Complexation of Cu and Pb by DOC in polluted groundwater: A comparison of experimental data and predictions by computer speciation models (WHAM and MINTEQA2)

Abstract The ability of dissolved organic carbon (DOC) from landfill leachate-polluted groundwater to form complexes with copper (Cu) and lead (Pb) was investigated using an equilibrium method based on cation exchange. The DOC was studied in the original matrix with a minimum of manipulation. In two leachate-polluted groundwater samples (S1 and S2) containing 180 and 80 mg C l −1 , respectively, more than 95% (in S1) and 90% (in S2) of Cu and Pb in solution were bound in DOC complexes. Even at relatively low DOC concentrations (40 mg C l −1 ) in a leachate-polluted environment more than 85% of the total Cu and Pb content in the solution were bound in DOC complexes. These data were used for evaluation of two speciation models and their default database (WHAM and MINTEQA2). The WHAM model overestimated the complexation of Cu and Pb in the groundwater samples, particularly at high degrees of complexation, where the predicted free metal ion activity was an order of magnitude lower than that observed in experiments. Much better agreement was obtained, however, by adjusting the default metal binding constants for fulvic acids in the WHAM model for Cu and Pb to 1.3. The MINTEQA2 model and its default database provided excellent prediction of Cu in sample S2 and Pb in sample S1 compared to the experimental results. For Cu in sample S1 and Pb in sample S2, however, the MINTEQA2 predictions deviated from the experimental results by amounts corresponding to a difference in the free metal ion activity of up to a factor of 2. Overall, the WHAM model with an adjusted database and the MINTEQA2 model with a default database offer very useful predictions on the extent of Cu and Pb complexation by dissolved organic carbon in leachate-polluted groundwater.