Effect Mechanism Of Humic Acid On Hydrothermal Pretreatment Of The Waste Of Wheat Straw Pulping

The objective of this paper making the waste of wheat straw pulping（waste wheat straw）as the research object,in-depth analyzing the problem of waste wheat straw pretreatment technology in preparation fermentability sugar.Putting forward using the natural surfactant of the ash in waste wheat straw---humic acid as the promoter to improve the efficiency of the hydrothermal pretreatment,and revealing the mechanism of promoting lignin dissolved by humic acid in the process of hydrothermal pretreatment.The effect of humic acid on enzymatic hydrolysis of waste wheat straw was also studied.Finally,based on waste wheat straw as the raw material of biorefining and humic acid as the pretreatment additive,an efficient hydrothermal pretreatment technology was established to realize the strategy of efficient production of xylo-oligosaccharides and fermentable sugars from waste wheat straw.The main conclusions are as follows:（1）Hydrothermal pretreatment can effectively remove hemicellulose and lignin from wheat straw and waste wheat straw.The hemicellulose removal of wheat straw and waste wheat straw was increased from 38.26%to 99.99%and from 33.59%to 80.60%,and the lignin removal was increased from 13.87%to 47.62%and from 9.61%to 38.59%,respectively.Using xylo-oligosaccharide content as the evaluation index,the optimal hydrothermal pretreatment conditions of wheat straw and wheat bran were 180 ^oC,60 min and 180 ^oC,40 min,respectively.The content of xylo-oligosaccharide in prehydrolysate of 50 g wheat straw and waste wheat straw was 9.53 g/L and 7.43 g/L,respectively.The results of NMR analysis showed that the bond breaking,structural unit depolymerization and polycondensation of lignin in wheat straw and waste wheat straw occurred simultaneously during hydrothermal pretreatment,andβ-O-4 bond cracking was more obvious.After hydrothermal pretreatment,the space physical barrier structure was significantly damaged,and the enzymatic hydrolysis efficiency of wheat straw and waste wheat straw was increased from 11.64%and 10.63%to 80.82%and 65.77%,respectively.（2）Due to the residual surface lignin in the hydrothermal pretreated waste wheat straw affected the subsequent cellulase hydrolysis process,humic acid in the ash of waste wheat straw was used as additive for hydrothermal pretreatment to overcome this problem.It was found that adding 0～30 g/L humic acid in the hydrothermal pretreatment of waste wheat straw could effectively improve the lignin removal rate from 16.62%to 35.56%.However,humic acid also had a buffering effect on hydrothermal pretreatment media,resulting in the xylan removal was reduced from 80.53%to 68.70%.The double-edged effect of humic acid in hydrothermal pretreatment caused the enzymatic hydrolysis efficiency of waste wheat straw to increase first and then decrease.When the addition of humic acid was 10 g/L,the enzymatic hydrolysis efficiency of waste wheat straw reached the maximum value（81.79%）,and the continued addition of humic acid to 30 g/L reduced the enzymatic hydrolysis efficiency of waste wheat straw from 81.79%to 78.24%.Meanwhile,the analysis of structure of lignin showed that humic acid had no effect on the molecular weight,functional group content,bonding bond and structural unit of lignin in wheat bran during pretreatment.（3）In order to further explore the mechanism of humic acid promoting lignin removal during hydrothermal pretreatment,humic acid solution with surface-active properties was dissolved with enzymatic hydrolyzed lignin.The results showed that with the increase of humic acid concentration from 0 g/L to 20 g/L,the dissolution amount of 1 g lignin increased from 107.12 mg to 166.45 mg,indicating that humic acid had the ability to promote the dissolution of lignin.By analyzing the structure of dissolved lignin,it was found that the chemical structure of dissolved lignin in humic acid solution did not change.In addition,different hydrophilic and hydrophobic humic acids were obtained by sulfonation reaction and esterification reaction respectively,and then using the modified humic acid（10 g/L）dissolve 1 g lignin.It was found that the solubility of lignin in the solution was increased from 122.31 mg/g to 168.82 mg/g with the enhancement of hydrophobicity of humic acid,which indicated that humic acid enhanced the hydrophilicity of lignin through hydrophobic interaction,and then promoted the dissolution of lignin in the aqueous phase.（4）In order to investigate the effect of HA on enzymatic hydrolysis of lignocellulose,10 g/L HA assisted hydrothermal pretreatment of waste wheat straw was performed.It was found that the enzymatic hydrolysis efficiency of the unwashed and washed waste wheat straw was 66.14%and81.97%,respectively.It was concluded that HA on the surface of pretreated waste wheat straw has affected the cellulase hydrolysis.By measuring the cellulase activity and cellulase protein content in HA and cellulase solution,it was found that when the concentration of HA increased from 0 g/L to 2.5 g/L,the specific activity of cellulase was decreased from 100.00%to 63.75%.The concentration of free enzyme protein was also decreased from 442.64 mg/L to 65.92 mg/L.These results indicated that humic acid can inhibit cellulase activity and adsorb cellulase.The inhibition effect of humic acid on enzyme activity was solved by using different metal salt complex humic acid,and found that the specific enzyme activity of cellulase in humic acid solution recovered from63.75%to 98.40%,and the free protein concentration increased from 65.92 mg/L to 388.27 mg/L with higher Fe³⁺.When 10 mmol/L FeCl₃ was added in the enzymatic hydrolysis process of unwashed humic acid pretreated waste wheat straw,the enzymatic hydrolysis efficiency increased from 66.14%to 77.34%,indicating that FeCl₃ could effectively improve the enzymatic hydrolysis performance of the pretreated waste wheat straw containing humic acid.（5）Based on the superior delignification of humic acid in hydrothermal pretreatment,an optimized hydrothermal pretreatment technology was established using high-ash contented waste wheat straw as raw material,10 g/L humic acid and 20 mmol/L FeCl₃ as additives.The results showed that the pretreatment system could effectively remove xylan and lignin from waste wheat straw,and the removal of xylan and lignin were up to 72.91%and 38.78%,respectively.The enzymatic hydrolysis efficiency of pretreated waste wheat straw was up to 82.71%.The results showed that the ethanol production rate and yield of the pretreated waste wheat straw were the highest,and the ethanol concentration was 4.82 g/L,while the ethanol concentration of the hydrothermal pretreated waste wheat straw was only 2.78 g/L,which was increased 73.38%.Material balance showed that 0.8 g xylose,3.1 g xylo-oligosaccharide and 11.8 g glucose could be obtained from 100 g waste wheat straw after hydrothermal pretreatment and enzymatic hydrolysis.2.8 g ethanol was obtained by simultaneous saccharification fermentation.When 100 g wheat bran was treated with humic acid and FeCl₃,1.4 g xylose,7.4 g xylo-oligosaccharide and 21.2 g glucose could be obtained.4.7 g ethanol was obtained by simultaneous saccharification fermentation.