Preparation Of Graphene-based Monolithic Composites And Its Applications In Bisphenol A Removal By Adsorption/Photocatalysis

BPA（Bisphenol A）,a frequently detected environment endocrine-disrupting chemical in many kinds of water bodies,could lead to many diseases such as reproductive toxicity,cancers,and autoimmune deficiency.Thus,it is in urgent demand to remove BPA.At present,adsorption and photocatalysis are two main remediation methods to remove BPA.However,recycle and regeneration of adsorbents and nano photocatalysis are the bottleneck for adsorption and photocatalytic application.It is an effective approach to remove trace organic pollutants by the combination of adsorption and photocatalysis,in which the pollutants can be trapped by adsorption to improve degradation driving force and be photocatalytic degraded to regenerate adsorption sites.However,the preparation strategy of adsorption-photocatalytic materials,the interactions between adsorption and photocatalysis,and especially the adsorption-photocatalysis synergistic effects are still lack of in-depth and systematic studies.Graphene-based monoliths（GBMs）and its composites is a new type of environmental functional material and can be recycled and regenerated effectively.However,many problems need to be solved about pollutants removal using GBMs.Besides,the effective structural regulation of GBMs and its composites and the pollutant decontamination mechanisms is still challenging.Moreover,most studies by now still focus on mg/L level BPA removal,which is far away from the fact that BPA exhibits strong endocrine disrupting effects at much lower concentrations.Therefore,more studies could be conducted to treatμg/L BPA.Based on the problem above,three aspects were conducted in this dissertation and the main contents lie as below:（1）Sodium ascorbate（Vc`Na）was used to reduce graphene oxide（GO）to prepare shell-free GBM.The microstructures and properties of the produced Vc`Na-GBM were characterized by SEM,BET-N₂,FTIR,Raman,XRD and XPS analysis.The results showed that Vc`Na-GBM was a graphene porous network monolith with a pore size ranging from submicrometer to several micrometers.The shell-free Vc`Na-GBM,compared with un-modified Vc-GBM,exhibited a higher BET surface area of 248.3 m²/g for an increase of21.4%,recovered 43.3%of sp^2-hybridized domains for an increase of 24.4%.The graphene in Vc`Na-GBM showed a C/O ratio of 7.95,and a Raman I<sub>D</sub>/I<sub>G</sub> ratio of 1.06.Strong electrostatic repulsion induced by Vc`Na promoted the removal of the shell of GBM.Vc`Na-GBM showed higher adsorption rate constants of 0.267 g/（mg·h）and 0.230 g/（mg·h）for MB and BPA respectively,with 1.60 and 2.47-fold increases,and possessed higher saturated adsorption capacities of 154 mg/g and 204 mg/g,for MB and BPA respectively for an increase of 1.34 and 1.20-fold.（2）A non-shell graphene-based monolith（GBM）with huge surface area was synthesized by chemical reduction of Graphene oxide（GO）using magnesium ascorbyl phosphate（MAP）and tested as an adsorbent for trace Bisphenol A（BPA）removal.The properties of MAP-GBM was characterized by SEM,BET-N₂、FTIR、Raman、XRD analysis.The results showed that MAP-GBM was a graphene porous network monolith with a BET surface area of294.3 m²/g and a graphene Raman I_D/I_G ratio of 1.2 and a pore size ranging from submicrometer to several micrometers.Mg²⁺and larger molecular weight of MAP make MAP-GBM larger surface area and higher BPA adsorption capacity.The adsorption mechanism could be ascribed to the unique nonporous surface adsorption,hydrogen bonding,andπ-πinteraction characteristics of MAP-GBM.After desorption with methanol,the adsorption efficiency remains as high as 88%after five regenerations.Batch adsorption experiments showed that the adsorption of BPA on MAP-GBM reach equilibrium at 10¹5 h and could be well fitted with the pseudo-second-kinetic model.The saturated adsorption capacity of MAP-GBM was 339 mg/g for BPA,which was 2.54 times of Vc-GBM.Thermodynamic results showed that the adsorption of BPA by MAP-GBM is a spontaneous endothermic process.A MAP-GBM adsorption column completely removed BPA from solution at low concentration（50μg/L）for⁴50 mL,and mg/L level of natural organic materials could decrease the low concentration BPA removal efficiency with a completely remove volume decreased to²50 m L.（3）TiO₂ nanotube（TNT）Pillared graphene-based monoliths（TPGBM）was prepared using TNT as physical pillars.SEM and TEM images showed that TPGBM was a 3D porous monolith and consisted of graphene and TNT.Elements mapping,TEM images,and XRD pattern suggested that TNT was an anatase nanotube with a diameter of¹0 nm and uniformly distributed in TPGBM.FTIR spectrum,XRD pattern and XPS analysis results revealed that the conjugateπregion was almost recovered which favored the adsorption of BPA.BET-N₂ showed that TNT could effectively prevent graphene from being excessive stacked with a larger BET surface area of 394.5 m²/g.The band gap of TPGBM was calculated to be 2.8 eV and it has good UV and visible light absorption capacity,higher than that of TNT and commercial TiO₂.Both h⁺,·O_2-and·OH were reactive species in BPA photocatalytic degradation,in the catalytic activity order of h⁺>·O_2->·OH.TPGBM exhibited good cycle performance,the adsorption sites in TPGBM can be regenerated by photocatalysis.After 5 cycles,BPA removal efficiency can maintain 92%.Both batch experiments and fluidized bed experiments result exhibited that TPGBM had good adsorption-photocatalysis synergistic removal efficiency toward BPA and the BPA adsorption on graphene improved photocatalytic driving forces.TPGBM could remove 97%of 5 mg/L BPA in 30 min in the batch reaction system and remove 86%of 50μg/L BPA under continuous flow system.Alternating dark-light cycles suggested that TPGBM continuous-flow fluidized bed exhibited a good transient response to light.