Reduction of lignin color via one-step UV irradiation

The dark color of industrial lignin is the main obstacle for their high value-added use in areas such as sunscreen or dyestuff dispersants. Here, a one-step method for reducing the color of lignin is presented and the structural changes of the whitened lignin are characterized by UV, FTIR, NMR, GPC, DLS, AFM, HS-GC and potentiometric titration. The results show that the content of aromatic ring, methoxyl and phenolic hydroxyl groups in the alkali lignin (AL) decreases, while the content of carboxylic groups increases when it is exposed to UV in THF. AL experiences a darkening process before it is gradually whitened. The blue shift of the UV spectrum and the decrease in size and molecular weight indicate AL is not only intermolecular and intramolecular disaggregated, but also partially depolymerized when light colored lignin is obtained. This simple but efficient whitening method is successfully practised in bleaching. The white degree of raw paper increases from 0.438 to 0.917 after being irradiated under sunshine for 3 days.