Photocatalytic efficiency toward volatile organic compounds (VOCs) decomposition has crucially relied on the nature of their stereochemical structures, in which the complicated decomposition mechanism has not been unveiled. As typical cases of VOCs pollutants, m-, p-, and o-xylene isomers share the identical molecular formula with discrepant methyl positions at the benzene ring. The essential contribution of the methyl position to the decomposition mechanism of xylene isomers, especially the rate-determining step for benzene ring-opening, is unraveled in this work. It is identified that the decomposition rate of xylene isomers on the SnO2 catalyst is decreased in the order of o-xylene > m-xylene ≈ p-xylene. The durability of SnO2 photocatalyst is also accomplished for a superior o-xylene decomposition performance. By combining the experimental and theoretical investigation, it is manifested that the regulation of methyl positions in the ortho-sites is an appealing route for reducing the ring-opening energy barriers and guiding the complete mineralization of the hazardous xylene. This work could provide insights into unraveling the unique role of the stereochemical structure of xylene on ring-opening barriers for efficient and stable VOC decomposition.
Fe2O3, as an earth-abundant photocatalyst for water purification, has attracted great attention. However, the high-spin FeIII in traditional Fe2O3 restricts its catalytic performance. In this work, based on the nanocrystal size alteration strategy, cubic Fe2O3 nanoclusters (3–4 nm) with low-spin FeIII were successfully anchored on six-fold cavities of the supramolecular condensed g-C3N4 rod (FCN) through the impregnation-coprecipitation method. FCN showed high photocatalytic activity, as the d band center of Fe 3d orbital (−1.79 eV) in low-spin FeIII shifted closer to Femi level, generating a weaker antibonding state. Then, the enhanced bonding state strengthened the interaction between Fe and O, further accelerating the charge carrier separation and enhancing its ability to capture OH−. Thus, low-spin FeIII enhanced the production of dominant reactive oxygen species (•OH/•O2−), promoting diclofenac photocatalytic degradation under solar light, with a kinetic rate constant (0.206 min−1) of 5 times compared with that of pristine g-C3N4.
Persulfate-based advanced oxidation processes (persulfate-AOPs) offer great promise for environmental remediation, with heterogeneous catalysts providing the backbone of many wastewater purification technologies. Unlike conventional nanocatalyst heterogeneous systems, the immobilized-catalyst system can bypass the separation problem to reduce scour and prevent aggregation by anchoring nanoparticles onto porous or large-particle carriers. This review presents the state-of-the-art of knowledge concerning immobilization methodologies and reactors, reaction mechanisms, and activation performance. Immobilization techniques onto supports are summarized and discussed, including membrane-based reaction systems (immersion mode, and filtration mode), electrocatalytic auxiliary systems, and alternative supports (metallic glasses, aerogels, hydrogels, and specific materials). Key scientific problems and important prospects for the further development of immobilized catalysts are outlined.
Fe3(PO4)2·8H2O (Vivianite) is one of the potential phosphorus recovery products from wastewater treatment plant (WWTP). In this study, we first discovered that vivianite can effectively photoactivate peroxodisulfate (PDS) to produce some reactive oxygen species (ROS) for tetracycline antibiotics (TCs) degradation. The results demonstrated that vivianite could efficiently activate PDS to achieve 100% removal of TCs under LED UV light (UVL), visible light (VL) or real solar light (SL) irradiation within 10 min, respectively. More importantly, ca. 80%, 78% and 40%∼58% of TOC removal efficiencies were achieved under UVL, VL and SL irradiation within 30 min, respectively. As well, toxicological simulation and antibacterial studies showed that the aquatic toxicity of the TCs intermediates was lower than those of the original TCs. This work provided new insights into the application of photoactivated sulfate radical-advanced oxidation process (SR-AOP) for organic pollutants degradation over vivianite, which may encourage the recovery and utilization of vivianite in the wastewater treatment process.
