Fabrication Of Graphene Oxide-based Materials For Removing Phenolic Pollutants In Wastewater Treatment:Properties And Mechanisms

The effective removal of aromatic pollutants in wastewater treatment constitutes a key and difficult issue recently.Among the aromatic pollutants,phenolic pollutants are toxic substances and environmental endocrine,posing a seriously threaten for the health of human and animals.Meanwhile,traditional biochemical treatment methods are restricted because of refractory to bildegradation for phenolic pollutants.Thus,it is necessary to develop effective methods for removal of phenolic pollutants.Based on the adsorption properties of graphene oxide-based materials,this work developed different graphene oxide-based materials,which possessed adsorption,oxidation,and catalytic oxidation properties for removing phenolic pollutants.The main research conclusions are listed as follows.(1)This work optimized the preparation processes of graphene oxide and prepared magnetic graphene oxide(MGO)to enhance adsorption and separation properties.During synthesis of GO,the dosage of KMnO4 and graphite,reaction temperature,and reaction time were optimized to obtain uniform GO products.MGO composites were further prepared by co-precipitating iron salts onto GO sheets,which could be separated and recycled by adding magnetic field.Considering the structural properties,GO possessed various oxygen-containing functional groups,such as hydroxyl,carbonyl,carboxyl,and epoxy groups,which combined iron oxides by Fe-O-C covalent bonds for MGO.Then,p-nitrophenol(p-NP)and p-tert-butylphenol(p-TBP)were selected as typical phenolic pollutants.For p-NP,the nitro groups were strong electron-withdrawing groups,which could reduce the electron cloud density as electron acceptors.The sp2 regions on GO sheets contained abundant ? electrons as electron donors.Thus,GO possessed strong ? electrons transfer interactions for p-NP.The maximum adsorption capacity of GO was 268.5 mg/g for p-N P.For p-TB P,the tertiary butyl groups were electron-donating groups,which could increase the electron cloud density as electron donors.The sp3 regions,edge,and defect on MGO sheets contained electron vacancy sites as electron acceptors.Thus.MGO also possessed ? electrons transfer interactions for p-TBP.Moreover,the hydrophobic interaction and hydrogen-bond interaction between GO,MGO and phenolic pollutants could enhance the removal properties.(2)This work prepared in-situ graphene oxide/manganese oxide composites(GO/MnOx)by synchronously utilizing manganese ions and acidic liquid waste during synthesis of GO.Compared with traditional ?-MnO2,the manganese oxides in composites were ?-MnO2 and Mn3O4,which would combine GO by Mn-O-C covalent bonds.Then,p-cresol(p-CR)and p-TBP were selected as typical phenolic pollutants.During removal process,GO in composites could adsorb phenolic pollutants by ?bonding interactions firstly.Then,manganese oxides in composites oxidized phenolic pollutants and the oxidation products could be re-adsorbed on GO sheets.It should be noted that GO sheets could re-oxidize Mn(II)of Mn3O4,thus enhancing the oxidation properties of composites.Compared with p-CR,the tertiary butyl groups in p-TBP possessed stronger electron-donating capacities.Thus,GO/MnOx composites possessed stronger adsorption and oxidation capacities for p-TBP than p-CR.Specially,cleavage of aromatic rings for p-TBP was detected for the oxidation products,indicating the potential of mineralization of GO/MnOx composites for phenolic pollutants.(3)This work modified nitrogen-doped graphene(NG)with amino-functionalized mesoporous silica(NH2-MCM-41)to constructe unique catalytic and adsorption cells for removing p-cresol(p-CR)and bisphenol A(BPA)in advanced oxidation processes(AOPs).In comparison with pristine NG,the nitrogen content,surface area,and dispersibility in water of NG/NH2-MCM-41 composites achieved an obvious promotion after combining with NH2-MCM-41.During removal process for phenolic pollutants,NG in composites could catalyze peroxymonosulfate(PMS)to generate abundant reactive oxygen species(ROS),such as sulfate radicals,hydroxyl radicals.and singlet oxygen.Then,the ROS could oxidize phenolic pollutants to achieve efficient degradation and mineralization.Meanwhile,the strong adsorption properties of NG in composites drived the oxidation process and ensured the removal of phenolic pollutants.In compared with p-CR,BPA possessed stronger ? bonding interactions and hydrophobic interactions with composites.And the ROS in AOPs possessed stronger oxidation capacities for BPA than p-CR,thus achieving better removal properties for BPA.Considering the removal mechanism,radical and nonradical pathways are involved in the catalytic oxidation of phenolic pollutants.Specilly,the synergistic effect between adsorption and catalytic oxidation was founded.The ? electron transfer interactions could enhcance the oxidation capacities for phenolic pollutants,and the effective oxidation degradation of phenolic pollutants could promote the adsorption of phenolic pollutants,thus achieving efficient removal of phenolic pollutants.(4)Based on above research,this work summarized suitable treatment methods for various phenolic pollutants.For the phenolic pollutants with electron withdrawing groups,graphene oxide-based materials possessed strong adsorption capacity for removing the phenolic pollutants.Centralized disposal of adsorbed materials could achieve effective treatment after adsorption.For the phenolic pollutants with electron-donating groups,graphene oxide-based materials possessed not only well adsorption capacity but also proper catalytic property in AOPs.The adsorption-oxidation and adsorption-catalytic oxidation processes could achieve efficient degradation of phenolic pollutants.