Adsorption of Nickel(II) Cations by Natural Zeolites

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1 ISSN 27-25, Protection of Metals and Physical Chemistry of Surfaces, 24, Vol. 5, No. 3, pp Pleiades Publishing, Ltd., 24. Original Russian Text O.I. Pomazkina, E.G. Filatova, Yu.N. Pozhidaev, 24, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 24, Vol. 5, No. 3, pp PHYSICOCHEMICAL PROCESSES AT THE INTERFACES Adsorption of Nickel(II) Cations by Natural Zeolites O. I. Pomazkina, E. G. Filatova, and Yu. N. Pozhidaev Irkutsk State Technical University, ul. Lermontova 83, Irkutsk, Russia efila@list.ru Received August 26, 23 Abstract The composition of a natural specimen of zeolite-containing rock of a Transbaikal field including Ca[Al 2 Si 7 O 8 ] 6H 2 O calcium heulandite and KAlSi 3 O 8 potash fieldspar impurity is confirmed with X-ray phase analysis and infrared spectroscopy. The time necessary for attaining adsorption equilibrium in a zeolite aqueous nickel sulfate solution system is estimated. The limiting adsorption at different temperatures is found from the Langmuir equation. Thermodynamic and kinetic characteristics of the adsorption of nickel (II) cation on natural minerals are calculated. DOI:.34/S INTRODUCTION The main pollutants contained in sewage of galvanic plants are heavy-metal ions. According to investigations carried out by the World Health Organization, judging from the Korte index, which expresses the level of hazard for living organisms, heavy metals have been at the top of the list of dangerous contaminants since 963, far ahead of dioxins and radionuclides []. Heavy metals are hazardous contaminants, monitoring the content of which in any environment is obligatory. This is especially important in the case of metals, which are used in large-scale industry. Being accumulated in the environment, the metals are dangerous because of their biological activity and toxicity. The most typical contaminants in the sewage of galvanic, chemical metallurgical, and hydrometallurgical plants are nickel, copper, zinc, chromium, and iron ions. In analytical review [2], ecological and economic aspects of the reduction and extraction of nickel from hydrometallurgical sewage exhaust batteries, and other sources were considered. Ecological danger of pollution of sewage with heavy-metal ions can be prevented by increasing the purification degree of sewage, reusing extracted metal ions, and developing reused water-distribution systems. In order to solve the problems, it is reasonable to use promising adsorption methods for sewage purification. Nickel ions can be extracted with natural and artificial zeolites. For example, the results of the extraction of copper and nickel from aqueous solutions with Y zeolites can be found in [3]. Based on the calculated free Gibbs energy values, the authors of [4] have shown that extraction of heavy-metal ions (Cr, Cu, Cd, Ni, and Pb) with the use of klinoptilolith can be considered as physical adsorption. It is also known that nickel(ii) ions can be removed from aqueous solutions with X zeolites modified with a biofilm in a bioreactor [5]. The adsorption mechanism was studied in detail with the use of infrared spectroscopy, Fourier-transform analysis, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). Natural klinoptilolith zeolite from Serbia field was studied [6] as an adsorbent that can be used in nickel (II) extraction. The sorption capacity of the zeolite was.9 mg/g at 298 K and increased to 3.8 mg/g with an increase in temperature to 338 K. The adsorption kinetics was approximated by a pseudo-first-order equation. The activation energy was found to be 7.44 kj/mol. The removal of heavy-metal ions from aqueous solutions with the use of synthetic A zeolites was considered in [7]. The results obtained were analyzed based on the Langmuir, Freundlich, and Dubinin Radushkevich equations. The free Gibbs energy of the process was found to correspond to ion-exchange adsorption. The synthetic zeolite was shown to be an efficient ionexchange