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The sorption of [Sr.sup.2+] and [Pb.sup.2+] on zeolitized tuffs from the Nevada Test Site (NTS) was investigated using macroscopic batch sorption experiments and X-ray absorption spectroscopy (XAS) as a function of geochemical parameters, including pH, ionic strength, and type of background electrolyte. The sorption of [Sr.sup.2+] is dependent on the ionic strength of the medium and independent of pH, suggesting that [Sr.sup.2+] sorption is controlled by ion exchange at permanent charge sites. At higher ionic strengths, background electrolyte cations compete effectively with [Sr.sup.2+] for cation-exchange sites and [Sr.sup.2+] sorption is suppressed. At the two lower ionic strengths (0.01 and 0.1 M), [Pb.sup.2+ ]sorption is also consistent with adsorption by cation exchange. At the highest ionic strength (1.0 M), however, exclusion of [Pb.sup.2+] from cation-exchange sites resulted in pH dependent adsorption, consistent with sorption on amphoteric surface hydroxyl sites or formation of surface precipitates. XAS was used to test these hypotheses. Based on XAS data, [Sr.sup.2+] formed hydrated surface complexes coordinated with approximately eight O atoms at an average distance of 2.60 ([+ or -]0.02) A, regardless of conditions, consistent with the formation of mononuclear, outer-sphere surface complexes at the Ca2 site in the B channel of clinoptilolite. The coordination environment of sorbed [Pb.sup.2+] was more complex and a function of pH and ionic strength. The first shell consisted of two to three O atoms at an average distance of 2.20 ([+ or -] 0.02) A. At low pH and ionic strength, XAS data were consistent with [Pb.sup.2+] adsorption at the Na1 and Ca2 cation exchange sites in channels A and B of clinoptilolite, respectively. At the highest ionic strength (1.0 M) and low pH, XAS provides evidence for formation of [Pb.sup.2+] monodentate, corner-sharing inner-sphere complexes, whereas at higher pH, XAS analysis is consistent with formation of edge-sharing bidentate inner-sphere complexes. As surface coverage increased, appearance of a second [Pb.sup.2+] peak suggests the formation of polynuclear, inner-sphere surface complexes. These results have significant implications for the transport of radionuclides and other contaminants at the NTS and other nuclear test sites and for the modeling of these processes.

The transport of cadmium and selenite in porous and alumina particles with different particle size was studied. Experiments were performed wherein the rates of cadmium and selenite uptakeof three transition aluminas were measured. The measured values were further analyzed by means of a rate diffusion model. Results indicate that the uptake behavior of the transition aluminas are influenced by their physical and chemical characteristics, such as pore structure, particle size and crystal structure.

Greg Pohll [a] Ahmed E. Hassan [a] Jenny B. Chapman [a] Charalambos Papelis [a] Roko Andricevic [a] Abstract Three-dimensional numerical modeling is used to characterize ground water flow and contaminant [...]

Heterogeneous media consisting of segregated flow regions are fractional-order systems, where the regional-scale anomalous diffusion can be described by the fractional derivative model (FDM). The standard FDM, however, first, cannot characterize the Darcy-scale dispersion through repacked sand columns, and second, the link between medium properties and model parameters remains unknown. To fill these two knowledge gaps, this study applies a tempered fractional derivative model (TFDM) to capture bromide transport through laboratory repacked sand. Column transport experiments are conducted first, where glass beads and silica sand with different diameters are repacked individually. Late-time tails are observed in the breakthrough curves (BTC) of bromide even in relatively homogeneous glass beads. The TFDM can capture the observed subdiffusion, especially the late-time BTC with a transient declining rate. Results also show that both the size distribution of repacked sand and the magnitude of fluid velocity can affect subdiffusion. In particular, a wider sand size distribution or a smaller flow rate can enhance the subdiffusion, leading to a smaller time index and a higher truncation parameter in the TFDM. Therefore, the Darcy-scale dispersion follows the tempered stable law, and the model parameters might be related to the soil size and flow conditions. [ABSTRACT FROM AUTHOR]

