The Effect of 60Co γ-Irradiation on the Structure and Thermostability of Alkaline Lignin and Its Irradiation Derived Degradation Products

Abstract

To elucidate the degradation mechanism of lignin treated by irradiation, alkaline lignin was used as the material treating with different dosage (0, 100, 200, 400, 600, 800, 1000, 1200 kGy). The morphology, molecular weight, structure, thermostability and degradation products of alkaline lignin irradiated by gamma ray were investigated by a set of experiments. The results observed in scanning electron microscopy showed that there were more cracks and small protuberances on the morphological surface of irradiated-lignin in comparison with untreated lignin. The gel permeation chromatography results showed that weight-average molecular weight and number-average molecular weight of lignin decreased from 17829 and 590 Da to 13526 and 444 Da, respectively, when irradiation dose increased from 0 to 1200 kGy. The absorption bands of hydroxyl group at 3237 cm−1 detected by Fourier transform infrared (FT-IR) analysis decreased with irradiation dose increased from 0 to 1200 kGy. FT-IR and solid state 13C nuclear magnetic resonance (13C CP/MAS NMR) confirmed that benzene ring skeleton structure of lignin disrupted when the absorbed dose was > 800 kGy. Thermogravimetry/differential thermogravimetry curves revealed thermostability and activated energy (Ea) of lignin were slightly decreased after irradiated. The irradiation derived degradation products of lignin were analyzed by gas chromatography and mass spectrometry, indicated presence of 22 aromatic compounds and 18 aliphatic acids. Moreover, the relative peak area of aromatic compounds and aliphatic acids in un-irradiated lignin (0 kGy) were 4.48 ± 0.42 and 23.81 ± 1.85%, respectively, and these two types of degradation products reached a maximum of 23.81 and 6.10% at 800 and 1200 kGy, respectively. In summary, the findings in this work provide a basic scientific support on the mechanism of irradiation and full utilization of lignin.