Constructed wetlands: fundamental processes and mechanisms for heavy metal removal from wastewater streams

Abstract

Metals are immobilised and retained within the sediment and filter materials of constructed wetlands through a range of processes. Major processes include: 1) sedimentation; 2) sulphate reduction with subsequent precipitation of metal sulphides; 3) hydrolysis and precipitation of Al and Fe hydrous oxides with co-precipitation and/or adsorption of other metals; 4) metal adsorption to the active surfaces of the filter material. Their importance varies depending on wastewater composition, the type of wetland and its operational parameters. Whereas adsorption has a finite capacity for metal sequestration, precipitation reactions can continue to remove metals as long as conditions remain favourable. The possible remobilisation of sequestered metals under changed conditions, during wetland operation or afterwards, is of environmental concern and is an aspect that requires future research. For improved performance, designers and operators need to tailor wetlands to favour the most effective sequestration reactions taking into consideration the composition of the wastewater.