Surface binding site analysis of Ca2+-homoionized clay–humic acid complexes

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 pKa values ranging from ∼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 0.01 and 0.1 M respectively. pKa values ranged from ∼4.4 to 9.6 for humic acid concentration of 0.01 and 0.1 mg/mL. For illite, four or five binding sites were found with pKas ranging from ∼4.1 to 9.4. 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 illite 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.

FTIR measurements of peat humic acids (PHA) sorbed to kaolinite clay showing the presence of organic functional groups from the adsorbed PHA.Figure optionsDownload high-quality image (55 K)Download as PowerPoint slideResearch highlights
► Humic substances sorb to the surface of calcium homoionized kaolinite and illite clay minerals.
► The overall surface reactivity (binding site concentration) of the clay–humics complex increases with respect to that of clay alone upon sorption of organic matter in a non-additive manner.
► An increase in the number of functional groups available for deprotonation and potential heavy metal sorption is observed upon humics sorption with respect to the number of functional groups on the bare clay surface.