Cryogenic-transmission electron microscopy of metal complexes with cyanobacterial polysaccharides, sacran, extracted from Aphanothece sacrum biomaterials reveals that Nd3+ complexes form networks composed of thick strings with a thickness ranging 10-20 nm while Fe2+ ones make dense entanglement of very thin strings. When Fe2+ and Nd3+ double complexation occurs, dense nanonetworks composed of thick strings with a thickness around 10 nm are formed. Next we prepared heterogels by crosslinking poly(vinyl alcohol) chains in the presence of sacran and investigated the effects of double complexation on metal sorption. The amount of Nd3+ sorbed into the heterogels in the 1 : 1 miscible solution of Fe2+ and Nd3+ is higher than that in the solution containing only Nd3+, suggesting that Fe2+ assists the Nd3+ sorption. In addition, it is found that the amount of sorbed Nd3+ is much higher than that of sorbed Fe2+ even in the acidic condition of pH 2. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 128: 676-683, 2013