Calvin C. Ainsworth

CHROMATE ADSORPTION BY KAOLINITE

Chromate (CrO42−) adsorption was investigated on kaolinite (0.2–2 μm) saturated with NaClO4 over a range of pH. Adsorption increased with decreasing pH because of protonation of chromate and/or variable charge sites on kaolinite. Chemical pretreatment to remove noncrystalline and crystalline oxide contaminants affected the magnitude of CrO42− adsorption, but not the pH range over which CrO42− adsorbed. Chromate adsorption at different sorbate and sorbent concentrations increased below the pHzpc for the kaolinite edge, suggesting the formation of weak surface complexes. If CrO42− and SO42− were present at equal concentration (5.0 × 10−7 M), the two solutes sorbed independently, suggesting binding to separate sites. The presence of excess SO42− (5.0 × 10−4 M), however, unexplainably enhanced CrO42− adsorption. The adsorption of both Chromate and sulfate can be described in terms of a site-binding model of the kaolinite edge, in which the edge is viewed as composite layers of Al and Si oxide. Surface complexation constants for CrO42− on kaolinite were similar to those for alumina, pointing to the importance of Al-OH edge sites in Chromate adsorption.

Desorption kinetics of radiocesium from subsurface sediments at Hanford Site, USA

Abstract The desorption of 137 Cs + was investigated on sediments from the United States Hanford site. Pristine sediments and ones that were contaminated by the accidental release of alkaline 137 Cs + -containing high level nuclear wastes (HLW, 2 × 10 6 to 6 × 10 7 pCi 137 Cs + /g) were studied. The desorption of 137 Cs + was measured in Na + , K + , Rb + , and NH 4 + electrolytes of variable concentration and pH, and in presence of a strong Cs + -specific sorbent (self-assembled monolayer on a mesoporous support, SAMMS). 137 Cs + desorption from the HLW-contaminated Hanford sediments exhibited two distinct phases: an initial instantaneous release followed by a slow kinetic process. The extent of 137 Cs + desorption increased with increasing electrolyte concentration and followed a trend of Rb + ≥ K + > Na + at circumneutral pH. This trend followed the respective selectivities of these cations for the sediment. The extent and rate of 137 Cs + desorption was influenced by surface armoring, intraparticle diffusion, and the collapse of edge-interlayer sites in solutions containing K + , Rb + , or NH 4 + . Scanning electron microscopic analysis revealed HLW-induced precipitation of secondary aluminosilicates on the edges and basal planes of micaceous minerals that were primary Cs + sorbents. The removal of these precipitates by acidified ammonium oxalate extraction significantly increased the long-term desorption rate and extent. X-ray microprobe analyses of Cs + -sorbed micas showed that the 137 Cs + distributed not only on mica edges, but also within internal channels parallel to the basal plane, implying intraparticle diffusive migration of 137 Cs + . Controlled desorption experiments using Cs + -spiked pristine sediment indicated that the 137 Cs + diffusion rate was fast in Na + -electrolyte, but much slower in the presence of K + or Rb + , suggesting an effect of edge-interlayer collapse. An intraparticle diffusion model coupled with a two-site cation exchange model was used to interpret the experimental results. Model simulations suggested that about 40% of total sorbed 137 Cs + was exchangeable, including equilibrium and kinetic desorbable pools. At pH 3, this ratio increased to 60–80%. The remainder of the sorbed 137 Cs + was fixed or desorbed at much slower rate than our experiments could detect.

