James L. Krumhansl

Synthesis, Characterization, and Ion Exchange Properties of Hydrotalcite Mg6Al2(OH)16(A)x(A‘)2-x·4H2O (A, A‘ = Cl-, Br-, I-, and NO3-, 2 ≥ x ≥ 0) Derivatives

A series of monovalent anion-containing hydrotalcites (HTCs) with the general formula Mg6Al2(OH)16(A)x(A‘)2-x·4H2O (A, A‘ = Cl-, Br-, I-, and NO3-, 2 ≥ x ≥ 0) have been studied. Samples were synthesized by three different methods:  ion exchange (IE), hydrothermal (HT), and recrystallization based on “memory effect” (ME). The physical, structural, and chemical characteristics and properties of the HTCs have been studied as a function of the synthetic methods and anions used. All three synthetic methods produced HTCs of good crystallinity and uniform particle size. When a second (A‘) or all four (Cl-, Br-, I-, NO3-) anions were present together with an A-HTC in an aqueous medium, the order of ion exchange preference was Br- > Cl- > NO3- > I-. When using one-pot synthetic methods (HT, ME), the same order of anion incorporation preference, Br- > Cl- > NO3- > I-, was observed.

A process model of natural attenuation in drainage from a historic mining district

A process model was used to better understand the controls on the chemical evolution of drainage in a historic mining district. At the Pecos Mine Operable Unit, New Mexico, drainage near the waste rock pile is acidic (pH varies from 3.0--5.0) and carries high concentrations of Zn, Al, Cu and Pb. As drainage flows toward the Pecos River, pH increases to greater than 7 and heavy metal content decreases. A process model of natural attenuation in this drainage shows the main controls on pH are reaction with a local bedrock that contains limestone, and concurrent mixing with tributary streams. Models that account for both calcite dissolution and mixing reproduce the observed decrease in aqueous metal concentrations with increasing pH. Contaminant concentrations attenuate primarily via two distinct pathways: Al, Cu, Fe and Pb precipitate directly from solution, whereas Zn, Mg, Mn and SO{sub 4} concentrations decrease primarily through dilution. Additionally, Pb adsorbs to precipitating hydroxide surfaces.

Surface complexation clues to dolomite growth

Calcium and magnesium adsorb in near-stoichiometric proportions to dolomite over wide ranges in [Ca{sup 2+}]/[Mg{sup 2+}], ionic strength, and solution composition pointing to minimal mixing of metal cations between the CaCO{sub 3} and MgCO{sub 3} layer edges exposed at the dolomite surface. Near-neutral pH Mg and Ca adsorb as hydrated ions, or, in sulfate-rich solutions, as metal sulfate complexes. Near-stoichiometric adsorption of Ca and Mg points to dehydration and subsequent carbonation of adsorbed Mg as the likely rate-limiting step for dolomite growth at near-Earth surface conditions. We propose that one path for dolomite growth from low-temperature natural waters is through the initial adsorption of Mg-sulfate complexes onto either (1) growing dolomite crystals or (2) rate-limiting dolomite nucleii. Field relations, as well as homogeneous synthesis at low temperatures (25{degrees}C < T < 100{degrees}C) support this hypothesis and provide a mechanistic explanation for dolomite growth from sulfate-rich natural waters. 36 refs.

A model for the evolution of brines in salt from the lower Salado Formation, southeastern New Mexico

