Kinetics And Mechanism Of Photochemical Degradation Of Imidazolium And Pyridinium Ionic Liquids And Acesulfame In Water

Ionic liquids（ILs）is a kind of molten salt in ionic state at room temperature or below 100℃,which is mainly composed of organic cations and inorganic anions.In recent years,imidazolium and pyridinium ionic liquids（Imi ILs and Pyr ILs）have been widely studied and applied because of their excellent properties.As a kind of non nutritive flavoring agent,artificial sweetener acesulfame potassium（AK）is widely used in low calorie food,drinks,toothpaste and drugs because of its good light and thermal stability,wide p H adaptation range（3～7）and appropriate price.A large number of studies have reported the presence of AK at the final outlet of the sewage treatment process.After the above Imi ILs,Pyr ILs and AK are discharged into natural water directly or indirectly,they will undergo adsorption（particulate matter and sludge in water）,biodegradation and photochemical degradation.Some studies have shown that Imi ILs,Pyr ILs and AK have low biodegradability,so the photochemical degradation process has become their main transformation.Therefore,the study of the aqueous photochemical transformation of Imi ILs,Pyr ILs and AK is very important to understand their reactivity and fate in natural water,and can provide guidance for their ecological risk assessment,harmless design and treatment.The photochemical degradation in this paper refers to the indirect photolysis process participated by the transient reactive species(·OH,CO₃^·–,³CDOM^*（triplet chromophoric dissolved organic matter）,¹O₂).These four transient reactive species were selected as they are common reactive species in natural surface water.Firstly,the 2^nd-order reaction rate coefficients of Imi ILs,Pyr ILs and AK with transient reactive species were obtained by competitive kinetics experiments,and their half-life and fate in natural water can be predicted by the Apex photochemical model.Then,products in the degradation process were obtained by liquid chromatography-mass spectrometry（LC-MSMS）and nuclear magnetic resonance hydrogen spectroscopy（¹H-NMR）,and based on the products,the degradation pathways and mechanism were deduced.Finally,the biodegradability and toxicity of Imi ILs,Pyr ILs,AK and their products were pre-evaluated by EPI software suite Ecosar.Here are the results and conclusions:（1）Photochemical degradation of Imi ILsFirstly,the 2^nd-order rate coefficients of six Imi ILs with different paired anions and varied alkyl chain lengths in reaction with·OH,CO₃^·–,³CDOM^*,¹O₂ measured by competitive kinetics method were respectively in ranges of（1.13～5.45）×10⁹,（2.17～11.2）×10⁹,（1.85～36.1）×10⁹ and（1.81～23.3）×10⁶ M^–1 s^–1,while the rate coefficients of the Imi ILs with CO₃^·–measured by laser flash photolysis-transient absorption spectroscopy are（1.21～6.22）×10⁶ M^–1 s^–1.The CO₃^·–rate coefficients measured by the two methods differ by two orders of magnitude,and datum by the laser method are more accurate（which are used for Apex）.With the increasing length of alkyl chain on imidazolium ring,the rate coefficients of·OH,CO₃^·–and ¹O₂ increased,while the effect of anions on the rate coefficients was not obvious.Secondly,Apex simulation results show that the main contributors for degradation of Imi ILs in natural water are ³CDOM^*and·OH,followed by CO₃^·–.The maximum half-life of Imi ILs can reach 6～70 days(k3_CDOM*=k3_AQ2S*),even up to 75～670 days under conditions of k3_CDOM*=0.05 k3_AQ2S*.The half-life in descending order is C4-BF4>C4-Cl>C10-Cl>C4-PF6>C8-Cl>C6-Cl.Thirdly,from the results of LC-MSMS and ¹H-NMR,it was deduced that the reaction mechanism of Imi ILs was hydrogen-atom transfer（HAT）.Taking C4 imidazolium cation as an example,it reacts with·OH to produce five products（m/z 155.12,m/z 153.10,m/z 137.11,m/z139.09,m/z 171.11）by dehydrogenation and oxygenation on the butyl chain.The reaction of C4 imidazolium cation with ³AQ2S^*produced not only the oxygenation product on the butyl chain（m/z 155.12）,but also the dihydroxy addition product of double bond on the ring（m/z171.12）.Finally,the Ecosar prediction results show that although the toxicity of the products is lower than that of the parents,Imi ILs（C4-Cl as the model）and its products still have acute and chronic toxicity to fish,daphnia,green algae and mysids.Moreover,except for product m/z139.09,the other products,Imi ILs itself and the hypothetical dealkylation product have low biodegradability.The above results show that Imi ILs have a certain threat to the water environment,so it needs to be considered carefully before large-scale industrial application.