Photo-electrochemical reduction of PFAS in complex water matrices

Domain: neuroscience
Published: 2026-03-28
Editorial outcome: major_revision

Abstract
Per- and polyfluoroalkyl substances (PFAS) are among the most persistent and challenging environmental pollutants. Compared with conventional oxidative or homogeneous reduction processes, electrochemical and photoelectrochemical reduction (ER and PER) offer reagent-free routes for PFAS destruction but remain limited by poor PFAS removal and sluggish defluorination under cathodic conditions. Here, we report a Pd-decorated TiO2 cathode that enables highly efficient photoelectrochemical degradation of perfluorooctane sulfonic acid (PFOS) under ambient conditions. Operando spectroscopy and density functional theory (DFT) calculations uncover a previously unrecognized mechanism in which cathodic potentials promote PFAS adsorption on TiO2, while UV-excited Pd sites generate hot electrons that directly drive C–F bond cleavage or form hydrated electrons for indirect reduction. This dual-electron pathway leads to rapid and deep defluorination, outperforming previously reported ER and PER systems even in complex matrices such as reverse osmosis concentrate (ROC) and aqueous film-forming foam (AFFF)-impacted water. The study further demonstrates scalable single-chamber reactor designs and mesh-type cathodes, advancing the practical implementation of photoelectrochemical PFAS destruction technologies.

Introduction
Per- and polyfluoroalkyl substances (PFAS) are synthetic chemicals that have been widely used since the 1940s due to their desirable chemical properties. However, they pose a significant threat to the environment because of their toxicity and persistence.1,2 Oxidative destruction processes, such as the hydrothermal treatment3,4 and electrochemical oxidation (EO),5,6 have been extensively investigated. Despite their effectiveness, these approaches often require elevated temperatures and pressures or form harmful byproducts. Reductive defluorination offers an alternative treatment paradigm capable of achieving comparable effectiveness under mild conditions while minimizing or eliminating the formation of oxyanion byproducts.7

Homogeneous photochemical reductive treatment, such as UV activation of sulfite (UV/S), generates hydrated electrons (e-aq) to achieve broad-spectrum reactivity toward various PFAS.8 However, there remains a critical need for catalytic systems capable of continuously generating reductants without the need for chemical supply. For chemical-free treatment, particulate photocatalysts such as silicon carbide and boron nitride were employed to degrade perfluorooctanoic acid (PFOA) under UV254 nm irradiation.9,10 However, these catalytic systems have not been demonstrated to effectively degrade the more recalcitrant perfluorooctane sulfonic acid (PFOS) unless more energy-intensive vacuum UV185 nm is irradiated.11

While the EO of PFAS on the anode has been extensively investigated, cathodic PFAS degradation remains a fundamental challenge. The research community has long sought breakthroughs in the electrochemical reduction (ER) or photoelectrochemical reduction (PER) pathways. Compared with the EO, far fewer investigations have been dedicated to this area. To date, reactive cathode materials, including carbon nanotubes modified by cationic surfactants,12–14 noble metals (Au and Pt),15,16 and diamond17 (summarized in Supplementary Table 1) have been explored. These studies revealed several fundamental challenges. It is widely recognized that the negatively charged PFAS molecules are typically repelled from the cathode.18,19 For example, a study clearly showed a three-order-of-magnitude drop in the PFOA partition coefficient between the aqueous phase and the Au surface when a negative potential was applied.15 Noticeable cathodic PFOS adsorption has previously been observed only on carbon nanotubes functionalized with cationic surfactants, where electrostatic attraction between the anionic PFOS headgroups and positively charged surfactant layers occurs.12 Even with successful PFAS adsorption on the cathode, achieving reductive defluorination is challenging. Under cathodic potential, water reduction readily occurs as a competing reaction, generating hydrogen and consuming a substantial portion of the electrons that could otherwise drive PFAS defluorination. This process may decrease the overall current efficiency but also hinders PFAS adsorption by occupying surface-active sites. To date, there exist only a few instances of confirmed PER removal of PFOS and limited defluorination in laboratory-prepared water.12,13 Key knowledge gaps remain, including the need for insights and solutions for the selective adsorption and reduction of perfluoroalkane sulfonic acids (PFSAs) and other PFAS in complex real-world water matrices, the optimal design of cathodes and reactors, and comprehensive assessments and demonstrations of scalability for practical applications.

Our central hypothesis is that a highly efficient PER cathode must contain reactive centers capable of generating strongly reducing species. Prior studies have shown that UV irradiation can excite 4d-band electrons in Pd through interband transitions, producing energetic hot electrons (e-hot).20 Based on our preliminary electronic structure analysis (Supplementary Fig. 1), the transient energies of these e-hot on Pd correspond to an effective reduction potential of approximately -4.2 V versus normal hydrogen electrode (VNHE), which is substantially more negative than the -2 V threshold required for PFOS reduction.7 Therefore, Pd is expected to function as the defluorination-active center. However, because PFAS adsorption is intrinsically challenging under cathodic polarization, we hypothesize that Pd must be integrated with an appropriate substrate to enable strong PFAS adsorption and efficient electron transfer.

This study marks a significant breakthrough in PER and ER treatment for PFAS. We report the scalable and facile synthesis of a Pd-decorated TiO2 cathode with dual functionality for PFAS adsorption and defluorination. Using a combination of operando characterization and theoretical calculations, we reveal a mechanism whereby cathodic current enhances PFAS adsorption on TiO2. The adsorbed PFAS is subsequently defluorinated either directly by e-hot generated at UV-excited Pd sites or indirectly via e-aq derived from these e-hot. The PER process demonstrates exceptional efficacy in degrading PFOS, even in the presence of complex, competitive matrices. Moving beyond proof-of-concept, we designed and validated a scalable PER system for the effective treatment of PFAS mixtures in membrane concentrates and simulated firefighting equipment cleaning solutions.

Results
Catalyst Design, Electrode Synthesis and Characterization
The proposed catalyst used in ER and PER systems consists of Pd nanoparticles (NPs) embedded in TiO2 matrices. As will be revealed later, TiO2 not only acts as a support to disperse and immobilize Pd NPs but is a prerequisite for PFAS adsorption through a novel cathodic mechanism. The Pd NPs are designed as active sites to generate e-hot for PFAS defluorination. As shown in Supplementary Fig. 1 and the associated extended discussion, we estimated the energy level of these e-hot on Pd to be approximately -4.2 V. The application of a cathodic potential can shift the Fermi level of Pd to a more negative value, thereby endowing the e-hot with even greater reductive power to facilitate reductive defluorination of PFAS (Supplementary Fig. 1).

We prepared self-standing electrodes composed of a Ti metal substrate and a Pd-TiO2 catalyst layer. The electrodes were fabricated by coating a Ti metal plate with sol-gels of metal-citrate complexes, followed by calcination in air and subsequent electrochemical reduction in electrolytes. The calcination of Ti-citrate sol-gel yielded anatase TiO2, as revealed by X-ray diffraction (XRD) analysis (Fig. 1a). The calcination of mixed Ti- and Pd-citrate sol-gel produced palladium oxide (PdO)-doped TiO2 as indicated by the PdO peaks in XRD and the Pd2+feature in X-ray photoelectron spectroscopy (XPS) (Fig. 1b). Upon applying 15 mA cm-2 cathodic current for 10 min in a 100 mM Na2SO4 electrolyte, Pd was electrochemically reduced to Pd0 (Fig. 1b). XRD analysis revealed well-defined Pd0 crystallinity with a dominant Pd (111) facet and the anatase TiO2 with a dominant TiO2 (101) facet. Scanning transmission electron microscopy (STEM; Fig. 1c) imaging confirmed the presence of Pd nanoparticles (NPs) embedded in the TiO2 matrix, and the electron diffraction pattern confirmed the presence of Pd (111) and Pd (200) facets. The average diameter of Pd NPs is 11.6 nm.

The molar ratio of Pd/Ti in Pd-TiO2 was controlled by modifying the Pd/Ti ratio in sol-gel precursors. For example, Fig. 1d shows a scanning electron microscopy (SEM) image of Pd-TiO2 with a pre-designed Pd/Ti molar ratio of 0.2 in the sol-gel. Energy dispersive spectrometry (EDS) mapping measured a 0.22 Pd/Ti ratio, which is close to the target value. Cross-sectional SEM-EDS mapping of the Pd-TiO2 plate cathode (Pd/Ti = 0.2) reveals a Pd signal corresponding to a coating thickness of about 2-3 μm (Fig. 1e). Note that the Ti substrate and the Pd-TiO2 layer contribute to the Ti emission, the thickness cannot be accurately derived from the Ti signal. Lastly, the Pd-TiO2 cathode surface is hydrophilic, as evidenced by the spreading of a water droplet and the small contact angle (Fig. 1f). This observation implies that PFAS adsorption on Pd-TiO2 via hydrophobic interactions is unlikely.

Cathodic defluorination of PFOS
We first investigated the cathodic removal of PFAS via ER and PER using a dual-chamber electrolytic cell (Fig. 2a). A proton exchange membrane (Nafion 117) separated the anodic water oxidation reaction from the cathodic PFAS treatment. All electrode geometric areas reported in the manuscript refer to the double-sided projected area submerged in the electrolyte. Either the plate-type TiO2 or the Pd-TiO2 cathode (16 cm2) was placed in the cathodic chamber, which contained 100 mL of Na2SO4 catholyte spiked with PFOS (10 µM). A dimensionally stable iridium tantalum oxide anode (Ir0.3Ta0.7Ox; 16 cm2) served as the counter electrode. The anolyte was 100 mL of 100 mM H2SO4. To introduce UV254 nm irradiation, a quartz jacketed 18-W low-pressure Hg lamp (Pen-Ray; Analytik Jena) was inserted into the catholyte, positioned 0.5 cm from the cathode. During the treatment, catholyte samples were collected for PFOS analysis, as detailed in the Method section. Defluorination efficiency (DeF) was calculated as the percentage of recovered fluoride relative to the total amount of fluorine in the original PFOS.

We conducted a series of control tests (24 h) to rule out possible PFOS removal pathways that are unrelated to ER and PER reactions (Supplementary Fig. 2). We then investigated the ER treatment of PFOS. As an n-type semiconductor, TiO2 exhibits metallic conductivity under cathodic potentials.21,22 Cyclic voltammetry profiles (Supplementary Fig. 3) indicate an onset potential of hydrogen evolution at -1 and -0.5 V versus reversible hydrogen electrode (VRHE) for TiO2 and Pd-TiO2, respectively. While open circuit TiO2 did not adsorb PFOS (Supplementary Fig. 2a), we surprisingly observed approximately 20% PFOS removal on TiO2 at -15 mA cm-2 after 8 h (Fig. 2b). This PFOS removal was not accompanied by defluorination, as no F- was detected (Fig. 2c). Therefore, we attribute PFOS removal by the TiO2 cathode to adsorption under cathodic potentials. This finding is unexpected because PFOS is an anion in solution and should be repelled by the negatively charged cathode. Thus, an uncovered mechanism other than the common electrostatic attraction or hydrophobic interactions must be responsible for PFOS adsorption.

Palladium doping further enhances the ER removal of PFOS After 8 hours of applied cathodic current, 55% of PFOS was removed (Fig. 2b). Methanol extraction of the used Pd-TiO2 cathode recovered 25% of PFOS, indicating that at least 25% of the original PFOS was removed through non-reactive adsorption. To evaluate SO42- formation resulting from desulfonation, the ER treatment was performed in NaClO4 catholyte, and the yield of SO42- was observed (Supplementary Fig. 4a). We estimate that approximately 28% of PFOS underwent C–S bond cleavage. When combined with the ~25% assigned to non-reactive adsorption, a total of 53% of PFOS is accounted for by adsorption-related pathways. This value closely matches the 55% PFOS removal determined from the decrease in aqueous-phase PFOS concentration (Fig. 2b and Supplementary Fig. 4b). Since no defluorination was detected in ER mode (Fig. 2c), we posit that the remaining perfluoroalkyl tails were adsorbed on the Pd-TiO2 surface after C–S bond cleavage. Further, we found no PFOS release upon discontinuation of the cathodic current (Supplementary Fig. 5), suggesting that while the cathodic current initiated the adsorption, it did not require sustained charges.

Under UV254 nm irradiation and cathodic currents (PER mode), we observed substantial PFOS removal and defluorination on the Pd-TiO2 cathode (Fig. 2b and c). Methanol extraction of the spent cathode did not detect any adsorbed PFOS, indicating that removal was due to degradation rather than adsorption. A performance comparison of Pd-TiO2 electrodes with Pd/Ti molar ratios ranging from 0.1 to 0.8 in PER mode (Supplementary Fig. 6) demonstrated that a 0.2 Pd/Ti ratio achieved the fastest PFOS removal and the highest DeF. Therefore, the Pd-TiO2 cathode with a 0.2 Pd/Ti ratio was selected for use in all subsequent experiments. The increase of cathodic current (5-20 mA cm-2) promotes PFOS removal in PER mode (Supplementary Fig. 7). A current density of 15 mA cm-2 was identified as the optimal condition, balancing fast PFOS removal kinetics and low electric energy consumption.

