Removal Performances And Mechanisms Of Lead By Phosphate-riched Crab Shell Biochar

Biochar,a carbon-riched porous solid,obtained from biomass pyrolysis at medium temperature under air-limited environment.The functional applications of biochar were closely related to its inherent physicochemical properties,which were greatly controlled by the feedstock types,pyrolysis conditions and pre-treatment or post-treatment methods.In this study,crab shell(CS),a chitin-based biomass,was used as raw material to prepare crab shell biochar(CB)under different pyrolysis temperature.The main objects of this study were to investigate the influence of temperature on the physicochemical properties of CB,explore the adsorption performance of phosphate onto CB and the underlying adsorption mechanisms,and discuss the potential removal of lead via pyrolysis P-loaded CB(CBP)and its possible mechanisms.Results showed that:(1)The physicochemical properties of CB were closely associated with the pyrolysis temperature.Overall,with increasing pyrolysis temperatures(400-700 ?),the yield and volatile matters content of CB decreased,while the ash content,fixed carbon,pH,electrical conductivity(EC),pHzpc and special surface area(SSA)increased.Higher pyrolysis temperature was,the greater aromaticity and stronger hydrophobicity of CB,which were indicative of a stronger environment stability and carbon sequestration potential.As the temperature increased,the nutrient mineral contents presented increase at first,then increase and last increase again.The cation exchange capacity(CEC)increased and then decreased as the temperature increased.Scanning electron microscopy(SEM)revealed that the development of pores in CB enhanced with increasing temperature and the surface was rougher,which resulted in a higher SSA.Thermogravimetric-differential thermogravimetric(TG-DTG)analysis showed that the weight loss of CB decreased with increasing pyrolysis temperature,and mainly weight dropped at 600-800 ? temperature range,which showed that CB characterized with a higher thermal stability.Fourier infrared spectroscopy(FT-IR)and X-ray diffraction(XRD)showed that with the increase of temperature,the stretching vibrations of O-H(3430cm-1)and-CH2(2926/2850 cm-1)on CB weakened or disappeared,while the adsorption peaks at 1400 cm-1 and 874 cm-1(CO32-)strengthened at first and then diminished.In additional,the diffraction peaks represented CaCO3 crystal region also reflected initial increase to decrease later.In conclusion,pyrolysis temperature had a significant influence on physicochemical properties of CB.Pyrolysis temperature at 600 ?,the CB exhibited maximum degree crystalline of mineral components.When the pyrolysis temperature above 600 ?,the structure of CB was destroyed and recrystallized.Compared with agricultural and forestry based waste biochar,CB characterized with a higher ash and pH,a lower carbon and SSA.Therefore,the study,the influence of pyrolysis temperature on the physicochemical properties of CB,could provide basic support for the application of CB.(2)Batch experiment showed that,pyrolysis temperature at 500? for CB(CB500)exhibited a good performance for P043--P enrichment.The enrichment characteristics of PO43--P onto CB500 were investigated under various operating variables like initial concentration of phosphate,reaction time,reaction temperature and solution pH.Langmuir model could better fit the isothermal equilibrium data than the Freundlich model.The correlation coefficient(R2)were all above 0.983 and the maximum adsorption capacities(Qm,cal)were 252.04-262.29mg P/g via the Langmuir model,which indicated that the absorption feature of PO43-P onto CB500 was homogeneous monolayer.All separation factors(RL)values at different reaction temperatures fall within the range of 0-1.0,suggesting that the adsorption of PO43--P by CB500 was benefit.In addition,pseudo-second-order model(R2>0.993)could well describe the adsorption process of PO43--P via CB500,and the maximum theoretical calculated adsorption amount(Qe,cal)were 95.71-118.51mg P/g,which were and closer to the experiment obtained maximum adsorption amount(Qe,exp,90.97-117.71 mg P/g).The PO43--P-riched efficient by CB500 increased