Sorption of Uranium(6+) and Neptunium(5+) by Surfactant-Modified:Natural Zeolites
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Recent studies have shown that treatment of zeolite minerals, such as clinoptilolite, with cationic surfactants yields sorbents that have a strong affinity for selenate and chromate [ I, I I 1,as well as nonpolar organics, such as benzene, 1,1,I-trichloroethane, and perchloroethylene, but which 2 also retain their ion exchange selectivity for heavy metal cations, such as Pb + [12-14]. Thus,
surfactant-modified zeolites (SMZs) have the potential for use in the remediation of different classes of soil and groundwater contaminants. However, no data are available to evaluate the potential effectiveness of SMZs in removing actinides from contaminated waters. Previous studies have shown that unmodified clinoptilolite can effectively sorb U(6+) and Np(5+) through a surface complexation mechanism [15,161, but this mechanism is ineffective when carbonate ligands, commonly present in groundwater, complex and keep the U and Np in solution. The objective of this study is to determine whether surfactant-modification would enhance the ability of clinoptilolite to sorb U(6+) and Np(5+). Enhanced sorption of actinides would expand the potential application of SMZs in the treatment and remediation of contaminated water.
1035 Mat. Res. Soc. Symp. Proc. Vol. 556 01999 Materials Research Society
EXPERIMENT Zeolite Preparation and Characterization Clinoptilolite-rich tuffaceous rocks from three localities in California-Death Valley Junction, Barstow, and Hector-were used in the study. The specimens were obtained from Minerals Research Company (Clarkson, New York). X-ray diffraction (XRD) analysis, scanning electron and optical microscopy indicate that these tuffaceous rock specimens are composed predominantly of well crystallized clinoptilolite, with montmorillonite clay as the second most abundant phase. Other minor phases identified include quartz, pyrite, calcite, feldspar, iron oxides, and unaltered volcanic glass. The material used in the experiments was prepared by breaking the as-received rock samples with a hammer into pieces less than 0.75 cm in diameter. These pieces were then crushed using a ceramic mortar and pestle, and sieved using a Ro-Tap sieve shaker and 8-inch diameter stainless steel sieves. The 35-200 mesh size (500-75 pm) fraction of the powdered and sieved Hector and Barstow clinoptilolite was used in the experiments without further treatment. A "purified" clinoptilolite material was also prepared from the 100-200 mesh size (150-75 pm) fraction of crushed and sieved Death Valley Junction clinoptilolite. This material was prepared by removing clay and other fine particles from clinoptilolite surfaces by ultrasonic cleaning, washing, and decantation; treatment with I N sodium acetate buffer to dissolve carbonate minerals; treatment with sodium dithionite-citrate-bicarbonate mixture to dissolve iron oxide minerals; and density separation with heavy liquids to separate any remaining mineral impurities (e.g., quartz and feldspar) [ 17]. The purified sample is greater than 98 volume % clinoptilolite, based on XRD an
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