Study On The Directional Regulation Of Biochar Structure And Its Adsorption And Mass Transfer Mechanism For Organic Pollutants

At present,China is mainly based on non renewable energy such as coal,oil and natural gas.In order to achieve carbon peak by 2030 and carbon neutralization by 2060,developing renewable biomass energy with zero carbon emission is one of the most important means.Converting solid wastes into biochar can reduce greenhouse gas emissions and achieve carbon sequestration,which is conducive to promoting the development of circular economy.There are many kinds of preparation and application of biochar,among which chemical activation of waste biomass and its application to the removal of emerging pollutants are the most common.The preparation of biochar and the diffusion and adsorption of pollutants are essentially the construction of micro-nano interface and the process of mass transfer of pollutants in the main phase and micro-nano interface.In recent years,significant progress has been made in the preparation and development of biochar,there are still shortcomings such as environmental pollution,poor removal effect of emerging pollutants,and unclear understanding of mass transfer mechanism of adsorption process.Based on this,this paper uses cotton straw and alfalfa as biomass materials,and tetracycline,norfloxacin,methylene blue,methyl orange and Rhodamine B as pollutant models.Combined with the co-activation method to improve the adsorption performance of biochar,the micro-nano interface construction to enhance the catalytic degradation performance of biochar,the computer simulation to study the mass transfer behavior of adsorbate molecules at the micro-nano interface,and the systematic research on the preparation of biochar,adsorption,catalytic degradation,pore volume and surface diffusion model and the interaction mechanism of adsorbent and adsorbate were carried out.The main research contents are as follows:（1）Cotton straw hydrothermal carbon（CSHC）was modified by H₂SO₄,KOH and KMn O₄to remove tetracycline（TC）and norfloxacin（NOR）.The adsorption mechanism was revealed by experiments and density functional theory（DFT）.The experimental results show that CSHC-KMn O₄has the strongest adsorption capacity for TC,while CSHC-H₂SO₄has the best adsorption capacity for NOR,and the adsorption performance of the two adsorbents is less affected by the coexistence of antibiotics.Pseudo-second-order kinetics and Sips isotherm model can well fit the adsorption process of TC and NOR on the surface of cotton straw hydrothermal carbon.DFT results showed that the interactions between adsorbent and TC（or NOR）includedπ-πstacking,electrostatic interaction and hydrogen bonding.In addition,during the adsorption process,HC as an electron acceptor has an electrophilic site,while NOR as an electron donor（O atom）has a nucleophilic site.（2）Density functional theory（DFT）,Monte Carlo simulation（GCMC）and molecular dynamics（MD）were used to study the adsorption and diffusion properties of benzene and its derivatives in the graphite slit model.The effects of temperature,slit width and functional groups on the adsorption and diffusion properties of benzene and its derivatives were discussed.DFT results show that the introduction of functional groups significantly changes the charge distribution of the graphite slit model and enhances its electronegativity.GCMC results showed that the adsorption capacity of guest molecules was proportional to the pressure and decreased with the increase of temperature.At saturated vapor pressure,the graphite slit model with an effective pore size of 10.2（?）has the largest adsorption capacity for guest molecules.In the graphite slit model modified by different functional groups,for benzene,ethylbenzene and chlorobenzene,the order of functional groups on the adsorption performance is-COOH>-OH>-NH₂,and the effect of amino groups on nitrobenzene is greater than that of hydroxyl groups.When graphite slits are modified by functional groups,Coulomb interaction plays a major role and hydrogen bonds are formed.The results of molecular dynamics show that the diffusion coefficient of guest molecules is proportional to the temperature and has little change in the mesoporous model.More importantly,the guest molecules are favorable for adsorption in micropores and diffusion in mesopores.It provides theoretical guidance for the preparation of adsorbents.