Estudo voltamétrico da adsorção de Cd(II) e Zn(II) na microalga marinha Tetraselmis gracilis

Abstract

dos Santos Maguerroski, K.. A voltammetric study on the adsorption of Cd(II) e Zn(II) on marine microalgae Tetraselmis gracilis Proton and metal binding capacities of living cells of the microalgae Tetraselmis gracilis (Kylin) butcher were determined by potentiometric titration and anodic stripping voltammetry, respectively. Data of alkalimetric titration of the microalgae suspension obtained in 0.60 mol L NaCl at 25.0 ± 0.1C was treated by a discrete site distribution model based on the linearization of the titration curve by modified Gran functions. A total of 3.4 x 10 mmol g ionizable sites were determined, divided in four classes of groups with pKa 4.4, 5.2, 7.0 and 9.3, whose relative abundances were 13, 5.8, 8.2 and 73%, 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 free metal concentrations from the voltammetric data considered the cell-metal aggregates with diffusion coefficient significantly lower than that of free metal ions. Adsorption capacities and the logarithm of the conditional equilibrium constants were 7.9 ± 0.9 μmol g and 6.9 ± 0.3 L g for Cd(II) and 18.1 ± 0.4 μmol g and 8.8 ± 0.2 L g for Zn(II). For titrations with binary mixtures of Cd(II) and Zn(II) the adsorption capacity of Cd(II) were 0.129 ± 0.008 μmol g and 18.1 ± 0.4 μmol g, respectively. The results suggest that binding of Zn(II) inhibits that of Cd(II). However, even in the presence of Zn(II), the alga surface has some minor sites that can bind Cd(II) (log Kads = 7.7 ± 0.2), a process that can lead to bioaccumulation of this toxic metal ion.