Preparation Of Camellia Sinensis Waste Branch-derived N-doped Carbon Materials And Its Applications On Heavy Metal Adsorption And Detection

The plantation area of tea?Camellia sinensis?L.?Kuntze?in China is the largest among the world.Every year,during the process of tea cultivation,processing and the varieties upgrading,a large amount of tea waste resources?about 6 million tons/year?are generated.However,these huge amounts of tea waste resources have not been utilized with high value at present.On the contrary,they were burned or discarded,causing serious air pollution and soil acidification to the environment.More importantly,unlike other biomass wastes,tea has a high content of caffeine?1-3%?and amino acids?2-5%?,making it to be a natural nitrogen-rich material.Therefore,tea waste resources are considered to be the ideal precursor for in situ preparation of N-doped biomass-derived carbon materials.In recent years,due to the rapid industrialization,there have been growing concerns over heavy metals pollution,which could seriously endanger the health of animals,plants and human beings.It has shown that N-doped carbon materials are excellent choices for adsorb anddetect heavy metal ions,owing to their cheap,available and unique physiochemical properties.In this study,tea branches were used as biomass-based precursor,and new N-doped carbon materials were successfully prepared by in situ hydrothermal carbonization method,which not only provides technical support for adsorption and detection of heavy metal ions in sewage,but also can realize high-value utilization of tea waste branches and broaden the application field and industrial value of tea tree resources.The main conclusions of this study are as followed:1. N-doped biochar was successfully prepared via urea/Zn Cl₂ in situ hydrothermal carbonization of Camellia sinensis branches waste at 120–280°C for 2 h under 1.0–9.8 MPa.The structural analysis were characterized by SEM,BET,Raman spectra,XRD and XPS,etc.,and the adsorption properties and mechanism of the biochar for heavy metal ions biochar samples increased significantly,with the maximum doping amount of 7.79%.The pyridinic N,pyrollic N,and graphitic N were the major N species on the surface of biochar.Moreover,the N-doped biochar belong to mesoporous materials.Furthermore,among the biochar samples,UHC-240 showed good adsorption capabilities for heavy metal ions Cu²⁺,adsorption isotherm model and the pseudo-second-order kinetic fitted the adsorption data well.2. N-doped biochar were successfully prepared by KOH/NH₄Cl in situ hydrothermal carbonization at 120-280°C for 2h under 0.4-6.5 MPa using tea branches waste as raw material.The structural analysis was characterized and the adsorption properties and mechanism were studied.The consequences showed that the maximum content of N reached up to 6.18%.Moreover,biochar has a large number of mesoporous structures.In addition,behavior had a better adjustment to the Langmuir adsorption isotherm and the pseudo-second-order kinetic models.3. N-doped carbon fibers were successfully prepared by urea/Zn Cl₂ hydrothermal carbonization of tea branches waste at 160-280°C for 2h under 0.8-8.9 MPa.The structural analysis and the adsorption properties and mechanism for heavy metal ions of the carbon fibers were studied.The characterization results showed that carbon fibers have rich pore structures,belonging to mesoporous structure with the pore diameters ranging in 2-30 nm.Under the high temperature and high pressure conditions,the N content in carbon fibers significantly increased to 8.96%,with graphitic N and pyridinic N being the main forms.Furthermore,carbon fiber NCF-240 has a higher adsorption capacity for heavy metal ions Langmuir isotherm model and the pseudo-second-order kinetic can well fit the adsorption data.4. N-doped carbon quantum dots were prepared by urea/Zn Cl₂ in situ one-pot hydrothermal method derived from tea branches at 240°C for 6h under 6.5 MPa.The characterization of N-doped carbon quantum dots was analyzed by TEM,Raman spectra and XPS,etc.,while UV-vis and fluorescence spectrophotometer were applied for fluorescence spherical nanoparticles with uniform size and good dispersibility.The CQDs surface contained with abundant oxygen-containing and nitrogen-containing functional groups.Furthermore,a broad shoulder peak at 310 nm was obtained via UV absorption spectra.The fluorescence properties showed that the excitation spectrum depend on the emission wavelength.The maximum excitation wavelength and the optimal emission wavelength are310 nm and 400 nm,respectively.Moreover,N-doped carbon quantum dots have good fluorescence stability in the solutions with p H ranging from 5.0 to 9.0.Additionally,heavy N-doped carbon quantum dots with the increase of ions concentration.