ION-EXCHANGE SYSTEMS IN PROTON-METAL REACTIONS WITH ALGAL CELL-WALLS

Algal cell walls contain acidic polysaccharides, e.g. alginic acid in many marine species, and pectin in fresh-water species. At neutral pH these provide anionic sites which bind ionically to metals, and acidification releases metals when covalent bonds to H are made. Added Cu releases an equivalent amount of Ca, Mg, H from Vaucheria, and protonated amines also displace metals. Interestingly, diamines, which function as efficient growth promoters, are sorbed more strongly than monoamines based on Langmuir adsorption constants. Sorption of Pb and Cd on Rhizoclonium from the solid hydroxides releases Na, Ca, Mg from the alga and OH from the hydroxide in essentially stoichiometric amounts. Sorbed heavy metals can be removed by precipitation of the sulfide (Cd), hydroxide (Cu, Al, and Ph), or as an EDTA complex (Pb) in ion exchange processes. Values of ion exchange constants for displacement of Ca by Mg, Zn, Cd, Pb, and Cu on Vaucheria are independent of pH and increase with the (charge)(2)/radius of metal ions, as also observed for K and Al. These results indicate a non-cooperative behavior of anionic sites which interact with these metals by electrostatic attractions. Rates of metal desorption and ion exchange constants with alginates as model systems were also investigated.