Design Of Biochar And Its Removal Of Hexavalent Chromium From Water

The development of human civilization has been accompanied by the emergence of environmental issues.Hexavalent chromium(Cr(Ⅵ))pollution of water is particularly severe,and its mobility and toxicity seriously threaten the environment and human health.Owing to its unique advantages,such as being pollution-free and low-cost,biochar has been widely used to remove Cr(Ⅵ)from water.However,the adsorption capacity of biochar is lower than that of traditional adsorbents,such as activated carbon;therefore,the modification of biochar has become a research hotspot.At present,most modification methods are complex in operation and expensive,and some even cause environmental pollution.Therefore,the development of efficient and environmentally friendly modification methods for biochar is of great significance.This study used different modification methods to adjust the surface structure,elemental content,and functional groups of microwave-pyrolyzed soybean residue biochar.Based on this,biochar ’s mechanism of Cr(Ⅵ)removal was explored.In addition,the adsorption performance of biochar is improved through modification,and the modification cost is reduced,thereby promoting the application of biochar in the treatment of heavy metal wastewater.(1)Biochar IBC-18 was prepared by freezing water-bearing soybean residue in liquid nitrogen for 30 min,followed by microwave pyrolysis for 18 min.Rapid freezing generates interconnected ice inside the soybean residue,which sublimates during microwave pyrolysis,forming interconnected pores in the carbonized soybean residue.Compared to unfrozen microwave pyrolysis biochar BC-18,IBC-18 exhibited a sheet-like structure rather than a block structure,and its specific surface area increased from 1.1 m2/g to 14.3 m2/g,while the micropore content also increased.Moreover,the uniform distribution of the hierarchical pores in IBC-18 promoted ion transfer and adsorption.The adsorption experiment showed that the theoretical maximum adsorption capacities of BC-18 and IBC-18 for Cr(Ⅵ)were 52 mg/g and 70 mg/g,respectively,indicating a significant improvement in the adsorption performance of IBC-18.The Ice-template/Microwave method is simple and efficient,and the prepared soybean residue biochar exhibits excellent removal performance.Therefore,this method is suitable for industrial applications.(2)N2,NH3·H2O and H2O were used to physically activate BC-15 under different conditions,and A900-0.5 modified with NH3·H2O at 900℃ for 0.5 h showed the best adsorption performance.The adsorption capacity of A900-0.5 for Cr(Ⅵ)reached 67 mg/g,which was 6.7 times higher than that of BC-15,while the adsorption capacities of N900-0.5(N2 atmosphere)and W900-0.5(H2O atmosphere)under the same conditions were 14 mg/g and 22 mg/g,respectively.Correspondingly,the specific surface areas of N900-0.5,A900-0.5,and W900-0.5 were 75.5 m2/g,386.5 m2/g,and 332.5 m2/g,respectively,and micropores were mainly found in all three biochars.In addition,the NH3·H2O treatment increased the N content of the biochar,resulting in more pyrrole and pyridine.Their peripheric sp2-hybridized Cconjugated structure produced a p-type doping effect by donating p-electrons and reducing Cr(Ⅵ)to Cr(Ⅲ).A900-0.5 also had a higher graphitization degree,which is conducive to electron transfer between biochar and Cr(Ⅵ).The adsorption experiment showed that the adsorption process of Cr(Ⅵ)by A900-0.5 followed the pseudo-second-order kinetic model and Langmuir isotherm model,with a maximum adsorption capacity of 92 mg/g.This adsorption process is a spontaneous endothermic reaction involving both chemical adsorption and physical adsorption.(3)FNS-X,a Fe/N co-doped biochar,was prepared within 10 min using a one-step microwave method.This method can improve the production efficiency and adsorption performance of biochar while reducing costs.This preparation method involves uniformly mixing the water-bearing soybean residue with Fe(NO3)3 and NH4Cl for microwave pyrolysis.Fe(NO3)3 can increase the microwave absorption efficiency of biomass,while the gas generated by the high-temperature decomposition of NH4Cl can etch biochar.In addition,NO3and NH4-also react to generate N2,increasing the specific surface area and pore content of the biochar.During this process,Fe and N were doped into biochar.Among the samples prepared under different conditions,FNS-7 exhibited the best removal performance at pH=1,with a maximum adsorption capacity of 210 mg/g for Cr(Ⅵ).This adsorption process followed the pseudo-second-order kinetic model and Freundlich isotherm model,mainly involving chemical adsorption.The characterization results showed that a large number of mesopores(71.80%)and N doping(16.60%)contained in FNS-7 played an important role in the removal of Cr(Ⅵ)by FNS-7.The above method can not only rapidly prepare low-cost and highperformance biochar but also does not produce waste solutions during the preparation process,avoiding environmental pollution.Therefore,this method has great potential for practical application.