Biogeochemical Processes in Hydrosoil of a Pilot-Scale Constructed Wetland Treatment System Designed for Treatment of Metals

Environmental conditions, metal (Cd, Cu, Ni, and Zn) distribution, and metal fractionation in hydrosoil (sediment and overlying organic detritus) of a free water surface pilot-scale constructed wetland treatment system (CWTS) were investigated to determine treatment processes. Chemical and mineralogical analyses were performed on organic detritus samples and sediment cores collected from two cells planted with Schoenoplectus californicus and two cells planted with Typha angustifolia. T-tests indicate that cells planted with S. californicus and those planted with T. angustifolia were equally effective in promoting conditions in the detritus favorable for sorption of metals and complexation with acid-volatile sulfide (AVS). For the sediment, cells planted with S. californicus were more effective than those planted with T. angustifolia in promoting AVS formation, which is favorable for metal complexation with AVS and attributed to addition of hydrosoil amendments (hay and gypsum) to cells planted with S. californicus. In all four cells, concentrations of Cd, Cu, Ni, and Zn in hydrosoil decreased gradually with depth from the detritus to 15 cm below the top of the sediment. Concentrations of Cd, Cu, Ni, and Zn in hydrosoil correlated significantly with organic matter content. Using a sequential extraction procedure, metals were detected from five operationally defined geochemical fractions: exchangeable, bound to carbonates, bound to Fe-Mn oxides, bound to organic matter or sulfides, and residual. The high percent of metals, particularly Zn, associated with the Fe-Mn oxide fraction indicates that oxidation and hydrolysis occurred in the wetland cells. High AVS concentration in the hydrosoil indicates that dissimilatory sulfate reduction occurred, and the presence of metal sulfides was confirmed by scanning electron microscopy. The vertical distribution of measured hydrosoil conditions and metal fractions investigated in this study provides information on the effect of wetland characteristics on treatment processes for sequestering metals in the hydrosoil. This research provided a framework for evaluating a CWTS to improve understanding of the capability and versatility of wetland cells to renovate water containing Cd, Cu, Ni, and Zn