The substantial defluorination of PFOS by Pd-TiO2 under PER mode suggests that this removal was primarily due to destruction. The 37% DeF represents one of the highest values reported to date for ER and PER systems (Supplementary Table 1).12,13 In addition to defluorination, we observed 90% desulfonation (Supplementary Fig. 4a). The remaining 10% gap in sulfur balance is likely due to the formation of products that retain the sulfonate structure, as will be discussed in subsequent sections. Throughout the PER treatment, the catholyte pH was not actively controlled. To prevent pH elevation in the catholyte, an effect commonly observed when using a neutral salt solution as the anolyte,23 we employed 100 mM H2SO4 as the anolyte. In this configuration, excess protons migrated through the membrane, generating a proton flux that surpassed the rate of consumption via cathodic hydrogen evolution. As a result, the catholyte pH decreased from approximately 6 to 3 within the first two hours and then stabilized (Supplementary Fig. 8). Nevertheless, the acidification had little influence on PFOS removal and defluorination, as comparable performance was observed when phosphate buffer was used as the catholyte to maintain a constant pH of 7 throughout the PER treatment (Supplementary Fig. 8). The acidification is undesirable for practical water treatment applications, a single-chamber reactor design is introduced in subsequent sections to mitigate this issue.

Furthermore, we prepared Pd-loaded carbon cloth (Pd/CC), in which TiO2 was replaced by carbon cloth as the substrate, and evaluated its performance in the PER treatment of PFOS. The Pd/CC electrode exhibited PFOS removal under open-circuit conditions owing to adsorption by the CC substrate. However, it did not facilitate further PFOS removal or defluorination under PER operation (Supplementary Fig. 9). These results indicate that Pd NPs alone are insufficient to drive PFOS adsorption or reductive destruction, and that the TiO2 support plays an essential role in enabling PER activity.

Mechanisms of Promoted PFOS Adsorption Under Cathodic Potentials.
The used Pd-TiO2 cathode after ERC was characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Compared with the fresh Pd-TiO2 before the reaction with its essentially complete transmission, the used cathode has broad dips at 1000-1400 cm-1 and 600-800 cm-1 (Fig. 3a). These features are attributed to -CF2- (1150 cm-1 and 1250 cm-1), -CF3 (1200 cm-1), and -S=O (1200 and 650 cm-1).24–26 The results of ex-situ FTIR support the conjecture of retained PFOS and other polyfluorinated structures on the Pd-TiO2 surface.

We conducted operando FTIR measurements of Pd-TiO2 during the ER reaction to probe the adsorption mechanism (Fig. 3b and c). The time-zero FTIR reference at open circuit showed a broad peak between 1000-1400 cm-1, corresponding to -CF2- and -CF3 bond vibration in solution. The application of cathodic current led to the decrease of transmittance (i.e., enhanced IR absorbance) for -CF3 at 1200 cm-1 and -CF2- at 1150 cm-1, indicating the enrichment of PFOS on the cathode surface under the cathodic current. Upward peaks in the 1200-1400 cm⁻¹ range were observed, which corresponds to the inhibition of specific -CF2- bond vibrations. Such inhibition would lead to reduced IR light absorption and increased transmittance compared to the time-zero reference (without applied current). This result strongly suggests that cathodic adsorption inhibited the vibration of some C-F bonds on the side of the fluoroalkane chain, suggesting a co-planar adsorption configuration. Similar FTIR patterns were consistently observed across various cathodic current densities and potentials (Supplementary Fig. 10), confirming that PFOS adsorption is enhanced under cathodic polarization.

Density functional theory (DFT) calculations provide detailed thermodynamic evidence and mechanistic insight into the cathodic adsorption mechanism. Periodic boundary condition DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP) to investigate the interactions between PFOS and Pd [111] and TiO2 [101] surfaces. These surface terminations were chosen because they are the experimentally dominant facets, as demonstrated by XRD, and are the thermodynamically expected surfaces. Given that the experimental catholyte pH of 3 (Supplementary Fig. 8), PFOS was modeled in its deprotonated state using a Hess cycle, referenced to the gas-phase PFOS and accounting for experimental solvation and deprotonation energies. The experimental cathodic potential of -0.5 V vs NHE was accounted for by placing an electron in the conduction band of TiO2 or, in the case of Pd, using the computational hydrogen electrode. Solvation effects were included using a hybrid method with an explicit H2O monolayer on the slab and implicit solvation filling the vacuum space between the slab and its periodic image, following the method of our previous work.27

The process of PFOS adsorption under cathodic current first requires the desorption of surface water molecules that form a hydrated layer on TiO2 and Pd. As shown in Fig. 4a, water binds moderately to the titanium sites on the TiO2 [101] surface, with desorption energies of 0.12 eV for one and 0.49 eV for two water molecules, but binds more strongly to Pd, requiring 0.46 eV and 0.73 eV, respectively.

Only after the water molecules are displaced does PFOS adsorption become possible. The DFT calculations predict that PFOS adsorbs on TiO2 with an exothermic binding energy of -3.81 eV (excluding water removal) and -3.69 eV after accounting for the water desorption energy penalty. The driving force for this water-replacement step arises from the stronger electrostatic and coordination interactions between the sulfonate oxygens of PFOS and the exposed Ti4+ sites, which compensate for the small energy penalty of removing surface water. Conversely, on Pd, the adsorption energy is -1.36 eV before and -0.74 eV after water removal, indicating that PFOS adsorption is unlikely on cathodic Pd, and therefore, that adsorption is restricted to TiO2 under cathodic conditions (-0.5 V vs NHE, pH 3). The large energy penalty for displacing water on Pd is attributed to the strong H-bonding network of the hexagonally arranged H2O monolayer. This theoretical prediction agrees well with experimental observations that when Pd is loaded on carbon cloth (Pd/CC) in the absence of TiO2, the Pd centers do not contribute to PFOS removal (Supplementary Fig. 9).

Supplementary Fig. 11a shows two possible adsorption configurations on TiO2. In the monodentate mode, PFOS replaces one water molecule; in the bidentate mode, it replaces two, with adsorption energies differing by 0.6 eV, making the monodentate configuration thermodynamically more favorable. The PFOS molecule adopts a coplanar geometry, where the perfluoroalkyl tail lies parallel to the surface (Fig. 4b), which is in agreement with our operando FTIR measurements. A similar geometry is observed for adsorption on Pd (Fig. 4c), although this pathway is not energetically favorable. It is important to note that the DFT-predicted coplanar adsorption of PFOS represents the lowest-energy configuration among multiple simulated geometries and does not imply hydrophobic interactions between the perfluoroalkyl tail and the TiO2 or Pd surface or hydrophobic repulsion of the PFOS from solution. The DFT simulations used the polarizable continuum model (PCM) implicit solvation to represent the water phase, and PCM methods do not model the configurational fluctuations and the disruption of H-bonding networks that lower the entropy of the solvated system and drive hydrophobic exosolvation.28,29 Similar coplanar adsorption has been reported for PFOA on gold surfaces,15 which is known to be hydrophilic,30 illustrating that adsorption geometry alone should not be interpreted as evidence of hydrophobic interactions.

Bader charge analysis (Supplementary Table 2) provides insight into the electronic origin of this co-planar adsorption preference. The surface titanium cations carry a charge of +2.04 e-, corresponding to a formal +4 oxidation state, while surface oxygens exhibit partial negative Bader charges (-1.35 to -1.40 e-), corresponding to a formal charge of -2. As shown in Supplementary Table 3, upon desorption of one surface water molecule, the exposed Ti atom gains 0.23 e-, and neighboring O atoms lose 0.15 e⁻, creating an electron-deficient region that strongly attracts the negatively charged sulfonate oxygen. During PFOS adsorption, about 0.15 e- is transferred from PFOS into the TiO2 lattice, distributing across the bonding Ti atoms and the partially empty surface and subsurface Ti-O states (Supplementary Table 3 and Supplementary Fig. 11b). These charge-transfer and coordination effects explain why Ti4+/Ti3+ centers act as the primary adsorption sites.

Desulfonation and Defluorination Mechanisms
As demonstrated above, PFOS adsorption only requires TiO2 filled with conduction band electrons, but defluorination occurs only in the presence of both Pd and UV irradiation. Our estimation based on band structure and photon energy suggests that UV excitation of Pd generates highly reductive e-hot (Supplementary Fig. 1). These energetic e-hot may react directly with adsorbed PFAS on the catalyst surface. We relied on multiple e-aq quenchers to prove that e-aq are critical in the PER reaction. The addition of SeO42- (5-10 mM; rate constant with e-aq: k = 1.1×109 M-1 s-1)31 and Cd2+ (5-10 mM; k = 4.8×1010 M-1 s-1)31 noticeably inhibited the PFOS removal and defluorination (Supplementary Fig. 12). Perchlorate (ClO4-) and chlorate (ClO3-) are inert to ER. Thus, they cannot be reduced by the Pd-TiO2 cathode under ER mode. However, these oxyanions are moderately reactive toward e-aq,32 and their removal under PER mode (Supplementary Fig. 13) confirms the existence of e-aq.

We further introduce dissolved oxygen via air purging into the PER system. The removal of PFOS was slowed due to the scavenging of e-aq by dissolved oxygen at 4.1 mg L-1 (Supplementary Fig. 14).33 However, PFOS removal and defluorination by PER continued effectively. A critical observation was that the presence of Cd2+ and SeO42- almost completely halted defluorination, whereas defluorination still occurred in the presence of dissolved oxygen. Since the cathodic electrodeposition of Cd2+ to Cd0 and SeO42- to Se0 has been well documented,34,35 and EDS mapping shows clear Cd and Se signals on the Pd-TiO2 cathode surface after reaction (Supplementary Fig. 12), it is reasonable to infer that the deposited Cd and Se poison Pd active sites responsible for defluorination.

The use of dissolved oxygen as a quencher should not alter the surface properties of Pd-TiO2. Kinetic analysis revealed that 4.1 mg L-1 dissolved oxygen could quench over 99% of the e-aq in the aqueous phase (Supplementary Fig. 14). Therefore, the observed defluorination of PFOS in the presence of dissolved oxygen must be attributed to e-hot residing on the Pd-TiO2 surface. By comparing the PFOS removal kinetics with and without dissolved oxygen, we estimated that e-aq and e-hot contributed 63% and 37% to PFOS removal, respectively (Supplementary Fig. 14).

The ability of the PER system to sustain PFOS degradation in the presence of oxygen overcomes a key limitation of conventional homogeneous advanced reductive processes, which require strictly oxygen-deficient environments.33,36 This unique feature enhances the versatility of PER and facilitates scale-up using membrane-less reactor configurations that are simpler and more practical than traditional dual-chamber designs, as discussed in later sections.

The reaction energy profiles of intermediates along various possible PFOS degradation pathways on Pd-TiO2 calculated by DFT are illustrated in Fig. 4d. The interaction of PFOS with the Pd-TiO2 surface is visualized in Fig. 4e. As discussed above, PFOS adsorption on TiO2 (state 2TiO2) is significantly more exothermic than on Pd (state 2Pd), and therefore thermodynamically more favorable. We further calculated various possible binding configurations of perfluoroalkyl tail groups (state 3) and sulfonate head group (state 5) interacting with TiO2 and Pd. The results show that the tail group on Pd (3Pd) and the head group on TiO2 (3TiO2) is the most thermodynamically favorable scenario.

Bader charge analysis (Supplementary Table 4) provides rationales for this finding. When the C-S bond breaks on TiO2, approximately 0.48 e⁻ is transferred from the surface to the fluorinated tail, leaving the surface Ti and O atoms electron-deficient by about 0.33 e⁻. When the tail subsequently migrates to a Pd site, TiO2 regains those 0.33 e⁻, while Pd donates electrons to the perfluoroalkyl group instead. On Pd, the tail receives an even greater charge transfer (0.62 e⁻) due to the abundance of Pd d electrons that stabilize the fragment. In contrast, the sulfonate moiety is stabilized on TiO2 after the C–S bond cleavage, gaining approximately 0.54 e⁻ from the surface once the electron-withdrawing tail departs. If the sulfonate group were to transfer from TiO2 to Pd, it would lose this electron density to Pd, making the process energetically unfavorable.

The DFT results rationalize the experimental observation that Pd-TiO2 outperforms bare TiO2 in PFOS adsorption and defluorination (Fig. 2b). This enhanced performance arises because the Pd-TiO2 interface facilitates C-S bond cleavage and tail-group migration to Pd, thereby regenerating TiO2 sites for continued PFOS adsorption and reaction while also preventing the reverse C-S bond formation reaction. This mechanism also explains why, under ER mode, Pd–TiO2 exhibits both reactive and non-reactive adsorption (Supplementary Fig. 4b). Once the Pd surface becomes fully covered by perfluoroalkyl tail groups, it can no longer sustain the C-S bond cleavage required for reactive adsorption. In contrast, under PER mode, the UV-excited Pd sites promote continuous tail-group destruction, enabling cyclic regeneration of adsorption and degradation sites at the Pd–TiO2 interface.

