Preparation Of Graphite Phase Carbon Nitride Composites And Study On Photocatalytic Reduction Of Uranium

The overuse of fossil resources has led to serious environmental pollution,forcing a desperate search for clean energy,especially nuclear energy.However,uranium resources are scarce on land and reserves in seawater are relatively abundant.At present,the most widely used method of seawater uranium extraction is adsorption method,but its limited saturation adsorption capacity and complex post-processing steps are a problem that cannot be ignored.The above problems can be overcome by reducing soluble hexavalent uranium U（VI）to insoluble tetravalent uranium U（IV）by photocatalytic technology.In this paper,a high dispersible graphitic carbon nitride（WSGCN）was prepared by molten salt method,which had a high uranium adsorption capacity in uranium solution,but its photocatalytic reduction ability was weak.Therefore,in order to improve the photocatalytic ability of WSGCN,and it was also easy to form a homogeneous composite material by taking advantage of its high dispersion in aqueous solution,a heterojunction material was formed by compounding WSGCN with a second semiconductor or cocatalyst,improving the photocatalytic activity.And copper nanoparticle-supported graphitic carbon nitride composites（Cu-WSGCN）,graphitic carbon nitride homotypic heterojunction composites（g-C₃N₄/WSGCN）,and graphitic phase carbon nitride composites(C_ring-WSGCN)compounded with carbon layers were prepared using WSGCN as the base,and the effect of different composite ratios,initial concentrations of uranium solution,p H value of uranium solution on uranium removal rate of composites and stability of composites in photocatalytic reduction uranium cycle experiments were explored.The Cu-WSGCN composite was obtained by loading copper nanoparticles on WSGCN by chemical reduction.The results showed that Cu-WSGCN composites have higher uranium removal rates than WSGCN.The composite had the best uranium removal rate when the copper nanoparticle loading was 1%.With an increase in the initial concentration of uranium solution,1%Cu-WSGCN had the highest removal rate of uranium at 500 mg·L^-1 uranium solution.When p H value of uranium solution was 5,1%Cu-WSGCN had the best uranium removal at 100 m L uranium solution of 100 mg·L^-1,and the removal rate reached 81.07%,when the photocatalyst input amount was 20 mg.And after three photocatalytic reduction cycles of uranium,the removal rate of uranium by 1%Cu-WSGCN remained at 75.96%.The photoelectrochemical test results showed that after the copper nanoparticles were loaded on WSGCN,there was a Schottky barrier at the interface between the two,which promoted the transfer of charge and the separation of photogenerated carriers,thereby improving the photocatalytic ability.The g-C₃N₄/WSGCN composites were prepared by blending WSGCN with bulk graphitic carbon nitride（g-C₃N₄）and calcination at high temperature.Photocatalytic reduction of uranium experiments showed composite materials had positive advantages for photocatalytic reduction of uranium when WSGCN and g-C₃N₄ rate was 1:3,the initial concentration of uranium solution was 500 mg·L^-1,and the p H value of uranium solution was 5.At 100 m L uranium solution of 100 mg·L^-1,after 2 h dark reaction stage and 2 h light reaction stage,the removal rate of uranium by g-C₃N₄/WSGCN（WSGCN:g-C₃N₄=1:3）increased from 26.96 to94.46%compared with WSGCN,when the p H value of uranium solution was 5.And after three photocatalytic reduction cycles of uranium,the removal rate of uranium by composite materials was maintained 85.39%.Combining PL spectroscopy,transient photocurrent response and electrochemical impedance spectroscopy,g-C₃N₄/WSGCN（WSGCN:g-C₃N₄=1:3）had a faster charge transfer rate and a more efficient photogenerated carrier separation rate than WSGCN,which was in line with the electron transfer mechanism of I-type heterojunction.The C_ring-WSGCN composites were prepared by blending glucose with WSGCN and calcining at high temperature.The results showed that the removal rate of uranium from C_ring-WSGCN composites was higher than that of WSGCN.When the carbon layer composite ratio was 5%,the initial concentration was 500 mg·L^-1 and the p H value of uranium solution was 8,it was more favorable for the photocatalytic reduction of uranium by composites.At 100 m L uranium solution of 100 mg·L^-1,the removal rate of uranium of 5%C_ring-WSGCN after adsorption and photocatalysis was as high as 92.72%when the p H value of uranium solution was 8.And the removal rate of uranium by composite materials remained above 87.15%after three photocatalytic reduction cycles of uranium.Since the best p H value of 5%C_ring-WSGCN photocatalytic reduction of uranium was 8,it was suitable for uranium extraction in seawater environments.From PL spectroscopy,transient photocurrent response and electrochemical impedance spectroscopy,it can be seen that WSGCN was recombined with a carbon layer withΠconjugated structure,which inhibited the recombination rate of photogenerated electrons and photogenerated holes,and increased the charge transport rate,thereby improving the photocatalytic reduction ability of uranium.