Sulfite(S(IV))-induced advanced oxidation processes (AOPs) have great prospect in the field of removing organic pollutants, yet developing highly efficient sulfite activation systems and optimizing active sites for favorable catalytic processes are important but still challenging. Herein, we have achieved a composite catalyst with modulated Co electron structure for efficient AOPs by decorating Co(OH)2 on ultrathin graphitic carbon nitride (g-C3N4) nanosheet through an adjustable strategy, which exhibits high catalytic performance in S(IV) activation system. At optimal pH 9, 92% of paracetamol (APAP) (0.005 mM) is removed with the degradation rate constant of k1 = 0.193 min−1 within 30 min in presence of the composite material. The in-situ synthesis mode introduces strong heterogeneous interface interaction, resulting in directional electron transfer from cobalt hydroxide layer to g-C3N4 sheet revealed by X-ray photoelectron spectroscopy and density functional theory (DFT) calculations. The underlying activity enhanced mechanisms for APAP in S(IV) activation system using Co(OH)2/g-C3N4 are proposed: (i) The ultrathin g-C3N4 nanosheets provide more anchoring centers for generating small Co(OH)2 nanoparticles with abundant active sites which benefit to form metastable intermediates of Co(II)-SO3; (ii) The strong interface interaction induces charge redistribution between Co(OH)2 and g-C3N4 conformed by DFT calculation, which modulates the d-band center of Co site and strengthens the bind of Co(II)-SO3, thereby giving rise to radicals (•OH, SO4• and O2•) and nonradicals (1O2 and electron transfer) oxidation for highly-efficient removal APAP. Our work will pave the way to build an environmentally friendly strategy for emerging organic pollutant degradation in water through building efficient catalysts in sulfite activation system.

A new type of solar active carbon-doped Bi3O4Br catalyst was synthesized by combining hydrothermal and post-thermal treatment. The activity of the material under sunlight and visible light was 3.3 times and 2.7 times that of Bi3O4Br, respectively. The C-doping on Bi3O4Br nanosheets increased the built-in electric field strength, thus significantly promoted the migration of charge carriers and enhanced the photocatalytic activity. In addition, replacing Br with C with a smaller atomic radius can shorten the interlayer spacing, which is beneficial to carrier separation. Experiments showed that the doping of C shortened the semiconductor band gap by 9.8% and expanded the absorption range of visible light. Among the photogenerated reactive species, h+ played a major role in the degradation of 1-methylpyrene (a typical polycyclic aromatic hydrocarbons), followed by O2∙- and •OH. Based on intermediate analysis and DFT calculation, we proposed the degradation mechanism and pathways. Quantitative structure–activity relationship (QSAR) analysis showed that some toxic intermediates were produced during the photocatalysis process, but the overall environmental risk was greatly reduced. This work provides new perspective for understanding non-metallic doping in semiconductor photocatalysts to enhance the built-in electric field, and this technology can be extended to other semiconductor materials.

The efficiency and mechanism of heterogeneous catalytic O3 and UV/O3 for municipal solid waste (MSW) incineration leachate advanced treatment was systematically compared. Prior to comparison, catalyst used in heterogenous catalytic O3 and operation parameters for each technology were optimized. The COD removal of CuO@Al2O3/O3 under its optimal parameters was 57.2%, which failed to meet the standard (≥75%). In contrast, the COD removal by UV/O3 could be 82.3%. The superior efficiency of UV/O3 over CuO@Al2O3/O3 could be summarized into three aspects: (I) Cu bounded ·OH (≡Cu–O·) preferentially attacked hydrophilic groups, while free hydroxyl radical (·OH) was non-selective, thus UV/O3 exhibited a unique three-stage mechanism; (II) The oxidation potential of ≡Cu–O· was higher than that of ·OH, therefore was more vulnerable to the negative effect of radical self-quenching; (III) The existence of UV-induced excited states made organics in UV/O3 more active than in CuO@Al2O3/O3 system, thus high concentration of anions enhanced COD removal in UV/O3 but affected that in CuO@Al2O3/O3. The study further revealed the characteristics of heterogeneous catalytic O3 and UV/O3, and UV induced excited state should be considered in UV-based advanced oxidation processes (AOPs).

The successful preparation of immobilized nitrogen-doped carbon/cobalt @ porous spherical substrate (N-C/Co@PSS) catalyst derived from ZIF-67 was reported in this work. The oxytetracycline (OTC), tetracycline (TTC), and chlortetracycline (CTC) in the simulated wastewater were decomposed via the peroxymonosulfate (PMS) activation by N-C/Co@PSS. The degradation of TCs was initially investigated by batch-type experiments, in which ca. 100% TCs with an initial concentration of 10.0 mg L-1 can be degraded over N-C/Co@PSS + PMS system within 15 min for 30 runs’ operation. In addition, detailed non-radical dominating degradation mechanism was explored by active species capture experiments, electron spin resonance (ESR) tests and electrochemical technology. Furthermore, continuous degradation of TCs antibiotics for up to 200 h in the packed N-C/Co@PSS fixed bed reactors could be accomplished. This work provides theoretical and technical support for the application of MOFs-based catalysts in large-scale wastewater remediation.

FeSx@MoS2-x (FM-x, x implied real Mo/Fe content ratios) in which FeSx derived from MIL-88A deposited on the surface of MoS2 with a tight heterogeneous interface were synthesized for peroxymonosulfate (PMS) activation to degrade atrazine (ATZ). The catalytic performance of FM-0.96 was greatly improved due to the rapid regeneration of Fe2+ resulting from the interfacial interaction. FM-0.96 could completely degrade 10.0 mg/L ATZ within 1.0 min, and the toxicities for most of its intermediates were greatly reduced. The k value of FM-0.96 was 320 and 40 times higher than that of the MoS2 and FeSx, respectively. The SO4·−, ·OH and 1O2 were mainly responsible for ATZ degradation in FM-0.96/PMS system, and the conversion pathway of 1O2 was analyzed. Furthermore, the long-term continuous operation for ATZ degradation was achieved using a fixed membrane reactor. This work provides deep insights into metal sulfide composites derived from metal-organic frameworks for removing pollutants by activating PMS.

In this study, magnetic iron-nickel sulfides biochar composites (MINBs) were successfully prepared via one-step solvothermal method and applied to Cr(VI)-containing wastewater treatment. X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed that synthesized iron-nickel sulfides anchored and dispersed on biochar surface. Cr(VI) removal efficiency and capacity by MINB-5 (molar ratio of Ni to Fe was 5%) were more than 97% and 24.8 mg g-1 within 20 min respectively, when the initial concentration of Cr(VI) was 20 mg L-1. Effects of different operational parameters on Cr(VI) removal efficiency were investigated, including molar ratio of Ni to Fe, dosage of catalyst, initial concentration of Cr(VI), pH value of solution, coexisting ions, natural organic matters (NOMs) and temperature. X-ray photoelectron spectroscopy (XPS) and flame atomic absorption spectrometric (FAAS) analysis demonstrated that Cr(VI) was removed through reduction process by Fe(Ⅱ), which was released from MINBs. Persistent free radicals (PFRs) of biochar, Ni(Ⅱ) and S(-Ⅱ) in MINBs jointly accelerated Fe(Ⅱ)/Fe(III) circulation, instead of direct reduction of Cr(VI) directly. These novel findings provide a new prospect of application of magnetic iron-nickel sulfides biochar composites for Cr(VI)-polluted wastewater remediation.

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.