After the C-S scission, the perfluorooctyl chain could be subjected to C-F scission or chain-shortening by C-C scission (intermediates “4a-b” in Fig. 4d and Supplementary Fig. 15). However, these steps are predicted to be highly endothermic, which is in agreement with the experimental results that Pd-TiO2 in ER mode alone did not lead to defluorination (Fig. 2c). The C-F scission may only be facilitated by energetic electrons (e-aq or e-hot). Biswas et al. have shown that the barrier to break the C-F bond of PFOS by e-aq was only about 0.08 eV (8.16 kJ mol-1).37 We suggest, therefore, that the C-F bond scission of the perfluoroalkyl tail is initiated by energetic electrons on Pd sites under the PER mode. Biswas et al. also found that the defluorination by e-aq was limited by the diffusion of the e-aq to the PFOS group.37 In our system, the perfluoroalkyl tail is directly bound to the Pd sites, placing it in proximity to the generated e-aq and e-hot. Consequently, we propose that C-F bond scission occurs through the action of photoexcited electrons. This mechanism accounts for the observed defluorination of PFOS exclusively on Pd-TiO2 under PER conditions.

These insights further underscore the critical role of cathodic adsorption in the PER system. Without cathodic current, PFOS removal was not observed on Pd-TiO2 under UV irradiation (Fig. S2b). The implications are two-fold: first, photoexcited TiO2 alone does not sustain PFOS adsorption, as photogenerated electrons rapidly migrate to Pd sites via Schottky contact,21 which, unlike TiO2, are unfavorable for PFOS adsorption, as discussed above. Second, without the cathodic adsorption of PFOS and C-S cleavage on TiO2 as the initial step, even if e-hot are generated on Pd sites, these excited electrons remain within the solid state and will rapidly relax to the Fermi level of Pd within femtoseconds to picoseconds.38,39

In PER mode, several synergistic effects are involved. Specifically, cathodic adsorption of PFOS and C-S bond cleavage on TiO2, along with the transfer of perfluoroalkyl groups to Pd, facilitate intimate contact between the perfluoroalkyl group and highly reducing e-hot on Pd. Meanwhile, the cathodic potential elevates the energy of e-hot on Pd beyond the vacuum level (Supplementary Fig. 1c), enabling electron emission from the solid phase into the aqueous phase. This process generates e-aq at the electrode-solution interface to react with adsorbed PFOS and Pd immobilized perfluoroalkyl group.40 Therefore, e-hot and e-aq jointly contribute as active reductive species responsible for PFOS defluorination.

We employed high-performance liquid chromatography coupled with either a triple quadrupole mass spectrometer (HPLC-MS/MS) or quadrupole time-of-flight mass spectrometry (HPLC-Q-ToF-MS) to analyze the transformation products (TPs) of PFOS during the PER treatment. Our targeted analyte list included 30 C1-C10 PFAS (Supplementary Table 5), and our suspect screening protocol was capable of detecting H/F exchange products across various chain lengths.5 Despite the comprehensive approach, only one H/F exchange product retaining the sulfonate group was identified, with an estimated concentration of ~0.8 μM (Supplementary Fig. 16). This accounted for approximately 8% of the remaining gap sulfur mass balance (Supplementary Fig. 4a), thereby closing the overall sulfur balance. The formation of H/F exchange TPs strongly supports an electron-mediated hydrodefluorination pathway,41 driven by both e-aq and e-hot. PFOA was the only shorter-chain perfluorinated compound detected, and it was further degraded at the end of the reaction. The transient appearance of PFOA supports a chain-shortening degradation pathway, which could stem from the reaction between perfluoroalkyl groups and both e-aq and e-hot.

Environmental Applications and Scale Up
The presence of UV-absorbing natural organic matter (NOM), introduced into the catholyte at concentrations of 10-100 mg L-1 as total organic carbon (TOC), slowed the PER reaction kinetics. Nevertheless, complete PFOS removal was achieved within 8 h when the NOM concentration was below 50 mg L-1 as TOC (Supplementary Fig. 17). Notably, the light transmittance (i.e., visual clarity) of the NOM-spiked catholyte increased significantly during PER treatment, likely due to the accelerated transformation of NOM by continuously generated energetic electrons, in addition to direct photolysis. These results highlight the system’s robust performance in the presence of light-absorbing matrices.

The presence of NO3- (10-100 mg L-1) compromised the PFOS treatment (Supplementary Fig. 18) due to the reaction with e-aq. However, since e-aq was continuously produced and NO3- is highly susceptible to both e-aq and electrochemical reduction, the PFOS removal performance should fully recover once NO3- is depleted in a batch operation. The PER process was tested in the presence of CaCl2 (50-150 mg L-1). PFOS removal remained unaffected by Ca2+ (Supplementary Fig. 19). Lastly, the robustness of Pd-TiO2 was demonstrated by its consistent PFOS removal performance, with endpoint efficiencies ranging from 94-98% and first-order rate constants between 0.35-0.42 h-1, remaining stable over five consecutive 8-hour batch operations spanning a total of 40 hours (Supplementary Fig. 20).

All the above results were obtained using a dual-chamber reactor. This design aims to exclude interference from anodic reactions. However, for scale-up, we envision a membrane-less single-chamber configuration as a more practical approach, as it features a simpler reactor configuration and avoids the use of costly ion-exchange membranes. Motivated by the observation that the PER reaction is resistant to dissolved oxygen, we hypothesized that PER can still proceed alongside the anodic water oxidation reaction in a single-chamber reactor.

Further, to maximize UV irradiation efficiency, we propose a tube cathode sheath that encases a UV lamp (Fig. 5a). Before finalizing this concept, we first demonstrated that the placement of the UV lamp, whether positioned between (UV_Mid) or outside (UV_Side) the anode and cathode, does not significantly impact PFOS removal kinetics (Fig. 5b). Based on these findings, we proposed the "UV-in-tube" configuration, which is analogous to the "UV_side" configuration. Another control test was conducted in a single-chamber reactor using a bare titanium tube without a Pd-TiO2 coating under UV lamp irradiation, coupled with an Ir0.3Ta0.7Ox anode (which remained in plate-type as purchased). The results showed no PFOS removal (Supplementary Fig. 21), ruling out adsorption by the Ti substrate and PFOS destruction by the Ir0.3Ta0.7Ox anode via EO. When a tube-type Pd-TiO2 cathode was used, effective PFOS removal (Fig. 5b) and defluorination (DeF = ~40% after PER, Supplementary Fig. 22) were observed. The inner tube surface subjected to both cathodic potential and UV light contributed predominantly to PFAS removal (Supplementary Fig. 23). The full utilization of UV irradiation and the increased electrode area enabled PFOS removal kinetics that were significantly faster than those in the dual-chamber design with a plate-type electrode. Another benefit of a single-chamber reactor is maintaining the pH between 6 and 7 throughout the reaction, thereby avoiding catholyte acidification in the dual-chamber reactor.

During PER treatment using a tubular Pd-TiO2 cathode in the presence of Cl⁻ (1 mM), no negative impact of Cl⁻ on PFOS removal or defluorination was observed (Fig. 5b). Furthermore, free chlorine (< 0.5 mg L-1 detection limit), ClO3- (< 5 μg L-1 detection limit), and ClO4- (< 5 μg L-1 detection limit) were not detected. These results suggest that even if chlorine was formed due to anodic oxidation, it was readily reduced by the Pd-TiO2 cathode alongside PFOS reductive destruction.

To facilitate system scale-up, the Ti tube substrate was replaced with a Ti mesh, which can be more readily fabricated into various geometries and offers greater flexibility in adjusting the available electrode area. We then constructed a mesoscale reactor (Fig. 6a), which is composed of a 16-W low-pressure Hg lamp wrapped by a Pd-TiO2 mesh cathode (256 cm2) and an Ir0.3Ta0.7Ox anode (196 cm2). The mesh Pd-TiO2 enables equally efficient removal of PFOS compared with the tube cathode, benchmarked by specific charge (Ah L-1; Supplementary Fig. 24). The specific energy consumption per order of PFOS removal (SEC) in the lab-prepared electrolyte was calculated as 178 Wh L-1, representing the lowest value reported to date for a PER-based system (Supplementary Table 1). Since the energy consumption of UV (68 Wh L-1) accounted for 38% of the energy consumption, it is reasonable to believe that, should a more energy-efficient UV source (e.g., LED UV) be adopted, the energy consumption could be further reduced.

The mesoscale reactor was applied for the treatment of reverse osmosis membrane concentrate (ROC) derived from the reclamation of municipal wastewater. The ROC contained redox-active anions such as 1950 mg L-1 of Cl- and 37 mg L-1 of NO3-, and background organics at a TOC of 50 mg L-1 (full list of composition see Supplementary Table 6). The ROC was treated as received without adjustment of pH (8.4) or conductivity (8.1 mS cm-1). As demonstrated in Fig. 6b, fluorotelomer sulfonic acid (FTS), PFSA, and PFCA detected in ROC were readily removed after 6 h of PER treatment.

Previously, we used combustion ion chromatography to quantify total fluorine in water.5,42 However, this method is unsuitable for ROC because its high salt content causes matrix interference during combustion, forming stable fluorides (e.g., CaF2, MgF2) that suppress the fluoride signal to near-background levels.43 Nonetheless, the direct analysis of F- in ROC by IC shows that the PER treatment led to an increase of F- concentration, indicating the substantial defluorination of organofluorine (Fig. 6c). No free chlorine was detected throughout the reaction, and no formation of chlorate and perchlorate was identified (Fig. 6d and Supplementary Table 6). The SEC of each target PFAS ranged from 80 to 150 Wh L-1 (Fig. 6e).

Coupling PER with EO toward complete PFAS mineralization
Recently, we discovered that PFAS subjected to homogeneous e-aq-mediated reduction in the UV/S process forms polyfluorinated intermediates that are more susceptible to EO, enabling complete defluorination in a UV/S-EO tandem process.5 Given the excellent performance of PER on driving electron-mediated PFAS reduction, we hypothesize that PER may serve as a chemical-free alternative to UV/S and be effectively integrated with EO. As shown in Supplementary Fig. 22, the PFOS spiked electrolyte was first treated with PER and then subjected to EO with a boron-doped diamond (BDD) anode. The PER-EO tandem process achieved complete destruction of PFOS and ~100% DeF. The results also imply that organic fluorine intermediates were retained in the aqueous phase after PER treatment rather than being released into the gas phase, as complete defluorination in the subsequent EO step would not have been possible otherwise.

The efficacy and practicality of the PER-EO process were further challenged in the treatment of aqueous film-forming foam (AFFF) diluted by tap water, representing firefighting equipment cleaning solutions. This wastewater contains 2 mg L-1 total organofluorine, 40 mg L-1 TOC, 7.4 mg L-1 of NO3-, and μg L-1 level of target PFAS (Supplementary Table 6). The diluted AFFF was treated as is throughout the PER-EO process without chemical addition or conductivity adjustment. The PER using the mesoscale single-chamber reactor with a Pd-TiO2 mesh cathode successfully destroyed most of the FTS, PFSA, and PFCA (Fig. 7a-c). Upon transferring to EO treatment, residual PFAS and precursors were further destroyed to reach 100% DeF (Fig. 7d and Supplementary Fig. 25). Removal of 99% of the TOC was achieved after PER-EO treatment (Supplementary Table 6).

In summary, this study reveals mechanisms of cathodic adsorption and energetic electron-mediated defluorination of PFAS on the Pd-TiO2 cathode within a robust, scalable, and highly efficient PER reaction system. These findings advance the fundamental understanding of heterogeneous catalytic reduction of PFAS, a field that has historically lagged behind EO strategies in both mechanistic insight and treatment efficacy. The PER system functions as a standalone treatment capable of achieving substantial PFAS degradation without generating oxyanion byproducts. Compared to homogeneous advanced reduction processes (e.g., UV/S), the PER process eliminates the need for continuous sulfite dosing and operates effectively across a broad native pH range (3–8, as tested in this study) without the need for pH adjustment, whereas UV/S systems typically require strongly alkaline conditions. If complete mineralization of PFAS in chemically complex or non-potable waste streams is prioritized, the integration of PER with EO enables near-complete defluorination in challenging wastewater matrices at ambient conditions, offering flexibility to address different treatment objectives.

