Biotite dissolution and Cr(VI) reduction at elevated pH and ionic strength

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Abstract

The effects of elevated pH, ionic strength, and temperature on sediments in the vadose zone are of primary importance in modeling contaminant transport and understanding the environmental impact of tank leakage at nuclear waste storage facilities like those of the Hanford site. This study was designed to investigate biotite dissolution under simulated high level waste (HLW) conditions and its impact on Cr(VI) reduction and immobilization. Biotite dissolution increased with NaOH concentrations in the range of 0.1 to 2 mol L-1. There was a corresponding release of K, Fe, Si, and Al to solution, with Si and Al showing a complex pattern due to the formation of secondary zeolite minerals. Dissolved Fe concentrations were an order of magnitude lower than the other elements, possibly due to the formation of green rust and Fe(OH)2. The reduction of Cr(VI) to Cr(III) also increased with increased NaOH concentration. A homogeneous reduction of chromate by Fe(II)aq released through biotite dissolution was probably the primary pathway responsible for this reaction. Greater ionic strengths increased biotite dissolution and consequently increased Fe(II)aq release and Cr(VI) removal. The results indicated that HLW would cause phyllosilicate dissolution and the formation of secondary precipitates that would have a major impact on radionuclide and contaminant transport in the vadose zone at the Hanford site.

Introduction

Chemical reactions between minerals and alkaline solutions occur in a variety of natural and engineered environments. Examples of high pH environments created by human activity include the stabilization of soils with lime (Golden et al., 1985), the alkaline flooding of sandstone reservoirs (Novosad and Novosad 1984, Mohnot et al 1987, Bauer and Berger 1998), and the formation of concrete (Andersson et al 1989, Savage et al 1992, Choquette et al 1991). The U.S. Department of Energy Hanford Site, located in the state of Washington, is an extreme example of human-induced alkalinity. At the Hanford site, hyperalkaline, high ionic strength waste fluids were generated as part of the Pu and U extraction process. These fluids contained large quantities of dissolved NaOH, NO3-, NO2-, and Al3+; possessed pH values >13; and had temperatures in excess of 50°C (Zachara et al., 2004). Since 1960, some of the single shell, high level waste (HLW) storage tanks designed to contain these solutions have leaked, releasing 1,920,000 to 3,456,000 L of HLW into the underlying vadose zone (Hanlon, 1996) where it has interacted with the mineral assemblage in the sediment. Major minerals in the Hanford sediment include quartz, feldspars, micas, illites, vermiculites, smectites, and iron (hydr)oxides.

Chromium is one of the major contaminants in the Hanford HLW because hexavalent Cr [Cr(VI)] was used as an oxidant to manipulate the valence states of Pu and U. The HLW and residual Cr(VI) were discharged to tank farms designated as S and SX. The estimated Cr concentration at the time of the original leak(s) ranged from 5.09 × 10-2 to 4.13 × 10-1 mol L-1 (Qafoku et al., 2002). Jones et al. (2000) estimated the total mass of Cr lost to the vadose zone from tanks S-104, SX-107, SX-108, SX-109, SX-113, and SX-115 to be ∼2685 kg. In the surface and subsurface environment, Cr transport and fate are mainly controlled by precipitation, sorption, and redox processes. Cr(III) is only sparingly soluble from about pH 6.0 to 11.5, whereas Cr(VI) is mobile, and its transport is normally controlled by sorption to mineral surfaces. Because Cr(VI) is an oxyanion, its sorption is severely limited at alkaline pH. This fact makes chemical reduction even more critical to the fate and transport of Cr at circum neutral and higher pH values (Rai, 1987). Under the hyperalkaline conditions associated with HLW at the Hanford site, redox and precipitation reactions are likely to be the dominant factors in attenuating dissolved Cr(VI).

