Abstract

The use of constructed wetlands for acid mine drainage amelioration has become a popular alternative to conventional treatment methods, but the metal attenuation processes of these systems are poorly understood. Precipitates from biotic and abiotic zones of a staged constructed wetland treating high metal load (~1000 mg L⁻¹ Fe) and low pH (~3.0) acid mine drainage were characterized by chemical dissolution, x-ray diffraction, and thermal analyses and speciated by geochemical modeling. Characterization of sediments from abiotic/aerobic zones within the treatment system showed an abundance of crystalline iron oxides and hydroxides. At the air/water interface of initial abiotic treatment basins, SO₄/Fe ratios were low enough (<2.0) for the formation of jarosite and goethite, but as the ratio increased due to treatment and subsequent reductions in iron concentration, jarosite was transformed to other Fe-oxyhydroxysulfates and goethite formation was inhibited. Amorphous iron minerals such as ferrihydrite and Fe(OH)₃ were dominant in biotic wetland cell substrates. Apparently, low Fe³⁺ activity, redox potential, and oxygen diffusion rates in the anaerobic subsurface environment inhibited crystalline iron precipitation. However, some crystalline iron minerals formed near the surface of biotic areas due to increased oxygen levels from surface aeration and/or oxygen transport by plant roots. Alkalinity generation from limestone dissolution within the substrate also played a significant role in metal retention processes. The formation of gypsum, rhodochrosite, and siderite as byproducts of alkalinity-generating reactions in this system appeared to have an impact on S, Mn, and Fe solubility controls. Therefore, sustaining alkaline conditions within the wetland is necessary for maintaining metal retention consistency and long-term treatment efficiency.