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

Complexation of cadmium (Cd), nickel (Ni) and zinc (Zn) by dissolved organic carbon (DOC) in leachate-polluted groundwater was measured using a resin equilibrium method. Metal-DOC complexation was measured at different DOC concentrations over a range of pH values ((5), (6), (7) and (8)). The results were compared to simulations made by two speciation models (WHAM and MINTEQA2). Of these models, WHAM came closest to simulating the experimental observations although it systematically overestimated the pH dependence of metal-DOC complexation. Accepting a variation in the free metal ion activity of a factor of 3–4 the WHAM model provided useful predictions of the complexation of Cd and Zn by DOC 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 dependence for DOC complexation of Cd, Ni and Zn and is thus in very poor agreement with the experimental results. As an alternative approach, relations between the conditional complex formation constant (log Kc) and pH were estimated 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.