Ion interaction with carboxymethylagar: a polarographic study

Abstract

Synthetic polycarboxylates are currently used to complex and eventually precipitate metal ions for different purposes, e.g. in waste-water treatments [l]. High affinity complexes and the occurrence of specificity may allow the selective removal of ions. The search for more specific complexing agents and the need for biodegradable systems have driven the attention of researchers to natural polycarboxylates. Among them, carboxylate polysaccharides are known to form strong complexes with specific ions. Alginates, extracted from brown algae 121, and pectins, extracted from higher plants [3], are known to interact strongly with a number of divalent cations. In particular, calcium ions are able to induce the formation of strong gels in alginate or pectate aqueous systems [4]. The mechanism of ion-induced gel formation in these polysaccharide systems implies polymer chain rearrangements; in fact, parallel to polymer chain association, a disorder-to-order conformational transition occurs [5,6]. In addition to Ca2+, other divalent ions (e.g. Cu2+ and Ni2+) can form gels with the above-mentioned polysaccharides. These gels exhibit different gel strengths with respect to that measured with calcium ions, indicating different energetics of complexation. One of the advantages of natural polymers is the presence of peculiar stereochemical motives on the back-bone chain. These features can favour the occur-