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

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Abstract
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), respectively. Selectivity studies with mixed-metal pair alginate systems highlight the importance of the specific macromolecular conformation of the alginate polymer in determining metal binding behavior in multiple-metal systems. Furthermore, they demonstrate the importance of the conformation of the alginate as it occurs within the tissue of Sargassum in determining the metal binding behavior of this algal biosorbent. The unique composition of the alginates present in species of Sargassum may represent a distinct advantage over other brown algal species when considering their implementation for the strategic removal of toxic heavy metals from contaminated and industrial wastewaters.