Mechanisms of lead and zinc removal from lead mine drainage in constructed wetland

Lab-scale constructed wetlands have been treating synthetic mine effluent for 28 to 78 months, with 90% removal of lead and 65% removal of zinc observed at hydraulic residence times of 0.45 to 4.5 days. The following major mechanisms of lead and zinc removal in lab-scale constructed wetlands were studied: adsorption, sulfide precipitation, and iron/manganese oxide co-precipitation. Individual major wetland substrates (chip bark, peat moss and hay) showed a large adsorption capacity for lead and zinc. Zinc was adsorbed more rapidly than lead. A pseudo-second-order equation best fitted the adsorption kinetics of lead while a pseudo-first-order equation described the adsorption kinetics of zinc. In the aerobic gravel lens area, lead and zinc were dominantly associated with Fe/Mn hydr(oxides). Kinetic test showed that lead and zinc can be co-precipitated with Fe/Mn hydr(oxides) in 20–30 minutes. Sulfide precipitation is a desired removal mechanism in constructed wetlands because of the low solubility of metal sulfides. Large numbers of sulfate reducing bacteria (SRB) and active sulfide generation were observed in all wetlands studied. Average concentrations of sulfide were 0.33 mg/L and 2.3 mg/L in the wetland effluents and in pore water, respectively. Sulfide can be generated continuously if sulfate is supplied to a wetland and biodegradable organic material is available. The rate of lead removal through sulfide precipitation was also determined. Based on the ratio of simultaneously extracted metals and acid volatile sulfide (AVS), lead retained in wetlands appeared less bioavailable than zinc.