The effect of pH on the complexation of Cd, Ni and Zn by dissolved organiccarbon from leachate-polluted groundwater

Complexation of cadmium (Cd), nickel (Ni) and zinc (Zn) by dissolved organi c carbon (DOC) in leachate-polluted groundwater was measured using a resin equilibrium method. Metal-DOG complexation was measured at different DOG co ncentrations over a range of pH values. The results were compared to simula tions made by two speciation models (WHAM and MINTEQA2). Of these models, WHAM came closest to simulating the experimental observati ons although it systematically overestimated the pH dependence of metal-DOG complexation. Accepting a variation in the free metal ion activity of a fa ctor of 3-4 the WHAM model provided useful predictions of the complexation of Cd and Zn by DOG in the pH range 5-8, and of Ni in the pH range 5-7. At pH 8, however, the model overestimates the extent of Ni-DOC complexation to an unacceptable degree. The MINTEQA2 model predicts virtually no pH depend ence for DOC complexation of Cd, Ni and Zn and is thus in very poor agreeme nt with the experimental results. As an alternative approach, relations bet ween the conditional complex formation constant (log K-c) and pH were estim ated for each metal. Using these relations for estimating the complexation of Cd, Ni and Zn by DOC a deviation in the free metal ion activity up to a factor of 2 can be expected. (C) 2000 Elsevier Science Ltd. All rights rese rved.