Complexation of Al{sup}(3+) by NOM will change the speciation of Al and influence its toxicity. Competitive binding data between Al and metal ions to humic acids are needed to understand changes in speciation and mobility. We also need to investigate the effect of changes in electrolyte concentration on the binding of Al, Pb and Cd to humic acid. Competition experiments were performed at various pH and electrolyte concentrations using Cd and Pb ion selective electrodes to measure free metal ion concentrations. The effect of aluminum on the lead binding is important; it can reduce the amount of lead bound by a factor of 2 to 3. In the absence of Al, an increase of ionic strength also leads to a reduction of Pb bound. For cadmium, similar results are obtained. The NICA-Donnan model is used to describe the Al and Pb binding to humic acids and to predict Pb-Al as well as Cd-Al competition without parameter adjustment. With NICA-Donnan model, we calculate the speciation of the metal ions in the system and quantify the contribution of specific binding and electrostatic binding as pH, electrolyte and metal ion concentrations change during the experiments. In a natural environment, Al competition will have an effect on Pb toxicity and on Cd transport.
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