Mechanism and performance of trace metal removal by continuous-flow constructed wetlands coupled with a...

Constructed wetlands coupled with a micro-electric field (CW-MEF) is a novel and efficient water treatment technology. The objective of this study was to investigate the mechanism and performance of trace metals (TMs) removal for CW-MEF systems during summer and winter. The mass distribution of TMs in plants and biofilms, physiological indices of wetland plants, and bacterial community structures on electrodes and in the rhizospheres were analyzed as well as to explore further the TM removal mechanism. Results show that the electric field intensities (EFI) of 100 and 200 mV cm^-1 had a significantly promoting effect on TM removal. Maximum removal efficiencies for Cu, Zn, Cd, Co, Ni and Pb were 95.6, 80.1, 74.0, 67.1, 69.8 and 99.6%, respectively, in summer with a 5d-hydraulic retention time (HRT). An EFI of 100 mV cm^-1 could alleviate the oxidative damage in plant cells by promoting the synthesis of reduced glutathione and an activity increase of catalase, thus increasing the phytoextraction for Cu, Zn and Cd. For biofilms, the MEF caused shifts in the bacterial community structures, and an EFI of 50 to 200 mV cm^-1 significantly promoted the enrichment of Cu, Zn, Cd and Co by biofilms. Moreover, microorganisms related to TM tolerance and enrichment exhibited a high abundance with an EFI of 100 and 200 mV cm^-1. It can be concluded that introducing MEF to CWs could intensify the TMs removal via the biological process and result in more efficient purification for TM-containing wastewater.