PFOA Degradation Under UV And Solar Light

Perfluorooctanoic acid (PFOA) has been detected in various environmental mediaand even in human beings due to its extensive use in manufacturing industrial medicaland commercial products. It is the subject of increasing regulatory interest because of itspotential to bioaccumulate growing evidence of its toxicity to animals andenvironmental persistence. US EPA consider PFOA to be a “likely carcinogen” based onavailable toxicity studies. Unlike most other persistent and bioaccumulative organicpollutants PFOA is water-soluble and do not bind well to soil or sediments thusit migrates easily and causes serious contamination to underground and surface watersources.To minimize the potential harm to human and environmental health there is acritical need to understand the fate and transport of PFOA and other perfluorinatedcompounds (PFCs) in the environment. Due to the presence of multiple highly stabileC-F bonds (116kcal/mol) PFOA has been long believed to be extremely recalcitrantand does not degrade under natural conditions. Consequently developing effectivetreatment technologies has proven difficult.Conventional abiotic treatment methods (e.g. advanced oxidation using H2O2andpersulfate) have demonstrated only marginal success in degrading PFOA. Importantlydegradation in these studies occurs only in relatively harsh conditions. Recently in anattempt to elucidate PFOA photodegradation mechanisms it is reported that PFOA and some shorter chain perfluorocarboxylic acids (PFCAs) can be degraded by UV light inthe presence of Fe(Ⅲ). In these previous studies PFOA degradation pathway waspresumed to be initiated by light induced organic radicals (from the Fe(Ⅲ)-PFOAadduct) but the role of the inorganic radicals (e.g. OH) which can be generated inthe Fe(Ⅲ) mediated degradation system was not investigated.A simple alternative photocatalytic approach involves the use of Fe(Ⅲ) incombination with natural sunlight as a source of UV irradiation. The reaction inputs arenaturally abundant in the environment (e.g. groundwater discharge into a surface waterbody) such that attenuation would be sustainable assuming proper conditions existed.However in this research we demonstrate that PFOA decomposes in thepresence of sunlight and ferric iron (Fe(Ⅲ)). Under such conditions97.8%of PFOAdecomposed within28days (4.4days half-life) with a defluorination ratio (fluoridereleased relative to fluoride originally present in PFOA) of13.6%. The addition ofcommon oxidants H2O2or persulfate significantly decreased (p <0.05) the PFOAdecomposition rate (74.8%for H2O2and88.9%for persulfate). This was presumably dueto the disruption of the Fe(Ⅲ)-PFOA adduct a precursor of C7F15COO radical that iscritically important for the Fe(Ⅲ)-mediated PFOA degradation by H2O2and persulfate.The addition of persulfate did increase the defluorination ratio from13.6%to50.3%presumably through the degradation of (and release of fluoride from) shorter-chainperfluorinated compounds indicating a potential enhancement effect of persulfate forthe Fe(Ⅲ)-sunlight system. Electron paramagnetic resonance (EPR) was used to characterize the radicals in the various reaction systems and OH was identified as keycomponent in promoting PFOA degradation in the Fe(Ⅲ)-sunlight system. The results ofthis result provide evidence for a set of simultaneous reaction pathways that differ fromprevious studies:1) Fe(Ⅲ) combines with PFOA to form Fe(Ⅲ)-PFOA adducts whichwhen irradiated form a C7F15COO radical that is critically important for subsequentdefluorination processes; and ii) Fe(Ⅲ) reacts with water to form OH which oxidizesPFOA to form the same C7F15COO radical. The finding that PFOA can be degraded byFe(Ⅲ) under natural sunlight provides mechanistic insight on PFOA natural attenuationusing a novel natural attenuation process. Because Fe(Ⅲ) is abundant in naturalenvironments and sunlight is essentially free this work also represents a potentiallyimportant step in developing simple and inexpensive remediation and managementstrategies for PFOA-contaminated water.