Effective blockage of chloride ion quenching and chlorinated by-product generation in photocatalytic wastewater treatment

Effective inhibition of chloride ion interference in photocatalytic process by negatively charged molecularly imprinted photocatalyst: Behavior and mechanism

The ubiquitous chloride ions (Cl-) in water seriously interfere with pollutant oxidation and inevitably generate undesirable chlorinated byproducts. In this study, we report for the first time that a negatively charged molecularly imprinted photocatalyst (MIP) can effectively inhibit Cl- interference and suppress the production of chlorination byproducts (the yield of chloroacetic acid was only 16 % of the bare photocatalyst system) while ensuring efficient degradation of target pollutants, thereby greatly improving the safety of the pollutant degradation process. Taking antibiotics as target pollutant, we investigated the mechanism of action of MIP by comparing the antibiotic degradation pathways, fate of photogenerated active species and production of reactive chlorine species (RCS) in the MIP and bare photocatalyst system. The mechanism by which MIP inhibits Cl- interference was mainly based on a synergy between electrostatic repulsion and steric hindrance induced by the specific capture of antibiotics in imprinted cavity, which effectively suppressed the production of RCS and hindered the participation of RCS in antibiotics degradation. In addition, MIP showed good compatibility with common cations, anions and organic matter, and performed well within a broad pH range in various water environments. Thus, the negatively charged MIP provides a feasible approach for the safe and efficient removal of pollutants in Cl- containing water.

Simulated sunlight/periodate-triggered formation of toxic halogenated bisphenols in highly saline water

Periodate (PI)-based oxidation using the activators, such as metal ions and light irradiation, has emerged as a feasible treatment strategy for the effective remediation of contaminated water and wastewater. Given the pervasive nature of PI residues and solar exposure during application, the role of solar light in remediating the challenging highly saline water matrices needs to be elucidated. In this study, bisphenol A (BPA) was selected as the targeted micropollutant, which can be efficiently eliminated by the simulated sunlight (SSL)/PI system in the presence of high-level Cl- (up to 846.0 mM) at pH 7.0. The presence of different background constituents of water, such as halides, nitrate, and dissolved organic matter, had no effect, or even accelerated BPA abatement. Particularly, the ubiquitous Br- or I- appreciably enhanced the BPA transformation efficiency, which may be ascribed to the generation of high-selective reactive HOBr or HOI. The in silico predictions suggested that the transformation products generated by halide-mediated SSL/PI systems via halogen substitutions showed greater persistence, bioaccumulation, and aquatic toxicity than BPA itself. These findings highlighted a widespread phenomenon during PI-based oxidative treatment of highly saline water, which needs special attention under solar light illumination.

Ferrocene crosslinked and functionalized chitosan microspheres towards bio-based Fenton-like system for the removal of organic pollutants

Dye-containing wastewater treatment has been a major long-term global challenge. For this purpose, a novel bio-based microspheres (CS-FC) with high specific surface area (63.24 m2·g-1) and nano-channels (17.95 nm) was prepared using chitosan as the framework and ferrocene as a crosslinking active group. CS-FC not only has the ability to rapidly enrich methyl orange (MO) through hydrogen-bonding and electrostatic attraction, but also almost completely degrades it in the presence of H2O2/K2S2O8 through a synergistic radical/non-radical mechanism under the activating effect of ferrocene. Without H2O2/K2S2O8, the maximum MO adsorption capacity of CS-FC is in the range 871-1050 mg·g-1, and conforms to a Langmuir isothermal model with pseudo-second-order kinetics. In the presence of H2O2/K2S2O8, the removal of MO dramatically increased from 32 % to nearly 100 % after incubation for 60 min, due to the simultaneous formation of highly reactive 1O2 and ·OH. The significant contribution from 1O2 endowed CS-FC/H2O2/K2S2O8 with high universality for degrading various organic pollutants (including azo dyes and antibiotics), a wide pH window (2-8), and low sensitivity to co-existing ions. Such cost-effective, recyclable porous bio-based microspheres are suitable for heterogeneous Fenton-like catalysis in organic wastewater treatment that rely on synergistic radical/non-radical reaction pathways.

Iron-doped bismuth oxybromides as visible-light-responsive Fenton catalysts for the degradation of atrazine in aqueous phases

Pesticides and its degradation products, being well-known residues in soil, have recently been detected in many water bodies as pollutants of emerging concerns, and thus there is a contemporary demand to develop viable and cost-effective techniques for the removal of related organic pollutants in aqueous phases. Herein, a visible-light-responsive Fenton system was constructed with iron-doped bismuth oxybromides (Fe-BiOBr) as the catalysts. Taking the advantage of sustainable Fe(III)/Fe(II) conversion and optimized H2O2 utilization, the optimal Fe-BiOBr-2 catalyst showed an excellent atrazine removal efficiency of 97.61% in 120 min, which is superior than the traditional homogeneous Fenton and the majority of heterogeneous processes documented in the literature. In this photo-Fenton system, hydroxyl (·OH) and superoxide (·O2-) radicals were dominant active species contributed to the oxidative degradation of atrazine. Due to the production of various active radicals, five degradation pathways were proposed based on the identification of intermediates and degradation products. Overall, this work not only demonstrates a fundamental insight into creating highly efficient and atom economic photo-Fenton systems, but also provides a complementary strategy for the treatment of organic pollutants in water.

