Photodegradation of milled wood lignin

The photodegradation of lignin has been investigated using milled wood lignin (MWL) as a model. Solutions of MWL were exposed to ultraviolet (UV) light under aerobic and anaerobic conditions. Changes in the molecular weight distribution (MWD) of the lignin were assessed by gel permeation chromatography (GPC). To elucidate the chemical changes induced in the lignin by irradiation, infrared (IR) and UV spectra of the irradiated solutions were also studied. Irradiation under nitrogen causes the formation of both high molecular weight (MW) condensation products and low MW fragments; whereas, irradiation under oxygen yields only low MW fragments, similar to those found under nitrogen. Since the formation of condensation products can be attributed to free radical coupling, it follows that oxygen is probably reacting with free radicals to form peroxyl free radicals thereby avoiding condensation to higher MW products. Finally, the chemical basis of the photoinduced changes in the MWD of the lignin are discussed