Extraction, isolation and cadmium binding of alginate from Sargassum spp.

Sargassum brown algal species have recently shown promise for use in flow-through column systems that rely on a passive ion-exchange mechanism for the remediation of toxic heavy metals such as Pd, Cd, and Zn from contaminated waters. To elucidate the metal binding mechanism and optimise this so-called biosorption process, detailed information on the biochemistry of the raw biomass and the alginate in particular is essential. This study focuses on the detailed characterisation ( e. g., percentage of yield, block co-polymer structure) of the various fractions of material isolated from S. fluitans and S. oligocystum following a (i) standard neutral, (ii) alkaline ( NaOH) and (iii) high-temperature alkaline alginate ( 80degreesC; Na2CO3) extraction. Results indicate that the alginate yield was independent of the temperature or the extraction method employed ( 21.1 to 22.8% and 18.9 to 20.5% yields for S. fluitans and S. oligocystum, respectively). Furthermore, H-1-nuclear magnetic resonance ( NMR) analyses revealed that the alginates isolated by the three methods displayed nearly identical doublet alpha-L-guluronic acid frequencies ( F-GG; between 0.55 to 0.58 for both S. fluitans and S. oligocystum). Cadmium binding experiments ( pH 4.5) further demonstrated that the three alginate extracts have similar metal binding capacities ( uptake ranging from 1.59 to 1.81 mmol Cd/gram). The implementation of the high-temperature alkaline extraction procedure resulted in the isolation of a new acid-soluble fraction ( ASF), capable of binding cadmium at pH 4.5, which cannot be isolated by the standard neutral extraction protocol. A preliminary characterisation of this ASF revealed the presence of minor quantities of proteins and sulphated polysaccharides, as well as traces of alginate and possibly other low-molecular weight uronic acid-containing polymers.