The involvement of pectin in Sr2+ biosorption by Azolla

An effective removal process of Sr2+ from polluted solution has been demonstrated by its binding to Azolla biofilter at high affinity, involving ion exchange with Na+, K+, Mg-2+ and Ca2+. Maximal Sr2+ binding capacity was as high as 3.5% per Azolla dry weight. The biosorption of Sr2+ was optimal at neutral and high pHs, but lower at acidic pH, due to a competition between Sr2+ and H+ 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 Sr2+ ions. Treatment with pectinase reduced the binding capacity of Azolla to Sr2+. Methylation of Azolla biomass, known to block the carboxyl groups of pectin by esterification markedly reduced the Sr2+ binding. Optimal Sr2+ binding by Azolla at high pH values indicates deesterification of pectin and exposure of additional available carboxyl groups for metal binding.