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

Abstract

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 O 2 retarded the oxidation of pyrite most notably in the alkaline region but not in the acidic and circumneutral pH. The absence of CO 2 increased the pH of samples with significant calcite content but did not affect those containing substantial amounts of pyrite. Increasing the CO 2 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.

Mobilization and speciation of arsenic from hydrothermally altered rock containing calcite and pyrite under anoxic conditions

“…Arsenic content of the sample is 23.6 mg/kg, with low sulfur content of 0.20% by weight (Tabelin et al, 2012b). Pyrites in hydrothermally altered rocks contain substantial amounts of As, sometimes reaching 1.07 wt% (Tabelin et al, 2012a). The sample also contains trace amounts of Mn and organic matter at 0.07% by weight as MnO and 0.23% by weight as organic C. In the bulk excavated rock used in this study, mineral phases of As like arsenopyrite were not detected and pyrite was only found in trace amounts.…”

“…In addition, mass balance calculations indicated that the oxidation of pyrite and dissolution of calcite increased at longer water residence time, which was consistent with the lower pH values observed at lower infiltration rates (Figure 2(a)). Relatively high concentrations of Ca 2+ and SO 4 2-were also observed at the start of the experiment and their flushing-out trends could be attributed to the presence of soluble phases like Ca-sulfates (e.g., gypsum) and Fe-sulfates in the rock, which are most likely secondary mineral products of pyrite oxidation prior to sampling (Figures 2 and 10B) (De Donato et al, 1993;Tabelin et al, 2012a;Todd et al, 2003). Substantial SO 4 2-concentration decrease in all cases under anoxic conditions suggests that pyrite oxidation is minimized.…”

“…First, rapid dissolution and transport of As from soluble phases at shorter water residence time minimize the immobilization of As through adsorption and/or co-precipitation reactions. Second, water residence time affects the pH of the rock-water system through its interactions with pyrite and calcite, which are the primary pH controlling minerals of altered rocks (Tabelin et al, 2012a).…”

“…The authors have identified several important factors controlling the release of As and Pb from altered rocks and have proposed several mechanisms in their previous papers (Igarashi et al, 2008;Tabelin and Igarashi, 2009;Tabelin et al, 2010;Tabelin et al, 2012a). However, most of the data in these studies were obtained using batch experiments close to equilibrium conditions.…”

Removal of Arsenic, Boron, and Selenium from Excavated Rocks by Consecutive Washing

“…However, very few have tackled the problem associated with toxic element release from excavated rocks. Our previously published works have explained the importance of pH, redox potential (Eh) and dissolved oxygen (DO) in the leaching behaviors of As and Pb from altered rocks and the strong influence of minor or trace minerals like pyrite and calcite on the mechanisms controlling their release (Tabelin and Igarashi 2009;Tabelin et al 2010;Tabelin et al 2012a). Furthermore, results of long-term column experiments by the authors showed that a considerable amount of As in altered rocks was released during the early stages via the dissolution of soluble secondary minerals, which were mostly products of pyrite oxidation (Tabelin et al 2012b).…”