New insight into PFOS transformation pathways and the associated competitive inhibition with other perfluoroalkyl acids via photoelectrochemical processes using GOTiO2 film photoelectrodes
Journal
Water Research
Journal Volume
207
Date Issued
2021
Author(s)
Abstract
The global distribution and environmental persistence of perfluoroalkyl acids (PFAAs) has been considered a critical environmental concern. In this work, we successfully fabricated a graphene oxide-titanium dioxide (GOTiO2) photoelectrode for perfluorooctane sulfonate (PFOS) degradation in a photoelectrochemical (PEC) system. The results reveal that a 5 wt.% GOTiO2 anode possesses the optimal PEC performance, with a band gap (Eg) of 2.42 eV, specific surface area (SBET) of 72.6 m2 g?1 and specific capacitance (Cs) of 4.63 mF cm?2. In the PEC system, PFOS can be efficiently removed within 4 h of reaction time, with a pseudo-first-order rate constant of 0.0124 min?1, under the optimized conditions of current density = 20 mA cm?2, electrode distance = 5 mm, solution pH = 5.64, [PFOS]0= 0.5 ?M and NaClO4 electrolyte concentration = 50 mM. The electron transfer pathway, hydroxyl radicals and superoxide radicals are all responsible for PFOS decomposition/transformation. New degradation pathways were identified; a total of 25 PFOS byproducts are reported in this work; and perfluoroalkane sulfonates (PFSAs), perfluorinated aldehydes (PFALs) and hydrofluorocarbons (HFCs) were identified for the first time. PFOS degradation involves the desulfonation pathway as the first step, followed by oxidation and subsequent defluorination, decarboxylation, decarbonylation, sulfonation, defluorination and hydroxylation. The results from this work also show that the reactivity of PFAAs is related to their carbon chain length, with shorter-chain PFAAs exhibiting a lower degradation rate. In a PFAA mixture, a decline in the degradation rate was observed for the shorter-chain-length PFAAs, suggesting stronger competitive inhibition and indicating stronger environmental recalcitrance during the treatment process. Novelty statement: Although many efforts have been made to identify perfluorooctane sulfonate (PFOS) degradation byproducts, previous studies were only able to identify byproducts that are related to perfluorinated carboxylic acids (PFCAs). This is the first study to elucidate the new PFOS degradation pathway; furthermore, this is the first report to identify byproducts containing sulfonate groups (perfluoroalkane sulfonates, PFSAs), aldehyde groups (perfluorinated aldehydes, PFALs), and hydrofluorocarbons (HFCs). This study further systematically explores how perfluoroalkyl acid (PFAA) degradation may be affected in the mixture system: shorter-chain-length PFAAs suffer stronger competitive inhibition in the photoelectrochemical (PEC) system. By utilizing the graphene oxide-titanium dioxide (GOTiO2) photoelectrode fabricated in this work, PFOS can be successfully decomposed during the PEC process for the first time. ? 2021
Subjects
Competitive inhibition
PFOS
Photoelectrochemical
Reactive oxygen species, Byproducts and reaction pathways
Carboxylation
Chain length
Chlorine compounds
Electrodes
Electrolytes
Energy gap
Free radical reactions
Graphene
Oxygen
Rate constants
Sodium compounds
Titanium dioxide
Oxygen reaction
Perfluoroalkyl acids
Perfluorooctane sulfonates
Photoelectrochemical system
Photoelectrochemicals
Photoelectrode
Reaction pathways
Reactive oxygen species
Reactive oxygen species, byproduct and reaction pathway
Degradation
aldehyde
carbon
electrolyte
graphene oxide
hydroxyl radical
perfluorooctanesulfonic acid
reactive oxygen metabolite
sodium perchlorate
sulfurous acid
superoxide
alkanesulfonic acid
carboxylic acid
fluorocarbon
biotransformation
chemical composition
chemical compound
detection method
electrochemical method
inhibition
oxidation
surface area
transformation
Article
comparative study
competitive inhibition
controlled study
current density
decarboxylation
decomposition
defluorination
degradation
electron transport
first order rate constant
fluorination
hydroxylation
reaction analysis
reaction time
sulfonation
environmental monitoring
Missouri
Novelty
United States
Alkanesulfonic Acids
Carboxylic Acids
Environmental Monitoring
Fluorocarbons
Type
journal article