Applications And Mechanisms Of Graphene Based Adsorbent/Photocatalysts For Organic Pollutants Removal From Water

With the rapid development of industry and economy,the problems of environmental pollution have become more and more serious,especially the organic pollution in water.Among many wastewater treatment methods,adsorption and photocatalytic technology have great application prospects for wastewater treatment.Thus,the development of new materials with excellent adsorption performance and high visible light photoactivity becomes the hotspot for organic pollutants removal.Graphene,as a single atomic layered honeycomb two-dimensional carbon material with unique physical and chemical properties,has aroused the research upsurge in wastewater treatment application.The graphene based materials include in the graphene oxide,reduced graphene oxide,graphene functionalization and graphene based composites,etc..The graphene-based adsorbent can effectively remove the inorganic/organic pollutants from wastewater,but the process of preparing the graphene by chemical method involves in the extensive use of toxic/highly toxic agents,which is liable to cause secondary pollution.At the same time,the graphene-based adsorbents are very easy to agglomerate in water,resulting in the sharp decrease in specific surface area and adsorption performance.Furthermore,the recovery of graphene oxide is very difficult.As a kind of zero band gap semiconductor,graphene has the excellent charge transfer and separation ability which make it can be used as a high efficiency carrier in photocatalytic reaction coupling the prominent adsorption properties of graphene.Based on the structure and properties of graphene,this study attempts to apply graphene-based materials to adsorption and photocatalysis of organic pollutants,in order to lay the theoretical foundation for its application in practical wastewater treatment.The graphene oxide and reduced graphene oxide used here are prepared by chemical method by considering the requirement of large-scale preparation of graphene-based materials in practical application.In view of the problems that the easy secondary pollution in the preparation of graphene caused by chemical reduction method and the easy agglomeration of graphene layer during wastewater treatment,the reduction of graphene oxide by green chemical agent,and the modification of graphene oxide with biosurfactant,the combination metal organic frameworks or photocatalyst with graphene are proposed.Thus,five different graphene-based adsorbent/photocatalyst were synthesized and characterized in order to obtain the graphene-based materials with the great potential in industrial application.Firstly,the reduction of graphene oxide were carried out by using the green and biodegradable ethanediamine disuccinic acid?EDDS?as reducing agent under the facil microwave irradiation.The adsorption properties of antibiotics tetracycline onto the as prepared reduced graphene oxide were studied.Secondly,from the viewpoint of inhibiting the agglomeration of graphene laminates,the water-soluble rhamnolipid was used to functionalize graphene oxide,and then the rhamnolipid functionalized graphene oxide was used as the adsorbent for methylene blue removal.Thirdly,from the perspective of reinforcing solid-liquid separation,the aluminum metal organic framework MIL-68?Al?with larger particles and density ratio was combined with the reduced graphene oxide,and applied to the adsorption of p-nitrophenol.Foutthly,in order to resource utilization the adsorption saturated graphene-based materials and simultaneous removal of various organic pollutants,the reduced graphnee oxide was used to decorated photoresponsive NH₂-MIL-68?Al?.The adsorption performance of congo red in single/binary solution of reduced graphene oxide decorated NH₂-MIL-68?Al?was investigated,and the saturated reduced graphene oxide decorated NH₂-MIL-68?Al?adsorbent was used as the photocatalyst for chromium?VI?reduction.Finally,to completely eliminate the toxicity of adsorbed pollutants,the metal sulfide Sn_0.215In_0.38S with visible photocatalytic activity site was introduced into the reduced graphene,and used as the adsorptive photocatalyst for the mineralization of rhodamine B under visible light irradiation.The specific research contents and results are as follows:?1?The ethylene diamine disuccinic acid?EDDS?was used as as reducing agent for reduced graphene oxide preparation under the facil microwave irradiation,which avoids the occurrence of secondary pollution during the graphene oxide?GO?reduction.The adsorption properties of tetracycline?TC?onto EDDS reduced graphene oxide?ERG?under different conditions were investigated.The results shown that the removal rate of oxygen functional groups of GO depends on the dosage of EDDS,and EDDS can be used to prepare ERG.The ERG5 prepared at EDDS/GO mass ratio of 5 had the best TC adsorption performance,and the maximum adsorption capacity was 558.66mg/g.The TC adsorption is a rapid spontaneous endothermic process,which could reach the adsorption quilibrium within 10 min,and the lower the initial p H was more conducive to adsorption.The interaction between van der Waals force,?