Contribution of sulfonate groups and alginate to heavy metal biosorption by the dry biomass of Sargassum fluitans

The mechanism of heavy metal complexation by the dry biomass of the brown seaweed Sargassum fluitans was investigated at the molecular level using different techniques. Simultaneous potentiometric and conductimetric titrations gave some information concerning the amount of strong and weak acidic functional groups in the biomass (0.25 +/- 0.05 mequiv/g and 2.00 +/- 0.05 mequiv/g, respectively). Those results were confirmed by the chemical identification of sulfonate groups (0.27 mequiv/g +/- 0.03) and alginate (45% of the dry weight) corresponding to 2.25 mmol of carboxyl groups/g of biomass. Modification of these functional groups by methanolic hydrochloride or propylene oxide demonstrated the predominant role of alginate in the uptake of cadmium and lead. However, sulfonate groups can also contribute, to a lower extent, to heavy metal binding, particularly at low pH. Eventually, FTIR spectrophotometry on protonated or cadmium-loaded alginate and S. fluitans biomass physically demonstrated that cadmium binding arises by bridging or bidentate complex formation with the carboxyl groups of the alginate.