Cadmium removal by constructed wetlands containing different substrates: performance, microorganisms and mechanisms

This study compared the cadmium (Cd) removal performance of constructed wetlands (CWs), including gravel (G-CW), magnetite (M-CW), coconut shell (C-CW) and biochar (B-CW). C-CW exhibited superior removal efficiencies for Cd compared to other CWs, with efficiencies of 93.18 %.C-CW benefited from the rich organic matter of coconut shells and enhanced DO consumption levels, which facilitated microbial and plant removal of Cd. The total accumulation of Cd in the substrate increased from 9.16 mg/kg to 30.66 mg/kg. Concurrently, the percentage of Cd in the organic matter-bound and residue states increased from 20.52 % to 37.56 %, which effectively reduced the bioavailability of Cd. All CWs can ensure that the plant antioxidant system is not subjected to Cd stress. Saccharimonadales and Micropruina became the dominant genera in all CWs, exhibiting a high tolerance to Cd. This study provides new understanding and theoretical support for selecting substrates to effectively treat heavy metals wastewater with CWs.