Cadmium biosorption and selectivity of sargassum spp. and their alginates in relation to their [alpha]-L-guluronic acid content and conformation

A series of equilibrium cadmium-binding experiments with raw species of Sargassum was performed in order to extend the database on the biosorption performance of this brown algae. The results indicate that their maximum cadmium uptake ranges from approximately 0.7 to 1.0 mmol Cd/gram biomass. A classical method for the extraction of alginate from brown algae was modified and applied to Sargassum species under high-temperature alkaline conditions (80°C, 2.0% Na2CO3). It yields alginate samples of low viscosity suitable for chemical characterization by direct acquisition of well-resolved NMR spectra. This new method also allowed the identification of an acid-soluble fraction not previously isolated by traditional methods, yet capable of binding cadmium.The 1H-NMR characterization of the alginates, extracted from Sargassum by the modified method, reveals a consistent and unusual enrichment in homopolymeric alpha-L-guluronic acid (G-blocks). These alginates display G-block diad frequency values (FGG) between 0.37 and 0.81. This translates into an enhanced selectivity for cadmium and calcium relative to monovalent ions such as sodium and the proton as well as smaller divalent ions such as magnesium. Results of competitive exchange experiments in the Cd-Ca-alginate system yield selectivity coefficients, K* CdCa, between 0.43 +/- 0.10 and 1.32 +/- 0.02 for a range of FGG, from 0.23 to 0.81. In contrast, much larger selectivity coefficients were obtained for the Mg-Ca-alginate (K*MgCa ≤ 18.0 +/- 1.4) and Mg-Cd-alginate (K*MgCa ≤ 16.0 +/- 0.9) systems with alginates extracted from Sargassum fluitans (FGG = 0.81) and Sargassum thunbergeii (F GG = 0.75). Selectivity studies with mixed-metal pair alginate systems, as opposed to the more common single metal methods, highlight the importance of the specific macromolecular conformation of the alginate polymer in determining metal binding behaviour in multiple-metal systems as well as the performance of Sargassum as an algal biosorbent. The unique composition of the alginates present in species of Sargassum and the resilience of the algae to degradation provide this bio-material with a distinct advantage over other brown algal species when considering its implementation for the strategic removal of toxic heavy metals from contaminated and industrial waste waters