Graduate Institute of Environmental Engineering, National Taiwan University, Taipei 106, Taiwan, ROC.
Department of Environmental Science and Engineering, Tunghai University, Taichung 407, Taiwan, ROC.
Water Res. 2021 Dec 1;207:117805. doi: 10.1016/j.watres.2021.117805. Epub 2021 Oct 26.
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 (GOTiO) photoelectrode for perfluorooctane sulfonate (PFOS) degradation in a photoelectrochemical (PEC) system. The results reveal that a 5 wt.% GOTiO anode possesses the optimal PEC performance, with a band gap (E) of 2.42 eV, specific surface area (S) of 72.6 m g and specific capacitance (Cs) of 4.63 mF cm. 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, under the optimized conditions of current density = 20 mA cm, electrode distance = 5 mm, solution pH = 5.64, [PFOS]= 0.5 µM and NaClO 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 (GOTiO) photoelectrode fabricated in this work, PFOS can be successfully decomposed during the PEC process for the first time.
全氟烷基酸（PFAAs）的全球分布和环境持久性被认为是一个关键的环境问题。在这项工作中，我们成功地制备了一种氧化石墨烯-二氧化钛（GOTiO）光电化学（PEC）系统中用于全氟辛烷磺酸（PFOS）降解的电极。结果表明，5wt%GOTiO 阳极具有最佳的 PEC 性能，带隙（E）为 2.42eV，比表面积（S）为 72.6mg，比电容（Cs）为 4.63mFcm。在 PEC 系统中，在电流密度为 20mAcm、电极距离为 5mm、溶液 pH 值为 5.64、[PFOS]=0.5µM 和 NaClO 电解质浓度为 50mM 的优化条件下，PFOS 可以在 4 小时内有效去除，其准一级反应速率常数为 0.0124min。电子转移途径、羟基自由基和超氧自由基都有助于 PFOS 的分解/转化。确定了新的降解途径；在这项工作中报道了总共 25 种 PFOS 副产物；并且首次鉴定了全氟烷磺酸（PFSAs）、全氟代醛（PFALs）和氢氟碳化物（HFCs）。PFOS 的降解涉及脱磺化途径作为第一步，然后是氧化和随后的脱氟、脱羧、脱羰、磺化、脱氟和羟化。这项工作的结果还表明，PFAAs 的反应活性与其碳链长度有关，短链 PFAAs 的降解速率较低。在 PFAA 混合物中，较短链长 PFAAs 的降解速率下降，表明在处理过程中存在更强的竞争抑制，表明更强的环境持久性。创新点：尽管已经做了很多努力来识别全氟辛烷磺酸（PFOS）的降解副产物，但以前的研究只能识别与全氟羧酸（PFCAs）有关的副产物。这是首次阐明新的 PFOS 降解途径；此外，这是首次报道鉴定含有磺酸盐基团（全氟烷磺酸，PFSAs）、醛基团（全氟代醛，PFALs）和氢氟碳化物（HFCs）的副产物。本研究进一步系统地探讨了在混合体系中，全氟烷基酸（PFAA）的降解可能受到的影响：在光电化学（PEC）体系中，短链长 PFAAs 受到更强的竞争抑制。利用本工作制备的氧化石墨烯-二氧化钛（GOTiO）光电化学电极，首次成功地在 PEC 过程中分解 PFOS。
