Fig. 3. Model A applied to metal binding data; the bound species considered included: monodentate Cd–carboxyl complex (CdC), bidentate Cd–carboxyl complex (CdCC), and bidentate Cd–(carboxyl + phenolic hydroxyl) complex (CdC). The background electrolyte was 0.1 M NaNO₃. Data are presented as the natural log of the ratio of bound to free Cd concentrations ([Cd]_bound/[Cd]_free), modeled vs. measured. The concentration of Cd_bound includes all Cd–humic acid complex forms and Cd held as a counter ion; Cd_free includes all free inorganic species. Different symbols show individual data subsets. The solid line indicates a 1:1 relationship. The values of the optimized Model A parameters were: –log intrinsic stability constant for metal complexation to a monodentate carboxyl binding site = 0.290, –log intrinsic stability constant for metal complexation to a monodentate phenolic hydroxyl binding site = 4.76, proportion of carboxyl groups capable of forming bidentate chelates = 0.051, proportion of monodentate binding carboxyl groups capable of forming bidentate chelates to a phenolic hydroxyl group = 0.950, and proportionality factor F_W = 0.169; residual standard deviation = 0.519, n = 102.