The Application Of Graphite Carbon Nitride In The Photocatalytic Uranium Extraction From Seawater

Uranium extraction from seawater is of significant importance for the sustainable development of nuclear energy.Given the low uranium concentration,high salinity and complex matrix of seawater,current techniques for recovering uranium face several challenges.Therefore,it is crucial to develop new techniques for efficient uranium extraction from seawater(UES).Heterogeneous photocatalysis,during which highly soluble U(Ⅵ)is reduced to immobile U(Ⅳ),has recently emerged as a thriving strategy for UES.This study focused on the key challenges and difficulties in the application of photocatalytic method in UES,and developed a series of carbon nitride-based photocatalysts for the efficient U(Ⅵ)reduction.The main achievements were as follows:(1)The U(Ⅵ) reduction efficiency of intrinsic g-C3N4(BCN)is limited by the low quantum conversion efficiency and high electron-hole recombination rate.In this work,mesoporous g-C3N4(mpg-C3N4)with higher surface area and dispersibility was prepared by introducing abundant mesopores onto BCN.Compared with BCN,both light utilization and the separation efficiency of photo-generated charge carriers were significantly improved for mpg-C3N4.Under visible light,mpg-C3N4 showed significantly enhanced photocatalytic performance for the reduction of U(Ⅵ).A high uranium reduction efficiency(0.01 mmol g-1 min-1)and capacity(～2,990 mg g-1)could be achieved by mpg-C3N4,which was 6.75 times higher than that of BCN.In the presence of co-existing ions,mpg-C3N4 exhibited relatively high selectivity for the photo-reduction of U(Ⅵ),and a satisfied selectivity for U(Ⅵ)was also achieved.The deposited uranium on mpg-C3N4 could be recovered simply by exposure in air for 24 h,and further eluted with a 0.1M Na2CO3 solution.This strategy provides guidance for the application of photocatalytic method in UES.(2)Considering the serious inhibition of seawater carbonates on the photocatalytic reduction of U(Ⅵ),carboxylated carbon nitride catalysts(CCN)were prepared by oxidating bulk carbon nitride(BCN).The modification of carboxyl groups on g-C3N4 largely promote the affinity for U(Ⅵ),and tuned the species of uranium on solid-water interface,making it easier to be photocatalyticlly reduced.The introduction of electronwithdrawing carboxyl groups also reduced the conduction band position and promoted the separation efficiency of electrons and holes,facilitating the photocatalytic performance for the reduction of U(Ⅵ).Under visible-light,CCN exhibited high reactivity(0.0867 min-1)for the photocatalytic U(Ⅵ)reduction in the presence of 0.2 mM CO32-,being～33 times of that for BCN(0.0026 min-1).·O2-was proved to be the main reactive reduction species for U(Ⅵ).Compared with BCN,CCN could generate more ·O2-and fewer oxidizing ·OH radicals,which is beneficial for the U(Ⅵ)reduction.U(Ⅵ)was finally reduced to UO2+x(x<0.25)nanoparticles and deposited on the surface of CCN.This work confirmed the possibility of extracting uranium from carbonates-containing solutions via photocatalytic methods for the first time,and effectively improved the applicability of photocatalytic methods in seawater.(3)In most photocatalytic systems,the photocatalytic reduction of U(Ⅵ)was strictly suppressed in open air,which limits the practical application of photocatalytic method.In this work,a type Ⅱ CdS/g-C3N4 nanocomposites were constructed by coupling narrow-bandgap CdS with g-C3N4.CdS/g-C3N4 can tune the generation of the reactive reduction/oxidation species by adjusting the band structure.More dissolved oxygen was reduced to form ·O2-,and the generation of ·OH was suppressed in this system.Remarkably high photocatalytic efficiency was realized without the aid of any electron sacrifice,where uranyl was reduced to U(Ⅳ)only within 6 min(reaction rate of 0.641 min-1),being the fastest among the ever-reported researches.Most importantly,a satisfying performance was achieved for the photocatalytic U(Ⅵ)reduction in open air,under solar irradiation and a 4 W LED lamp,and in spiked seawater.The reduction capacity for uranium reached 2,379 mg g-1,and the recovered CdS/g-C3N4 material after uranium elution could keep high stability.This study realized the efficient photocatalytic reduction of U(Ⅵ)in open air,further promotes the application of photocatalytic method in UES.(4)The photocatalytic reaction is strongly dependent on solar light,and the uranium product is easy to be oxidized and dissolved again without illumination.To solve this problem,K+and cyano group co-decorated poly(heptazine imide)(K-CNPHI)is developed to realize the persistent U(Ⅵ)reduction upon/beyond light.Under visible light irradiation,K-CN-PHI achieved the photocatalytic reduction of U(Ⅵ)with a reaction rate of 0.89 min-1,being 46.8 times greater than that over BCN(0.019 min-1).This system can further be triggered by light to form long-living radicals(over 3 d)and drive the fast reduction of U(Ⅵ)in the dark(within 1 min),where the flexible structural K+as counterions could stabilize the electrons trapped by cyanamide groups.The results collectively proved K-CN-PHI to be a novel and efficient photocatalyst enabling persistent U(Ⅵ)extraction around the clock,broadening the practical applications of photocatalytic method in UES.(5)The carboxylated mesoporous g-C3N4/CdS hydrogel film(CMCF)catalyst was fabricated by combing the advantages of the above-mentioned photocatalytic systems.The as prepared CMCF hydrogel film possessed rich porous channels,which can act as electron transfer channels for the photocatalytic reduction of U(Ⅵ).In the open air,the CMCF hydrogel film exhibited excellent reactivity for the photocatalytic U(Ⅵ)reduction in the presence of CO32-,where 80%of U(Ⅵ)can be extracted within 150 min irradiation without the aid of electron sacrifice.Moreover,the CMCF hydrogel film has good mechanical strength,stability and antibacterial property,and was easy to be recovered and reused.The fabricated photocatalyst is expected to meet the requirements of the photocatalytic uranium extraction from real seawater in practical applications,and greatly promotes the application of the photocatalysis method in UES.