Study On Preparation And Uranium Adsorption Performance Of Hydroxyapatite-based Composites

Hydroxyapatite(HAP)is a highly efficient uranium adsorption material with unique structure and excellent media adaptability.However,the reported HAP adsorbents still exist problems,containing complex preparation process,large potential safety hazards,small specific surface area,poor recycling and so on,which greatly limits the application of HAP in the remediation of uranium-containing wastewater.Freeze-drying-calcination method is widely used to prepare aerogel with porous structure and large specific surface aera because of its low cost,simple operation and easy control of product morphology in recent years.Konjac glucomannan(KGM),Xanthan gum(XG)and chitosan(CS)are three typical polysaccharides,which have rich groups(primary and secondary hydroxyl),biocompatibility and environmental friendliness.They could be chemically modified to obtain derivatives with improved performance and is widely used as template materials.In this work,HAP-based aerogels were prepared by freeze-drying-calcination and in-situ synthesis with the help of templates,which were used for the treatment of uranium containing wastewater.This study not only provided a green and environmental-friendly new method to synthesize porous HAP with large specific surface area,but also supplied a new strategy for developing efficient,green and economical HAP-based materials.The specific research contents were as follows:(1)To explore the influence of template types on the morphology and structure of HAP aerogel and uranium adsorption performances,three common sugars(KGM,XG and CS)with rich groups,good biocompatibility and environmental friendliness were selected as templates to synthesize HAP aerogel with different template sources(K-HAP,X-HAP and C-HAP)by freeze-drying-calcination method.Compared with X-HAP and C-HAP,K-HAP showed better pore structure and particle distribution,providing more available active site.Under the optimal experimental conditions,K-HAP showed better uranium adsorption performances,like good selectivity,short equilibrium time(10 min),high removal efficiency(99.2%)and large removal capacity(2070.3 mg/g).The adsorption process of uranium on K-HAP satisfied with Langmuir and Pseudo-second-order models,indicating that the adsorption process on K-HAP was monolayer chemical adsorption.This study further clarified the effect of template types on the morphology and structure of products and provided a new idea for the preparation of green and efficient porous materials.(2)In order to further evaluate the possibility of practical application of HAP aerogel prepared by freeze-drying-calcination method,the uranium adsorption performances of HAP aerogel were compared with commercially HAP.the prepared HAP aerogels possessed porous structure with large specific surface area(118.4 m~2/g),which could provide abundant available active sites.the removal efficiency of uranium by HAP aerogel reached99.4%within 10 minutes and the maximum adsorption capacity reached 2087.6 mg/g,which was much higher than the commercial HAP.Langmuir and Pseudo-second-order models were better for the description of the uranium removal process on HAP aerogel,suggesting that uranium removal on HAP aerogel was uniform single-layer chemical adsorption.According to the results of characterization before and after adsorption,it can be inferred that uranium was loaded on the HAP aerogel surface through complexation,dissolution precipitation and ion exchange.In this study,HAP aerogel with porous and large specific surface area was successfully synthesized by freeze-drying-calcination method,which not only improved the morphology and structure of HAP,but also greatly enhanced the uranium adsorption performance of HAP,providing potential materials for the remediation of uranium containing wastewater.(3)To solve the problem of poor recovery of HAP,magnesium doped hydroxyapatite(MHAPA)was successfully prepared by combining in-situ synthesis and freeze-drying-calcination method.Characterization results demonstrated that magnesium doping had changed the crystal structure of HAP and reduced the crystallinity and grain size.At p H=4.0 and T=298 K,the maximum removal capacity of MHAPA fitted by Langmuir model(2685.6 mg/g)was about twice that of commercially HAP(1299.8 mg/g),showing excellent uranium removal performances.In addition,MHAPA also performed good anti-ion interference ability,dynamic removal performance and recycling stability.After three cycles,the desorption efficiency of MHAPA still exceeded 85%.The excellent uranium removal performance of MHAPA was mainly attributed to its low crystallinity and grain size,fast ion exchange rate and partial ionization under acidic conditions,thus accelerating the chemical reaction between surface active sites and uranyl ions,including electrostatic attraction,ion exchange and complexation.This study not only enhanced the remediation ability of HAP-based materials for uranium-containing wastewater,but also prepared a hydroxyapatite composite material with recovery and recycling,providing a new direction for the design of HAP-based uranium adsorption materials.