material for extraction of heavy-metal ions from industrial sewage. Dowex HCR S/S cationexchange polymer was studied in [8] as a material for extracting nickel and zinc from aqueous solutions. Under optimal conditions, the efficiency of the extraction of heavy-metal ions was 98%. As follows from the above brief review of investigations in the field, natural and synthetic zeolites are widely used for extracting heavy-metal ions from sewage. There are no literature data on the use of calcium heulandite for the same purpose. The object of this work was to study the possibility of using natural zeolites for the extraction of heavymetal ions, including nickel(ii), from sewage of galvanic plants. EXPERIMENTAL Natural zeolites from the Eastern Transbaikal field were taken as specimens. Zeolites belong to framework aluminosilicates, crystal lattices of which are 32

2 ADSORPTION OF NICKEL(II) CATIONS 33 Heulandite-Ca L in (Counts) d = d = d = d = d = d d = d = d = d = d = d = d = d = d = d = d = d = d = d = d = d = d = d = d = d = d = d = d = d = d = d = θ, scale d = d = d = d = d = d =.6678 d =.644 d =.592 d = Fig.. X-ray-diffraction pattern of a zeolite-containing specimen. formed by [SiO 4 ] 4 and [AlO 4 ] 5 tetrahedra joined via common vertices in a three-dimensional structure [9]. The existence of voids, cavities, and channels in zeolite microstructures, as well as the relative freedom in motion of cations and water molecules, predetermines the unique properties of zeolites. In an aqueous environment, cations present in zeolites (Са 2+, Na +, К +, etc.) can easily be replaced with foreign cations present in the solution. Zeolites belong to microporous adsorbents with pore sizes of.5.5 nm []. The properties of zeolite specimens were studied with the use of X-ray phase analysis with a D8- ADVANCE automated powder diffractometer and infrared spectroscopy with a BRUKER Tensor 27 system. For the tests, preliminarily sieved zeolite fraction with a granule size of..5 mm was taken. Adsorption of nickel(ii) cations was studied from aqueous solutions prepared from chemically pure NiSO 4 7H 2 O and distilled water. The initial concentration of nickel(ii) cations in the solutions was selected on the basis of the actual compositions of sewage of galvanic plants. The content of nickel(ii) cations in aqueous solutions was determined spectrophotometrically with the use of dimethylglyoxime []. The adsorption properties of zeolite specimens with respect to nickel(ii) ions were determined under stationary conditions. Adsorption isotherms were constructed for constant samples ( g) and variable nickel(ii) concentration (from 4 to 83 mg/l). The adsorption (А, mg/g) was calculated from the following relation: c ceq A = V, () m where с and c eq are the initial and equilibrium concentrations of nickel in the solution (mg/l), m is the zeolite sample (g), and V is the volume of solution (L). The volume of solution was. L. The acidity of model solutions was measured with a ph-34 ph-meter with a conventional technique [2]. The temperature was maintained with a UTU-4 thermostat, and investigations were carried out at 298, 38, and 338 K. Solutions were stirred with a magnetic Transmitiance [%] Wavenumber cm Fig. 2. Infrared spectrum of the natural zeolite PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 5 No. 3 24

3 34 POMAZKINA et al. A, mg/g К 5 2 τ, min Fig. 3. Kinetic-adsorption curves obtained at different temperatures. A, mol/kg К c eq, mol/l Fig. 4. Adsorption isotherms of nickel(ii) cations. agitator, and the conditions of stirring were the same in all experiments. The content of heavy metals, including nickel(ii) cations, in sewage of galvanic plants was determined with the use of atomic-absorption analysis [2]. For analysis of electrolytes, which contained mixtures of different cations (Ni 2+, Сu 2+, Zn 2+, and Fe 3+ ), a systematic approach specifically, the ammonia method, was used. When determining the content of the main component, the interfering effect of residual ions was eliminated by