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Article

Enhanced Degradation of Carbamazepine from Constructed Wetlands with a PEC System Based on an Anode of N-TiO2 Nanocrystal-Modified TiO2 Nanotubes and an Activated Carbon Photocathode

by
Xiongwei Liang
1,2,†,
Shaopeng Yu
1,*,†,
Bo Meng
1,2,
Jia Liu
1,2,
Chunxue Yang
1,2,
Chuanqi Shi
1 and
Junnan Ding
1
1
Cold Region Wetland Ecology and Environment Research Key Laboratory of Heilongjiang Province, Harbin University, Harbin 150086, China
2
State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150086, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Separations 2024, 11(7), 216; https://doi.org/10.3390/separations11070216
Submission received: 25 May 2024 / Revised: 6 July 2024 / Accepted: 11 July 2024 / Published: 19 July 2024
(This article belongs to the Special Issue Advanced Nitrogen Removal Process in Adsorption and Removal of Pollutants)
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Versions Notes

Abstract

:
We used the Vienna ab initio Simulation Package (VASP), X-ray photoelectron spectroscopy (XPS) and diffuse reflectance (DRS) to optimize anode material for a photoelectric catalytic system. After screening how the doping of TiO2 by N and S affects its photoelectric properties, N-doped TiO2 was selected to fabricate the photoelectron catalytic (PEC) system. TiO2 nanotubes modified by N-doped TiO2 nanocrystals and activated carbon were used as an anode and as a photocathode, respectively, to decompose carbamazepine in water samples from the constructed wetlands. The calculations showed that the N-TiO2 NCs/TNTAs-AC/PTFE system had the highest content of •OH. The highest carbamazepine removal rate under the N-TiO2 NCs/TNTAs-AC/PTFE composite presence was at pH = 8, and 69% of carbamazepine was removed within 180 min of the constructed wetland water treatment at pH = 7.8. The PEC system containing modified (with nano N-TiO2) TiO2 nanotubes as an anode and activated carbon as a photocathode can effectively decompose carbamazepine in the constructed wetlands.

