Preparation Of Biochar From Reed Roasting Coupled With Pyrolysis And Its Adsorption Performance On Tetracycline In Water

In this study,we focused on the resource utilization of Phragmites australis(PAS)and the problem of tetracycline pollution in water,and carried out two parts of research content:"Torrefaction coupled pyrolysis of PAS and biochar preparation"and"Tetracycline adsorption".Specifically,the PAS was prepared into biochar through"atmospheric pressure torrefaction coupled pyrolysis","gas-pressurized torrefaction coupled pyrolysis"and"direct slow pyrolysis",and the physicochemical properties of the torrefied PAS and biochars derived from raw and torrefied PAS were analyzed.Biochars were used in the tetracycline adsorption test in water,the adsorption effect under different conditions was investigated,thermodynamic and kinetic analysis was carried out,and biochars were characterized by FTIR,SEM,XRD and EDS.The main results are as follows:(1)After atmospheric pressure(AP)torrefaction,the physicochemical properties of PAS were significantly improved.Proximate analysis results showed that the high heating value(HHV)increased from 17.55 MJ/kg to maximum 23.32 MJ/kg,and the volatile content(VM)decreased from 69.99%to minimum 29.64%.The fixed carbon(FC)and ash content increased from 13.00%and 17.01%to maximum 20.67%and53.45%,respectively.Elemental composition data showed that the O/C and H/C were reduced from 1.081 and 0.129 to minimum 0.582 and 0.069,respectively.The fiber composition data showed that the hemicellulose content decreased from 29.6%to minimum 13.7%,and the lignin and cellulose content increased from 12.7%and 22.3%to maximum 27.1%and 42.4%.The bulk density decreased from 281.04 kg/m~3 to minimum 132.31 kg/m~3,and the hydrophobicity was significantly enhanced.In addition,proximate analysis and fiber composition showed a good correlation with color value,and R~2 can reach up to 0.9506 and 0.9834,respectively.After gas-pressurized(GP)torrefaction,the physicochemical properties of PAS were further improved,and the physicochemical properties of GP torrefied PAS was more uniform than that of AP torrefied PAS.Specifically,the HHV increased to maximum 22.93MJ/kg,the O/C and H/C were reduced to minimum 0.560 and 0.052,the cellulose and hemicellulose content decreased to minimum 5.5%and<0.1%,the lignin content increased to maximum 70.3%,the VM content decreased to minimum 24.28%,the FC and ash content increased to 46.05%and 29.67%,the hydrophobicity was enhanced and better than that of AP torrefied PAS.The bulk density was slightly reduced(minimum is 204.03 kg/m~3),and higher than that of AP torrefied PAS.Proximate analysis,elemental composition and fiber composition showed a good correlation with color value,and R~2 can reach up to 0.9959,0.9999,and 0.9999,respectively.(2)After AP and GP torrefaction,the combustion and pyrolysis performance of PAS were significantly improved.Specifically,the combustion stability was reduced,and the combustion performance of the GP torrefied PAS was lower than that of the AP torrefied PAS,indicating that the GP torrefied PAS is easier to burn and more suitable for fuel applications than AP torrefied PAS.According to the KAS and Coats-Redfern analysis,after AP and GP torrefaction,the pyrolysis performance of PAS was more stable,and the pyrolysis performance of the GP torrefied PAS was more stable than that of the AP torrefied PAS.(3)The results of biochar preparation and characterization showed that AP torrefaction was beneficial to increase the specific surface area of biochar,while GP torrefaction showed the opposite trend.Compared to biochars derived from GP torrefied PAS,biochars derived AP torrefied PAS have a better hydrophilicity,and the amount of oxygen-containing functional groups of biochars derived AP torrefied PAS were more than that of biochars derived from GP torrefied PAS.The types and amount of functional groups of P-Raw(biochar prepared by pyrolysis of PAS at 675?)and P-AP9-270(PAS was first torrefied with 9%oxygen-containing nitrogen gas at 270?,and then pyrolyzed at 675?)were not much different,mainly including hydroxyl,carboxyl,aldehyde and phenolic hydroxyl.P-AP9-270 had a larger number,more regular arrangement and more uniform pore structure than P-Raw.The crystallinity of biochars was low,and the contents of C and O on the surface of P-Raw and P-AP9-270were not much different.It was speculated from the data that the ash content of P-Raw was higher than that of P-AP9-270.(4)The results of the tetracycline adsorption test showed that the removal rate of tetracycline by biochars showed a continuous downward trend with the increase of the initial concentration,and the adsorption amount showed an upward trend;the removal rate and the adsorption amount showed a continuous upward trend with the increase of temperature,and the continuous time increased;under all conditions,the adsorption performance of P-AP9-270 was better than P-Raw.Both Langmuir and Freundlich models can fit well the adsorption process of biochars to tetracycline(R~2 can reach up to 0.9990 and 0.9970,respectively).The?G of biochar adsorption to tetracycline was lower than 0,and the value of?G decreased with temperature increasing,indicating that the adsorption process is a spontaneous endothermic reaction.The?H of the adsorption reaction of biochars to tetracycline was higher than 0,indicating that the adsorption process is an endothermic reaction.The?S of the adsorption reaction of biochars to tetracycline was higher than 0,indicating that the adsorption is an irreversible adsorption process.The adsorption kinetics results showed that the adsorption kinetics of biochar to tetracycline was more in line with the intra-particle diffusion equation(R~2 was in range of 0.9572?0.9947).