Competitive metal ion adsorption in goethite systems using in situ voltammetric methods and potentiometry

Competitive complexation with respect to the binary-metal ion combinations Cu(II)-Zn(II), Cu(II)-Pb(II), and Pb(II)-Zn(II) were studied at the goethit e (alpha-FeOOH)-water interface (25 degrees C, I = 0.1 M NaNO3). In additio n to potentiometric titrations and batch adsorption experiments, an in situ voltammetric technique was utilized. Sorption studies were performed withi n the ranges 3.5 less than or equal to pH less than or equal to 8.5, 10(-6) less than or equal to [M(II)](tot) less than or equal to 10(-3) M, and wit h solid concentrations between 0.09 and 9 g dm(-3). The results obtained (m etal uptake and proton release curves) were compared with corresponding pre dicted values, based upon previously determined thermodynamic surface compl exation models for the different single-metal ion goethite systems. The sur face complexation modeling was based upon the constant capacitance concept, The results showed that the Cu(II) and Zn(II) surface complexation submode ls were combinatory in all two-metal systems studied, both at 10(-3) M and at 10(-6) M concentrations of the metals. For Pb(II) a somewhat stronger ad sorption to goethite than predicted by the previously determined submodel w as observed in all two-metal systems studied, These deviations could be cor rected for by a moderate adjustment of the equilibrium constant for the mos t acidic surface complex =FeOHPb2+. It can also be concluded that the in si tu voltammetric technique can be applied to measure trace concentrations of at least two metal ions simultaneously occurring in goethite suspensions u nder varying experimental conditions with good accuracy. (C) 1999 Academic Press.