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

Abstract

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 (80 °C; 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, ¹H-nuclear magnetic resonance (NMR) analyses revealed that the alginates isolated by the three methods displayed nearly identical doublet α-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.