The adsorption kinetics of metal ions onto different microalgae and siliceous earth

In the present work the adsorption kinetics of the six metal ions aluminum, zinc, mercury, lead, copper. and cadmium onto living microalgae were measu red. The freshwater green microalga Scenedesmus subspicatus. the brackish w ater diatom Cyclotella cryptica the seawater diatom Phaeodactylum tricornut um, and the seawater red alga Porphyridium purpureum were the subject of in vestigation. In most cases the adsorption rate of the metals could be well described by using the equation of the Langmuir adsorption rate expression. Inverse parameter estimation allowed the determination of the rate constan ts of the adsorption process and the maximum metal content of the algae. Th e highest values for the rate constant were obtained for Porphyridium purpu reum followed by Phaeodactylum tricormutum. High values for the maximum con tent were obtained for Cyclotella cryptica and Scenedesmus subspicatus. The maximum rate constant was 24.21 h(-1) for the adsorption of Hg to Porphyri dium purpureum whereas the maximum metal content (0.243 g g(-1)) was obtain ed for Zn on Cyclotella cryptica. A comparison of these values with those o btained for the mineral siliceous earth exhibiting low maximum content and high adsorption rates reveals that the mechanism of adsorption onto the alg ae is a mixture of adsorption and accumulation. (C) 2001 Elsevier Science L td. All rights reserved.