1. Introduction

Pharmaceuticals and personal care products (PhCs and PCPs, respectively) are often detected worldwide in the environment, including water [1,2,3]. The annual production of PhCs and PCPs is several hundred tons [4], and their consumption constantly increases [5,6,7]. Even with strict environmental regulations, these substances still end up in the environment, either through human excretions by simply being thrown away by consumers or disposed by corporations because of their expiration or lack of use [8]. Ultimately, these chemicals end up in effluents of wastewater treatment plants or constructed wetlands and eventually in rivers and lakes, seas and oceans [9]. As a result, they are detected in waste-, ground-, surface- and even drinking waters [1,10,11,12].
Wastewater treatment plants (WWTPs) and constructed wetlands are not efficient enough to neutralize PhCs [10,13]. Consequently, the discharge of wastewater effluents is the main PhC “supplier” to the environment [14,15,16]. PhCs are often detrimental to water organisms, which is widely discussed in the scientific literature.
Carbamazepine (CBZ), 5H-dibenzo[b,f]azepine-5-carboxamide, is a drug used to treat trigeminal neuralgia, epilepsy and psychiatric conditions. However, its presence in the environment was shown to severely affect embryonic cells, as well as digestive and central nervous systems, even at low concentrations [17,18,19,20]. Its presence in rivers around Madrid was detected in 70% of the sampling sites [21]. The range of CBZ concentration was 20–1160 ng/L, with an average of ~82 ng/L. Even such small contents are toxic to some organisms [22]. Thus, efficient removal of CBZ from the environment, especially from waters, is needed. CBZ can be removed using techniques involving bacteria or fungi, as well as by sorption-, oxidation- and reduction-based methods [23,24,25]. Nevertheless, all these methods could not completely eliminate CBZ, especially on a large scale, because of their high energy consumption, as well as complex, bulky and equipment-involved preparation procedures. The photocatalytic degradation of organic pollutants using solar energy is very promising, because it is inexpensive, does not produce any toxic or environmentally dangerous by-products and is also very efficient in decomposing organics.
Titanium dioxide (TiO2) is well known for its non-toxicity, excellent chemical stability and low cost. These and other excellent properties promote its widespread application in photocatalysis [26,27,28]. Currently, a lot of research efforts are focused on enhancing the photocatalytic performance of TiO2 even further. However, the application of TiO2 is limited by many disadvantages. First, the spectral response range of the TiO2 photocatalyst is limited to wavelengths shorter than 387 nm [29], which belong to the UV range of sunlight. TiO2 can only utilize a small part of the solar spectrum. Recent studies have shown that TiO2 nanocrystals (NCs) doping, e.g., S, F, C and I [30,31,32,33] could improve the TiO2 response in the visible light. Second, the rapid recombination of electron hole pairs on the surface of the TiO2 photocatalyst restricted the widespread application of TiO2 photocatalysis technology. How to effectively inhibit the recombination rate of photocarriers and improve the photocatalytic activity have become a research hotspot. Photoelectric catalysis (PEC) [34] is a catalytic oxidation technology using photoelectric synergy technology. It effectively inhibits the recombination of electron hole pairs, degrades quickly, has little selectivity, consumes little energy and has no secondary pollution [35]. Recently, TiO2 nanotube arrays (TNTAs) were reported to show higher photoelectron catalytic activity than immobilized TiO2 films [36]. Activated carbon (AC) is considered to be an ideal cathode material for the PEC system [37]. The PEC system has been applied for organics degradation [38,39], and N-S-TiO2/AC has realized excellence in photoelectrocatalysis in experiments [37]; however, there was little research on carbamazepine removal from constructed wetlands. Therefore, it was necessary to study carbamazepine degradation of constructed wetlands.
VASP (Vienna ab initio Simulation Package, 5.4.4) is a software package developed by the Hafner group at the University of Vienna for electronic structure calculation and quantum mechanics–molecular dynamics simulation. It is one of the most popular commercial softwares in material simulation and computational material science [40]. Many researchers use VASP to simulate electronic migration, band migration and optical properties of materials [41,42]. VASP (version number is 5.4.4) calculation has advantages for crystalline materials with repetitive structures, such as TiO2 and X-doped TiO2.
Atrazine has been degraded by a PEC system of N-S-TiO2/AC in previous studies with good performance [37]. In a subsequent study, it was found that N-TiO2 was more effective than N-S-TiO2; in that study, it was used to treat atrazine from the riparian zone with PEC [37]; in this paper, it was used to treat carbamazepine in the wetland, and in this study, the DRS assay was added in order to better demonstrate its catalytic effect. In this paper, VASP, XPS and DRS were used to filter the N- and S-doped TiO2, and the screened elements were doped into TiO2 to form a PEC system with TATAs and AC. We studied the degradation of carbamazepine by the PEC system. To our knowledge, carbamazepine has not been studied in the N-doped PEC system. This study can provide new ideas for the degradation of carbamazepine and also provide the basis for the composition of the PEC system.
This study aims to explore the degradation of carbamazepine in constructed wetlands using a novel PEC system. The methodology involves doping TiO2 with nitrogen (N) to enhance its photocatalytic activity under visible light. The PEC system is then integrated with TNTAs and AC to maximize efficiency. Advanced simulation tools such as VASP, along with experimental techniques like X-ray photoelectron spectroscopy (XPS) and diffuse reflectance spectroscopy (DRS), are employed to analyze and optimize the doped TiO2.
To our knowledge, the novelty lies in the integration of doped TiO2 with TNTAs and AC, coupled with the application of PEC technology, to achieve high degradation efficiency. This study provides new insights into the degradation mechanisms of carbamazepine and offers a potential solution for mitigating pharmaceutical pollutants in aquatic environments. The findings could pave the way for developing more effective and sustainable water treatment technologies.

