Colloid Formation in Hanford Sediments Reacted with Simulated Tank Waste

Abstract

Solutions of high pH, ionic strength, and aluminum concentration have leaked into the subsurface from underground waste storage tanks atthe Hanford Reservation in Washington State. Here, we test the hypothesis that these waste solutions alter and dissolve the native minerals present in the sediments and that colloidal (diameter < 2 microm) feldspathoids form. We reacted Hanford sediments with simulated solutions representative of Hanford waste tanks. The solutions consisted of 1.4 or 2.8 mol/kg NaOH, 0.125 or 0.25 mol/kg NaAlO4, and 3.7 mol/kg NaNO3 and were contacted with the sediments for a period of 25 or 40 days at 50 degrees C. The colloidal size fraction was separated from the sediments and characterized in terms of mineralogy, morphology, chemical composition, and electrophoretic mobility. Upon reaction with tank waste solutions, native minerals released Si and other elements into the solution phase. This Si precipitated with the Al present in the waste solutions to form secondary minerals, identified as the feldspathoids cancrinite and sodalite. The solution phase was modeled with the chemical equilibrium model GMIN for solution speciation and saturation indices with respect to sodalite and cancrinite. The amount of colloidal material in the sediments increased upon reaction with waste solutions. At the natural pH found in Hanford sediments (pH 8) the newly formed minerals are negatively charged, similar to the unreacted colloidal material present in the sediments. The formation of colloidal material in Hanford sediments upon reaction with tank waste solutions is an important aspect to consider in the characterization of Hanford tank leaks and may affect the fate of hazardous radionuclides present in the tank waste.

Remediation of Uranium in the Hanford Vadose Zone Using Ammonia Gas: FY 2010 Laboratory-Scale Experiments

“…A 2.8 general mineral transformation pathway was observed in these studies: poorly crystalline aluminosilicate fi Linde Type A zeolite fi cancrinite/sodalite, with cancrinite and sodalite being the two stable phases (Deng et al 2006a). In earlier studies, the same group found that cancrinite, sodalite, LTA zeolite and allophane were identified as the new formation in these sediments (Mashal et al 2004;Mashal et al 2005b;Mon et al 2005). The soil mineral dissolution followed the order of quartz fi kaolinite fi illite, and although cancrinite, sodalite, and zeolite were formed, the zeolite was not detected after 25 days of reaction time (Mashal et al 2005a), indicating that this mineral underwent either dissolution or phase transformation.…”

Subsurface Behavior of Plutonium and Americium at Non-Hanford Sites and Relevance to Hanford

“…Our review suggests that colloidfacilitated transport has generally been overstated in site assessments described in this study. This position also is supported by Hanford-specific studies demonstrating that colloid facilitated transport of highly sorptive contaminants in groundwater is minimal (Flury et al 2002;Cherrey et al 2003;Zhuang et al 2003Zhuang et al , 2004Marshal et al 2004;Dai et al 2005;Czigany et al 2005). Colloid-facilitated transport of highly sorptive contaminants in the vadose zone would be expected to be even less than in saturated groundwater, due to the much higher ratio of surface area to water volume and thin water-film thicknesses, which would be conducive to filtration of particles from solution.…”

Geochemical Processes Data Package for the Vadose Zone in the Single-Shell Tank Waste Management Areas at the Hanford Site

“…Important micaceous sorbents (including biotite) may also dissolve in this zone Samson et al 2005). Hydroxide alkalinity is transformed to silica and aluminate alkalinity in this zone (Marshal et al 2004). Below this zone, where depth is controlled by the volume of tank waste release, exists a pH neutralization zone (pH 6.5 -10), where protons are released for additional base neutralization by the secondary precipitation of complex suites of zeolitic phases including cancrinite; feldspathoids, such as sodalite, ettringite, and gibbsite; and other unnamed aluminosilicates (Ainsworth et al 2005;Chorover et al 2003;Deng et al 2006;Qafoku et al 2004;Wan et al 2004a;Wan et al 2004b;see Figure 6.8).…”

“…Hydroxide and other anion concentrations are critical variables determining secondary mineral precipitate phase identity and morphology. The precipitates exist as both grain coatings (Ainsworth et al 2005;Qafoku et al 2004) and suspended colloids in the aqueous phase exhibiting negative surface charge (Marshal et al 2004). …”

“…However, these issues have only been partially explored in the laboratory, with variable effects noted in mineral (Catalano et al 2005;Choi et al 2005a;Choi et al 2005b;Chorover et al 2003;Mon et al 2005) and sediment systems (Ainsworth et al 2005;Marshal et al 2004). Zhuang et al (2003) observed that 137 Cs + adsorbed to feldspathic colloids resulting from tank waste-sediment reaction, but sorption strength was below that of the native micaceous fraction.…”