Study On Two-step Hydrothermal Pretreatments Of Vinegar Residue For The Co-production Of Multi Products

Vinegar residue is a by-product produced during the solid-state fermentation of agricultural waste such as rice husk and wheat bran to produce vinegar.It has a highly acidic and stubborn lignocellulosic structure.Landfill and incineration are the main ways of large-scale utilization,but they not only result in resource waste but also pollute the environment.Based on the idea of full component utilization of lignocellulose and"hemicellulose-first"preparation of high value-added product xylooligosaccharides（XOS）,this thesis adopts a two-step hydrothermal pretreatment technology.Firstly,the residual organic acids in vinegar residue are used to realize the organic acid catalytic assisted hydrothermal preparation of XOS,and the effective components of XOS are further improved through the assistance of endoxylanase;Secondly,alkali assisted hydrothermal artificial humification technology is used to prepare humic-like acid（HLA）,fulvic-like acid（FLA）and hydrochar（HC）from the solid residue after the first-step hydrothermal pretreatment.The two-step hydrothermal pretreatment conditions were optimized and the formation mechanism of HLA and FLA was explored.The prepared HLA was subjected to a series of structural characterization such as Fourier transform infrared spectrometry（FT-IR）,elemental analysis（EA）,X-ray photoelectron spectroscopy（XPS）,scanner electron microscopy and energy-dispersive spectroscopy（SEM-EDS）,as well as structure parameters（E₄/E₆）determination and total acid functional group determination.The structure of the prepared HC and its influence on methylene blue adsorption properties were studied.The main research results are as follows:（1）The first-step hydrothermal pretreatment of vinegar residue to prepare XOS is as follows:solid-liquid ratio 1:10,170℃,50 min.Under these conditions,the yield of XOS（X₂-X₆）is 29.10%.Further use of endoxylanase with an enzyme dosage of 1.0 IU/m L resulted in a yield of 36.22%for XOS（X₂-X₆）.The ratio of（X₂-X₄）/XOS increased by 0.16 compared to that without enzymatic hydrolysis.（2）Firstly,after the first-step hydrothermal pretreatment of vinegar residue,the solid residue separated from the solid liquid was washed to p H 7,and the second-step of alkali assisted hydrothermal pretreatment was used to prepare HLA,FLA and HC.Taking the highest HLA yield as the indicator,the second-step of alkali assisted hydrothermal pretreatment conditions are as follows:solid-liquid ratio 1:10,0.6 mol/L KOH,210℃,13 h.The yields of HLA,FLA,and HC were 15.31%,13.46%,and 31.67%,respectively.Secondly,the effect of different washing degrees on the preparation of HLA and FLA by alkaline assisted hydrothermal pretreatment was investigated.The yield of HLA gradually decreased with the increase of water washing degree,indicating that water washing removed water-soluble precursor substances that promote the formation of HLA and FLA.Similar HLA and commercial humic acid（CHA）have certain similarities and differences in structure.Both of them have aromatic structure and aliphatic structure,but compared with CHA,the content of aromatic structure in HLA is low,while the content of aliphatic structure is high.The structural characterization results indicated that the content of aromatic structures and oxygen-containing functional groups in HLA decreases with increasing water washing degree.（3）The effects of different ratios of water-soluble mimetic mixtures/monomers on HLA and FLA preparation were investigated.The research results indicated that the yield of HLA and FLA decreased with the increase of the proportion of water-soluble simulation mixture.When the ratio is 1,the yield of HLA and FLA is the highest,at 3.83%and 29.42%,respectively.HLA and FLA contents of glucose,xylose and arabinose in water-soluble analog monomers are 1.64-2.05%and35.77-39.21%,3.51-6.28%and 41.80-54.87%,0.41-0.61%and 28.58-63.70%,respectively.The yield of HLA increases with the increase of monosaccharide concentration,while the yield of FLA is the opposite.The degradation products of monosaccharides,such as formic acid,acetic acid,lactic acid,furfural,and HMF,did not produce HLA and FLA.The structural characterization results indicate that as the ratio increases,the aromatic structures and oxygen-containing functional groups content of HLA gradually increase.And the content of aromatic frameworks and oxygen-containing functional groups in HLA derived from xylose was the highest.（4）The structure of HC obtained by two-step hydrothermal pretreatments of vinegar residue and its adsorption properties for methylene blue were studied.The SEM results indicated that the surface structure of HC become more regular than that of vinegar residue;FT-IR,XPS and elemental analysis showed that the content of aromatic structures decreased and the content of oxygen-containing functional groups increased in HC compared with vinegar residue.BET results showed that the two-step hydrothermal pretreatments increased the specific surface area,pore volume and pore diameter of vinegar residue.HC-T210t13 had the largest specific surface area,25.37 m²/g,and its adsorption capacity and adsorption rate of methylene blue were 19.68 mg/g and98.44%,respectively.For the HHV results,SR-T170 derived from acid hydrolysis yielded the highest HHV of 18.86 MJ/kg,while HC exhibited the lowest HHV of 16.45 MJ/kg.