全氟羧酸在185nm真空紫外光下的降解研究

Photodegradation of Perfluorinated Carboxylic Acids by 185 nm VUV Light

以全氟辛酸为代表的全氟羧酸是一类新的持久性有机污染物，广泛地存在于各种环境介质．研究了全氟辛酸、全氟庚酸、全氟己酸、全氟戊酸和全氟丁酸等5种全氟羧酸在185nm真空紫外光下的光降解行为，以发展1种有效降解全氟羧酸的方法．结果表明，全氟羧酸在185nm紫外光照下发生显著地降解并生成氟离子，而在254nm紫外光照下降解不明显．反应6h后，全氟丁酸降解率达到60％以上；而其它4种全氟羧酸的降解率达到90％以上，脱氟率在21％～71％之间，表现出随碳链增长而降低的趋势．氮气、空气、氧气等3种反应气氛对全氟羧酸在185nm紫外光下的降解与脱氟没有显著影响．LC／MS分析表明，全氟辛酸光降解时逐级生成短链的全氟庚酸、全氟己酸、全氟戊酸和全氟丁酸．全氟羧酸在185nm光照下首先发生脱羧反应，脱羧后的自由基与水反应生成少1个碳原子的全氟羧酸和氟离子．

Perfluorinated carboxylic acids （PFCAs） especially perfluorooctanoic acid （PFOA） are a new type of persistent and bioaccumulative organic pollutants, and they have been widely detected in various environmental media. Photodegradation of five types of PFCAs i.e. PFOA, perfluoroheptanoic acid （PFHpA）, perfluorohexanoic acid （ PFHxA）, perfluoropentanoic acid （PFPeA） and perfluorobutanoic acid （PFBA） by 185 nm vacuum ultraviolet （VUV） were examined to develop an effective method to deal with PFCAs pollution. All five PFCAs degrade significantly under the irradiation of 185 nm light and fluoride ion correspondingly forms, though PFCAs hardly degrade under irradiation of 254 nm UV light. After 6h irradiation by 185 nm light, more than 60% of PFBA degrades, while other four PFCAs degrade more than 90%, and defluorination ratio range from 71% to 21% decreasing with increasing carbon atoms contained by PFCAs. Three reaction atmosphere i. e. nitrogen, air and oxygen have no significant effect on degradation and defluorination of PFCAs. LC/MS measurement shows, PFOA degrades to PFHpA, PFHxA, PFPeA and PFBA step by step. Under irradiation of 185 nm light, PFCA firstly is decarboxylated, and the radical thus formed reacts with water to form shorter-chain PFCA and fluoride ion.