Dynamic interactions of Hg(II) with the surface of green microalgae Chlamydomonas reinhardtii studied by stripping chronopotentiometry

Green microalgae are well known to bioaccumulate potentially toxic metals, which can be magnified along the trophic chain. Therefore, they also have the potential to be used as bioadsorbents for removal of toxic metals. Interaction of Hg(II) with live Chlamydomonas reinhardtii was investigated for the first time by stripping chronopotentiometry, enabling the quantification of the reactive metal fractions in presence of the microalgae. This approach allowed the computation of the Hg(II) fraction able to dissociate from metal-cell surface complexes due to the concentration gradient imposed by the deposition potential applied to the working electrode. Equilibrium was reached in 120min. Biosorption isotherms at pH4.0, 5.0 and 6.0 were fitted using the Langmuir-Freundlich equation, showing the highest biosorption capacity, (3.3±0.9) mmolg−1, at pH4.0. The mass balance of Hg(II) in filtered solution plus in digested microalgae (102±8%) indicated that most Hg(II) was retained in the cells instead of being released to the surrounding environment, an interesting feature for biosorbents. However, for initial Hg concentrations >20μmolL−1 the determined Hg(II) concentrations in presence of the cells were 2 to 6 times higher than those determined in the filtered solutions, suggesting a partial lability of the surface-bound Hg(II) which should be considered in bioremediation technologies.