PROTON AND COPPER-BINDING BY HUMIC-ACID - APPLICATION OF A DISCRETE-SITE ELECTROSTATIC ION-BINDING MODEL

A discrete-site/electrostatic model of ion binding by humic substances has been applied to proton- and copper-binding data for a soil humic acid. The proton data cover the pH range 3-9 and the ionic strength ra nge 0.001 M-0.1 M, while those for copper refer to the pH range 4-5, i onic strengths between 0.005 M and 0.1 M, and p[Cu] values between 3 a nd 8. The model is able to explain the major observed trends, includin g the dependence of proton and copper binding on ionic strength and th e binding of copper as a function of pH. However, the calculated ionic strength dependence of copper binding is slightly less than that obse rved. In addition, the model has been used to predict ratios of proton s released to copper bound under different conditions, on the basis of the separately estimated parameters for proton and copper binding, Th e model correctly predicts the ratios to be between 1 and 2, and to de crease with increasing bound copper and with pH.