Preparation Of Structure-controllable Porous BN Materials And The Study Of Its Adsorption Properties

With the development of society,water pollution is getting worse.It has become a major obstacle to the sustainable development of economy and society.How to achieve effective sewage treatment has become a worldwide challenge.During the study of sewage treatment technologies,the adsorption method using porous materials as carriers is superior to other methods in timeliness and economy.Hexagonal boron nitride has attracted a great deal of attention due to its excellent physical and chemical properties.As an environment-friendly technology,freeze-drying has been widely used to produce novel porous materials.In this paper,we prepared porous BN materials using this method.First we studied the effects of pH,PVA concentration and freezing rate on the pore structures,and then discussed the pore-forming mechanism under different conditions.The results showed that porous BN materials with lightweight,high porosity,aligned channels and dendritic pores were fabricated.With the addition of PVA,dendritic bridges appeared between the aligned channels.Once the PVA concentration exceeded an critical value,the pores disappeared.The dispersion stability of the suspension improved with the increase of pH of the slurry.Under the action of the driving force of ice crystal growth,the particles were repulsed by the moving solidification front.In the meanwhile the microstructure of the porous BN materials gradually changed from the dendritic pore to the aligned channel.When the pH value was constant,the supercooling degree of the slurry increased with the increase of the freezing rate,and then the nucleation rate and the growth rate of the ice crystals changed.When the nucleation rate was close to the growth rate,the microstructure of the porous BN materials changed from randomly distributed pores to aligned channels.Based on the above studies,the sintering densification and adsorption properties of porous BN materials were further investigated.Firstly,the combination of the carbothermal reduction method and the freeze-drying method achieved the carbothermal reduction of B2O3 at a relatively low temperature and obtained porous BN ceramics with lightweight&high porosity.The obtained samples were characterized by XRD,SEM,TEM,XPS and FT-IR.It was found that the crystallinity of B4C increased with the increase of sintering temperature.The pore structures of the porous BN ceramics were aligned channels,with pore diameters of about 39-195?m and porosities of 92.10-94.65%.HRTEM analysis showed that the BN and B4C particles were not tightly-bonding.Subsequently,B2O3-SiO2 and metal Al were used as sintering aids for preparing porous BN ceramics under nitrogen or air atmosphere,respectively.The phase composition and content were analyzed.Using B2O3-SiO2 as a sintering aid,the Al4B2O9 phase appeared after 900 ? in air atmosphere could effectively inhibit the oxidation of BN;if metal A1 was added,the AIN phase appeared in the product at 1000?.As the sintering temperature increasing,the content of AIN phase would increase.This AIN phase could improve the wettability of the Al/BN system at the solid-liquid interface,which should be beneficial for the sintering performance of porous BN ceramics.Finally,the adsorption properties of the as-prepared porous BN materials were investigated.The results showed that malachite green was adsorbed on the BN layers through a,'?-?'stacking interaction between the aromatic ring and the in-plane six-membered ring of BN.The adsorption isotherm matched well with the Langmuir isotherm model.It revealed the monolayer adsorption type and the saturated adsorption amount was 339.8 lmg/g;The maximum adsorption capacities of porous BN materials for Cu2+ was 435mg/g.Pt particles were uniformly adsorbed on porous BN materials via Van der Waals force.The adsorption amount of Pt particles was approximately 1.5wt%,and the particle size was approximately 10nm.These results indicate that porous BN materials with aligned channels prepared through freeze-drying are promising adsorption materials for wastewater treatment.