Ecient Mineralization of Organic Pollutants Using Visible-light-induced PTCDA Anions Electronic Reservoir and Photogenerated Holes

Introducing the anion intermediate found with PTCDA into advanced oxidation processes (AOPs) overcomes the limitation of visible-light degradation. Stabilized PTCDA anionic intermediates act as electron reservoir to activate PMS to generate reactive oxygen species, thus improving the degradation rate of organic pollutants driven by visible light. At the same time, the photogenerated holes of PDI induce α and β scission with the unshared electron in organic molecules, and realize the deep mineralization of converting organic molecules to CO 2 . The BPA degradation rate of PTCDA /PMS is over 125.8 and 2.8 times as high as PTCDA photocatalysis and Co 3 O 4 /PMS, respectively. The BPA mineralization of PTCDA /PMS reaching ~ 88% outclasses Co 3 O 4 /PMS (~ 25%). In continuous ow reactor, it has a ~ 100% degradation and ~ 80% mineralization of BPA. The outstanding degradation in real water under solar light excitation indicates that PTCDA/PMS would be an intriguing system for non-toxic and harmless elimination of organic pollutants.


Introduction
Advanced oxidation processes (AOPs) decompose complex organic pollutants into CO 2 and H 2 O via reactive oxygen species (ROS) 1, 2, 3 . However, the toxicity of low-concentration intermediates produced without complete mineralization may threaten humans, ecosystems and the environment 4,5,6,7,8,9 . Therefore, deep mineralization must be realized for water or water reuse. Nowadays, catalysts/cocatalysts that signi cantly promote the activation of oxidants (H 2 O 2 , peroxymonosulfate (PMS), etc.) have been widely investigated due to its fast rate of ROS production 10 . These ROS can decompose organic pollutants with high ux, but are di cult to completely oxidize to CO 2 11 . Besides, PMS, for example, has a low utilization rate due to the limitations of poor invertibility of transition metals. The singlet oxygen ( 1 O 2 ) dominates the non-radical pathway 12,13,14,15 . Especially for carbonaceous materials, 1 O 2 produced by PMS selfdecomposition is the main active species for degradation 16 . However, the oxidation potential of 1 O 2 is not powerful enough to mineralize pollutants 17,18,19,20,21 . It is still a challenge to activate PMS by non-metallic catalysts with deep mineralization capabilities.
As another vital oxidation species, the photogenerated holes (h + ) produced in photocatalysis can react with organic compounds via dynamic control thus avoid the problem of sluggish kinetic to form CO 2 . A deeper valence band (VB) often produce h + with stronger oxidation ability, 22 which can be controlled by adjusting the molecular structure of organic semiconductors. 23,24 As for the degradation process caused by photogenerated holes in the photocatalytic process, there has always been vague questions: how does h + interact with organic pollutants? What is the mechanism responsible for mineralization? There is little in-depth literature on this issue. Therefore, it is of great signi cance to gure out how the h + plays a mineralize role in photocatalytic degradation process, which currently presents a key barrier to achieve deep mineralization of AOPs.
Recently, PTCDA-based supramolecular catalysts have been introduced in photocatalytic oxidation of water, producing oxygen by its h + with strong oxidation capacity generated by the low VB position 23,25,26 . It is reasonable to suspect that h + of PTCDA promotes the mineralization of organic pollutants. PTCDA anions generated by PTCDA under the light excitation have been used as an e cient electron donor in electron transfer system 27 . Fortunately, this hints that the production and elimination of anions in PTCDA is reversible. To be inspired, it is expected that the PTCDA anions generated by wide wavelength visible light will act as electrons reservoirs to activate PMS, triggering a chain reaction. Meanwhile, h + participates in degradation reaction to realize deep mineralization.
