Distribution and migration of heavy metals in the sediment-plant system: Case study of a large-scale constructed wetland for sewage treatment

Constructed wetlands are extensively applied in wastewater treatment and water ecosystem restoration. How-ever, the characteristics of heavy metals accumulation and migration in a long-running large-scale constructed wetland for wastewater treatment remain unclear. In this study, the variation of heavy metals (Cu, Zn, Pb, Cd, Cr, and As) in the sediment-plant system of a wetland that has been operating for 14 years was quantified. Results show that the sediments of the constructed wetland were the sink for heavy metals. All heavy metals, except As, significantly increased (P < 0.05) in sediments within 0-40 cm depth, and Zn and Cr had leaked to 40-60 cm depth (P < 0.05). Along with the surface flow direction, heavy metal concentrations mostly showed a declining trend, and in comparison, Cu and Cr transported longer distances. Bioconcentration factors show that the two common wetland plants, Phragmites australis and Typha latifolia, exhibited obvious differences in enrichment performance of heavy metals, with the orders of Zn > Cr > Cd > Cu > Pb > As and Cd > Zn > Cr > Cu > As > Pb, respectively. The translocation factors of the two kinds of plants were less than 1 suggesting that they are suitable for phytostabilization. Redundancy analysis indicates that sediment organic matter was the primary environ-mental factor affecting the distribution and migration of heavy metals in the wetland system. The discrepancy in the migration characteristics of pollutants, especially heavy metals, should be seriously considered in the design and management of wetland systems, including highly-enrichment plants, appropriate hydraulic residence time, and effective surficial filling medium.

Automated summary

This study investigated the variation of heavy metals in a constructed wetland that has been operating for 14 years. The results showed that the sediments of the wetland acted as a sink for heavy metals, with concentrations decreasing along the flow direction. The two common wetland plants exhibited different enrichment performances of heavy metals, and were suitable for phytostabilization. Sediment organic matter was identified as the primary factor affecting the distribution and migration of heavy metals in the wetland system.