2. Materials and Methods

2.1. Materials

5H-dibenzo[b,f]azepine-5-carboxamide (carbamazepine, CBZ), hydrofluoric acid (HF), tetrabutyl titanate (TiO(Bu)4), nitric acid (HNO3), thiourea (CS(NH2)2) and absolute ethanol (EtOH) were all of analytical grade, purchased from Sinopharm Chemical Reagent Co and used as received. We used distilled water (DI) for all tests. The concentration of carbamazepine in the constructed wetland samples (collected in Qinggang, China) was 125 µg/L.

2.2. Quantum Chemical Simulation

Calculations related to the local structural relaxations and electronic properties were performed using density functional theory (DFT) within the generalized gradient approximation (GGA) of the VASP code. The 3d34s1, 2s22p2, 2s22p3 and 3s23p4 atomic orbitals were treated as valence states for Ti, O, N and S, respectively. The cut-off energy for the expansion of wavefunctions into plane waves was set at 500 eV. According to the Monkhorst-Pack scheme, 5 × 5 × 1 Γ-centered k-point meshes were selected for the structural optimization and electronic properties calculations, and 9 × 9 × 1 k-point grids were used during the optical spectrum simulations. Absorption spectra of pure and doped (with S and N) TiO2 were simulated.

2.3. Fabrication of the N-TiO2 NCs/TNTAs Photoelectrodes

Highly ordered TiO2 nanotube arrays (TNTAs) were obtained using a two-electrode configuration consisting of Ti and Pt foil as an anode and as a cathode, respectively. Ti foil was ultrasonicated for 10 min in a mixture of ethanol, acetone and DI water. After the foil was dried at room temperature (RT), it was then soaked for 30 s in HF/HNO3 mixture (at 1:4 volume ratio, diluted in equal amount of water). After the Ti foil was rinsed with DI water, it was anodized for 2 h at 20 V, first in 0.5 wt% NH4F and then in 40 vol% glycerol aqueous solutions. The resulting anodized TiO2 nanotube arrays were rinsed with DI water and dried first at RT and then at 70 °C for 4 h.
Yellow Ti4+ precursor sol was prepared as described elsewhere [43]. A mixture containing 10 mL of absolute EtOH, 12 mL of HNO3 (diluted with DI water at a 1:5 volume ratio) and thiourea was added dropwise to a beaker containing 40 mL of absolute EtOH and 10 mL of Ti(OBu)4 under constant vigorous stirring. Photoelectrodes based on N-doped TiO2 NC-decorated TNTAs (N-TiO2 NCs/TNTAs) were prepared using the evaporation-induced self-assembly (EISA) technique [44,45]. After 120 min of constant stirring, as-prepared amorphous TiO2 nanotube arrays were mixed with yellow Ti4+ precursor sol for 30 min and then washed with DI water. This step was repeated three times; after which, the modified TNTA electrodes were dried first for 4 h at 70 °C and then heated for 2 h at 500 °C.

2.4. Cathode Preparation

Activated carbon was immersed for 48 h in 40% NaOH solution; after which, it was rinsed with DI water (until the pH became equal to ~7) and then kept in a dry box until further use. An AC/PTFE cathode was prepared as described by [43] using the following steps. As-obtained dry AC powder was first sieved to 50 mesh, and 0.45 g of the resulting powder was mixed in a beaker placed in a water bath (kept at 333 K) with various amounts (1.125, 1.5 and 2.25 g) of 10 wt% PTFE latex under vigorous stirring. The resulting paste was then mixed with absolute ethanol, which was added dropwise for further dispersion. The resulting suspension was then rolled with the 60-mesh stainless steel powder (used as a support) using a MT-10-160 machine (Xiongji Machine Factory, Zhongshan, China). The final product was an AC/PTFE composite electrode.