-?electron donor-acceptor and cation-?was responsible for excellent TC removal from water.?2?The biosurfactant rhamnolipid?RL?was used as modifier to prevent the agglomeration of graphene lamellae in graphene oxide?GO?,and the as-prepared rhamnolipid funtionalized graphene oxide?RL-GO?was used for the adsorption of methylene blue?MB?in wastewater.It was found that the presence of RL in the preparation process can significantly reduce the wrinkles on the surface of graphene layer and improve the adsorption performance of MB.The MB adsorption was affected by the dose,p H,temperature and initial concentration of MB.The maximum adsorption capacity of MB was 529.10mg/g under the condition of 298K,p H7 and the dosage of 400mg/L.The migration process of MB was controlled by film diffusion and internal diffusion,which was a spontaneous endothermic reaction process on the non-uniform surface driven by electrostatic attraction,?-?interaction and hydrogen bonding.?3?To improve the settling performance of graphene based material,the MIL-68?Al??MA?with large particle size was composited with reduced graphene?RG?.The adsorption property of 4-nitrophenol?PNP?on MA/RG was evaluated under different p H,ionic strength and other phenols.The results shown that MA/RG had the enhanced specific surface area and PNP adsorption ability compared with monomer.Under the condition of 200mg/L and 303K,the optimum p H value of PNP adsorption was 5.0,and the maximum adsorption capacity was 332.23mg/g,which was is 1.64and 2.23 times higher than that of RG and MA.The mass transfer of PNP was controlled by film diffusion and intraparticle diffusion.The adsorption process was a spontaneous endothermic reaction that occurs on a uniform surface and was mainly driven by hydrogen bonds and?-?interactions.The presence of o-nitrophenol and m-nitrophenol has little effect on the adsorption,but phenol has a strong inhibitory effect on PNP adsorption.The MA/RG has good settling and regeneration performance,and the adsorption capacity of PNP after five recycles by methanol is still 271.82mg/g.?4?We used the reduced graphene oxide?RGO?to decrodated the NH₂-MIL-68?Al??NMA?,and the synthesized RGO/NMA was used for the removal of congo red?CR?in the single/binary solution.To realize the resource utilization of RGO/NMA after CR adsorption,the CR loaded RGO/NMA was used as a photocatalyst for chromium?VI?removal.It was found that the adsorption of CR on RGO/NMA was the exothermic adsorption on the homogeneous surface.The maximum single layer adsorption capacity is 473.93mg/g,which is 14.37 and 2.21 times of that of RGO and NMA respectively.In binary solution,the synergistic effect of adsorption was found,in which the removal efficiency of MB,MV,MG,GV and Rh B was 99.97,98.80,96.44,97.07 and 89.93%,respectively,and the CR removal rates were higher than97.34%attributing to the electrostatic attraction,?-?interaction and hydrogen bonding between dyes and RGO/NMA.Under visible light irradiation for 2h,the photocatalytic efficiency of chromium?VI?by CR loaded RGO/NMA was 93.6%.?5?In order to completely eliminate the toxicity of rhodamine B?Rh B?,the Sn_0.215In_0.38S/reduced graphene?SIS/RGO?heterojunction was synthesized for Rh B degradation under visible light irradiation.The results shown that the adsorption and photocatalysis of Rh B were affected by p H,ionic strength,etc..When the ionic strength was 0.04 M,the p H was 3.0,the graphene loading was 2.5%and the SIS/RGO dose was 200 mg/L,the best Rh B adsorption capacity was 173.97mg/g,and the degradation efficiency was as high as 96.25%.The adsorption process was a single layer adsorption on the homogeneous surface,which was mainly driven by electrostatic attraction,?-?interaction and surface complexation.The enhanced photocatalytic properties are mainly due to efficient mass transfer efficiency,dye self-sensitization and efficient transfer of photogenerated carriers.Furthermore,free radicals·O₂^-and hole h⁺were the main active species for the degradation of Rh B.