紫外光致氯生水合电子对全氟辛酸的降解

doi:10.7503/cjcu20160156

摘要/Abstract

摘要：

研究了185 nm紫外光激发氯离子生成的水合电子还原降解全氟辛酸(PFOA)的效果. 结果表明, 该体系中氯离子、紫外光和绝氧环境是保证PFOA高效降解的必要条件; 当PFOA的浓度为0.03 mmol/L时, 最佳反应条件为氯离子与PFOA摩尔浓度比( $c C l -$ /c_PFOA)为10.0, 溶液初始pH值为10.0, 体系温度为25 ℃. 该条件下反应23 h后PFOA的降解率和脱氟率分别达到99.6%和65.0%. PFOA在该体系中0~8 h的降解符合一级反应动力学, 反应速率常数为6.3×10^-3 min^-1. PFOA在该体系中降解的主要产物有氟离子、短链全氟羧酸、甲酸和乙酸. PFOA的降解有2种途径: (1) 氯离子在紫外光照射下产生水合电子, 水合电子进攻PFOA, 造成C—F键及C—C键的断裂; (2) 具有较高能量的185 nm紫外光光解PFOA, 发生脱羧反应, 并通过水解作用逐步脱氟.

关键词: 全氟辛酸, 水合电子, 氯离子, 185 nm紫外光

Abstract:

Based on the characteristic of perfluorooctanoic acid(PFOA) that it is vulnerable to be nucleophile attacked, we developed a new method for the degradation of PFOA in aqueous phase with chloride as a mediator. In this study, 185 nm ultraviolet photolysis of chloride leads to the generation of hydrated electrons, which contribute to the defluorination of PFOA. Chloride, ultraviolet, and anaerobic environment are all the necessary factors to ensure the effective degradation of PFOA. In this system, when the concentration of PFOA is 0.03 mmol/L, the optimal reaction conditions are $c C l -$ /c_PFOA=10.0, pH=10.0, with temperature being 25 ℃. Under these conditions, the degradation and defluorination rates of PFOA after 23 h’s reaction are 99.6% and 65.0%, respectively. Kinetic analysis indicated that the decomposition of PFOA fits the first order model with a rate constant of 6.3×10^-3 min^-1. The degradation products are fluorinion, perfluorinated carboxylic acid with short-carbon-chains, formic acid, and acetic acid. According to the degradation products, we proposed two major degradation pathways of PFOA: direct cleavage of C—F bonds and C—C bonds due to the attack by hydrated electrons; and decarboxylating by ultraviolet irradiation and defluorinate by hydrolysis. This method is of great significance to eliminate the PFOA in wastewater.

Key words: Perfluorooctanoic acid, Hydrated electron, Chloride, 185 nm Ultraviolet

中图分类号:

O644

TrendMD:

郭睿, 张超杰, 张庚, 周琪. 紫外光致氯生水合电子对全氟辛酸的降解. 高等学校化学学报, 2016, 37(8): 1499.

GUO Rui,ZHANG Chaojie,ZHANG Geng,ZHOU Qi. Degradation of Perfluorooctanoic Acid by UV/Chloride Process^†. Chem. J. Chinese Universities, 2016, 37(8): 1499.

图/表 9

Fig.1 Equipment diagram 1. UV light; 2. quartz tube; 3. aerator pipe; 4. seal cover; 5. circulating water inlet valve; 6. circulating water outlet valve; 7. sampling place; 8. temperature probe; 9. magnetic stirring apparatus.

Table 1 Reaction conditions of three systems*

System	c_NaCl/(mmol·L^-1)	Time(He)/min	pH	UV185
PFOA+He+NaCl+UV185	0.3	30	10.0	Irradiate
PFOA+He+UV185	0	30	10.0	Irradiate
PFOA+He+NaCl	0.3	30	10.0	None

Fig.2 Degradation(A) and defluorination(B) rates of PFOA under different conditions a. PFOA+He+NaCl+UV185; b. PFOA+He+UV185; c. PFOA+He+NaCl.

Fig.3 Degradation kinetics of PFOA

Table 2 Degradations of PFOA in different reaction systems

System	Reaction kinetic	10³k_obs/min^-1	Half-life/h	Reference
PFOA+NaCl+He+UV185	First-order	6.3	1.77	This study
PFOA+KI+N₂+UV254	First-order	7.0	1.58	[30]
PFOA+K₂S₂O₈+O₂+UV185	First-order	1.4	8.19	[31]
PFOA+N₂+UV185	First-order	1.9		[27]

Fig.4 Degradation(A) and defluorination(B) rates of PFOA with different cCl -/cPFOA cCl -/cPFOA: a. 0; b. 1.0; c. 10.0; d. 122.0.

Fig.5 Degradation(A) and defluorination(B) rates of PFOA with different initial pH values pH: a. 5; b. 6; c. 7; d. 8; e. 9; f. 10.

Fig.6 Concentration of PFOA and intermediate products during the degradation

Fig.7 C-balance(A) and F-balance(B) after different reaction time

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