Relationship between Physical Parameters of Various Metal Ions and Binding Affinity for Alginate

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We investigated the relationship between the physical parameters of various metal ions, including toxic metal ions, and the binding affinity of these metal ions for alginate (Alg). The binding constant, K, of Sr²⁺ was the highest among all tested metal ions. The order of K values was: Sr²⁺>Pb²⁺>Tb³⁺>Dy³⁺>Ca²⁺>Cd²⁺>Mg²⁺>Fe²⁺>Fe³⁺>Co²⁺>Al³⁺>Ni²⁺>Cs⁺>Cu²⁺>Ag⁺>Li⁺>K⁺. The metal ions showing the highest K values had ionic radii within the range of about 90–120 pm. Moreover, the K values of divalent or trivalent metal ions tended to be higher than those of monovalent ions. The number of binding sites per 1 mg of Alg (n) was highest for K⁺, followed by Pb²⁺ and Cs⁺. The order of affinity (calculated as the product of n and K) was Pb²⁺>Dy³⁺>Tb³⁺>Sr²⁺>Ca²⁺>Mg²⁺>Cd²⁺>Fe²⁺, Fe³⁺>Cs⁺>Al³⁺>Co²⁺>Ni²⁺>Cu²⁺>Ag⁺>K⁺>Li⁺. Our results support the idea that Alg would be effective as an excretion accelerator and/or absorption inhibitor for various toxic metal ions.