Clay-humic substance complexes play a major role in controlling the mobility of toxic metals in contaminated soils However our understanding of the underlying mechanisms is limited Binding site analysis of clay and clay-mineral-humic composites in this study revealed an enhanced surface reactivity for the clay surface by the sorbed humic substances Kaolinite and illite had three binding sites with pK(a) values ranging from similar to 4 5 to 9 6 at Ca2+ concentrations of 0 01 and 0 1 M respectively In the presence of peat humic acid (PHA) four or five binding sites were observed for humics sorbed kaolinite surface at Ca2+ concentrations of 001 and 01 M respectively pK(a) values ranged from similar to 4 4 to 96 for humic acid concentration of 001 and 0 1 mg/mL For illite four or five binding sites were found with pK(a)s ranging from similar to 4 1 to 94 From zeta potential measurements of PHA-kaolinite or PHA-illite suspensions the already negative potential decreased by 30 mV from pH 4 to 7 and by 10 mV for pH values greater than 7 For illite the initial negative surface potential decreased by 15 mV up to a pH of 9 Above this pH the potential decrease diminished to 2 or 5 mV These changes in surface potential confirm the adsorption of PHA to the clay mineral surface FTIR measurements of clay samples were able to identify the kaolinite and like phases In addition FTIR absorption bands found in the range of 1950-1800 cm(-1) suggest the interaction of PHA with kaolinite and illite surfaces The results of this study indicate that the sorption of humic substances increases the availability of clay surface functional groups for deprotonation and potential sorption of toxic metal cations (C) 2010 Elsevier Inc All rights reserved