Surface Complexation Modeling of Proton and Cd Adsorption onto an Algal Cell Wall

This study quantifies Cd adsorption onto the algal species Pseudokirchneriella subcapitata's cell wall by applying an equilibrium thermodynamics approach to model the observed adsorption behavior. Potentiometric titrations are used to determine deprotonation constants and site concentrations for the functional groups on the algal cell wall. Adsorption experiments conducted as a function of pH and total Cd concentration yielded the stoichiometry and site-specific stability constants for the important Cd-algal surface complexes. The acid/base properties of the cell wall are modeled best by invoking four discrete surface functional group types. Application of this surface complexation modeling approach enables quantitative assessment of algal adsorption effects on aqueous metal mobilities. The results suggest that the stabilities of the Cd-surface complexes are high enough for algal adsorption to affect the fate and transport of Cd, but that on a per gram basis, bacteria exhibit higher buffering capacities and stabilities of the Cd-cell wall surface complexes.