Freeze-thaw Assisted Alkali Treatment For Efficient Separation Of Hemicellulose From Bamboo

Hemicellulose is a natural polymer abundant in the earth and has shown great promise in food packaging,biomedicine and other fields due to its green and degradable properties and high chemical activity.The most traditional and applied hemicellulose extraction method is alkali treatment,but alkali extraction of hemicellulose often uses a higher concentration of sodium hydroxide solution,which is environmentally polluting,and the high concentration of alkali solution also makes more lignin leach out,thus affecting the purity of hemicellulose samples.Therefore,this paper promotes the separation and extraction of bamboo hemicellulose by freeze-thaw assisted alkali treatment.And the freeze-thaw assisted alkali treatment process was optimized by adding alkali during the freezethaw process to enhance the efficiency of alkali permeation and to realize the efficient separation and extraction of hemicellulose.We also compared the components and structures of hemicellulose samples obtained by conventional alkali treatment and optimized freeze-thaw-assisted alkali treatment before and after the process,and characterized the physicochemical properties of the remaining solids to clarify the advancedness of the freeze-thaw-assisted alkali treatment method.The main studies and results of this paper are as follows.The effect of freeze-thaw pretreatment on the separation of hemicellulose by alkali treatment was analyzed using bamboo as raw material.Firstly,the effects of alkali concentration,alkali treatment time,freezing temperature and number of freeze-thaw cycles on the separation of each component of bamboo were investigated.The results showed that the bamboo was frozen at-30°C for 12 h,thawed at room temperature for 12 h,and then treated with alkali solution at a concentration of 7% for 90 min at 75°C.The isolation yield of hemicellulose reached 64.71%.Compared with the conventional alkali treatment process,the alkali concentration of freeze-thaw pretreatment-assisted alkali treatment was reduced from 9.0% to 7.0%,and the alkali treatment time was reduced from 120 min to 90 min,while the separation yield of hemicellulose was increased by18.51%.The results of the study confirmed the auxiliary effect of freeze-thaw pretreatment on the separation of hemicellulose by alkali treatment.In order to further reduce the amount of chemicals used in the alkali treatment process and improve the permeation efficiency of the alkali solution,the freeze-thaw pretreatment-assisted alkali treatment process was optimized,and the optimal process conditions for the separation of hemicellulose by freeze-thawassisted alkali treatment were obtained by single-factor experimental analysis,and the physicochemical properties of the remaining solids of bamboo were characterized.The optimized process conditions for freeze-thaw assisted alkali treatment were: freezing temperature-40°C,freezing time 10 h,alkali concentration 5.0% and alkali treatment time 2 h.The hemicellulose separation yield was increased to 73.26% under this process condition.Compared with the conventional alkali treatment and the unoptimized freeze-thaw pretreatmentassisted alkali treatment process,the crystallinity and thermal stability of the remaining solids were improved by the optimized freeze-thaw-assisted alkali treatment process.The intensity of the characteristic absorption peaks of acetyl groups was significantly reduced after the freeze-thaw assisted alkali treatment.And the percentage of C1 signal peak area of the remaining solids of bamboo decreased,with the highest percentage of C1 peak area of the optimized process of freeze-thaw assisted alkali treatment,and its O/C ratio was lower than that of the unoptimized freeze-thaw pretreatment assisted alkali treatment and conventional alkali treatment processes.The optimized freeze-thaw-assisted alkali treatment process further reduced the concentration of sodium hydroxide and increased the separation yield of hemicellulose by 8.58%,which shows that the optimized freeze-thaw-assisted alkali treatment process has higher selectivity for the separation of hemicellulose,and improved the thermal stability of the remaining solid,which is beneficial for its subsequent high-value utilization.The fractional and structural properties of hemicellulose samples obtained by conventional alkali treatment,freeze-thaw-assisted alkali treatment and optimized freeze-thaw-assisted alkali treatment were compared and analyzed.The optimized freeze-thaw-assisted alkali treatment process yielded a hemicellulose sample with a purity of 94.73% and a heavy average molecular weight of 48928g/mol,which was higher than the other two hemicellulose samples.The O/C ratio of this hemicellulose sample was 0.672,which was higher than that of the hemicellulose sample from the unoptimized freeze-thaw pretreatment-assisted alkali treatment and the conventional alkali treatment processes.The stronger signal of the 4-O-methyl-D-glucuronide unit of the hemicellulose sample obtained by the freeze-thaw assisted alkali treatment optimized process indicated that its side chain structure was protected to some extent.Therefore,compared with the conventional alkali treatment process,the optimized freeze-thaw assisted alkali treatment process greatly reduced the amount of chemicals used in the alkali treatment process,while the hemicellulose separation effect was significantly improved.The reduced alkali concentration inhibited the lignin leaching to a certain extent and reduced the damage of hemicellulose structure by the alkali solution,thus improving the purity and molecular weight of hemicellulose samples and providing some theoretical guidance for the separation of components of woody fiber biomass and extraction of hemicellulose.