Effects Of Coexisting Arsenate And Humic Acid On The Sorption Behaviors Of U(?) At Goethite/water Interface

The macroscopic sorption trends and microscopic speciation of heavy metal ions and radionuclides at the natural mineral/water interfaces have a significant influence on their migration,transformation,ecotoxicity and fate in the geological environment.In view of this,the effects of arsenate and humic acid?HA?on the sorption behaviors of U?VI?at goethite/water interface were investigated in this thesis by combining the batch experiments,theoretical speciation modeling,powder X-ray diffraction?PXRD?,Fourier transform infrared spectroscopy?FTIR?and X-ray photoelectron spectroscopy?XPS?analysis.The main research contents and findings are listed as follows:1)Solution pH plays an important role in the sorption behaviors and microscopic interaction mechanisms of U?VI?.Within the acidic pH range,the presence of arsenate and HA facilitated U?VI?sequestration due to the formation of ligand-bridging ternary surface complexes in type B.Typically,PXRD analysis showed that the positive effect of arsenate on U?VI?immobilization was partly attributed to the potential precipitation of the tr?egerite?UO₂HAs O4·4H₂O?and Na-autunite?Na2?UO₂As O4?2·3H₂O?phases.While at alkaline pH,the coexisting arsenate and HA tended to form UO₂?As O4?n2-3n or UO₂?CO3?HA complexes with U?VI?in the aqueous solution,which strongly inhibited the sorption of U?VI?onto the goethite surfaces due to electrostatic repulsion.2)Contact time is also an important factor that affects the sorption trends and speciation of arsenate and U?VI?.In the ternary U?VI??50 ?M?/arsenate?133 ?M?/goethite system,the sorption percentage of arsenate gradually increased with increasing contact time,while the sorption percentage of U?VI?maintained at a high level of ¹00%.According to the theoretical speciation modeling and PXRD analysis results,the tr?egerite and Na-autunite phases formed after 1 day.As the contact time prolonged to 3 days,a metastable UO₂?H₂As O4?2·H₂O?denoted as UAs2?was observed and then this solid completely transformed into well-crystallized Na-autunite phase after 7 days.3)The sorption behaviors of U?VI?within different reaction systems exhibit disparate responsivity to the temperature variation.Specifically,the increase of temperature had no obvious effect on the sorption trends of U?VI?in the ternary U?VI?/arsenate/goethite system.Therefore,more attentions should be paid to the impacts of other environmental factors?e.g.,pH,ionic strength,concentration of coexisting pollutants,contact order,etc.?.In contrast,the rising of temperature facilitated U?VI?sorption in the ternary U?VI?/HA/goethite system,implying that the immobilization of U?VI?at goethite/water interface was an endothermic process.4)The uptake behaviors and microstructure of U?VI?are highly correlated with the contact orders of different pollutants.At an acidic pH value of 4.5,the sorption percentages of U?VI?in the ternary U?VI?/HA/goethite system showed a decreasing trend as follows: pre-equilibration of U?VI?with HA before the addition of goethite> simultaneous addition of U?VI?,HA and goethite>pre-equilibration of HA with goethite before the addition of U?VI?.The pre-equilibrated U?VI?and HA could form complexes that were easily adsorbed on the positively charged goethite surfaces.In contrast,the pre-equilibration of HA with goethite possibly caused the aggregation of solid particles and the decrease of specific surface area,which would correspondingly reduce the sorption percentage of subsequetly added U?VI?.For the sorption samples prepared at different contact orders,FTIR and XPS analysis suggested the formation of binary and/or ternary surface complexes with different microstructure and proportions.In summary,this thesis reveals the influencing rules of different environmental factors on the sorption trends of U?VI?at goethite/water interfaces.Based on theoretical speciation modeling and spectroscopic analysis,the microscopic species and retention mechanisms of U?VI?in the absebce/presence of arsenate and HA are clarified.The experimental results can provide a scientific basis for accurately understanding and predicting the physicochemical behaviors and ecotoxicity of U?VI?in the actual water environment.Meanwhile,the research findings can also provide essential reference data for developing effective remediation strategies towards radioactive wastewater.