(6)--2023 Chemical Engineering Jouran核化学与放射化学.pdf

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1、Chemical Engineering Journal 461(2023)142012Available online 20 February 20231385-8947/2023 Elsevier B.V.All rights reserved.Uranium extraction by a graphene-based asymmetric electrode configuration through combined complexation,electro-adsorption,and photocatalytic reduction Chuan Yana,Yun Liaob,c,

2、*,ChengJin Shenb,Xiaofang Wengb,Ruilin Leib,Chenlin Liaoa,Yuxiang Zhoua,Meng Wanga,*aSchool of Nuclear Science and Technology,University of South China,Hengyang,Hunan 421001,China bSchool of Chemistry and Chemical Engineering,University of South China,Hengyang,Hunan 421001,China cHunan Key Laborator

3、y for the Design and Application of Actinide Complexes,University of South China,Hengyang,Hunan 421001,China A R T I C L E I N F O Keywords:U(VI)Electro-adsorption Photocatalytic reduction Complexation Graphene A B S T R A C T Recently,capacitive deionization has become a promising approach for uran

4、ium extraction owing to its energy-efficiency and environmental benign.However,the adsorbed charged uranium(VI)(U(VI)woul-d repulse the incoming U(VI)ions,and might re-enter into solution,vastly impeding the improvement of electro-adsorption performance.Reducing the adsorbed U(VI)ions into U(IV)prec

5、ipitation is an ideal way to address above issues.Here,a coupling approach of photocatalytic reduction and electro-adsorption(PEA)was proposed.An asymmetric electrode configuration,composed of phosphate functionalized graphene(GP)cathode and graphene/TiO2 nanocomposite(GT)anode was rationally design

6、ed,and originally proved its enhanced U(VI)extraction via the PEA method.The asymmetric electrode device enables the fast transport of photo-electron and the rapidly directed migration of U(VI)ion to the GP cathode.More importantly,it combines three synthetic mechanisms of complexation,electro-adsor

7、ption,and photocatalytic reduction to extract U(VI)ions.As a consequence,the as-designed PEA method shows a higher removal rate of 91.3%in comparison with conven-tional photocatalytic reduction(PA)and electro-adsorption(EA)methods.Meanwhile,its kinetics rate is 225%and 50%faster than PA and EA metho

8、ds.Furthermore,an enhanced reduction efficiency of U(VI)to U(IV),good selectivity as well as reusability for PEA method were also obtained.The results provide a potential approach to combine multiple mechanisms for efficient U(VI)extraction from aqueous solution by purposeful design of asymmetric el

9、ectrodes.1.Introduction As a carbon-free and sustainable energy source,nuclear power has emerged as an alternative to polluting fossil fuels.Currently,there exists more than 440 nuclear plants as well as above 60 ones under con-struction1.Uranium is a core element for nuclear fuel,which might cause

10、environmental contamination if it is leaked or discharged into water during the mining and utilization process on account of its chemical and radiological toxicity.Meanwhile,hexavalent uranium(U(VI)is surprisingly abundant in aqueous solution(e.g.sea and salt lake)with reserves of 5001000 times grea

11、ter than those on land2.Hence,the extraction and recovery of U(VI)from aqueous solution have become one of the central issues from the perspective of energy saving and environmental protection.To extracting U(VI)from aqueous solution,various methods are adopted,mainly involving precipitation3,solven

12、t extraction4,ion exchange5,membrane separation6 and adsorp-tion7.However,these methods have serious drawbacks,such as complex process,easy to produce secondary pollution,harsh service conditions,high cost and low efficiency.Recently,capacitive deionization(CDI),i.e.,electro-adsorption,has gradually

13、 become a promising approach for U(VI)extraction.Under the influence of external voltage(2.0 V),positively charged U(VI)ions migrate,accumulate,and absorb into electrical double layer(EDL)on the surface of negatively charged electrode,and thus the concentration of U(VI)in aqueous solution drops rapi

14、dly.Through reversing or cutting off the external voltage,the captured-U(VI)ions would be released,*Corresponding authors at:School of Chemistry and Chemical Engineering,University of South China,Hengyang,Hunan 421001,China(Y.Liao).E-mail addresses:(Y.Liao),(M.Wang).Contents lists available at Scien

15、ceDirect Chemical Engineering Journal journal homepage: https:/doi.org/10.1016/j.cej.2023.142012 Received 19 December 2022;Received in revised form 3 February 2023;Accepted 16 February 2023 Chemical Engineering Journal 461(2023)1420122along with the regeneration of electrode material.Apparently,this

16、 approach possesses the virtues of energy-efficiency,environmental benign and simplicity,etc 811.It has been proved that electro-adsorption shows great advantages in capacity and kinetics rate in contrast to physico-chemical adsorption11.For example,Liu et al.,12 reported under the influence of elec

