Synthesis Of Bacterial Cellulose And Its High-valued Application

Bacterial cellulose?BC?is a natural and biodegradable nanostructured high molecular polymer produced by certain bacteria.It has been widely used in food,biomedical,nanometer functional materials and other fields due to its unique structure and excellent performance.However,in the traditional method,the cost of using the monosaccharide as carbon source in the medium to synthesize BC is too high,which limits its scale production.Thus,it is especially important to search abundant and economical natural media for low-cost preparation of BC as well as its high value utilization.The low-cost preparation of bacterial cellulose and high value-added applications were studied in this thesis.To begin with,BC was prepared by using the hydrolyzed sugar in the prehydrolysate of agroforestry biomass?corn stalk?as the carbon source.The effect of temperature and time on prehydrolysis was investigated.The optimal treatment conditions for corn stalk were as bellow:The mass loss of corn stalk raw materials could reach to 34.75%under treatment temperature of 160°C,time of 70 min.Furfural substances had an inhibitory effect on the synthesis of BC from Acetobacter xylinum.The double detoxification process of activated carbon and ion exchange resin on the effect of corn straw hydrolysate was carried out.The result showed that the detoxification method had a high removal rate of substances?such as furfural,lignin,and acetic acid?which inhibit the growth of microorganisms.When the addition of activated carbon and ion exchange resin was 5.0%?m/V?and 1/10?w/w,ion exchange resin/hydrolyzate?,the removal rate of furfural,lignin and acetic acid reached to88.58%,89.73%,and 82.97%,respectively.This indicated that double detoxification process of activated carbon and ion exchange resin had a very good detoxification effect on corn straw hydrolysate,and the sugar content in the hydrolysate was 31.90 g/L,which had the potential to produce BC.Furthermore,the effects of initial pH value,culture temperature,fermentation time,inoculum size and different medium on BC yield were also investigated.And the obtained optimum conditions were as follows:initial pH value of 6.8,culture temperature of 30?,fermentation time of 7 days,inoculum size of 8.0%.The yield of BC prepared by using hydrolyzed sugar from corn stalk as the carbon source could be up to 2.86g/L under the best conditions,and the fiber diameter was in the range of 20⁸0 nm.And the differences in preparation methods,morphology,chemical structure,surface charge and crystallinity between bacterial cellulose and plant-derived nanocellulose?nanocrystals,nanofibrils?were systematically compared.Bacterial cellulose was obtained by bottom-up biosynthesis,while cellulose nanocrystals?CNC?and cellulose nanofibrils?CNF?were separated by top-down chemical or physical methods.Compared to plant-derived nanocellulose,BC had a range of advantages and especially the preparation process of BC was more green and environmentally friendly.It was found that BC had a higher crystallinity of 88.70%,higher thermal stability,and more elongated nanofibers.In addition,BC pellicle was studied.It was a natural hydrogel with high water content?>90%?,and can be obtained with various thickness by controlling the culture time.The BC aerogel was obtained by liquid nitrogen directed freeze-drying method.BC aerogel had the unique advantages of light weight?5.8 mg/cm³?,high porosity?99.5%?and large specific surface area(436.25 m²·g^-1).The pores of aerogel were mostly small pores below 100 nm.It had super-hydrophilic and water-absorbing swell characteristics,and the absorbed water was about 160 times to its own weight.The aerogel exhibited good recovery properties in water,and the volume restored to its original thickness within 150 minutes,which was fast.Moreover,BC aerogel had good water repellency and reusability.The film compressed from BC aerogel had good flexibility and mechanical properties,and the tensile strength of 3.02MPa and tensile elongation of 10.0%.Furthermore,the BC aerogels were carbonized at different temperatures?800°C,1000°C,1100°C?to obtain the BC carbon aerogels,corresponding to the specific surface areas of 642.83 m²·g^-1,745.61 m²·g^-1 and 778.75 m²·g^-1,respectively.The internal structure of the carbon aerogels were well maintained during the carbonization process.The carbon aerogel had super compressibility?90%thickness was compressed?,high elasticity?restored to its original thickness after releasing pressure?,and fatigue resistance?the structural stability was maintained after 1000 compressions?,and graded porous structure.The flowers on the top of BC carbon aerogel can not be burnt on the high temperature alcohol lamp,which proved to be good heat insulation and thermal stability.Meanwhile,the morphology and structure of BC carbon aerogel was not damaged when burnt on high-temperature alcohol burner,which proved to be excellent fire resistance.The BC carbon aerogel has hydrophobic properties,and the higher pyrolysis temperature will lead to the better hydrophobic properties.The water contact angles of carbon aerogel obtained by carbonization at 800°C,1000°C and 1100°C were 131.832°,145.057°and 156.273°,respectively.And carbonization at 1100°C gave a superhydrophobic property.BC carbon aerogel has the characteristics of hydrophobic and lipophilic,which had ultra-strong and ultra-fast adsorption capacity for various oils and organic solvents,up to 132²74 g/g,and could achieve adsorption saturation in just a few seconds.Last but not the least,the low-cost BC carbon aerogel was applied as the flexible negative electrode material to a potassium ion battery.The electrode made from BC carbon aerogel had ultra-high compression properties and nanofiber properties of multi-stage pores,which could be directly used as the self-supporting electrode.The potassium ion battery exhibited excellent rate performance and ultra-long cycle stability.At a current density of1000 mA/g,the specific capacity as high as 158 mAh/g after 2000 cycles.And the specific capacity attenuation per revolution was only 0.006%.After continuing to cycle 1500 cycles at a current density of 2000 mA/g,the specific capacity of the BC carbon aerogel electrode remained at 141 mAh/g.When the current density was increased to 5000 mA/g,the specific capacity could still be maintained at 122 mAh/g after 1000 cycles.The mechanism research was investigated,which showed that the hierarchical porous structure on carbon nanofibers had a strong adsorption effect on potassium ions,and this storage mechanism for potassium ion played a key role in improving the rate performance and cycle stability of the battery.