with increasing reaction temperature,suggesting the enrichment process be chemisorption.The rate parameter of intra-particle diffusion,Kd1>>Kd2,the constant C was not zero,indicated that the enrichment of PO43-P mainly controlled by the surface adsorption process and intra-particle diffusion process,and intra-particle diffusion process was not rate-limiting step.The thermodynamic parameters(AG°,AH0 and AS°)showed that the PO43--P-riched onto CB500 was a non-spontaneous endothermic reaction.The activation energy(Ea)was 9.46KJ/mol.As the solution pH increased,enrichment capability of PO43--P decreased,and the solution pH declined after P043--P-riched(compared to 0.01M NaCl solution).Compared the FT-IR and XRD analysis before and after PO43--P-riched,after PO43--P-enriched,the stretching vibrations of CO32-(1400 cm-1 and 874 cm-1)and the diffraction peak attributed to CaCO3 crystal region weakened,while the peak at 1062 cm-1corresponed to PO43-vibration and diffraction peak assigned to Cas(PO4)3OH crystal region strengthened,showed that PO43-replaced CO32-forming more stable Cas(PO4)3OH and Ca3(PO4)2.nH2O.In summary,CB exhibited a high enrichment capacity of PO43-P and the adsorption process was non-spontaneous endothermic reaction,which controlled by the homogeneous mono-layer chemical adsorption.Potential enrichment mechanisms included ion exchange,electrostatic interaction,surface complexation and surface precipitation.Consequently,CB derived from CS waste possessed a higher potential to enrich PO43--P from aqueous solution when applied as a functional environmental absorbent.(3)CB showed a good enrichment capability of P043"-P.In order to realize the functional application for phosphate-riched biochar,we discuss the removal performance of lead onto phosphate-riched CB(CBP)through pyrolysis.When the pyrolysis temperature was 600 0C,CBP displayed an effective lead removal performance(up to 92.44%lead removal for pristine CB)and a lower release of solution phosphate(0.72mg P/g,0.67 mg P/g for primary CB).The influences of reaction temperature,contact time,solution pH and initial concentration of lead on removing lead onto CBP were investigated.Results showed that,with the temperature,time,pH and the initial concentration increased,the removal efficiency of lead by CBP increased.By adsorption isotherm models,kinetic model and thermodynamic models were used to fit the experimental data,Langmuir model(R2=0.998)and pseudo-second-order kinetic model(R2?0.984)could well describe the lead removal process,indicated that the removal process of lead by CBP was mainly dominated by homogeneous monolayer chemical adsorption.The maximum adsorption(Qm,cal)calculated from Langmuir model was 1052.02mg Pb/g(Qm,exp was 1047.25mg Pb/g).Thermodynamic parameters AG°(-0.59-0.22KJ/mol),?H°(8.23KJ/mol),?S°(27.56J/(mol·K))and Ea(27.94KJ/mol),suggested that the removal of lead onto CBP was endothermic reaction processes of non-spontaneous at low-temperature,spontaneous at high temperature.Langmuir-Langmuir model(R2=0.990,Qe=1047.25mg/g)indicated that 65.6%lead removal was attributed to precipitation,while 34.4%was attributed to surface adsorption.After lead adsorption,the solution pH(compared with controls,0.01M NaCl)declined.According to FT-IR and XRD for before and after lead adsorption,after lead adsorption,the absor:ption vibration peaks at 3347cm-1(O-H),1729cm-1(C=O),1420cm"1(CO32-),938cm-1(internal bending vibration of O-H),873cm-1(CO32-)changed and appeared new stretching vibration peaks near 1051cm-1,839cm-1 and 678cm-1.In addition,the region corresponded to CaCO3 and Ca5(PO4)3OH or Ca3(PO4)2 nH2O crystal diffraction peaks diminished or disappeared,diffraction peaks presented Pb5(PO4)3OH and PbO crystal region added after lead adsorption.Thus showed that the removal of lead largely contributed to replacing Ca2+ formed more stable Pb5(PO4)3OH precipitate.In conclusion,CBP possessed a huge potential for lead removal,and the possibility removal mechanism involved with complexation,ion exchange,surface precipitation and intra-particle diffusion.Hence,CBP used to lead removal,could reduce the environmental risk of phosphorus-riched sediment,and realized harmless use of resources.