Performance and mechanism investigation on the enhanced photocatalytic removal of atrazine on S-doped g-C3N4

Developing efficient method for removing low-concentration atrazine, a poisonous chlorinated triazine herbicide with poor biodegradability, was an important measure to control its risk. In this work, highly efficient photocatalytic oxidation of atrazine was achieved on S-doped g-C3N4 (S-g-C3N4). Approximate 99.6% of atrazine was removed in 2 h with a reaction rate constant of 2.76 h-1, nearly 2.44 times that on g-C3N4. The mechanism investigation indicated the improved photocatalytic performance of S-g-C3N4 could be attributed to the enlarged specific surface area, extended light absorption as well as the accelerated separation of the photogenerated charge carriers, which was brought about by the successful doping of sulfur in g-C3N4. Meanwhile, the influence of sulfur doping on the generation and contribution of different reactive species in atrazine removal were also elucidated. It revealed that compared with g-C3N4, the more positive valence band potential of S-g-C3N4 was beneficial to produce more singlet oxygen, which could react synergistically with the superoxide radicals, leading to the improved atrazine removal efficiency. The S-g-C3N4 based photocatalytic system also showed preferential photocatalytic oxidation capability in removing other triazine pesticides compared with 3-chlorophenol (3-CP). The potential applicability of the S-g-C3N4 based photocatalytic system in removing atrazine in high salty water was also investigated, which exhibited superior anti-interference ability towards virous coexistent ions. This work will provide essential and fundamental information for establishing efficient photocatalytic system for triazine type pollutants in waters.

Insight into the synergistic mechanism of sonolysis and sono-induced BiFeO3 nanorods piezocatalysis in atenolol degradation: Ultrasonic parameters, ROS and degradation pathways

Herein, BiFeO₃ nanorods (BFO NRs) was synthesized as the piezoelectric catalyst. The synergistic mechanism of sonolysis and sono-induced BFO-piezocatalysis in atenolol degradation was revealed and the effect of ultrasonic parameters on it was investigated for the first time. The results indicated that 100 kHz was the optimal frequency for the sonolytic and sono-piezocatalytic degradation of atenolol in ultrasound/BFO nanorods (US/BFO NRs) system, with the highest synergistic coefficient of 3.43. The piezoelectric potential differences of BFO NRs by COMSOL Multiphysics simulations further distinguishing that the impact of cavitation shock wave and ultrasonic vibration from sonochemistry reaction (i.e., 2.48, -2.48 and 6.60 V versus 0.008, -0.008 and 0.02 V under tensile, compressive and shear stress at 100 kHz). The latter piezoelectric potentials were insufficient for reactive-oxygen-species (ROS) generation, while the former contributed to 53.93% •OH yield in US/BFO NRs system. Sono-piezocatalysis was found more sensitive to ultrasonic power density than sonolysis. The quenching experiments and ESR tests indicated that the ROS contribution in atenolol degradation followed the order of •OH > ¹O₂ > h⁺ > O₂^•- in US/BFO NRs system and ¹O₂ generation is exclusively dissolved-oxygen dependent. Four degradation pathways for atenolol in US/BFO NRs system were proposed via products identification and DFT calculation. Toxicity assessment by ECOSAR suggested the toxicity of the degradation products could be controlled.

Photocatalytic O2 activation by metal-free carbon nitride nanotube for rapid reactive species generation and organic contaminants degradation

Advanced oxidation processes (AOPs) using oxygen (O₂) as an oxidant represent a low-cost and sustainable wastewater treatment process. Herein, a metal-free nanotubular carbon nitride photocatalyst (CN NT) was prepared to activate O₂ to degrade organic contaminants. The nanotube structure allowed for sufficient O₂ adsorption, while the optical and photoelectrochemical properties enabled photogenerated charge to be efficiently transferred to the adsorbed O₂ to trigger the activation process. The developed CN NT/Vis-O₂ system based on O₂ aeration degraded various organic contaminants and mineralized 40.7% of chloroquine phosphate within 100 min. In addition, the toxicity and environmental risk of treated contaminants were reduced. Mechanistic studies suggested that the enhanced O₂ adsorption capacity and fast charge transfer behavior on CN NT surface led to reactive·O₂^-, ¹O₂ and h⁺ generation, each of which played a distinct role in contaminants degradation. Importantly, the proposed process could overcome the interference from water matrices and outdoor sunlight, and the energy and chemical reagent savings reduced the operating cost to about 1.63 US$·m^-3. Altogether, this work provides insights into the potential application of metal-free photocatalysts and green O₂ activation for wastewater treatment.

