Comparative evaluation of different substrates compiled in column-type vertical-flow constructed wetlands to remove heavy metals

Heavy metals (HMs), like chromium (Cr), manganese (Mn), and nickel (Ni), are toxic pollutants frequently detected in industrial wastewater. In this study, column-type vertical-flow constructed wetlands (CWs) filled with different substrates were examined for their ability to remove Cr, Mn, and Ni, focusing on their applicability to high HM concentrations (tens of mg/L) and short hydraulic retention times (2 days). Five candidate substrates-gravel, limestone, reddish soil, wood pellets, and activated carbon-were screened for their ability to remove 6 mg/L of the three HMs, revealing that reddish soil and activated carbon removed all three HMs efficiently. Further experiments to remove 60 mg/L of HMs were conducted using CWs filled with reddish soil or activated carbon with or without planting Phragmites australis. The results indicated that (1) substrate adsorption contributed exclusively to HM removal and the effects of planting may be negligible, and (2) activated carbon (removal efficiency of 74-99%, 98-99%, and 99% for Cr, Mn, and Ni, respectively) is likely a more promising substrate than reddish soil (removal efficiency of 90-99%, 48-99%, and 61-99% for Cr, Mn, and Ni, respectively) in light of its ability to achieve efficient HM removal under high loading conditions.