Isotherms of adsorption of Cu(II) and Ni(II) onto solid Azraq humic acid (AZHA) were studied at different pH (2.0-3.7) values and 0.1 M NaClO4 ionic strength. The Langmuir monolayer adsorption capacity was found to range from 0.1 to 1.0 mmol metal ion/g AZHA, where Cu(II) has higher adsorptivity than Ni(II). The previously reported NICA-Donnan parameters for sorption of Cu(II) on HA fit the amount of Cu(bound) determined in the present study at pH 3.7 but underestimates those at pH values of 3.0, 2.4, and 2.0. The contribution of low affinity sites to binding of metal ions increases with decreasing pH and increasing metal ion loading. The aggregation of HA, which is facilitated by decreasing pH and increasing metal loading, may increase the ability of low-affinity sites to encapsulate metal ions. The binding of Ni(II) to HA exhibits less heterogeneity and less multidentism than that of Cu(II). AZHA loaded with Cu(II) and Ni(II) was found to be insoluble in water with no measurable amount of desorbed metal ions. (c) 2006 Elsevier Inc. All rights reserved.