The Active Center Construction On Collagen Fiber And Its Capture And Fixation Behavior For Airborne Iodine

A large amount of biomass waste will be generated in the process of tanning,such as peeling,shaving and trimming.The main component is Collagen Fiber（CF）with multi-layer structure,which is rich in nitrogen and oxygen functional groups,and has been widely used in food,medical treatment,cosmetics and industry.As a new type of clean energy,the large-scale development and utilization of nuclear energy is the inevitable choice of the"Peak carbon dioxide emissions"strategy.However,how to dispose of spent fuel safely and economically has become a major issue that must be solved for the sustainable development of nuclear power in China.Radioactive iodine is a key pollutant in nuclear fuel cycle,spent fuel reprocessing and radioactive waste disposal.This dissertation takes skin collagen fiber of biomass leather waste as the research object,using the theories and techniques of protein chemistry,tanning chemistry and phytochemistry,starting from the two issues of high-efficiency capture and stable fixation of airborne iodine,and the specific active center for the action of airborne iodine was constructed.The performance and mechanism of the above CF active center for efficient capture and stable fixation of airborne iodine were systematically studied.The main research results are as follows:（1）In view of the characteristics of tight cross-linking and clustering between polypeptide chains of CF multi-layer structure,resulting in low specific surface area and partial shielding of active functional groups,this chapter firstly used Na OH as the activator to activate the space and structure of CF to prepare adsorption material with high specific surface area and many active sites conducive to iodine capture.The optimized activation process conditions were obtained:the p H of the system was adjusted to 12 with 10 mol·L^-1 Na OH,and activated at 60℃for 6 h.Under the conditions of this activation process,dark yellow loose activated CF（ACF）was obtained,with a specific surface area of 54.80 m^<sup>2·g^-1,which was 195.10%higher than that of 18.57 m^<sup>2·g^-1 without activation,indicating that the activation treatment of CF with alkali can open the multi-layer structure of CF to a certain extent,so that its physical space can be effectively expanded and activated,and it has more pore structure,thereby exposing more functional groups,and realizing the activation of CF space and structure.On this basis,the capture behavior of iodine before and after CF activation was compared.The study showed that the capture capacity of ACF for iodine vapor at 14 h reached 1068.60 mg·g^-1,which was significantly higher than that of CF(428.30 mg·g^-1).It had high adsorption capacity and rapid adsorption kinetics.In iodine-cyclohexane solution,the adsorption process conforms to the pseudo-first-order kinetics and Langmuir isotherm model.Through functional group modification and shielding,combined with FT-IR,EDX,XPS and TGA,it was considered that the capture of airborne iodine vapor by ACF was mainly due to the exposure of-OH,-CONH₂ and-NH₂ in ACF,in which hydroxyl group contributed the most,followed by amine group.Further analysis showed that the active site leads to the polarization of the iodine molecule to form I₃^-,and finally forms a stable coordination complex with the active site and was stably captured.The research results indicated that the capture strategy of airborne iodine vapor after strong alkali activation treatment of leather waste is feasible.（2）According to the research in the previous chapter,it was found that the hydroxyl group is an effective group to capture iodine.Therefore,this chapter will utilize the principles of tanning chemistry to assemble catechin rich in phenolic hydroxyl groups on the surface of ACF,enrich the surface active sites of ACF,construct specific active center for airborne iodine,and improve the iodine capture performance.The results showed that the activated CF increased the amount of assembly and binding with catechin due to the increase of exposed functional groups,and further enriched the content of iodine active recognition sites on the surface of ACF.The experimental results of airborne iodine vapor capture showed that its maximum capture capacity was 2122.68 mg·g^-1,an increase of 98.64%compared with 1068.60mg·g^-1 of ACF,and an increase of 395.61%compared with 428.30 mg·g^-1 of CF.Compared with other biomass adsorbents,it had obvious advantages,indicating that the introduction of phenolic hydroxyl groups can greatly improve the capture performance of airborne iodine.In iodine-cyclohexane solution,the adsorption process conforms to the pseudo-first-order kinetics and Langmuir isotherm model.Through functional group modification and shielding,combined with FT-IR,EDX,XPS and TGA,the aromatic ring,phenolic hydroxyl and imine groups in catechin assembled on the surface of ACF were effective capture sites for iodine,among which the phenolic hydroxyl group contributes the most,followed by amine group.Further analysis suggested that the iodine molecules eventually formed I^-and I₃^-polyiodine complexes were captured.These results indicated that the assembly of stronger phenolic hydroxyl group on the surface of ACF is one of the feasible strategies to enhance the capture of iodine vapor.