Fabrication And Luminescence Mechanism Of Carbon Quantum Dots From Tofu Wastewater

In general,tofu yellow serofluid produced during the production of tofu was discharged as wastewater.Because the tofu yellow serofluid was composed high concentrations of organics(such as protein,amino acids,lipids,sugars,citric acid,organic acids,water-soluble vitamins,etc.),the direct discharge of tofu yellow serofluid was caused environmental pollution.In this paper,Shi Ping tofu wastewater(yellow serofluid)was used as the carbon source,through the hydrothermal or pyrolytic reaction,organic matters in the yellow slurry were carbonized at high temperatures to form carbon quantum dots(CQDs).Chemical modifiers passivate them to water-soluble,highly fluorescent carbon quantum dots.This method was used a simple process to utilize the yellow serofluid of tofu to produce high value-added carbon quantum dot luminescent materials,turning waste into treasure,and at the same time greatly reducing chemical oxygen demand COD,making it meet the national emission standards,from the source.It solved the pollution problem that the Shi Ping Yi Long Lake National Wetland Park suffered from tofu wastewater pollution for many years.The innovative research results in this paper were as follows:1.Using Shi Ping’s tofu wastewater(yellow serofluid)as a carbon source,the organic matter in the yellow pulp water was carbonized by pyrolysis to form carbon quantum dots(CQDs),which were passivated and regulated by different chemical modifiers.The fluorescence wavelength makes it a wavelength-tunable water-soluble high-fluorescence carbon quantum dot.2.Using deionized water-dispersed CQDs to make them functionalized,XPS test results show that there are many kinds of hydrophilic oxygen-containing functional groups on the surface of CQDs,such as-COOH,-O-C=O,-OH,and these functional groups lead to superior water solubility of CQDs.The bright blue fluorescence emits from CQDs under UV light of 365 nm,so the fluorescence intensity and wavelength of CQDs can be modulated by changing the modifier.3.Deionized water was used to disperse CQDs to make them functionalized.TEM test results show that the size distribution of carbon quantum dots is focused on the 2 to 10 nm,and the average size of 3.5 nm,with uniform distribution,high crystallinity,and clear crystal lattice stripe,lattice spacing of about 0.22 nm,corresponding to the crystal orientation(100).Bright green fluorescence emits from CQDs under ultraviolet irradiation of 365 nm wavelength.4.The CQDs modified with sodium hydroxide solution exhibit multiple absorption peaks.There were obvious characteristic absorption peaks near 220 nm and262 nm,which were caused by the π-π* transition in the C=C double bond.The weak absorption was observed near 325 nm.It was closely related to the emission fluorescence of CQDs,which was derived from the n-π* transition of C=O double bond.Bright green fluorescence emits from CQDs under ultraviolet irradiation of 365 nm wavelength.