Processes of Transformation and Removal of Metals in Constructed Wetlands for the Treatment of Effluents from Metallurgical Plants

Abstract

This chapter describes the processes of transformation and removal of metals in constructed wetlands for wastewater treatment.These processes are described focusing on the role of wetland sediment/substrate. Moreover, two study cases of treatment wetlands are presented. The bottom sediment is primarily responsible for the removal of contaminants from water. Due to the electrochemical properties of the sediment surface, the solid phase of the wetlands controls the adsorption, transformation, and release of metals to water. All these processes are governed by the properties of the sediment, being redox potential and pH the most important ones. Other properties of the sediment that play important roles in the behavior of trace elements are: cation exchange capacity, carbonates, Fe and Mn oxyhydroxides, clay minerals and fine size fractions. Two study cases in the metallurgical industry are described: 1) A wetland constructed for the treatment of effluents from a tool-manufacturing plant, and 2)A wetland constructed for the treatment of effluents from a chrome-plating plant for large pieces. Both systems are free-water surface (FWS) constructed wetlands. Wastewater (with high pH and salinity containing Fe, Cr, Ni and Zn) and sewage were treated together. Contaminant removal efficiencies and contaminant retention by the sediment and/or plants are compared.