Envlron. Sci. Technol. 1993, 27, 1 108- 1 1 16
Effects of Conditioning and Treatment of Chabazite and Clinoptilolite Prior to Lead and Cadmium Removal Sabhha Kesraoul-Oukl,’ Christopher Cheeseman, and Roger Perry
Centre for Environmental Control and Waste Management, Imperlal College of Science, Technology and Medicine, Imperial College Road, London SW7 2BU, United Kingdom This paper characterizes two natural zeolites, chabazite and clinoptilolite, and investigates the effects of treatment and conditioningon the removal of Pb and Cd from effluent wastewaters. The results demonstrated that the asreceived zeolites contained a complement of exchangeable K, Ca, and Na ions but exposing them to concentrated NaCl solutions converts them to a homoionic state in the Na form, which improves significantly their exchange capacity. Particle size was found to have no effect on the exchange, suggesting that the mechanism is not surface controlled. Pb and Cd removal results using as-received, conditioned, and treated zeolites showed that conditioning and treatment improved both the exchange capacity and the removal efficiency when operating at metal concentrations greater than 250 mg/L. NaOH-treated chabazite and clinoptilolite performed best with optimum Pd and Cd exchange capacities exceeding 200 mg/g. The chabazite exchange capacity performance was superior to the clinoptilolite’s for both Pd and Cd. This result was expected as chabazite characterization showed that it had a higher A1 substitution of Si, which in turn will produce a negative framework favorable to higher cation exchange capability. Overall, metal removal efficiencies exceeding 99% ’ were achieved,revealing the natural zeolites potential in treating effluent contaminated with high levels of P b and Cd.
Introduction Zeolites have been intensively studied in the last half century, although the attention has been concentrated mainly on synthetic zeolites, and it is only in recent years that natural zeolites have started gaining interest (1). Natural zeolites are capable of removing trace quantities of cations from aqueous solutions by utilizing the phenomenon of ion exchange (2). Clinoptilolite, for example, has received extensive attention due to its attractive selectivities for certain heavy metal ions such as Pb2+, Zn2+,Cd2+,Ni2+, Fez+,and Mn2+ (3-8). However, application of natural zeolites on an industrial scale has been limited to the removal of ammonia from municipal wastewaters (9-11) and, to a smaller extent, for the removal of Cs and Sr from radioactive wastes (12-14). Two parameters are important prior to any ion-exchange evaluation behavior of a natural zeolite: (a) ita chemical composition in the as-receivedform, and (b) its conversion to a single ionic form to enable an accurate determination of its operating exchange capacity. The objective of this study was to perform an accurate characterization of the natural zeolites, chabazite from Arizona and clinoptilolite from California, and to investigate the effects of conditioning and treatment on the removal performance of lead and cadmium from effluent contaminated with metal levels ranging from 100 to 1000 mg/L. 1108
Environ. Sci. Technol., Vol. 27, No. 6, 1993
Experimental Section Naturally occurringzeolites, clinoptiloliteand chabazite, were provided for study by the American Colloid Co. The as-received zeolites were lightly ground and sieved. Fine particles with a diameter of
