Enhancement Of Lignin Removal Selectivity From Pre-hydrolysis Liquor Of Aspen By Pectinase Treatment

Extracting hemicellulose from wood by auto-hydrolysis before wood pulping is considered to be an efficient approach for the biorefinery,and the utilization of the hemicellulose separated and purified from the Pre-hydrolysis liquor(PHL)is considered to be a vital method of biorefinery.Most of the hemicellulose would dissolve in the PHL by auto-hydrolysis,and some high value-added products,such as hydrogels,functional oligosaccharide and food packaging film can be produced by the hemicelluloses separated and purified from the PHL.During the separation and purification of hemicellulose from the PHL,the removal of the various of impurities especially lignin is very essential.Based on the concept of biorefinery,in present work,the effects of the recycling of the PHL on the dissolution and conversion of hemicellulose under different temperature and hydrolysis time were studied;the negative charge of the PHL,mainly the polyanion,the main source of cationic demand in the PHL,was preliminarily analyzed and discussed.A new method for the removal of lignin from the PHL by pectinase combined with cationic polymer treatment was discussed.The optimum conditions(including temperature,treatment time and enzyme dosage)of pectinase and the effects of the pectinase treatment on the selectivity of lignin removal were studied.The results showed that the concentration of hemicellulose saccharides in the PHL depended on the pre-hydrolysis temperature and PHL recycling times.Compared to the unrecycled PHL,the concentration of hemicellulosic saccharides in PHL increased significantly when recycling PHL once or twice at 170°C.Furthermore,the amount of high-molecular-weight hemicelluloses(HMHs)in PHL recycled once at 170°C increased from 2.58 g/L(unrecycled)to 6.18 g/L,but the corresponding average molecular weight of HMHs decreased from 9.2 kDa to 7.6 kDa.The concentration of hemicellulosic saccharides in PHL decreased with PHL recycling time at 180°C,accompanied by the formation of a significant amount of furfural.The analysis results of the negative charge in the PHL showed that the contribution of the colloidal lignin to the cationic demand of the PHL was negligible.For the PHL s obtained at the same temperature PHL,with the increasing of pH,its cationic demand increased.Poly-galacturonic acid was the main source of the cationic demand of the PHL obtained at low hydrolysis temperature,because most of the galacturonic acidwould be exist in the polymeric form at low hydrolysis temperature.With the increase of the hydrolysis temperature,most of the poly-galacturonic acid would be degraded to the oligomers and monomers,so its contribution to the cationic demand became negligible.The effect of dissolved lignin on the polyanionic charge in the hydrolyzate was also related to the hydrolysis temperature.The contribution of dissolved lignin to the cationic demand of the PHL obtained at low hydrolysis temperature was negligible,which was due to its small content in the PHL.The effects of the dissolved lignin on the cationic demand of the PHL became significant for the PHL obtained at high hydrolysis temperature due to the obvious increase of the dissolved lignin content.Pectinase was used to treat the PHL followed by the cationic polymer treatment.The results showed that the optimum conditions for pectinase treatment were: the enzyme dosage 1000 u/L,treatment temperature 40°C and treatment time 8 h.The cationic demand of PHL decreased by 37% after pectinase compared to the PHL without pectinase treatment.The lignin removal with pectinase treatment followed by cationic polymer treatment increased from 8.7%(only with cationic polymer treatment)to 17.9% at 33% reduced cationic polymer addition,and the lignin removal selectivity was improved more than doubled.