| In order to alleviate the shortage of traditional fossil fuels and the increasingly serious environmental problems,clean and low-pollution nuclear energy has been rapidly developed.Nuclear power production has been used on a large scale in many countries,but the radioactive pollutants generated by it have aroused wide concern.Among them,radionuclide iodine(129I and 131I)produced by nuclear fission are major pollutants with very long half-lives,which can quickly disperse into the air and solution,causing serious impacts on the ecological environment and human health.At present,the most commonly used treatment method for radioactive iodine is solid adsorption,so it is necessary and meaningful to find green renewable,low-cost and high-performance iodine capture agents.Aiming at the problems of high cost of adsorbent,lack of regulation of performance and unclear mechanism of action in the capture of radioactive iodine,a series of lignin carbon/nano-clay composites were prepared by pyrolysis,using waste enzymatic lignin as carbon source and natural nano-clay as template,and the adsorption properties of the materials for iodine in different environments and different states were investigated.The main contents are as follows:The lignin carbon/nano-clay composites(ELC-X)were synthesized by pyrolysis using enzymatic lignin as carbon source,and natural nano-clay of halloysite(H),palygorskite(P)and sepiolite(S)as template,respectively.The physical and chemical properties of the composite were investigated and its adsorption effect on iodine in air and solution was explored.The results indicated lignin based porous carbon(ELC)well coated on the surface of nano-clay,and made the surface areas of origin nanoclay increased to 202.3～251 m2/g.These composites appeared the micro-mesoporous hierarchical structure,considerable N doping(1.10～1.49%)and good chemical stability.Three phase iodine element(iodine vapor,iodine in n-hexane,and iodine ions)were selected for the adsorption performance of composites.The results showed that the introduction of ELC could well promote iodine vapor uptake of nano-clay,and up to 435.0 mg/g.More importantly,the synergistic effect between lignin based carbon and nano-clay was very significant for the adsorption of iodine in n-hexane and iodine ion,their capacity were far exceed those of a single(bare)material,and reached up to 332.0 mg/g(ELC-P)and 28.5 mg/g(ELC-H),respectively.In addition,the X-ray diffractometer(XRD),Fourier transform infrared spectroscopy(FT-IR)and Raman spectroscopy(Raman)analysis of ELC-P before and after adsorption showed that the adsorption mechanism of lignin carbon/palygorskite composites(ELC-P)for iodine n-hexane was related to the combined effects of hydrogen bonding and charge transfer.To further explore the influence of preparation conditions on the structure and properties of ELC-P,the optimal preparation conditions were obtained.In other words,the effects of the ratio of carbon source to template and carbonization temperature on the structure and iodine capture of lignin carbon/palygorskite composites were investigated by using enzymatic lignin as the carbon source and palygorskite as the template.The results showed that ELC-P has a controllable specific surface area and pore size structure.When the ratio of carbon source to template dosage was 6/1,the composite material(ELC-P-6/1)prepared had the highest specific surface area(379.2 m2/g)and microporous fraction(76.7%).The performance test showed that ELC-P-6/1(620.5 mg/g)had the largest adsorption capacity of iodine vapor among series of composites with different dosage ratios,followed by the composite material(ELC-P-3/1)obtained by the carbon source to template dosage ratio of 3/1,which had an iodine vapor adsorption capacity of 435.0 mg/g and had the best adsorption effect on iodine n-hexane(327.9 mg/g).Among the series of composites obtained at different carbonization temperatures,the composite material(ELC-P-800)prepared at 800℃ had the best adsorption effect on iodine vapor and iodine n-hexane.The iodine vapor capture capacity of ELC-P-800 was 435.0 mg/g,and the maximum adsorption capacity of iodine n-hexane is 327.9 mg/g,the iodine vapor capture capacity was 435.0 mg/g and the maximum adsorption capacity of iodine n-hexane was 327.9 mg/g.It can be seen that the main structural factors affecting the adsorption performance of iodine molecules in different phase states were different.For iodine vapor,the specific surface area and the proportion of micropores had a significant impact,while for iodine n-hexane,it may be related to the relative ratio of micropores to mesopores,and the nitrogen and oxygen content,which provides an important reference for the subsequent design and preparation of iodine element adsorbents for specific phase.To further improve the iodine adsorption performance of ELC-P,a series of lignin carbon/palygorskite composites(ELC-P-Y)doped with different metal elements(Y=Bi,Zn,Fe)were designed and prepared for the capture of radioactive iodine.The results showed that the iodine vapor adsorption effect of ELC-P-Bi,ELC-P-Zn and ELC-P-Fe was enhanced,and the adsorption capacity increased by 20.4%,49.4%and 37.9%respectively,compared with ELC-P,reaching 523.8 mg/g,650.0 mg/g and 600.0 mg/g.The adsorption of iodine in n-hexane by ELC-P-Bi,ELC-P-Zn and ELC-P-Fe showed that the adsorption capacity of ELC-P-Fe reached 362.2 mg/g,and the adsorption equilibrium reached about 180 min.The adsorption process was better matched with the quasi-second-order kinetics and Freundlich model,indicating that it was a multi-molecular layer adsorption and the adsorption process was mainly controlled by chemical adsorption.The adsorption mechanism of ELC-P-Fe for iodine in n-hexane was analyzed by X-ray photoelectron spectroscopy(XPS),FT-IR and Raman.The results showed that ELC-P-Fe mainly converted iodine molecules into charge transfer complexes through Lewis acid-base interactions and charge transfer. |
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