Accumulation of heavy metals in aboveground biomass of Phragmites australis in horizontal flow constructed wetlands for wastewater treatment: A review

• Amount of heavy metals in plant tissue in constructed wetlands is highly variable.
• Heavy metals in aboveground plant tissue may represent up to 60% of the inflow load.
• Heavy metals in plant tissue may represent a large part of the removed elements.
• The highest accumulation in aboveground plant tissues was observed for zinc.

Wastewater treatment in constructed wetlands is a biotechnological process which has been used for more than five decades for wastewater treatment. It is generally agreed that plants are important part of the treatment system, however, the direct role of plants is usually restricted to plant uptake of nutrients and heavy metals. The purpose of this study was to evaluate the amount of heavy metals sequestered in the aboveground biomass of Phragmites australis and thus, available for harvest and removal. The survey revealed that the amount of heavy metals accumulated in the aboveground plant biomass (aboveground standing stock) represents often only small fraction of the inflow annual load but in some studies, this fraction is quite high, especially for zinc (up to 59%), more rarely for cadmium (55%) and chromium (38%). The amount of heavy metals sequestered in the plant shoots as a fraction of total removed heavy metal in the constructed wetland is variable with values as high as 71% for cadmium, 55% for chromium or 49% for zinc in some studies. However, there is still a large gap in our knowledge on heavy metal accumulation in aboveground tissues, namely the conditions that would promote heavy metal uptake and subsequent translocation to aboveground biomass.