Temporal distribution of Cr, Ni and Zn in the sediment of a constructed wetland

DI LUCA, G.A.; MAINE, M.A.; MUFARREGE, M.M.; HADAD,H.R.

YORK

Simposio; 6th International Symposium on Wetland Pollutant Dynamics and Control. Constructed Wetland Association. WETPOL; 2015

The aim of this work was to study the accumulation and speciation of Cr, Ni and Zn in the sediment of a free water surface wetland constructed for the wastewater treatment at a metallurgical industry along time to predict whether metals continue to be retained or could be released to the water. The studied period covers 2007 to the present. Surface sediment samples (0-3cm) were collected in the inlet and outlet areas of the wetland by duplicate using a 3cm diameter, 10cm long PVC corer. Redox potential (Eh) and pH of the sediment, Organic matter content (LOI) were determined. The constructed wetland continued removing metals after 13 years of operation. Metals have been accumulated in the inlet sediment while the outlet concentrations do not differ significantly with those at the beginning of the operation period. It would be suggested that there is still a long effective life time because even the inlet sediment is still retaining, and apparently, it has not reached saturation yet. However, the wetland removal capacity seems more related with the incoming wastewater composition and the macrophyte development than with the substrate nature: in the inlet, Zn was mainly bound to the carbonate fraction; Cr and Ni were mainly bound to the Fe-Mn oxide fraction. Such pattern was determined by the high pH, Ca2+, carbonate and Fe concentration of the wastewater that caused CaCO3 and Fe(OOH)n precipitation and the subsequent metal sorption to their surfaces. Metals have been retained by sediment fractions that will not release them into the water while the chemical and environmental conditions remain unchanged. In consequence, the removal would be sustainable in time as long as the incoming wastewater composition is maintained.