The adsorption characteristics of δ-manganese dioxide: a collection of diffuse double layer constants for the adsorption of H +, Cu 2+, Ni 2+, Zn 2+, Cd 2+ and Pb 2+

Abstract

Thermodynamic data for all fate-determining processes are needed in order to predict the fate and transport of metals in natural systems. The surface complexation properties of a synthetic MnO 2, δ-MnO 2, have accordingly been investigated using glass electrode potentiometry. Experimental data were interpreted according to the surface complexation model in conjunction with the diffuse double layer model of the solid/solution interface. Adsorption constants were determined using the non-linear optimisation program FITEQL. Surface complexation parameters determined in this way were validated against results obtained from the literature. Best fits of alkalimetric titration data were obtained with a 2-site, 3 surface-species model of the δ-MnO 2 surface. Site concentrations of 2.23×10 −3 mol g −1 and 7.66×10 −4 mol g −1 were obtained. Corresponding logarithms of formation constants for the postulated surface species are −1.27 (≡XO −), −5.99 (≡YO −) and 3.52 (≡YOH 2 +) at I=0.1 M. The surface speciation of δ-MnO 2 is dominated by ≡XO − over the pH range investigated. Metal adsorption was modelled with surface species of the type ≡XOM +, ≡XOMOH, ≡YOM +, ≡YOMOH (M=Cu, Ni, Zn, Cd and Pb) and ≡XOM 2OH 2+ (M=Pb). For Cu, Ni and Zn, titration data could be modelled with ≡XOM +, ≡XOMOH, ≡YOM + and ≡YOMOH, whereas for Cd, ≡XOM + and ≡YOM + were sufficient. Lead data were best modelled by assuming the dinuclear species ≡XOM 2OH 2+ to be the only surface species to form. Adsorption constants determined for Ni, Cu and Zn follow the Irving-Williams sequence. The model suggests an adsorption order of (Pb, Cu) > (Ni, Zn) > Cd. The discrepancy between model predictions and published adsorption results is similar to the variability observed in experimental results from different laboratories.