In this study, vanadium trioxide (V2O3) was adopted to activate PMS via a Fenton-like reaction to degrade metronidazole (MNZ). The V2O3-PMS system can almost completely degrade MNZ at 30 min with 42.4% TOC removal. Comparative tests reveal that V2O3 stands out among a variety of heterogeneous catalysts, including metallic oxides and carbon materials. Sulfate radicals (SO4•−) and hydroxyl radicals (•OH) derived from PMS decomposition are major reactive oxygen species, based on quenching tests, electron spin resonance (ESR) analysis, the steady-state concentrations of radicals ([SO4•−]ss = 5.1 × 10-13 M and [•OH]ss = 4.0 × 10-14 M), and kinetics model. The process of stepwise electron transfer from vanadium species to PMS to produce reactive radicals was proved by small-molecule simulation experiments and pickling experiments of vanadium oxides. Possible pathways of MNZ degradation were proposed based on the results of LC-MS and Fukui function, including two stages of the hydroxylation and bond cleavage of nitro and the subsequent ring-opening. This study reveals the high reusability and practicability of the V2O3-PMS system over a relatively wide pH range, which puts forward a new vision on V2O3 induced Fenton-like reactions and a new reference method for the removal of medical organic contaminants in water.
In this study, the previously overlooked effects of contaminants’ molecular structure on their degradation efficiencies and dominant reactive oxygen species (ROS) in advanced oxidation processes (AOPs) are investigated with a peroxymonosulfate (PMS) activation system selected as the typical AOP system. Averagely, degradation efficiencies of 19 contaminants are discrepant in the CoCaAl-LDO/PMS system with production of SO4•–, •OH, and 1O2. Density functional theory calculations indicated that compounds with high EHOMO, low-energy gap (ΔE = ELUMO – EHOMO), and low vertical ionization potential are more vulnerable to be attacked. Further analysis disclosed that the dominant ROS was the same one when treating similar types of contaminants, namely SO4•–, 1O2, 1O2, and •OH for the degradation of CBZ-like compounds, SAs, bisphenol, and triazine compounds, respectively. This phenomenon may be caused by the contaminants’ structures especially the commonly shared or basic parent structures which can affect their effective reaction time and second-order rate constants with ROS, thus influencing the contribution of each ROS during its degradation. Overall, the new insights gained in this study provide a basis for designing more effective AOPs to improve their practical application in wastewater treatment.
Accurate identification of radicals in advanced oxidation processes (AOPs) is important to study the mechanisms on radical production and subsequent oxidation-reduction reaction. The commonly applied radical quenching experiments cannot provide direct evidences on generation and evolution of radicals in AOPs, while electron paramagnetic resonance (EPR) is a cutting-edge technology to identify radicals based on spectral characteristics. However, the complexity of EPR spectrum brings uncertainty and inconsistency to radical identification and mechanism clarification. This work presented a comprehensive study on identification of radicals by in-situ EPR analysis in four typical UV-based homogenous AOPs, including UV/H2O2, UV/peroxodisulfate (and peroxymonosulfate), UV/peracetic acid and UV/IO4− systems. Radical formation mechanism was also clarified based on EPR results. A reliable EPR method using organic solvents was proposed to identify alkoxy and alkyl radicals (CH3C(=O)OO·, CH3C(=O)O· and ·CH3) in UV/PAA system. Two activation pathways for radical production were proposed in UV/IO4− system, in which the produced IO3·, IO4·, ·OH and hydrated electron were precisely detected. It is interesting that addition of specific organic solvents can effectively identify oxygen-center and carbon-center radicals. A key parameter in EPR spectrum for 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin adduct, AH, is ranked as: ·CH3 (23 G) >·OH (15 G) >IO3· (12.9 G) >O2·− (11 G) ≥·OOH (9–11 G) ≥IO4· (9–10 G) ≥SO4·− (9–10 G) >CH3C(=O)OO· (8.5 G) > CH3C(=O)O· (7.5 G). This study will give a systematic method on identification of radicals in AOPs, and shed light on the insightful understanding of radical production mechanism.