Methods
Chemicals and Materials
Chemicals used as received include sodium sulfate ( J.T.Baker, ≥98%), sulfuric acid (J.T.Baker, 96.7%), palladium chloride (Sigma-Aldrich, 99%), titanium butoxide (Ti(OBu)4, Sigma-Aldrich, 99%), sodium perchlorate monohydrate (Thermal Fisher, 98%), citric acid (Alfa Aesar, 99%), ethylene glycol (Sigma Aldrich, 99%), cadmium perchlorate (Sigma Aldrich, 97%), sodium selenate (Sigma Aldrich, 95%), Heptadecafluorooctanesulfonic acid potassium salt (PFOS, Sigma Aldrich, 98%). ROC samples generated from RO filtration of municipal wastewater were provided by Trussell Tech. The AFFF product diluted by tap water is Platinum Plus C6 3% AFFF Buckeye MIL-SPEC. The Ir0.3Ta0.7Ox anode was purchased from Entrustech, China.

Preparation of Cathode
Titanium (Ti) plates (Grade 2 Titanium sheet, McMaster-Carr, 2.5 cm × 3.2 cm) were polished prior to use. The metal oxide coatings on the Ti plate were deposited by dip-coating in sol-gel precursor solutions followed by calcination. To prepare the coating solutions, we initially prepared a sol-gel by combining 11.5 mg of citric acid with 15.6 ml of ethylene glycol and mixing for 30 min at 60 °C. Subsequently, 0.3 mL of Ti(OBu)4 was added to 5 ml of ethanol, which was then mixed at room temperature. Subsequently, 35 mg PdCl2 and Ti(OBu)4 mixture were added to the sol-gel and mixed for 1 hour at 90 °C. Finally, dilute the coating solution to 50 ml with ethylene glycol. The precursor solution contained 20 mM Ti(OBu)4 and 4 mM PdCl2. The Ti plate was dipped in the coating solution for 20 s, dried at room temperature, and calcinated at 550 °C for 10 min. This cycle was repeated ten times, after which the plate was calcinated at 550 °C for 2 h. The Pd loading amount was adjusted by altering the ratio of Pd to TiO2 in the coating solutions. The coating of Pd-TiO2 on the Ti tube (Grade 9 Titanium tube; Ø 0.55") and mesh (Grade 2 100 mesh opening) follows the same protocol.

Characterization of Cathode
To characterize the catalyst coating without interference from the Ti metal substrate, the catalyst powders were collected by scraping them off the electrode. The powder XRD spectra were recorded with a Bruker instrument (2° to 40°, step 0.5° at 5.0 s per step) using Mo Kα1. SEM imaging was conducted using a JEOL JSM 7600 FEG-SEM with an EDX module. STEM images were performed using a Cs-Corrected Jeol 2100FC Microscopy operating at 200 kV. XPS was measured using a PHI VersaProbe III X-ray Photoelectron Spectrometer (Physical Electronics, USA) equipped with a monochromatic 1486.7 eV Al Kα X-ray source.

PFAS Treatment
The dual-chamber cell configuration was first used to investigate the ER and PER reaction without the interference of the anodic reaction. All electrode areas reported in the manuscript refer to the double-sided projected area submerged in the electrolyte. The Pd-TiO2 cathode (16 cm2) was placed in the cathodic chamber, which contained 100 mL of Na2SO4 catholyte spiked with PFOS (10 µM). An Ir0.3Ta0.7Ox anode (16 cm²) served as the counter electrode. The anolyte was 100 mL of 100 mM H2SO4. To introduce UV254 nm irradiation, a quartz jacketed mini low-pressure Hg lamp (18 W; 5 cm (L) × 0.7 cm (D); Pen-Ray Analytik Jena) was inserted into the catholyte, positioned 0.5 cm from the plate-type cathode.

The scaled-up single-chamber reactor configuration used a tube-type Pd-TiO2 cathode (48 cm2) and a mesh-type Pd-TiO2 cathode (256 cm²). The tube and mesh cathodes were paired with Ir0.3Ta0.7Ox anodes with surface areas of 16 cm2 and 196 cm2, respectively. The tube-type cathode was wrapped around a mini low-pressure Hg lamp (18 W; 5 cm (L) × 0.7 cm (D); Pen-Ray Analytik Jena), while the mesh-type cathode was wrapped around a larger low-pressure Hg lamp (16 W; 26 cm (L) × 1.2 cm (D)).

In the final section of the manuscript, we demonstrate a PER-EO tandem treatment process designed to illustrate a scalable workflow. This approach utilizes an EO flow cell to treat 0.7 L of diluted AFFF following mesoscale PER treatment using a mesh-type Pd-TiO2 cathode. The EO flow reactor, provided by Element Six, is equipped with two BDD disks (4.4 cm in diameter, 0.8 cm inter-electrode spacing) serving as the anode and cathode. The internal volume of the flow cell chamber is 95 mL. During EO treatment, the 0.7 L of PER-treated AFFF was circulated through the cell at a flow rate of 100 mL min-1 under a current of 5 A. The current and flow rate parameters were adopted from our previous study,5 where they were optimized for PFAS degradation, and are applied without modification in this work.

Analysis
Targeted analysis of PFAS was conducted on ultra-high-performance liquid chromatography (UPLC, Thermo Vanquish) coupled to a triple quadrupole mass spectrometer (QQQ MS/MS, Thermo Altis). The analytical method includes 30 PFAS (Table S5). A 10 μL aliquot of the sample was injected and separated on a Thermo Scientific Hypersil GOLD PFP column (2.1 mm × 100 mm, 1.9 μm), maintained at 40 °C. Chromatographic separation utilized a mobile phase A of 5 mM ammonium acetate in water and a mobile phase B of acetonitrile, flowing at a 0.4 mL min-1 rate. The LC gradient was programmed as follows: 0 min (20% B), 0.5 min (20% B), 3 min (80% B), 5 min (100% B), 5.5 min (100% B), 5.6 min (20% B), and 8 min (20% B). The MS parameters were optimized based on selected reaction monitoring (SRM) transitions (precursor/product fragment ion pair). Sample acquisition was conducted using Thermo XCalibur 4.5 software, and subsequent data analysis employed Thermo TraceFinder 5.1 software.

Suspect screening of PFAS TPs was conducted using high-performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (HPLC/QToF-MS, SCIEX). For the HPLC separation, a 50 μL sample was injected onto a Phenomenex Gemini column (3 μm particle size, 100 × 3 mm) and eluted with ultrapure water (A) and methanol (B), both containing 5 mM ammonium acetate, at a flow rate of 0.6 mL min-1. The gradient was as follows: 90% A-10% B from 0−1.5 minutes, 35% A-65% B from 1.5−8 minutes, 5% A-95% B from 8−8.1 minutes, 1% A-99% B from 8.1−12.5 minutes, and 90% A-10% B from 12.5−15.5 minutes. Data acquisition was turned off for the first 3 min of the run to prevent salts from entering the detector. High-resolution mass spectrometry analysis was performed on a SCIEX X500 QToF mass spectrometer in dual electrospray ionization (ESI) negative mode. A SWATH acquisition method was used in LC-QToF-MS analysis in negative ion mode with ion spray voltage at −4500 V. The QToF-MS/MS was set to scan fragment ions (ranging from 50-2500 Da) for parent ion masses between 100-2500 Da. The validation of polyfluorinated TPs and precursors is based on the following criteria: (i) mass tolerance < 5 ppm; (ii) peak area > 102; (iii) peak area increased with time from time zero.

The analysis of anions (F-, NO3-, SO42-, Cl-, ClO3-, and ClO4-) was performed on a Dionex Aquion ion chromatography system equipped with an anion-exchange column (Thermo Fisher Scientific, RFIC IonPac AS18). The AS18 column operated with a 35 mM KOH solution as the eluent at a flow rate of 1 mL min-1, with a suppressor current set to 57 mA.

ATR-FTIR and in situ FTIR Measurement
The ATR–FTIR measurements were conducted using a Bruker INVENIO-R FTIR spectrometer equipped with a room temperature Deuterated Lanthanum α Alanine doped TriGlycine Sulphate (RT-DLaTGS) detector. A platinum attenuated total reflectance (ATR) accessory with a diamond crystal window was used to collect the FTIR spectra of samples. The scanner velocity is set to 10 kHz. The transmittance spectra ranged from 4,000 to 400 cm−1 and were recorded by averaging 16 scans at a resolution of 4 cm−1.

The operando FTIR experiments were conducted on a Bruker INVENIO-R FTIR spectrometer equipped with an A530/IP electrochemical cell using a liquid nitrogen-cooled mercury-cadmium-telluride (MCT) detector.

Theoretical Calculation
DFT calculations of the PFOS degradation mechanism on Pd/TiO2 were performed with the Vienna Ab initio Simulation Program (VASP; version 6.4) software1,2 using the PBE functional. Wave functions were composed of a summation of plane waves with kinetic energies up to 500 eV. A gamma-point-centered 4×4×1 k-point mesh samples the Brillouin Zone. Electronic structures and energy converged, and energy changes were below 2×10-3 eV when the cut-off energy and k-point mesh were increased to 4×4×1. DFT-D3 corrections based on Grimme et al.3 were used to calculate the nonlocal van der Waals interaction. PAW pseudopotentials reduced the explicitly calculated electrons to the 1s of H; 2s and 2p of O and F, 3s and 3p of S; 3d,3s, and 4p of Ti; 4d,5s, and 5p of Pd. A Hubbard correction of 3.5 eV was placed on the 4d electrons of Ti, following previous studies 6. The Pd [111] and anatase TiO2 [101] facets were used to model the Pd and TiO2 surface, as the XRD results demonstrate that these were the most prevalent in the synthesized materials. The supercells contained 6 × 6 × 5 primitive cell slabs that were separated by at least 15 Å of vacuum space. The middle layer of the Pd slab was frozen, while the entire TiO2 slab was allowed to relax. Atomic geometries of the slabs and PFOS were relaxed until atomic energies changed by less than 0.001 eV. Electronic charge density was calculated using Bader charge analysis7. The solvation effects were included via implicit H2O solvation using VASPsol.44