Fe(II) is an important inorganic reductant that is abundant in soils and sediments. Fe(II) is found in many silicates, and oxidation-reduction reactions between aqueous species and structurally bound Fe on or beneath the surface of both silicates and oxides can control the redox state of associated solutes. In particular, studies have demonstrated the heterogeneous reduction of Cr(VI) by Fe(II) in biotite, vermiculite, illite, smectites, chlorite, magnetite, ilmenite, Fe(II)-hematite, Fe(II)-goethite, and sulfides (Eary and Rai 1989, Ilton and Veblen 1994, Gan et al 1996, White and Peterson 1996, Patterson et al 1997). However, it is not known if this mechanism is active at alkaline pH, and it is not clear what role, if any, host mineral dissolution has on Cr transport and fate.

Biotite dissolution has been investigated in several studies, but most were conducted under acid to neutral pH conditions (Acker and Bricker 1992, Malmstrom et al 1996, Malmstrom and Banwart 1997, Taylor et al 2000). These experiments indicated that biotite dissolved incongruently, and that different sites within the sheet structure reacted at different rates. In general, interlayer cations were released more rapidly than octahedral cations, which reacted slightly faster than tetrahedral cations. Information for phyllosilicate dissolution under alkaline pH conditions is limited (Velde 1965, Mohnot et al 1987, Chermak 1992, Chermak 1993, Eberl et al 1993, Bauer and Berger 1998, Bauer et al 1998), and the specific effect of hyperalkaline, high ionic strength solutions on biotite stability has not been previously reported. The available literature indicates that dissolution may be significant under highly alkaline pH conditions. Acker and Bricker (1992) showed that the dissolution rate of silicates was two to three orders of magnitude greater at pH 12 to 13 than at neutral pH. Brady and Walther (1989) suggested that dissolution rates of aluminosilicates at pH >12 were comparable to those at pH <3.

The objectives of this study were to (1) investigate biotite dissolution in solutions simulating Hanford HLW, and (2) examine the potential role of biotite in Cr(VI) reduction and immobilization under conditions similar to those at the Hanford site.

Section snippets

Materials and methods

Solutions were prepared using reagent grade NaOH (5 mol L-1; J. T. Baker Mallinckrodt Baker Inc., Phillisburg, NJ, USA), Na2CrO4 (Sigma-Aldrich, Milwaukee, WI, USA; 98%), and NaNO3 (Fisher Scientific, Pittsburgh, PA, USA; crystal, 99.2%). Deionized water was boiled while being purged with Ar and then stored in a plastic container. Reagents were mixed in a glove box filled with high purity (99.998%) Ar, transferred to high density polypropylene reaction vessels, and sealed under Ar.

The biotite

Results and discussion

Biotite (ideal formula [K(Mg,Fe)3(Si3Al)O10(OH)2]) is a 2:1 phyllosilicate mineral with tightly held, nonhydrated interlayer cations. The 2:1 layer has octahedrally coordinated cations sandwiched between two sheets of Si, Al tetrahedra. The main cations in the octahedral layer are Mg and divalent iron Fe(II), and the interlayer ion is mainly K. Under hyperalkaline conditions, biotite may be altered in a number of ways including (1) expansion of the interlayer regions due to an exchange of

Conclusions and implications

The results of this study indicate that biotite dissolution and the release of K, Si, Al, and Fe to solution increased with NaOH concentrations. Fe concentrations were an order of magnitude lower than the other elements measured. The chemical reduction of Cr(VI) to Cr(III) also increased with NaOH concentration, and higher ionic strengths increased biotite dissolution and Cr(VI) removal. Collectively, these results indicated that Fe(II) released during biotite dissolution was the primary agent

Acknowledgments

We thank three anonymous reviewers for their constructive comments that have greatly improved the manuscript. The authors would like to acknowledge Dr. Chia-chen Chen during the collection of XAS spectra. Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by the

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    Associate editor: G. Helz

    Present address: 4044 Derring Hall, Department of Geosciences, Virginia Tech, Blacksburg, VA 24061 USA.

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