**5.6. Reduction**

In this process, the metal interacts with the functional groups like carboxyl, gets reduced, and leads to the growth of crystals. Elements like gold and palladium have been obtained by the process of reduction. The metal gets reduced once it binds to the biosorbent at discrete places. Removal of toxic hexavalent chromium can be done by the process of reduction. Many organisms remove Cr (VI) by reduction to Cr (III) by biosorption from the aqueous solution [50–52].

The mechanism of biosorption can be studied using different techniques. The acidic and basic properties of the functional groups that are present on the material surface and ion exchange properties can be determined by Boehm method or potentiometric titration [53]. Fourier transform infrared spectrometry (FTIR) offers important information about the functional groups that are present on the surface of biosorbents like carboxyl, amino, amide, hydroxyl, sulfate, carbonyl, ether, ester, and the nature of the bond that are involved in biosorption [54]. Scanning electron microscope (SEM) is a powerful technique for qualitative evaluation of the structure and morphological changes of the biosorbent before and after metal biosorption. Energy dispersive X-ray (EDX) technique provides valuable information about the availability of various elements on the surface of the biosorbent. X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique for analyzing the surface chemistry of the biosorbent, that is, electronic state and empirical formula of the elements present and oxidative state of the biosorbed metal ion [55].
