| Recently, with the rapid development of nuclear industry, all the countries will have a great demand of uranium resources. During the process of the uranium mining, these wastewater including uranium,which has chemical toxicity and radioactivity, has a serious affect on the environment and human health, therefore, the removal and recyle of uranium from the aqueous solution is specially important.The MOFs--- Zn-MOF-74, because of its high porosity, high surface area, adjusting pores and shapes and various coordination structure, has became one of the best adsorbing materials.In the research, design Zn as metal center and 2,5-dihydroxybenzene-1,4-dicarboxylic acid as organic ligand, respectively in the solvothermal method for synthesizing microporous Zn-MOF-74 crystal material(A-1) and fast synthesizing method for synthesizing microporous-mesoporous Zn-MOF-74 nanometer material(A-2); loading different ratio of photoresponsive coumarin on the sample(A-2), the coumarin-modified materials(samples A-3 series: 11.7wt%?20.6wt%?28.2wt% and 35.7wt%) would be obtained. By the N2 adsorption-desorption, FTIR, XRD, TG and other characterization methods, characterize the samples A-1, A-2 and A-3 series. Study the influence of the pH of aqueous solution, reaction time, reaction tempture, initial concentration of uranium and elution solution over the uranium over the samples A-1 and A-2, in order to analyze their adsorption model and regularity. For analyzing the behaviors of the samples A-3 series for photo-switched storage/release of uranium, the affect of ultraviolet light and elution solution on the adsorption of uranium over them were studied.The experiments indicated that the adsorption sites in the surface of samples A-1 and A-2 could be coordinated with the uranyl ion and the adsorption process was spontaneous. The pseudo-second-order kinetics model and Langmuir isotherms model for adsorption of uranium over the samples A-1 and A-2 were fitted well, which suggested these adsorptions were chemical adsorption and monolayer adsorption. But the maxium adsorption capacity of sample A-2 was 1. 64 times that of sample A-1, which indicated that microporous-mesoporous Zn-MOF-74 nanometer material completely improved the adsorption ability of uranium. In the optimal experiment conditions, the adsorption capacity of samples A-3 series were 360mg/g, 356mg/g, 350mg/g and 280mg/g; in comparision to the sample A-2, coumarin-modified samples provided enough adsorption sites, which made a contribution to the adsorption capacity of uranium, increased by 15%. After the ultraviolet light, samples A-3series would have the photodimerization, then close the “open-close double door” and decrease the adsorption capacity, whose adsorption capacity were 360 mg/g, 325 mg/g, 300 mg/g and 245 mg/g. The adsorbed uranium in A-3:28.2%wt could be easily eluted by 0.1 mol/L HCl with elution efficiency of 80%, whereas identical samples after UV irradiation(320nm), under the same eluting conditions, gave only a 60% elution efficiency, indicative of 20% uranium(equal to 70mg/g) that were sealed inside sorbent by means of such photo-switching technique. Similarly,reirradiating the same samples under 250 nm completely rerelease the sealed 20% uranium. To sum up, the sorbent whose structure was themicroporous-mesoporous and modifying coumarin will improve the adsorption abilities; this reaearch will provide a way to enhance the uptake capacity of uranium and demonstrate a means of such ptoto-switched technique for storage and release of uranium. |
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