2.5. Characterization

Phase compositions of N-TiO2 NC and N-TiO2 NC/TNTA materials were analyzed by X-ray diffraction (XTR) performed using a Rigaku D/Max IIIB instrument (Wilmington, MA, USA) operated using Cu K radiation as an X-ray source. Samples were also studied by field emission scanning electron microscopy (FE-SEM) performed by Quanta 200F and Ultra 55 (ZEISS, Oberkochen, Germany) instruments. Chemical composition was determined using energy-dispersive spectroscopy (EDS) conducted by an IE450X-Max80 (Oxford, UK) instrument. X-ray photoelectron spectroscopy (XPS) measurements were performed by the PHI-5700 instrument (Physical Electronics, Chanhassen, MN, USA) using a monochromatic Al Kα source and a charge neutralizer. Photoluminescence (PL) tests were performed to determine the hydroxyl (•OH) radical yield at the photo-illuminated TNTAs/water interface and to understand the properties of TNTAs, N-TiO2 NCs/TNTAs–AC/PTFE and TNTAs–AC/PTFE. PL spectra were collected using the FP-6500 (Jasco Inc., Easton, MD, USA) instrument. The UV–Vis diffuse reflection spectrum (DRS) of the samples were recorded with a Model Shimadzu UV-2550 spectrophotometer equipped with an integrating sphere and used BaSO4 as the reference.

2.6. Photoelectrochemical (PEC) Measurements

Photoelectrochemical properties of N-TiO2/TiO2 NTs-AC/PTFE PEC, TiO2 NTs-AC/PTFE PEC and TiO2 NTs PC were obtained using the standard three-electrode configuration. Measurements were performed using a PQSTA128N electrochemical workstation. TNTAs served as a photoanode [46].

2.7. PC and PEC Performances

CBZ concentrations were measured by the HPLC Shimadzu LC-2000 instrument (Tokyo, Japan) using a 5TC-C18 column (250 × 4.6 mm, 5 μm) manufactured by Agilent (Santa Clara, CA, USA). The UV detector was operated at the 230 nm excitation wavelength. The mobile phase, consisting of acetonitrile and water (at a 55:45 volume ratio), was passed through the column at a 0.8 mL/min constant flow rate. The injection volume was 100 μL. An external standard was used to determine the CBZ contents.

3. Results

3.1. Filtering of Mixed Elements

3.1.1. VASP Calculation

The photocatalytic performance of TiO2 is affected by its structure. Figure 1 shows Ti coordination in TiO2 for ideal and for S- and N-doped lattices. The software used the (221) plane of the anatase structure. The simulations indicated that the N atom substituted O in the lattice, and S substituted Ti. These lattice configurations were used to simulate the corresponding absorption (see Figure 2a–c, respectively). The simulations demonstrated that the maximum absorbance of N-TiO2 was at 143, 206, 270 and 780 nm in the UV region, while its average absorbance in the visible region was 10,000 higher than undoped TiO2. The absorption signal of S-TiO2 was determined to be higher than of TiO2. N-TiO2 demonstrated the strongest absorption; thus, it is very likely that it will exhibit the best photocatalytic properties.

3.1.2. XPS and DRS Analysis

In order to select a better catalytic-doped element, XPS of N-TiO2 and S-TiO2 were carried out to study the physical and chemical state and quantitative composition of the elements, as shown in Figure 3. Figure 3c showed the valence state distribution of S in S-TiO2. After fitting the curve, the binding energy in 167.1 eV and 170.2 eV was observed in two distinct peaks, indicating that the S in the TiO2-doped S6+ [47] transformed into S4+ [48]. This phenomenon is related to the environment in which the catalyst is prepared [49]. Table 1 lists the measured XPS data of the S-TiO2 catalyst, and it can be seen that element S accounts for 0.84% of S-TiO2 while element N accounts for 0.76% of N-TiO2. Figure 3b showed the valence state distribution of N in N-TiO2: two peaks were observed in the N1s, 401.2 and 399.6 eV, respectively. In the 401.2 eV peak, the state of the N is Ti-O-N or Ti-N, indicating that N replaced TiO2 in crystal O [50]. At 399.6 eV, the N was replaced by N in the state of the anion [51]. This phenomenon shows that N is replaced by O in TiO2 crystal, and this phenomenon has been studied in previous studies [37,49].
The optical properties of the TiO2, S-TiO2 and N-TiO2 samples were measured by UV–Vis diffuse reflectance spectra. The results are shown in Figure 4a, and all samples showed a typical absorption in the UV region because of the intrinsic band gap absorption of TiO2. In the visible light region, the absorbance of all the samples changed, especially that of N-TiO2, N-TiO2 and S-TiO2, shifting to the visible-light region, and the optical band edge exhibited a remarkable red shift with respect to that of TiO2, indicating electron promotion from the valence band to conduction band in N-TiO2 and S-TiO2 at the visible light region. Additionally, the Kubelka–Munk function [52] was used to calculate the band gap energies of the as-synthesized TiO2 samples by plotting [F(R)·E]1/2 vs. energy of light, and the results are shown in Figure 4b. The band gap energies were 2.73, 3.04 and 3.21 eV for N-TiO2, S-TiO2 and TiO2, respectively, indicating that the band gap of TiO2 was narrowed by N doping and S doping, and N doping showed better performance.
Combining the results of VASP, XPS and DRS, both N doping and S doping could improve the photocatalytic performance of TiO2, and N doping has a better photocatalytic performance than S doping. N doping TiO2 was used as the doped element in photoelectric catalysis.

