Fate and transport of dissolved/colloidal metals in an aging passive treatment system for coal mine drainage: Divergent behavior of trace metals and iron

Abstract

Passive treatment systems are widely used to mitigate the low pH and high concentration of Fe (and often Al) that are found in abandoned mine drainage (AMD), but few studies have focused on understanding how Fe and Al removal – and the possible concurrent formation of colloids - affects the fate of trace metals in these systems. We measured trace metals at multiple locations across a 17-year-old passive treatment system in the middle western anthracite field of Pennsylvania, USA, to understand their partitioning between the dissolved and colloidal fractions and the extent to which the behavior of the major metal species controls the fate of the trace metals. The treatment system, consisting of an oxidation pond followed by three vertical flow wetlands, removes more than 90 % of the Fe, while the trace metals Co, Ni, and Zn remain at more than 70 % of their influent concentrations. Under the oxic conditions of surface water at the spillways between ponds, ferric oxyhydroxide precipitates form, which settle rapidly and do not create mobile colloids. The trace metal concentrations remain high under these conditions as the low pH (< 4) inhibits adsorption to the precipitates. Standpipes discharging anoxic water from the underdrains for each upstream vertical flow wetland exhibit higher pH, leading to the precipitation of Al hydroxysulfates and the formation of colloids. The total (dissolved + colloidal) concentrations of Co and Ni correlate well with Al concentrations, suggesting adsorption or co-precipitation with the Al precipitates. Total Zn concentration is not well correlated with Al, and the colloidal fraction is significant in some, but not all standpipe samples, suggesting that Zn is forming a separate precipitate phase, likely ZnS. Overall, the ability of an aging treatment system to increase pH – not its Fe removal efficiency – is a predictor of the mobility of dissolved trace metals. However, the behavior of each metal varies substantially along different flow paths within the system.