Competitive sorption of copper and lead at the oxide-water interface: Implications for surface site density

The competitive sorption of Cu(II) and Pb(II) to colloidal hematite was inv estigated as a function of pH and total metal concentration. Acid-base titr ations of the hematite and single-metal sorption experiments for Cu and Pb at low to medium surface coverages were used to calibrate two surface compl exation models, the triple layer model, and a 2-pK basic Stern model with i on-pair formation. The surface site density was systematically varied from 2 to 20 sites/nm(2). Three different metal surface complexes were considere d: (1) an inner-sphere metal complex; (2) an outer-sphere metal complex; an d (3) an outer-sphere complex of singly hydrolyzed metal cations. Both mode ls provided excellent fits to acid-base titration and single-metal sorption data, regardless of the surface site density used. With increasing site de nsity, Delta pK of the stability constants for protonation reactions increa sed and metal surface complexes decreased steadily. The calibrated models b ased on different site densities were used to predict competitive sorption effects between Cu and Pb and single-metal sorption at higher total metal c oncentrations. Precipitation of oversaturated solid phases was included in the calculations. Best predictions of competitive sorption effects were obt ained with surface site densities between 5 and 10 sites/nm(2). The results demonstrate that surface site density is a key parameter if surface comple xation models are exposed to more complex, multicomponent environments. We conclude that competitive metal sorption experiments can be used to obtain additional information about the relevant surface site density of oxide min eral surfaces. Copyright (C) 1999 Elsevier Science Ltd.