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The effect of pH on chlorite dissolution rates at 25°C

Published online by Cambridge University Press:  11 February 2011

Åsa B. Gustafsson  and
Ignasi Puigdomenech
Åsa B. Gustafsson
Affiliation:
Royal Institute of Technology, Inorganic Chemistry, Stockholm, Sweden
Ignasi Puigdomenech
Affiliation:
Swedish Nuclear Fuel & Waste Management Co., Stockholm, Sweden.
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Abstract

The Swedish nuclear industry is planing to dispose spent nuclear fuel encapsulated in copper canisters in granitic bedrock at a depth of approx. 500 m. The reducing capacity of the geo-sphere is important for canister corrosion and radionuclide migration. The main inorganic reduc-tants are Fe(II)-containing minerals: biotite in the bedrock, and pyrite and chlorite in the fracture filling minerals as well as in the buffer and backfill. As a consequence, the dissolution of chlorite is an important parameter when considering the reducing capacity available to groundwaters around the repository.

The weathering rates of a chlorite sample from Taberg in central Sweden has been studied in 0.01 M NaCl media at (25±1)°C in the pH region 2–12. The chlorite was characterized before and after grinding and sieving, and was found to have a relatively low Fe-content, with the formula (Mg9.44 FeII0.72Al1.68)(Si6.47Al1.4FeIII0.13)O20(OH)16. Kinetic experiments were performed using a thin-film continuous flow-through technique with flow rates of 5 mL/h or 2.7 mL/h. This technique prevents the precipitation of secondary phases such as iron(III) hydroxides. Solution samples were taken after a reaction time of 0–30 days. The contents of released Al, Fe, Mg and Si were analyzed using an Inductively Coupled Plasma emission spectroscopy (ICP-AES) technique and then normalized to the Brunauer-Emmet-Teller (BET) surface area and flow-rate in order to obtain rates of dissolution. The dissolution rate of the chlorite studied was pH-dependent, and a minimum was observed in the neutral pH region. The dissolution rate (based on Si data) decreased from 7×10-11 mol/(m2 s) at pH=2 to 4×10-13 mol/(m2 s) at pH≈7 and then in-creased to 3×10-12 mol(m2 s) at pH=12.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 757: Symposium II – Scientific Basis for Nuclear Waste Management XXVI , 2002 , II3.16
Copyright
Copyright © Materials Research Society 2003

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