Effect Of Semiconductor Mineral Potoelectrons On Uranium Biomineralization

The pollution treatment of heavy metals and radionuclides is a hotspot of global concern.Compared with other methods,photoelectron cooperated with microorganisms have significant advantages for the reduction and solidification of heavy metal ions.In order to investigate the effect and mechanism of semiconductor minerals photoelectrons cooperated with microorganisms on uranium removal and mineralization,in this work,semiconductor mineral TiO₂ and microorganism-Alcaligenes faecalis were chosen as the research object.Citric acid was used as hole scavenger to explore the effect of photoelectrons cooperated with A.faecalis on uranium removal and mineralization in the light source-semiconductor mineral-microorganism-variable valency heavy metal system.XPS,XRD,FTIR,SEM and other test methods were used to reveal the synergistic effect of photoelectrons and A.faecalis and how to control the photocatalytic removal and mineralization of uranium in solution systems.Ultimately,this study provided a theoretical basis for a remediation and treatment way of uranium environmental pollution.The results were showed as below:?1?The results of heavy metal pollution of surface soil samples around a uranium mine showed that the contents of seven heavy metals?Cr,Cu,Ni,Pb,Sr,U,Zn?in the uranium mine were higher than the control area,and the heavy metal accumulation was revealed.The results of PLI and Nemero comprehensive index method showed that the soil around the uranium mine was in the medium-heavy pollution level.Furthermore,the occurrence state of measured heavy metals showed that the main chemical speciation was residual fraction,but the exchangeable fraction of Sr and the organic binding fraction of U and Zn were significantly higher than other elements.The RAC results of the whole investigation area indicated that Sr,U and Zn were in the medium-high pollution risk level,and the soil samples of U close to the uranium mine has been in a high ecological pollution risk.?2?Compared to sodium citrate,sodium acetate and ethanol,the photocatalytic removal rate of U???in solution system was significantly increased by adding citric acid as hole scavenger,wich reached 53.48%.At the same time,the removal of U?VI?concentration was consistent with the quasi-first order kinetic equation when citrate was used as hole scavenger in pH=4.0.The photocurrent was the highest when TiO₂ was 2.00 g/L under the ultraviolet light irradiation.The best experimental condition was obtained:the concentration of TiO₂ was 2.00 g/L,pH=4.0,and organic hole scavenger was citrate.?3?The result of the photocatalytic removal of U???under different U???and CA molar ratio found that the removal rate of U???and uranium crystal decreased with the molar concentration ratio of U???and CA;The removal concent of U???was consistent with the quasi-first order kinetic equation when the ratio of U???and CA was equal to 1,and the reaction rate constants was 3,2 times of control group?CA:U???=0:1?.In addition,the removal rate of U???was up to 98.60%after 64 h illumination,and a rod-like uranium crystals was found in the system.?4?The fitting results of different molar ratio of U???and CA showed that it was mainly in the form of complexes[?UO₂??Cit?]-.According to the changes of the concentration of U???and CA in each reaction group,and the oxided uranium crystals found in the XRD and TEM-EDS,the complex structure of[?UO₂??Cit?]-was destroyed by photo-induced holes?h+?.The oxidized product of CA was complexed with U?VI?to form precipitation on the surface of TiO₂.In addition,the results of i-t test showed that the photocurrent of TiO₂ was obviously improved by CA.The photoelectron?ecb-?increased opportunities to reduce U???on the surface of TiO₂;according to the redox potential of ecb-and h+,ecb-and h+ reacted with H2O to produce H2O₂.Then,H2O₂ and O₂ from the environment reacted with UO₂2+ to generate the arc-like crystals of UO4·2H2O.The result of XPS and pH found that the addition of CA effectively promoted U???to be reduced in the system,the valence of uranium and pH was changed in different reaction process,so the new morphology of uranium crystals was found in the system.?5?In the A.faecalis-TiO₂ coexistence system,as the result of FTIR showed that U???mainly reacted with the active groups of-OH,-C=O,N-H,C-N,-PO42-and other groups in the protein and phosphoric acid groups from A.faecalis,which can accelerate the adsorption of U???in the system.The removal rate of U?VI?was above 95.54%and 53.59%respectively in CA:U???=0:1 and CA:U???=1:1 system after 24 h of dark treatment.The removal rate of U???was 98.63%in CA:U???=1:1 system after 12 h photocatalysis,and the final reduction rate of U???reached 27.80%.In addition,the morphology of the crystal on the surface of TiO₂ was changed from thin film flakes to complete fish scales.?6?In this study,the mechanism of the photoelectron coordinated with A.faecalis to U???photocatalytic removal and reduction of mineralization in the solution system was proposed as follows:under the dark conditions,due to CA and UO₂2+ formed a complexe,the effect of A.faecalis was enhanced due to the adsorption of TiO₂ surface was decreased,which accelerated its death decomposition.The photo-induced holes attacked CA and destroyed the complex structure of[?UO₂??Cit?]-under the ultraviolet light.The released of UO₂2+reacted with the organic groups of A.aecalis cells to produce complexes and mineralization and to promote the photocatalytic removal of U???in the system.In no A.faecalis system,CA cooperated with A.faecalis to significantly improve the photocatalytic removal of U???.CA played a role as hole scavenger to release more photoelectrons from TiO₂,so more U???was reduced to U???by the photoelectrons.Fianly,due to the changes of uranium valence and the interaction betweenh+ and the organic active groups of A.faecalis,the morphological of uranium on the surface of TiO₂ was changed.Based on the results of this research it can conclude that CA played a role as hole scavenger,which can effectively cooperate with A.faecalis to promote U?IV?photocatalytic removal and mineralization in solution system.The coordination of photocatalytic semiconductor mineral and microbial is an sustainable,efficient and low energy consumption way for heavy metals treatment.This research may provide a theoretical basis for uranium pollution treatment method.