binding the ions in strongly bound complexes [2]. RESULTS AND DISCUSSION According to the data of X-ray phase analysis, a specimen of zeolite-containing rock from the Eastern Transbaikal field (Fig. ) is composed of Ca[Al 2 Si 7 O 8 ] 6H 2 O calcium heulandite and КАlSi 3 O 8 potash fieldspar impurity. The structure of heulandite is close to that of phyllosilicates, which makes it different from other zeolites []. The presence of structural groups typical of heulandite [3] in the zeolites studied was confirmed by data of infrared spectroscopy (Fig. 2). The Al O stretching vibrations are related to absorption bands at 794 and 727 cm. The intense absorption band at 4 cm can be assigned to asymmetric stretching vibrations of Si O Si atomic groups. The absorption band at 344 cm confirms the presence of O H bonds. Adsorption activity of zeolite with respect to nickel(ii) cations was estimated from the analysis of kinetic curves and adsorption isotherms. The time necessary for attaining adsorption equilibrium, which corresponds to the constant nickel(ii) concentration in the solution, was found to be about 2 min (Fig. 3). With an increase in temperature, the adsorption in the initial period increases, which is obviously caused by the increase in the diffusion rate in zeolite granules. When the adsorption equilibrium is reached, the adsorption decreases with an increase in temperature. Figure 4 shows adsorption isotherms recorded at temperatures of 298, 38, and 38 K. The shape of isotherms corresponds to monomolecular adsorption. With an increase in temperature, the adsorption of nickel(ii) cations on zeolites decreases, which is typical of exothermal adsorption processes. Therefore, the amount of adsorbed nickel(ii), which corresponds to the equilibrium between the liquid phase and the adsorption layer, decreases with an increase in temperature. The Langmuir adsorption isotherm can adequately describe the limiting adsorption: = +, (2) A A A Kceq where А is the current adsorption value (mol/kg), А is the limiting adsorption (mol/kg), K is the adsorptionequilibrium constant, and c eq is the concentration of solution (mol/l). Figure 5 shows the linearized Langmuir adsorption isotherm for nickel(ii) cations on natural zeolites. The А limiting adsorption was determined from the intercepts of the linearized dependences with the ordinate axis, and the К adsorption-equilibrium constant was found from the slopes of the lines (Table ). Based on the adsorption-equilibrium constant, the free Gibbs energy of the adsorption process was estimated from the Van t Hoff isotherm, Δ G = RT ln K, and the results obtained are listed in Table. PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 5 No. 3 24

4 ADSORPTION OF NICKEL(II) CATIONS К /A К ln(c /c) /c eq 5 5 τ, min Fig. 5. Linearized Langmuir adsorption isotherm of nickel(ii) ions on natural zeolite. Fig. 6. Integral variant of the first-order kinetic dependence. With an increase in temperature, the free Gibbs energy increases in absolute value, which means that the adsorption interaction becomes stronger with an increase in temperature. Adsorption kinetics is usually described by a pseudo-first-order equation: ln c ln c = kτ, c 2+, Ni or,ni 2 + ln = kτ, Ni 2+ c 2+ Ni where k is the rate constant of pseudo-first-order adsorption (min ). Adsorption-rate constant k and correlation coefficient R 2 were estimated from the ln( c 2+ c 2+ ) = f( τ) linear dependence (Fig. 6)., Ni Ni Rate constants were found from the slopes of the lines at different temperatures (Table 2). Based on the rate constants, the activation energy of the adsorption was calculated as follows: ( ) R ln kt ln k T T 2 2T E =. T2 T The obtained estimate of the activation energy (2. kj/mol) means that the process studied is physical adsorption. In the thermodynamics of adsorption, important information is presented in adsorption isosteres, lnc eq /T mol/kg.6 mol/kg.5 mol/kg.4 mol/kg Fig. 7. Isosteres at different adsorption of nickel(ii) cations. which are temperature dependences of the concentration of the equilibrium phase at a constant adsorption value (Fig. 7). The linear character of the isosteres constructed means that the physical adsorption process does not significantly change with an increase in temperature. The experimental results obtained were used for calcu- Table. Limiting adsorption, adsorption-equilibrium constant, and free Gibbs energy of adsorption T, K Linearized Langmuir equation mmol/g A, mg/g K 3 ΔG, kj/mol 298 /A =.7 /c /A =.8 /c /A =. /c PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 5 No. 3 24

