Research On The Mechanism Of Adsorptive Ash During Wheat Straw Hydrothermal Pretreatment

The rational utilization of crop straws is one of the essential avennues to realize rural revitalization and sustainable development in our country.Wheat straw(WS)and its pulp waste could be used for generating various bio-based chemicals such as cellulosic ethanol through biorefinery.The natural and compact structure of the lignocellulose restricts the direct cellulose efficient enzymatic hydrolysis and enzymatic hydrolysis efficiency is low.Mild and green lignocellulose pretreatment could effectively enhance the cellulase hydrolysis efficiency of lignocellulose and coproduce value-added xylo-oligosaccharide for improving the economic benefits of biorefinery.Waste wheat straw(WWS)is a considerable amount of cellulose waste from WS pulp mill.However,a huge amount of adsorptive ash contained in the WWS is the key factor that restricts WWS hydrothermal pretreatment efficiency during WWS hydrothermal pretreatment.Although the impacts of adsorbtive ash on the performance of WWS hydrothermal pretreatment could be removed by pre-washing,the water consumption is large and the process is complicated.Developing one or more high material utilizing,simple and practicable hydrothermal pretreatments to enhance the cellulose enzymatic hydrolysis efficiency and coproduce xylo-oligosaccharide is the effective avenue to efficient hydrothermal pretreatment based WWS biorefinery.In this thesis,the mechanism of adsorptive ash on the hydrothermal pretreatment is studied,methods of eliminating the buffering system during adsorptive ash riched WWS hydrothermal pretreatment are developed and the corresponding mechanisms are also explained.The results of this study will provide theoretical basis and application guidance for the technical methods of high ash content biomass hydrothermal pretreatment for efficiently co-producing biomass sugar and xylo-oligosaccharides.The main findings are as follows:(1)The formations of the weakly acidic medium of the WS hydrothermal pretreatment are inhibited with the additions of adsorprtive ash which is conducive to the xylan removal.As a result,the xylo-oligosaccharide productions and the enzymatic hydrolysis efficiency of the pretreated materials are reduced.Under the parameters of the WS hydrothermal pretreatment are solid to liquid ratio of 1:10,temperature of 180°C and residence time of 40 min,with the concentrations of adsorptive ash increased from 0 to 100 g/L,the p H value of the pretreatment medium increased from 3.8 to 4.8,the concentration of xylo-oligosaccharides decreased from the initial 8.8 g/L to 5.0 g/L,the cellulose accessibility was greatly reduced,and enzymatic hydrolysis efficiency was decreased from 84.9%to 66.3%.Insoluble mineral is the main component of adsorptive ash,accounting for 82.4 wt%.The K⁺,Na⁺,Ca²⁺,Mg²⁺cations adsorbed by insoluble minerals could undergo cation exchange reaction with the free H⁺generated by hydrothermal pretreatment to be buffered,which is about 60.4%of the buffering capacity of the adsorptive ash.The xylo-oligosaccharides productions and enzymatic hydrolysis efficiency obtained from WS hydrothermal pretreatments are gradually decreased.Insoluble minerals have the greatest impact on the hydrothermal pretreatment performance of WS.Adding the insoluble minerals in 100 g/L adsorptive ash to the WS hydrothermal pretreatment,the cellulose enzymatic hydrolysis efficiency of the pretreated WS is only 58.4%.(2)Select silica,the main component of insoluble minerals,the main component of soluble salt,sodium phosphate,and the main component of organic matter,sodium humate as typical adsorption-type ash model compounds to better understand the effects of different buffering systems on the WS hydrothermal pretreatment mechanism.Among these three model compounds,the buffering capacity in solution is in the order of sodium phosphate>sodium humate,and silica has no buffering effect.The main component of insoluble minerals,silica,has almost no effect on the WS hydrothermal pretreatment due to lack of exchangeable K⁺,Na⁺,Ca²⁺and Mg²⁺cations.The buffering effect of the adsorptive ash model compound resulted in the reductions of the weakly acidic medium of WS hydrothermal pretreatment,the