Preparation Of Novel Carbon-coated Nano Zero-valent Iron And Its Application And Mechanism In Organic Pollution Remediation

Nano zero-valent iron（nZVI）,as a low-cost and environmentally friendly remediation material,has been widely used in the field of environmental pollution remediation in recent years.However,the agglomeration and passivation problems caused by its high surface energy and high activity seriously hinder its application in environmental remediation.Supporting materials and stabilization modification can effectively overcome its agglomeration problem,but its easy passivation defect has not been effectively solved.In this paper,carbon-coated nZVI（Fe⁰@C）with antioxidant properties was successfully prepared for the first time by combining the hydrothermal method and the carbothermic reduction method.In addition,the synthesis mechanism of Fe⁰@C and its reaction mechanism for activating persulfate（PS）to remove 4-chlorophenol（4-CP）in water were investigated in detail by combining various characterization methods.Not only that,the enhancement mechanism of carbon-coated structure to nZVI in situ synthesis of hydrogen peroxide（H₂O₂）and its reaction mechanism of mineralization of nitrobenzene（NB）,the reaction mechanism of the composite activated PS to degrade anthracene（ANT）in soil and the mechanism of its acting as a reducing agent in combination with surfactants to reduce the adsorption of NB in the soil has been investigated in detail.The above research provides some theoretical basis for the application of nZVI and its composite materials in the actual environmental pollution remediation.The main innovative researches in this paper include:（1）For the first time,carbon-coated nZVI with antioxidant properties was prepared by combining the hydrothermal method and carbothermal reduction method with glucose and nano-ferroferric oxide as raw materials.In the preparation process,the mixture of ferroferric oxide and glucose solution was firstly reacted at 180°C for 10 h by hydrothermal method to encapsulate the nano-ferroferric oxide in the carbon shell.Then,the hydrothermally synthesized composite material was calcined at a high temperature in a tube furnace for carbothermal reduction reaction to obtain carbon-coated nZVI.The synthesis mechanism of carbon-coated nZVI was also explored.Through scanning electron microscopy（SEM）and transmission electron microscopy（TEM）characterizations,it can be concluded that the carbon-coated nZVI has a spherical solid structure and the nZVI particles are embedded in the mesoporous carbon spheres.Through X-ray diffraction（XRD）,thermogravimetric analysis（TGA）,and Fourier transform infrared（FTIR）spectra characterization,it can be concluded that the synthesis of carbon-coated nano-zero valent iron requires sufficient carbon source（carbon-iron molar ratio greater than 14:1）and temperature（greater than 520°C）.If the carbon source concentration is too low,the ferroferric oxide cannot be completely reduced to zero valent iron,and if the carbon source concentration is too high（for example,the carbon-iron molar ratio is 26:1）,carbon deposition will be generated on the surface of the composite material to inhibit the activity of the material.The unique structure of the carbon coating not only suppressed the agglomeration of nZVI,but also made nZVI stable in air for more than 120 days.Not only that,Fe⁰@C can efficiently degrade 4-CP by activating PS.Among them,the composite with a carbon-iron ratio of22:1 showed the best activation effect for PS,and 82%of 4-CP could be removed after15 min of reaction.Moreover,unlike the traditional nZVI-activated PS to generate sulfate radicals,Fe⁰@C selectively induced the generation of hydroxyl radicals.In addition,Fe⁰@C can also reduce the dissolved oxygen in the water to generate hydroxyl radicals to oxidatively degrade 4-CP.（2）Based on the previous research,it can be known that the synthesis of carbon-coated nZVI requires sufficient carbon sources（carbon-iron molar ratio greater than14:1 and less than 26:1）and a specific synthesis temperature（more than 520°C）.The in-situ synthesis of hydrogen peroxide using nZVI requires the participation of Fe⁰,so the synthesis process of carbon-coated nZVI was optimized and the carbon-iron molar ratio in the synthesis process was adjusted to 16:1,18:1,and 20:1 to increase the content of Fe⁰ in the composites.The results show that the contents of Fe⁰ in the composites with carbon-iron ratios of 20:1,18:1,and 16:1 are 47.6%,51.1%,and 53.2%,respectively.Not only that,the composite materials under all ratio conditions can efficiently synthesize hydrogen peroxide in situ,and the higher the iron content in the composite materials,the higher the synthesis amount of hydrogen peroxide within the same reaction time.Not only that,when the composite material with a carbon-iron ratio of 18:1 was used to remove NB in water,91%of NB in the reaction system was removed by adsorption,reduction,and oxidation within 120 min,and the proportion of oxidative degradation was 42.92%.In addition,carbon-coated nZVI also breaks through the strict p H limitation of the traditional Fenton reaction,and it can achieve effective mineralization of NB in the range of p H 3-7.Quantitative experiments on hydroxyl radicals showed that the composite with a carbon-iron ratio of 18:1 could generate 185.64μM hydroxyl radicals within 120 min without any energy consumption.In addition,the reaction mechanism study showed that the carbon-coated structure enhanced the single-electron transfer process on the nZVI surface by changing the electron transfer pathway on the nZVI surface,so that nZVI could efficiently synthesize H₂O₂ through the superoxide radical pathway under neutral conditions.