Effects of Surfactant Modification on the Sorption Behavior of Natural Zeolites for Strontium(2+) and Uranium(6+)
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281 Mat. Res. Soc. Symp. Proc. Vol. 608 ©2000 Materials Research Society
anions in solution [7,12]. In this study, experiments were conducted to evaluate the ability of surfactant-modified zeolite to sorb both Sr2+and U6+ when exposed to a mixed radionuclide solution. The surfactantmodified clinoptilolite was reacted with strontium solutions having a range of strontium concentration, and with a mixed solution of strontium and uranium. EXPERIMENTAL METHODS Material Preparation Clinoptilolite used in this study was obtained from Minerals Research Company (Clarkson, New York) and originated as a clinoptilolite-rich tuffaceous rock from Death Valley Junction, CA. The material was prepared as described in Prikryl and Pabalan [11 ] and Pabalan [13]. The internal and external cation exchange capacities (CECs) of the clinoptilolite were measured using the method of Ming and Dixon [141 as modified by Haggerty and Bowman [1]. The external CEC was determined to be 0.11 meq/g (milliequivalents per gram), while the internal CEC was determined to be 1.86 meq/g, for a total calculated CEC of 1.97 meq/g of clinoptilolite. Surfactant-modified zeolite (SMZ) was prepared by reacting the clinoptilolite samples with reagent-grade HDTMA (Fisher Scientific). A previous study [1] suggested that anion sorption on the SMZ is maximized when the external CEC of the zeolite is fully satisfied by HDTMA. Therefore, the clinoptilolite samples were treated with a quantity of HDTMA approximately equal to 100 % of the zeolite external CEC. However, recent investigations have concluded that maximization of SMZ sorption capacity may require additional loading and is influenced not only by the initial concentration of HDTMA in solution, but also the time allowed for reorganization of HDTMA on the zeolite surface [15,16]. In this study, 10 g of clinoptilolite powder was treated with 120 mL of a 0.016 M HDTMA solution. The mixture was mechanically shaken for 24 hr at 25 'C, centrifuged, and the supernatant decanted. The solid was then rinsed with deionized water several times, filtered, air-dried, and stored in plastic containers prior to sorption experiments. Batch Sorption Experiments Batch sorption experiments were conducted by reacting weighed amounts of clinoptilolite or SMZ with Sr 2+ solutions or Sr2+- and U 6+-bearing solutions over a range of radionuclide concentration and pH (Table 1). Experimental solutions containing Sr2+ were prepared by mass from reagent grade Sr(N0 3)2. Uranium-bearing solutions were prepared by diluting a purchased 233U standard (Isotope Products, Inc., Burbank, CA). The pH of experimental solutions was varied in -0.5 unit increments, typically in the range from 2.5 to 9, by addition of HN0 3 or NaHCO 3 . Solution ionic strength depended on pH adjustment and varied from 0.003 to 0.009 M. The experiments were conducted in loosely capped polycarbonate bottles in equilibrium with air and at a solid mass to solution volume (MN) ratio of 4 g/L. The mixtures were allowed to react for 14 days to allow equilibrium with respect to
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