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Heavy Metals Uptake by Sardinian Natural Zeolites: Experiment and Modeling Alberto Cincotti,*,†,‡ Anna Mameli,† Antonio Mario Locci,† Roberto Orru` ,†,‡ and Giacomo Cao*,†,‡,§ Dipartimento di Ingegneria Chimica e Materiali, and Unita` di Ricerca del Consorzio InteruniVersitario Nazionale “La Chimica per l'Ambiente” (INCA), Piazza d’Armi 09123 Cagliari, Italy, Centro Interdipartimentale di Ingegneria e Scienze Ambientali (CINSA), and Laboratorio del Consorzio INCA, Via San Giorgio 12, 09123 Cagliari, Italy, and CRS4, Parco Scientifico e Tecnologico, POLARIS, Edificio 1, 09010 Pula, Cagliari, Italy
Sardinian natural clinoptilolites are examined to evaluate their performance for heavy metals, i.e., lead, cadmium, copper, and zinc, removal. The natural material is either used as received or once converted into the sodium homoionic form. Equilibrium data for heavy metals in aqueous solutions and the natural material are obtained. The corresponding behavior is quantitatively correlated using classical adsorption isotherms as well as the ion-exchange equilibria model developed by Melis et al. (Ind. Eng. Chem. Res. 1995, 34, 3916), whose parameters are estimated by fitting the equilibrium data. To evaluate removal performance under dynamic conditions, breakthrough experiments are also conducted. The latter ones are compared with different ionexchange rate laws (namely, equilibrium model, linear driving force (LDF), and film model rate approximations, along with Nernst-Planck model) with the aim of pointing out the importance of the hindrance effect of intra- and interphase mass transport when using the natural materials examined in this work. On the basis of the results reported and discussed in the present paper, the natural zeolite named ROM1 may be selected, from the equilibrium point of view as well as in terms of bed volumes at breakthrough point, for application in a potentially competitive low-cost process of lead removal from aqueous solutions. 1. Introduction Zeolites have been recognized for more than 200 years, but only during the middle of the twentieth century have they attracted the attention of scientists and engineers who demonstrated their technological importance in several fields.2 Although most of the effort was devoted to synthetic zeolites, in recent years increasing attention has been directed toward natural zeolites, whose status changed from that of museum curiosity to an important mineral commodity. Several hundred thousands tons of natural zeolite-bearing materials are mined in the United States, Japan, Italy, Bulgaria, Cuba, former Yugoslavia, Mexico, Korea, and Germany, but only those containing chabazite, clinoptilolite, erionite, ferrierite, phillipsite, mordenite, and analcime are available in sufficient quantity and purity to be considered as exploitable natural resources.3 The main reason for the interest in natural zeolite-bearing materials is the increasing demand of low-cost ion-exchange and adsorbent materials in fields such as energy production, pollution control, and metal recovery as well as their wide availability on the earth. In this regard, naturally occurring zeolites have a great potential for application as packing materials in subsurface reactive barriers intercepting groundwater plumes and as fixed-bed adsorbers designed to remove several metals from industrial wastewaters.2 In particular, the economically advantageous application of natural zeolites in the treatment of effluent polluted by heavy metals has gained renewed attention only recently. In fact, the affinity of natural zeolites for lead and ammonium ions is acknowledged in the literature2 and applied, to some extent, on the industrial scale to the removal of * To whom correspondence should be addressed. † Dipartimento di Ingegneria Chimica e Materiali, and Unita` di Ricerca del Consorzio Interuniversitario Nazionale “La Chimica per l'Ambiente” (INCA). ‡ Centro Interdipartimentale di Ingegneria e Scienze Ambientali (CINSA), and Laboratorio del Consorzio INCA. § CRS4, Parco Scientifico e Tecnologico, POLARIS.
ammonia from municipal wastewaters. In a previous work,4 we tested two Italian, specifically Sardinian, natural zeolites, namely Z1 and Z2, characterized by different clinoptilolite content equal to 20 and 50 wt %, respectively. We found that, by increasing clinoptilolite content, heavy metals and ammonium adsorption capacities increase but remain invariably lower than those reported in the literature for similar materials. Only once converted in sodium form, the material (Z2), which shows the most interesting adsorption capability, is characterized by lead and cadmium removal comparable to that of other natural clinoptilolite-based adsorbents reported in the literature. Therefore, our investigation on the practical exploitation of Sardinian natural zeolites as low-cost ion-exchange and adsorbent materials for heavy metal ions removal from aqueous solutions is extended in this work, where a number of natural zeolites taken from different sites in Sardinia are specifically tested. To this aim, equilibrium and dynamic experiments have been performed in order to make a selection of the Sardinian zeolites in terms of heavy metals removal capacity (i.e., equilibrium isotherm) as well as uptake time scales (i.e., breakthrough bed volumes). Uncertainties, like incomplete ion exchange and dependence of cation-exchange capacity (CEC) determination on the particular experimental procedure adopted, are highlighted and tentatively discussed in the light of the experimental results reported in the corresponding technical literature. 2. Experimental Section The material investigated was collected at different sites in the Bonorva and Romana neighborhood, northwest Sardinia (Italy). Correspondingly, in what follows, the natural material is identified by the symbols Z3, Z4, and Z5 (from Bonorva site) and ROM1, ROM2, ROM6, ROM8, MR7, and MR7bis (from Romana site). Pretreatment of zeolites consists of washing with bi-distilled water flowing at 2.5 L/h through 80 g of fixed-bed adsorbent material in order to remove fines. As an illustrative example,
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Figure 1. Leachable cations measured during washing of Z4.