17、trical field,fourfold faster ki-netics than conventional physicochemical was obtained.Zhang et al.,13 also proved that the adsorption capacity of U(VI)could vary from 294 mg g 1 to 626 mg g 1 under a voltage of 0.7 V.Meanwhile,our groups14,15 have found that through introducing functional groups int

18、o the structure of electrode materials,to offer complexation active site with U(VI)ions,could further improve the extraction efficiency of U(VI).Huang et al.,9,10 have proved that the selectivity for U(VI)could also be improved through the formation of inner-sphere complexation between PO group and

19、UO22+.Obviously,combining complexation and electro-adsorption together shows a huge potential in U(VI)extraction.However,several shortcomings hinder its practical applica-tion:(1)The adsorbed-U(VI)still possess high toxicity and mobility,which may cause secondary pollution;(2)The adsorbed-U(VI)is st

20、ill positively charged,which would expel the incoming U(VI)ions because of Coulomb repulsion(co-ions expulsion effect),thus making them inaccessible to the inner structure of electrode;(3)Other undesirable cations might compete the active site with U(VI)ions,leading to the reduce of its selectivity

21、and capacity.Recently,reduction and solidification of soluble U(VI)into insoluble U(IV)during the adsorption process have attracted extensive attention.After being reduced,electroneutral uranium deposits(e.g.,UO2)are solidified and separated from water,which can effectively avoid sec-ondary pollutio

22、n and release active sites to capture incoming U(VI)ions.In addition,the reduction induced by electron can also impede the competition of other ions16.Hence,reducing the adsorbed U(VI)ions into U(IV)precipitation is an ideal way to address the above challenges during the electro-adsorption process.A

23、t present,there are mainly-four reduction methods:chemical reduction,micro-biological reduction,electrodeposition,and photo-catalytic reduction.Among them,the adsorbent of chemical reduction is always easy to aggregate and thus affect the reducing efficiency,while the microbial reduction shows a slo

24、w rate and uncertain long-term ef-fect.Electrodeposition is easy to operate and with relatively high reduction efficiency.However,the applied voltage is always very high(several to tens of voltage),which would give rise to the water elec-trolysis,energy waste as well as structural failure of the ele

25、ctrode ma-terial17.By contrast,photocatalytic reduction has the advantages of effective solar energy utilization,high efficiency,simple operate,and reusable catalyst18,19.To date,many researches have successfully realized the photocatalytic reduction of U(VI)to U(IV)through a serial of photocatalyst

26、s such as TiO2,g-C3N4,MOF,MoS2 and their composites 1821.Nevertheless,the recombination of photoelectron-holes has been an exasperating problem19,22.To address this issue,besides tailoring the structure of photocatalysts by introducing heterojunctions,element doping,defect engineering,etc.,the most

27、common way is to add sacrificial agents(e.g.,methanol,ethanol,sodium citrate,etc.)into the Fig.1.(a)Schematic illustration of the PEA coupling method.Preparation of(b)GP cathode and(c)GT anode.C.Yan et al.Chemical Engineering Journal 461(2023)1420123electrolyte,but it inevitably increases the cost a

28、nd even brings envi-ronmental pollution19,23.In addition,the most current researches performed with suspension-type photocatalysts were not practical,because it is highly challenging to separate these catalysts from aqueous solution or reduced U(IV)deposits24,25.In brief,every single method fails to

29、 perfectly meet the high demands of U(VI)extraction.Based on the above consideration,here we integrate the superiority of above methods and demonstrate an efficient coupling approach of photocatalytic reduction and electro-adsorption(PEA)for uranium capture.The scheme of the PEA coupling method main

30、ly involves four steps(Fig.1a):Step I,driven by an external electrical field,U(VI)ions fast migrate from aqueous into EDL;Step II,U(VI)ions complex with functional groups onto the surface of cathode;Step III,the excited photoelectrons transport through external circuit and reduce the adsorbed-U(VI)i

31、nto U(IV);Step IV,the electroneutral uranium species further deposit and grow,and thus the active sites are released.The PEA method possesses combined advantages including fast kinetics by elec-trical field-driven migration of U(VI)ion in place of random diffusion,high capture rate by three synerget

32、ic mechanisms of complexation,electro-adsorption,and photocatalytic reduction working simulta-neously,as well as the high energy efficiency contributing to the extra photoelectrons from anode lowering the applied bias on cathode to avoid the water splitting,while the applied bias,in turns,guiding th

33、e photoelectrons fast transport to cathode to avoid the recombination with holes.To realize the idea of PEA method,an asymmetric electrode configuration composed of phosphate functionalized graphene(GP)cathode with electro-adsorption characteristics and graphene/TiO2 nanocomposite(GT)anode with phot

34、ocatalytic activity was first con-structed(Fig.1b and 1c).For both electrodes,graphene was selected as a support material and efficient charge transport material owing to its excellent electron mobility,large specific surface area,excellent optical transmittance,easy modification as well as good che

