Acidification of Reservoir and Leaching of Aluminum by Oxidation-reduction Reactions of Pyrite

Abstract

Water quality in a reservoir was investigated to identify the causes of acidification of the reservoir water and the change in the water color to light blue. The results showed that the sulfate concentration was high at the sampling points in the reservoir where the pH was low, and that there was sufficient dissolved oxygen to keep the redox condition in the reservoir aerobic. A small plot of land reclaimed by using excavated rocks which contained considerable quantities of pyrite was constructed in the center of the reservoir, suggesting that the acidification of the reservoir was caused by the oxidation-reduction reaction of pyrite, to produce sulfate and hydrogen ions.
Soluble aluminum concentration only corresponded to the pH of the reservoir water in spite of the sampling points. The geochemical evaluation by the commonly used geochemical model PHREEQE showed that an aluminum hydroxide, gibbsite, was in equilibrium with the precipitation-dissolution reaction. This suggests that the acidified reservoir water leached aluminum from the surrounding soils, rocks and suspended solids, and that the precipitation-dissolution reaction of gibbsite occurred. It is thought that the reservoir looked light blue due to the white precipitate of the formed aluminum hydroxide.