Mn^(2+)催化亚硝酸氧化破坏H_(2)C_(2)O_(4)机理研究

Reaction Mechanism of H_(2)C_(2)O_(4) Oxidized by HNO_(2) via Mn^(2+) as Catalyst

为明确硝酸溶液中以Mn^(2+)作催化剂时,亚硝酸氧化破坏H_(2)C_(2)O_(4)的具体化学行为和反应机理,本文考察了在硝酸和硫酸体系中以Mn^(2+)作催化剂时亚硝酸氧化H_(2)C_(2)O_(4)的差异、Mn^(2+)与草酸络合对亚硝酸氧化Mn(Ⅱ)到Mn(Ⅲ)的作用以及Mn(Ⅲ)破坏H_(2)C_(2)O_(4)过程中产生的自由基,获得了具体的催化反应历程,推测了反应机理。结果表明,亚硝酸在催化反应过程中起主导作用,加入亚硝酸可有效消除反应初期存在的诱导期;反应过程中,溶液中游离的Mn^(2+)与H_(2)C_(2)O_(4)络合生成了MnC_(2)O_(4),而作为配体的草酸降低了Mn(Ⅱ)被氧化到Mn(Ⅲ)的反应活化能,使得亚硝酸能氧化MnC_(2)O_(4)并生成[Mn(C_(2)O_(4))_(3)]^(3-),Mn(Ⅲ)会将所络合的草酸氧化生成·OOC—COOH并被还原成Mn(Ⅱ),·OOC—COOH在酸性溶液中稳定性差,会迅速分解并释放出还原性物质,最终实现了H_(2)C_(2)O_(4)的氧化分解。

H_(2)C_(2)O_(4)is widely used in spent fuel reprocessing.For example,in order to recovery plutonium oxalate precipitated mother liquor before returning,it is necessary to reduce the H_(2)C_(2)O_(4)concentration from 0.1 mol/L to about 10^(-4)mol/L.Among the methods of H_(2)C_(2)O_(4)destroyed in waste liquid,the process of H_(2)C_(2)O_(4)oxidized in nitric acid solution with Mn^(2+)as a catalyst is the most mature and suitable for industrialization,and its research is very rich.Nitrite plays an important role in destroying oxalic process in the process of H_(2)C_(2)O_(4)destroyed with Mn^(2+)as a catalyst in nitric acid solution was mentioned in many past studies.However,the specific reaction chain is still ambiguous and controversial on nitrite oxidation of H_(2)C_(2)O_(4)with Mn^(2+)as a catalyst,and the catalytic reaction mechanism is not clear.To solve these issues in the area of post-processing,the optimization of the treatment of the mother liquor of plutonium oxalate precipitation in the Purex process and the acquisition of H_(2)C_(2)O_(4)deep destruction technology in spent fuel reprocessing are to be benefited.Firstly,sodium nitrite solutions were continuously added into reaction solutions,at the same time the concentration of H_(2)C_(2)O_(4)in solutions was determined.H_(2)C_(2)O_(4)is obviously oxidized in the presence of nitrite and Mn^(2+)in sulfuric acid solutions.Secondly,Mn(Ⅱ)complexed oxalate oxidized to Mn(Ⅲ)with nitrous acid as a oxidant was studied.The results demonstrate that the complexation of Mn^(2+)with H_(2)C_(2)O_(4)reduces the reaction activation energy of Mn(Ⅱ)oxidized to Mn(Ⅲ),and MnC_(2)O_(4)can be oxidized to[Mn(C_(2)O_(4))_(3)]^(3-)by nitrous acid.Finally,free radicals and substates in reaction process were detected by EPR’s spin capture technique.H_(2)C_(2)O_(4)is oxidized to·OOC—COOH with the Mn(Ⅲ)complexed with itself,and Mn(Ⅲ)is reduced to Mn(Ⅱ).The·OOC—COOH radicals with poor stability rapidly decompose and release reducing substances in acidic solution,and decomposition of H_(2)C_(2)O_(4)is completed.With the increase of acidity of nitrous acid,the concentration of HNO3molecule increases.Thus,the concentration of HNO3molecule in the solution can no longer be neglected compared with the concentration of nitrous acid.Therefore,when the concentration of nitric acid is more than 6 mol/L,the decomposition rate of H_(2)C_(2)O_(4)in the nitric acid solution with Mn^(2+)as a catalyst may be controlled by various reactions,which needs to be further studied.