35、mical stability 15,26,27.To prepare GP cathode,amino trimethylene phosphonic acid(ATMP)was chosen as a reductant to crosslink with graphene and as a modifier to offer phosphate groups.A 3D interconnected conductive network with hydrophilic phosphate active sites was finally formed.The as-produced GP

36、 electrode is expected to exhibit good electron transport ability and intensified selectively U(VI)capture.To prepare GT anode,TiO2 was selected as photocatalytic material due to its high stability,non-toxicity,high photocatalytic activity,and excellent dielectric properties28,29.Through incorporati

37、on with graphene,it expected to show enhanced photocatalytic quantum yield and broaden photo-response range since graphene could act as an electron capture/trans-port medium and thus reducing the charge recombination.Recently,the preparation and application of graphene/titanium dioxide composite pho

38、tocatalysts have been widely reported,however,to the best of our knowledge,this is the first time that GT anode and phosphate func-tionalized GP cathode are coupled to assemble an asymmetric electrode configuration and combine three mechanisms of complexation,electro-adsorption,and photocatalytic re

39、duction for uranium capture.Eventu-ally,the as-designed PEA method shows a higher removal rate of 91.3%and faster kinetics rate in comparison with photocatalytic reduction(PA)and electro-adsorption(EA)methods by the asymmetric graphene-based electrode configuration.In addition,an enhanced reduction

40、ratio of U(VI)to U(IV),good selectivity as well as reusability was also obtained.2.Experimental 2.1.Preparation of GP cathode Unless otherwise noted,the following reagents were provided by Macklin Regent Co.Ltd.All purchased reagents were used directly without further treatment.A modified Hummers ap

41、proach was adopted to produce GO nanosheets beforehand15.Phosphate functionalized graphene(GP)monolith was first prepared through a hydrothermal reaction under a low temperature of 90.Usually,a certain amount of GO was added into DI water under ultra-sonic to obtain a homogenous suspension with a co

42、ncentration of 4 mg mL 1.0 g,0.5 g,1.0 g and 2.0 g amino trimethylene phosphonic acid(ATMP,50%aqueous solution,v/v)were separately added into 20 mL GO suspension and mixed sufficiently under agitation.The above mix-tures were then put into four 30 mL glass vial and kept at 90 for 24 h to produce GP

43、hydrogels.Subsequently,the as-produced GP hydrogels were put into a 50 cryogenic refrigerator for 4 h.After thawing at room temperature,the GP hydrogels were then subjected to a vacuum drying process at 60 for 24 h.According to the mass of ATMP added,the as-obtained samples were denoted as GP-0,GP-0

44、.5,GP-1.0 and GP-2.0,respectively.The preparation of GP electrodes involves three steps:slurry prepa-ration,spin-coating,and drying.To obtain electrode slurries,GP active material,conductive carbon black(TIMCAL Spain)and polyvinylidene fluoride binder were mixed with a mass ratio of 8:1:1 and ground

45、ed into powder in an agate mortar.N-methyl pyrrolidone(NMP,99%)was dropwise added into above mixture under magnetic stirring with a stirring speed of 200 rpm.The stirring process further continued for 48 h to obtain uniform slurries.Then the slurries were spin-coated onto four graphite plates with a

46、 thickness of 5 mm and maintained a gap height of 200 m.Afterwards,the GP electrodes were put into a vacuum oven and kept at 80 for 12 h to remove remanent solvents.2.2.Preparation of GT anode Graphene/TiO2 nanocomposite was first fabricated to obtain GT anode.Typically,a certain amount of GO(0 mg,8

47、 mg,24 mg,40 mg,48 mg)was mixed with 10 mL DI water,and ultrasonic dispersed for 30 min in advance to form a homogenous suspension.6.8 g(or 0.02 mol)tet-rabutyl titanate(TBOT,98%)was dissolved in 30 mL ethanol and added dropwise to the above GO suspension.After continuing stirring for another 2 h,th

48、e mixture was transferred to a Teflon hydrothermal reactor,and heated at 200 C for 10 h.The obtained white or black-white mixtures were centrifuged and washed with DI water and ethanol for several times to get the precipitates,and then dried in vacuum at 80 C for 12 h.The mass ratio of GO to TiO2 wa

49、s 0,0.5%,1.5%,2.5%and 3.0%,and the GT nanocomposites are marked as GT-0,0.5,1.5,2.5 and 3.0,correspondingly.To obtain GT anode,the as-prepared GT photocatalyst active mate-rials were evenly mixed with ethanol(99%)with a concentration of 200 mg mL 1.Next,the obtained paste was spin-coated onto the co

50、n-ducting fluorine-doped SnO2 glass substrate(FTO,with a sheet resis-tance of 15)that had been cleaned,ultrasonicated,and rinsed with alcohol and deionized water.Eventually,the resultant films with an active area of 2 cm2 and a thickness of 2 m were calcinated at 450 for 2 h in argon to gain good el