The pH effects on the adsorption/desorption and surface reaction energies were included using a thermodynamic Hess cycle (Supplementary Fig. 26). The effect of potential on the Pd surface was implemented by means of the computational hydrogen electrode formalism45 and is referenced to the reversible hydrogen electrode (RHE). This approach calculates the reaction free energy at potential U using equation 2:
ΔGprotonation = G0-2.303kT(pKa-pH) (1)
ΔG(U) = ΔG(U0)-ne(U-U0) (2)
where the reaction free energy at 0 V (G(U0)) shifts linearly with potential by -eU (e is the electron charge). Free energies do not include calculated entropies, but rather, only include the entropic contributions due to the potential, entropy of the gas phase molecules as derived from the Hess cycle, and pH effects. Coordinates of all optimized PFAS structures used in the DFT calculations are provided in Supplementary Data 1.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript the authors describe the use of a PER destructive pathway for the degradation of PFOS using a TiO2/Pd cathode. Overall, the manuscript is well-written and clear. However, I have a few questions about the suggested mechanism and TEA:\n1) What is the thickness of the TiO2 layer deposited on the Ti substrate?\n2) The authors state that PFOS ER relies on the unique combination of TiO2 and Pd, and suggest a complex mechanism that involves a multi-step adsorption/migration/destruction process. Perhaps that is the case (using DFT is welcome, but the computation used here does not tell this entire story, which is based on conjecture). However, a negative control would be welcome. For example, a hydrophobic cathode material (e.g., activated carbon) with Pd surface particles, should offer multiple hydrophibic surface sites amenable to PFOS adsorption. If the authors are correct in their analysis, no PFOS destruction should occur.\n3) The “UV in Tube” configuration does not make sense to me. As the anode is placed outside of the tube, the electric field would extend from the outside of the tube to the anode. The inner portion of the tube should not experience charge, or at the minimum, it should be greatly attenuated. How does this square with the DFT analysis that states that PFOS adsorption is facilitated by the charging of the TiO2?\n4) The material costs are typically only a small part of the total CapEx of a system. I would suggest to either make this analysis more robust (i.e., what is the total production costs, at scale, of these materials) or remove this from the manuscript.\n5) A more robust TEA is needed to make any comparisons to other PFOS treatment approaches (e.g., FF + thermal oxidation, adsorption on AC + thermal oxidation). The main issue associated with electrochemical approaches to PFAS destruction is the cost of treatment. Unfortunately, this is missing from this analysis. If such an analysis is beyond the scope (as I suspect it is), I suggest removing any discussion of costs and focus on the science. In reality, the utility of these electrochemical approaches is constrained by their cost – there is a reason very few (if any) electrochemical treatment approaches for water have been scaled.", "reviewer_2": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_3": "Building on previous PER findings from lab, this manuscript demonstrates a compelling and well-executed study on the photo-electrochemical reduction (PER) of PFAS using Pd-decorated TiO2 cathodes for practical application. The integration of experimental results with detailed DFT calculations to uncover underlying mechanisms is particularly strong. Before it can be accepted, several areas could be improved for better clarity, scientific rigor, and broader applicability:\nTwo major points need further explanation\n• \"Energetic Electrons\" vs. Hydrated Electrons: While the manuscript differentiates between \"photo-generated energetic electrons from Pd active sites\" and \"hydrated electrons (eaq− ),\" the exact nature and roles of these species could be more precisely clarified. The text suggests ehot can react directly or relax to form eaq−. A clearer discussion on their formation pathways, importance, and reactivity would help—for example, whether ehot transfers directly to PFAS or mainly generates eaq−. (Could be added to Introduction part). Additionally, suggest moving the calculation of ehot’s energy level into the manuscript from the SI, as this is a critical hypothesis.\n• Role of TiO2 in C-S Scission: Since TiO2 plays a major role in initial adsorption, further clarification on how this adsorption occur and its mechanism, where the C-S bond breaking occurs—on TiO2 surface or at the Pd/TiO2 interface—and why would strengthen the defluorination. Are there specific TiO2 features that promote this bond cleavage before the tail migrates to Pd? Also, the authors claimed that desorption of H2O has to occur before PFOS adsorption. But, through what kind of force? Why sulfonate group will tend to be joint to the surface, not the carbon chain? Will be surface hydrophobicity analysis helpful?\nSome other Points for Improvement:\n1. Please improve the Abstract, make it more attractive to readers. There aren’t exciting findings and mechanistic details now.\n2. In Introduction, discussion on cathodic PFAS adsorption, and how to avoid water reduction will be more comprehensive.\n3. Mass Balance and Byproduct Identification: The 10% sulfur balance gap and transient PFOA presence are noted, but identifying other transformation products (TPs) remains a challenge. Explaining why TPs are difficult to detect—due to low concentrations, diverse structures, or short lifespans—and future analytical strategies would be helpful.\n4. Impact of pH on Adsorption and Reactivity: The pH drops from 6 to 3 in the dual-chamber reactor (Fig. S8). Since the single-chamber setup avoids this, discussing how this pH shift might influence PFAS adsorption and defluorination would be relevant, given the pH dependence of PFAS chemistry and electrode behavior.\n5. Control Experiment Duration: For the statement \"Negligible PFOS removal in the presence of cathodes without current and UV irradiation\" (Line 132), it would be helpful to specify the duration of this control to confirm that equilibrium adsorption remained negligible over the timescale considered.\n6. Figure 2 and Figure S4a, the electrolytes are different, indicating they are different reaction systems. In Figure 2, 100 mM Na2SO4 were used, and the authors detected low level of sulfate from degraded 10uM PFOS. This is confusing.\n7. Grammar and Phrasing: A thorough proofread to fix minor grammatical errors and improve phrasing is recommended. For example, rephrasing \"The process of PFOS adsorption under cathodic current first requires the desorption of water molecules from the existing surface water monolayer and then the subsequent adsorption of PFOS on the vacated sites\" (Lines 265-267) would enhance flow.", "reviewer_4": "This manuscript presents an innovative approach for the mineralization of PFASs in complex water matrices through photoelectrochemical reduction. It also explore the environmental applications and scale up. This research offers new insights into the reductive degradation of PFAS and demonstrates certain innovative aspects and practical application. While the study is well-designed and addresses a critical environmental challenge, several issues need improvement before publication.\n1. Regarding the proposed PFOS adsorption mechanism on Pd-TiO2, that the current discussion primarily relies on DFT analysis of pure TiO2 (Fig. 4) without explicitly addressing how Pd doping modifies the adsorption behavior. The direct extrapolation of TiO2's adsorption mechanism to the Pd-TiO2 system requires experimental or theoretical validation, as the introduction of Pd sites may significantly alter the surface chemistry and adsorption characteristics. More importantly, the DFT calculation results presented in Lines 265-313 lack sufficient supporting data to substantiate the key conclusions. For instance, the energy barrier differences between various binding configurations (e.g., bidentate vs. monodentate, Lines 265- 267) are discussed textually without showing corresponding energy profiles or structural visualizations; and the critical computational details are missing.\n2. Lines 312-313: The authous state that “the negatively charged Pd0 metal surface electrostatically repels PFOS”. However, why the electroreductive adsorbtion of PFOS on Pd-TiO2 cathode is stronger than that on TiO2 cathode?\n3. The basis for the statement in Lines 357-359 regarding \"the migration of the tail group...the thermodynamic driving force\" needs to be clarified. The explanation provided in Lines 368-382 for why the alkyl chain preferentially binds to the Pd surface while the sulfonate group tends to remain on TiO2 is currently limited to textual description without sufficient supporting evidence. We recommend supplementing with additional evidences and supporting calculation data to provide more convincing validation for these key mechanistic conclusions.\n4. Lines 339-340 claim that PFOS can still be reductively defluorinated by energetic hot electrons even after complete quenching of hydrated electrons. However, Fig. S9 clearly shows that defluorination was nearly completely inhibited when sufficient quencher was added to eliminate hydrated electrons. Can the quencher also eliminate energetic hot electrons (currently no evidence supports this)? What is the function of energetic hot electrons in PFOS defluorination? Could you exactly quantify the contribution ratio of energetic hot electrons and hydrated electrons? Similarly, Line 417: the state of “The formation of H/F exchange TPs strongly suggests the formation of eaq-” should be further explore as it may contribute by energetic hot electrons.\n5. Line 84: The authors state that \"The Pd NPs are designed as active sites for PFAS defluorination,\" but fail to provide supporting references. The introduction section also does not explain why Pd was selected as the defluorination active site or elaborate on the rationale behind choosing and designing the Pd-TiO2 material. A literature review summarizing previous relevant studies should be added to address these points.\n6. In Line 119, the authors indicate that Nafion 117 was used as the ion-exchange membrane to separate the anodic and cathodic chambers. To further validate the experimental system, it would be beneficial to include a control experiment without PFOS under the same photoelectroreduction conditions, which could help determine whether Nafion 117 itself contributes to fluoride ion generation.\n7. The authors have explained in Lines 192-194 that neutral salts in the anolyte typically lead to pH increase in the catholyte. However, experimental results demonstrate that when H2SO4 was used as the anolyte in this system, the catholyte pH failed to stabilize and instead became acidic. This acidification phenomenon could potentially interfere with both the defluorination efficiency and mechanistic pathways. It is therefore recommended to further discuss how such pH variations might specifically impact the defluorination performance.\n8. The data interpretation in Lines 168-177 requires further clarification. The reported 22% non-reactive adsorption (Line 170) appears inconsistent with the 25% PFOS recovery (Line 166), necessitating explicit justification of these values. In addition, the 27% C-S bond cleavage calculation assumes all sulfate originates from PFOS decomposition and remains detectable, thus these critical assumptions should be clearly stated and validated. Furthermore, the sum of adsorption (22 or 25%) and bond cleavage (27%) doesn't fully account for the 55% degradation efficiency, suggesting contributions from unidentified intermediates or mass balance errors, as potentially indicated by the unaccounted portion in Fig. S4b. A more comprehensive discussion of these discrepancies and underlying assumptions would strengthen the manuscript's rigor.\n9. In Lines 185-186, the authors observe that the PFOS removal rate increases with current density within the tested range of 5–15 mA/cm2. This raises an question: Would further increasing the current density beyond 15 mA/cm² lead to additional enhancement in PFOS degradation efficiency?\n10. In Lines 214-215, the authors attribute PFOS removal on TiO2 and Pd-TiO2 cathodes to potential-dependent adsorption. While in situ FTIR characterization during electroreduction (at different times) is presented to support this claim, it would further strengthen the mechanistic understanding if comparative FTIR spectra at different applied potentials could be provided (e.g., as Supplementary Information). Such data could more directly correlate adsorption behavior with cathode potential.\n11. The authors investigated the effect of chloride ions on defluorination in a single-chamber reactor (Lines 485-491), but no specific explanation was provided for this focused study. A brief justification for this experimental design choice would strengthen the logical flow of the study.\n12. In Lines 514-515, the authors note that high salt content in ROC samples may suppress fluoride ion signal detection during measurement. However, the manuscript does not specify how this analytical interference was addressed. A brief description of the solution (either in Methods or Supplementary Information) would ensure the reliability of the reported fluoride data is properly validated.\n13. Regarding the supplementary discussion of Fig. S1 (Lines 156-157), the statement \"Specifically, cathodic adsorption...of perfluoroalkyl groups to Pd\" appears unsupported by the presented data. Fig. S1 does not actually demonstrate the Potential-dependent PFOS adsorption on TiO2, the C-S bond cleavage, and the perfluoroalkyl chain migration to Pd sites. These claims represent mechanistic inferences that should properly be developed in the main text's conclusion section through comprehensive discussion of all experimental evidence, rather than being asserted prematurely in the SI based on limited data.\n14. Comments on figures optimization:\n1) Fig. 1a and 1b exhibit inconsistent image sizes and font styles. Uniform formatting is recommended for better visual coherence.\n2) Fig. S4b: The color label \"pink\" is incorrect and should be corrected to \"purple.\"\n3) Line 147: The statement \"While open-circuit TiO2 did not adsorb PFOS\" should reference Fig. S2a for supporting evidence.\n4) Fig. 4a: The rationale for selecting the cathodic potential (−0.5 VNHE) should be explicitly stated.\n5) Fig. 4e: The resolution appears suboptimal. Additionally, consider isolating this figure and directly annotating the meanings of labels 1, 2, 3, 4, 5 within the figure (rather than only in the caption) for more intuitive interpretation."}, "author_response": "RESPONSES TO REVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nIn this manuscript the authors describe the use of a PER destructive pathway for the degradation\nof PFOS using a TiO2/Pd cathode. Overall, the manuscript is well-written and clear. However, I\nhave a few questions about the suggested mechanism and TEA:\n1. What is the thickness of the TiO2 layer deposited on the Ti substrate?\nResponse 1.1: The revised manuscript provided the thickness of the Pd-TiO2 layer.