The massive use of antibiotics has led to their omnipresence in aquatic environments, and the photodegradation was found to be the dominant transformation process for antibiotics in the natural river system. Herein, we investigated the photodegradation kinetics of 77 antibiotics in 7 classes in water under simulated sunlight. Using the quantum chemical descriptors predicted by the density functional theory calculation, the quantitative structure-activity relationship (QSAR) models were established to explore the main chemical descriptors determining the photodegradation of antibiotics. The results showed that the photodegradation kinetics of antibiotics conformed to the pseudo-first order kinetic model. The photodegradation rate constants of different antibiotics varied 4 orders of magnitude, and the photodegradation rate constants of quinolones were significantly higher than those for other classes of antibiotics due to the F atoms in their molecular structures. The developed QSAR models revealed that the energy gap (Egap) between ELUMO and EHOMO was the main chemical descriptor determining the photodegradation of antibiotics, and it was negatively correlated with lgk. In addition, the number of F atom was also included in the QSAR models due to the great contribution of F atom to the direct photolysis of quinolones. This study ordered the photodegradation rate constants of 77 antibiotics, and revealed the major chemical descriptors determining the photodegradation of antibiotics. The results provide the basic information for the photolysis of antibiotics, which is significant for predicting the environmental behaviors and evaluating the ecological risks of antibiotics in aquatic environments.
Photocatalysis is an efficient technology for water decontamination and purification. Development of photocatalysts with high activity becomes the key to this research area. In recent years, titanate nanotubes (TNTs), derived from TiO2 nanoparticles through hydrothermal treatment with NaOH/KOH, have been attracting great attention. TNTs are composed of edge-sharing [TiO6] octahedrons as the skeleton and Na+/H+/K+ in the interlayers, which exhibit a uniform tubular microstructure, a large specific surface area, high photoelectric conversion properties, and good stability. Therefore, TNTs and their modified materials are widely used for removal of heavy metals and organic contaminants through photocatalytic oxidation or reduction. In this perspective, we systematically summarize cutting-edge research on the application of TNT-based photocatalysts in the water treatment area, illustrate the challenges for fundamental research and practical applications, and reveal the critical knowledge gaps and research needs for the future. In particular, preparation and specific properties of TNT-based photocatalysts are presented. Modification of TNTs to promote photocatalytic activity is discussed as well as their applications for contaminants removal from water. The latest advances in theoretical calculations on materials and contaminants in this photocatalysis system are clarified. In the future, strategic programs on both fundamental research and practical applications of TNT-based photocatalysts are proposed.
Peracetic acid (PAA)-based system is becoming an emerging advanced oxidation process (AOP) for effective removal of organic contaminants from water. Various approaches have been tested to activate PAA, while no previous researches reported the application of metal-organic frameworks (MOFs) materials for PAA activation. In this study, zeolitic imidazole framework (ZIF)-67, a representative MOFs, was facile synthesized via direct-mixing method at room temperature, and tested for PAA activation and sulfachloropyridazine (SCP) degradation. The as-synthesized ZIF-67 exhibited excellent performance for PAA activation and SCP degradation with 100% of SCP degraded within 3 min, owing to the specific MOFs structure and abundant Co2+ sites. The pseudo-first-order kinetic model was applied to fit the kinetic data, with rate constant k1 of ZIF-67 activated PAA system 34.2 and 156.5 times higher than those of conventional Co3O4 activated PAA and direct oxidation by PAA. Radical quenching experiments and electron paramagnetic resonance (EPR) analysis indicated that CH3C(O)OO• played a major role in this PAA activation system. Then, the Fukui index based on density functional theory (DFT) calculation was used to predict the possible reaction sites of SCP for electrophilic attack by CH3C(O)OO•. In addition, the degradation pathway of SCP was proposed based on Fukui index values and intermediates detection, which mainly included the S-N bond cleavage and SO2 extrusion and followed by further oxidation, dechlorination, and hydroxylation. Therefore, ZIF-67 activated PAA is a novel strategy and holds strong potential for the removal of emerging organic contaminants (EOCs) from water.