（3）Cotton straw porous carbon（Fe@PGHC）was prepared by co-activation method,was used to adsorb methylene blue（MB）and methyl orange（MO）,and its adsorption mechanism was revealed by experiments and density functional theory（DFT）.The experimental results show that Fe@PGHC has a large specific surface area,micropores and mesopores are evenly distributed on the surface,and has a certain graphitization structure.Sips and pseudo-second-order kinetics models can describe the adsorption process more accurately.DFT results show that there areπ-π,electrostatic and hydrogen bond interactions between MB/MO and Fe@PGHC.More importantly,the increase of oxygen-containing functional groups can improve the adsorption of MB and reduce the adsorption performance of MO.In the adsorption process,MO as an electron donor（O atom）has nucleophilic sites,and MB as an electron acceptor has electrophilic attack sites.（4）Using alfalfa as raw material,potassium hydroxide and ferric chloride as activators,nitrogen doped porous graphitized biochar（NPGBCs）were prepared by co-pyrolysis,which was used to adsorb methylene blue（MB）and methyl orange（MO）.The adsorption mechanism was revealed by experiments and density functional theory（DFT）,and the effects of different N element types were proved.The characterization and adsorption performance results showed that NPGBCs had a large specific surface area（950.52 m²/g）,graphitization structure and excellent adsorption performance for MB（326.90 mg/g）and MO（906.52 mg/g）.Sips isotherm and Pseudo-second-order kinetic model can better describe the adsorption process.DFT results show that the interactions between MB/MO and NPGBCs includeπ-πinteraction,electrostatic interaction and hydrogen bond interaction.In addition,it was also found that pyrrole N and pyridine N played a decisive role in the removal of MO.（5）Biochar（BC）was prepared by co-activation of alfalfa,potassium hydroxide and calcium hydroxide.Then a new BC/2ZIS/WO₃photocatalyst was prepared by hydrothermal method for synergistic adsorption and photocatalytic degradation of methylene blue（MB）and Rhodamine B（Rh B）.Through experiments and density functional theory（DFT）,the mechanism of adsorption and degradation was revealed,and the possible degradation path was revealed.In the adsorption process,BC/2ZIS/WO₃follows the Sips isothermal model,and the maximum adsorption capacity of MB and Rh B is 120.04 and 466.55 mg/g respectively.A series of characterization and photoelectrochemical analysis confirmed that the introduction of BC improved the separation efficiency of photogenerated carriers and inhibited the recombination of photogenerated electron hole pairs.The photocatalytic results showed that the removal rates of MB and Rh B by BC/2ZIS/WO₃were 80.5%and 99%,respectively.In addition,DFT calculation shows that S-19（MB）and N-18（Rh B）atoms are the most vulnerable sites to free radical attack in catalytic degradation.（6）NBCs/MnFe₂O₄composite was prepared by hydrothermal method.It was used to catalyze the degradation of tetracycline（TC）in the ultraviolet activated sulfite system.The adsorption and degradation mechanism were studied by means of experiment and DFT.The characterization results show that NBCs have large specific surface area（985.20 m²/g）,graphite like structure and developed pore structure.The adsorption experiments show that the adsorption process can be better described by the Sips isotherm and the Pseudo-second-order kinetic model.The maximum adsorption capacity of NBCs/Mn Fe₂O₄-2:1 for TC is 347.56 mg/g.The photocatalytic results showed that NBCs/Mn Fe₂O₄-2:1 had the highest degradation efficiency of TC（84%）,and Na Cl and Na₂SO₄had obvious inhibitory effect on TC degradation under alkaline conditions.Electrochemical experiments show that the introduction of NBCs improves the separation efficiency and transfer rate of photogenerated electrons and holes.The results of DFT and LC-MS showed that the C-1（TC）atom was the most vulnerable reaction site to free radical attack,and the degradation process of TC included demethylation,C-N bond breaking,quinonization and ring opening reaction.（7）The pore volume and surface diffusion model was used to analyze the organobentonite-phenol system and activated carbon fiber-pentachlorophenol system.In the initial stage of adsorption,there is a high quality absorption gradient in the adsorbent.With the increase of adsorption time,the mass gradient gradually decreases,and the mass transfer direction of the adsorbent remains toward the core of the adsorbent.It can be seen from the percentage of surface diffusion flux that surface diffusion plays a dominant role in the diffusion of the whole particle.The Biot numbers（Bi）of organic bentonite and activated carbon fiber are between 0.5 and 30,indicating that external mass transfer diffusion and internal particle diffusion simultaneously control the adsorption process.