\nRevision in Fig. 1:\nRevision in the Main Text:\n2) The authors state that PFOS ER relies on the unique combination of TiO2 and Pd, and\nsuggest a complex mechanism that involves a multi-step adsorption/migration/destruction\nprocess. Perhaps that is the case (using DFT is welcome, but the computation used here does not\ntell this entire story, which is based on conjecture). However, a negative control would be\nwelcome. For example, a hydrophobic cathode material (e.g., activated carbon) with Pd surface\nparticles, should offer multiple hydrophibic surface sites amenable to PFOS adsorption. If the\nauthors are correct in their analysis, no PFOS destruction should occur.\nResponse 1.2:\nWe appreciate the reviewer’s insightful comment. To further validate this point, we\nconducted a set of negative control experiments by loading Pd onto carbon cloth (CC) and\nevaluating its performance in PFOS defluorination. The results were consistent with the\nreviewer’s hypothesis: although PFOS could be adsorbed by the CC substrate, no\ndefluorination was detected under PER conditions. We selected the PER mode rather than\nER mode for these tests, as it represents the optimal condition for observing PFOS\ndegradation and defluorination. These findings reinforce our conclusion that effective\nPFOS defluorination requires the synergistic combination of TiO and Pd.\nRevision in the Supporting Information:\n3) The “UV in Tube” configuration does not make sense to me. As the anode is placed outside of\nthe tube, the electric field would extend from the outside of the tube to the anode. The inner\nportion of the tube should not experience charge, or at the minimum, it should be greatly\nattenuated. How does this square with the DFT analysis that states that PFOS adsorption is\nfacilitated by the charging of the TiO ?\nWe appreciate this insightful comment. It is important to note that both the top and bottom\nends of the tube cathode are immersed in the electrolyte, thereby electrically connecting both\nthe inner and outer surfaces to the same electrochemical circuit with the anode. To directly\naddress this concern, we measured the potentials at the inner and outer surfaces of the tube\ncathode by placing Ag/AgCl reference electrodes inside and outside the tube during operation\nat a cathodic current density of 15 mA cm-2. The measured potentials were -0.42 V (inner\nRHE\nsurface) and –0.63 V (outer surface), indicating that both surfaces experienced sufficiently\nRHE\nnegative (cathodic) potentials, albeit with slight attenuation inside the tube. Thus, the DFT\nresults are consistent with the experimental observations.\nFurthermore, we performed an additional experiment to provide direct experimental\nevidence. The external surface of the tube cathode was covered with duct tape so that only the\ninner surface was exposed to the electrolyte and UV irradiation. Under these conditions, the\nPER treatment of PFOS exhibited removal kinetics identical to those obtained with an\nunmasked tube. These results confirm that the inner surface of the tube cathode indeed\nexperienced cathodic potential and actively contributed to PFOS removal under UV irradiation,\nthereby supporting the validity of the “UV-in-tube” configuration.\nRevision in the Main Text:\nRevision in the Supporting Information:\n4) The material costs are typically only a small part of the total CapEx of a system. I would\nsuggest to either make this analysis more robust (i.e., what is the total production costs, at scale,\nof these materials) or remove this from the manuscript.\nResponse 1.4: We appreciate the reviewer’s constructive suggestion. We agree that a\ncomplete cost assessment would require a detailed techno-economic analysis that accounts\nfor total capital and operational expenditures, which is beyond the current study’s scope.\nAccordingly, we have removed the discussion of material cost and any cost-related\ncomparison from the revised manuscript to maintain focus on the mechanistic insights and\nperformance evaluation of the PER process.\nRevision in Supporting Information: Table S4 of the submitted version, which compared\nthe electrode costs, was removed.\nRevision in Main Text: The preparation of the mesh Pd–TiO₂ electrode was motivated by\nscalability and design flexibility rather than cost reduction in the revised manuscript.\n5) A more robust TEA is needed to make any comparisons to other PFOS treatment approaches\n(e.g., FF + thermal oxidation, adsorption on AC + thermal oxidation). The main issue associated\nwith electrochemical approaches to PFAS destruction is the cost of treatment. Unfortunately, this\nis missing from this analysis. If such an analysis is beyond the scope (as I suspect it is), I suggest\nremoving any discussion of costs and focus on the science. In reality, the utility of these\nelectrochemical approaches is constrained by their cost – there is a reason very few (if any)\nelectrochemical treatment approaches for water have been scaled.\nResponse 1.5: We appreciate the reviewer’s thoughtful comment. We agree that a\ncomprehensive TEA is beyond the scope of this study. In response, we have removed the\ndiscussion of material cost and economic comparison from the revised manuscript to maintain\nfocus on the scientific mechanisms and performance of the PER process. Our intention is to\npresent the fundamental advances in cathodic PFAS reduction, while recognizing that large-\nscale economic evaluation will require future work integrating engineering design and system\noptimization.\nRevision: All discussions pertaining to cost have been removed from the manuscript\nrevision. Additional details are provided in Response 1.4.\nReviewer #2 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is\npart of the Nature Communications initiative to facilitate training in peer review and to provide\nappropriate recognition for Early Career Researchers who co-review manuscripts. (Additional\ncomments from reviewer #2 that were shared with the editor) I have several minor suggestions\nthat could further enhance the clarity and completeness of the manuscript:\n1. Pd Loading and Adsorption Capacity\nSince Pd nanoparticles play a crucial role in enhancing PFAS adsorption and facilitating\ndefluorination, it would be valuable to include a discussion or data on how varying the Pd\nloading influences adsorption capacity. Is there an optimal Pd loading that balances\nperformance and material cost?\nResponse 2.1: We appreciate Reviewer 2’s comments and greatly value the efforts of senior\nreviewers in supporting early-career researchers.\nThe effect of Pd loading on PFOS adsorption and defluorination was systematically examined\nby preparing Pd–TiO₂ electrodes with Pd/Ti molar ratios ranging from 0.1 to 0.8 (Fig. S6). The\nresults show that increasing Pd loading from 0.1 to 0.2 enhances both PFOS removal and\ndefluorination efficiency, likely due to improved electron transfer and an increased density of\nactive Pd sites. However, further increasing Pd loading beyond 0.2 led to a decline in\nperformance, which we attribute to partial shading of the TiO₂ surface, reduced UV light\npenetration, and possible aggregation of Pd nanoparticles. Therefore, a Pd/Ti ratio of 0.2\nrepresents an optimal balance between catalytic performance and material cost. However, as\nsuggested by Reviewer #1, the revised manuscript should focus on the science. The\nimplications for cost are not discussed in detail.\nRevision: an extended discussion was added in the Supporting Information.\n2. Control Test Duration in Fig. S2\nThe control experiments in Fig. S2 have inconsistent time durations across different conditions.\nStandardizing these tests to a uniform duration (e.g., 24 h) would make comparisons more\nrigorous and persuasive.\nResponse 2.2: We have extended all control tests to 24 h.\nRevision in the Supporting Information:\n3. Cathodic Potential Optimization (Lines 149–150)\nThe manuscript states that PFOS removal is enabled by TiO₂ cathodes under cathodic potential.\nCould the authors specify the optimal applied potential for maximizing removal efficiency?\nAdditionally, a discussion on the trade-off between removal efficiency and energy consumption\nwould help identify the most practical operating conditions for real-world deployment.\nResponse 2.3: Impact of potential and current density and the implications on energy\nconsumption are provided in the revised manuscript.\nRevision in the Main Text:\nNew data are available in the Supporting Information:\nReviewer #3 (Remarks to the Author):\nBuilding on previous PER findings from lab, this manuscript demonstrates a compelling and\nwell-executed study on the photo-electrochemical reduction (PER) of PFAS using Pd-decorated\nTiO2 cathodes for practical application. The integration of experimental results with detailed\nDFT calculations to uncover underlying mechanisms is particularly strong. Before it can be\naccepted, several areas could be improved for better clarity, scientific rigor, and broader\napplicability:\nTwo major points need further explanation\n1. \"Energetic Electrons\" vs. Hydrated Electrons: While the manuscript differentiates between\n\"photo-generated energetic electrons from Pd active sites\" and \"hydrated electrons (eaq− ),\" the\nexact nature and roles of these species could be more precisely clarified. The text suggests ehot\ncan react directly or relax to form eaq−. A clearer discussion on their formation pathways,\nimportance, and reactivity would help—for example, whether ehot transfers directly to PFAS or\nmainly generates eaq−. (Could be added to Introduction part). Additionally, suggest moving the\ncalculation of ehot’s energy level into the manuscript from the SI, as this is a critical hypothesis.\nResponse 3.1:\nWe appreciate the insightful comments from Reviewer #3. The roles and contributions of hot\nelectrons and hydrated electrons have been clarified and emphasized in multiple sections of\nthe revised manuscript.\nUnfortunately, due to the word limit, it is challenging to move Figure S1 and the associated\ndiscussion into the main text at this stage. We will consult with the Editor for permission to\ndo so during the final formatting process.\nRevision in Introduction:\nRevision in the Discussion to clarify the role of the hydrated electron and hot electron:\n2. Role of TiO2 in C-S Scission: Since TiO2 plays a major role in initial adsorption, further\nclarification on how this adsorption occur and its mechanism, where the C-S bond breaking\noccurs—on TiO2 surface or at the Pd/TiO2 interface—and why would strengthen the\ndefluorination. Are there specific TiO2 features that promote this bond cleavage before the tail\nmigrates to Pd? Also, the authors claimed that desorption of H2O has to occur before PFOS\nadsorption. But, through what kind of force? Why sulfonate group will tend to be joint to the\nsurface, not the carbon chain? Will be surface hydrophobicity analysis helpful?\nResponse 3.2:\nWe appreciate the reviewer’s thoughtful and constructive comments. We have expanded and\nclarified the mechanistic discussion accordingly.\nFirst, the experimentally observed release of sulfate after ER treatment confirms that C–S\nbond cleavage occurs on Pd-TiO , and our revised DFT calculations now provide stronger\n2\nsupport to this observation. The DFT analysis explicitly compares multiple binding\nconfigurations of the sulfonate headgroup and the perfluoroalkyl tail on TiO₂ and Pd surfaces.\nThe most thermodynamically favorable configuration is one in which the sulfonate group binds\nstrongly to TiO while the liberated perfluoroalkyl fragment subsequently associates with Pd.\nmoiety is stabilized by Ti4+ sites, an intrinsic feature of TiO that facilitates both adsorption\n2\nand activation of the C–S bond.\nSecond, regarding the driving force for displacement of interfacial H O: our calculations\n2\nshow that the desorption energies of surface water (0.12–0.49 eV) are small compared to the\nstrong electrostatic and coordination interactions between the sulfonate oxygens and exposed\nTi /Ti4+ sites. These interactions render PFOS adsorption on TiO highly exothermic (~ –3.5\n5c 2\neV, Fig. 4a), providing a clear thermodynamic basis for water replacement. Thus, the\nmechanism is driven by specific coordination forces—not hydrophobic attraction.\nThird, the reviewer raised the question of why the sulfonate headgroup preferentially\nanchors to the surface instead of the fluorinated tail. Both the DFT results and Bader charge\nanalysis show that adsorption is dominated by charge transfer and coordination between the\nsulfonate group and Ti4+ centers. In contrast, the perfluoroalkyl tail interacts only weakly with\nTiO and forms no stable adsorption configuration. This explains why the headgroup binds\n2\nfirst and why C–S activation occurs at the TiO interface before the fluorinated fragment\n2\nmigrates toward Pd.\nFinally, we considered whether surface hydrophobicity influences the process. The DFT\nand Bader charge results indicate that PFOS adsorption on Pd–TiO is governed primarily by\n2\nelectrostatic/coordination interactions rather than hydrophobic effects. Therefore,\nhydrophobicity analysis would not significantly alter the mechanistic interpretation.\nWe have revised the DFT section and mechanistic discussion to incorporate these\nexplanations and provide a clearer, more comprehensive rationale aligned with the reviewer’s\nquestions.\nRevision in the Main Text to improve the readability of DFT section on PFOS adsorption:\nRevision in the Main Text to improve the readability of DFT section on C-S cleavage:\nSome other Points for Improvement:\n3. Please improve the Abstract, make it more attractive to readers. There aren’t exciting findings\nand mechanistic details now.\nResponse 3.3:\nWe improved the abstract to highlight the new mechanistic insights and significant\nengineering merits.\nRevision:\n4. In Introduction, discussion on cathodic PFAS adsorption, and how to avoid water reduction\nwill be more comprehensive.\nResponse 3.4: The Introduction has been strengthened according to the reviewer’s\nsuggestion.\nRevision in Introduction:\n5. Mass Balance and Byproduct Identification: The 10% sulfur balance gap and transient PFOA\npresence are noted, but identifying other transformation products (TPs) remains a challenge.\nExplaining why TPs are difficult to detect—due to low concentrations, diverse structures, or\nshort lifespans—and future analytical strategies would be helpful.\nResponse 3.5: Please allow us to provide clarification. In the submitted version, the 10% gap\nin the sulfur balance, noted in the early section of the manuscript and shown in Fig. S4, had\nbeen resolved in the latter section through the semi-quantification of sulfonate group–\ncontaining transformation products in Fig. S13 (renumbered as Fig. S16 in the revised\nmanuscript).\nHowever, we agree with the reviewer that the limitations of TP analysis, especially for\nthose formed after C-S cleavage, should be acknowledged. An extended discussion on this\naspect has been added to the Supporting Information.\nRevision in the Supporting Information:\n6. Impact of pH on Adsorption and Reactivity: The pH drops from 6 to 3 in the dual-chamber\nreactor (Fig. S8). Since the single-chamber setup avoids this, discussing how this pH shift might\ninfluence PFAS adsorption and defluorination would be relevant, given the pH dependence of\nPFAS chemistry and electrode behavior.\nResponse 3.6:\nWe have added additional data showing that when PBS was used as the catholyte in a dual-\nchamber reactor, the pH decrease was effectively prevented. The pH remained stable at\napproximately 7, consistent with that observed in the single-chamber reactor. This indicates\nthat pH has a negligible effect on PFOS removal. The corresponding data and discussion\nhave been included in the revised manuscript.