（2）Photochemical degradation of Pyr ILsFirstly,the 2^nd-order rate coefficients of the reaction of four Pyr ILs with different substituents and varied alkyl chain lengths with·OH,CO₃^·–,³CDOM^*,¹O₂ measured by competitive kinetics method are（2.75～16.2）×10⁸,（1.31～3.49）×10⁷,（2.46～8.30）×10⁹and（1.28～2.96）×10⁶ M^–1 s^–1.The rate coefficients of Pyr ILs with CO₃^·–measured by laser method are（0.96～3.25）×10⁷ M^–1 s^–1,which are close to those measured by competitive kinectics method.With the increasing length of alkyl chain,the rate coefficients of·OH and³CDOM^*also show an increasing trend,while the rate coefficients of CO₃^·–and ¹O₂ have no obvious trend.The introduction of methyl at C3 position of pyridinium ring is also conducive to the increase rate coefficients of·OH,CO₃^·–,³CDOM^*,¹O₂.Secondly,the Apex simulation results show that the main contributors for degradation of Pyr ILs in natural water in summer sunny days are ³CDOM^*and CO₃^·–,followed by·OH and¹O2.The maximum half-life of Pyr ILs can reach 18～55 days(k3 _CDOM*=k3_AQ2S*),even up to210～600 when with k3 _CDOM*=0.05 k3_AQ2S*.The half-life in descending order is NB-Pyr⁺>NHe-Pyr⁺>NDo-Pyr⁺>NB3M-Pyr⁺.Thiredly,it can be inferred from the results of LC-MSMS and ¹H-NMR that NB-Pyr⁺and NB3M-Pyr⁺react with·OH through HAT mechanism to generate alkyl radical and peroxy radical intermediates,and then generate dehydrogenation products（P134 and P148respectively）,hydroxylation products（P152 and P166s respectively）and products further oxidized with·OH.Finally,the Ecosar prediction results show that NB-Pyr⁺may have acute and chronic toxicity to fish,daphnia,green algae and mysids,and products P150a and P150b of NB-Pyr⁺have higher toxicity than itself.Compared with NB-Pyr⁺,NB3M-Pyr⁺has higher acute and chronic toxicity to the organisms,and the products P178,P164a and P164b of NB3M-Pyr⁺also have higher toxicity than itself.Therefore,their potential risks to natural water environment should be fully considered before industrial application.（3）Degradation of AK by simulated sunlightThe simulated solar photodegradation results of AK show that acidic conditions are more conducive to the photodegradation of AK than alkaline conditions,and common inorganic cations and anions can also improve the pseudo 1^st-order rate coefficient of AK.Radical scavenging experiments show that the main contributor to the photolysis of AK is its triplet state（³AK^*）,which can also be verified by the good photolysis performance of AK in the absence of radical source.In addition,kinetics experiments show that the 2^nd-order rate coefficients of AK in reaction of with·OH,CO₃^·–,³AQ2S^*and ¹O₂ are 2.97×10⁹ M^-1 s^-1,2.82×10⁸ M^-1 s^-1,1.15×10⁹ M^-1 s^-1 and 2.94×10⁶ M^-1 s^-1 respectively.The Apex simulation results show that the main contributors to the degradation of AK are CO₃^·–,³CDOM^*and·OH.The maximum half-life of AK is 0.012～28.5 days（taking water depth and DOC as variables）.The results of LC-MSMS showed that the water addition product m/z 179.99A（[C₄H₆NO₅S]^–）and isomerization product m/z 161.99A/B（[C₄H₄NO₄S]^–）were produced in the process of simulated sunlight degradation of AK,while the other three products were detected in the reaction between AK and·OH:the isomer of water addition product m/z 179.99A/B（[C₄H₆NO₅S]^–）,the ring opening product m/z 136.99（[C₃H₅O₄S]^–）by losing-N-C=O-group of AK,and the ring opening product m/z 95.98（[H₂NO₃S]^–）by losing-C=O-CH=C（-CH₃）-group of AK.The above results show that the direct photodegradation of AK only occurs hydrolysis and isomerization,but when it reacts with·OH,there will be addition reaction and ring opening and cracking reaction in two different ways.Finnaly,Ecosar prediction results show that the toxicity of products of AK is greater than that of AK itself,and it is not easy to be biodegradable.This study is of great significance to the ecological risk assessment of AK.