Reports that the exploitation of metal-free carbon materials to enhance permanganate (PM) oxidation to abate organic pollution in water have emerged in recent publications. However, the activation mechanism and active sites involved are ambiguous because of the intricate physicochemical properties of carbon. In this study, reduced graphene oxide (rGO) as a typical carbon material exhibits excellent capability to boost permanganate oxidation for removing a wide array of organic contaminants. The simultaneous two reaction pathways in the rGO/PM system were justified: i) rGO donates to electrons to decompose PM and produce highly reactive intermediate Mn species for oxidizing organic contaminants; ii) rGO mediates electron transfer from organics to PM. Oxygen-containing groups (hydroxyl, carboxyl, and carbonyl) were justified as electron-donating groups, while structural defects (vacancy and edge defects) were shown to be critical for rGO-mediated electron transfer. Therefore, the oxidation pathway of the rGO/PM system can be controlled by regulating oxygen functional groups and structural defects. The changeover from electron donor to electron mediator by decorating surface active sites of carbon materials will be of great help to the design and application of carbocatalysts.
ABSTRACT Numerous approaches have been used to modify graphitic carbon nitride (g-C3N4) for improving its photocatalytic activity. In this study, we demonstrated a facial post-calcination method for modified graphitic carbon nitride (g-C3N4-Ar/Air) to direct tuning band structure, i.e., bandgap and positions of conduction band (CB)/valence band (VB), through the control of atmospheric condition without involving any additional elements or metals or semiconductors. The synthesized g-C3N4-Ar/Air could efficiently degrade sulfamethazine (SMT) under simulated solar light, i.e., 99.0% removal of SMT with rate constant k1 = 2.696 h−1 within 1.5 h (4.9 times than pristine g-C3N4). Material characterizations indicated that the damaged/partial-collapsed structure and decreased nanosheet-interlayer distance for g-C3N4-Ar/Air resulted in the shift of band structure due to the denser stacking of pristine g-C3N4 through oxidative exfoliation and planarization by air calcination. In addition, the bandgap of g-C3N4-Ar/Air was slightly shrunk from 2.82 eV (pristine g-C3N4) to 2.79 eV, and the CB was significantly upshifted from −0.44 eV (pristine g-C3N4) to −0.81 eV, suggesting the powerful ability for donating the electrons for O2 to form •O2−. Fukui index (f –) based on theoretical calculation indicated that the sites of SMT molecule with high values, i.e., N9, C4 and C6, preferred to be attacked by •O2− and •OH, which is confirmed by the intermediates’ analysis. The tuning method for graphitic carbon nitride provides a simple approach to regulate the charge carrier lifetime then facilitate the utilization efficiency of solar light, which exhibits great potential in efficient removal of emerging organic contaminants from wastewater.
This work demonstrates the successful immobilization of MIL-88A(Fe) MOF on cotton fibers to fabricate MIL-88A(Fe)/cotton fibers (MC) by an eco-friendly method. The prepared MC is used to activate peroxydisulfate for eliminating multiple tetracycline antibiotics, such as oxytetracycline (OTC), tetracycline (TTC), and chlortetracycline (CTC) in simulated wastewater under UV-light irradiation. The photoactivated sulfate radical-advanced oxidation processes (SR-AOPs) towards the removal of tetracycline antibiotics matrix (initial concentration of 10.0 mg/L) using MC were initially investigated using a batch method. The results reveal that 97.5% OTC, 95.2% TTC, and 100.0% CTC can be degraded in the MC/UV/PDS system in the presence of 2 g/L of MC and 1 mM of PDS. The degradation pathways of OTC, TTC, and CTC were clarified via liquid chromatography-mass spectrometry analysis and DFT calculations. The quantitative structure–activity relationship analysis shows that the tetracycline antibiotics are transformed into their corresponding intermediates with lower toxicity within 8.0 min. A self-designed fixed bed reactor, in which the MC was packed into the annular channel, was adopted to test the long-term operation possibility of the MC in the continuous photoactivated SR-AOP system. The findings demonstrate that the whole antibiotics matrix can be removed completely within 22 h. This work is the first to demonstrate the use of MOFs as catalysts for SR-AOP to achieve continuous purification of simulated wastewater. The findings highlight a new possibility for the use of MOFs in large-scale wastewater treatment over.
