Basalt oxygen consumption

Preliminary Assessment of Oxygen Consumption and Redox Conditions in a Nuclear Waste Repository in Basalt... 

Since crushed basalt has been recommended as a major backfill component (1), experiments were completed to evaluate the rate of dissolved oxygen consumption and the redox conditions that develop in basalt-water systems under conditions similar to those expected in the near-field environment of a waste package. Two approaches to this problem were used in this study (l)the As(III)/As(V) redox couple as an indirect method of monitoring Eh and (2) the measurement of dissolved oxygen levels in solutions from hydrothermal experiments as a function of time. The first approach involves oxidation state determinations on trace levels of arsenic in solution (4-5) and provides an estimate of redox conditions over restricted intervals of time, depending on reaction rates and sensitivities of the analyses. The arsenic oxidation state approach also provides data at conditions that are more reducing than in solutions with detectable levels of dissolved oxygen. 

Dissolved Oxygen. The experimental results demonstrate that in the absence of basalt, DO is maintained at high levels, while in the presence of basalt, oxygen is effectively removed (see Figure 1). Although the ferrous iron content of Umtanum basalt mesostasis is not well known, estimates from bulk ferrous/ferric iron data and from microcharacterization of mesostasis phases (7, 20) indicate that the amount of Fe2+ available for oxygen consumption by mesostasis dissolution is large relative to the amount of DO (>.10 on a mole/mole basis).Thus, the available Fe2+ concentration should remain constant over the duration of the experiments. 

An apparent first order rate constant of 1.5 x 10 2 hr 1 was derived from the DO data from the 150°C experiment. For the 100°C experiment, the rate constant is about 4.5 x 10 4 hr"1. Further experiments are needed to determine the full rate law for oxygen consumption. Because reaction mechanisms and/or rates can change with time, extrapolation to conditions under which basalt controls Eh may not be justified. 

The relationship of the stirring rate in these experiments to the rates of hydrolysis reactions of basalt phases is indicative of surface-reaction controlled dissolution (21). First order kinetics are not inconsistent with certain rate-determining surface processes (22). Approximate first order kinetics with respect to dissolved oxygen concentration have been reported for the oxidation of aqueous ferrous iron (23) and sulfide (24), and in oxygen consumption studies with roll-type uranium deposits(25). 

Applications. In the following paragraphs, the conditions (temperature, time, water/rock mass ratio, surface area) and the results on closed system oxygen consumption and redox conditions of the basalt-water experiments are compared to expected conditions in the open system backfill and near-field environment of an NWRB. Crushing of basalt for pneumatically emplaced backfill could result in a substantial fraction of finegrained basalt with a variety of active surface sites for reaction similar to the crushed basalt used in the experiments. The effects of crushing on rates of mineral-fluid reactions are well documented (10,26). 

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