\nRevision in Main Text:\nRevision in Supporting Information:\n7. Control Experiment Duration: For the statement \"Negligible PFOS removal in the presence of\ncathodes without current and UV irradiation\" (Line 132), it would be helpful to specify the\nduration of this control to confirm that equilibrium adsorption remained negligible over the\ntimescale considered.\nResponse 3.7:\nWe have standardized all control tests to a duration of 24 h. This information has been clarified\nin the revised manuscript. Please also refer to our Response 2.2 to Reviewer 2 for additional\ndetails.\nRevision in the Main Text:\n8. Figure 2 and Figure S4a, the electrolytes are different, indicating they are different reaction\nsystems. In Figure 2, 100 mM Na2SO4 were used, and the authors detected low level of sulfate\nfrom degraded 10uM PFOS. This is confusing.\nResponse 3.8:\nWe would like to clarify that PER conducted in Na₂SO₄ catholyte (Fig. 2) was used to evaluate\ndefluorination. In contrast, to assess desulfonation, PER was performed in a NaClO catholyte\n4\n(Fig. S4). This clarification has been added to the revised manuscript.\nRevision in the Main Text:\nClarification is provided in the figure caption:\n9. Grammar and Phrasing: A thorough proofread to fix minor grammatical errors and improve\nphrasing is recommended. For example, rephrasing \"The process of PFOS adsorption under\ncathodic current first requires the desorption of water molecules from the existing surface water\nmonolayer and then the subsequent adsorption of PFOS on the vacated sites\" (Lines 265-267)\nwould enhance flow.\nResponse: 3.9:\nWe have thoroughly checked and correct grammatical errors.\nTo improve readability, the DFT sections have been rewritten.\nReviewer #4 (Remarks to the Author):\nThis manuscript presents an innovative approach for the mineralization of PFASs in complex\nwater matrices through photoelectrochemical reduction. It also explore the environmental\napplications and scale up. This research offers new insights into the reductive degradation of\nPFAS and demonstrates certain innovative aspects and practical application. While the study is\nwell-designed and addresses a critical environmental challenge, several issues need\nimprovement before publication.\n1. Regarding the proposed PFOS adsorption mechanism on Pd-TiO2, that the current discussion\nprimarily relies on DFT analysis of pure TiO2 (Fig. 4) without explicitly addressing how Pd\ndoping modifies the adsorption behavior. The direct extrapolation of TiO2's adsorption\nmechanism to the Pd-TiO2 system requires experimental or theoretical validation, as the\nintroduction of Pd sites may significantly alter the surface chemistry and adsorption\ncharacteristics. More importantly, the DFT calculation results presented in Lines 265-313 lack\nsufficient supporting data to substantiate the key conclusions. For instance, the energy barrier\ndifferences between various binding configurations (e.g., bidentate vs. monodentate, Lines 265-\n267) are discussed textually without showing corresponding energy profiles or structural\nvisualizations; and the critical computational details are missing.\nResponse 4.1:\nWe appreciate the reviewer’s insightful comments. In the revised manuscript, we have\nsubstantially strengthened the discussion on PFOS adsorption on Pd-TiO by incorporating\n2\nnew experimental evidence, additional DFT analyses, and clearer mechanistic rationales.\n1) We added control experiments showing that Pd loaded on carbon cloth does not promote\nPFOS removal or defluorination. This confirms that Pd alone does not provide favorable PFOS\nadsorption sites.\n2) We expanded the DFT section with explicit adsorption energies, structural visualizations,\nand Bader charge analysis. The results show that PFOS adsorption on Pd sites is\nthermodynamically unfavorable, whereas adsorption on TiO remains favorable. The\n2\ndiscussion in Lines 265-313 has been rewritten for clarity and is now fully supported by new\ndata.\n3) The enhanced PFOS adsorption observed on Pd-TiO under cathodic current is now\n2\nexplained through a synergistic mechanism: cathodic C–S bond cleavage at TiO releases the\n2\nperfluoroalkyl tail, which can migrate to Pd. This migration frees new TiO₂ adsorption sites,\nenabling continuous PFOS capture. We have clarified this mechanism and linked it to both the\nexperimental and computational observations.\n4) We acknowledge that Pd doping may alter the TiO surface; however, our control tests\n2\nshow that under UV irradiation without applied current (open circuit), PFOS adsorption\non Pd-TiO is negligible (Fig. S2). This indicates that intrinsic structural modification from\nthe dominant role in modulating PFOS adsorption behavior via TiO .\nalone does not enhance PFOS adsorption, (ii) providing complete computational justification,\nand (iii) clarifying that the observed adsorption enhancement arises from catalytic synergy\nunder PER conditions rather than Pd-induced structural changes.\nRevision in the Main Text and Supporting information to show that Pd did not play a\nrole in adsorption.\nRevision of L 265-313 in the Main Text to improve readability regarding PFOS\nadsorption mechanism.\nNew Figure (S11) and Table (S3) are provided to support the discussion.\n2. Lines 312-313: The authous state that “the negatively charged Pd0 metal surface\nelectrostatically repels PFOS”. However, why the electroreductive adsorbtion of PFOS on Pd-\nTiO2 cathode is stronger than that on TiO2 cathode?\nResponse: 4.2:\nThe original statement that “a negatively charged Pd0 metal surface electrostatically repels\nPFOS” was based on DFT results showing that PFOS adsorption on Pd0 is thermodynamically\nunfavorable. We agree that describing this as electrostatic repulsion is inaccurate, and the\nstatement has therefore been removed from the manuscript.\nRegarding the observation that Pd-TiO₂ exhibits stronger PFOS adsorption than TiO under\n2\nboth ER and PER conditions, our experimental data, together with DFT calculations,\nconsistently indicate that the enhanced performance of Pd-TiO originates from interfacial\n2\neffects. Specifically, the Pd-TiO interface promotes C-S bond cleavage and facilitates the\n2\nmigration of the PFOS tail group to Pd. This interfacial mechanism regenerates TiO\n2\nadsorption sites for repeated PFOS uptake and reaction, while simultaneously suppressing the\nreverse C-S bond formation. This synergy explains the greater PFOS removal observed for Pd-\nTiO compared to TiO alone.\n2 2\nRevision in the Main Text:\n3. The basis for the statement in Lines 357-359 regarding \"the migration of the tail group...the\nthermodynamic driving force\" needs to be clarified. The explanation provided in Lines 368-382\nfor why the alkyl chain preferentially binds to the Pd surface while the sulfonate group tends to\nremain on TiO2 is currently limited to textual description without sufficient supporting evidence.\nWe recommend supplementing with additional evidences and supporting calculation data to\nprovide more convincing validation for these key mechanistic conclusions.\nResponse 4.3:\nWe performed rigid DFT calculations to evaluate the thermodynamics of PFOS fragmentation\nat the Pd-TiO interface. The results show that a configuration in which the perfluoroalkyl tail\n2\nbinds to Pd while the sulfonate group adsorbs on TiO is the most thermodynamically favorable\n2\nscenario. This conclusion is supported by the calculated reaction energy profile (now presented\nas Fig. 4d in the revised manuscript).\nIn addition, Bader charge analysis confirms that interfacial electron redistribution stabilizes\nthe corresponding fragments on the TiO and Pd sites. The Bader charge values are now\n2\nprovided in the Supporting Information.\nThe entire DFT section has been rewritten to improve readability. All reported energy\ndescriptors and numerical values are now directly supported by figures or tables for clarity and\ntransparency.\nRevision in the Main Text:\nNew Table (S4) to support the discussion.\n4. Lines 339-340 claim that PFOS can still be reductively defluorinated by energetic hot\nelectrons even after complete quenching of hydrated electrons. However, Fig. S9 clearly shows\nthat defluorination was nearly completely inhibited when sufficient quencher was added to\neliminate hydrated electrons. Can the quencher also eliminate energetic hot electrons (currently\nno evidence supports this)? What is the function of energetic hot electrons in PFOS\ndefluorination? Could you exactly quantify the contribution ratio of energetic hot electrons and\nhydrated electrons? Similarly, Line 417: the state of “The formation of H/F exchange TPs\nstrongly suggests the formation of eaq-” should be further explore as it may contribute by\nenergetic hot electrons.\nResponse 4.4: We further analyzed the quencher experiment results to evaluate the roles of\nCd2+, SeO 2-, and dissolved oxygen (via air purging). Kinetic analysis shows that dissolved\n4\noxygen serves as an effective quencher for differentiating the contributions of hot electrons\nand hydrated electrons to PFOS degradation. Regarding the reviewer’s final point, our new\ndata demonstrate that both hydrated electrons and hot electrons participate in the reaction. Thus\nshould jointly contribute to the formation of H/F products. We have revised the main text and\nSupporting Information accordingly.\nRevision in the Main Text to explain the different defluorination behaviors using\ndifferent quenchers:\nRevision in the Supporting Information to provide kinetic analysis.\nRevision in the Main Text regarding the H/F TPs product formation:\n5. Line 84: The authors state that \"The Pd NPs are designed as active sites for PFAS\ndefluorination,\" but fail to provide supporting references. The introduction section also does not\nexplain why Pd was selected as the defluorination active site or elaborate on the rationale\nbehind choosing and designing the Pd-TiO2 material. A literature review summarizing previous\nrelevant studies should be added to address these points.\nResponse 4.5:\nTo the best of our knowledge, this is the first study to explore using Pd to generate hot\nelectrons via the PER approach for PFFAS destruction. Thus, the reference we find is a\ncross-disciplinary source to show that the hot electron can be generated by UV excitation on\nPd. The introduction has been expanded.\nRevision in the Main Text:\n6. In Line 119, the authors indicate that Nafion 117 was used as the ion-exchange membrane to\nseparate the anodic and cathodic chambers. To further validate the experimental system, it would\nbe beneficial to include a control experiment without PFOS under the same photoelectroreduction\nconditions, which could help determine whether Nafion 117 itself contributes to fluoride ion\ngeneration.\nResponse 4.6: Additional control test was provided to prove that no F- can be released from\nNafion 117 under the PER condition.\nRevision in the Main Text:\nRevision in the Supporting Information:\n7. The authors have explained in Lines 192-194 that neutral salts in the anolyte typically lead to\npH increase in the catholyte. However, experimental results demonstrate that when H2SO4 was\nused as the anolyte in this system, the catholyte pH failed to stabilize and instead became acidic.\nThis acidification phenomenon could potentially interfere with both the defluorination efficiency\nand mechanistic pathways. It is therefore recommended to further discuss how such pH\nvariations might specifically impact the defluorination performance.\nResponse 4.7:\nWe have added additional data showing that when PBS was used as the catholyte in a dual-\nchamber reactor, the pH decrease was effectively prevented. The pH remained stable at\napproximately 7. The results show that pH has a negligible effect on PFOS removal. The\ncorresponding data and discussion have been included in the revised manuscript.\nRevision in Main Text:\nRevision in Supporting Information:\n8. The data interpretation in Lines 168-177 requires further clarification. The reported 22%\nnon-reactive adsorption (Line 170) appears inconsistent with the 25% PFOS recovery (Line\n166), necessitating explicit justification of these values. In addition, the 27% C-S bond cleavage\ncalculation assumes all sulfate originates from PFOS decomposition and remains detectable,\nthus these critical assumptions should be clearly stated and validated. Furthermore, the sum of\nadsorption (22 or 25%) and bond cleavage (27%) doesn't fully account for the 55% degradation\nefficiency, suggesting contributions from unidentified intermediates or mass balance errors, as\npotentially indicated by the unaccounted portion in Fig. S4b. A more comprehensive discussion\nof these discrepancies and underlying assumptions would strengthen the manuscript's rigor.\nResponse: We appreciate the reviewer’s comment. We have carefully re-examined the mass\nbalance calculation and corrected the numbers. The results show that approximately 28% of\nPFOS underwent C–S bond cleavage and was likely adsorbed onto the cathode surface as\nperfluoroalkyl fragments; 25% of PFOS was adsorbed on the cathode with its molecular\nstructure intact; and 45% of PFOS remained in the aqueous phase. Based on the decrease of\nPFAS concentration in the water phase, the overall removal efficiency was 55%. When\ncombining both reactive and non-reactive adsorption fractions (28% + 25%), the total PFOS\nremoval by adsorption is 53%. The small 2% discrepancy may result from analytical\nuncertainties during PFOS quantification by LC–MS and fluoride measurement by IC. Such\nerrors could arise from the dilution steps required to fit the sample concentrations within the\nrespective calibration ranges of PFAS in LC–MS and F⁻ in IC analyses. Numbers in the main\ntext were corrected, and an extended discussion was provided in the supporting information.\nRevision in the Main Text:\nRevision in the Supporting Information:\n9. In Lines 185-186, the authors observe that the PFOS removal rate increases with current density\nwithin the tested range of 5–15 mA/cm2. This raises an question: Would further increasing the\ncurrent density beyond 15 mA/cm² lead to additional enhancement in PFOS degradation efficiency?\nResponse 4.9: Impact of potential and current density and the implications on energy\nconsumption are provided in the revised manuscript. A higher current of 20 mA/cm2 was\nprovided. The results suggest that 15 mA/cm2 is the optimal current density that resulted in the\nlowest EEO.\nRevision in the Main Text:\nRevision in the Supporting Information:\n10. In Lines 214-215, the authors attribute PFOS removal on TiO2 and Pd-TiO2 cathodes to\npotential-dependent adsorption. While in situ FTIR characterization during electroreduction (at\ndifferent times) is presented to support this claim, it would further strengthen the mechanistic\nunderstanding if comparative FTIR spectra at different applied potentials could be provided\n(e.g., as Supplementary Information). Such data could more directly correlate adsorption\nbehavior with cathode potential.\nResponse 4.10: New data were provided as suggested.\nRevision in the Main Text:\nRevision in the Supporting Information:\n11. The authors investigated the effect of chloride ions on defluorination in a single-chamber\nreactor (Lines 485-491), but no specific explanation was provided for this focused study. A brief\njustification for this experimental design choice would strengthen the logical flow of the study.\nResponse 4.11: A brief justification has been added to improve the logical flow of the\ndiscussion.