Recycling clamshell as substrate in lab-scale constructed wetlands for heavy metal removal from simulated acid mine drainage

Abstract

This study aimed to evaluate the feasibility of clamshells as a substrate in constructed wetlands (CWs) for removing heavy metals from acid mine drainage (AMD). Column-type CWs (ID 12.5 cm, H 50 cm) filled with clamshells or gravel were planted with cattails. Synthesized AMD containing 20 mg/L zinc (Zn), 0.3 mg/L cadmium (Cd), 20 mg/L copper (Cu), 1.1 mg/L lead (Pb), 0.6 mg/L manganese (Mn), and other minerals (pH=4.1) was fed to CWs (1 L/column) with hydraulic retention times of 2–7 days in sequencing batch mode. Results indicated higher metal removal in clamshell-based CWs with the pH neutralization than in gravel-based CWs. The removal efficiencies were 85.3–92.6%, 84.1–98.3%, 96.5–99.7%, 98.3–99.1%, and 64.0–83.8%, respectively, for Zn, Cd, Cu, Pb, and Mn. During 6 months of operation, 790.7 mg of Zn, 10.6 mg of Cd, 762.4 mg of Cu, 40.1 mg of Pb, and 19.9 mg of Mn were fed to each CW. Of those, retaining in the substrate was the main metal removal route, representing 49.7–82.5%, followed by plant uptake (16.1–39.0%), and other processes (0.6–3.2%). The metal contents in the clamshell were increased, although that of calcium was lowered after treatment. The clamshell-based CWs extended the cattail root and incubated sulfate-reducing bacteria in a larger population. These findings suggest a recycling way of seashells as filter media in CWs for heavy-metal-rich wastewater treatment.