Trace Metal Retention in LBOS-Amended, Vertical-Flow Constructed Wetlands Treating Low pH, Ferric Iron-Dominated Acid Rock Drainage

Abstract

Remediation strategies employed for mitigation of low-pH, metal-sulfate drainage (i.e., acid rock drainage, ARD) may include passive treatment systems. While shown effective for neutralization of acid and removal of major ions (e.g., iron, aluminum, and manganese), trace element removal during acid neutralization in passive systems is rarely investigated. Therefore, in this presentation, we will focus on the fate of trace metals in one common type of passive treatment system, the vertical flow wetland (VFW). Our experimental VFW contained a limestone buffered organic substrate (LBOS) and received low pH (<3), ferric iron-dominated ARD for two years. During this time, trace elements (As, Cd, Cr, Cu, U, Co, Ni, and Zn) were removed along a pH gradient controlled by a series of reaction zones that developed above a dynamic limestone dissolution front. Therefore, for the practical purpose of implementing LBOS to treat low pH, ferric iron-dominated ARD, high trace element removal efficiency can be expected as long as the limestone dissolution front does not pass completely through the substrate. With the exception of uranium, trace metal attenuation largely occurred above the limestone dissolution front in the transitional and oxide reaction zones. Moreover, trace metal removal exhibited a strong dependence on pH. Based largely on increasing pH with depth, trace metal removal within the LBOS follows the sequence: