Preparation Of Biochar And Magnetic Biochar And Their Adsorption And Removal Of Cu²⁺, Pb²⁺ And Zn²⁺ In Aqueous Solution

When heavy metals are released into water bodies through human activities and other means,they will cause serious water pollution.At the same time,due to their non-degradability,heavy metals may accumulate in human bodies through the food chain,and subsequently cause serious harm to human health.The solving of heavy metal pollution problem in water bodies has become one of the most important research topics.As an important adsorbent for the treatment of heavy metal polluted wastewater,biochar has vantages of high efficiency and simplicity,but is difficult to recycle and utilize,which makes biochar limited when it is actually put into use.When magnetic media are loaded on the surface of biochar,the adsorbent may be completely separated from the adsorption system by external magnetic filed after adsorption,which can greatly reduce the cost of wastewater treatment.In this paper,waste grapefruit peel was carbonized at different pyrolysis temperatures to produce biochar adsorption materials(BC),which was then loaded by magnetic ferroferric oxide(Fe3O4)to obtain magnetic biochar adsorption materials(MBC)that could be recycled by external magnetic field.At the same time,BC and MBC prepared at different pyrolysis temperatures were characterized by means of fourier transform infrared spectrometer,X-ray diffractometer and specific surface area and gap analyzer,and the kinetic and thermodynamic characteristics of the adsorption of Cu2+,Pb2+and Zn2+on BC and MBC and the competitive adsorption of them were studied.In addition,the effects of pyrolysis temperature,initial concentration of metal ions,ambient temperature,pH of solution and addition amount of adsorbents on the adsorption of Cu2+,Pb2+and Zn2+on BC and MBC were studied in a single solution system.The following results and understandings were obtained:(1)The results of characterization showed that the specific surface area of biochar decreased slightly after magnetic treatment,and the higher the pyrolysis temperatures were,the larger the specific surface area.MBC was more abundant in surface functional groups than BC,which was rich in unsaturated carbon double bonds,Fe-O bonds and other active groups,which were conducive to MBC adsorbing metal ions through chelation.The characteristic peak of amorphous carbon appears in BC with no magnetic treatment,which was an amorphous structure.MBC had obvious Fe3O4 crystal phase.The higher the pyrolysis temperatures were,the sharper the characteristic peak and the higher the strength of Fe3O4 crystal phase.(2)The kinetic test results showed that the adsorption processes of Cu2+,Pb2+and Zn2+in aqueous solution by BC and MBC basically reached equilibria at 60 min,and the adsorption process of Cu2+,Pb2+and Zn2+ in aqueous solution by BC and MBC could be well fitted with the quasi second-order kinetic model,and the correlation coefficients(R2)were greater than 0.990 and 0.995,respectively.The calculated equilibrium adsorption capacities(qe,cal)were close to the experimental values(qe,exp).(3)The thermodynamic test results showed that the adsorption processes of Cu2+,Pb2+and Zn2+by BC and MBC were more consistent with the Langmuir isothermal adsorption model,and the reactions mainly occurred in monolayer adsorption.The adsorption capacities of BC600 to Cu2+,Pb2+and Zn2+were maximum at 45℃,and the theoretical maximum saturated adsorption capacity was 8.19,32.19 and 5.65 mg/g,respectively;the adsorption capacities of MBC600 to Cu2+,Pb2+ and Zn2+ were maximum at 45℃,and the theoretical maximum saturated adsorption capacity was 8.25,27.68 and 5.30 mg/g,respectively.These indicated that the adsorption effects of BC and MBC to three kinds of heavy metal ions were different;BC and MBC were more suitable for Pb2+polluted water,followed by Cu2+,while Zn2+had relatively weak adsorption effect.Thermodynamic parameter analysis results showed that the adsorption of Cu2+,Pb2+and Zn2+by BC and MBC was a spontaneous endothermic reaction due to ΔGO<0 and ΔH0>0 in the adsorption system,and the high temperature was conducive to the adsorption reaction.(4)In a single heavy metal ion adsorption system,the adsorption capacities of Cu2+,Pb2+and Zn2+in aqueous solution by BC and MBC increased rapidly at first and then tended to be gentle with the increase of initial concentration.At the same initial concentration,the adsorption effect of BC on Cu2+,Pb2+and Zn2+ was BC600>BC500>BC400,while MBC500 and MBC600 were MBC500>MBC600 at the same initial concentration,and MBC400 was relatively weak.For Zn2+,the higher the pyrolysis temperature was,the higher the adsorption effect of MBC,being MBC600>MBC500>MBC400.On the whole,the increasing of ambient temperature and solution pH value were beneficial to the adsorption of Cu2+,Pb2+and Zn2+by BC and MBC.With the continuous increasing of BC and MBC dosage,the adsorption capacity of Cu2+,Pb2+and Zn2+showed a trend of sharp increasing first and then slow increasing,and the change curve of Ct/C0 showed a trend of rapid decreasing first and then gradual gentle.Moreover,BC and MBC with higher pyrolysis temperature removed Cu2+,Pb2+and Zn2+ more thoroughly.(5)Since the co-existence system of a variety of ions often occurs in actual water pollution,this study carried out ion competition adsorption experiments.The results of binary competition adsorption experiments showed that in the Cu2+-Pb2+ system,Pb2+was more competitive adsorption advantages than Cu2+;in the Cu2+-Zn2+ system,Cu2+was more competitive than Zn2+;in the Pb2+-Zn2+ system,Pb2+had more competitive adsorption advantages than Zn2+.In the co-existence system of Cu2+,Pb2+and Zn2+,the adsorption selection order was Pb2+>Cu2+>Zn2+on BC and MBC.