Proton and metal binding capacities of living cells of the microalgae Tetraselmis gracilis were determined by potentiometric titration and anodic stripping voltammetry, respectively. Binding of Cd(II) and Zn(II) was studied in seawater (pH 8.2) by additions of either single metallic species or binary mixtures. Computation of metal concentrations considered the cell-metal aggregates as slow diffusing. Adsorption capacities and conditional equilibrium constants were 2.9 ± 0.3 µmol g-1 and 6.9 ± 0.3 L g-1 for Cd(II) and 18.5 ± 0.9 µmol g-1 and 8.8 ± 0.2 L g-1 for Zn(II), respectively. For titrations with binary mixtures of Cd(II) and Zn(II), the adsorption capacity of Cd(II) decreased to 0.14 ± 0.01 µmol g-1 and that for Zn(II) to 15.9 ± 1.1 µmol g-1. The results suggest that binding of Zn(II) inhibits that of Cd(II). Under conditions of depleted Zn(II), the alga surface binds Cd(II), a process that can lead to bioaccumulation.
anodic stripping voltammetry; biosorption; dynamic speciation; marine microalgae
