| Heavy metals,due to ultra-high biological toxicity and non degradability,would enter human body through food chain enrichment,leading to the occurrence of various diseases such as headache,anemia,dermatitis,and cancer.Adsorption has the advantages of simple operation,high efficiency,and low cost,and is an important technology for the treatment of heavy metal-contaminated wastewater.The high specific surface area,rich pore structure and functional groups of biomass carbon make it a potential adsorbent for wastewater remediation.The conversion of waste biomass into biochar can realize the dual benefits of waste treatment and environmental remediation.However,biochar is light in weight,and small in particle size,thus is difficult to separate from the liquid.Therefore,the preparation of magnetic biochar by loading magnetic groups and applied in heavy metal adsorption has become a research hotspot in recent years.However,due to the differences of magnetization methods,biomass raw materials,magnetization conditions and target heavy metals,etc.,it is still unclear whether magnetization modification can improve the adsorption efficiency.Based on this,in this paper,a series of magnetic biochar materials were prepared using typical agricultural and forestry waste biomass.The magnetization method（impregnation method and precipitation）,precursor iron solution concentration（0.01-1mol/L）,pyrolysis temperature（300-700℃）,and raw material type（fir wood,bamboo and rice straw）on the characteristics of magnetic biochar and its adsorption capacity for target heavy metal（Pb（Ⅱ）,Cr（Ⅵ））were systematically studied.The materials were characterized by physical and chemical means such as specific surface area（BET）,Fourier transform infrared spectroscopy（FTIR）,X-ray diffraction（XRD）and X-ray photoelectron spectroscopy（XPS）.The relationship between preparation methods,physical and chemical properties,adsorption capacity and mechanism of biochar was systematically revealed,aiming at the directional preparation of magnetic biochar for the treatment of target pollutants.The main content and achievements of this paper are as follows:1.A series of fir wood（FW）magnetic biochars were prepared under different magnetization conditions and pyrolysis temperature,which were used for the removal of Pb（Ⅱ）.The main magnetic products on the surface of magnetic biochar synthesized by impregnation（expressed as Fe（3））and coprecipitation（expressed as Fe（2））are Fe3O4.The adsorption of Pb（Ⅱ）on different biochar conforms to the quasi second-order kinetic model and Langmuir model.The adsorption capacity of magnetic biochar prepared by coprecipitation method for Pb（Ⅱ）is much higher than that by impregnation method.The maximum adsorption capacity is found for FWFe（2）1M700C,which achieves 817.64 mg/g and is about 5-13 times higher than that of FWFe（3）sample and original biochar sample.The mechanism study shows that the adsorption process mainly includes electrostatic,ion exchange,coprecipitation and complexation.The adsorption capacity of Pb（Ⅱ）mainly depends on the p H value and zeta potential of biochar,yet is less affected by the specific surface area and morphology of biochar.Precipitation can improve the p H value of biochar,resulting in stronger electrostatic attraction and ion exchange,and thus higher Pb（II）adsorption capacity.2.The effects of different biomass raw materials on Pb（II）adsorption characteristics were investigated.The adsorption of Pb（II）on biochar prepared from fir wood（FW）,bamboo（BW）and rice straw（RS）accorded with quasi second-order kinetics and Langmuir model.For RS biochar,the maximum adsorption capacity（360.65 mg/g）of RS700C is 3 times and 5.3 times that of BW700C（118.72 mg/g）and FW700C（67.50mg/g）.The main reason is that RS700C has higher alkali metal content,higher p H and richer oxygen-containing functional groups.Magnetization can significantly improve the adsorption capacity of biochar,among which FW biochar increases the most significantly after magnetization（FW700C:67.5 mg/g;FWFe（2）0.1M700C:692.95 mg/g）,BW biochar takes the second place（BW700C:118.72 mg/g;BWFe（2）0.1M700C:780.68 mg/g）,while RS biochar increases the least（RS700C:360.65 mg/g;RSFe（2）0.1M700C:650.65 mg/g）.According to the mechanism analysis,ion exchange and physical adsorption are the main mechanisms.The magnetization process can significantly enhance the precipitation of FW biochar,and enhance the complexation reaction of FW and BW biochar.3.The adsorption of Cr（Ⅵ）by prepared biochars were investigated.The results showed that the magnetic biochars,prepared by impregnation method（RSFe（3）sample）,have excellent removal capacity of Cr（Ⅵ）.The removal rate can reach more than 95%at different p H,and the optimal dosage is 2 g/L.The maximum adsorption capacity of rice straw magnetic biochar RSFe（3）1M700C for Cr（Ⅵ）is 88.94 mg/g.The adsorption process conforms to the quasi second-order kinetics and Langmuir model.After recycling for 5 times,the removal rate of Cr（Ⅵ）can still reach 74.7%.In the alkaline environment（p H=8）within 72 hours,the iron dissolution of RSFe（3）1M700C is only 0.04 mg/L,indicating that RSFe（3）1M700C material has good stability and good reusability. |