3.2. Morphology of N-TiO2 NTs Photoelectrode as a Function of Its Fabrication Parameters

The morphology of TiO2-based nanomaterials often defines their catalytic properties. Therefore, we analyzed N-TiO2 NCs/TNTAs and N-TiO2 NCs by SEM (see Figure 5a). Numerous uniformly distributed N-TiO2 nanocrystals, 10–15 nm in diameter, are clearly visible on the surface of the TiO2 nanotubes (see Figure 5a,b). The surface of the NaOH-treated AC showed large 10 µm pores (Figure 5c), which should facilitate the (photo)catalytic performance. The SEM of the AC-PTFE composite electrode showed that AC is mostly distributed on the electrode surface (see Figure 5d). Oxygen in the air would catalytically decompose to forms H2O2 and •OH according to previous studies [43,53].
EDS showed that the N-TiO2 NCs/TNTAs—AC nanocomposite contained 46.21 wt% of C, 1.31 wt% of N, 35.16 wt% of O and 17.5 wt% of Ti. Elemental mapping confirmed the uniform distribution of these elements on the AC surface.

3.3. XRD Analyses

The XRD spectra of TiO2 and N-TiO2 corresponded to the anatase structure according to the JCPDS card number 1286-84 (see Figure 3a). No changes in the XRD peak positions were observed for the N-doped TiO2, very likely because of the low dopant content, which agrees with the previously reported results [43,45,54]. The intensity of the N-TiO2/TNTA XRD peaks was higher than of the TNTA peaks because of the presence of N-TiO2 nanocrystals on the TNTA surface. The XRD results agreed well with the data obtained by FE-SEM (Figure 3a).

3.4. Photoelectrochemical Properties

Understanding a material’s electrochemical properties is important to assess its photocatalytic efficiency. Thus, we tested the separation of photogeneration of electron and hole (e−/h+) pairs with TiO2 NT and N-TiO2 NT electrode assistance. The photocurrent densities of N-TiO2 NCs/TiO2 NT and S-TiO2 NCs/TiO2 NT photoelectrodes are shown in Figure 6a. Photoelectrodes based on TiO2 NTs, N-TiO2 NCs/TiO2 NTs and S-TiO2 NCs/TiO2 NTs showed reproducible and stable performances under intermittent light illumination by the Xenon lamp. N-TiO2 NC/TiO2 NT electrodes showed a transient photocurrent response with a photocurrent density equal to 7.48 mA/cm2, which is ~1.46 and 1.2 times higher than the photocurrent densities of TiO2 NCs/TiO2 and S-TiO2 NCs/TiO2 NT photoelectrodes (which are equal to 5.12 and 6.23 mA/cm2), respectively. Thus, the N-TiO2 NCs/TiO2 NTs photoelectrode could provide a higher PC and separation efficiency of photogenerated charge carriers than other photoelectrodes, which agrees with our quantum chemical calculations of absorbance. The high photocurrent density of the PEC system based on N-TiO2 NCs/TiO2 NTs(anode)-AC/PTFE(cathode) agrees with the data reported in the literature [53].