Chromium Speciation and Mobility in a High Level Nuclear Waste Vadose Zone Plume

Radioactive core samples containing elevated concentrations of Cr from a high level nuclear waste plume in the Hanford vadose zone were studied to asses the future mobility of Cr. Cr(VI) is an important subsurface contaminant at the Hanford Site. The plume originated in 1969 by leakage of self-boiling supernate from a tank containing REDOX process waste. The supernate contained high concentrations of alkali (NaOH ≈ 5.25 mol/L), salt (NaNO3/NaNO2 >10 mol/L), aluminate [Al(OH)4− = 3.36 mol/L], Cr(VI) (0.413 mol/L), and 137Cs+ (6.51 × 10−5 mol/L). Water and acid extraction of the oxidized subsurface sediments indicated that a significant portion of the total Cr was associated with the solid phase. Mineralogic analyses, Cr valence speciation measurements by X-ray adsorption near edge structure (XANES) spectroscopy, and small column leaching studies were performed to identify the chemical retardation mechanism and leachability of Cr. While X-ray diffraction detected little mineralogic change to the sediments from waste reaction, scanning electron microscopy (SEM) showed that mineral particles within 5 m of the point of tank failure were coated with secondary, sodium aluminosilicate precipitates. The density of these precipitates decreased with distance from the source (e.g., beyond 10 m). The XANES and column studies demonstrated the reduction of 29–75% of the total Cr to insoluble Cr(III), and the apparent precipitation of up to 43% of the Cr(VI) as an unidentified, non-leachable phase. Both Cr(VI) reduction and Cr(VI) precipitation were greater in sediments closer to the leak source where significant mineral alteration was noted by SEM. These and other observations imply that basic mineral hydrolysis driven by large concentrations of OH− in the waste stream liberated Fe(II) from the otherwise oxidizing sediments that served as a reductant for CrO42−. The coarse-textured Hanford sediments contain silt-sized mineral phases (biotite, clinochlore, magnetite, and ilmenite) that are sources of Fe(II). Other dissolution products (e.g., Ba2+) or Al(OH)4− present in the waste stream may have induced Cr(VI) precipitation as pH moderated through mineral reaction. The results demonstrate that a minimum of 42% of the total Cr inventory in all of the samples was immobilized as Cr(III) and Cr(VI) precipitates that are unlikely to dissolve and migrate to groundwater under the low recharge conditions of the Hanford vadose zone.

Rapid Kinetics of Cu(II) Adsorption/Desorption on Goethite.

The kinetics of Cu[sup 2+] adsorption/desorption on goethite ([alpha]-FeOOH) was evaluated using the pressure-jump (p-jump) relaxation technique. This technique provides both kinetic and mechanistic information for reactions occurring on millisecond time scales. A double relaxation event was observed for Cu[sup 2+] adsorption/desorption on goethite. The rate of these relaxations ([tau]) decreased with an increase in pH, along the adsorption edge. The mechanism ascribed to the relaxations is the formation of a monodentate innersphere Cu[sup 2+]/goethite surface complex. The calculated intrinsic rate constant for adsorption (k[sub 1][prime]int) was 10[sup 6.81] L mol[sup [minus]1] s[sup [minus]1] and was about 2 orders of magnitude larger than the intrinsic rate constant for desorption (k[sub 1][prime]int = 10[sup 4.88] L mol[sup [minus]1] s[sup [minus]1]). Using results from this study and others, it was established that the rate of adsorption of divalent metal cations on goethite was directly related to the rate of removal of a water molecule from the primary hydration sphere of a particular divalent metal cation. 30 refs., 7 figs., 1 tab.

Quinoline sorption to subsurface materials: role of pH and retention of the organic cation

The sorption of quinoline (pK/sub a/ = 4.94) was investigated on low-organic-carbon subsurface materials that varied in pH. Sorption isotherms were measured from 10/sup -7/ to 10/sup -4/ M quinoline and were found to be nonlinear. The resulting Freundlich constant (K/sub F/), based on total aqueous quinoline concentration, were poorly correlated with subsoil properties, including organic carbon. Higher sorption in the acidic subsoils and favorable coefficients for regression of K/sub F/, normalized to cation-exchange capacity vs. the ionization fraction (Q), point to the importance of ion exchange of the protonated compound. When the subsoil pH is adjusted, it is shown that sorption parallels the ionization fraction and retention of the organic cation far exceeds that of the neutral species. Calculations of surface speciation and thermodynamic parameters of sorption (..delta..H/sup 0/, ..delta..S/sup 0/) point to ion exchange and/or surface protonation at pH, levels exceeding pK/sub a/ by greater than 2 log units. It is suggested that in subsurface materials of low carbon content, quinoline sorption is controlled by pH the nature and capacity of the exchange complex, and groundwater ion composition. 46 references, 5 figures, 4 tables.

Quinoline sorption on Na-montmorillonite; contributions of the protonated and neutral species

Dilute aqueous solutions of quinoline were contacted with Na-montmorillonite to elucidate the sorption process of the neutral and protonated species. Sorption occurs via a combination of ion exchange and molecular adsorption and yields S-type isotherms. Exchange between the quinolinium ion (QH+) and Na can be described by means of Vanselow selectivity coefficients and a thermodynamic exchange constant (Kex). Due to the apparent adsorption of the neutral species at high mole fractions (x) of the solid phase, the thermodynamic standard state was defined as 0.5 mole fraction. The selectivity at pH ~4.95 of the QH+ species over Na (at XQH+ = 0.5) was determined to be Kv = 340. At pH ≥ 5.5 surface mole fractions of 0.5 could not be obtained without adsorption of the neutral species. This study suggests that at dilute solution concentrations quinoline is sorbed preferentially as the cation even at pHs ≫ pKa. A critical surface-solution concentration is apparently necessary for adsorption of the neutral species.

