The kinetics of arsenate and chromate adsorption/ desorption on goethite (R-FeOOH) were investigated using a pressure-jump (p-jump) relaxation technique. Information provided by this technique was used to elucidate the fate of arsenate and chromate in natural environments. Chemical relaxations resulting from rapidly induced pressure changes were monitored via conductivity detection. The adsorption/desorption of these oxyanions on goethite involved a double relaxation event. The proposed mechanism for the adsorption of arsenate and chromate on goethite is a two-step process resulting in the formation of an inner-sphere bidentate surface complex. The first step, associated with the fast τ values, involved an initial ligand exchange reaction of aqueous oxyanion species H 2 AsO 4or HCrO 4 -with OH ligands at the goethite surface forming an inner-sphere monodentate surface complex. The subsequent step, associated with the slow τ values, involved a second ligand exchange reaction, resulting in the formation of an inner-sphere bidentate surface complex. Overall, the results suggest that chromate may be the more mobile of the two oxyanions in soil systems.

The adsorption‐desorption of the divalent metal cations (Me2+) Co2+, Cd2+, and Pb2+ to hydrous ferric oxide (HFO) was investigated as a function of oxide aging and Me2+‐oxide residence time. The HFO was produced and stored for up to 86 wk. Periodically, Me2+ sorption was determined across the pH range of 2.5 to 12. In addition, the Me2+ ions were contacted with freshly produced HFO and stored at a pH that dictated that 80 to 100% of the Me2+ would be in the sorbed state; desorbability of the Me2+ was determined as a function of Me2+‐oxide residence time. The change in the crystallinity of the HFO as a function of time was also monitored. The HFO aged without the Me2+ ions displayed no hysteresis between the adsorption‐desorption curves and no substantial shifts in fractional Me2+ adsorption were observed with pH throughout 21 wk of aging. The HFO aged with the Me2+ ions displayed increasing desorption hysteresis with time for Co2+ and Cd2+, but not Pb2+. The magnitude of hysteresis followed the order Co > Cd > Pb, which is the inverse of the ionic radii of the metal sorbates. While oxalate‐extractable Fe decreased with time during a 20‐wk period, powder x‐ray diffraction was unchanged during the same period. The data presented here suggest that Co and Cd are being incorporated into the metal oxide structure via recrystallization, but Pb remains associated with the surface and excluded from incorporation.

Chromate adsorption was measured with and without reactive cosolutes on four subsurface soil horizons differing in pH and mineralogy, and on clay fractions from two of the oxide‐containing subsoils. Chromate adsorption was greatest in lower pH materials enriched in kaolinite and crystalline iron oxides. Over a range in pH, chromate adsorption to subsoil was similar to that observed for pure‐phase oxides. Chromate binding was reversible to pH and was depressed in the presence of SO2‐4 and dissolved inorganic C, which compete for adsorption sites. A surface site density for crystalline Al‐substituted iron oxides in the subsoils was estimated from chromate adsorption on the clay fractions using the model FITEQL and outer sphere surface coordination constants from Al‐goethite. The estimated site density for soil crystalline iron oxides was well below that of clean oxides, suggesting surface saturation by indigenous soil ions. The calculated site density and surface binding constants for Al‐goethite were used in the Triple Layer Model to calculate the effect of ionic strength, cosolutes, solids concentration, and sorbate concentration on CrO2‐4 adsorption. Model calculations were in good qualitative agreement with experimental results.

