DEVELOPMENT OF ANOXIC MINERAL-DISSOLUTION APPARATUS AND ANOXIC DISSOLUTION KINETICS OF BIOTITE

Mineral dissolution experiments have been intensively done for the past two decades whereas few dissolution experiments have been done under anoxic conditions. Recently, atmospheric evolution in the Precambrian has been a controversial subject, and modern, anoxic weathering deep in the subsurface has been another subject for high-level radioactive waste disposal. For both subjects, anoxic, mineral-water interactions should be well understood. We have developed an apparatus that enables us to carry out mineral dissolution experiments under anoxic conditions to examine the effects of dissolved oxygen on dissolution kinetics. With the apparatus, dissolution experiments of biotite were carried out. The results were compared with those obtained by similar dissolution experiments under the present atmosphere, i.e., oxic conditions.

All processes, from solution supply to solution sampling of flow-through type experiments are carried out in one glove box. A solution is bubbled by Ar gas in advance of dissolution experiments, and the glove box is filled with the Ar gas coming out of the solution. When dissolved O2 (DO) in the solution becomes less than 0.001 mg/l, equivalent to 3 x 10(-5) atm of PO2, the solution flows into a mineral-solution reaction vessel at a rate of 30 ml/day and the dissolution experiment is started. Effluent is finally sampled in a bottle. The concentration of DO is monitored during the experiments. The temperature of the reaction vessel can changed from 20 to 40 degrees C. We design the apparatus to control PO2 and PCO2 in the ranges of 10(-5) to 2 x 10(-1) atm and 10(-3.5) to 1 atm, respectively.

Anoxic biotite dissolution was carried out at 3 x 10(-5) atm of PO2, pH3 (with hydrochloric acid) and 40 degrees C for 30 days. The dissolution rate was faster by a factor of two under anoxic conditions than oxic conditions.