Synthesis And Application Of Graphene Based Composite Materials In Adsorption Of Cr,Hg Ions And Hydrogen Producing Performance

Water pollution caused by toxic heavy metals poses a serious threat to the health of human beings and the entire ecosystem,and it has become an environmental issue of concern to the world.Although heavy metals exist naturally,human activities introduce excessive amounts of heavy metals into the environmental matrices.According to the investigation of the Pearl River Delta,industrial wastewater is still the important factor affecting heavy metal pollution in the Pearl River Delta.As the largest river flowing into the South China Sea,the Pearl River is of great significance to the social,economy and livelihood in southern China.Therefore,how to better deal with heavy metal wastewater and recycling has become a key issue for researchers.In heavy metal water pollution treatments,adsorption is generally considered to be the most efficient and economical method for removing heavy metals.And among many adsorbents,graphene is widely studied as a promising adsorbent because of its unique physicochemical properties.Studies show that heteroatom-doped can regulate and control the structure and properties of graphene.However,the current researches on heteroatom-doped graphene-based materials are mainly focused on catalytic and electrochemical fields,and relatively little attention is paid to adsorption.Therefore,based on the preliminary survey of the Pearl River Delta,a series of graphene-based materials by polymer crosslinking,doping and nanoparticles loaded had been synthesized to remove the pollutants of Cr and Hg in the industrial wastewater and the removal mechanisms were analyzed in detail.It provides theoretical and technical basis for the application of graphene-based materials in heavy metal pollution remediation.At the same time,the cooperative hydrogen production performance of the waste from adsorbent containing heavy metal was studied,which laid the foundation for the recycling of the adsorbent-waste and the generation of new energy.The main research contents are as follows:（1）Polyethyleneimine cross-linked graphene oxide composites were synthesized by using polyethyleneimine as graphene oxide crosslinking agent,using the amide reaction and non-covalent bond between graphene oxide and polyethyleneimine.Increasing the molecular weight and loading ratio of polyethyleneimine can improve the adsorption performance of graphene oxide to some extent,but excessive polyethyleneimine will affect the exposed adsorption sites,thus reducing the adsorption capacity of composites.The polyethyleneimine cross-linked graphene oxide exhibited good adsorption performance for Cr（VI）and the capacity was 436.20 mg g^-1.The adsorption process followed to the pseudo-second-order kinetic model.The mechanism under different p H were discussed in detail.When the initial concentration of Cr（VI）was 50 mg L^-1 and the p H was 3,the experimental adsorption capacity was 117.42 mg g^-1,the removal efficiency was as high as 99.97%.The residual Cr（VI）and total Cr concentrations were 0.014 mg L^-1 and 1.11 mg L^-1,respectively,which can meet industrial wastewater discharge standards.（2）High-nitrogen-doped reduced graphene oxide（NRGO）was synthesized in situ by hydrothermal method to remove Hg（II）.It was found that since polyethyleneimine can be decomposed into C-substituted ethylpyrrole,pyrrole,ethylamine and ammonia,nitrogen can be better doped into the lattice of graphene at a content of 12.69%.The pyridinic-N:amide/amine-N:pyrrolic-N:graphitic-N was 0.75:1.05:1:1.09.Due to nitrogen doping,NRGO has good adsorption performance for Hg（II）over a wide range of p H,especially when the p H was 2.The adsorption process followed the pseudo-second-order kinetic,and the Langmuir and Sips models fitted the experiment well.Through the ternary mixed experiment design,we found that the affinity of NRGO to Cr（VI）,Cr（III）and Hg（II）was Cr（VI）>Hg（II）>Cr（III）.There is no competition between Cr（VI）and Hg（II）in the presence of sufficient adsorption sites.It indicates that NRGO has different adsorption mechanisms for Hg（II）and Cr（VI）.The doping of nitrogen changed the electron distribution of the system and increased the affinity of the system to Hg（II）.Meanwhile,because the electronegativity of oxygen was higher than that of nitrogen,Hg（II）and generated Hg（I）in aqueous solution were more easily combined with oxygen of oxygen-containing functional groups to form stable metal complexes,which were conducive to the removal of Hg.The adsorbent is a good adsorbent because of its good ability of independent and mixed adsorption.（3）To make the nitrogen functions better,the dispersion and properties of sodium lignosulfonate was used to control the preparation of nitrogen-doped graphene（N-LEGO）by hydrothermal method.This method can obtain a relatively higher proportion of pyridinic-N and graphitic-N in graphene（pyridinic-N:amide/amine-N:pyrrolic-N:graphitic-N is 2.3:1.6:1:2.3）,and the adsorbent is more conducive to the adsorption of Cr（VI）.The maximum experiment capacity of N-LEGO was 416.91 mg g^-1.The adsorption process accorded with pseudo-second-order kinetic and Freundlich model.The adsorption of HCr O4^-and Cr2O7^2-by five doping structures by DFT simulation show that the adsorption processes are exothermic and the adsotpiton systems are stable.The five N-doped structures preferentially adsorb Cr2O7^2-.The sequence is amine functional graphene>pyridinic-N doped graphene>pyrrolic-N doped graphene>graphitic-N doped graphene>graphene;for HCr O4^-,the adsorption order is pyridinic-N doped graphene>amine functional graphene>graphitic-N doped graphene>graphene>pyrrolic-N doped graphene.These results show that N-doped in the lattice system is advantageous to the adsorption of Cr（VI）.At the same time,the charge density difference diagrams show that the charge near nitrogen consumes after adsorption,and the charge transfer is between nitrogen and Cr（VI）.In conclusion,the removal mechanism of Cr（VI）by N-LEGO is mainly the reduction-oxidation reaction between nitrogen and Cr（VI）,and the main products of nitrogen after the reaction are nitrate（NO3^-）and nitro.The adsorbent maintains excellent removal performance for Cr（VI）under the condition of the coexistence of heavy metals and organic matters.（4）The synthesis of nanoparticles supported on graphene was used to remove Cr（VI）from aqueous solution,and the recycling of chromium-containing adsorbent waste was considered.First,reduced graphene oxide/molybdenum disulfide（r GO/Mo）nanocomposites were synthesized by hydrothermal method.r GO and Mo S2 were reassembled in the process of reaction to form a new form and r GO/Mo S2 presents nano-flake structure when the ratio of r GO and Mo S2 is 1:7.Mo S2 is uniformly dispersed on the surface of r GO under the action of polyethylene glycol.The adsorption capacity of this new basal edge enrichment feature of r GO/Mo S2 was 124.65 mg g^-1.Then electrochemical tests were carried out on adsorbents with different initial concentrations and adsorbents after calcination.The test results showed that with the increase of chromium content in adsorbent wastes,HER performance of r GO/Mocould be gradually improved.After the waste was calcined by N2（600℃）,the hydrogen production performance of the r GO/Mo S2-600℃containing 50 mg L^-1 Cr（VI）was significantly improved.When the current density was 10 m A cm^-2,the overpotential of r GO/Mo S2-600℃was 74.5 m V,and the Tafel slope was 50.59 m V dec^-1.The hydrogen production mechanism was Volmer-Heyrovsky mechanism.This study showed that the introduction of chromium enhanced the effective electron transfer in r GO/Mo S2 and played a synergistic role in the HER performance of r GO/Mo S2,making more active hydrogen production sites.The Cr（VI）-contained adsorbent has good HER performance,indicating that this method could recover and reuse the adsorbed wastes.