Department of Civil Engineering, University of North Dakota, 243 Centennial Drive Stop 8115, Grand Forks, North Dakota 58202, United States.
Department of Civil Engineering, University of North Dakota, 243 Centennial Drive Stop 8115, Grand Forks, North Dakota 58202, United States; Department of Chemistry, University of North Dakota, 151 Cornell Street Stop 9024, Grand Forks, North Dakota 58202, United States.
Water Res. 2021 Jul 15;200:117271. doi: 10.1016/j.watres.2021.117271. Epub 2021 May 19.
Thermal treatment is routinely used to reactivate the spent granular activated carbon (GAC) from water purification facilities. It is also an integral part of sewage sludge treatment and municipal solid waste management. This study presents a detailed investigation of the fate of per- and polyfluoroalkyl substances (PFAS) and one PFAS alternative (GenX) in thermal processes, focusing on the effect of GAC. We demonstrate that the thermolysis of perfluoroalkyl carboxylic acids (PFCAs), including perfluorooctanoic acid (PFOA), and GenX can occur at temperatures of 150‒200 °C. Three temperature zones were discovered for PFOA, including a stable and nonvolatile zone (≤90 °C), a phase-transfer and thermal decomposition zone (90‒400 °C), and a fast decomposition zone (≥400 °C). The thermal decomposition began with the homolysis of a C‒C bond next to the carboxyl group of PFCAs, which formed unstable perfluoroalkyl radicals. Dual decomposition pathways seem to exist. The addition of a highly porous adsorbent, such as GAC or a copolymer resin, compressed the intermediate sublimation zone of PFCAs, changed their thermal decomposition pathways, and increased the decomposition rate constant by up to 150-fold at 250 °C. The results indicate that the observed thermal decomposition acceleration was linked to the adsorption of gas-phase PFCA molecules on GAC. The presence of non-activated charcoals/biochars with a low affinity for PFOA did not accelerate its thermal decomposition, suggesting that the π electron-rich, polyaromatic surface of charcoal/GAC played an insignificant role compared to the adsorbent's porosity. Overall, the results indicate that (1) substantial decomposition of PFCAs and GenX during conventional thermal GAC/sludge/waste treatment is very likely, and (2) the presence or addition of GAC or other highly porous materials can accelerate thermal PFAS decomposition and alter decomposition pathways.
热处置通常用于使水净化设施中的废颗粒状活性炭（GAC）再活化。它也是污水处理和城市固体废物管理的重要组成部分。本研究详细研究了全氟和多氟烷基物质（PFAS）以及一种 PFAS 替代品（GenX）在热过程中的归宿，重点关注 GAC 的影响。我们证明，全氟烷基羧酸（PFCAs），包括全氟辛酸（PFOA）和 GenX 的热解可以在 150-200°C 的温度下发生。发现 PFOA 有三个温度区，包括稳定且不可挥发区（≤90°C）、相转移和热分解区（90-400°C）以及快速分解区（≥400°C）。热分解始于 PFCAs 羧基旁的 C-C 键的均裂，形成不稳定的全氟烷基自由基。似乎存在两种分解途径。添加高度多孔的吸附剂，如 GAC 或共聚树脂，会压缩 PFCAs 的中间升华区，改变它们的热分解途径，并使 250°C 时的分解速率常数增加高达 150 倍。结果表明，观察到的热分解加速与气相 PFCA 分子在 GAC 上的吸附有关。具有低亲和力的非活化炭/生物炭的存在并没有加速其热分解，这表明与吸附剂的多孔性相比，炭/ GAC 的富π电子、多环芳烃表面的作用并不重要。总体而言，结果表明：（1）在常规热 GAC/污泥/废物处理过程中，PFCAs 和 GenX 会发生大量分解；（2）GAC 或其他高多孔材料的存在或添加可以加速热 PFAS 分解并改变分解途径。