Herein, the PTCDA anion produced by photocatalysis was accurately used as an electron reservoir for activating PMS. The wide spectrum up to 700nm induced numerous PTCDA anions, providing an electronically activated PMS to realize a high-ux degradation process. Due to the strong adsorption of PMS and rapid reaction on PTCDA, the self-decomposition of PMS e ciency decreased to achieve high utilization of PMS. The timely removal of epromoted the effective participation of h + in the degradation process, leading to the occurrence of deep mineralization. The α and β scission of organic pollutants caused by photogenerated holes is the main mechanism of deep mineralization. As an AOP technology, its universal applicability to pollutants, super stability, excellent performance in the ow system, real water and solar light, indicating it is an extremely potential environmental remediation technology.

Results And Discussion
High-ux Degradation with Deep Mineralization for BPA by PTCDA/PMS system. With Bisphenol A (BPA) as a target pollutant, the catalytic degradation activity of PTCDA/PMS system was investigated. Compared with PTCDA photocatalysis, PTCDA/PMS has the advantage of high-ux degradation. The BPA removal in PTCDA photocatalysis was less than 10%. Interestingly, the coexistence of PMS and PTCDA boosted BPA removal e ciency signi cantly up to 100%. The highest degradation rate of PTCDA/PMS (0.629 min -1 ) is 125.8 times that of PTCDA photocatalysis system (Fig. 1a). The PTCDA with different side chains such as alanine has the same phenomenon ( Figure S1). A high concentration of 100 ppm BPA was entirely degraded by PTCDA/PMS system in 5 h, far better than the conventional Co 3 O 4 /PMS system (~ 50%) ( Figure S2).The addition of PMS signi cantly increased the degradation abilities and 0.3 mM was chosen as the concentration for subsequent work considering the cost ( Figure S3). Besides, the PTCDA/PMS system decomposed different organic pollutants rapidly, verifying that PTCDA/PMS was effective for the remediation of wastewater with various recalcitrant organic pollutants ( Figure S4). After 5 cycles in batch reactor, ~ 100% of BPA was still removed within 10 min, and the morphology and structure after the reaction remained unchanged ( Figure S5), indicating the excellent recirculation of PTCDA. Furthermore, a series of characterization showed that the superb charge transport ability of PTCDA laid a foundation for obtaining high-ux degradation performance (see Figure S24-S26 for detailed analysis).
Mineralization capacity is an important indicator to evaluate AOP. The removal of TOC reached ~ 88% for PTCDA/PMS system. However, the conventional single AOP is considered to be unable to completely oxidize and degrade organic pollutants. 28,29,30 For comparison, conventional Fenton-like cocatalysts such as Co 3 O 4 , Fe 2 O 3 and multi-walled carbon nanotubes (CNT) were selected to conduct BPA degradation and TOC removal tests (Fig. 1b), where PTCDA/PMS showed an absolute advantage. At a lower PMS dosage of 0.3 mM, Co 3 O 4 and Fe 2 O 3 could only completely degrade ~ 25% and ~ 13% of BPA into CO 2 respectively ( Figure S7). The adsorption of CNT removed 39% of TOC, but only 5% comes from the activation of PMS ( Figure S6). By further increasing the dosage of PMS to 1.0 mM, the mineralization for Co 3 O 4 , Fe 2 O 3 and CNT obtained limited improvement ( Figure S8), which was still far from that in PTCDA/PMS system. Homogeneous Fenton (Fe 2+ /H 2 O 2 ) is widely regarded as a powerful tool for removing organic pollutants, but its mineralization ability is poor. When 20ppm, 50ppm and 100ppm BPA as target pollutants, Fe 2+ /H 2 O 2 can only mineralize ~ 28%, ~ 30% and ~ 27% respectively. Surprisingly, the PTCDA/PMS system is ~ 88%, ~ 69% and ~ 55%, respectively ( Figure S9). Admittedly, homogeneous Fenton can quickly remove the target pollutants, but it cannot completely oxidize and degrade them, making the target pollutants only stay in the intermediate stage. The PTCDA/PMS system obtained high-ux degradation with deep mineralization.
