Clay Mineralogical Transformations over Time in Hanford Sediments Reacted with Simulated Tank Waste

mineral dissolution and precipitation may occur. The effect of alkaline solutions on the transformation of Buried waste storage tanks at the USDOE Hanford Reservation clay minerals has been the subject of intensive research in Washington State have released solutions containing high concentrations of Na, OH, NO3, and Al into the vadose zone. When such (Cuadros and Linares, 1996; Bauer and Berger, 1998; solutions contact vadose zone sediments, mineral transformations will Bauer and Velde, 1999; Taubald et al., 2000). Bauer and change the sediment matrix. We hypothesized that Si, dissolved from coworkers (1998, 1999) studied the reaction of kaolinite primary and secondary minerals, will combine with Al from the tank in KOH solutions and reported the formation of new waste to form crystalline or poorly crystalline network silicates such solid phases. Buhl et al. (1997) found that kaolinite was as zeolites and feldspathoids. In this study, we characterized the colloitransformed to sodalite at pH 10. Upon the reaction dal ( 2 m equivalent diam.) minerals formed when simulated tank of kaolinite with a NaOH-NaNO3 mixture, the following solutions reacted with vadose zone Hanford sediments. Variables mineral transformation sequence was found: kaolinite → studied included simulated tank waste (STW) composition, reaction fly ash → montmorillonite → natural zeolite (Park et al., time, and temperature. Hanford sediments were reacted with a series 2000). of simulated tank solutions in batch experiments at 25 and 50 C for 1, 10, 25, 40, and 50 d. The mineralogical, structural, and chemical Many studies have been conducted on the transformaproperties of the resulting colloidal fractions and bulk solutions were tion of aluminosilicates at high pH, but few reports are determined by x-ray diffraction (XRD), Fourier transform infrared available on the type of minerals that could form in (FTIR), 27Aland 29Si-magic angle spinning-nuclear magnetic resositu under the leaking Hanford waste tanks. Hanford nance (MAS-NMR), scanning electron microscopy (SEM), energysediments consist of primary phases such as quartz and dispersive x-ray analysis (EDAX), colorimetry, atomic absorption feldspars and secondary phases such as aluminosilicate spectroscopy, and inductively coupled plasma–atomic emission specclays and iron oxides (Serne et al., 2002). Recently, troscopy (ICP–AES). Upon contact with STW, Si was released from minerals representative of Hanford sediments were subthe sediments and a portion was incorporated into poorly crystalline ject to mineral transformation studies. For example, solids. The amount of poorly crystalline solids increased initially and when quartz was reacted with STWs, NO3–cancrinite reached maximum quantities between 0 and 25 d. Lability of minerals in the presence of NaOH followed the order quartz → kaolinite → was found to precipitate on the quartz surface (Bickillite. New secondary minerals, NO3–cancrinite, NO3–sodalite, and more et al., 2001). Chorover et al. (2003) examined the zeolite A, were formed at the expense of the original clay minerals. dissolution of kaolinite reacted with STW and reported Zeolite A was labile and disappeared after about 25 d of reaction time. the formation of network aluminosilicates, including Cancrinite and sodalite, however, appeared to be stable and increased zeolite (chabazite), NO3–cancrinite, and NO3–sodalite. in abundance with time. Zhao et al. (2004) corroborated these results, finding that both cancrinite and sodalite formed when kaolinite reacted with STWs. A high-level radioactive wastes have been Similar reactions occurred when Hanford sediments produced as a byproduct of Pu production at the reacted with STWs at elevated temperatures (60–90 C) Hanford nuclear site in Washington state. The highwhere silicate minerals dissolved and zeolitic phases level waste was stored in 177 steel-lined underground precipitated (Kaplan et al., 1998; Nyman et al., 2000; tanks and has an estimated volume of about 65 million Qafoku et al., 2003a, 2004; Mashal et al., 2004). The gallons (Gephart and Lundgren, 1998). These wastes mineral dissolution rates and the morphology of the are alkaline and high in ionic strength. The composition secondary precipitates were found to be dependent on of the waste includes radionuclides and high concentraSi/Al aqueous molar ratios (Qafoku et al., 2003a). In tions of NaOH, NaNO2, NaNO3, and NaAlO2 (Serne another study, STWs in contact with Hanford sediments et al., 1998). Following leakage of the tanks, the heat precipitated cancrinite and sodalite in the colloidal size generated by radioactive decay resulted in temperatures fraction (Mashal et al., 2004). beneath the tank in excess of 50 C (Pruess et al., 2002). Here, we want to expand the existing knowledge of When highly alkaline solutions contact clay minerals, colloidal material formed in Hanford sediments reacted with STW. The objective of this work was to investigate and characterize the colloidal materials ( 2 m in Dep. of Crop and Soil Sciences, Center for Multiphase Environmental Research, Washington State Univ., Pullman, WA 99164-6420. ReAbbreviations: AAO, acidic ammonium oxalate; AES, atomic emission ceived 5 July 2004. *Corresponding author ([email protected]). spectroscopy; EDAX, energy-dispersive x-ray analysis; FTIR, Fourier transform infrared; ICP, inductively coupled plasma; MAS, magic angle Published in Soil Sci. Soc. Am. J. 69:531–538 (2005). © Soil Science Society of America spinning; NCA, noncarbonate alkalinity; SEM, scanning electron microscopy; STW, simulated tank waste; XRD, x-ray diffraction. 677 S. Segoe Rd., Madison, WI 53711 USA