Metal and proton binding onto the roots of Fescue rubra

This study quantifies Pb and Cd adsorption onto the root cell walls of the grass species Festuca rubra by applying a surface complexation approach to model the observed adsorption behavior. We use potentiometric titrations to determine deprotonation constants and site concentrations for the functional groups on the root material. We model the acid/base properties of the root cell wall with a non-electrostatic model involving three discrete surface functional group types, with pK_a values of 4.2 ± 0.1, 6.2 ± 0.2, and 8.8 ± 0.2. Pb adsorption kinetics experiments indicate that the experimental systems reach equilibrium within 3 h, and Pb desorption experiments indicate that the adsorption reactions are fully reversible on this time scale. Adsorption experiments conducted as a function of pH yield site-specific stability constants for the important Pb- and Cd-root surface complexes. The results of the Pb and Cd adsorption experiments indicate that the first two sites contribute to metal adsorption under the experimental conditions, with calculated log stability constant values of 4.1 ± 0.3 and 5.9 ± 0.3 for the Pb system; and 3.5 ± 0.4 and 5.0 ± 0.5 for the Cd system. Our results suggest that the stabilities of the Pb- and Cd-surface complexes are comparable to those involving other biomass surfaces, and therefore these complexes may affect the transport and distribution of metals in soil systems.