Study On Synthesis Of Hierarchical Porous Biochar By KHCO₃-activated Sludge Pyrolysis And Its Removal Performance For 4-Chlorophenol

As a by-product of urban sewage treatment plants,sewage sludge has a huge output.So its reduction,stabilization and recycling are urgent needs in the field of urban organic solid waste treatment and disposal.Pyrolysis,as a potential sludge treatment and recycling technology,has attracted widespread attention.In this paper,porous sludge-derived biochar with three-dimensional hierarchical pores was prepared using KHCO₃ as the pyrolysis activation reagent.Its adsorption performance for 4-chlorophenol（4-CP）and the catalytic degradation performance for 4-CP coupled with peroxymonosulfate（PMS）were studied.The pore-formation mechanism of hierarchical porous biochar,as well as its adsorption mechanism for 4-CP and catalytic degradation mechanism coupled with PMS for 4-CP were investigated.The main research contents are as follows:1.Preparation of hierarchical porous biochar by KHCO_3-activated sludge pyrolysisTaking the dewatered sludge cake obtained by conditioning municipal sludge with Fe Cl₃ as the research object,and using KHCO₃ as the pyrolysis activation reagent,combined with the post-treatments of water leaching and acid leaching,hierarchical porous sludge-derived biochar with micropores,mesopores and macropores was successfully prepared.The effect of pyrolysis activation temperature on the pore structure of biochar was investigated.The results showed that the hierarchical porous biochar（PSB-900）prepared at900°C had the highest BET specific surface area（1476 m²/g）and total pore volume（1.138cm³/g）.In the activation process of KHCO₃,firstly,the CO₂ generated by the decomposition of KHCO₃ was extruded to form a macropore structure;with the increase of the pyrolysis temperature,K₂CO₃ and K₂O eroded the carbon structure to from micropores,and at the same time,the inorganic components in the sludge（quartz,aluminosilicate minerals such as muscovite）reacted with potassium-containing compounds to form various water-soluble potassium aluminum silicates.During the water leaching process,the dissolution of the water-soluble potassium aluminum silicate in the biochar resulted in the formation of a large amount of mesopores.The remaining inorganic compounds mainly composed of iron and aluminum compounds were removed in the subsequent acid leaching process,and further produced abundant micropores.The analysis of pyrolysis gas and pyrolysis oil showed that KHCO₃-activated sludge pyrolysis could increase the yield of combustible gases such as H₂and CH₄,reduce the yield of SO₂ gas,and lower the release temperature of nitrogen-containing pollutants,which is convenient for centralized treatment.The presence of KHCO₃ could promote the catalytic cracking of tar and improve the quality of pyrolysis oil.2.Adsorption performance of hierarchical porous biochar for 4-CPThe adsorption performance of the hierarchical porous biochar samples prepared at different activation pyrolysis temperatures for 4-CP was investigated.Due to its well-developed pore structure and strong aromaticity,the adsorption capacity of PSB-900 for 4-CP was up to 192.35 mg/g.The adsorption mechanism research showed that the adsorption of 4-CP by hierarchical porous biochar was mainly physical adsorption,and the strong aromaticity of biochar could strengthen its interaction with aromatic pollutants through theπ-πbond.Adsorption kinetics studies showed that when the initial 4-CP concentration was100 mg/L,all the porous biochar samples prepared at different activation pyrolysis temperatures could quickly remove 4-CP in a short time.Especially the adsorption rate of PSB-900 reached 0.020 g/（mg·min）,which could remove 70%of 4-CP in 5 min,far exceeding the adsorption rate of carbon materials reported in literature.The rapid adsorption rate of PSB-900 was due to its large pores and lamellar pore walls,which was beneficial to overcome the resistance of the liquid film during the adsorption process,while the mesoporous structure could reduce the internal diffusion resistance.In addition,PSB-900showed good adsorption performance under a wide p H range（3-11）.The results of adsorption-desorption cycle experiments showed that the adsorption capacity of PSB-900for 4-CP remained above 95%of the initial adsorption capacity after 5 cycles.3.Catalytic degradation performance of hierarchical porous biochar coupled with PMS for 4-CPThe performance of hierarchical porous biochar samples prepared at different activation pyrolysis temperatures as catalysts to activate persulfate（PMS）to degrade high concentration 4-CP（500 mg/L）was studied.With a low PMS dosage（molar ratio of PMS to 4-CP=2）,PSB-900 could still achieve 100%removal of 4-CP in a short time.Catalytic degradation mechanism research showed that the dominant role was electron transfer in the PSB-900/PMS catalytic degradation system,and the increase in pyrolysis temperature enhanced the local density of the electron cloud of the biochar,making the biochar more conductive.In addition,superoxide anion radicals and hydroxyl radicals also played a role in catalytic degradation process of 4-CP.The research results in this paper provide new ideas for the preparation of hierarchical porous sludge-derived biochar and the efficient and rapid removal of organic pollutants from wastewater.