Computer simulation of reactive transport in heterogeneous systems remains a challenge due to the multiscale nature of reactive dynamics and the non-Fickian behavior of transport. This study develops a fully Lagrangian approach via particle tracking to describe the reactive transport controlled by the tempered super- or subdiffusion. In the particle-tracking algorithm, the local-scale reaction is affected by the interaction radius between adjacent reactants, whose motion can be simulated by the Langevin equations corresponding to the tempered stable models. Lagrangian simulation results show that the transient superdiffusion enhances the reaction by enhancing the degree of mixing of the reactants. The proposed particle-tracking scheme can also be extended conveniently to multiscale superdiffusion. For the case of transient subdiffusion, the trapping of solutes in the immobile phase can either decrease or accelerate the reaction rate, depending on the initial condition of the reactant particles. Further practical applications show that the new solver efficiently captures bimolecular reactions observed in laboratories. [ABSTRACT FROM AUTHOR]

Because of the hundreds of nuclear weapon tests conducted on the Nevada Test Site (NTS) during the Cold War, the migration of radionuclides and contaminants is a potential concern. The mobility of these compounds and our ability to remediate contaminated sites are controlled by sorption and desorption processes, which depend frequently on the nature of the contaminant, the mineralogy of the site, and the geochemical conditions. The sorption and desorption behavior of strontium (Sr) and lead (Pb), two metal cations with different chemistries, commonly found on nuclear test sites were studied. Strontium showed pH-independent and ionic-strength-dependent sorption, consistent with ion exchange processes at permanent charge sorption sites. The sorption uptake of Sr increased with decreasing ionic strength of background solution. Strontium desorption from the adsorbents was enhanced by increased background electrolyte concentration and was a function of background electrolyte composition. The fractional uptake of Pb was higher, compared to that of Sr, and was only pH dependent at the highest ionic strength used (1.0 M). This pH-dependent sorption behavior, consistent with formation of surface complexes at amphoteric surface hydroxyl sites or formation of surface precipitates, could explain the decreased Pb desorption, compared to that of Sr. especially at increased background electrolyte concentrations. Under conditions typical for the groundwater at the NTS (I = 0.003 M, pH = 8.0), both Pb and Sr are expected to bind strongly on tufts with composition similar to the zeolitized tuffs used in this study. Any increase in the dissolved ion concentration of the groundwater, however, may result in, at least partial, release of Sr and enhanced Sr mobility. [ABSTRACT FROM AUTHOR]

The sorption of Sr&sup2+; and Pb&sup2+; on zeolitized tuffs from the Nevada Test Site (NTS) was investigated using macroscopic batch sorption experiments and X-ray absorption spectroscopy (XAS) asaction of geochemical parameters, including pH, ionic strength, and type of background electrolyte. The sorption of Sr&sup2+; is dependent on the ionic strength of the medium and independent of pH, suggesting that Sr&sup2+; sorption is controlled by ion exchange at permanent charge sites. At higher ionic strengths, background electrolyte cations compete effectively with Sr&sup2+; for cation-exchange sites and Sr&sup2+; sorption is suppressed. At the two lower ionic strengths (0.01 and 0.1 M), Pb&sup2+; sorption is also consistent with adsorption by cation exchange. At the highest ionic strength (1.0 M), however, exclusion of Pb&sup2+; from cation-exchange sites resulted in pH dependent adsorption, consistent with sorption on amphoteric surface hydroxyl sites or formation of surface precipitates. XAS was used to test these hypotheses. Based on XAS data, Sr&sup2+; formed hydrated surface complexes coordinated with approximately eight O atoms at an average distance of 2.60 (± 0.02) Å, regardless of conditions, consistent with the formation of mononuclear, outer-sphere surface complexes at the Ca2 site in the B channel of clinoptilolite. The coordination environment of sorbed Pb&sup2+; was more complex and a function of pH and ionic strength. The first shell consisted of two to three O atoms at an average distance of 2.20 (±0.02) Å. At low pH and ionic strength, XAS data were consistent with Pb&sup2+; adsorption at the Na1 and Ca2 cation exchange sites in channels A and B of clinoptilolite, respectively. At the highest ionic strength (1.0 M) and low pH, XAS provide evidence for formation of Pb&sup2+; monodentate, corner-sharing inner-sphere complexes, whereas at higher pH, XAS analysis is consistent with formation of edge-sharing bidentate... [ABSTRACT FROM AUTHOR]