| With the accelerating process of urbanization and the rising standard of living of the people,the production of domestic and industrial wastewater is increasing rapidly,which leads to the production of sludge from sewage water treatment plants has increased sharply.According to the data from the Ministry of Housing and Construction,nearly half of the sludge has not been harmlessly treated,and the sludge problem has become a bottleneck in the field of sewage treatment.The existing treatment and disposal technologies have deficiencies and cannot meet the requirements of sustainable development.The sludge hydrothermal carbonization technology is the hotspot of the research of sewage sludge treatment technology at home and abroad,and it is also a harmless,reducing,and resource treatment technology of sludge which is the direction of development.However,there still has a lot of technical and economic problems to be solved before successfully realizing the industrial-scale application of hydrothermal carbonization technology.Besides,with the development of industry,great amount of industrial wastewater containing heavy metal ions is discharged into water and soil without treatment to meet the standard,which is very harmful to human beings.Using biomass hydrochar as an adsorbent to treat wastewater containing heavy metals has a good treatment effect.It is also an effective way to utilize waste biomass resources,and has the advantages of waste biomass,the wide range of raw materials,low cost and easy operation,and treating waste with waste.Based on the above background,this project adopts the hydrothermal carbonization method to treat the residual sludge of urban wastewater treatment plant,and investigates the effect of different hydrothermal conditions(reaction temperature and residence time)on the solid phase product of sludge hydrothermal carbonization,and innovatively investigated the adsorption properties and mechanism of sludge hydrochar on Cr(Ⅵ).The paper investigated the morphology,structure,chemical composition,and properties of sludge hydrochar by physical and chemical characterization.And BCR three-step continuous extraction method is used to analyze the distribution of heavy metals in sludge hydrochar and conduct environmental risk assessment.Static adsorption experiments were designed to screen out the adsorbed Cr(Ⅵ)effect,and the preparation conditions for the optimal Cr(Ⅵ)adsorption properties and adsorption mechanism of the preferred materials were investigated.The following conclusions:(1)Hydrothermal carbonization can effectively achieve sludge reduction.The yield of solid phase products after hydrothermal carbonization of sludge is 45.60%～68.07%,and the dehydration performance is improved.Elemental analysis,scanning electron microscope,specific surface area and pore structure analysis,Zeta potential analysis and other characterization results show that the content of C,H,N,S,O in the sludge hydrochar decreases,the ash content increases,and the HHV of hydrochar is improved compared with sludge.The sludge hydrochar is a mesoporous material,and hydrothermal carbonization changes the surface morphology of the sludge,forming a rich pore structure on the surface,which increases its specific surface area.FTIR spectrum shows that the sludge hydrochar has retained the original functional groups such as hydroxyl and carboxyl groups after hydrothermal treatment.In acidic solution,the surface of the sludge hydrothermal charcoal is positively charged,and in neutral or alkaline solution it is negatively charged.(2)Hydrothermal carbonization realizes the redistribution of heavy metals between the solid and liquid products of the sludge.Some heavy metals migrate into the liquid products,but most of the heavy metals remain in the solid products,and with the quality of the solid products decreasing,the relative content of heavy metals increases,which are higher than the original sludge content.The heavy metals in the sludge are transferred from the weakly bioavailable binding state(exchangeable and weak acid soluble state,reducible state)to the more stable oxidizable state and stable residue state,which greatly reduces the migration ability of heavy metals in sludge.And it means the heavy metals has stabilized in the sludge.The environmental risk assessment results of heavy metals show that hydrothermal carbonization of sludge will not only increase the potential ecological risk of heavy metals in the sludge but also reduce environmental pollution risk of heavy metals in sludge.by decreasing the mass fraction of heavy metals present in the exchangeable and weak acid soluble state.(3)Within the range of hydrothermal conditions investigated in this study,the preparation condition with the best adsorption performance for Cr(Ⅵ)is HTC240-4.The adsorption effect of sludge hydrochar is affected by factors such as p H,dosage,and coexisting ions.For a solution with an initial Cr(Ⅵ)concentration of 50 mg/L,when the temperature is 25°C,p H=2.5,the dosage of hydrotchar is 10 g/L,the removal rate reaches 99.81%,and the adsorption capacity of hydrochar is 4.991 mg/g.The introduction of competing ions in the solution has a certain inhibitory effect on the adsorption and removal rate of Cr(Ⅵ)by the sludge hydrochar.The order of the influence of the four common anions is CO32->SO42->H2PO4->Cl-.(4)The quasi-second-order kinetic model can better describe the adsorption kinetics of hydrothermal carbon,indicating that a chemical reaction has occurred during the adsorption process.The adsorption isotherm of Cr(Ⅵ)by HTC240-4 conforms to the Langmuir model.At three temperatures,R~2 are all greater than 0.99,indicating that the process of adsorption of Cr(Ⅵ)by hydrochar follows a single layer of uniform adsorption.In the calculation results of thermodynamic parameters,Δ~Θ>0,Δ~Θ<0,Δ~Θ>0,indicating that the adsorption reaction of hydrochar on Cr(Ⅵ)is an endothermic heat that can be carried out spontaneously and the entropy increases reaction.(5)Based on the material characterization analysis of HTC240-4 before and after Cr(Ⅵ)adsorption,the adsorption of HTC240-4 to Cr(Ⅵ)is a combination of physical adsorption and chemical adsorption,and the chemical adsorption includes multiple mechanisms such as ion exchange,complex coordination,and redox adsorption. |
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