ACID MINE DRAINAGE KINETICS

A review of reported acid mine drainage (AMD) facies reaction rates shows that iron transformation rates set the tempo of AMD development. Iron release by dissolved oxygen (DO) oxidation of pyrite at high pH is relatively slow. Nevertheless, unless pyrite contents are very low, rates of chemical weathering of silicate rocks are too slow to generate alkalinity as fast as acid is produced by the pyrite-DO reaction. When the rate of alkalinity delivery is lower than the rate of hydrogen ion production, the pH declines until at around pH 4 runaway AMD commences with Fe(III) becoming the principle pyrite oxidant. At low pH, microbes regenerate Fe(II) to make Fe(III) faster than it reacts with pyrite and the excess Fe(III) reprecipitates as iron oxyhydroxides, producing even more acid. At pH > 4, microbial oxidation of Fe(II) is relatively unimportant compared to abiotic homogeneous and heterogeneous rates. Because carbonate rocks are much more reactive than silicate rocks, they can neutralize acid produced by the pyrite-DO reaction faster than it can be produced so that runaway AMD seldom develops. Instead, the iron liberated from the pyrite is quickly oxidized to Fe(III), which immediately precipitates and coats the pyrite grain thereby slowing access of oxygen to its surface. Eventually the pyrite is replaced by a limonite pseudomorph. Analysis of the rates of the reactions that lead to coating development suggests that addition of excess bicarbonate alkalinity during early stages of mine waste exposure might cause enough of a coating to form to avoid runaway AMD development. A number of additional kinetic factors are important in AMD settings. Marcasite oxidation rates are very similar to pyrite oxidation rates. Pyrite grain size strongly affects reaction rates but trace element content seems to have little or no effect. The trace element content of AMD solutions is controlled by the combined rates of release from pyrite along with rates of oxidative and nonoxidative dissolution of other host sulfides.