Sorption of metals by extracellular polymers from the cyanobacterium Microcystis aeruginosa fo. flos-aquae strain C3-40

The sorption of cadmium (II), copper (II), lead (II),manganese (II), and zinc (II) by purified capsularpolysaccharide from the cyanobacterium Microcystis aeruginosafo. flos-aquae strainC3-40 was examined by four methods: equilibriumdialysis, metal removal from solution as detected byvoltammetry, metal accumulation by capsule-containingalginate beads, and calorimetry. The polysaccharide'ssaturation binding capacities for these metals rangedfrom 1.2 to 4 mmol of metal g^-1 of capsule, whichcorresponds to 1 metal equivalent per 2 to 4saccharide subunits of the polymer. Competitionbetween paired metals was tested with simultaneous andsequential additions of metal. Cadmium (II) andlead (II), as well as lead (II) and zinc (II), competedrelatively equally and reciprocally for polymerbinding sites. In contrast, manganese (II) stronglyinhibited the binding of cadmium (II) and lead (II), butitself was not substantially inhibited by either theprior or simultaneous adsorption of cadmium (II) or lead (II).The data are interpreted with respect to overlap ofbinding sites and possibilities of altered polymerconformation or solvation. Calorimetric studies oflead (II) and cadmium (II) association reactions withthe polysaccharide suggest that the enthalpies aresmall and that the reactions may be driven by entropy.