（3）In the previous two chapters,it was proved that alkali treatment can effectively activate the space and structure of CF,and the strategy of exogenously introducing plant polyphenols rich in iodine-specific recognition sites is feasible.This chapter aims at the problem that the multi-layer structure space and structural activation of ACF is not complete,and the phenolic hydroxyl assembly cannot penetrate into the interior of ACF.Through enzymatic hydrolysis of CF,its three-dimensional structure was further opened to obtain the collagen aerogel（3DCF aerogel）with a three-dimensional network structure with ultra-high porosity,high content of flake structure,and good channel effect.The surface was modified with catechin active sites to prepare a polyphenol-functionalized three-dimensional porous collagen aerogel（Catechin@3DCF aerogel）.The results showed that the catechin assembly amount of the aerogel was 366.34 mg·g^-1,which was 101.01%higher than that of ACF,which further confirmed that the deep activation of CF space and structure was conducive to the assembly of phenolic hydroxyl groups in exogenous active center.The capture capacity of3DCF aerogel for airborne iodine was 885.0 mg·g^-1,an increase of 106.63%compared with428.30 mg·g^-1 of CF,which confirmed that enzymatic directed hydrolysis was beneficial to the formation of iodine active center.The maximum capture capacity of Catechin@3DCF aerogel for airborne iodine vapor was 2226.70 mg·g^-1,which was 5.42%higher than that of Catechin@ACF(2122.68 mg·g^-1),which was mainly due to the increase of catechin assembly.In iodine-cyclohexane solution,the adsorption process conforms to the pseudo-first-order kinetics and Langmuir isotherm model.Catechin@3DCF aerogel was irradiated by ⁶⁰Coγ-ray in the high dose range of 10 to 350 k Gy,the capture performance of airborne iodine was1738.10 mg·g^-1,which decreased by only 21.94%.The main reason was that the free radicals generated by irradiation promoted the internal cross-linking of material,resulting in a reduction in the number of exposed iodine affinity sites.And the material structure did not change significantly,indicating excellent anti irradiation stability.Through functional group modification and shielding,aromatic ring,phenolic hydroxyl group and imine group were effective capture sites of airborne iodine,which could convert iodine molecules into I₃^-polyiodine complex to achieve high efficiency capture.The research results showed that Catechin@3DCF aerogel was an airborne iodine capture material with low cost,ultra-high porosity,high capture capacity,high radiation stability and green environmental protection.（4）In the previous chapter,the as-prepared Catechin@3DCF aerogel material exhibited ultra-high porosity,good channel effect,and high irradiation stability,and has ultra-high capture performance for iodine vapor.However,the captured iodine was easy to resolve at high temperature,and the thermal stability of the material also faced certain problems.In this chapter,bismuth compounds were immobilized on the surface of ACF,and Bi³⁺was converted into Bi⁰,and then hydrothermally carbonized to prepare a porous bismuth-based collagen fiber hydrothermal carbon adsorption material（Bi-BT@ACF HC）.The experimental results showed that the Bi-BT@ACF HC had a bismuth loading of 25.92 mg·g^-1,was rich in porous structure,and had good high temperature resistance and stability（only 26.28 wt.%weight reduction at 500℃）.The maximum iodine vapor capture capacity of the adsorbent was2066.80 mg·g^-1,and the optimal chemical fixed iodine content was 1274.80 mg·g^-1,accounting for 61.68%,while the maximum chemical fixed iodine content of other bismuth-containing adsorbents was only 986.0 mg·g^-1（Bi₂S₃@PAN）.At the same time,Bi-BT@ACF HC-I₂ has no obvious degradation below 310°C,and a small amount of weight loss was mainly caused by the decomposition of Bi I₃,indicating that the material has good thermal stability and can achieve efficient capture of airborne iodine vapor and stable iodine fixation.Combined with the material structure and valence analysis of Bi,the mechanism of Bi-BT@ACF HC capture airborne iodine was mainly the chemical reaction between iodine and Bi⁰ to generate stable Bi I₃ phase and achieve stable chemical adsorption.Secondly,the hydroxyl group in the material also has a certain induction adsorption effect on iodine,which further increases the chemical adsorption of iodine.The above studies indicated that the introduction of bismuth active center by ACF will help to prepare airborne iodine capture material with low cost,strong chemical iodine fixation ability,and strong thermal stability.（5）In the previous chapter,Bi-BT@ACF HC had good high temperature resistance and good chemical iodine fixation ability.However,the pores were easily blocked after bismuth was immobilized,which led to a low specific surface area and low effective utilization rate of bismuth.In this chapter,Bi-BT@ACF was prepared into activated carbon with large specific surface area by oxidation-calcination,which not only improved the high temperature resistance of the material,but also realized the efficient capture and stable iodine fixation of airborne iodine.The experimental results showed that bismuth-based porous collagen fiber activated carbon（Bi-BT@ACF AC）,rich in pore structure and high specific surface area,had good high temperature resistance and stability 270°C（weight only decreased by 8.05 wt.%at270℃）.Bi-BT@ACF AC had a maximum capture capacity of 3302.30 mg·g^-1 for airborne iodine vapor,and the optimal chemical iodine fixation was 1133.70 mg·g^-1.Combined with material structure and valence analysis of bismuth,there were three main types of capture mechanism of airborne iodine by Bi-BT@ACF AC:one was the chemical reaction between iodine and Bi⁰ to form a stable Bi I₃ phase;the other was the adsorption induced by the active site of-OH and amide C=O,which led to the polarization of iodine molecules into I^-or I₃^-and be captured;the third was that the iodine vapor was retained in the pores of the material by physical adsorption.The above studies indicated that using ACF as the carrier and introducing exogenous phenolic hydroxyl group to induce adsorption of iodine molecules can be further prepared bismuth-based porous adsorbent with low cost,strong chemical iodine fixation ability,good iodine capture performance,and thermal stability.The research results of this subject are expected to effectively solve the problem of recycling biomass leather waste.At the same time,CF biomass composite loaded with bismuth-based functional active center is expected to replace the common silver-based materials in the current post-processing process,and provide a reference for the capture and fixation of radioactive airborne iodine.