Fluid inclusions were collected from a bedded salt horizon in the lower Permian Salado Formation in the Delaware Basin, southeastern New Mexico. The sampling horizon, at a depth of approximately 645 meters, consists primarily of recrystallized halite, with thin layers of anhydrite. Other trace minerals, dispersed throughout the salt, include quartz, polyhalite, gypsum, K-feldspar, magnesite, and clays. Large fluid inclusions (up to several mm on an edge) are common in the halite; in addition, bands of microscopic (<10 μm) fluid inclusions are present as primary (“chevron”) structures in fragments of unrecrystallized salt. We sampled 109 large inclusions by individual extraction of the fluids, which were analyzed for Ca, Mg, K, Na, Cl, Br, and SO4. The chemistry of the inclusion fluids and the associated mineralogy suggest that these brines represent Permian seawater that has undergone evaporation and subsequent modification by diagenetic reactions, dominated by the alteration of calcium sulfate to polyhalite and magnesite formation. The range of fluid inclusion compositions suggests a significant departure from a simple seawater evaporation model. Other brines from the same horizon in the Salado Formation were sampled and analyzed for the same elements as the fluid inclusions, and differed significantly from them primarily by the depletion of Mg relative to K. The association of these brines with argillaceous and/or anhydritic halite containing a suite of authigenic minerals (quartz, magnesite, and Mg-rich clays) suggests that these are intergranular brines with compositions determined over a much longer time scale than that required by the fluid inclusions. The principal reactions affecting intergranular brine chemistry are dehydration of gypsum, dewatering of detrital clays, and uptake of Mg during clay diagenesis. Overall, the observed variation in brine compositions implies that, if large-scale hydrologie circulation is occurring in the Salado halite, the time scale is limited by the rate required for low-temperature silicate diagenesis.

Reactive barriers for 137Cs retention

137Cs was dispersed globally by cold war activities and, more recently, by the Chernobyl accident. Engineered extraction of 137Cs from soils and groundwaters is exceedingly difficult. Because the half-life of 137Cs is only 30.2 years, remediation might be more effective (and less costly) if 137Cs bioavailability could be demonstrably limited for even a few decades by use of a reactive barrier. Essentially permanent isolation must be demonstrated in those few settings where high nuclear level wastes contaminated the environment with 135Cs (half-life 2.3 x 10(6) years) in addition to 137Cs. Clays are potentially a low-cost barrier to Cs movement, though their long-term effectiveness remains untested. To identify optimal clays for Cs retention, Cs desorption was measured for five common clays: Wyoming Montmorillonite (SWy-1), Georgia Kaolinites (KGa-1 and KGa-2), Fithian Illite (F-Ill), and K-Metabentonite (K-Mbt). Exchange sites were pre-saturated with 0.16 M CsCl for 14 days and readily exchangeable Cs was removed by a series of LiNO3 and LiCl washes. Washed clays were then placed into dialysis bags and the Cs release to the deionized water outside the bags measured. Release rates from 75 to 139 days for SWy-1, K-Mbt and F-Ill were similar; 0.017% to 0.021% sorbed Cs released per day. Both kaolinites released Cs more rapidly (0.12% to 0.05% of the sorbed Cs per day). In a second set of experiments, clays were Cs-doped for 110 days and subjected to an extreme and prolonged rinsing process. All the clays exhibited some capacity for irreversible Cs uptake. However, the residual loading was greatest on K-Mbt (approximately 0.33 wt.% Cs). Thus, this clay would be the optimal material for constructing artifical reactive barriers.

Anion Scavengers for Low-Level Radioactive Waste Repository Backfills

Minimization of 129/− and 99 TcO4 − transport to the biosphere is critical to the success of low level radioactive waste (LLRW) storage facilities. Here we experimentally identify and classify potential sorbent materials for inclusion in LLRW backfills. For low pH conditions (pH 4-5), Cu-sulfides and possibly imogolite-rich soils provide Kds (distribution coefficients) of roughly 103 mL g−1 for /−, and 102 mL g−1 for TcO4 −. At near neutral pH, hydrotalcites, Cu-oxides, Cu-sulfides, and lignite coal possess Kds on the order of 102 mL g−1 for both /− and TcO4 −. At high pH (pH>10), such as might occur in a cementitious LLRW facility, calcium monosulfate aluminate Kds are calculated to be roughly 102 mL g−1 for both both /− and TcO4 −.

