Dissolution kinetics of heulandite at pH 2--12 and 25{degrees}C

Journal Article · Tue Jun 01 04:00:00 EDT 1993 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/0016-7037(93)90408-O· OSTI ID:139166

Ragnarsdottir, K V ^[1]

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1. Univ. of Bristol (GB)

Because of their favourable cation exchange reactions, heulandite and clinoptilolite have been suggested as being capable of immobilizing radionuclides and therefore could possibly act as an important barrier for nuclear waste. Recent studies of laboratory-reacted minerals indicate, however, that hydrated surface layers tend to accumulate highly hydrolyzable heavy elements. These hydrated layers may therefore be the most important retardants for radionuclides. The dissolution rate of heulandite depends strongly on pH. Based on silica release, the logarithm of the steady-state dissolution rate at pH 2 is {minus}13.1 mol cm{sup {minus}2} s{sup {minus}1}. The logarithm of the rate decreases to {minus}15.8 mol cm{sup {minus}2} s{sup {minus}1} at pH 7.2 and increases again to {minus}14.6 mol cm{sup {minus}2} s{sup {minus}1} at pH 12.2. At low pH, Al is released preferentially to silica; but at intermediate and high pH, the release of silica appears to be congruent relative to Al. The change in dissolution rate with pH indicates that at low pH, the dissolution mechanism is controlled by the detachment of a positively charged Al species, >Al-OH{sub 2}{sup +}. Below pH 5, however, a silica-rich surface layer is formed requiring diffusion through the layer. At intermediate and high pH, it is likely that the dissolution rate is controlled by the detachment of a negatively charged silica species, >Si{sup {minus}}O{sup {minus}}. The reaction order of the hydrogen ion under low pH conditions is 0.7, and the reaction order of the OH{sup {minus}} ion is 0.3 at high pH. The measured dissolution rates indicate that a 1 mm heulandite crystal would dissolve in 300,000 yrs if the solution composition is maintained undersaturated. 75 refs., 11 figs., 3 tabs.

• OSTI ID: 139166
• Journal Information: Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: 11 Vol. 57; ISSN GCACAK; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

05 NUCLEAR FUELS
ALUMINIUM
CHEMICAL REACTION KINETICS
DISSOLUTION
HEULANDITE
PH VALUE
RADIOACTIVE WASTE DISPOSAL
SILICON OXIDES
TIME MEASUREMENT
YUCCA MOUNTAIN
Yucca Mountain Project