\nRevision in the Main Text:\n12. In Lines 514-515, the authors note that high salt content in ROC samples may suppress\nfluoride ion signal detection during measurement. However, the manuscript does not specify how\nthis analytical interference was addressed. A brief description of the solution (either in Methods\nor Supplementary Information) would ensure the reliability of the reported fluoride data is\nproperly validated.\nResponse 4.12: We would like to clarify that, as stated in our submitted manuscript, the\ndetermination of total fluorine in ROC samples using combustion ion chromatography (CIC)\npresents analytical challenges. A screenshot of the original text is provided below for reference.\nThe measurement of fluoride ions in PER-treated laboratory water, ROC, and diluted AFFF\nsolutions by direct ion chromatography (IC) does not exhibit any interference.\nThe difference arises because, in CIC analysis, samples undergo a combustion step prior to IC\ndetection. High salt content in ROC samples can interfere with fluoride quantification during\nthis combustion process through several matrix effects. In particular, abundant cations such as\nCa2+ and Mg2+ may react with released fluoride to form thermally stable, non-volatile salts\n(e.g., CaF , MgF ), which retain part of the fluorine in the solid residue and thereby reduce the\n2 2\nfraction converted to HF for subsequent absorption and IC measurement. Such matrix-related\ninterference has been reported in previous studies (e.g., Aro et al., iScience, 24, 102968, 2021).\nIn summary, we did not encounter interference in fluoride measurements performed by\nconventional IC. The observed interference pertains specifically to CIC-based total fluorine\nanalysis, and we have further elaborated on this issue in the revised manuscript.\nRevision in the Main Text:\n13. Regarding the supplementary discussion of Fig. S1 (Lines 156-157), the statement\n\"Specifically, cathodic adsorption...of perfluoroalkyl groups to Pd\" appears unsupported by the\npresented data. Fig. S1 does not actually demonstrate the Potential-dependent PFOS adsorption\non TiO2, the C-S bond cleavage, and the perfluoroalkyl chain migration to Pd sites. These\nclaims represent mechanistic inferences that should properly be developed in the main text's\nconclusion section through comprehensive discussion of all experimental evidence, rather than\nbeing asserted prematurely in the SI based on limited data.\nResponse 4.13: In the submitted version, the extended discussion under Fig. S1 included\nexperimental evidence that was presented in later sections in the main text. We appreciate the\nreviewer’s suggestion. In the revised manuscript, the discussion associated with Fig. S1 has\nbeen streamlined to focus only on the information directly supported by the figure, while the\nremaining content has been integrated into the relevant sections of the main text.\nRevision: Part of the Fig. S1 discussion was relocated to the Main Text:\n14. Comments on figures optimization:\n1) Fig. 1a and 1b exhibit inconsistent image sizes and font styles. Uniform formatting is\nrecommended for better visual coherence.\n2) Fig. S4b: The color label \"pink\" is incorrect and should be corrected to \"purple.\"\n3) Line 147: The statement \"While open-circuit TiO2 did not adsorb PFOS\" should reference\nFig. S2a for supporting evidence.\n4) Fig. 4a: The rationale for selecting the cathodic potential (−0.5 VNHE) should be explicitly\nstated.\n5) Fig. 4e: The resolution appears suboptimal. Additionally, consider isolating this figure and\ndirectly annotating the meanings of labels 1, 2, 3, 4, 5 within the figure (rather than only in the\ncaption) for more intuitive interpretation\nResponse 4.14: We greatly appreciate these detailed comments. Adjustments were made\ncorrespondingly.\n1) Figure 1 was readjusted\n2) Figure S4b caption was corrected.\n3) Revision was made in the Main Text.\n4) Revision was made in the Main Text.\n5) More details were added in Fig. 4e (now Fig. 4d in the revised version)"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed my concerns.", "reviewer_2": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_3": "The authors did nice revision and have addressed the comments successfully. The manuscript can be accepted now.", "reviewer_4": "The authors have greatly provided the responses to the previous comments. However, several key issues still require further clarification or discussion.\n1. Line 356-357, “This suggests that Cd2+ and SeO42- may have been electrochemically deposited on the cathode surface, thereby poisoning Pd active sites responsible for defluorination.” This statement is conjectural and lacks direct evidence. More importantly, the selection of Cd2+ and SeO₄2- as quenchers for hot electrons (e-hot) lacks a clearer scientific reason.\n2. Lines 90-91 and 337-338, please clearly elucidate the proposed mechanism and/or experimental evidence for how e-hot generates eaq-.\n3. In Response 3.2, the authors suggest that hydrophobicity does not influence the adsorption of PFOS on TiO2. However, the description in Lines 322–323 \"The PFOS molecule adopts a coplanar geometry, where the perfluoroalkyl tail lies parallel to the surface\" — appears to indicate a configuration that may well be driven by hydrophobic interactions. Such an arrangement is typical for hydrophobic alkyl chains aligning parallel to surfaces to minimize contact with the aqueous environment. Therefore, it seems inconsistent to entirely dismiss the role of hydrophobic interactions in this adsorption process. The authors should clarify whether hydrophobic forces are considered negligible or if their interpretation requires further elaboration.\n4. Regarding the Response 2.1, the experimental conditions remain unclear. In the performance comparison of electrodes with varying Pd/Ti molar ratios, was this evaluation conducted under electrochemical conditions, or was it a photoelectrochemical process? In the main text (Lines 204–205) and in the response, the authors attribute the decline in Pd performance to \"shading of the TiO2 surface and reduced UV light penetration.\" This explanation inherently implies that the tests were performed under UV illumination. However, the caption of Fig. S6 does not specify the measurement conditions. If the data were indeed obtained under purely electrochemical (dark) conditions, the provided explanation would be invalid. The authors should clarify and explicitly state the experimental conditions in the figure caption and relevant text.\n5. In Lines 352–353, the statement, \"However, PFOS removal and defluorination by PER continued effectively. Nevertheless, PFOS degradation and defluorination continued effectively under these conditions\" is semantically repetitive. Please revise this sentence to convey the intended meaning clearly and concisely.\n6. In Response 4.14, the consistency issues in Figure 1a and 1b regarding scale and font style appear to remain unresolved. Please ensure both panels are uniformly formatted to meet the journal's publication standards"}, "author_response": "Response to Comments\nReviewer #4\nThe authors have greatly provided the responses to the previous comments. However, several key issues\nstill require further clarification or discussion.\nResponse: We sincerely appreciate the reviewer’s careful evaluation of our revised manuscript. In\nresponse to Comments 1 and 3, we have provided new experimental data. Extended discussion and\nclarification have been added as suggested in Comments 2 and 4. Errors have been corrected in\naccordance with Comments 5 and 6.\n1. Line 356-357, “This suggests that Cd2+ and SeO42- may have been electrochemically deposited on the\ncathode surface, thereby poisoning Pd active sites responsible for defluorination.” This statement is\nconjectural and lacks direct evidence. More importantly, the selection of Cd2+ and SeO₄2- as quenchers\nfor hot electrons (e-hot) lacks a clearer scientific reason.\nResponse:\nPlease note that in the submitted manuscript, we explicitly stated that Cd2+ and SeO - were introduced\n4\nas common quenchers for the e- , rather than e- A screenshot from the manuscript highlighting\naq hot.\nthis clarification is provided below.\nThe corresponding rate constants were cited to demonstrate that the quenching reactions of e- by Cd2+\naq\nand SeO 2- are well established in the literature. However, our experimental results indicate that the use\n4\nof these two e- quenchers in the PER system is not ideal, as both species tend to deposit on the cathode\naq\nsurface, leading to poisoning of active sites and interference with catalytic performance. This limitation\nhas now been explicitly addressed and discussed in the revised manuscript according to the comment.\nIt is also important to note that, later in the manuscript, we succefully used dissolved oxygen as an e-\naq\nquencher without the concern of poisoning the active sites. The exploration of Cd2+ and SeO 2- as\n4\nquenchers therefore represents an informative methodological insight rather than a limitation, and it\ndoes not affect the central novelty or significance of this work on PFAS removal and defluorination.\nGiven the above, our revision focuses on providing more discussion and data on the\nelectrodeposition of Cd2+ and SeO 2-.\nRevision in Supporting Information:\n2. Lines 90-91 and 337-338, please clearly elucidate the proposed mechanism and/or experimental\nevidence for how e-hot generates eaq-.\nResponse: We carefully considered multiple options to improve Lines 90–91. However, this sentence\nappears in the final paragraph of the Introduction and is intended to briefly highlight the key findings\nof this study. Expanding the discussion of the e- generation mechanism at this stage would add\naq\nexcessive detail and risk disrupting the narrative flow, potentially diminishing reader engagement with\nthe subsequent sections where the mechanism is presented in depth. We therefore respectfully suggest\nretaining Lines 90-91 in their current form as an effective hook for the reader.\nWe appreciate this comment and have provided a more in-depth discussion in Lines 337-338. To\nour knowledge, this is the first study to employ PER for e- generation, and therefore, the available\nhot\nprior literature is limited. We identified a relevant study demonstrating that UV irradiation at 213 nm\ncan excite electrons in diamond to a reduction potential of approximately −5 V vs NHE. That work\nshowed that such high-energy electrons can be injected into water to form hydrated electrons as their\nreduction potential readily exceed thermodynamic requirement for the reaction: H O + e- → e- (E° ≈\n2 aq\n−2.9 V vs NHE). The reported mechanism closely parallels our system, in which e- are generated at\nhot\napproximately −4.7 V vs NHE. Together, this literature provides supportive evidence for our proposed\nhypothesis.\nRevision in the Main Text:\nNew Reference Cited:\n3. In Response 3.2, the authors suggest that hydrophobicity does not influence the adsorption of PFOS on\nTiO2. However, the description in Lines 322–323 \"The PFOS molecule adopts a coplanar geometry,\nwhere the perfluoroalkyl tail lies parallel to the surface\" — appears to indicate a configuration that may\nwell be driven by hydrophobic interactions. Such an arrangement is typical for hydrophobic alkyl chains\naligning parallel to surfaces to minimize contact with the aqueous environment. Therefore, it seems\ninconsistent to entirely dismiss the role of hydrophobic interactions in this adsorption process. The\nauthors should clarify whether hydrophobic forces are considered negligible or if their interpretation\nrequires further elaboration.\nResponse: We appreciate the follow-up comment. The response and revision give us the opportunity\nto provide further clarification and additional data.\nWe feel confident that the hydrophobicity of the perfluoroalkyl tail is not the driver of the co-planar\ngeometry, owing to the physics of the simulation and those that govern hydrophobic repulsion. Our\nDFT simulations utilized polarizable continuum model (PCM) implicit solvents rather than explicit\nsolvation in the space above the surfaces to represent the aqueous solution, and PCM methods cannot\ndrive hydrophobic-based segregation. Hydrophobic driven exovlation arises from entropic fluctuations\nand the disruption of H-bonding networks, neither of which exists in implicit solvation. Implicit\nsolvation treats the space as a polarized continuum, reproducing the averaged bulk-like properties of,\nin this case, water, without explicitly resolving the H-bonding sites or structural variation. Because of\nthe continuum approach, these models do not resolve for the entropic, fluxtional, and structural effects\nof the H-bonding networks, which are the basis of hydrophobic/hydrophilic behavior (J. Phys. Chem.\nB 2009, 113(18), 6378-6396 & Biomolecules 2025, 15(9), 1218). Further, we note that the TiO surface\n2\nis itself hydrophilic; as such, the coplanar configuration alone should not be interpreted as evidence of\nhydrophobic interactions. We provide photos to demonstrate the hydrophilic surface of Pd-TiO (Fig.\n2\n1f) and additional discussion in L329-337. As a similar case, the simulation of adsorption of PFOA on\nAu plate also shows a coplanar as the most favorable configuration (J. Am. Chem. Soc. 2024, 146 (15),\n10687–10698), and Au surface is known as hydrophilic (Journal of Colloid and Interface Science 1980,\n75 (1), 51–55.).\nRevision in the Main Text:\nRevision to Fig. 1\n4. Regarding the Response 2.1, the experimental conditions remain unclear. In the performance\ncomparison of electrodes with varying Pd/Ti molar ratios, was this evaluation conducted under\nelectrochemical conditions, or was it a photoelectrochemical process? In the main text (Lines 204–205)\nand in the response, the authors attribute the decline in Pd performance to \"shading of the TiO2 surface\nand reduced UV light penetration.\" This explanation inherently implies that the tests were performed\nunder UV illumination. However, the caption of Fig. S6 does not specify the measurement conditions. If\nthe data were indeed obtained under purely electrochemical (dark) conditions, the provided explanation\nwould be invalid. The authors should clarify and explicitly state the experimental conditions in the figure\ncaption and relevant text.\nResponse: We confirm that the investigation of the Pd/Ti molar ratio on PFOS removal was conducted\nin PER mode. The relevant specifications are provided in both the main text and the figure caption of\nFig. S6. We have carefully reviewed all other figure captions. Wherever experiments were conducted\nin PER mode, the corresponding UV irradiation conditions are explicitly stated.\nRevision in the Main Text:\nRevision of Fig. S6:\n5. In Lines 352–353, the statement, \"However, PFOS removal and defluorination by PER continued\neffectively. Nevertheless, PFOS degradation and defluorination continued effectively under these\nconditions\" is semantically repetitive. Please revise this sentence to convey the intended meaning clearly\nand concisely.\nResponse: Thank you so much for pointing it out. We have corrected the sentence as suggested.\nRevision in the Main Text:\n6. In Response 4.14, the consistency issues in Figure 1a and 1b regarding scale and font style appear to\nremain unresolved. Please ensure both panels are uniformly formatted to meet the journal's publication\nstandards.\nResponse: We sincerely appreciate the detailed review. Figure 1a and b are revised.\nRevision:"}, {"round": 3, "reviewer_comments": {"reviewer_4": "My comments are addressed. I have no further questions"}, "author_response": ""}]

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