Constructed wetlands (CWs) have been applied to remediate heavy metal pollution effectively in practice. However, the heavy metal release from CWs has not been paid enough attention. In this study, a 5-month experiment was carried out with three parallel lab-scale vertical flow constructed wetlands (VFCWs) with zeolites as fillers. The artificial rainwater was pumped into VFCWs to study the release characteristic and mechanisms of heavy metals (Cu, Zn, Cr, and Pb). The results showed that significant amounts of Zn and Cu were released from the VFCWs at the end of the experiment while Pb and Cr rarely escaped. The upper layer (0-30 cm) of the VFCWs was the most effective area for heavy metal removal due to the presence of sediments, but it was also the most active area for heavy metal release. To explain this result, the sediments were analyzed before and after being leached by the tap water. The results indicated that Zn and Cu existed mainly in the exchangeable state, and they had strong leachability and bioavailability, causing its releases. Also, competitive adsorption of different metals meant that the metal ions with strong adsorption to zeolite caused the metal ions with weak adsorption to be desorbed from zeolites, and thus, a large amount of Zn escaped from VFCWs. The escape of heavy metals from CWs illustrated that it should be paid more attention in the management.
Keywords: Heavy metals; Release; Road runoff; Vertical flow constructed wetlands.