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⁺, Mg²⁺ 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.