Adsorption Mechanisms Of U???onto Fe₃O₄ Nanoparticles Coated With Lignite Humic Acid

With environmental pollution caused by the use of fossil energy becomes increasingly severe,the development and utilization of nuclear energy is therefore a matter of extreme urgency.However,uranium-ore mining and refinement,nuclear power plant accidents and long-term storage of nuclear waste have inevitably contributed to a rise in uranium contamination of surface and ground waters.Therefore,in order to minimize the detrimental impact on the environment and to successfully press forward our nation's strategic nuclear energy plan,it is of strategic need to search for and develop efficient and economical approaches for treating U?VI?contaminated water.This dissertation is based upon the extraction,characterization and stability evaluation of lignite humic acid to obtain a type of magnetic humic acid nanomaterial with great affordability,satisfactory stability,high solid-liquid separability and high adsorption capacity.Through a series of characterization analysis techniques,batch experiments,and surface complexation models,the adsorption properties and mechanism of this material for U?VI?were studied.The main findings are as follows:?1?The extraction methods of humic acid from lignite were studied,and the stability of three kinds of lignite humic acid and a commercial humic acid were evaluated.The results showed that the extraction methods for maximum yield are as follows,0.5 mol L^-1Na OH,0.07 mol L^-1Na₄P₂O₇,3 hours reaction time and 65?.The functional groups in lignite humic acid and commercial humic acid are hydroxyl group,carboxyl group and aromatic group.The total acid groups content is C-HA>MW-HA>DSZ-HA>DZ-HA from high to low.The release amount of heave metals in MW-HA did not exceed the limit value of drinking water under all p H conditions.The order of TOC concentration due to humic acid dissolution is C-HA>DZ-HA>DSZ-HA>MW-HA.To sum up,MW-HA has exhibited high content of total acid groups,low metal release amounts,low solubility and good structural stability in water.?2?The adsorption behavior of U?VI?on lignite humic acid and commercial humic acid was studied.The results showed that p H had a significant effect on the adsorption of U?VI?onto humic acid,and the optimal p H was about 5.0?8.0.The speed of reaching adsorption equilibrium is MW-HA>C-HA>DZ-HA>DSZ-HA from fast to slow.All humic acid samples reach adsorption equilibrium within 5 hours,and the experimental data were found to comply with the pseudo-second-order kinetic model.At 298K,the adsorption capacity of MW-HA,C-HA,DSZ-HA and DZ-HA are 129.9,111.3,86.7 and 50.5 mg g^-1,respectively.The thermodynamic parameters showed that the adsorption reaction was a spontaneous and endothermic process.After three adsorption and desorption cycles,the adsorption capacity of MW-HA,DZ-HA,DSZ-HA and C-HA decreased by 10.2%,9.5%,35.6%and 33.5%respectively.In conclusion,MW-HA showed fast adsorption reaction rate,large adsorption capacity and good recycling performance.?3?The adsorption mechanism of U?VI?onto LHA-coated Fe₃O₄NPs was studied.The results showed that MW-HAcoating can significantly reduce the agglomeration of Fe₃O₄NPs.The LHA-coated Fe₃O₄NPs has good recycling performance and can be easily separated.U?VI?adsorption is p H dependent,with an optimum p H range from 5.0 to 8.0.At 298K,the maximum adsorption capacity of 0.5 LHA-Fe₃O₄NPs,1.5 LHA-Fe₃O₄NPs and 2.5LHA-Fe₃O₄NPs are 42.5,55.6 and 68.7 mg g^-1,respectively.XPS results indicated that no significant U?VI?reduction occurred on the surface of the LHA-coated Fe₃O₄NPs,possibly due to the formation of maghemite on the magnetite surface and/or because the MW-HA coating inhibits U?VI?reduction.The results of potentiometric titrations showed that the site concentrations of the LHA-coated Fe₃O₄NPs increase with increasing extent of LHA coating on the NPs.U?VI?adsorption onto the Fe₃O₄NPs with the highest extents of LHA coating is dominated by binding onto the LHA-associated binding sites?e.g.,carboxyl sites?,while>Fe-OH sites are also involved in U?VI?adsorption onto Fe₃O₄NPs with the lowest extent of LHA coating.?4?We measured the adsorption of U?VI?onto 0.5 LHA-Fe₃O₄NPs as a function of p H,ionic strength,adsorbent concentration,DIC concentration and dissolved calcium concentration.The observed U?VI?adsorption onto 0.5 LHA-Fe₃O₄NPs is ionic strength independent,and below p H 5,the extent of U?VI?adsorption increases with increasing p H and adsorbent concentration.The extent of U?VI?adsorption decreases dramatically with increasing concentration of added Na HCO₃and Ca Cl₂above p H 5 and p H 6 respectively.The adsorption data were modeled using a non-electrostatic surface complexation modeling approach to determine the identities and thermodynamic stabilities of possible uranyl complexes on the LHA-coated Fe₃O₄NP surfaces.The modeling results indicate that a series of uranyl-hydroxide,uranyl-hydroxide-carbonate,and uranyl-carbonate are involved in the adsorption under different p H and DIC concentration conditions.The model can be applied to predict the distribution of U?VI?in complex LHA-coated Fe₃O₄NP-bearing systems under a wide range of p H,ionic strength,DIC,Ca²⁺,and solute:sorbent conditions.In summary,this dissertation has systematically evaluated the stability of lignite humic acid in aqueous solution,and helped lay a foundation for the preparation of magnetic humic acid nanomaterials with great affordability,stable structure and excellent adsorption capacity.The adsorption properties and mechanism of Fe₃O₄nanoparticles coated with lignite humic acid for U?VI?were clarified.A surface complexation model suitable for complex water environments is established,which entails important practical significance for the engineering application of Fe₃O₄nanoparticles coated with lignite humic acid.This dissertation research can provide experimental and theoretical basis for practical applications of functional magnetic nanomaterials in the treatment of U?VI?pollution.