Degradation of PFAS during the reactivation process of activated carbon
Jul 31, 2025
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The reason for reactivating activated carbon
The use of activated carbon to remove PFAS from water requires more frequent reactivation of the activated carbon. Under an oxygen-free atmosphere, at temperatures of 500°C and above, typical substances such as PFOA and PFOS are completely removed from the activated carbon and may decompose. This helps address the "forever chemicals" issue. However, some degradation products are formed at lower temperatures, so post-treatment of the gas flow during the heating process is crucial. More frequent reactivation requires significant energy consumption, as well as additional transportation and raw material usage, thereby increasing the carbon footprint of industries such as drinking water production.
Most water purification processes, such as aeration, coagulation/flocculation/precipitation, rapid filtration, softening, (advanced) oxidation, and slow sand filtration, are ineffective in removing per- and polyfluoroalkyl substances (PFAS). In contrast, adsorption using granular activated carbon (GAC) or powdered activated carbon (PAC) exhibits high efficiency in PFAS removal and is currently the most widely used PFAS removal technology next to membrane filtration. Both methods can effectively adsorb PFAS from water but cannot decompose them. Therefore, measures must be taken to prevent the adsorbed PFAS from re-entering the environment as much as possible.
The removal efficiency of PFAS is affected by their chemical structure and the load of organic compounds on the activated carbon. Long-chain PFAS (with a carbon chain length of ≥6 carbon atoms) have stronger adsorption properties compared to short-chain PFAS, so short-chain PFAS usually decompose faster. Common types of PFAS include sulfonic acids (such as perfluorooctanesulfonic acid, PFOS) and carboxylic acids (such as perfluorooctanoic acid, PFOA). Sulfonic acids have better adsorption performance than carboxylic acids, while branched-chain PFAS generally have inferior adsorption effects compared to non-branched PFAS.
When an activated carbon filter breaks through (i.e., its adsorption capacity is saturated), GAC needs to undergo reactivation treatment. For this purpose, the saturated GAC is transported to the reactivation facilities of activated carbon suppliers for processing. A multi-hearth furnace (a type of vertical calcining kiln) is usually used, where GAC adsorbed with pollutants is fed from the top. The treatment process includes the following steps: first, drying the GAC at 105°C, then pyrolyzing it at 650-850°C to remove impurities, and finally reactivating it in an oxygen-free atmosphere at 800-900°C to restore the adsorption activity of the activated carbon surface [1]. The gas generated during the reactivation process is collected through a trapping system and heated and oxidized multiple times in an external forced burner at a high temperature above 1000°C. The gas is then purified in a gas scrubber, and solid particles are trapped. The treated clean activated carbon is discharged from the bottom of the kiln and can be reused in applications such as drinking water production.
During the reactivation process, PFAS undergo various changes, including removal, destruction, transformation, and mineralization. When PFAS are not detected on GAC, it indicates that PFAS have been "removed". However, this does not mean that the final fate of PFAS is clear - they may be transformed through evaporation, degradation, or mineralization. During the destruction of PFAS, the parent molecules degrade and disappear. If degradation is incomplete, transformation products may be generated, and even other types of PFAS may form, which may also be environmentally toxic. Therefore, it is crucial to accurately assess the degree of degradation and whether transformation products are generated. The presence of transformation products can be verified through suspect analysis of known or expected products, or through non-target analysis. If PFAS are completely degraded, i.e., mineralization is achieved, all C-F bonds are broken, generating fluoride. Fluoride then reacts with water or calcium to form hydrogen fluoride (HF) or calcium fluoride (CaF₂), thereby minimizing its environmental hazards. The remaining parts of the PFAS molecules are converted into carbon dioxide (CO₂) or water (H₂O).
Although the mineralization process increases the consumption of raw materials and energy, it helps address the pollution problem of "forever chemicals" in the environment by completely decomposing PFAS into non-toxic elements (such as fluoride, CO₂, and H₂O) [1]. By measuring the fluoride ion concentration, an attempt can be made to establish a mass balance to verify whether PFAS are completely mineralized. Theoretically, completely mineralized PFAS should be 100% converted into detectable fluoride ions. However, in practice, verification of this process usually faces challenges, partly because the analytical detection limit of fluoride is relatively high (usually about 20µg/L), while the concentration of PFAS in water is often at the nanogram per liter (ng/L) level, making analysis difficult.
It is known that granular activated carbon (GAC) has a catalytic effect on the mineralization of per- and polyfluoroalkyl substances (PFAS) under heating conditions. Watanabe et al. (2018) [2] conducted studies in an oxygen-free nitrogen (N₂) atmosphere at 700°C and found that the fluoride generation rates during the mineralization of perfluorooctanoic acid (PFOA), perfluorohexanoic acid (PFHxA), and perfluorooctanesulfonic acid (PFOS) were 30%, 46%, and 72% respectively. When PFAS were adsorbed onto GAC, the fluoride generation rates significantly increased to 51%, 74%, and 70% respectively. Furthermore, after adding sodium hydroxide to the reactivation column, the fluoride generation rates were further improved to 74%, 91%, and 90%, while the residual amount of the original PFAS components dropped to less than 1%. These results indicate that GAC plays an important role in the mineralization process of PFAS. However, for efficient application, it is crucial to deeply understand the physicochemical processes occurring on the surface of GAC. GAC can effectively "retain" PFAS until the reactivation column reaches a temperature suitable for mineralization. In addition to thermochemical reactions, the physicochemical properties of GAC (such as surface pore structure and chemical activity) are likely to have a significant impact on the mineralization efficiency of PFAS [1].
