Modeling equilibrium and kinetics of metal uptake by algal biomass in continuous stirred and packed bed adsorbers

Physical and chemical characterization of algae Gelidium particles shows a gel structure, with two major binding groups, carboxylic and hydroxyl groups, with an affinity constant distribution for protons, well described by a Quasi-Gaussian distribution suggested by Sips. A continuous model, considering a heterogeneous distribution of the carboxylic groups, determined by potentiometric titration experiments, was able to predict equilibrium data at different pH. The metal uptake capacity decreases with the solution pH, suggesting that competition exists between hydrogen ions, present in high concentrations for low pH values, and metal ions. For high ionic strengths, adsorption sites will be surrounded by counter ions and partially lose their charge, which weakens the contribution of the electrostatic binding and decreases the overall adsorption. A small influence of the temperature in the adsorption process was observed. Batch kinetic experiments were also performed, at different pH values, and results were well fitted by a mass transfer model, considering the intraparticle diffusion resistance given by the linear driving force model (LDF). Continuous stirred adsorber (CSTA) and packed bed column configurations were also tested for metal adsorption. The biosorbent regeneration was achieved by contacting it with strong acid (0.1 M HNO₃). A mass transfer model was applied with success to describe the biosorption/desorption process in CSTA and packed bed column, considering the equilibrium given by the Langmuir equation/mass action law and film and intraparticle diffusion resistances.