A review of research trends on the usage of photocatalysis for wastewater treatment: bibliometric analysis

Photocatalysis is seen as a viable alternative to treating water pollution, due to its flexibility, low cost, and ability to use visible light which is a plentiful and free energy source. Hence, determining the topics of interest and widening collaboration networks will go a long way in improving research in this field. In this study, we aimed to analyze the global research trends on the usage of photocatalysis for wastewater treatment using bibliometric analysis, centered on the outputs of publications, co-authorships, countries of affiliation, and author's keyword co-occurrences. Bibliometric analysis is a review method that is well-known and more conversant to Social Science. Employing it in Physical Science, which is rarely seen, will provide an avenue and yet another method of determining common research topics as well as the potential opportunities and future research in the field. A potential hybrid review paper of great importance to future research in the area will be produced. A total of 1373 articles published within 27 years between 1993 and 2020 were extracted from the Scopus database. In the beginning, less attention was given to the said topic, because after the oldest article was published in 1993, there was no record of other publications until after 5 years (1998). However, from 2002 there was a growing interest in research in that field, with a cumulative increase every year to date, except for a few years with fewer publications. Meanwhile, the number of publications has risen significantly from 2017 to 2020, with an increase of more than 70 publications every year; this is expected to increase rapidly in the coming years. Recently researchers are focusing on developing efficient photocatalysts for contaminants of emerging concern, like pharmaceutical and refinery wastewater, however, the usage of conducting polymers to produce nanocomposite which was found to be very effective is still lagged in wastewater treatment, as such it will be a good area of future research on effective photocatalysts for wastewater treatment.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40899-023-00868-5.

Natural manganese sand activates sodium hypochlorite to enhance ionic organic contaminants removal: Optimization, modeling, and mechanism

Although sodium hypochlorite acting as an oxidant has been investigated for the role it plays in the degradation of organic contaminants, little attention has been paid to its activation and efficient utilization. In this study, natural manganese sand (NMS) was verified to be effective for activation of sodium hypochlorite (NaClO). Due to the generation of O₂^-, the removal efficiency of ionic organic contaminants in NMS/NaClO system was 1.9-4.1 times higher than that in NMS or NaClO alone. Hence, NMS activated NaClO system performed ~96.6 % contaminants removal efficiency at a wide pH range (pH 5-9). Kinetic modeling yielded that the NMS dosage was more important than NaClO dosage. Long-term stability was observed in the presence of various salts (bicarbonate, sulfate, phosphate, and chloride). Characterization results revealed that electron transfer among NMS, NaClO, and organic contaminants was responsible for NaClO activation. Then NaClO-based Fenton-like process was proposed by tracing the degradation intermediates of methyl orange (MO) and generations of reactive oxygen species in the MO/NMS/NaClO system. This study presents the potential of NMS to activate NaClO and enhance ionic organic contaminants removal from aquatic environments.

Enhanced Heterogeneous Catalytic Activity of Peroxymonosulfate for Rhodamine B Degradation via a CoII-Based Metal-Organic Framework

A stable metal-organic framework with the formula {[Co(BBZB)(IPA)]·H₂O}_n (JXUST-23, BBZB = 4,7-bis(1H-benzimidazole-1-yl)-2,1,3-benzothiadiazole and H₂IPA = isophthalic acid) was constructed by incorporating Co²⁺ ions and two conjugated ligands under solvothermal conditions. JXUST-23 takes a dinuclear cluster-based layer structure with a porosity of 2.7%. In this work, JXUST-23 was used to activate peroxymonosulfate (PMS) to degrade rhodamine B (RhB), a difficult-to-degrade pollutant in water. Compared with pure PMS or JXUST-23, the JXUST-23/PMS system displays the best degradation ability of RhB in neutral solution. When the mass ratio of JXUST-23 to PMS was 2:3, 99.72% of RhB (50 ppm) was removed within 60 min, and the reaction rate was 0.1 min^-1. Furthermore, free radical quenching experiments show that SO₄^•- was the main free radical during the process of RhB degradation. In addition, JXUST-23 exhibits good reusability for the degradation of the organic dye RhB, making it a potential candidate for environmental remediation.

