Adsorption characteristics of delta-manganese dioxide: a collection of diffuse double layer constants for the adsorption of H+, Cu2+, Ni2+, Zn2+, Cd2+ and Pb2+

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 MnO2, delta -MnO2, 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 delta -MnO2 surface. Site concentrations of 2.23 x 10(-3) mol g (-1) and 7.66 x 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 (=YOH2+) at I = 0.1 M. The surface speciation of delta -MnO2 is dominated by =XO- over the pit range investigated. Metal adsorption was modelled with surface species of the type =XOM+, =XOMOH, =YOM+, =YOMOH (M = Cu, Ni, Zn, Cd and Pb) and =XOM2OH2+ (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 =XOM2OH2+ 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.