Figure 2. SEM microstructure of Sardinian natural clinoptilolite-bearing material named ROM1: (a) clinoptilolite and (b) mordenite.
Figure 1 reports the temporal profile of ion concentrations measured during Z4 washing at column exit by atomic absorption spectrophotometer (Video 12, Instruments Laboratory). The other zeolites examined in this work present an analogous behavior. As can be seen, while Na+ concentration takes about 35 min to reach a value of 100 h, as reported in the Experimental Section. Following this conclusion, the lead-removal performance of Sardinian natural zeolites would increase significantly by using zeolite particles finer than those here adopted, thus further supporting their practical exploitation as competitive lowcost materials for heavy metal ions removal from aqueous solutions. 4. Concluding Remarks In this work, we present the extension of our investigation on the practical exploitation of Sardinian natural zeolites as lowcost materials for heavy metal ions removal from aqueous solutions. Batch equilibria isotherms have been experimentally determined for as-received natural materials or once converted into sodium form. The corresponding fitting parameters of adsorption as well as ion-exchange isotherms are given. The analysis of removal performance under dynamic conditions by means of different ion-exchange rate laws points out the importance of the hindrance effect of intraphase transport when using the Sardinian natural zeolites. For the latter ones, the selectivity scale Pb > Cu > Cd ≈ Zn based on maximum metal removal in terms of meq/L can be proposed. The system ROM1/ Pb2+ may be selected as a potentially competitive low-cost adsorption process from the thermodynamic point of view as well as in terms of bed volumes at breakthrough point. Nomenclature C ) concentration in the liquid phase, mol m-3 CEC ) cation-exchange capacity, meq gzeolite-1 D ) diffusivity, m2 s-1 dp ) diameter of zeolite particle, m ED ) dispersion coefficient, m2 s-1 feq ) equilibrium isotherm kf ) interparticle diffusion coefficient, m s-1 K ) constant of equilibrium isotherm L ) zeolite bed height, m n ) equilibrium parameter N ) solution normality, eq m-3 q ) concentration in the solid phase, mg gzeolite-1 qmax ) monolayer coverage of adsorbent of Langmuir isotherm, mg gzeolite-1 r ) radial coordinate in the particle, m R ) correlation coefficient Re ) Reynolds number (Vintdp/V)
Sc ) Schmidt number (V/Dm) t ) time, s Vs ) volume of the solution, L Vint ) interstitial fluid velocity, m s-1 W ) zeolite weight, kg X ) equivalent fraction in liquid phase Y ) equivalent fraction in zeolitic phase z ) distance through the bed, m Greek Symbols ) bed void fraction γ ) level of heterogeneity of zeolitic functional groups η ) average percentage error Fp ) zeolite particle density, kg m-3 V ) kinematic viscosity, m2 s-1 Superscripts 0 ) initial conditions F ) feed conditions * ) fluid-solid interface condition - ) volume average over the particle or geometric average Acknowledgment The financial support of Progemisa SpA, Italy, is gratefully acknowledged. The authors thank Dr. M. Palomba, Dr. S. Naitza, and Mrs. F. Sini for helpful collaboration when performing the experimental part of this work. Literature Cited (1) Melis, S.; Cao, G.; Morbidelli M. A new model for the simulation of ion exchange equilibria. Ind. Eng. Chem. Res. 1995, 34, 3916-3924. (2) Mumpton, F. A. The Natural Zeolite Story. In Proceedings of 3° Congresso Nazionale AIMAT, De Frede, Ed.; Naples, XXXI-LXIV, 1996. (3) Kesraoui-Ouki, S.; Cheeseman, C.; Perry, R. Effects of conditioning and treatment of Chabazite and Clinoptilolite prior to lead and cadmium removal. EnViron. Sci. Technol. 1993, 27, 1108-1116. (4) Cincotti, A.; Lai, N.; Orru`, R.; Cao, G. Sardinian Natural Clinoptilolites for Heavy Metals and Ammonium Removal: Experimental and Modeling. Chem. Eng. J. 2001, 84/3, 275-282. (5) Langella, A.; Pansini, M.; Cappelletti, P.; De Gennaro, B.; De Gennaro, M.; Colella, C. NH4+, Cu2+, Zn2+, Cd2+ and Pb2+ exchange for Na+ in a sedimentary clinoptilolite, north Sardinia, Italy. Microporous Mesoporous Mater. 