Metal Selectivity of Sargassum spp. and Their Alginates in Relation to Their α-l-Guluronic Acid Content and Conformation

The discovery of a consistent and unusual enrichment in homopolymeric α-l-guluronic acid G-blocks in alginates extracted from a suite of Sargassum brown algae is described in this study. 1H NMR spectroscopy was used to characterize these alginates which display homopolymeric guluronic acid block (G-block) frequency values (FGG) between 0.37 and 0.81. The presence of these G-blocks results in an enhanced selectivity for cadmium or 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 for the Cd−Ca−alginate system yield selectivity coefficient, K*CdCa, values between 0.43 ± 0.10 and 1.32 ± 0.02 for a range in FGG of 0.23 to 0.81. In contrast to the Cd−Ca−alginate system, the Mg−Ca−alginate and Mg−Cd−alginate systems yielded maximum values of K*MgCa (18.0 ± 1.4) and K*MgCd (16.0 ± 0.9) for the alginates extracted from Sargassum fluitans (FGG = 0.81; Cuba) and Sargassum thunbergii (FGG = 0.75; Korea), r...