3.5. PL Properties

The hydroxyl radical is a major participant in the PC reaction [55,56]. Typically, the PL intensity correlates with the number of generated •OH radicals. Therefore, to understand the PEC and PC activities of the TiO2 NTs, S—TiO2/TiO2 NTs, N—TiO2/TiO2 NTs and N—TiO2/TiO2 NTs (anode)—AC/PTFE (cathode) systems, we measured the formation rate of •OH radicals at the photo-illuminated TiO2 NTs/water interface using fluorescence of 2-hydroxy terephthalic acid (TAOH), which forms upon •OH reaction with terephthalic acid (TA) [57]. Figure 6b shows PL intensities of TAOH catalytically produced from 0.5 mM TA solution 30 min after the system was exposed to light. A strong PL peak was observed at ~425 nm for the systems containing N-TiO2/TiO2 NT and S-TiO2/TiO2 NT catalysts. However, the PL intensity of the system containing N-TiO2/TiO2 NTs was higher than the system containing S-TiO2/TiO2 NTs, which agrees with our quantum chemical computation results and with the experimentally determined photoelectrochemical properties. The system containing N—TiO2/TiO2 NTs (anode)—AC/PTFE (cathode)-PEC demonstrated the highest PL intensity, indicating that more •OH formed and that this active material configuration has excellent catalytic efficiency.

3.6. Influence of pH

The photoelectric catalysis efficiency of TiO2-based materials strongly depends on the pH [58]. Additionally, even though CBZ is a neutral molecule, the pH affects its photoelectric catalytic degradation [59]. Thus, the photoelectric catalytic activity of N-TiO2 NCs/TiO2 NTs—AC/PTFE towards 3 mg/L CBZ was tested at pH values equal to 5, 7, 8 and 9 for 75 min. It was reported that CBZ degradation induced by free radicals is not affected by changing the solution pH in the 2–8 range, because the ionized state of the photoelectric catalyst surface can be protonated and deprotonated under acidic and alkaline conditions, respectively [59,60]. At acidic pH values, positively charged holes are the major oxidation species, while •OH radicals dominate oxidation reactions at neutral or high pH values [61,62]. More •OH is expected at high amounts of hydroxyl ions present on the catalyst surface. Therefore, the efficiency of catalytic CBZ degradation is expected to increase at high pH values. The data shown in Figure 6c on CBZ degradation at different pH values confirm this assumption: the amount of the decomposed CBZ increased in the 5–8 pH range. CBZ decomposition slowed down above pH = 8. These results agree with previous studies [60].