Since visible light accounts for ~ 45% of the total radiant energy of sunlight, a wide-spectrum response is particularly important 31 . Contributed by the π-π accumulation induced extended conjugated electron cloud, the edge of the intrinsic absorption band of PTCDA covers the entire visible range of 630nm, further calculating the bandgap of 1.97ev, the position of CB and VB is -0.33V and 1.64V, respectively ( Figure S10). Expanding the absorption is bene cial to improve the ideal solar energy conversion e ciency since photon absorption is the rst step of photocatalysis 32,33 . In this regard, PTCDA with a large conjugate that can powerfully capture light is suitable. Moreover, the photogenerated charge on the PTCDA surface can be detected by the steady-state surface photovoltage (SPV) spectrum. The PTCDA produced an obviously positive signal in the 300-700 nm, with a maximum of 163 µV. The wavelengthdependent photocatalytic degradation activity of PTCDA/PMS system closely matches with SPV change trends (Fig. 1c). A positive signal indicates that photogenerated holes are transferred to the irradiated surface to oxidize BPA as the main reactive species.
How to improve the mineralization without signi cantly increasing the concentration of PMS is a crucial problem for the application of persulfate in the eld of water and wastewater treatment 34,35 , which can be solved by PTCDA/PMS system. A low concentration of 0.3 mM PMS is added to PTCDA/PMS system, and 58.7mg BPA can be mineralized per millimole of PMS (Table S2) High-ux Degradation Mechanism. Because of the extensive system of conjugate π, PTCDA semiconductor is bene cial to store photogenerated electrons to form electron reservoir. First, a bias voltage was applied to prove that PTCDA could be converted to PTCDA anions to form electronic reservoirs. The PTCDA showed an onset reduction potential of -1.015 V versus ferrocene ( Figure S11), which corresponded to the formation of its anion (PTCDA -), while the second onset reduction potential at -1.269 V indicated the transformation from PTCDAto the dianion state (PTCDA 2-). Whether it was PTCDAor PTCDA 2-, such a low potential can reduce PMS. Meanwhile, light excitation could produce PTCDA anion as well 36,37 .
The PTCDA anions exhibited a different color from their own that enabled them to be tested by ultraviolet-visible (UV-Vis) light irradiation 38,39 . New absorption bands of PTCDA anion appeared at ~ 668, ~755 and ~ 788 nm under irradiation of λ 350 nm, and the maximum amount of PTCDA anions was obtained when irradiated with simulated sunlight (Fig. 2a). With the prolongation of the irradiation time, the absorption of PTCDA anions increased sharply ( Figure S12a). However, UV-vis absorption of PTCDA anion in PMS solution was relatively weak even for more extended periods of light irradiation ( Figure   S12b). To give more evidence, the PTCDA anion could be observed by electron paramagnetic resonance (EPR) since the unpaired electrons on the PTCDA anion. As shown in the illustration in Fig. 2b, a strong resonance with a g = 2.00359 was detected after illumination, and no hyper ne splitting information indicated complete delocalization of unpaired electrons on PTCDA 40,41 . In order to quantify the content of PTCDA anions, the EPR signal peak and the standard manganese peak were integrated separately, and the ratio of the two was taken 42,43 . After turning off the light, the signal was signi cantly weakened and stabilized ( Figure S13a). With the extension of irradiation, the PTCDA anion signal gradually increased. Turning off the light for 17 min, the PTCDA free radical signal was still stronger than initial dark state ( Figure S13b), indicating that PTCDA anion forms a stable electronic reservoir. Surprisingly, PTCDA anions tested in water could almost be reduced to a dark state (Figure S13c-d) due to electrons were captured by abundant electron acceptors (H 2 O, O 2 ). In the presence of electron acceptors, the electron reservior provided electrons, making the generation and quenching of PTCDA anion a reversible process. As expected, the PTCDA anion in PMS solution is weak ( Figure S14). Similar to H 2 O or O 2 , the PMS as an electron acceptor takes electrons from the PTCDA anion electron reservior. Besides, Alanine-PTCDA has the same phenomenon ( Figure S15-16). The above results strongly proved that PTCDA anion served as an electron reservoir for PMS under wide spectrum excitation, resulting in the activation of PMS and occurrence of Fenton-like reaction. Besides, the order of the degradation rate in different atmospheres was as follow: k 2 (N 2 ) > k 2 (Air) > k 2 (O 2 ), which emphasized the importance of PMS activation by PTCDA anion electron reservoir ( Figure S17). The intense adsorption energy between PMS and PTCDA (-87.35kJ/mol) suggested that PTCDA could inhibit the selfdecomposition of PMS ( Figure S18), which was bene cial to improving the utilization rate of PMS.