“Concentrate-and-degrade” is an effective strategy to promote mass transfer and degradation of pollutants in photocatalytic systems, yet suitable and cost-effective photocatalysts are required to practice the new concept. In this study, we doped a post-transition metal of Indium (In) on a novel composite adsorptive photocatalyst, activated carbon-supported titanate nanotubes (TNTs@AC), to effectively degrade perfluorooctanoic acid (PFOA). In/TNTs@AC exhibited both excellent PFOA adsorption (>99% in 30 min) and photodegradation (>99% in 4 h) under optimal conditions (25 °C, pH 7, 1 atm, 1 g/L catalyst, 0.1 mg/L PFOA, 254 nm). The heterojunction structure of the composite facilitated a cooperative adsorption mode of PFOA, i.e., binding of the carboxylic head group of PFOA to the metal oxide and attachment of the hydrophobic tail to AC. The resulting side-on adsorption mode facilitates the electron (e‒) transfer from the carboxylic head to the photogenerated hole (h+), which was the major oxidant verified by scavenger tests. Furthermore, the presence of In enables direct electron transfer and facilitates the subsequent stepwise defluorination. Finally, In/TNTs@AC was amenable to repeated uses in four consecutive adsorption-photodegradation runs. The findings showed that adsorptive photocatalysts can be prepared by hybridization of carbon and photoactive semiconductors and the enabled “concentrate-and-degrade” strategy is promising for the removal and degradation of trace levels of PFOA from polluted waters.
The photochemical behavior of a model PAH, naphthalene, was investigated under simulated sunlight irradiation with different dissolved organic matter (DOM) in seawater. The results revealed that naphthalene was prone to direct photolysis (Φd = 1.34 × 10-3) and could be degraded by 3DOM*/1O2-induced reactions with fulvic acid (FA) and humic acid (HA) at low concentrations. However, the DOM at a high level dramatically decreased the kobs due to the higher light attenuation and radical competition effect. The presence of FA resulted in lower 3DOM*/1O2 generation and quantum yield compared with HA, but it achieved higher degradation kinetics due to the higher reactivity between 3FA* and naphthalene and their lower binding effect. The naphthalene degradation in natural water with different depths and DOM were modeled based on the experimental results, which revealed the important role of indirect photolysis initiated by inorganic constituents. Moreover, several degradation intermediates were identified by GC-MS and three possible pathways were proposed. The Quantitative Structure Activity Relationships (QSAR) evaluation revealed that some intermediates are more toxic than original naphthalene. This study offers further insights into the photochemical behavior of PAHs, which will facilitate our understanding of the persistence and ecological risks of organic contaminants in natural waters.
The widely spilled diclofenac (DCF) in water has attracted broad attention because of its potential environmental risk. In this work, palladium quantum dots (PQDs) deposited g-C3N4 photocatalysts (PCNs) were fabricated through a two-step process, i.e., initial thermal polymerization followed by an in-situ reduction for PQDs deposition. In addition, the synthesized g-C3N4 (43.09 m2/g) composing of ultrathin sheets had 4 times larger specific surface area than bulk g-C3N4 (8.73 m2/g), thus offered abundant sites for reaction. The optimized material (PCN2) with 1 wt% PQDs loading content achieved the highest cost-efficiency for DCF degradation, and exhibited a kinetic rate constant (k1) of 0.072 min−1, which was 8 times higher than bulk g-C3N4. The mechanisms on enhanced photocatalytic activity of PCN are interpreted as: (1) decoration of PQDs can alter the optical band structure of g-C3N4, leading to a narrowed bandgap; (2) PQDs can act as electron transfer mediator to retard the recombination of photogenerated charge carriers; and (3) a photosensitization-like electron transfer pathway occurs from highest occupied molecular orbital (HOMO) of DCF to conduction band (CB) of g-C3N4 by means of PQDs. Radical quenching experiments and electron spin resonance (ESR) analysis indicated •O2- was the primary radical for DCF degradation. Density functional theory (DFT) calculation combined intermediates identification further revealed that the Cl11 and N12 atoms with high Fukui index (f 0) were more venerable to attack. PCN2 also remained good stability after five continuous cycles for DCF degradation, showing the great potential for practical application in water treatment area.
