Samples in ampules with a diameter of3 mm were placed in a resonator. The parameters of theanisotropic spectra were calculated from the position ofthe lines in parallel orientation of the  g factor ( g || ) withan accuracy of 0.005 or estimated from the lines in per-pendicular orientation of the  g  factor ( g ⊥ ) according to[16]. IR spectra of samples were recorded on aNexus-470 FTIR spectrophotometer in a region of400 − 4000 cm –1 .RESULTS AND DISCUSSIONTo confirm PAR immobilization on the surface layerof the silica gel, we compared the IR spectra of the syn-thesized sorbent and of the initial reactant (Table 1).The IR spectrum of the modified adsorbent in regions<1200 and 3000–3800 cm –1  exhibits intense absorptionbands of the framework and hydrate layer of silica,which hamper identification of some functional groupsof immobilized molecules. At the same time, the bandof the stretching vibrations of the OH groups of thePAR molecules at 3360 cm –1  can be identified ratherreliably in the spectrum of the adsorbent. The region of1000–1600 cm –1  in the IR spectra is most informativeto confirm the presence of PAR in the modifying silicalayer. This region contains absorption bands at1580 cm –1  corresponding to skeletal vibrations of theC–N bonds at 1600 cm –1  (the most intense band) andbands at 1340 and 1440 cm –1  caused by vibrations ofthe aromatic rings.The studies of the dependence of the degree ofextraction of the metals on the acidity of the solution(Table 2) showed that the silica gel with chemicallyimmobilized PAR extracts quantitatively the Pb 2+ , Cd 2+ , and Ni 2+  ions in neutral and weakly alkalinemedia. The sorption capacity of the obtained sorbenttoward these metals is rather high and makes it possibleto extract them quantitatively from aqueous solutions ina wide concentration interval (0.1–100  µ g/ml). In thiscase, the Zn 2+ , ëÓ 2+ ,  Cu 2+ , and Fe 3+  ions are partially Table 1.   Absorption bands in the IR spectra of 4-(2-pyridy-lazo)resorcinol (PAR) and modified silica with grafted PAR ν , cm –1 AssignmentPAR modified silica with PAR1060 1060 ν (C–OH)1230 δ (OH)1340, 1440, 1600 1340, 1440, 1600 (C–C) ar 1580 1580 ν (C–N) ar 3360 3360 ν (OH) 618 RUSSIAN JOURNAL OF COORDINATION CHEMISTRY        Vol. 33        No. 8        2007 YANOVSKAYA et al. extracted. In acidic media at pH 1–4, the synthesizedadsorbent quantitatively extracts none of these metals.The results of studying the kinetics of adsorption ofthe metals in question in the static regime in a neutralmedium at pH 6.8–7.0 show that the silica gel withchemically immobilized PAR extracts quantitativelythe Pb 2+ ,  Cd 2+ , and Co 2+  ions for 2–5 min under theseconditions. The maximally possible (higher than 83%)degree of extraction of the Cu 2+  ions is attained withinthe same time. The achieved degree of adsorption of theFe 3+  and Zn 2+  ions is ~89 and 95%, respectively(10-min contact). The maximum degree of extraction ofthe Ni 2+  ions is achieved for 1 h.Thus, the silica adsorbent with covalently immobi-lized PAR synthesized by one-step aminomethylationreaction is characterized by satisfactory rate of adsorp-tion of toxic metal ions and can be used for their extrac-tion from solutions in the dynamic regime.The ESR spectra of the complexes formed on the sil-ica gel surface with covalently immobilized PAR mol-ecules during Cu 2+  ion adsorption are characterized bythe asymmetric shape, anisotropy, and hyperfine struc-ture of the signal  g ||   (Fig. 1, Table 3). The values of theparallel component of the  g  factor and hyperfine split-ting constant ( Ä || ) calculated from the ESR spectra ofsample  1  (in which the concentration of adsorbed cop-per is several times lower than the concentration ofgrafted PAR molecules) are characteristic of octahedraloxygen complexes with the Cu [é 4 é 2 ]  coordination unit(adducts of copper bis(chelates) with strong axial coor-dination to positions 5 and 6 [17]) or octahedral com-plexes similar to those formed in crystalline hydrateCuSO 4   ·  5 H 2 O [18, 19].These complexes can be formed by copper(II) coor-dination to the oxygen atoms of two hydroxyl groups oftwo PAR molecules and two water molecules or twohydroxyl groups of one 4-(2-pyridylazo)resorcinolmolecule and four water molecules.For a low degree of coverage of the PAR-modifiedsilica gel with copper(II), the complexes with the coor-dination mode different from those known for solutionsprevail. This is indirectly confirmed by the fact that atlow concentrations, copper is sorbed nonquantitativelyand more poorer than the Pb 2+ and Cd2+ ions (Table 2),although the stability constant of the copper complexeswith this reactant in solution is much higher than thosefor the latter [9].A comparison of the ESR spectrum of sample 2 inwhich the amount of adsorbed copper exceeds the con-centration of the grafted PAR molecules with the spec-Table 2.  