Deep mineralization Mechanism. The quench experiment was used to explore the active species in PTCDA/PMS system.  44,45 . The OCV attenuation rate of the PTCDA working electrode was K = 7×10 − 5 s -1 in PMS solution after turning off, which was 74% slower than that of K = 27×10 − 5 s -1 without PMS. (Fig. 3b. See support information for detailed analysis). The reason is that eof PTCDA anion was captured by PMS, resulting in the less e-h + recombination on PTCDA electrode surface, thus obtaining a long life of h + .
As mentioned above, the mineralization of PTCDA/PMS with h + is much better than that of other Fenton-like systems, which is explained from the perspective of degradation products next. For the conventional Fenton-like system without h + , a high concentration of hydroxyl admixture was captured by high resolution mass spectrometry (LC/MS). Due to the hydroxyl radicals increased into the π electrons of BPA, hardly opened benzene ring for further mineralization 46 . The possible addition sites of BPA were predicted by Fukui function, which was consistent with the hydroxyl addition products detected by high-resolution LC/MS ( Figure S19). On the contrary, small molecular weight substances were detected during the PTCDA/PMS (Fig. 3c) A N = 14.7 Gauss hyper ne division constant (one of which was covered by the PTCDA anion peak), which belongs to the alkane free radical (R • ) signal. The six-fold peak did not appear without BPA ( Figure S20). Although it is impossible to determine the organic molecules of R · , but su cient to account for the generation of R · in PTCDA/PMS system. The EPR of R · once again proving the rationality of the above speculation process. h + is positively charged, and their activation sites are atoms with unshared electrons, causing α and β scission, which is a process of rapid carbon reduction mineralization. All organic molecules with hydroxyl functional groups can be converted into small molecules by photogenerated holes in a similar way until mineralization.
So far, as Scheme 1 shows, PTCDA supramolecular generates PTCDA anion electron reservoir under wide spectral excitation. PMS obtains electrons from the electron reservoir to produce ROS ( Figure S21), which can form rapidly hydroxyl admixture with BPA and/or incompletely degradation. h + rapidly mineralize organic molecules containing lone electrons through α and β scission.
Continuous Flow,real water and solar light Puri cation. Besides batch reactor, continuous ow performance is a key indicator to verify that puri cation technology is potential for industrial application. The puri cation effect of PTCDA/PMS was tested in a continuous ow reactor (Fig. 4a), which was illuminated by a 7.0 cm×2.5 cm×1.0 cm quartz window with a light area of 17.5 cm 2 (Fig. 4b). Non-woven fabrics xed PTCDA powder is easy for recovery.