2000, 37, 337-343. (6) Roskill Information Services Ltd. The Economics of Zeolites, fourth ed.; Roskill Information Services Ltd.: London, 1995. (7) Townsend, R. P.; Loizidou, M. Ion exchange properties of natural clinoptilolite, ferrierite and mordenite. Part 1: Sodium-ammonium equilibria. Zeolites 1984, 4, 191-195. (8) Semmens, M. J.; Martin, W. P. The influence of pretreatment on the capacity and selectivity of clinoptilolite for metal ions. Water Res. 1988, 22, 537-542. (9) Pansini, M.; Colella, C.; Caputo, D.; De Gennaro, M.; Langella, L. Evaluation of phillipsite as cation exchanger in lead removal from water. Microporous Mesoporous Mater. 1996, 5, 357-364. (10) Curkovic, L.; Cerjan-Stefanovic, S.; Filipan, T. Metal ion exchange by natural and modified zeolites. Water Res. 1997, 31, 1379-1382. (11) Cerri, G.; Langella, A.; Pansini, M.; Cappelletti, P. Methods of determining cation exchange capacities for clinoptilolite-rich rocks of the Logudoro region in Northen Sardinia, Italy. Clays Clay Miner. 2002, 50 (1), 127-135. (12) Kurama, H.; Kaya, M. Removal of heavy metals from wastewater with bigadic (Turkiye) Clinoptilolite. Proc. Treat. Minimization HeaVy Metal Containing Wastes 1995, 113-125. (13) Cerjan-Stefanovic. S.; Curkovic, L.; Cerjan-Stefanovic, S.; Filipan, T. Metal ion exchange by natural and modified zeolites. Croat. Chem. Acta 1996, 69, 281-290. (14) Faghihian, H.; Ghannadi Marageh, M.; Kazemian, H. The use of clinoptilolite and its sodium form for removal of radioactive cesium, and strontium for removal of radioactive cesium, and strontium from nuclear wastewater and Pb2+, Ni2+, Cd2+, Ba2+ from municipal wastewater. Appl. Radiat. Isot. 1999, 50, 655-660.
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(15) Maliou, E.; Malamis, M.; Sakellarides, P. O. Lead and cadmium removal by ion exchange. Water Sci. Technol. 1992, 25 (1), 133138. (16) Zamzow, M. J.; Eichbaum, B. R.; Sandgren, K. R., Shanks, D. E. Removal of heavy metals and other cations from wastewater using zeolites. Sep. Sci. Technol. 1990, 25, 1555-1569. (17) Barrer, R. M.; Klinowski, J.; Sherry, H. S. Zeolite exchangers: Thermodynamic treatment when not all ions are exchangeable. J. Chem. Soc., Faraday Trans. 2 1973, 69, 1669-1676. (18) Semmens, M. J.; Seyfarth, M. The selectivity of clinoptilolite for certain heavy metals. In Natural Zeolites, Occurrence, Properties, Use. Sand, L. B., Mumpton, F. A., Eds.; Pergamon Press: Elmsford, NY, 1978; pp 517-526. (19) Dyer, A.; Enamy, H.; Townsend, R. P. The plotting and interpretation of ion-exchange isotherms in zeolite systems. Sep. Sci. Technol. 1981, 16 (2), 173-183. (20) Pansini, M.; Colella, C. Dynamic data on lead uptake from water by chabazite. Desalination 1990, 78, 287-295. (21) Helfferich, F. Ion exchange; McGraw-Hill: New York, 1962. (22) Carta, G.; Cincotti, A.; Cao, G. Film model approximation for particle-diffusion-controlled binary ion exchange. Sep. Sci. Technol. 1999, 34, 1-15.
(23) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The properties of gases & liquids; McGraw-Hill: New York, 1988. (24) Chung, S. F.; Wen, C. Y. Longitudinal dispersion of liquid flowing through fixed and fluidized beds. AIChE J. 1968, 14 (6), 857-866. (25) Ruthven, D. M. Principles of adsorption and adsorption processes; Wiley: New York, 1984. (26) Perry’s Chemical Engineers Handbook, 7th ed.; McGraw-Hill, New York, 1997. (27) CRC Handbook of Chemistry and Physics, 80th ed.; CRC Press: New York, 1999. (28) Booker, N. A.; Cooney, E. L.; Presley, A. J. Ammonia removal from sewage using natural Australian zeolite. Water Sci. Technol. 1996, 34 (9), 17-24. (29) Robinson, S. M.; Arnold, W. D.; Byers, C. H. Mass-transfer mechanisms for zeolite ion exchange in wastewater treatment. AIChE J. 1994, 40 (12), 2045-2054.
ReceiVed for reView March 24, 2005 ReVised manuscript receiVed October 25, 2005 Accepted November 3, 2005 IE050375Z
