**5.5. Precipitation**

negative, or neutral charge depending on the number of binding ligands involved. The complex formation to the monodentate ligand is more preferable than multidentate because the latter contains multiple ligands which may lead to multiple species binding. The metal ion interacts with the ligands by covalent bonds. The attenuated total reflection infrared spectral (ATR-IR) analysis of *Cyanobacterium microcystis* after the biosorption of antimony (III) suggested the involvement of carboxyl, hydroxyl, and amine groups through surface complexation [40]. A similar mechanism of biosorption was reported by other studies by using *Acidiphilium*, *Termitomyces clypeatus,* and alkali-modified sewage sludge for the removal of Cd

It refers to the process in which a chelating agent binds to the metal ion at more than one place at a time in order to form a ring structure and the complex is known as chelate. Mostly polydentate ligands participate in the reaction to form stable structures by multiple bonding. An increase in binding sites of the ligand increases the stability of the structure. Chelates are more stable than complexes because of multiple binding with the metal ion in more than one place. Rice straw was used as a potential biosorbent for the removal of Cd (II) from the effluent. The biosorbed Cd (II) chelates with the functional groups such as C=C, C–O, and O–H and carboxylic acids which are present on the surface of the biosorbent [44]. A similar mechanism of biosorption was reported in the removal of Cr (III) and Cu (II) by carboxyl and

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=,

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

, Na<sup>+</sup>

, Mg<sup>+</sup>

, and Ca<sup>+</sup>

and for the removal

hydroxyl groups present on the surface of soybean meal waste [45].

straw for the removal of cadmium by exchange with K<sup>+</sup>

of Cu (II), Zn (II), and Pb (II) using watermelon rind [44, 47].

(II), Cr (VI), and Cd (II), respectively [41–43].

**5.2. Chelation**

76 Biosorption

**5.3. Coordination**

**5.4. Ion exchange**

–OH, –S–, –O–R, and =NOH.

The metal ions form precipitates with the functional groups present on the surface of the microbial cells and remain intact or penetrate into the microbial cell. Most cases involve the formation of insoluble inorganic metal precipitates. Organic metal precipitates may be formed when microbial cells are used. Most of the extracellular polymeric substances excreted by the microbes are involved in the formation of organic precipitates. Precipitation of Cu (II) onto *Mesorhizobium amorphae* causes deformation, aggregation, and damage to the cell surface as shown by scanning electron microscope-energy dispersive X-ray (SEM-EDX) analysis [48]. This mechanism of precipitation for biosorption of metal ions was reported by other studies using soybean meal, watermelon rind, and green tomato husk (*Physalis Philadelphia lam*) for the removal of Cr (III) and Cu (II); Cu (II), Zn (II), and Pb (II); and Fe and Mn, respectively [45, 47, 49].
