Solid phase iron–sulfur geochemistry of a reactive barrier for treatment of mine drainage

Abstract

This paper discusses the solid phase Fe–S geochemistry of a reactive barrier at the Nickel Rim mine site (Ontario, Canada). The barrier, designed to treat groundwater contaminated by acid mine drainage, is composed of leaf and municipal compost and wood chips. This study shows that S is accumulating in the organic material as primarily acid volatile sulfides, at concentrations up to 195 μmol S g −1 d.w. (0.63 wt% S) in a zone of preferential flow. Pyrite and/or S 0 account for only a small fraction of the total reduced inorganic S, as oxidants are probably present at only low concentrations in the barrier system. The results of the solid phase analyses, the formation of disordered mackinawite (Fe 1+ x S) on piezometer tubing, and thermodynamic calculations indicate that the precipitation of poorly crystalline Fe monosulfides is the primary sink for Fe and S in the barrier. Siderite (FeCO 3) formation is proposed as an additional Fe sink in areas of high Fe flux. Minor accumulations of acid-soluble, organically-bound S in the reactive barrier occur in zones of low aqueous Fe concentration. After 23 months of operation, the average rate of S accumulation at the up-gradient edge of the barrier is calculated to be 87 μmol S g −1 a −1 (d.w. organic material), or 47 mol S m −2 a −1 in the direction of groundwater flow. Solid phase analyses from samples collected 3 and 14 months after installation indicate that the S accumulation rate declined by a factor of 3 over that time period.