With a treatment capacity of 34.3 L h − 1 m − 2 , the 10 mg/L BPA was completely puri ed in 50 h (Fig. 4c left). The mineralization maintained at ~ 80%. In contrast, for PTCDA photocatalysis, the removal of BPA is only ~ 2%. For Co 3 O 4 , the removal BPA reached ~ 74% within 1 hour. However, the removal rate was only ~ 25% and the mineralization was stable at ~ 5% after 9 h. Besides, in tap water, Songhua river and secondary sedimentation tank e uent tests with 20 mg/L BPA added, PTCDA/PMS system still obtained considerable degradation activity (Fig. 4c middle & Table S3). Although the xenon lamp used in the above experiment can simulate sunlight ( Figure S22), to further demonstrate the industrial application potential of PTCDA/PMS system, degradation performance was tested under solar light. The sunlight intensity on the ground at 116.33° east longitude and 40.0° north latitude in autumn is ~ 60 mW/cm 2 .
PTCDA/PMS can degrade 10 mg/L BPA rapidly in 10 minutes under outdoor natural light exposure (Fig. 4c right & Figure S23). Based on high-ux deep mineralization, high stability and cost considerations, we believe that the PTCDA/PMS con guration should be an ideal oxidation system for the removal and mineralization of organic pollutants with high potential.

Conclusion
In summary, PTCDA/PMS system, a new AOP technology that achieved both high-ux degradation and deep mineralization of organic pollutants has been established. The high utilization rate of PMS conduces to high-ux degradation rate and occurs e cient photogenerated e − -h + separation. h + with high oxidizing ability produced α and β scission contributing to deep mineralization process far superior conventional Fenton-like process. This work provides a new solution for water environmental remediation.

Materials And Methods.
Synthesis perylenetetracarboxylic dianhydride and their derivatives (PTCDA). In this work, we have synthesized two types of perylene-based organic catalysts: Preparation of perylenetetracarboxylic dianhydride (PTCDA): Herein, we selected concentrated sulfuric acid and water to form mixed solvent systems of H 2 SO 4 /H 2 O. Typically, 0.1g commercial PTCDA (perylenetetracarboxylic dianhydride, Alfa Aesar) was dissolved in 10mL H 2 SO 4 (good solvent) under ultrasonic treatment for 1 hour, and then deionized water (poor solvent) of 100 mL were added into the above solution all at once, respectively. Solid insoluble precipitates would appear instantly and keep these suspensions still for another 0.5 h, and these resulting bright red solids were collected by ltration through a 0.45µm membrane lter and washed with deionized water for several times, and then dried in oven at 60°C for subsequent use. It is marked as PTCDA.
Preparation Alanine substituted PTCDA (Alanine-PTCDA): Perylene-3,4,9,10-tetracarboxylic dianhydride, 18 g imidazole and 2.5 g (28.06 mM) 3aminopropionic acid (all supplied by Aldrich) were heated in a ask at 150°C for 4 h. Next, the reaction mixture was dispersed in 300 mL HCl (2 M) and 100 mL ethanol and stirred overnight. Then, the nal red solid was washed to neutrality with distilled water and ltered through a 0.22 µm membrane lter. Finally, the collected red solid was dried in an oven at 60°C under vacuum. Disperse 0.276 g of the above powder in 100 mL of water. Then add 417 µl of triethylamine solution and stir vigorously for about 30 minutes. Then add 20ml HCl (4M) and stir for another 3h. Centrifuge and wash the product to neutrality. Finally, the solid was collected and dried under vacuum at 60°C and made into powder. Labeled as alanine-PTCDA.
Non-woven fabric load of PTCDA photocatalysts. First, some clean hydrophilic non-woven fabrics are used as carriers for powdered PTCDA, and 20 mg of powdered PTCDA is dispersed in water and drip-coated on the non-woven fabric. The 5 oxygen atoms exposed by PTCDA can easily form hydrogen bonds with the hydrophilic non-woven fabric, which ensures that the PTCDA is rmly bonded to the non-woven fabric and avoids the leakage of the catalyst. Therefore, the successful loading of PTCDA on non-woven fabrics is a green and energy-saving way that does not require special catalyst recovery.
Non-woven fabric load of Co 3 O 4 photocatalysts. The catalyst on non-woven fabric was replaced with Co 3 O 4 .

Declarations
Funding. Con ict of interest statement. None declared.