Constructed wetlands for metallic wastewater treatment: An updated global profile

Constructed wetlands (CWs) are increasingly used to treat heavy metals (HMs) wastewater. To reveal the nature, behaviour and the removal mechanisms of HMs in CWs, literature of the last four years was reviewed to map the most recent profile on CWs treatment of HMs. Bibliometric analysis on the research trends shows that there is increasing interest with more concentrated research on Cu, Zn, Cr, and Pb. The mechanisms of HMs removal by CWs include adsorption, filtration, precipitation, co-precipitation, and ion exchange, while the substrates adsorption plays a major role in the removal of HMs. These substrates can be categorized as adsorption substrates and alkaline substrates. Forty-five species of wetland plants were used for treating HMs in the last four years. More HMs are usually accumulated in the plants roots compared to the aboveground parts. As an important part of CWs, microorganisms play a considerable role in HMs removal, including adsorption of HMs by biofilm, promotion of plant growth, enhancement of plant resistance to HMs, and oxidation-reduction reaction to generate HMs precipitation. In addition, the newly emerged coupling technique of CWs and microbial fuel cell owns a high removal rate of HMs. This provides a new way, deserving further investigation.