Effectiveness And Mechanism Of U(Ⅵ) Removal By Halloysite And Its Modified Composites

The progress and development of the world have led to the exploitation and utilization of nuclear energy,which inevitably produced a large amount of radioactive wastewater in the process.As an important element of nuclear energy,uranium is radioactive and chemically toxic,threatening environmental safety and human health.Adsorption method has been widely used for wastewater purification.In this paper,we investigated the adsorption performance of naturally widespread clay mineral halloysite nanotubes（HNTs）and their modified materials on uranium in wastewater.（1）The adsorption performance of U（Ⅵ）by acid-base activation and heat-treated HNTs were investigated.HNTs were activated with H₂SO₄ and Na OH,and three concentration gradients were set for both acid and base:1 mol/L,3 mol/L and 6 mol/L.The heat treatment temperatures were 300°C,600°C and 900°C.From the static adsorption experiments,it can be concluded that the adsorption performance of HNTs for U（Ⅵ）is poor after acid-base activation and heat treatment at p H=4.0.At p H=6.0 and 8.0,the adsorption performance of heat-treated HNTs for U（Ⅵ）was enhanced to some extent（more at p H=6.0）,and the removal efficiency of U（Ⅵ）gradually improved with increasing calcination temperature（69.46%for U（Ⅵ）by900CH compared to 49.53%for pristine HNTs under the same conditions）,and the adsorption of U（Ⅵ）was inhibited by acid-base activation.（2）Composite adsorbents（HNTs@PDA）with different polydopamine（PDA）layer thicknesses were synthesized by organic modification of HNTs through self-polymerization of dopamine（DA）.Characterization of HNTs@PDA demonstrated that the original structure of the HNTs was maintained.The thickness of the PDA layer increased with the mass ratio of DA to HNTs（thicknesses:HD2≈27.30 nm;HD5≈13.13 nm,HD10≈7.93 nm）.Adsorption experiments confirmed that the adsorption capacity of HNTs@PDA for U（Ⅵ）was significantly improved and the maximum adsorption capacity was specifically:HD2（72.51 mg/g）>HD5（60.48 mg/g）>HD10（42.00 mg/g）>HNTs（28.30 mg/g）.The adsorption process of U（Ⅵ）by HD5 was endothermic and affected by coexisting higher valence cations.The Pseudo-second-order kinetic model and Freundlich model matched well the experimental data of U（Ⅵ）removal by HD5.HD5 was recycled five times in the experiment,and the U（Ⅵ）removal rate decreased from 93.11%to 76.67%,a decrease of 16.44%,with good reusability.The adsorption mechanism of U（Ⅵ）by HD5 can be explained by considering electrostatic interactions and the complexation of C-O,-NH-and C-N/C=N in the PDA layer.（3）Two kinds of materials,HNTs loaded Fe₂O₃（He-HNTs）and HNTs loaded Fe₃O₄（Ma-HNTs）,were prepared by inorganic compounding of HNTs.The loaded Fe₂O₃ and Fe₃O₄ were in granular and spherical form,respectively,with the grain lengths in the range of about 30～60 nm and the spherical particle sizes in the range of about 10～20 nm.The Ma-HNTs exhibited good magnetic separation ability.The adsorption capacity of He-HNTs and Ma-HNTs on U（Ⅵ）was enhanced compared to that of raw HNTs.The adsorption rates of He-HNTs and Ma-HNTs on U（Ⅵ）were very fast,and the equilibrium was basically reached in the first 40 min.The Pseudo-second-order kinetic model and Freundlich isotherm model more accurately described the adsorption process of U（Ⅵ）on He-HNTs and Ma-HNTs.The maximum adsorption capacities were 74.64 mg/g for He-HNTs and 66.40 mg/g for Ma-HNTs.The adsorption mechanism of U（Ⅵ）on He-HNTs is mainly electrostatic interaction and surface complexation.The adsorption mechanism of uranium by Ma-HNTs includes electrostatic interaction,surface complexation and also the redox effect of Fe（II）on U（Ⅵ）in the material.