Lead and zinc removal by laboratory-scale constructed wetlands

Constructed wetlands have the potential to trap and remove metals in mine w astewater. To determine the effectiveness of constructed wetlands for treat ing selected heavy metals in neutral mine effluent typical of lead mines, e ight laboratory-scale constructed wetlands were set up to treat a synthetic , slightly alkaline, mine water containing 34.2 mg/L sulfate (SO42-), 50 mu g/L lead (Pb), and 300 mug/L zinc (Zn). After 45 days, one of the wetlands was switched to treat a synthetic smelter effluent with a much greater load of SO42-, sodium (Na+), and Pb. Temperature, hydraulic loading, and substr ate composition typically did not affect treatment efficiency. The pH of th e effluent was reduced from 8.0 to 8.5 to near neutral. The average removal in the eight wetlands was 90% for Pb and 72% for Zn. In wetlands operating on synthetic mine water, SO42- was completely removed, likely by conversio n to sulfide by sulfate-reducing bacteria. In the wetland operating on synt hetic smelter effluent, only approximately 25% of 6 g/L influent sulfate wa s removed, and a breakthrough period of 4 days for Na+ was observed. Whole effluent toxicity assays on undiluted wetland effluent from wetlands treati ng mine and smelter water had 100% survival of fathead minnows and Daphnia magnia. Survival of Ceriodaphnia dubia was zero in undiluted effluent, but 75 to 100% survival was observed when the effluent was diluted to one-half strength.