Preparation Of Fly Ash-Based Adsorbent And Study On Uranium Adsorption Properties

Economic development and social progress are inseparable from energy.Nowadays,countries urgently need to find alternative energy sources to cope with the ensuing energy crisis.Uranium（U（Ⅵ））,the main source of nuclear energy and the core of its development and application,is also the main component of radioactive wastewater.U（Ⅵ）is able to enter the organism via the food web and cause many diseases,therefore,it should be treated before industrial wastewater containing uranium is discharged into the environment.Various methods have been proposed for the removal of U（Ⅵ）from aqueous solutions,among which adsorption has been widely used because of its ease of operation,environmental friendliness,feasibility and cost effectiveness.Among the many adsorbent materials fly ash（CFA）is considered as a viable adsorbent for wastewater treatment due to its special structure,abundant tiny active channels and large specific surface area.1)In order to improve the U（Ⅵ）adsorption performance of CFA,fly ash aerogel（CFAA）with porous structure was prepared in this study with the help of ice template method.Characterization by TGA,FTIR,and SEM reveals that CFAA consists of many spherical and irregular particles of different sizes and has good thermal stability.The results of adsorption experiments showed that the removal rate of U（Ⅵ）on CFAA was as high as94.5%,which was much higher than that of untreated CFA（83.7%）.Meanwhile,the maximum removal capacity of CFAA was 110.73 mg g^-1,which was almost twice that of the untreated CFA.After five cycles,the removal rate of U（Ⅵ）by CFAA still exceeded 80%,indicating that CFAA has good cycling stability.the Pseudo-second-order kinetic model and Langmuir isothermal model are more suitable for describing the adsorption behavior of CFAA on U（Ⅵ）,indicating that the adsorption of U（Ⅵ）on CFAA is a homogeneous monolayer chemical adsorption.2)To investigate the effect of CFA size on the U（Ⅵ）adsorption performance of the prepared products,porous fly ash aerogels of different sizes（PCFAA-325,PCFAA-600 and PCFAA-1250）were prepared by the ice template method using different sizes of CFA（325mesh,600 mesh and 1250 mesh）as raw materials in this study.Compared with PCFAA-325and PCFAA-600,PCFAA-1250 has superior U（Ⅵ）adsorption performance,with a maximum U（Ⅵ）adsorption efficiency close to 96.5%and a maximum U（Ⅵ）adsorption capacity of about 123.09 mg g^-1.The Pseudo-second-order kinetic model and Langmuir isothermal model are more suitable for describing the removal behavior of U（Ⅵ）on PCFAA-1250,indicating that the adsorption process of U（Ⅵ）by PCFAA-1250 was a homogeneous monolayer chemisorption.In addition,after five cycles,the desorption efficiency of PCFAA-1250 for U（Ⅵ）was 84.9%and the adsorption efficiency was 87.5%,which was much higher than that of PCFAA-325 and PCFAA-600,indicating that PCFAA-1250 also has a more excellent recyclability.The changes of XRD,FTIR,SEM and XPS before and after adsorption can be found that the adsorption of U（Ⅵ）on PCFAA is achieved by the formation of complexes with U（Ⅵ）by groups on the surface of PCFAA（such as-OH,Si O-OH and Al O-OH）.3)To further improve the U（Ⅵ）adsorption performance（e.g.,adsorption capacity,equilibrium time,recyclability,etc.）of CFA,hydroxyapatite modified fly ash composite aerogel（HCFAA）was prepared by combining the ice template method and in situ synthesis method in this study,and it was used for the removal of U（Ⅵ）from wastewater.The experimental results showed that the maximum removal efficiency and removal capacity of U（Ⅵ）on HCFAA were 97.6%and 205.7 mg g^-1,respectively,which were much higher than those of CFA(83.6%and 59.1 mg g^-1)and most reported fly ash-based adsorbents.In addition,HCFAA also had a shorter adsorption equilibrium time（240 min）compared to CFA（720 min）.After five cycles,the desorption efficiency of HCFAA for U（Ⅵ）was still higher than 90%,indicating the good recyclability of HCFAA.The Langmuir isothermal model was in good agreement with the Pseudo-second-order kinetic model and experimental data,indicating that the adsorption process of HCFAA for U（Ⅵ）may be monolayer and homogeneous chemisorption.Meanwhile,the efficiency of U（Ⅵ）removal by HCFAA in simulated real wastewater was as high as 80.6%,and even the U（Ⅵ）could be completely separated from the real wastewater under dynamic adsorption conditions,further indicating that HCFAA also has good prospects for practical applications.The characterization results before and after adsorption showed that U（Ⅵ）was successfully attached to HCFAA by ion exchange,dissolution-precipitation and surface complexation reactions.