Dissolution Of Halogenated Biotite From An Acid-Mine-Drainage Site In Globe-Miami, Arizona, And Its Contributions To Ground-Water Chemistry

Abstract

Dissolution kinetics and ion-release rates were measured for a halogenated, trioctahedral biotite isolated from stream alluvium down-gradient from an acidic metal­enriched plume in Pinal Creek Basin, Arizona. Experiments were performed using flow-­through columns in which unground 150-250 µm biotite and bulk alluvial sediment from Pinal Creek basin were reacted with a solution of dilute sulfuric acid (pH 3.4 and 2.9) for five months under flowing argon gas at 25°C. The dissolution rate of the halogenated biotite was 2.16 x 10^-13 mol m^-2 s^-1 at pH 3. After 1400 hours, mineral dissolution was constant and congruent. The order of the biotite dissolution reaction with respect to hydrogen ion activity (n) was 0.52 for output pH values between 3.1 and 3.5. Dissolution rates for this study were slightly slower than rates previously measured for biotite and phlogopite in the same pH ranges, possibly due to sample composition, preparation and/or reactor design. This study is one of the few where Cl release rates were measured and averaged 0.08 picomol m^-2 s^-1. Inverse geochemical modeling of ground-water evolution between wells in Pinal Creek basin by Glynn and Brown (1996) suggested that an additional chloride source was necessary in some model simulations to achieve chloride mass balance. A model calculation of Cl input from biotite dissolution along the flow path modeled by Glynn and Brown (1996) using the biotite dissolution rate and the rate of Cl released determined in this study determined that Cl contribution from the dissolution of halogenated biotite can be as much as 10 mg/L.