Metal retention and distribution in the sediment of a constructed wetland for industrial wastewater treatment

A free water surface wetland was built in 2002 to treat wastewater from a tool factory containing metals (Cr, Ni, Zn and Fe), nutrients and organic matter. Until 2006, the last reported period, the wetland retained metals and stored them primarily in the bottom sediment and in the biomass of macrophytes secondarily. The aim of this work was to study metal retention and distribution in the sediment of a constructed wetland for industrial wastewater treatment. Total concentrations and fractions (exchangeable, carbonate-bound, Fe-Mn oxides-bound, organic matter-bound and residual) of metals in sediment were analyzed in this treatment wetland, in order to estimate the fate of metals over time. Metal concentrations were significantly higher in the inlet than in the outlet sediment; concentrations in the latter remained without significant differences throughout the testing period. Metal concentrations and redox potential decreased with depth within the sediment. The lowest metal concentrations and pH and the highest redox values were attained in spring, in agreement with the period of maximum macrophyte growth. Ni and Zn were mainly stored associated with the carbonate fraction; Cr was mainly associated with the Fe-Mn oxides fraction, while Fe was mainly associated with the residual fraction, probably as pyrite. The incoming wastewater composition containing high pH, carbonate, calcium and Fe concentrations favored the observed association in the surface sediment. It would be expected that sediment will continue retaining metals in fractions that will not release them into the water while the chemical and environmental conditions remain unchanged.