5 36 POMAZKINA et al. Table 2. Adsorption-rate constant T, K Rate constant, k 2, min R Table 3. Differential heat of adsorption A, mol/kg Q, kj/mol Table 4. Purification of sewage Heavy-metal ions Heavy-metal ion concentration before purification, mg/l Heavy-metal ion concentration after purification with zeolites, mg/l Ni Cu Zn ðí lation of differential heats of adsorption in accordance with the Clausius Clapeyron relation: ln ceq ( T) A= const Q =, R where Q is the isosteric heat of adsorption (kj/mol) and R is the universal gas constant (J/(mol K)). The differential heat of adsorption was estimated from the slopes of isosteres in accordance with the Clausius Clapeyron equation, and the results obtained are listed in Table 3. As can be seen (Table 3), in the initial period, the adsorption of heavy-metal ions on natural zeolites is accompanied by an increase in the differential heat of adsorption from 22 to 3 kj/mol. Such a jump is due to the interaction between adsorbed nickel(ii) cations and active sites of the zeolites studied. Subsequent increase in the adsorption results in the decrease in the heat of adsorption because of the increase in the repulsion between particles at their close arrangement in the system [4]. The possibility of using natural zeolites for extracting heavy-metal ions, including nickel(ii) cations, was studied with the use of sewage of a galvanic plant. The results obtained are summed in Table 4. Zeolites as absorbents enable one to decrease the concentration of heavy-metal ions in industrial sewage by more than 85%. CONCLUSIONS. Equilibrium in a zeolite aqueous NiSO 4 solution system is reached in 2 h. The adsorption of nickel(ii) cations on calcium heulandite is 5.9 mg/g at 298 K, which enables us to suggest that this zeolite be used as an adsorbent for purification of sewage of galvanic plants. 2. Adsorption of nickel(ii) cations on zeolites can formally be described by the first-order kinetic equation. The apparent activation energy of the process is 2. kj/mol. REFERENCES. Fomin, G.S., Voda. Kontrol khimicheskoi, bakterial noi i radiatsionnoi bezopasnosti po mezhdunarodnym standartam (Water. Monitoring Chemical, Bacterial, and Radiation Safety According to International Standards), Moscow: Protektor, Coman, V., Robotin, B., and Ilea, P., Resour., Conserv. Recycl., 23, vol. 73, p Keane, M., Colloids Surf., A, 998, vol. 38, no., p.. 4. Myroslav, S., J. Hazard. Mater., 29, vol. 6, p Quintelas, C., Pereira, R., Kaplan, E., and Tavares, T., Biores. Technol., 23, vol. 42, p Rajic, N., Stojakovic, D., Jovanovic, M., et al., Appl. Surf. Sci., 2, vol. 257, p El-Kamash, A.M., Zaki, A.A., and El Geleel, M.A., J. Hazard. Mater., 25, vol. 27, p Alyüz, B. and Veli, S., J. Hazard. Mater., 29, vol. 67, p Breck, D.W., Zeolite Molecular Sieves: Structure, Chemistry, and Use, New York: Wiley, Tsitsishvili, G.V., Andronikoshvili, T.G., Kirov, G.N., and Filizova, L.D., Prirodnye tseolity (Natural Zeolites), Moscow: Nauka, Marchenko, Z., Spectrophotometric Determination of Elements, New York: Wiley; Vasil ev, V.P., Analiticheskaya khimiya (Analytical Chemistry), Moscow: Drofa, Veisgeim, A.S., Nazarenko, O.B., and Zarubina, R.F., Vest. Nauki Sibiri, 22, no. 4(5), p Tsivadze, A.Yu., Rusanov, A.I., Fomkin, A.A., et al., Fizicheskaya khimiya adsorbtsionnykh yavlenii (The Physical Chemistry of Adsorption Phenomena), Moscow: Granitsa, 2. Translated by Y.V. Novakovskaya PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 5 No. 3 24