The sorption of Cs ϩ was investigated over a large concentration range (10 Ϫ9 -10 Ϫ2 mol/L) on subsurface sediments from a United States nuclear materials site (Hanford) where high-level nuclear wastes (HLW) have been accidentally released to the vadose zone. The sediment sorbs large amounts of radiocesium, but expedited migration has been observed when HLW (a NaNO 3 brine) is the carrier. Cs ϩ sorption was measured on homoionic sediments (Na ϩ , K ϩ , Ca 2ϩ ) with electrolyte concentrations ranging from 0.01 to 1.0 mol/L. In Na ϩ electrolyte, concentrations were extended to near saturation with NaNO 3(s) (7.0 mol/L). The sediment contained nonexpansible (biotite, muscovite) and expansible (vermiculite, smectite) phyllosilicates. The sorption data were interpreted according to the frayed edge-planar site conceptual model. A fourparameter, two-site (high-and low-affinity) numeric ion exchange model was effective in describing the sorption data. The high-affinity sites were ascribed to wedge zones on the micas where particle edges have partially expanded due to the removal of interlayer cations during weathering, and the low-affinity ones to planar sites on the expansible clays. The electrolyte cations competed with Cs ϩ for both high-and low-affinity sites according to the trend K ϩ ϾϾ Na ϩ Ն Ca 2ϩ . At high salt concentration, Cs ϩ adsorption occurred only on high-affinity sites. Na ϩ was an effective competitor for the high-affinity sites at high salt concentrations.In select experiments, silver-thiourea (AgTU) was used as a blocking agent to further isolate and characterize the high-affinity sites, but the method was found to be problematic. Mica particles were handpicked from the sediment, contacted with Cs ϩ (aq) , and analyzed by electron microprobe to identify phases and features important to Cs ϩ sorption. The microprobe study implied that biotite was the primary contributor of high-affinity sites because of its weathered periphery. The poly-phase sediment exhibited close similarity in ion selectivity to illite, which has been well studied, although its proportion of high-affinity sites relative to the cation exchange capacity (CEC) was lower than that of illite. Important insights are provided on how Na ϩ in HLW and indigenous K ϩ displaced from the sediments may act to expedite the migration of strongly sorbing Cs ϩ in subsurface environments.

No abstract

Previous studies suggest that soot formed in premixed flat flames can contain a substantial amount of aliphatic compounds. Presence of these compounds may affect the kinetics of soot mass growth and oxidation in a way that is currently not understood. Using an infrared spectrometer coupled to a microscope (micro-FTIR), we examined the composition of soot sampled from a set of ethylene-argon-oxygen flames recently characterized (A. D. Abid, et al. Combust. Flame, 2008, 154, 775-788), all with an equivalence ratio Φ=2.07 but varying in maximum flame temperatures. Soot was sampled at three distances above the burner surface using a probe sampling technique and deposited on silicon nitride thin film substrates using a cascade impactor. Spectra were taken and analyses performed for samples collected on the lowest five impactor stages with the cut-off sizes of D(50)=10, 18, 32, 56 and 100 nm. The micro-FTIR spectra revealed the presence of aliphatic C–H, aromatic C–H and various oxygenated functional groups, including carbonyl (C=O), C–O–C and C–OH groups. Spectral analyses were made to examine variations of these functional groups with flame temperature, sampling position and particle size. Results indicate that increases in flame temperature leads to higher contents of non-aromatic functionalities. Functional group concentrations were found to be ordered as follows: [C=O]<[C–O]<[aliphatic C–H]. Aliphatic C–H was found to exist in significant quantities, with very little oxygenated groups present. The ratio of these chemical functionalities to aromatic C–H remains constant for particle sizes spanning 10-100 nm. The results confirm a previous experimental finding: a significant amount of aliphatic compounds is present in nascent soot formed in the flames studied, especially towards larger distances above the burner surface.

Column experiments were conducted to investigate U(VI) desorption and sorption kinetics in a sand-textured, U(VI)-contaminated (22.7 micromol kg(-1)) capillary fringe sediment from the U.S. Department of Energy (DOE) Hanford site. Saturated column experiments were performed under mildly alkaline conditions representative of the Hanford site where uranyl-carbonate and calcium-uranyl-carbonate complexes dominate aqueous speciation. A U(VI)-free solution was used to study contaminant U(VI) desorption in columns where different flow rates were applied. Sorbed, contaminant U(VI) was partially labile (11.8%), and extended leaching times and water volumes were required for complete desorption of the labile fraction. Uranium-(VI) sorption was studied after the desorption of labile, contaminant U(VI) using different U(VI) concentrations in the leaching solution. Strong kinetic effects were observed for both U(VI) sorption and desorption, with half-life ranging from 8.5 to 48.5 h for sorption and from 39.3 to 150 h for desorption. Although U(VI) is semi-mobile in mildly alkaline, subsurface environments, we observed substantial U(VI) adsorption, significant retardation during transport, and atypical breakthrough curves with extended tailing. A distributed rate model was applied to describe the effluent data and to allow comparisons between the desorption rate of contaminant U(VI) with the rate of shortterm U(VI) sorption. Desorption was the slower process. We speculate that the kinetic behavior results from transport or chemical phenomena within the phyllosilicate-dominated fine fraction present in the sediment. Our results suggest that U(VI) release and transport in the vadose zone and aquifer system from which the sediment was obtained are kinetically controlled.