This study demonstrated heavy metal removal from neutral mine drainage of a closed mine in Kyoto prefecture in pilot-scale constructed wetlands (CWs). The CWs filled with loamy soil and limestone were unplanted or planted with cattails. The hydraulic retention time (HRT) in the CWs was shortened gradually from 3.8 days to 1.2 days during 3.5 months of operation. A short HRT of 1.2 days in the CWs was sufficient to achieve the effluent standard for Cd (0.03 mg/L). The unplanted and the cattail-planted CWs reduced the average concentrations of Cd from 0.031 to 0.01 and 0.005 mg/L, Zn from 0.52 to 0.14 and 0.08 mg/L, Cu from 0.07 to 0.04 and 0.03 mg/L, and As from 0.011 to 0.006 and 0.006 mg/L, respectively. Heavy metals were removed mainly by adsorption to the soil in both CWs. The biological concentration factors in cattails were over 2 for Cd, Zn, and Cu. The translocation factors of cattails for all metals were 0.5–0.81. Sulfate-reducing bacteria (SRB) belonging to Deltaproteobacteria were detected only from soil in the planted CW. Although cattails were a minor sink, the plants contributed to metal removal by rhizofiltration and incubation of SRB, possibly producing sulfide precipitates in the rhizosphere.
A lab-scale experimental system accommodating soil and plants was designed to evaluate the applicability of constructed wetlands (CWs) to mine drainage treatment. Synthetic wastewater containing Cd (0.11 mg/L) and other minerals (pH 6.8) was prepared based on the chemical composition of an actual neutral mine drainage (NMD). In lab-scale CWs consisted of a column (ID 12.5 cm, H 50 cm) filled with pumice stones and loamy soil were planted reed (Reed-CW) or cattail (Cattail-CW) plants. Some were left unplanted (Unplanted CW). The synthetic NMD (2.0 L) was treated in a 1-week cycle sequencing batch mode in the CWs in a greenhouse. The unplanted CW removed cadmium sufficiently to satisfy the effluent standard (0.03 mg/L) from the NMD, mainly by soil adsorption. Presence of the emergent plants, especially cattail, enhanced metal removal possibly by filtration with their elongated roots and metal sulfide precipitation by sulfate-reducing bacteria in the rhizosphere of the Cattail-CW.
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