3.7. PC- and PEC-Based Degradation of Carbamazepine in Water Samples Collected from Constructed Wetland

The CBZ content and pH value on the sample collected from the Qinggang constructed wetland were 0.121 mg/L and 7.8, respectively. CBZ degradation experiments were performed under simulated sunlight irradiation by a neon lamp (see Figure 6d). The system containing the N-TiO2 NCs/TiO2 NTs-AC/PTFE-PEC catalyst showed the fastest CBZ degradation rate, followed by N-TiO2 NCs-PC, S-TiO2 NCs-PC and TiO2 NTs-PC catalysts. We believe that the CBZ degradation rate correlated with the •OH concentration, which is clearly visible in Figure 6b. The CBZ degradation in these systems showed the same trend as the •OH concentration. CBZ decomposition assisted by the N-TiO2 NCs/TiO2 NTs-AC/PTFE PEC catalyst rapidly accelerated during the first 30 min, which can be explained by a relatively high initial CBZ content [63]. After 30, 60, 90, 120, 150 and 180 min, 81.7, 68.3, 55.3, 44.6, 36.5 and 31.0% of CBZ remained, respectively. Overall, 69% of CBZ was decomposed during the first 180 min of the reaction catalyzed by N-TiO2 NCs/TiO2 NTs-AC/PTFE-PEC, which agrees with previously reported results [60,64,65,66].
The CBZ removal rate reported in the literature was somewhat higher, mostly because those experiments were performed only under UV irradiation [67]. This is expected, because the absorption rate of TiO2 in the UV region is higher than in the visible (see Figure 2). Kumar et al. (2019) reported that over 71% of CBZ was decomposed during the 9 h reaction [68]. The PEC system obtained for the N-TiO2 NCs/TiO2 NTs (anode)—AC/PTFE (cathode) was the best among all the others tested in this work. Therefore, we considered this system as the most promising.
The mechanism of the processes occurring in the PEC system and assisted by the N-TiO2 NCs/TiO2 NT (anode)—AC/PTFE (cathode) catalyst is schematically shown in Figure 7. N-TiO2 NCs as a photoanode active material absorb and convert more solar energy to chemical energy than S-doped and undoped TiO2 (Figure 3). Photoelectric chemical reactions occurring at the photoanode can be presented as shown below.
O2 + 2H+ + 2e− → H2O2
O2 + 2H2O + 2e− → H2O2 + 2OH−
H2O2 + OH− → H2O + HO2−
H2O + HO2− → •OH + •O2− + H2O
Upon sunlight irradiation and catalytical influence, O2 and H2O transform into H2O2, •OH and •O2−. Thus, a significant amount of OH− accumulates at the photoanode (see Equations (5) and (6), which decreases the PC efficiency [69]. Application of an external voltage can accelerate OH- transfer and enhance PC efficiency [70]. Our PEC system, in which N-TiO2 NCs/TiO2 NT was the anode and AC/PTFE was the cathode, absorbed a substantial amount of visible (400–700 nm) light and demonstrated high transient PCR (equal to 0.115 mA/cm2) and OCP (equal to −0.312 mV/cm2) [45]. Photoelectrodes could generate a certain voltage to facilitate OH− transfer. AC/PTFE could catalyze O2 into H2O2 and •OH, improving PC performance. Thus, the catalyst would degrade carbamazepine into H2O-, CO2- and N-related inorganic ions [71].
Reaction occurring on a photoanode: 4OH− + 4e− → O2 + 2H2O2
Reaction occurring on a cathode: 4H2O + 4e− → 2H2 + 4OH−

4. Conclusions

We used VASP, XPS and EDS to screen which doping elements would provide the best photocatalytic performance of TiO2. The results showed that N-doped TiO2 would perform the best. Most CBZ was degraded at pH = 8.0 when N-TiO2 NCs/TiO2 NT was used as a catalyst, and 69% of CBZ was removed after 180 min of exposure of the CBZ-containing solution to the PEC system consisting of N-TiO2 NCs/TiO2 NT anode and activated carbon as a cathode at pH = 7.8. This system was then used to test CBZ removal in real-life samples collected from constructed wetlands.
The PEC system containing N-TiO2 nanocrystal-modified TiO2 nanotubes and activated carbon as electrodes is very promising for the removal of carbamazepine from the constructed wetlands. The method proposed in this work is inexpensive and uses widespread materials, as well as solar energy; thus, it can be expanded to large-scale decontamination efforts to remove other organic pollutants from water reservoirs.

Author Contributions

Conceptualization, X.L. and S.Y.; Methodology, X.L.; Software, X.L.; Validation, X.L., S.Y. and B.M.; Formal Analysis, X.L.; Investigation, X.L.; Resources, X.L.; Data Curation, X.L.; Writing—Original Draft Preparation, X.L.; Writing—Review and Editing, X.L., S.Y., B.M., J.L., C.Y., C.S. and J.D.; Visualization, X.L.; Supervision, S.Y.; Project Administration, S.Y.; Funding Acquisition, S.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This article was funded by the Heilongjiang Provincial Natural Science Foundation of China (LH2021E096 and LH2023C066).

