The roles of pyrite and calcite in the mobilization of arsenic and lead from hydrothermally altered rocks excavated in Hokkaido, Japan

This paper describes the enrichment of hydrothermally altered volcanic and sedimentary rocks with arsenic (As) and lead (Pb), and the effects of pyrite and calcite on the mobilities and release mechanisms of these toxic elements under oxic and anoxic conditions. Enrichment of the altered rock with As and Pb predominantly occurred in precipitated pyrite grains and not on the alumino-silicate minerals making up the matrix of the rock. Arsenic was incorporated in pyrite grains formed during alteration in both volcanic and sedimentary rocks, but Pb was only found in the pyrite grains of the volcanic rock samples. When in contact with water, altered volcanic rocks had acidic pH while altered sedimentary rocks had alkaline pH. The mobilities of both As and Pb from the altered rocks were enhanced at acidic and alkaline pH and a minimum was observed in the circumneutral pH under both oxic and anoxic conditions. The absence of O2 retarded the oxidation of pyrite most notably in the alkaline region but not in the acidic and circumneutral pH. The absence of CO2 increased the pH of samples with significant calcite content but did not affect those containing substantial amounts of pyrite. Increasing the CO2 also had insignificant effect on the concentrations of As and Pb in the leachate. The mechanisms controlling the mobilization of As and Pb from these rocks like dissolution of soluble secondary minerals, pyrite oxidation and adsorption were all related to pyrite while the pH of the rock when in contact with water was controlled by pyrite and calcite. Thus, excavated waste rocks that have been altered can be grouped based on the relative abundance of pyrite and calcite and their pH when in contact with water.