Dependence of the degree of extraction (%) of the metal ions on the silica gel with chemically grafted PAR on pHof the solution*Metal ionpH of solution1.0 1.7 4.0 6.8 7.0 8.0 9.2Zn2+ 55.5 84.6 78.8 92.9 87.5 89.3 71.3Cd2+ 0 0 0 92.0 99.9 92.0 92.4Pb2+ 0 0 21.2 99.9 99.9 99.9 95.6Cu2+ 36.4 69.7 76.6 85.1 83.0 86.4 82.3Fe3+ 0 92.6 93.0 97.5 88.8 68.8 67.6Co2+ 0 0 72.5 99.9 84.8 84.8 84.8Ni2+ 20.0 50.0 75.0 99.9 99.9 99.9 99.9* Experimental conditions: static regime, weight of the adsorbent 0.1 g, weight of the metal in the initial solution 100 µg, adsorption time 24 h.DPPHÇ100 mT21Fig. 1. ESR spectra of the Cu(II) complexes with PARchemically immobilized on the silica gel surface afteradsorption of (1) 0.013 and (2) 0.062 mmol/g copper.RUSSIAN JOURNAL OF COORDINATION CHEMISTRY      Vol. 33      No. 8      2007COMPLEX FORMATION OF HEAVY METALS 619trum of sample 1 shows that an increase in the adsorbedcopper concentration decreases the g|| and Ä|| values(Table 3). The calculated parameters of the spectrum ofsample 2 (concentration of the coordinated metal is0.062 mmol/g) correspond to those for the complexesof four-coordinate copper(II) with the [N2O2] coordina-tion unit [20–22], as it usually occurs for the complexformation of the PAR molecules with the Cu2+ ions insolutions.The DR spectra of the metal complexes with thePAR molecules chemically modified on the silica sur-face (Fig. 2) are characterized by absorption bands inthe visible region at 525, 510, 700, 520 and 700, 500 nmfor Cu(II), Cd(II), Fe(III), Pb(II), and Zn(II), respec-tively. These data indicate that the coordination modeof the Cd2+ and Pb2+ ions with the PAR moleculeschemically grafted to the silica gel surface by the one-step aminomethylation reaction is similar to that forcomplex formation in solutions. Also note that, accord-ing to published data [23], the complexes with the Zn2+ions formed on the surface of the PAR-containing sor-bent obtained by the sol–gel method are also character-ized by a band at 500 nm. The DR spectra of the Cu(II)complexes with PAR with different copper contents inthe samples are similar.Thus, the silica gel with covalently immobilizedPAR synthesized by one-step aminomethylationthrough the Mannich reaction extracts quantitatively atpH 6.8–7.0 particularly toxic metals such as Pb(II),Cd(II), and Ni(II) and sorbs partially the Zn2+, Cu2+,Fe3+, and Co2+ ions due to the formation of surface com-plexes. The data on the DR spectra showed that thecoordination mode of PAR chemically grafted to thesilica surface by the Cd2+ and Pb2+ ions is similar to thatfor the interaction of the components in solutions.The ESR study of the structures of the Cu2+ com-plexes with PAR immobilized on silica indicates thatthe octahedral oxygen complexes Cu[O4O2] predomi-nate at low coverages of the surface with the metal,whereas the complexes with the [N2O2] coordinationunit prevail when the content of the adsorbed copperincreases.The synthesized chemically modified silica is char-acterized by rather good ability to adsorb metal ionsdue to the pronounced complexing properties of thesurface groups. This makes the obtained adsorbentpromising for use in the processes of solid-phase con-centration of heavy metals during analyses of differentnatural objects.REFERENCES1. Basova, E.M., Bol’shova, T.A., Shpigun, O.A., andIvanov, V.M., Zh. Anal. Khim., 1993, vol. 48, no. 7,p. 1094.2. Ivanov, V.M., Zh. Anal. Khim., 1991, vol. 46, no. 4,p. 645.3. Sato, K., Monden, M., and Goto, T., Bunseki Kagaku,1999, vol. 48, no. 2, p. 261.Table 3.  Parameters of the ESR spectra of the complexes of Cu2+ ions with PAR covalently bound with the silica surfaceSample Amount adsorbed of Cu2+ ions, mmol/gParameters of ESR spectrag|| g⊥ A|| × 10–4, cm–11 0.013 2.380 2.039 1792 0.062 2.310 2.028 173Absorption500 λ, nm400 6657654312Fig. 2. Diffuse reflectance spectra of (1) PAR, (2) silica gelwith chemically grafted PAR, and (3–7) surface complexesof PAR with the Cu2+, Cd2+, Fe3+, Pb2+, and Zn2+ ions,respectively.620RUSSIAN JOURNAL OF COORDINATION CHEMISTRY      Vol. 33      No. 8      2007YANOVSKAYA et al.4. Tange, H., Yasuda, M., and Goto, T., Bunseki Kagaku,2000, vol. 49, no. 4, p. 239.5. Ivanov, V.M., Ershova, N.I, and Figurovskaya, V.N., Zh.Anal. Khim., 2002, vol. 57, no. 10, p. 1052 [J. Anal.Chem. (Engl. Transl.), vol. 57, no. 10, p. 890].6. 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Complex formation of heavy metals with 4-(2-pyridy-lazo) resorcinol chemically immobilized on silica gel

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Complex formation of heavy metals with 4-(2-pyridy-lazo) resorcinol chemically immobilized on silica gel

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