**5.3. Coordination**

The metal atom in the complex is bound to its immediate neighbors by a coordinate covalent bond by accepting a lone pair of electrons from the non-metal atom. The non-metal atom is known as the donor (coordinating atom) and the metal atom which accepts the electron pair is known as the acceptor. Compounds having such types of bonds in their structure are known as coordinate compounds. Some examples of coordinating groups are =O, –NH2, –NH, –N=, –OH, –S–, –O–R, and =NOH.

#### **5.4. Ion exchange**

Ion exchange is an important concept in biosorption which involves the exchange of binary metal ions during biosorption with the counter-ions present on the surface of the biosorbent. Most of the purification process works on the mechanism of ion exchange. Ion exchange can take place either by cation or anion exchange. Carboxyl groups can be a good example of cation exchangers while amino/imidazole groups represent anion exchangers. The process of biosorption of Cr (III), Cd (II), and Cu (II) by *Spirulina* was studied. Three functional groups capable of cation exchange were identified on the surface: phosphate, carboxyl, and hydroxyl groups [46]. Ion exchange mechanism of biosorption was reported in other studies using rice straw for the removal of cadmium by exchange with K<sup>+</sup> , Na<sup>+</sup> , Mg<sup>+</sup> , and Ca<sup>+</sup> and for the removal of Cu (II), Zn (II), and Pb (II) using watermelon rind [44, 47].
