Preparation Of Onion-like Carbon Composites And Their Adsorption And Degradation Properties For Organic Dyes

Dyes polluted wastewater treatment has become a research hotspot in recent decades,which has aroused the focus of scientists in many fields such as environment,materials,chemistry,and biology.Current mainstream treatment methods include physical method,chemical method,biological method,etc.,among which adsorption method and catalytic method have received the most attention.The adsorption method utilizes minerals,biomass,waste,activated carbon etc.to achieve the removal of pollutants through physical or chemical combination with the pollutants.Among them,new nano-carbon materials represented by graphene,carbon nanotubes,and fullerene have become the focus of attention due to their advantages such as high specific surface area,high adsorption capacity,chemical modification,and strong adsorption specificity.The catalytic method uses the input of external energy such as light,electricity,ultrasound,oxidants,etc.to achieve complete degradation of pollutants at the molecular level.Among them,nano-titanium dioxide(Ti O₂)have been the most favored photocatalyst due to their advantages of low toxicity,low cost and high efficiency.Although the above methods have been studied and applied on a large scale,there are still many bottlenecks.For nano carbon adsorbent materials,the main obstacles are:(1)Nano-adsorbent in the dispersed state can take the advantage of large specific area,but it is difficult to recycle and easily cause secondary pollution;(2)Adsorption can only realize pollutants enrichment,which requires further chemical treatment to achieve degradation at the molecular level.For nano-Ti O₂photocatalytic materials,the main obstacles are:(1)Similar recovery difficulties and secondary pollution problems as nano-adsorbents;(2)Band gap and carrier recombination efficiency of Ti O₂ are higher,and the adsorption performance is poor,restricting its photocatalytic efficiency and its application in the visible light range.Recent studies have found that nano-carbon materials not only have excellent adsorption properties for organic dyes,but also can improve their photocatalytic performance by compounding with Ti O₂.At the same time,carbon nanomaterials modified Ti O₂ can combine adsorption with the catalytic process to achieve the synergy of the two methods of"adsorption-catalysis".Therefore,the method of modifying nano-photocatalysts by nano-carbon materials to improve the efficiency of dye treatment has attracted the attention of researchers in recent years.Onion-like carbon(OLC)is a type of multilayer fullerene with C₆₀ as the core.Their physical and chemical properties are very similar with graphene and carbon nanotubes,and it also has a more stable material structure.OLC can be prepared in large scale by high-temperature treatment of nanodiamond in vacuum,which is expected to replace other forms of nano carbon materials in organic adsorption and catalytic enhancement.This article takes OLC as the research object and has carried out a systematic work including material preparation,molding,modification,and application.The chapters are briefly described as follows.(1)Preparation and adsorption properties of onion-like carbon powderThe commercially available detonation nanodiamond was purified and then OLC was prepared under vacuum and high temperature conditions.TEM results show a clear and perfect concentric ring structure of OLC,which is evenly distributed within a particle size range of 5-10 nm.The experimental results prove that with 5 mg OLC put into 50 ml of 10 mg/L methylene blue solution,and the adsorption equilibrium is reached at three temperatures of 293 K,313 K,and 333 K,their saturated adsorption amounts are:183.63 mg/g,300.00 mg/g,370.00 mg/g.The adsorption mechanism analysis shows that the adsorption force of OLC on methylene blue mainly comes from the van der Waals force,?-?interaction,electrostatic interaction and hydrogen bond interaction.This part of the study successfully prepared OLC with excellent adsorption performance,which constituted the main content of the second chapter of this thesis.(2)Research on film formation of OLC at room temperature and their adsorption propertiesAiming at the problem that OLC powders are likely to cause secondary pollution during the adsorption process,we have prepared OLC coatings using vacuum cold spray(VCS).The coating has the characteristics of dense inner layer and loose outer layer,which not only ensures the bonding strength of the coating,but also facilitates the adsorption of the dye.After the adsorption performance test,the coating can ensure the adsorption efficiency in several cycles,but because of the shedding of the coating,the removal rate gradually decreases.In the study of the deposition mechanism,the compaction between nanoparticles is achieved through room temperature compaction,and two forms of particle deformation occur during this process,thereby achieving the dual role of mechanical bonding and van der Waals force bonding.Due to the characteristics of room temperature deposition,the physical and chemical properties of OLC are maintained,which is conducive to the performance of its adsorption performance.The realization of OLC coating at room temperature not only promotes the practical application of OLC,but also can be extended to the preparation of other nano-carbon material functional films/coatings,which will promote the development of nano-carbon material functional devices.In this part of the study,the secondary pollution of powder adsorbents was solved by the room temperature film formation of OLC,which constituted the main content of Chapter 3 of this thesis.(3)Preparation of three-dimensional microspheres of OLC and their adsorption propertiesAiming at the problem that OLC powder is easy to cause secondary pollution during adsorption and the problem of shedding of VCS-OLC coating in Chapter 3,based on the traditional preparation method of calcium alginate(CA)gel,four porous aerogel microspheres,CA,CA-OLC,CA-Fe₃O₄,and CA-OLC-Fe₃O₄,were prepared via in situ ion polymerization method.Using the characteristics of Ca CO₃ generating Ca²⁺and CO₂ simultaneously in dilute HCl,the polymerization and pore formation of microspheres are realized at the same time.The microspheres have the characteristics of uniform size and abundant pores,and OLC and Fe₃O₄ are evenly distributed on the surface of the CA sheet,which is beneficial to increase the adsorption capacity of aerogel microspheres and can be recycled under the traction of magnetic field.The results of the cycle adsorption experiment show that with the increase of the cycle number,the dye adsorption capacity is basically maintained at a certain stable level,without significant capacity decrease.In this part of the study,porous CA aerogel microspheres loaded with OLC and Fe₃O₄ were successfully prepared,which constituted the main content of Chapter 4 of this thesis.(4)Preparation and photocatalytic performance of Ti O₂-OLC composite coatingIn Chapter 3 and Chapter 4,the adsorption properties of OLC powders was modified by room temperature film formation and magnetic recovery to avoid secondary pollution.However,adsorption still only plays a role in enrichment of pollutants.The enriched dyes have not been degraded at the molecular level,so further treatment is needed,and photocatalysis can achieve the degradation of pollutants at the molecular level.In this chapter,suspension plasma spray(SPS)is used to prepare Ti O₂-OLC composite coating,and a photocatalytic coating with long-term recycling and enhanced performance is obtained.During the preparation of the coating,due to the decomposition of the organic dispersant,the coating indicates a porous microstructure.And because the decomposition of organic matter reduces the deposition temperature,the original structure and physical and chemical properties of OLC and Ti O₂ can be retained in the coating.Analysis of the mechanism of photocatalytic enhancement shows that due to the presence of OLC,the catalytic reaction has been promoted from the three main processes of photocatalytic reaction.This part of the research has achieved the preparation of OLC-reinforced porous Ti O₂coating,which constitutes the main content of Chapter 6 of this thesis.In summary,we prepared OLC nanopowders in large scale,verified the adsorption performance of the powders and explained the adsorption mechanism.On this basis,two-dimensional thin films and three-dimensional microspheres were prepared.The two methods of fixed film formation and magnetic recovery effectively solved the problems of OLC recovery and secondary pollution.Subsequently,we prepared OLC modified Ti O₂composite coatings,which achieved a synergy interaction between adsorption and catalysis,effectively solving the limitations of single method.