Large quantities of fossil fuel combustion (FFC) wastes, such fly ash, bottom ash, flue gas desulfurization (FGD) sludge, and oil ash are being disposed of on land. There is a need to accurately assess the mobilization of elements that results from the weathering of these wastes. To help meet this need, available data on physical, chemical, mineralogical, extract, and leachate characteristics have been compiled and reviewed, and a comprehensive approach to understanding how major elements are mobilized from fossil fuel wastes is described. Many of the physical properties of fly ashes overlap those of bottom ashes; similar data for FGD sludge and oil ashes are sparse. One taxonomic and three utilitarian classification schemes have been proposed for fly ashes. However, no taxonomic system is applicable to all FFC wastes. The ranges of concentrations of AI, Ca, and Fe found in these wastes were within the ranges of their concentrations in soils. Concentrations of K, Na, Mg, and S in some FFC wastes may exceed their highest concentrations in soils. The matrix of fly ashes consists principally of quartz, glass, and mullite. As minor components in fly ashes, anhydrite, lime, periclase, hematite, and magnetite are reported frequently. A minor fraction of many of the major elements may also be present in a glassy phase. The principal compounds in FGD sludges are sulfites, sulfates, carbonates, and hydroxides of Ca. The oil ashes contain mainly sulfates and oxides of AI, Na, Fe, and Mg. Water extracts of FFC wastes contain significant fractions of Ca, Mg, K, and Na, a fact that confirms the presence of these elements in highly soluble forms. Leaching studies conducted using fly ash columns have shown that initial leachates contain high concentrations of Ca, Na, K, and S and that as leaching continues these concentrations decline and reach steady states. Usually only negligible fractions of Si, AI, Fe, and Mg are leached from fly ash columns. In this article, a comprehensive approach is suggested, one that combines thermochemical principles with chemical and mineralogical data to predict the upper limits of elemental concentrations that can be attained in leachates from weathering FFC wastes. This thermochemical approach has shown that secondary solid phases may control concentrations of AI, Ca, and S measured in extracts of fly ashes and FGD sludges and in pore waters from fly ash lysimeters at a field site. Characterization of weathered wastes and improved thermochemical data for secondary solid phases in these wastes will be useful for understanding and predicting the chemistry of leachates.All authors, Environ. Sci. Dep., Battelle, Pacific Northwest Lab.,

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 Ϸ 5.25 mol/L), salt (NaNO 3 /NaNO 2 Ͼ10 mol/L), aluminate [Al(OH) 4 Ϫ ϭ 3.36 mol/L], Cr(VI) (0.413 mol/L), and 137 Cs ϩ (6.51 ϫ 10 Ϫ5 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 Ϫ in the waste stream liberated Fe(II) from the otherwise oxidizing sediments that served as a reductant for CrO 4 2Ϫ. 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., Ba 2ϩ) or Al(OH) 4 Ϫ 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.

The desorption of 137 Cs ϩ 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 ϩ-containing high level nuclear wastes (HLW, 2 ϫ 10 6 to 6 ϫ 10 7 pCi 137 Cs ϩ /g) were studied. The desorption of 137 Cs ϩ was measured in Na ϩ , K ϩ , Rb ϩ , and NH 4 ϩ electrolytes of variable concentration and pH, and in presence of a strong Cs ϩ-specific sorbent (self-assembled monolayer on a mesoporous support, SAMMS). 137 Cs ϩ 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 ϩ desorption increased with increasing electrolyte concentration and followed a trend of Rb ϩ Ն K ϩ Ͼ Na ϩ at circumneutral pH. This trend followed the respective selectivities of these cations for the sediment. The extent and rate of 137 Cs ϩ desorption was influenced by surface armoring, intraparticle diffusion, and the collapse of edge-interlayer sites in solutions containing K ϩ , Rb ϩ , or NH 4 ϩ. Scanning electron microscopic analysis revealed HLW-induced precipitation of secondary aluminosilicates on the edges and basal planes of micaceous minerals that were primary Cs ϩ 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 ϩ-sorbed micas showed that the 137 Cs ϩ distributed not only on mica edges, but also within internal channels parallel to the basal plane, implying intraparticle diffusive migration of 137 Cs ϩ. Controlled desorption experiments using Cs ϩ-spiked pristine sediment indicated that the 137 Cs ϩ diffusion rate was fast in Na ϩ-electrolyte, but much slower in the presence of K ϩ or Rb ϩ , 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 ϩ was exchangeable, including equilibrium and kinetic desorbable pools. At pH 3, this ratio increased to 60-80%. The remainder of the sorbed 137 Cs ϩ was fixed or desorbed at much slower rate than our experiments could detect.

Past studies of the environmental aspects of fossil fuel waste disposal have focused on determining elemental concentrations, elemental distributions, and empirical rates of elemental extraction. The concentration data for the minor elements (i.e., As, B, Ba, Cd, Cr, Cu, Pb, Mn, Hg, Mo, Ni, Se, Sr, V, and Zn) are extremely All authors, Environ. Sci. Dep., Battelle, Pacific Northwest Lab.,

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