The high-throughput production of the eco-friendly MIL-88A(Fe) was achieved under mild reaction conditions with normal pressure and temperature. The as-prepared MIL-88A(Fe) exhibited efficient photo-Fenton catalytic ofloxacin (OFL) degradation upon visible light irradiation with good stability and reusability. The OFL (20.0 mg/L) was completely degraded within 50 min under visible light with the aid of MIL-88A(Fe) (0.25 g/L) and H2O2 (1.0 mL/L) in aqueous solution (pH = 7.0). The hydroxyl radicals (·OH) are the main active species during the photo-Fenton oxidation process. Meanwhile, the degradation intermediates and the corresponding degradation pathways were identified and proposed with the aid of both ultra-high performance liquid chromatography tandem quadrupole time-of-flight mass spectrometry (UHPLC-Q-TOF-MS) and density functional theory (DFT) calculations. Finally, the degradation product library was firstly established to identify intermediate transformation products (TPs) with their variation of concentration, and their corresponding toxicologic activities were assessed via Toxtree and T.E.S.T software as well. Finally, the MIL-88A is efficient and stable with four cycles’ catalysis operations, demonstrating good potential for water treatment.
A novel Z-scheme Ag/AgVO3/carbon-rich g-C3N4 heterojunction with excellent solar-light-driven photocatalytic activity was constructed via a facile hydrothermal-calcining method. The Ag/AgVO3/carbon-rich g-C3N4 composites displayed superior performance for the photocatalytic degradation of sulfamethiadiazole (SFZ) under solar irradiation. The optimal composite with a 10 wt% Ag/AgVO3 content showed the highest photocatalytic activity, its degradation rate constant (k) for SFZ degradation was ∼13 and 30 times than that of carbon-rich g-C3N4 (CCN) and Ag/AgVO3, respectively. Furthermore, •O2– was identified as the most crucial reactive species in the Z-scheme photocatalysis system. The greatly improved photocatalytic activities are derived from the built-in electric field (BIEF) of CCN and efficient Z-scheme charge transfer with Ag nanoparticles as charge transmission-bridge. The possible photocatalytic degradation mechanism and pathway over Ag/AgVO3/carbon-rich g-C3N4 were proposed based on LC-MS analysis and density functional theory (DFT) calculation, and the toxicity of intermediates was evaluated by Quantitative structure–activity relationship (QSAR) based prediction. In summary, this work provides new insight into constructing highly efficient Z-scheme photocatalyst, which is promising for implementation in surface water remediation.
Peracetic acid (PAA), an alternative disinfectant of chlorine, has drawn increasing attention in the application of wastewater treatment. However, little is known about the influence of water matrices on PAA-induced organic micropollutant (OMP) degradation. Here, we found that the coexisting bromide ions (Br–) in water can trigger the oxidation of OMP during PAA treatment but probably result in higher ecotoxicity. Br– can efficiently decompose PAA with a species-specific rate constant (kPAAH/Br–) of 0.198 ± 0.003 M–1·s–1. The thus generated HOBr led to a significant abatement (31.8–81.3%) of OMPs (17α-ethinylestradiol, sulfamethoxazole, naproxen, and phenol) after a 1 h reaction at pH 7.1. The coexisting H2O2 component in the PAA solution can competitively consume HOBr and inhibit OMP transformation. The OMP degradation in the PAA/Br– process was highly pH-dependent and preferred acidic conditions. Furthermore, a comprehensive model was established to simulate the reaction kinetics of the OMP degradation by the PAA/Br– process with good accuracy. High-performance/electrospray ionization-triple quadrupole mass spectrometry results indicated the generation of various brominated products, with higher model-predicted toxicity than their parent compounds. This work significantly advances the understanding of the role of Br– in OMP oxidation by PAA and alerts the possible environmental health risks.