Structural Modification Of Biochars And Their Application In The Dyeing Wastewater Treatment

Biochar is an adsorbent with low cost,high porosity and rich functional groups,which has a brilliant prospect in the application of printing and dyeing wastewater treatment.The application of biochars can not only realize the resource utilization of waste biomass,but also reduce the cost of dye wastewater treatment.However,due to the high proportion of inorganic components,the biochars prepared from waste biomass have some disadvantages,such as smaller surface area and lower adsorption capacities.The dye wastewater usually contains several pollutants,but the common biochars only have good adsorption capabilities for the dyes with specific charge,which have poor adsorption universality.In addition,biochar is usually deposited or buried after use,which may produce solid waste pollution for the environment.The above defects limit the large-scale application of waste biomass biochar in the printing and dyeing wastewater treatment.In this paper,the biochars with different specific surface areas,functional group modified biochars,metal oxide loaded porous biochars,metal oxide coated biochar microspheres and solar-thermal biochar cakes were prepared from tannery sludge and waste bamboo by regulating the pyrolysis conditions,introducing polar functional groups,in-situ loading on the pores of biochars and cladding biochar microspheres,respectively.The contents are as follows:To improve the adsorption capacity and universality of biochar,the structures and performances of the biochar were regulated.A functional equation was established by the response surface method to regulate the specific surface areas and Zeta potentials of biochars（BCs）.Then,the modified biochars（MBCs）were prepared by introducing polar functional groups to the BCs,such as amide group and cyanide groups.Finally,the magnetic biochars were obtained by loading Fe₃O₄ particles on the MBCs.The results showed that the removal rate of BCs（10 g/L）for the Cr（Ⅵ）（25 mg/L）was43.5%.The removal rates of BCs（8 g/L）for the reactive red X-3B,direct yellow RS,cationic blue X-GRL and acid blue 2GL（200 mg/L）were 0.1%,17.9%,99.1%and0.05%,respectively.The BCs only had good adsorption capability for the cationic dyes.The adsorption capability and adsorption universality of biochars for pollutants were significantly improved after the introduction of amide group and cyanide groups onto the BCs.The maximum removal rates of MBCs for the Cr（Ⅵ）,reactive red X-3B,direct yellow RS,cationic blue X-GRL and acid blue 2GL were all above 98%under the same adsorption condition,and no obvious desorption behaviors were observed in the simulated environment.In addition,after the magnetic modification for MBCs,the biochars can be separated from the solution quickly under the magnetic field,which improved the performances of biochars in the practical application.To improve the recycling ability of biochars and reduce the solid waste pollution that generated from the used biochars,the metal oxides in-situ loading porous biochars were prepared.The Fe-BC was prepared by the pyrolysis of the mixture of Fe and bamboo powder.Then,the TiO₂,Mg O and Zn O metal oxides were loaded on the Fe-BC.Biochar composites（TiO₂@Fe-BC and Mg O/Zn O/TiO₂@Fe-BC）were used for the treatment of printing and dyeing wastewater.The results showed that the removal capability of TiO₂@Fe-BC for Cr（Ⅵ）（93.29%）was better than that of Mg O/Zn O/TiO₂@Fe-BC.However,Mg O/Zn O/TiO₂@Fe-BC has greater adsorption-catalytic degradation capabilty for the methylene blue（MB）under the UV light irradiation.When the concentration of Mg O/Zn O/TiO₂@Fe-BC is 0.8 g/L,the MB solution（50 mg/L）can be decolorized completely.After the 4th use,the removal rate of Mg O/Zn O/TiO₂@Fe-BC for MB is still higher than 80%.The hollow catalytic microspheres were prepared from metal oxide coated biochar microspheres by regulating the structure of the adsorption-catalysis biochar composites to improve their utilization efficiency for sunlight.Biochar microspheres（BMs）were prepared by the waste bamboo slices.The Mg O-Zn O and TiO₂-Ag shells were coated on the BMs by sol-gel method respectively,and the metal oxide coated biochar microspheres（DSMs）were obtained.Then,the template inside（BMs）was removed by oxidation,and the hollow catalytic microspheres（HCMs）with inner layer of non-photosensitive catalytic materials（Mg O-Zn O）and outer layer of photosensitive catalytic materials（TiO₂-Ag）were obtained.The catalytic degradation capability of HCMs for the methylene blue（MB）was explored.The results showed that the light absorption wavelength of HCMs was extended to the visible light（580 nm）.The HCMs have excellent photocatalytic degradation ability for the MB due to the^·O₂^-,e^-and·OH that produced in the solution under the simulated sunlight irradiation（Xe lamp,150 W）.The MB solution（100 m L,5 mg/L）can be decomposed with 0.01 g HCMs,and after the 5th use,the removal rate of HCMs for MB can still reach 80%.Compared with the prepared Mg O/Zn O/TiO₂@Fe-BC,commercial anatase and rutile,the light utilization efficiency and catalytic degradation ability of HCMs were higher.The photo-thermal biochar cakes with solar-steam generation capability were prepared.Here,the waste acrylic fiber was used as the backbone,the Mg（OH）₂ was coated onto the fiber,and the bamboo powder or tannery sludge were filled among the fibers.The mixtures were shaped and pyrolyzed in airtight environment.Two kinds of biochar cake(BC_3-0and BC_3-1)with high graphite content and strong hydrophilicity were prepared under the optimal preparation conditions.The dye wastewater of reactive red X-3B was purified by the BC_3-0and BC_3-1.The results showed that the maximal solar utilization efficiency of BC_3-1was 78.98%.Under the simulated sunlight irradiation,most of the pollutants in the reactive red X-3B wastewater（10 g/L）can be removed by the photo-thermal biochar cakes though the solar-steam generation.Especially,the concentrations of reactive red X-3B,COD,TOC,TC and IC in the collected clean water that purified by the BC_3-0were 4.67,17.52,34.33,46.94 and 12.61mg/L,respectively,which achieved the direct emission standards of textile dyeing and finishing wastewater（GB 4287-2012）.