Insights into trivalent chromium biosorption onto protonated brown algae Pelvetia canaliculata: Distribution of chromium ionic species on the binding sites

Abstract

In the present study, biosorption of trivalent chromium by protonated brown algae, Pelvetia canaliculata, was studied in batch system. FTIR analyses provided information about the possible binding groups present in the algae, as carboxylic, hydroxyl and sulfonate groups. Potentiometric acid-base titrations showed a heterogeneous distribution of two major binding groups, carboxyl and hydroxyl ones, following the Quasi-Gaussian affinity constant distribution suggested by Sips, which allowed to estimate the maximum amount of acidic functional groups (2.26 +/- 0.02 mmol g(-1)) and proton binding parameters (pK'(H) = 3.18 +/- 0.01; m(H) = 0.78 +/- 0.03) for an ionic strength of 0.2 M. The trivalent chromium removal was found to increase with pH and the maximum chromium uptake was observed at pH 4.0 (similar to 0.6 mmol/g), corresponding to 1.34 meq/g, since at that pH, 25.8% of total chromium is in the Cr3+ form and 72.7% as CrOH2+. An equilibrium model incorporating the hydrolysis reactions that chromium undergoes in the aqueous phase was able to predict the chromium biosorption at different pH values and chromium concentrations, enabling the prediction of the distribution of chromium ionic species on the binding sites. A mass transfer model provided a good representation of the chromium biosorption kinetics, resulting in intraparticle homogeneous diffusion coefficients of 4.6 x 10(-7) cm(2)/s for Cr3+ and 1.8 x 10(-8) cm(2)/s for CrOH2+. The distribution of chromium ionic species in the solution and on the binding sites was also predicted by the kinetic model.