Data Availability Statement

Data are available through request to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Separations 11 00216 g001
Figure 1. The structures of TiO2 (a), N-TiO2 (b) and S-TiO2 (c).
Figure 1. The structures of TiO2 (a), N-TiO2 (b) and S-TiO2 (c).
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Separations 11 00216 g002
Figure 2. Absorption spectrum curves of TiO2 and N-TiO2 (a), S-TiO2 and N-TiO2 (b) and TiO2 and S-TiO2 (c).
Figure 2. Absorption spectrum curves of TiO2 and N-TiO2 (a), S-TiO2 and N-TiO2 (b) and TiO2 and S-TiO2 (c).
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Separations 11 00216 g003
Figure 3. XRD patterns of N -TiO2/TNTAs and TNTAs (a) and the results of the XPS analysis (b,c).
Figure 3. XRD patterns of N -TiO2/TNTAs and TNTAs (a) and the results of the XPS analysis (b,c).
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Separations 11 00216 g004
Figure 4. The DRS analysis of TiO2, N-TiO2 and S-TiO2 (a) and the band gap energies of TiO2, N-TiO2 and S-TiO2 (b).
Figure 4. The DRS analysis of TiO2, N-TiO2 and S-TiO2 (a) and the band gap energies of TiO2, N-TiO2 and S-TiO2 (b).
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Separations 11 00216 g005
Figure 5. FE-SEM images of N-TiO2 NCs/TNTAs (a), N-TiO2 NCs (b), active carbon (c), N-TiO2 NCs/TNTAs—AC/PTFE electrodes (d) and EDX analysis of N-TiO2 NCs/TNTAs—AC (e).
Figure 5. FE-SEM images of N-TiO2 NCs/TNTAs (a), N-TiO2 NCs (b), active carbon (c), N-TiO2 NCs/TNTAs—AC/PTFE electrodes (d) and EDX analysis of N-TiO2 NCs/TNTAs—AC (e).
Separations 11 00216 g005
Separations 11 00216 g006
Figure 6. The results of the photocurrent density (a), photoelectron catalytic system, and photocatalytic system on the fluorescence intensity (b), CBZ degradation under different pH conditions (c), and CBZ degradation under visible light in four different systems (d).
Figure 6. The results of the photocurrent density (a), photoelectron catalytic system, and photocatalytic system on the fluorescence intensity (b), CBZ degradation under different pH conditions (c), and CBZ degradation under visible light in four different systems (d).
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Separations 11 00216 g007
Figure 7. The mechanism of photoelectric catalysis.
Figure 7. The mechanism of photoelectric catalysis.
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Table 1. XPS element analyses of TiO2, N-TiO2 and S-TiO2 catalysts.
Table 1. XPS element analyses of TiO2, N-TiO2 and S-TiO2 catalysts.
SamplesElement Amount(%)n(Ti)/n(O) (%)
TiONS
TiO226.3773.630035.81
S-TiO225.5073.6600.8434.61
N-TiO225.5373.720.76034.63
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Liang, X.; Yu, S.; Meng, B.; Liu, J.; Yang, C.; Shi, C.; Ding, J. Enhanced Degradation of Carbamazepine from Constructed Wetlands with a PEC System Based on an Anode of N-TiO2 Nanocrystal-Modified TiO2 Nanotubes and an Activated Carbon Photocathode. Separations 2024, 11, 216. https://doi.org/10.3390/separations11070216

AMA Style

Liang X, Yu S, Meng B, Liu J, Yang C, Shi C, Ding J. Enhanced Degradation of Carbamazepine from Constructed Wetlands with a PEC System Based on an Anode of N-TiO2 Nanocrystal-Modified TiO2 Nanotubes and an Activated Carbon Photocathode. Separations. 2024; 11(7):216. https://doi.org/10.3390/separations11070216

Chicago/Turabian Style

Liang, Xiongwei, Shaopeng Yu, Bo Meng, Jia Liu, Chunxue Yang, Chuanqi Shi, and Junnan Ding. 2024. "Enhanced Degradation of Carbamazepine from Constructed Wetlands with a PEC System Based on an Anode of N-TiO2 Nanocrystal-Modified TiO2 Nanotubes and an Activated Carbon Photocathode" Separations 11, no. 7: 216. https://doi.org/10.3390/separations11070216

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