Natural Ground Water Colloids From the USGS J-13 Well in Nye County, NV: a Study Using SAXS and TEM
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Natural Ground Water Colloids From the USGS J-13 Well in Nye County, NV: a Study Using SAXS and TEM
Jeffrey A. Fortner, 1 Carol J. Mertz, 1 S. F. Wolf,1∗ and Peter R. Jemian2 1 Chemical Technology Division, Argonne National Laboratory 2 University of Illinois, Urbana-Champaign We report results from ultra small angle x-ray scattering (USAXS) and transmission electron microscopy (TEM) of dilute silicate colloids that occur naturally in groundwater from the USGS J13 well, located near the Yucca Mountain Site in Nevada. We also examined a separate sample of this groundwater that had been treated by heating to 90 degrees C in contact with crushed Topopah Spring Tuff from the Yucca Mountain. The USAXS measurements were done at the UNICAT undulator beamline at the Advanced Photon Source at the Argonne National Laboratory. Powerlaw plots (scattering intensity verses momentum transfer) were fitted to the USAXS data. Colloids in the untreated J-13 groundwater were shown to have a fractal dimension of nearly 3, whereas colloids in the treated groundwater ("EJ-13") have a dimensionality of approximately 2.4 over a length scale of approximately 3 to 300 nm. Similar power-law plots with dimension 3 characterized concurrent SAXS measurements from aqueous suspensions of Na-montmorillonite and NIST Brick Clay (NBS-67). We attribute these results to the sheet silicate layered structure of the clay colloids present in J-13 well water, montmorillonite, and "brick clay" systems. The differences between EJ13 and as-received J-13 are perhaps owing to exchange of calcium for sodium with the tuff. Radionuclide incorporation into, adsorption onto, or ion exchange with existing groundwater colloids may promote colloidal transport of radionuclides in groundwater. Such radionuclidebearing colloids could thereby increase the concentrations of actinides in groundwater and enhance migration into human-accessible aquifers. Our results demonstrate the first application of USAXS to study the physical nature of such groundwater colloids, and represent perhaps one of the most dilute systems ever studied by small angle scatering. INTRODUCTION Colloidal systems encompass a wide variety of surface-active agents and dispersed particles, typically with at least one length dimension in the size range of 1 nm to 1 µm. Because of their high surface area many insoluble components tend to sorb (adsorb and absorb) to colloidal particles. The stability of colloidal particles enhances the apparent solubility of many insoluble trace metals. The size, charge density and surface chemistry of the colloids are important parameters for determining transport properties. Groundwater-borne colloids have been recognized as potential agents in contaminant transport from a waste site. The incorporation, adsorption, or ion exchange of radionuclides onto pre-existing groundwater colloids can increase the apparent solubility of actinides in solution and may enhance migration in aquifer systems. Clay minerals, which readily form colloids, can either directly incorporate
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