Application of the Nernst–Planck approach to lead ion exchange in Ca-loaded Pelvetia canaliculata

Abstract

Ca-loaded Pelvetia canaliculata biomass was used to remove Pb 2+ in aqueous solution from batch and continuous systems. The physicochemical characterization of algae Pelvetia particles by potentiometric titration and FTIR analysis has shown a gel structure with two major binding groups – carboxylic (2.8 mmol g −1) and hydroxyl (0.8 mmol g −1), with an affinity constant distribution for hydrogen ions well described by a Quasi-Gaussian distribution. Equilibrium adsorption (pH 3 and 5) and desorption (eluents: HNO 3 and CaCl 2) experiments were performed, showing that the biosorption mechanism was attributed to ion exchange among calcium, lead and hydrogen ions with stoichiometry 1:1 (Ca:Pb) and 1:2 (Ca:H and Pb:H). The uptake capacity of lead ions decreased with pH, suggesting that there is a competition between H + and Pb 2+ for the same binding sites. A mass action law for the ternary mixture was able to predict the equilibrium data, with the selectivity constants α Ca H = 9 ± 1 and α Ca Pb = 44 ± 5, revealing a higher affinity of the biomass towards lead ions. Adsorption (initial solution pH 4.5 and 2.5) and desorption (0.3 M HNO 3) kinetics were performed in batch and continuous systems. A mass transfer model using the Nernst–Planck approximation for the ionic flux of each counter-ion was used for the prediction of the ions profiles in batch systems and packed bed columns. The intraparticle effective diffusion constants were determined as 3.73 × 10 −7 cm 2 s −1 for H +, 7.56 × 10 −8 cm 2 s −1 for Pb 2+ and 6.37 × 10 −8 cm 2 s −1 for Ca 2+.