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TECHNICAL REPORTS

Vadose Zone Processes and Chemical Transport

Aluminum Effect on Dissolution and Precipitation under Hyperalkaline Conditions

I. Liquid Phase Transformations

O.S. Qafoku, Pacific Northwest National Lab., Interfacial Geochemistry Group, 902 Battelle Blvd., P.O. Box 999, MSIN: K3-61, Richland, WA 99352

^* Corresponding author (nik.qafoku{at}pnl.gov).

Received for publication January 3, 2003. Substantial amounts of self-boiling, Al-rich, hyperalkaline, and saline high-level waste fluids (HLWF) were deposited to the vadose zone at the Hanford Site, in Washington State. The objective of this study was to investigate the effects of similar fluids on the extent of dissolution and precipitation in the sediments. Metal- and glass-free systems were used to conduct batch experiments at 323 K under CO₂ and O₂ free conditions. Base-induced dissolution of the soil minerals was rapid in the first 48 h as indicated by immediate releases of Si and Fe into the soil solution. Potassium release lagged behind and dissolution of K-bearing minerals (mica and K-feldspar) proceeded faster only after 2 to 3 d of the experiment. Silicon and Fe release exhibited high dependence on aqueous [Al] (rate orders <-1), because Al decreased free OH concentration in the contact solution and probably inhibited soil mineral dissolution. Initial K release exhibited low dependence on [Al] (fractional rate orders). Initial dissolution rates calculated based on Si release varied with aqueous [Al] from 29.47 to 4.35 x 10^-12 mol m^-2 s^-1. Aluminum participated in the formation of the secondary phases (precipitation rates of 10^-8 mol s^-1) but the overall precipitation rate of alumino-silicate secondary phases was probably controlled by aqueous [Si] (rates of 10^-9, and rate constants between 0.0054 and 0.0084 h^-1). The changes in the soil solution chemistry (release of K, Si, Fe, and other elements) may play a significant role in the fate of radionuclides and contaminants like Cs, Sr, Cr, and U in the Hanford sediments.

Abbreviations: DOE, U.S. Department of Energy • EC, electrical conductivity • EGME, ethylene glycol monoethylene • ERDF, Environmental Restoration Disposal Facility • HLWF, high level waste fluids • ICP-AES, inductively coupled plasma–atomic emission spectroscopy

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