Characterization of Vadose Zone Sediment: Borehole 41-09-39 in the S-SX Waste Management Area

This report was revised in September 2008 to remove acid-extractable sodium data from Table 5.15. The sodium data was removed due to potential contamination introduced during the acid extraction process. The rest of the text remains unchanged from the original report issued in February 2002. The overall goal of the of the Tank Farm Vadose Zone Project, led by CH2M HILL Hanford Group, Inc., is to define risks from past and future single-shell tank farm activities. To meet this goal, CH2M HILL Hanford Group, Inc., asked scientists from Pacific Northwest National Laboratory to perform detailed analyses on vadose zone sediment from within the S-SX Waste Management Area. This report is one in a series of four reports to present the results of these analyses. Specifically, this report contains all the geologic, geochemical, and selected physical characterization data collected on vadose zone sediment recovered from borehole 41-09-39 installed adjacent to tank SX-109.

Characterization of Salicylate-Alumina Surface Complexes by Polarized Fluorescence Spectroscopy

Speciation of salicylate anions (o-hydroxybenzoate) adsorbed on aqueous colloidal alumina (δ-Al2O3) was determined by polarized fluorescence excitation spectroscopy. Adsorption isotherms and pH and ionic strength edges suggest the existence of both inner-sphere and outer-sphere salicylate surface complexes. Spectroscopic characteristics of inner-sphere surface-salicylate complexes (one bidentate and two monodentate) were identified through comparison of suspension spectra with remarkably similar fluorescence and excitation spectra of solution phase Al-salicylate complexes. The large fluorescence Stokes gap of the aqueous salicylate anion is highly sensitive to complexation, resulting in spectral shifts characteristic of aluminum binding in the three inner-sphere salicylate complexes. These species appear to be present even at extremely low surface coverage, and the relative distributions are dependent on pH, ionic strength, and the relative concentrations of alumina and salicylate. The bidentate complex, however, is the predominant species at low surface coverages. Fluorescence anisotropy measurements, both steady-state and time-resolved, demonstrate that the bidentate and monodentate surface complexes do not undergo rotational reorientation on the time-scale of the fluorescence (τf = 4.0 ns), consistent with inner-sphere, polar covalent binding of these salicylate complexes to alumina surface sites. At high surface coverage, time-resolved anisotropy measurements suggest the existence of a surface salicylate species that is rotationally hindered (τr = 31 ps ) relative to free solution phase salicylate ions (τ r = 20 ps ). This behavior is consistent with an electrostatically bound outer-sphere complex suggested by the pH and ionic-strength sorption edges.

Characterization of Vadose Zone Sediment: Slant Borehole SX-108 in the S-SX Waste Management Area

This report was revised in September 2008 to remove acid-extractable sodium data from Table 4.17. The sodium data was removed due to potential contamination introduced during the acid extraction process. The rest of the text remains unchanged from the original report issued in February 2002. The overall goal of the of the Tank Farm Vadose Zone Project, led by CH2M HILL Hanford Group, Inc., is to define risks from past and future single-shell tank farm activities. To meet this goal, CH2M HILL Hanford Group, Inc., asked scientists from Pacific Northwest National Laboratory to perform detailed analyses on vadose zone sediment from within the S-SX Waste Management Area. This report is the fourth in a series of four reports to present the results of these analyses. Specifically, this report contains all the geologic, geochemical, and selected physical characterization data collected on vadose zone sediment recovered from a slant borehole installed beneath tank SX-108 (or simply SX-108 slant borehole).

The sorption of N-heterocyclic compounds on reference and subsurface smectite clay isolates☆

The sorption of the nitrogen heterocycle compounds (NHC) pyridine, quinoline and acridine was measured from Na and Ca electrolyte solutions on specimen montmorillonite (SWy1) and natural smectite clays isolated from subsurface materials. Sorption was dominated by highly selective exchange for the NHC cation with neutral species sorption noted only at high surface concentrations. The selectivity sequence acridine > quinoline > pyridine was observed on all smectites. Na-saturated SWy1 and natural smectites sorbed NHC compounds comparably. Exchange constants on the Na-smectites were fairly constant with surface coverage and correlated with the octanol-water partition coefficients of the NHC and calculated formal charge on specific atom centers in the molecules. In contrast, the exchange of NHC differed between Ca-saturated SWy1 and natural clays. Ca saturation decreased NHC sorption only on SWy1. Exchange constants for the Ca-smectites varied with surface coverage, and the natural Ca-smectites exhibited higher affinity for NHC than Ca-SWy1. Mineralogic differences between the specimen and natural smectites are believed to effect NHC exchange when the smectite clays are aggregated as quasi-crystals in Ca suspensions. Organic material and Fe-oxides associated with natural smectites have no observable effect on NHC adsorption.