Study On The Hydrothermal Synthesis Of Hemicellulose-Based Carbon Microspheres And Their Adsorption Properties

Since biomass energy occupies an increasingly prominent position in the field of alternative energy,it impels people continuously to study the technologies for expanding their applications.As the second largest natural polysaccharide,hemicelluloses is not fully utilized Iike cellulose,and is often wasted as a residual product in traditional industry.Therefore,the carbon microspheres were prepared through a hydrothermal route of hemicelluloses,which were from the pulping and papermaking process.On this basis,it was further modified to obtain magnetic carbon microspheres.Subsequently,two adsorbent materials were obtained by chemical activation and their adsorption properties were tested.?1?The carbon microspheres were prepared via a mild hydrothermal method from xylan.The effects of concentration,temperature and reaction time on the products were discussed.The optimum reaction conditions were determined as follows:the mass concentration of xylan was 15 wt%,the pH of solution was in the range of 2-3,the temperature was 180? and reaction time was 8 h.?2?The carbon microspheres were activated with KOH solution to obtain adsorbent materials.The results showed that the adsorbent had cracks on the surface but still keep spherical structure.Meanwhile,the crystal structures of carbon microspheres were changed,but the degree of graphitization and the contents of oxygen-containing groups on the surface were increased.Furthermore,the specific surface area of materials were increased to 352.2 m²/g,which was conducive to the subsequent adsorption experiments.?3?The magnetic carbon microspheres were prepared by two-step hydrothermal method.Firstly,Fe₃O₄ particles were prepared by solvothermal method from ferric chloride hexahydrate,subsequently,they were mixed into the system of the preparation of carbon microspheres as the second-step hydrothermal reaction.The effects of xylan concentration,temperature,reaction time and the amount of Fe₃O₄ on the product were studied.The optimum reaction conditions were as follows:the mass concentration of xylan was 15 wt%,the amount of Fe₃O₄ was 0.10 g,temperature was 180 oC,reaction time was 8 h.The magnetic carbon microspheres were regular,smooth and well dispersed.The particle size distribution was 0.5?7?m,and the saturation magnetization was 8.38 emu/g.?4?The magnetic carbon microspheres were activated with KOH solution to obtain adsorbent materials.The results showed that the adsorbent had cracks on the surface but still keep spherical structure.Meanwhile,the crystal structures of carbon microspheres were changed,but the degree of graphitization and the contents of oxygen-containing groups on the surface were increased.Furthermore,the specific surface area and saturation magnetization were increased to 478.1 m²/g and 13.14 emu/g,respectively.?5?In the adsorption experiments of Cu????T=303 K,pH=6,C0=20 mg/L,Dosage=0.5 g/L?,two adsorbents reached adsorption equilibrium within 400 min and 300 min with the maximum adsorption capacity of 15.20 mg/g and 21.20 mg/g,respectively.The adsorption process were both in accordance with the pseudo second-order kinetic model and Langmuir isotherm model.?6?In the adsorption experiments of Pb????T=303 Ks pH=5,C0=20 mg/L,Dosage=0.5 g/L?,two adsorbents reached adsorption equilibrium within 300 min and 250 min with the maximum adsorption capacity of 32.35 mg/g and 39.45 mg/g,respectively.The adsorption process were both in accordance with the pseudo second-order kinetic model and Langmuir isotherm model.?7?After five desorption experiments,two adsorbents still keep high removal rates and regeneration rates,but the adsorbent obtained from magnetic carbon microspheres were relatively more stable,which provides theoretical support for the reduction of cost in actual production.The above conclusions indicate that,as the residue of pulping and papermaking,the xylan can be successfully used to prepare carbon microspheres by hydrothermal method.After activation,it can be used as an adsorbent material in the field of heavy metal adsorption.Moreover,the carbon microspheres keep good adsorption performance and reusability after magnetization,which further provides a new way to broaden the utilization of hemicellulose.