FTIR is one of the most used techniques. Infrared Spectroscopy belongs to the group of molecular vibrational spectroscopies which are molecule-specific, and give direct information about the functional groups, their kind, interactions and orientations. Its sampling requirements allow the gain of information from solids, and in particular from solid surfaces. Even if historically IR has been mostly used for qualitative analysis, to obtain structural information, nowadays instrumental evolution makes non-destructive and quantitative analysis possible, with significant accuracy and precision. The shift of the bands and the changes in signal intensity allow the identification of the functional groups involved in metal sorption. Using this technique, carbonyl, carboxylic, aromatic, amine, and hydroxyl groups has been found to be involved in metal uptake by different biosorbents.

1.2.1 Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS)

DRIFTS occurs when light strikes on the surface of a material and is partially reflected and transmitted. The light that penetrates the material may be absorbed or reflected out again. The diffuse reflectance (radiation reflected from an absorbing material) is thus composed of surface-reflected and bulk re-emitted components, and contains information relative to the structure and composition of the sample. Even if DRIFTS has been not of large use, it has found interesting applications on verifying the enhancement of cadmium sorption capacity by juniper wood when carbonyl groups were substituted by sulfonic groups and on determining that Cr3+, Cu2+ and Zn2+ were sorbed onto the organic polymeric fraction of olive mill wastewater by ion exchange between alkaline and alkaline-earth metal ions and protons bound to carboxylic groups.