xylan removal rate and the cellulose accessibility were reduced.The enzymatic hydrolysis efficiency of the pretreated WS was low.When the sodium phosphate and sodium humate were 30 g/L,the enzymatic hydrolysis efficiency of pretreated WS were only 47.3%and 57.7%.In addition to the buffering effect of humic acid in the hydrothermal pretreatment system,the properties of its complex weakly acidic macromolecular compounds could reduce the activity of H⁺in the solution.When the sodium phosphate and sodium humate were 30 g/L,the xylan removal yields of pretreated WS without adding model compound were reduced from 81.0%to 61.4%and 43.0%,the concentrations of xylo-oligosaccharides decreased from the initial 8.8 g/L to 4.9g/L and 2.8 g/L,respectively.The addition of sodium humate could promote the dissolution of lignin in WS hydrothermal pretreatment.When the added concentration of sodium humate is 10g/L,the lignin removal yield of the hydrothermal pretreated WS could reach 43.7 wt%.(3)The cations adsorbed by the adsorptive ash could react with the H⁺generated during WWS hydrothermal pretreatment through cation exchange reaction and the free H⁺is buffered.Different cations exert different cation exchange capacity.The order of the cation exchange capacity from large to small is as follows:Fe³⁺>Al³⁺>H⁺>Ca²⁺>Mg²⁺>NH₄⁺>K⁺>Na⁺.To negate the self-buffering system of WWS hydrothermal pretreatment,minor amounts of exchangeable metal salts(Fe³⁺and Al³⁺)are applied.The results show that the introduction of exchangeable metal salts could effectively enhance the weakly acidic medium of the WWS hydrothermal pretreatment.As a result,the p H value of the prehydrolysate is decreasing,and the acid buffering capacity and cation exchange capacity of WWS were significantly reduced.The cation exchange reaction mechanism of the WWS metal salt pretreatment is revealed by tracking the migration of the metal salt between the prehydrolysate and the pretreated substrate.From the perspective of co-production of xylo-oligosaccharides,FeCl₃ is a suitable exchangeable metal salt additive.The concentration of xylo-oligosaccharides in the prehydrolysate could be increased from 2.8 g/L to 6.2 g/L after WWS hydrothermal pretreatment with 20 mmol/L FeCl₃,while the addition of AlCl₃ does not increase significantly.At the same time,when the concentrations of FeCl₃increased from 0 to 20 mmol/L,the enzymatic hydrolysis efficiency of the pretreated WWS increased from 49.7%to 66.6%.1.0 kg of raw WWS could generate 62.0 g of xylo-oligosaccharides and 160.6 g of glucose.The addition of exchangeable metal salt could effectively eliminate the adsorptive ash buffering system formed during the hydrothermal pretreatment.The process of metal salts pretreatment is simple and the yield of biomass sugar and xylo-oligosaccharide production is the highest.(4)The adsorptive ash forms a good colloidal buffer system during the WWS hydrothermal pretreatment,which could significantly reduce the quantity and activity of hydrogen ions during the WWS hydrothermal pretreatment.By adding 0-3 vol%hydrogen peroxide to the WWS hydrothermal pretreatment,the acidity of the WWS hydrothermal pretreatment is enhanced,and the buffer system is eliminated.When the additions of hydrogen peroxide increased from 0 to3.0 vol%,the glucan recovery yield decreased from 94.3%to 80.6%,xylan removal yield increased from 68.1%to 88.3%and the lignin removal yield increased from 16.5%to 28.6%,respectively.When the dosage of hydrogen peroxide is 2 vol%,the xylan removal yield of the pretreated substrate is 88.3 wt%,the concentration of xylo-oligosaccharides in prehydrolysate is6.6 g/L,and the enzymatic hydrolysis efficiency could reach 84.0%.On the view of co-producing of xylo-oligosaccharides,2 vol%is the optimum concentration.1.0 kg of WWS could obtain 66.0 g of xylo-oligosaccharides and 218.2 g of glucose.Under this pretreatment condition,the degradation products of hydrogen peroxide are only water and oxygen,which will not remain,making it more green and environmentally friendly.The method has a more obvious elimination effect on the WWS self-buffering system,and the dissolution rate of xylo-oligosaccharides and the cellulose enzymatic hydrolysis efficiency of the pretreated materials are greatly improved.