Transport-controlled kinetics of dissolution and precipitation in the sediments under alkaline and saline conditions
Millions of liters of high temperature, Al-rich, alkaline, and saline high-level waste (HLW) fluids were accidentally discharged onto the sediments at the Hanford Site, WA. Dissolution and precipitation are two processes that might occur when these fluids contact the sediments, but their occurrence and extent are not well studied under such extreme conditions. The objective, therefore, was to investigate the effects of geochemically stable, Al-rich, alkaline and saline solutions on the extent of soil mineral dissolution and precipitation during reactive transport through the sediments. Metal- and glass-free systems were used to conduct miscible-displacement experiments at 50 C under CO{sub 2} and O{sub 2} free conditions. Results showed that soil liquid phase composition changed significantly because of base-induced soil mineral dissolution and the subsequent releases of Si, K, Al, Fe(III), Fe(II), Ca, Mg, and Ba, into the aqueous phase. Transport-controlled release of these elements was time-dependent as evidenced by its extent varying with the fluid residence time. Initial dissolution rates calculated based on Si release in the column effluents in the second pore volume (PV) varied between 6.085 x 10{sup -11} and 5.377 x 10{sup -13} mol m{sup -2} s{sup -1}. They increased with base concentration and decreased with Al concentration in the leaching solution and the fluid residence time. Al precipitation rates (normalized to 1 kg of solution) varied in the range 0.4374 x 10{sup -6} ({+-} 0.019 x 10{sup -6}) and 1.069 x 10{sup -6} ({+-} 0.278 x 10{sup -6}) mol s{sup -1}. Al precipitation followed a first-order kinetics with an initial rate constant of 0.0701 h{sup -1} (half-life of 9.9 h at about 3 PV), which increased to 0.13706 h{sup -1} (half-life of 5.1 h at about 20 PV). The precipitates identified with SEM and confirmed from the modeling results, were mainly NO{sub 3}-cancrinite. NO{sub 3}-sodalite formation in the presence of high OH concentrations and gibbsite formation in the presence of high Al concentrations were also suggested from the modeling results. The secondary phases that are formed during these experiments have high specific and possibly reactive areas; they covered the soil mineral particles protecting them from further dissolution. Aqueous and solid phase transformations caused by base-induced dissolution and precipitation should be important determinants of the fate of contaminants and radionuclides in the vadose zone under alkaline and saline conditions.
- Research Organization:
- Pacific Northwest National Lab., Richland, WA (US), Environmental Molecular Sciences Laboratory (US)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 15011335
- Report Number(s):
- PNNL-SA-38913; GCACAK; 3230; KP1301020; TRN: US200506%%256
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 68, Issue 14; Other Information: PBD: 1 Jul 2004; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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