Degradation Of Spent Ion Exchange Resins Based On Fenton And PMS Advanced Oxidation Processes

Radioactive wastes are inevitably produced during the operation of nuclear power plants and the development of the nuclear industry.Ion exchange resins play a crucial role in nuclear fuel cycle,nuclear power plant operation,spent fuel storage and radioactive waste liquid treatment.The spent ion exchange resins are classified as medium or low-level radioactive waste,which needs a harmless method for treatment and disposal.In this work,Fenton system and permonosulfate（PMS）/transition metal system were used to degrade ion exchange resins,and the influencing factors of these systems were investigated to optimize the degradation conditions.The statistical molecular fragmentation（SMF）model and density functional theory（DFT）were applied to calculate the degradation process of ion exchange resins by advanced oxidation systems.Combined characterization with calculation results,the degradation mechanism of ion exchange resins was proposed.The PMS/heterogeneous Fenton-like system is proposed as an effective method for the treatment of anion exchange resin,and nano-Fe⁰/Co⁰ is an effective catalyst for this system.Under the conditions of reaction temperature 90°C,H₂O₂ dosage 50 ml,PMS dosage 3 g,and catalyst Fe⁰/Co⁰ dosage 0.1 g,solid reduction of anion resin reached 90%.In this system,PMS adjusted the p H of the system and introduced SO₄·^-with high oxidation activity,which is conducive to the degradation of anion exchange resin.The homogeneous Fenton system with H⁺/Fe²⁺as the catalyst is proposed as an effective method for the treatment of the mixed cation exchange resin and anion exchange resin.Under the conditions of initial temperature 96°C,H₂O₂ dosage 200 ml,H⁺/Fe²⁺concentration 1 M/0.1 M,and mixing ratio 1:1,mixed resins were completely dissolved.During the experiment,cation exchange resin and anion exchange resin showed different reactivity.Combined with DFT calculation results,this may be related to the different charge distribution and dissociation barrier energy of two kinds of resins.According to the characterization results,the main intermediate products were inferred,which indicated that the degradation of mixed resins included desulfonation,valence change of nitrogen atom,and the cleavage of long-chain carbon skeleton.Combined SMF model and DFT calculation,a simplified model of cation exchange resin was proposed,and the degradation processes of cation exchange resin initiated by hydroxyl radical（·OH）were clarified.The prediction of active sites,the results of SMF model and transition state calculation were in good agreement,which reflected that during the degradation process of cation resin,S-O bond and C-S bond of sulfonic acid group were more likely to react.The meta-position of the sulfonic acid group on the benzene ring was the most active site,and the benzene ring without the sulfonic acid group was reactive to some certain degree.The positions of C11-C14 and C17-C20 on the carbon skeleton were easier to be broken.It is also found that dihydroxy addition and elimination reaction played a main role in the process of desulfonation,carbon skeleton cleavage and benzene ring separation.The degradation results and mechanisms obtained in this paper not only propose new treatment processes for the reduction of spent ion exchange resins,but also provide a theoretical basis and technical reserve for the treatment and disposal of radioactive waste.