Boehmite Sorbs Perrhenate and Pertechnetate

Boehmite and Al-oxyhydroxide gels sorb ReO4-, a non-radioactive analogue of TcO4- from NaNO3 solutions. Sorption appears to be substantially electrostatic (though there appears to be a specific preference for ReO4- over NO3-) and is most effective at pH < 8. Measured Kd´s lie between 5 and 105 ml g-1, depending on the solid, pH, and ionic strength. ReO4- and TcO4- are both partially removed from high pH Hanford-type acid waste simulants upon neutralization and formation of Al-rich sludges. We therefore propose that sequestration of Tc by boehmite limits dissolved Tc levels in the near and sub-surface environment and for that purpose boehmite might be relied on as a backfill, or reactive barrier, to limit environmental transport of Tc.

Cs+ Removal from Seawater by Commercially Available Molecular Sieves

With more than 10 US nuclear reactors sited on a coast, there is interest in being able to effectively remove radiological cesium from seawater in case of an accident. This study addresses the relative ability of commercially available molecular sieves to remove Cs+ from seawater. Experiments using CSTs IONSIEV IE-911 show that for 3-hour and 1-day exposures, acidic pellets have very high Cs removal capability in both normal seawater and a 9-fold concentrate. The basic form of the IE-911 performs well relative to aluminosilicate materials, but only at significantly lower Cs loadings than the acidic form.

Chemical evolution of leaked high-level liquid wastes in Hanford soils

A number of Hanford tanks have leaked high level radioactive wastes (HLW) into the surrounding unconsolidated sediments. The disequilibrium between atmospheric C0{sub 2} or silica-rich soils and the highly caustic (pH > 13) fluids is a driving force for numerous reactions. Hazardous dissolved components such as {sup 133}Cs, {sup 79}Se, {sup 99}Tc may be adsorbed or sequestered by alteration phases, or released in the vadose zone for further transport by surface water. Additionally, it is likely that precipitation and alteration reactions will change the soil permeability and consequently the fluid flow path in the sediments. In order to ascertain the location and mobility/immobility of the radionuclides from leaked solutions within the vadose zone, the authors are currently studying the chemical reactions between: (1) tank simulant solutions and Hanford soil fill minerals; and (2) tank simulant solutions and C0{sub 2}. The authors are investigating soil-solution reactions at: (1) elevated temperatures (60--200 C) to simulate reactions which occur immediately adjacent a radiogenically heated tank; and (2) ambient temperature (25 C) to simulate reactions which take place further from the tanks. The authors studies show that reactions at elevated temperature result in dissolution of silicate minerals and precipitation of zeolitic phases. At 25 C, silicate dissolution is not significant except where smectite clays are involved. However, at this temperature CO{sub 2} uptake by the solution results in precipitation of Al(OH){sub 3} (bayerite). In these studies, radionuclide analogues (Cs, Se and Re--for Tc) were partially removed from the test solutions both during high-temperature fluid-soil interactions and during room temperature bayerite precipitation. Altered soils would permanently retain a fraction of the Cs but essentially all of the Se and Re would be released once the plume was past and normal groundwater came in contact with the contaminated soil. Bayerite, however, will retain significant amounts of all three radionuclides.

In-situ Formation of Bismuth-Based Iodine Waste Forms

We investigated the synthesis of bismuth oxy-iodide and iodate compounds, in an effort to develop materials for iodine recovery from caustic waste streams and/or final waste disposal if repository conditions included ambient conditions similar to those under which the iodine was initially captured. The results presented involve the in-situ crystallization of layered bismuth oxide compounds with aqueous dissolved iodine (which resides as both iodide and iodate in solution). Although single-phase bismuth oxy-iodide materials have already been described in the context of capturing radioiodine, our unique contribution is the discovery that there is a mixture of Bi-O-I compositions, not described in the prior work, which optimize both the uptake and the degree of insolubility (and leachability) of iodine. The optimized combination produces a durable material that is suitable as a waste form for repository conditions such as are predicted at the Yucca Mountain repository (YMP) or in a similar type of repository that could be developed in coordination with iodine production via Global Nuclear Energy Program (GNEP) production cycles.