Investigation Upon Preparation And Adsorption Properties For Heavy Metal Ions Of Porous Hexagonal Boron Nitride

Porous hexagonal boron nitride(h-BN)has excellent chemical stability,heat and corrosion resistance as well as good insulation and lubricity.It has a good application prospect in insulators,ceramics,composite reinforcements and high temperature solid lubricants,etc.Among them,porous h-BN has great superiority in the fields of' hydrogen storage,catalysis,and adsorption separation due to its high specific surface area and special pore structure.At present,the preparation methods of porous h-BN mainly include self-assembly method,pyrolysis method,template method and chemical reaction method.But there are some problems such as high toxicity of raw materials,high reaction temperature,difficulty in template removal,and complex preparation methods.It is urgent to develop efficient,cleaning and energy saving preparation methods.In this paper,porous hexagonal boron nitride materials were synthesized through a facile two-step roasting process by using boric acid and melamine as raw materials.Firstly,the optimum process conditions for the synthesis of porous hexagonal boron nitride were determined through exploration experiments and porous h-BN nanosheets with high specific surface area were prepared;Secondly,the adsorption performance of porous h-BN nanoplates on metal ions was systematically studied and the main influencing factors of the adsorption process were determined.Finally,the adsorption mechanism of porous h-BN nanoplates for Cu2+ was explored and the adsorption model was calculated and analyzed by experimental data.The main innovative results of this paper can be summarized as follows:(1)The optimal preparation process of porous hexagonal boron nitride with high specific surface area was investigated.The phase,composition and preparation method of the precursor were clarified.The effects of reactant ratio and calcination system(temperature,atmosphere and time)on the structure and morphology of the porous h-BN were systematically investigated and the optimum process conditions were determined:A precursor was prepared by cooling crystallization;the molar ratio of melamine/boric acid was 1:2.Then it was first calcined at 600°C for 2 h,and calcined at 700° C for 3 h in NH3 atmosphere.Under these conditions,the weakly crystalline porous h-BN nanosheets with a specific surface area of 321 m2/g were obtained.(2)The main influencing factors of metal ions on porous h-BN nanosheets were determined.The adsorption process is affected by pH,initial concentration,contact time,etc.Taking Cu2+ as an example,when the pH was between 2-5,the amount of adsorbed Cu2+increased with the increase of pH,and the adsorption equilibrium was reached at a pH of 4.5,initial concentration>3000 ppm,and adsorption time reaching 120 min.The maximum adsorption capacity for Cu2+ was 819 mg/g.And in a mixed metal solution containing Cu2+,Fe3+,Cr3+,and Ni2+,it had a good adsorption effect on each metal ion.(3)The adsorption mechanism of porous h-BN nanosheets on metal ions was proposed.By comparing the FTIR,SEM and XRD patterns before and after the adsorption of porous h-BN nanosheets,it was inferred that the higher adsorption capacity of porous h-BN nanosheets was related to the surface-linked-NH2.Zeta potential measurements showed that at pH>3,the surface of porous h-BN nanosheets is negatively charged,which may promote the adsorption of positive valence metal ions;the experimental data of porous h-BN nanosheets for Cu2+ accorded with the Langmuir isotherm adsorption model.And the kinetic adsorption process accorded with the pseudo second-order kinetic model;the adsorbed Cu2+by the porous h-BN nanosheets was in the form of Cu2Cl(OH)3.