In this study, a Donnan-type model for nonspecific binding of electrolyte ions has been combined with the nonideal competitive adsorption(NICA) model for specific binding to humic substances. The model considers site heterogeneity, non-ideality, multicomponent competition, and electrostatic interactions. The NICA-Donnan model predicted reasonably well the increase in Cd binding on changing from 0.025M Ca(NO3)2.4H2O background electrolyte to 0.006M Ca(NO3)2.4H2O and also the increase in Cd binding by rising pH. Results indicate the most of the cadmium binding by fertilizer humic acid is related to specific sorption (more than 85%). Increasing the ionic strength from 0.006M to about 0.025M leads to a significant decrease in Cd binding on carboxylic groups in compare with on phenolic groups. It can be related to prefer of calcium binding by carboxylic groups in comparison with phenolic groups.
