Preparation Of New Graphite Phase Carbon Nitride And Its Application In The Detection And Treatment Of Environmental Organic Pollutants

In recent years, water had been seriously polluted with the industrial wastewater and domestic sewage and other emissions. The United States Environmental Protection Agency ?USEPA? provided for the control of 129 toxic substances,including 114 organic pollutants, which based on the toxicity of pollutants, the possibility of biodegradation, and the probability of occurrence in water. Organic pollutants were in the water for a long time and they were accumulation and toxic.They had toxic and carcinogenic properties for humans. Because of their trace concentrations in the water environment, it is critical to develop effective analytical techniques to analyze the traceable compounds. What is more, when the organic pollutants were into the environment to reach a certain concentration in production and using process, they would bring unpredictable damage to agriculture and forestry and they would also affect the normal life of humans, animals and plants. Therefore,people not only need to detect the organic pollutants in the environment,but also they need to adsorb and degrade these pollutants. In order to achieve better enrichment,adsorption and degradation effects, the materials used in the pretreatment were usually required large surface area, easy to disperse, the better effect with the target analytes, and the fast processing speed.Graphene carbon nitride ?g-C₃N₄? exhibited great potential for enrichment,adsorption and degradation of organic aromatic compounds. The electron delocalization of g-C₃N₄ gave it multiple adsorption mechanisms, which included complexing effects, hydrogen bonds, redox reactions, ?-? conjugation effects,hydrophobic effects, acid-base reactions and electrostatic interactions and so on.However, the separation of g-C₃N₄ required filtration or centrifugation, which was cumbersome and time-consuming. Fe₃O₄ nanoparticles were one of the most widely used as magnetic materials. On the one hand, they had good stability and large surface area. On the other hand, they were easy to synthesize, easy to gather, easy to fix on the adsorbent surface, and they did not effectively adsorb the analytes. Therefore,Fe₃O₄ can be immobilized on the surface of g-C₃N₄ to overcome the drawback when g-C₃N₄ is used alone as an adsorbent. In addition, g-C₃N₄ also has a non-toxic and visible light response ?semiconductor bandgap 2.7eV?. The unique structure determined its wide range of applications in the field of photocatalysis. However, the catalytic activity of normal g-C₃N₄ was small. It was beneficial to the contact of the target analytes with g-C₃N₄ by increasing the specific surface area of the g-C₃N₄.Thereby improving the catalytic activity of g-C₃N₄ made it more suitable for use as a photocatalyst when it was used in practice. Thus,this paper used new synthetic velvet-like V-g-C₃N₄/Fe₃O₄ nanocomposites, which had large surface area and highly dispersed, for enrichment and detection polychlorinated biphenyls ?PCBs? and adsorption treatment chlorophenols ?CPs?. Finally, the synthesis of different specific surface area g-C₃N₄ were carried out by photocatalytic degradation of chlorophenols to select the best photocatalytic degradation material. The details and results were as follows.?1?Extraction of trace polychlorinated biphenyls in environmental waters by well-dispersed velvet-like magnetic carbon nitride nanocompositesThe velvet-like g-C₃N₄ ?V-g-C₃N₄? was prepared by water-assisted one-step heat-shrinkage method. The synthesis method was simple and easy to operate. The V-g-C₃N₄/Fe₃O₄ nanocomposites were prepared by chemical coprecipitation method.The results show that the nanocomposites were successfully synthesized by thermal analysis, elemental analysis, FT-IR, XRD, BET and TEM characterization. Then this nanocomposite was used to develop a highly efficient magnetic solid-phase extraction?MSPE? method for the pre-concentration of trace PCBs in water samples. In this process,the response surface method ?RSM? was used to optimize the factors that affected the MSPE, such as temperature, adsorption time, pH and salinity. The solution was detected by HPLC-UV to optimize the adsorption conditions and the elution conditions. A fast, selective and sensitive method using MSPE and Gas Chromatography-Mass Spectrometer ? GC-MS? was developed to analyze the ten PCBs in four real water samples. And the linearity and the limits of detection of the developed method for PCBs investigated were established by GC/MS. The results showed that the limits of detection ?LODs S/N=3? were in the range of 9.0×10^-6-5.8×10-5 ?g/mL, and good spiked recoveries over the range of 80.1-118.4%,and RSDs ?n=5? of 0.02-3.7% were obtained under optimal experimental conditions.This work demonstrated the potential of using this nanocomposite for pretreatment and preconcentration of PCBs and other different hydrophobic pollutants containing carbon-based ring structures in water samples.?2? Application of V-g-C₃N₄/Fe₃O₄ nanocomposites as adsorbents adsorption for chlorophenols in waterThe prepared V-g-C₃N₄/Fe₃O₄ nanocomposites were used as adsorbents to adsorb three kinds of chlorophenols in water. The effects of initial concentration of chlorophenols, adsorption time, salinity, temperature and pH were investigated. The influence factors were further optimized by the response surface method to select the best adsorption conditions. The analysis was performed using HPLC to evaluate the properties of the materials as adsorbents to adsorbe chlorophenols. The experimental results showed that the synthesized V-g-C₃N₄/Fe₃O₄ had the advantages of large specific surface area, fast adsorption rate ?30s?, and good effect on the adsorption of chlorophenols.?3? Synthesis of carbon nitride nanomaterials with different specific surface area and their application in rapid photodegradation of chlorophenols under visible lightThe preparation of g-C₃N₄ with different specific surface area through melamine and urea did not require the use of templates, toxic solvents and expensive chemicals during the synthesis process. And then these materials were characterized. In this experiment, three chlorophenols were selected as the target compounds. The analysis was performed using HPLC to evaluate the properties of these materials as photocatalysts. In addition, some key factors affecting photocatalytic degradation were studied, including the ionic strength, pH and temperature of the solution and so on. The optimum conditions for photocatalytic degradation of the target analytes were optimized, and then the materials were reused. Finally, the photocatalytic degradation of chlorophenols were carried out under the optimum conditions and analyzed by real time mass spectrometry ?DART?. The results indicated that the V-g-C₃N₄ ?N2, 3h?nanoparticles were proven significantly more efficient than other materials. It was due to the large specific surface area and good dispersion properties of this material.Furthermore, this work provided an attractive alternative method for production of highly active photocatalysts that have potential application in the degradation of chlorophenols and other similar nonpolar contaminants in water.