Diketoacid derivative of kappa-carrageenan (KCAR) was synthesized as an oxidation product of oxidation of KCAR with potassium permanganate in alkaline solution at pH's > 12 with a yield of 96.2%. The chemical structure of the coordination biopolymer macromolecule (DKA-KCAR) has been elucidated by elemental analysis and IR spectroscopy. The product has a high efficiency for removal of toxic heavy metal cations from the contaminated matters in environment. Suggested models for chelation were presented. A tentative reaction mechanism consistent with the experimental results for oxidation is suggested