If only the removal or destruction of PFAS occurs without complete mineralization, PFAS or their degradation products may only be transferred rather than completely eliminated. Without effective control measures, these PFAS or their degradation products may re-enter the environment and water systems, leading to re-contamination of surface water, and ultimately requiring secondary removal using GAC. In large-scale treatment, additional combustion steps and gas scrubbers are usually introduced to handle the gas flow released during the reactivation process, so as to prevent the emission of degradation products. However, the problem lies in the extreme chemical stability of PFAS. It is known that PFAS can only be completely "burned" and decomposed at high temperatures above 1200°C. Therefore, the actual effect of conventional combustion steps still needs further verification.
Reactivation experiments
KWR, in collaboration with the University of Bath in the UK, has developed an experimental device for reactivating activated carbon under controlled conditions and systematically studying the transformation behavior of PFAS. This device can accurately simulate the reactivation process, providing a reliable platform for evaluating the degradation efficiency of PFAS and optimizing process parameters.
First, a high-concentration PFAS "stock solution" was prepared, which is a solution containing various per- and polyfluoroalkyl substances (PFAS) at a relatively high concentration. In the experiment, this stock solution was diluted to the required concentration for subsequent tests.
A new granular activated carbon (model TL-830, Chemviron) was selected for the experiment. Two batches of activated carbon were loaded with perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) respectively. Subsequently, the concentration of PFAS in the solution after loading was analyzed by liquid chromatography-mass spectrometry (LC-MS) at the University of Bath, and the PFAS loading on the activated carbon was calculated.
The results showed that the loading of PFOS was 81 µg/g of activated carbon, and the loading of PFOA was 75 µg/g of activated carbon. In the experiment, the activated carbon was divided into groups of 10 grams each, and heat-treated in a tube furnace at 300°C, 500°C, 700°C, and 900°C respectively. At the beginning of the treatment, the activated carbon was first heated at 105°C for 30 minutes to evaporate the moisture contained in it, so as to prevent the microporous structure of the activated carbon from being damaged due to steam expansion during the subsequent high-temperature treatment. Then, the activated carbon was heated to the target temperature. During the entire experiment, nitrogen gas was continuously introduced into the tube furnace to maintain an oxygen-free environment. The outflowing gas stream was condensed through a cold trap and purified sequentially through two gas washing bottles.
After each reactivation was completed, the system was cleaned, and then a new batch of activated carbon was loaded to continue the experiment. The treatment under each temperature condition was repeated twice to ensure the reliability of the data. After reactivation, the activated carbon was subjected to extraction analysis to determine the content of PFAS remaining on the activated carbon. The results showed that PFOS and a very small amount of PFOA could be detected in the extract only under the reactivation condition of 300°C. When the temperature reached 500°C (a common temperature in the activated carbon reactivation process) and above, no residual PFOS or PFOA could be detected on the activated carbon.
Post-reactivation Treatment
PFOS
After the reactivation of PFOS-loaded activated carbon, only C8F16, perfluorooct-1-ene (m/z 399), was qualitatively detected in the activated carbon treated at 300°C. This component was not detected in the cold trap or gas washing bottles. After reactivation at 500°C (or higher), this substance was not detected in the activated carbon, condensates, or gas washing bottles. Perfluorooct-1-ene is a PFOS molecular chain without the characteristic sulfone group. It was also detected in the extract of the loaded activated carbon before reactivation but not in the stock solution. Therefore, this substance was already present on the activated carbon.
PFOA
After the reactivation of PFOA-loaded activated carbon, no degradation products were found on the activated carbon. However, various degradation products were found in the condensates of the cold trap and gas washing bottles. The transformation products formed at 300°C and found in the condensates or gas washing bottles may also form during the process of temperature rising to higher values and have condensed on the cold trap at lower temperatures. C8HF14O2 (m/z 395), which "lost" one fluorine atom during reactivation, can be found in the condensates and both gas washing bottles. This substance was also present in the stock solution but was removed from the activated carbon after reactivation.
C5F9 (m/z 231), C4F7 (m/z 181), C3F7 (m/z 169), C3F5 (m/z 131), and C2F5 (m/z 119) are sequential degradation products, in which carbon atoms or CF2 are broken off from the molecular chain. Due to the qualitative measurement of degradation products, it is impossible to determine whether their content increases or decreases during high-temperature treatment.
Conclusion
This study investigated the reactivation of activated carbon loaded with PFOS or PFOA. The results showed that after treating the activated carbon at a temperature of at least 500°C, PFOS and PFOA were completely removed from the activated carbon and might have been degraded. The degradation product of PFOS, C8F16, was only found on the activated carbon treated at 300°C. In addition, no degradation products were detected on the activated carbon or in the gas washing bottles. The degradation products of PFOA, including C8HF14O2, C5F9, C4F7, C3F7, C3F7, and C2F5, were not found on the activated carbon but were detected in the cold trap condensates and the two gas washing bottles. The signals of these substances were very low, so it was impossible to clarify the treatment process at different temperatures (i.e., whether they were formed only during heating at relatively low temperatures or also during treatment at higher temperatures). Reactivation under anoxic conditions at temperatures above 500°C seems to effectively remove PFOS and PFOA from activated carbon, which is consistent with the statements of activated carbon manufacturers. The transformation products found correspond to the degradation pathways identified in the literature [3] and [4]. However, the formation of other by-products cannot be ruled out. The fluoride measurement proposed in this paper has not yet yielded a final PFAS mass balance, which emphasizes the need for technologies with lower detection limits. Reactivation is an effective method to remove PFAS from activated carbon, but the more PFAS that need to be removed, the more reactivation cycles are required. This also means increased transportation, energy consumption, and raw material consumption, as well as reduced purification capacity. It will also increase the carbon footprint of drinking water production. Clean water is precious, but the means to obtain it come at a high cost.
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