Adsorption of metals and protons on Gloeocapsa sp. cyanobacteria: A surface speciation approach
Résumé
The purpose of the present work is to extend our knowledge of metal–cyanobacteria interactions and to contribute to the database on adsorption parameters of aquatic microorganisms with respect to metal pollutants. To this end, the surface properties of the cyanobacteria (Gloeocapsa sp. f-6gl) were studied using potentiometric acid–base titration methods and ATR-FTIR (attenuated total reflection infrared) spectroscopy. The electrophoretic mobility of viable cells was measured as a function of pH and ionic strength (0.01 and 0.1 M). Surface titrations at 0.01–1.0 M NaCl were performed using limited residence time reactors (discontinuous titration) with analysis of Ca, Mg and dissolved organic C for each titration point in order to account for alkali-earth metal–proton exchange and cell degradation, respectively. Results demonstrate that the cell-wall bound Ca and Mg from the culture media contribute to the total proton uptake via surface ion-exchange reactions. This has been explicitly taken into account for net proton balance calculations. Adsorption of Zn, Cd, Pb and Cu was studied at 25 °C in 0.01 M NaNO3 as a function of pH and metal concentration. The proportion of adsorbed metal increases as a function of culture age with cells of 44 days old having the largest adsorption capacities. A competitive Langmuir sorption isotherm in conjunction with a linear programming method (LPM) was used to fit experimental data and assess the number of surface sites and adsorption reaction constants involved in the binding of metals to the cyanobacteria surface. These observations allowed the determination of the identity and concentration of the major surface functional groups (carboxylate, amine, phosphoryl/phosphodiester and hydroxyl) responsible for the amphoteric behavior of cyanobacterial cell surfaces in aqueous solutions and for metal adsorption. Results of this work should allow better optimizing of metal bioremediation/biosequestration processes as they help to define the most efficient range of pH, cell biomass and duration of exposure necessary for controlled metal adsorption on cyanobacteria cultures. It follows from comparison of adsorption model parameters between different bacteria that technological application of cyanobacteria in wastewater bioremediation can be as efficient as other biological sorbents.