Heavy metal fates in laboratory bioretention systems

Key to managing heavy metals in bioretention is to understand their fates in bioretention facilities. In this study, pot prototypes filled with bioretention media were built to simulate the conditions of natural growth of plants. Synthetic runoff with different heavy metal loadings (copper, cadmium, lead, and zinc) was periodically applied. Metal accumulations in tissues of grasses – Panicum virgatum, Kentucky-31, and Bromus ciliatus, were investigated after 230 d of growth and multiple runoff treatment events. After 183 d of periodic runoff application, the concentrations of Zn, Cu, Pb and Cd with low and high loadings had the same trends in the plant tissues, Zn > Cu > Pb > Cd, following the trend of the input metal concentrations. The fates of input metals were 88–97% captured in soil media, 2.0–11.6% not captured by bioretention media, and 0.5–3.3% accumulated in plants. Compared to the metals retained by the soil, the percentages of input metals taken up by plants were relatively low due to the low plant biomass produced in this study. Greater biomass density would be required for the vegetation to have a valuable impact in prolonging the lifetime of a bioretention cell.