（3）Based on the full study of the characteristics of carbon-coated nZVI,Fe⁰@C wasused as a PS activator to remove anthracene in the soil to explore the feasibility and activation mechanism of its application in the soil environment.The results show that,compared with the traditional nZVI/PS system,the Fe⁰@C/PS system exhibits excellent performance in the process of removing ANT from the soil.After 0.5 h of reaction,51.6%of ANT in soil could be removed,and the removal rate was increased to 76.4%after 12 h of reaction.Not only that,the Fe⁰@C/PS system showed a good removal effect on ANT in the initial p H range of 3-9.In addition,increasing the dosage of PS can provide more reactive oxygen species for the degradation process of ANT to promote the degradation of ANT.Free radical scavenging experiments showed that sulfate radicals,hydroxyl radicals,superoxide radicals,and singlet oxygen existed simultaneously in the reaction system,among which superoxide radicals and singlet oxygen were the main reactive oxygen species for removing ANT.In addition,the results of quenching experiments and electron paramagnetic resonance spectroscopy showed that superoxide radicals may be mainly used as precursors to generate singlet oxygen.It is worth noting that Fe⁰@C has a synergistic effect with soil components during the activation of PS.The activation of PS by Fe⁰@C generates a large number of free radicals,and soil components（such asβ-Mn O₂）can promote the conversion of superoxide radicals to singlet oxygen in the reaction system.（4）After proving the feasibility of carbon-coated nZVI as a PS activator in soil,it was used as a reducing agent to reduce soil-adsorbed NB.It should be noted that the use of carbon-coated nZVI as a reducing agent relies heavily on the interfacial reaction process,and the adsorption of pollutants on the soil will hinder the contact process between the composite material and the pollutants.Therefore,in the experimental process,surfactants were used to enhance the desorption of NB adsorbed by soil to enhance the reduction process of NB.During the research,two different types of soil were used to study the effect of soil organic matter（SOM）content on NB desorption and reduction,including campus soil with low organic matter content and potting soil with high organic matter content.In addition,the research process selected three different types of surfactants for enhanced desorption,including biosurfactant rhamnolipid（RL）,anionic surfactant sodium dodecyl sulfate（SDS）,and nonionic surfactant polysorbate-80（Tween 80）.The results showed that the type and concentration of surfactant,water-soil ratio,temperature,and SOM content all affected the desorption of NB adsorbed by soil.RL and SDS showed the best desorption effect on NB adsorbed by campus soil and potting soil,respectively.The difference is that with the increase of surfactant concentration,the desorption effect of RL on NB adsorbed by campus soil first increased and then decreased,while the desorption effect of SDS on NB adsorbed by potting soil increased continuously.The combination of surfactants had no obvious strengthening effect on the desorption of NB adsorbed by campus soil.In addition,increasing the water-soil ratio can promote the desorption of NB in different soils.However,increasing the temperature only promoted the desorption of NB in potting soil.Furthermore,compared with the conventional nZVI,the Fe⁰@C composite exhibited superior performance in reducing soil-adsorbed NB.Whether it was campus soil or potting soil,the removal rate of Fe⁰@C to NB was about twice that of conventional nZVI under the same dosage conditions.However,both the alkaline environment and the adsorption of soil particles on the composite had a negative impact on the reduction of NB,especially in the campus soil system,the negative impact of the alkaline environment is particularly obvious.When the p H value was decreased from 9 to 3,the proportion of aniline（AN）formation in the campus soil system increased from 19.37%to 69.29%.The successful preparation of carbon-coated nZVI in this thesis substantially solved the defect of nZVI being easily passivated,and provided a theoretical basis for the application of nZVI with anti-oxidation properties in actual pollution remediation.Not only that,the characteristic of carbon-coated nZVI in situ efficient synthesis of hydrogen peroxide can apply nZVI to the field of mineralization of organic pollutants without additional oxidants.In addition,the synergistic effect of carbon-coated nZVI and soil components also provides some theoretical basis and new ideas for the selective oxidation of organic pollutants.Based on this study,exploring the selective oxidation process of free radicals induced by nZVI is an important research direction for the application of nZVI in the field of oxidation in the future.