Metal speciation in polluted waters and the role of EDTA

The environmental fate and effects of pollutant metals in aquatic systems depend on speciation. Complexation by organic ligands can enhance the mobility of pollutant metals in wastewater treatment plants (WWTPs) and reduce their toxicity to aquatic organisms in receiving waters. Although metal speciation techniques have shown that metals in WWTPs and polluted surface waters frequently are associated with strong ligands, it is difficult to improve metal removal efficiency within WWTPs or accurately predict the behavior of pollutant metals in the aquatic environment without determining the identity of the ligand.; New analytical methods are developed for quantifying the concentrations of metal-ethylenediaminetetraacetate (EDTA) complexes in wastewater and polluted surface waters. Measurements of CuEDTA2–, NiEDTA 2–, and ZnEDTA2– are compared with metal speciation results obtained using competitive ligand exchange techniques ( i.e., chelating resin-column partitioning and cathodic stripping voltammetry). This comparison indicates that the strong metal complexes of Ni and Zn detected in wastewater consist mainly of metal-EDTA complexes. Although complexation by EDTA is likely to account for the strong Cu complexes in wastewater, concentrations of Cu were generally too low to determine CuEDTA2– .; To examine the relationship between pH and EDTA speciation within a WWTP, a series of four municipal WWTPs are studied. Equilibrium calculations indicate that most of the EDTA should consist of CuEDTA2–, NiEDTA 2–, and ZnEDTA2– at pH values above 7.0, while FeEDTA complexes (i.e., FeEDTA– and FeOHEDTA2–) should dominate EDTA speciation at lower pH values. Measurements of metal-EDTA complexes in WWTPs suggest that the pH depression experienced during oxygen activated sludge treatment can convert CuEDTA2– and ZnEDTA2– into FeEDTA complexes, which remain stable even after the pH returns to higher values. No change in speciation is observed in air activated sludge plants, which lack this pH change. Since pollutant metal-EDTA complexes are nearly impossible to remove during wastewater treatment, this may lead to an overall improvement in metal removal during wastewater treatment.; Concentrations of NiEDTA2– are not affected by pH changes during wastewater treatment. Due to kinetic limitations, NiEDTA 2– is stable during wastewater treatment and after discharge to aquatic systems. Measurements of Ni speciation in San Francisco Bay indicate a seasonal variation consistent with the discharge of stable NiEDTA 2– from wastewater effluents mixing with weaker Ni complexes from surface runoff.