**5.5 Surface precipitation**

It is a process in which the metal ions present in the aqueous solution form precipitates with the functional groups that are present on the surface of the microbial cells due to which the metal ions remain intact with the microbial cell. Organic and inorganic metal precipitates are generally formed during the process of adsorption. Use of microbial cells forms the organic metal precipitates, that occur due to the excretion of extracellular polymeric substances. In several other cases, insoluble inorganic metal precipitates are also formed. Several studies have reported the involvement of surface precipitation mechanism using the husk of green tomato, soybean meal, and watermelon rind for the eradication of Cu (II), Cu (III), Pb (II), Zn (II), Fe and Mn [41, 42].

#### **5.6 Reduction**

Reduction is also an important mechanism of adsorption that plays a vital role in the biosorption of various heavy metals such as gold and palladium. During the process of reduction the metal connects with the functional group, gets reduced, and undergoes the growth of crystals. The metal that is reduced binds the biosorbent at various places. The eradication of numerous heavy metals such as chromium, gold, palladium etc. can be done easily by the process of reduction. For example, by using the process of biosorption, removal of Cr (VI) can be done easily by reducing it into Cr (III) from the aqueous solution [17].
