Phenolation, amination and cross-linking of lignin: synthesis and characterization of functionalized lignin

Lignin is an inexpensive bio-adsorbent and has many applications in the adsorption of heavy metals. The structure and size of the lignin molecular chain affect its properties. Introducing of amino functional groups to the structure of lignin can improve its physical and chemical properties and increase its reactivity and adsorption power. Lignin cross-linking also improves its performance and adsorption characteristics. In the first step, lignin as a black liquor was extracted and purified from the waste of the wood factory. The lignin was then reacted with phenol to obtain lignophenol. The obtained lignophenol was then aminated via reacting with the ethylene diamine. Finally, in order to increase the strength of the adsorbent structure and prevent the dissolution of the adsorbent at low pHs (suitable for adsorption), 37% aqueous formaldehyde was added to the aminated lignin, from which aminated and cross-linked lignophenol was then obtained. Different types of lignins including lignin, lignophenol and amine-functionalized (branched or cross-linked) lignophenol were well synthesized and identified. The results of FT-IR spectrum showed that the amine functional groups were well attached to the lignophenol branches and the lignophenol was converted to the amino-containing lignophenol. Also, the results of acid-base titration and 11HNMR show that phenol was successfully bonded onto the lignin and amine functional groups were observed in lignin structure. The results of acid-base titration, elemental analysis and zeta potential tests also confirmed amination of lignophenol.