Adsorption Performance And Mechanism Of Four Kinds Of Ions By Mg-loaded Biochar

Nutrition wastewater and electroplating wastewater are two major wastewater systems that the world needs to face at present.At present,there are many traditional treatment methods,many of which can produce practical effects.Typical wastewater treatment methods are electrolysis,physical chemistry,and ion exchange.As a general adsorption method,adsorption method has the advantages of efficient treatment,simple operation,recyclability,and effective adsorption of various pollutants,which conforms to the current concept of green recycling.Most scientific researchers are actively involved in research of the adsorption method,in which biochar is widely studied.Biochar is a kind of heat-limited pyrolysis product with high carbon content and a large number of functional groups.Due to its wide sources,common plant straw,animal manure,and sludge can be used as raw materials for biochar.Thus,biochar is very popular in the research field due to its low preparation cost and strong regeneration.In this study,MgCl₂·6H₂O was used in the wet impregnation process,Six kinds of magnesium modified biochar were prepared through impregnation modification:Banana Straw Biochar?BSB?,Cassava Straw Biochar?CSB?,Taro Straw Biochar?TSB?,Camellia Oleifera Shell Biochar?CFSB?,Maize Straw Biochar?MSB?,and Chinese Fir Straw Biochar?FSB?.The removal of ammonia nitrogen?NH₄⁺-N?,total phosphorus?TP?and heavy metals?Cd?II?,Cr?VI??was studied by means of the adsorption kinetics equation and adsorption isothermal model.The adsorption mechanism was investigated by means of Scanning Electron Microscope-Energy Spectrum Analysis?SEM-EDS?,X-ray powder diffraction?XRD?,Fourier infrared analysis?FTIR?,X-ray electron diffraction?XPS?,specific surface area?BET?,and High Resolution Transmission Electron Microscope?HTEM?.Six kinds of Mg-loaded biochars that were prepared in this study showed good crystallization.The XRD analysis showed that MgO and Mg₂?OH?₃Cl·4H₂O are the main crystal components attached to the biochar surface.According to Image J,the average particle sizes of BSB,FSB,MSB,CFSB,TSB,and CSB are 33.30 nm,6.44 nm,9.47 nm,3.27 nm,6.70 nm,and 5.46 nm,respectively.The experimental results of the adsorption of NH₄⁺-N and TP by the biochars showed that the pseudo-second order kinetic equation and Langmuir model can better describe the adsorption process of NH₄⁺-N and TP by the 6 biochars,which is approximately a single-layer chemical adsorption process.The adsorption of ions by the 6 kinds of biochar such as BSB is a multi-step process,including surface adsorption and intra-particle diffusion processes.The best adsorption effects for NH₄⁺-N and TP were seen in CSB and BSB,respectively where the theoretical saturated adsorption amounts reached 24.04 mg/g and 31.15 mg/g.Correlation analysis of magnesium loading,total pore volume,and average pore size showed that the adsorption capacity of NH₄⁺-N and TP by magnesium loading showed a strong correlation;the adsorption capacity of NH₄⁺-N and TP by total pore capacity showed strong correlation and moderate correlation,respectively;the correlation between the average pore size for NH₄⁺-N and TP adsorption has weak correlation and extremely strong correlation.According to the FTIR,XRD,SEM-EDS,and BET analyses,and the pH and pH_PZCZC results of the biochar solutions,the adsorption of NH₄⁺-N by the biochars is mainly dominated by ion exchange,complexation precipitation,and functional group;the adsorption of TP by the biochar is mainly dominated by electrostatic attraction and complexation.The experimental results of the adsorption of Cd?II?by the 6 biochars showed that the adsorption of Cd?II?by the 6 kinds of biochar were in accordance with the pseudo-second order kinetic equation and Langmuir model.The theoretical saturated adsorption amounts of Cd?II?by BSB,CSB,FSB,CFSB,MSB,and TSB were 333.33 mg/g,238.10 mg/g,75.19mg/g,96.15 mg/g,66.23 mg/g,and 185.19 mg/g respectively,as calculated by the Langmuir model.Based on the results of the SEM-EDS,FTIR,XRD,XPS,and adsorption contributions,the total contribution of mineral precipitation and cation exchange in BSB and CSB adsorption contributions accounted for 92.40%and 77.89%of the total adsorption amount.Cd-?interaction and functional groups.The sum of the contribution of complexation only accounted for 7.60%and 22.11%of the total adsorption.In the CFSB and TSB samples,the Cd-?interaction adsorption contributions accounted for 59.96%and 50.43%,respectively.The proportions of ion exchange,mineral precipitation,and functional complexation were13.45%,19.19%,and 10.41%;34.44%,8.86%,and 6.27%,respectively.For the FSB and MSB samples,the adsorption mechanisms are relatively uniform,as the sum of the contribution of mineral precipitation and ion exchange was 58.18%and 57.07%,respectively.Based on the 6 types of biochar adsorption mechanisms for Cd?II?,Cd-?interaction,ion exchange,mineral precipitation,and functional group controlled adsorption processes are integrated.Among them,mineral precipitation and cation exchange dominate.The experimental results for the adsorption of Cr?VI?by the 6 biochars showed that the adsorption of Cr?VI?by the 6 kinds of biochar was in accordance with the pseudo-second order kinetic equation and Langmuir model,which was approximately a monolayer chemisorption process.BSB had the largest adsorption capacity,up to 125.00 mg/g,which may be due to the large specific surface area and abundant functional groups of BSB.Through XRD analysis and comparing with the standard powder association card library,it was concluded that the complexes embedded in the biochar after adsorption of Cr?VI?in CFSB,CSB,TSB,FSB,and MSB mainly appear in the form of MgCrO₄·7H₂O,while BSB mainly appears in the form of Cr?OH?₃.Through SEM-EDS,FTIR,and XRD characterization,it was concluded that electrostatic attraction,functional group bonding,and complexation are the main mechanisms for Cr?VI?adsorption in the 6 biochars.