The involvement of pectin in Sr2+ biosorption by Azolla

An effective removal process of Sr^sup 2+^ from polluted solution has been demonstrated by its binding to Azolla biofilter at high affinity, involving ion exchange with Na^sup +^, K^sup +^, Mg^sup 2+^ and Ca^sup 2+^. Maximal Sr^sup 2+^ binding capacity was as high as 3.5% per Azolla dry weight. The biosorption of Sr^sup 2+^ was optimal at neutral and high pHs, but lower at acidic pH, due to a competition between Sr^sup 2+^ and H^sup +^ ions on the cation binding groups of Azolla. Pectin, constituting 8-10.5% (w/w) of the Azolla cell wall, is shown to bind a major portion of Sr^sup 2+^ ions. Treatment with pectinase reduced the binding capacity of Azolla to Sr^sup 2+^. Methylation of Azolla biomass, known to block the carboxyl groups of pectin by esterification markedly reduced the Sr^sup 2+^ binding. Optimal Sr^sup 2+^ binding by Azolla at high pH values indicates deesterification of pectin and exposure of additional available carboxyl groups for metal binding.[PUBLICATION ABSTRACT]