The Influence Of Oxidants On The Treatment Of Radioactive Organic Liquid Waste By Supercritical Water Oxidation

Supercritical water oxidation（SCWO）has developed rapidly since it was proposed in 1980,and has played an important role in waste treatment,especially in the treatment of organic hazardous waste.In recent years,with the deepening of SCWO process conditions exploration,degradation mechanism research and key techniques breakthroughs,SCWO has been promoted from laboratory research to small-scale and pilot-scale applications.Its research field has also expanded from national defense,pharmaceutical and municipal waste disposal to radioactive waste disposal.This work focuses on the problem of radioactive organic waste liquid treatment in China.Also,in view of the notable problems of the current SCWO process,especially the equipment problems caused by the high operating temperature and the wastewater increasing problem caused by the low proportion of organic matter in the system,etc.,the degradation behavior of SCWO system with Na NO₃,Na₂S₂O₃,KCl O₄,K₂Cr₂O₇ as coupling ionic oxidants was carried out.The degradation results of TBP under SCWO show that the reaction temperature is particularly important for the degradation of organic matter,which would directly affect the reaction rate of the degradation reaction and the density of the reaction medium.Below 500 ℃,the degradation efficiency of TBP under SCWO has a great temperature dependence.The temperature of the system increases from 450 ℃ to 500℃,at the feed concentration of 4%（V/V）,the TOC（total organic carbon）of the effluent decreases rapidly from～9000 ppm to～1000 ppm.Above the reaction temperature of 500 ℃,with the degradation of most of the organic matter in the system and the generation of refractory products,the degradation reaction rate of the organic matter in the system decreases rapidly,the concentration of organic matter was basically maintained at a relatively low level.The amount of H₂O₂ added also affects the degradation ability of the system.When the oxidation stoichiometric ratio is increased to 100%and above,the TOC value of the effluent decreases rapidly from～9000 ppm to～1000 ppm.In the TBP feed range of 1%to 4%（V/V）,the feed concentration has little effect on the removal rate of organic matter.Under the conditions of 500 ℃,45 s,and 100%oxidation stoichiometric ratio,the TOC removal remains above 90%.Otherwise,when the feed concentration of TBP was increased to5%（V/V）,the TOC value in the effluent rapidly increased to～4000 ppm.Therefore,the SCWO system with H₂O₂ as the sole oxidant is more suitable for treating the feed with TBP concentration below 4%（V/V）.In addition,the degradation of TBP under SCWO has obvious time dependence.In the reaction time interval of 15–45 s,the TOC value of the effluent decreases rapidly from>4400 to～1200 ppm,and the TOC removal rate reaches about 95%.After 45 s,the TOC value in the effluent is basically maintained at～1000 ppm.The addition of additional ionic oxidants,that is Na NO₃,Na₂S₂O₃,KCl O₄,K₂Cr₂O₇,can enhance the degradation ability of SCWO system to a certain extent. For example,for the SCWO system with 4%（V/V）organic,the TOC degradation rate of the effluent can be increased by more than 10%at 475 ℃.Moreover,the addition of Na NO₃,KCl O₄,K₂Cr₂O₇ can effectively reduce the TOC content in the system when the organic matter degradation equilibrium is reached.At 550 ℃,the final TOC value of the effluent is reduced from 550 ppm to below 80 ppm,resulting in a higher final TOC removal.Compared with the original SCWO,which can only maintain 4%（V/V）of the processing capacity of TBP,the addition of ionic oxidants can improve the oxidation capacity of the system to varying degrees.Compared with KCl O₄ and Na₂S₂O₃,Na NO₃ and K₂Cr₂O₇ have more obvious improvement effect on SCWO system,that could still maintain the processing capacity of more than 99%of the TOC in the system with the feed concentration of organic matter is increased to 10%（V/V）.During the degradation of SCWO,organic matter,different gaseous products are produced through steam reforming,water gas reaction,methanation and a small amount of hydrogenation reaction.The main gaseous product is CO₂,followed by CO,H₂ and CH₄.Especially above 500 ℃,the gas phase composition is almost CO₂.The effect of ionic oxidants on gas phase products is mainly below 500 ℃,which can promote the transformation of organic matter in the system to gas-phase products and generate more non-CO₂ components.It is worth mentioning that the addition of Na NO₃,Na₂S₂O₃ and K₂Cr₂O₇ has a certain inhibitory effect on the formation of CH₄and H₂.Especially when the organic matter concentration in the system is lower than10%,their addition can effectively reduce the generation of H₂ and CH₄.Under the experimental conditions of this study,the highest proportions that CO,CH₄,H₂ and other organic gases can reach are 38.25%,1.99%,5.26%and 6.54%,respectively.The by-products of TBP in the degradation of SCWO are very complex.It covers alkanes,alkenes,ethers,alcohols,aldehydes,ketones,acids and other types of organic compounds,both low molecular weight organics and large molecular weight organics.The addition of ionic oxidants can reduce the proportion of alcohols and organic acids,especially organic acids,in the intermediate products to varying degrees.Taking the free radical reaction as the main reaction,combined with the chemical reactions such as pyrolysis,hydrolysis and ionic reaction under SCWO as the main reaction process,the relationship between the by-products was reasonably correlated,the SCWO reaction pathway of TBP was preliminarily speculated and analyzed. During the reaction,ionic oxidants play different roles in the SCWO process.Some of them will participate in the degradation reaction and play a role in the oxidation of high-valent elements,and some will participate in the reaction.The nitro-and cyano-compounds in the by-products confirm this inference well.In general,this work proposes and verifies the idea of improving the oxidation system of SCWO by ionic oxidants to improve its processing capacity.This provided new data support for the oxidation enhancement study of SCWO,and it also has reference and guiding value for the development of SCWO.