Photo-induced Degradation Of Perfluorinated Carboxlic Acids And The Alternatives

Perfluorocarboxylic acids（PFCAs）have been widely used in commercial and industrial applications due to their strong surface-active effect,excellent thermal and chemical stability,and high light transparency.One of the key properties of PFCAs is their high stability towards natural environmental degradation.As a consequence,these compounds have been detected in environmental samples and wildlife worldwide.Given the potential toxic effects on humans and ecosystems,it is imperative to develop technologies for the effective removal of these compounds.Most of current treatment methods are not readily applicable for technical processes because of the harsh reaction conditions and the high energy consumption.Moreover,these works have largely been limited on perfluorooctanoic acid（PFOA）,leaving knowledge on other PFCAs species significantly lacking.In this work,we systematically evaluated the photo--induced decomposition of PFCAs and the alternatives hydroperfluorocarboxylic acids（H-PFCAs）and tetrahydroperfluorocarboxylic acids（2H,2H,3H,3H-PFCAs）,and photodegradation of PFOA on different soil solid particles,considering the possible effects of various parameters such as humic substances and solution p H.The reaction pathways were proposed by identifying reaction products and the reaction mechanisms were further elucidated by theoretical calculations.This work could provide new insights into the removal of PFCAs in water.The major contents and results are described as follows:（1）The photodegradation of a series of PFCAs（F（CF₂）_nCOOH,n=1-11）in water by a300 W medium pressure mercury lamp was experimentally and theoretically examined.Photolysis of PFCAs all followed pseudo-first-order kinetics with the rate constant(k_app)increasing with carbon chain lengths,except for trifluoroacetic acid（TFA）which cannot be degraded by the polychromatic irradiation.Product analysis showed that the PFCAs were mainly decomposed into shorter carbon chain length PFCAs in a stepwise manner,with the accumulation of TFA and fluoride ions as the end products.Moreover,a small amount of perfluoroolefins(C_nF_2n)was determined as gas-phase products.Wiberg bond order calculations confirmed the cleavage of the C-C bond between carboxylic carbon and the adjacent carbon as the first reaction step,and density functional theory-based calculations revealed that k_app value was correlated with some molecular structural parameters(D_C-C,E_LUMO-E_HOMO,BO_C-C and L_C-C).In the case of mixture irradiation,the evolution profiles of individual PFCAs were different from that in single-component systems,due to the dynamic balance between production and degradation.（2）The Hg lamp induced photodegradation of H-PFCAs(HC_nF_2nCOOH,n=4,6,10),alternatives to PFCAs,was studied to reveal the decomposition efficiencies and mechanisms.The photodegradation of H-PFCAs all followed pseudo-first-order kinetics,and the reaction rates increased with carbon chain length.The degradation rates of H-PFCAs were almost identical to those of PFCAs.The removal efficiency of H-PFCAs was enhanced by lowering p H and adding Fe³⁺,but inhibited by the presence of humic acid,carbonate and bicarbonate.According to the products identified by LC-MS and GC-MS analysis,decarboxylation,defluorination and elimination reaction were involved in the photodegradation of H-PFCAs.（3）The Hg lamp induced photodegradation of 2H,2H,3H,3H-PFCAs(C_nF_2n+1C₂H₄COOH,n=6,7,8),precursors to PFCAs,was studied to reveal the decomposition efficiencies and mechanisms.The photodecomposition of 2H,2H,3H,3H-PFCAs all followed pseudo-first-order kinetics,and the photolysis rate coefficients increased with the increasing carbon chain length.Under the same reaction condition,2H,2H,3H,3H-PFCAs degraded much faster than the corresponding PFCAs.The photodecomposition rate coefficient of C₈F₁₇CH₂CH₂COOH was accelerated by low p H and Fe³⁺addition,but decreased by the existence of humic acid,carbonate and bicarbonate.Compared with ultrapure water,a decreased removal of2H,2H,3H,3H-PFCAs was observed in four types of natural waters,i.e.,tap water,Jiuxiang river water,primary effluent and secondary effluent.According to mass analysis,C₈F₁₇CH₂CH₂COOH was mainly decomposed into 8:2 fluorotelomer acid(C₈F₁₇CH₂COOH),shorter-chain perfluorocarboxylic acids（PFCAs）,perfluoro-1-enes(C_nF_2n)and perfluoroketenes(C_nF_2n+1CF=C=O).ECOSAR predictions showed that photolysis generally decreased the aquatic toxicity of C₈F₁₇CH₂CH₂COOH.（4）The photodegradation kinetics and mechanisms of PFOA on six different solid particles were investigated.After 24 h of Hg lamp irradiation,significant degradation of PFOA was observed on paddy soil（PS）,black soil（BS）,yellow soil（YS）,red soil（RS）,Jiuxiang river soil（JXR）and 400 mesh quartz sand（QS）.The photodegradation of PFOA on solid matrixes all followed pseudo-first-order kinetics,and the order of PFOA loss was QS>RS>JXR>YS>BS>PS.The intrinsic properties of solid particles,including light-shielding effect,organic carbon contents,particle size distribution and fractal dimension,and silica contents all have some correlations with PFOA photodegradation.In addition,trifluoroacetic acid（TFA）on solid particles can be effectively photodegraded,while it was hardly degraded in aqueous solution.Mass spectrometry results showed that decarboxylation and elimination were the two main reactions for PFOA photodegradation,leading to the formation of various products.