Electrostatic self-assembled layered polymers form supramolecular heterojunction catalyst for photocatalytic reduction of high-stability nitrate in water

In this work, two polymers are connected by electrostatic self-assembly method to form a supramolecular heterojunction to remove pollutants. g-C₃N₄-Cl/PANI catalyst can be used for photocatalytic reduction of nitrate in water, and the nitrogen selectivity reaches 98.2%. Specially, charge density analysis and comparative experiments showed that the introduction of covalent chlorine increased in electron transfer conduction between layers. In addition, differential charge density and solid EPR tests reveal high electron density and electron transfer pathways for supramolecular heterostructures. The results of the work function give direct evidence for the high catalytic performance of the supramolecular heterojunction. The reasons and active species of photocatalytic reduction of nitrate by g-C₃N₄-Cl and g-C₃N₄-Cl/PANI are compared. The catalyst exhibits the performance of highly reducing nitrate to harmless nitrogen with the contribution of supramolecular heterojunction and covalent chlorine. In short, a new idea of constructing a supramolecular photocatalyst is proposed, which can be applied to efficiency reduce nitrate in water.

New insight into the regulation mechanism of visible light in naproxen degradation via activation of peroxymonosulfate by MOF derived BiFeO3

BiFeO₃ (BFO) nanocage prepared by metal-organic-framework derivatization (MOF-d) was adopted as activator to first investigate the effect mechanism of visible-light on naproxen-degradation via peroxymonosulfate (PMS) activation. MOF-d BFO expressed more excellent PMS activation ability than hydrothermal-synthetic BFO, due to highly ordered mesopores. A 3.0 times higher pseudo-first-order degradation rate constant was achieved after visible-light introduced. The quenching experiments indicated that the contribution of ROS in naproxen degradation followed the order of SO₄^•->¹O₂ ≈ •OH in MOF-d BFO/PMS/dark system, while changed into h⁺>¹O₂ > >O₂^•-≈SO₄^•-> •OH after visible-light introduced^. EPR tests first revealed that visible-light promoted ¹O₂ yield (non-radical pathway) but suppressed •OH and SO₄^•- generation (free-radical pathways). N₂-purging experiments further proved that ¹O₂ primarily originates from the reaction between h⁺ and PMS, equivalently to that between O₂ and e^--h⁺ in MOF-d BFO/PMS/vis system. Under visible-light, PMS activation via Fe (III) might be hindered by e^- filling on Fe 3d orbital and anion PMS preferred to approach h⁺ rather than e^-, resulting in the decrease of •OH and SO₄^•- yields. Moreover, PMS faces competition from adsorbed-O₂ and oxygen-vacancies for e^- capture. The degradation-pathways for naproxen in dark and under visible light were both proposed in MOF-d BFO/PMS system.

Evaluation of practical application potential of a photocatalyst: Ultimate apparent photocatalytic activity

Suffered from rapid recombination of electrons and holes, apparent photocatalytic activity (APA) of all photocatalysts can never achieve their theoretical ultimate values. But the upper limit of practical APA is of great significance to evaluate the practical application potential of a photocatalyst. Thus, in this work, the concept of ultimate apparent photocatalytic activity (UAPA) was firstly proposed and a convenient evaluation method was first established based on the nature that EDTA-2Na can exclusively scavenge photo-excited holes, and methyl orange (MO) is mainly attacked by superoxide radical (O₂^-) which is produced instantly by photo-excited electrons. From a macro perspective, six popular photocatalysts were designedly selected to verify the feasibility and application scope of the proposed UAPA evaluation method. Moreover, O₂^- production rate and photocurrent intensity were measured by spectroscopy and spectrum analyses, and theoretical carrier concentrations were calculated by density functional theory (DFT) to further confirm the rationality and reliability of the proposed method. Positive responses of all the tests guarantee that the proposed UAPA could precisely evaluate the application potential of a photocatalyst and rank the photocatalysts according to their practical potential.

Graphitic carbon nitride embedded with graphene materials towards photocatalysis of bisphenol A: The role of graphene and mediation of superoxide and singlet oxygen

Composite photocatalysts comprising graphitic carbon nitride (g-C₃N₄) and graphene materials were synthesized and evaluated in the photocatalysis of bisphenol A (BPA) with a focus on elucidating the reaction mechanism. Embedding reduced graphene oxide (rGO) to g-C₃N₄ significantly accelerated the photocatalysis rate of BPA by three folds under visible light irradiation at neutral pH. We showed that rGO synthesized in intimate contact with g-C₃N₄ increased the surface areas and electrical conductivity of the g-C₃N₄ composites and promoted the electron-hole pair separation. The BPA photodegradation mechanism involved selective oxidants as superoxide (O₂^•-) and singlet oxygen (¹O₂) that were formed through one-electron reduction of O₂ and the unique oxidation of O₂^•- by photogenerated hole (h⁺), respectively. The synthesized photocatalyst exhibited superior visible light photoreactivity to that of N-doped P25 TiO₂, good photo-stability and reuse potential, and was operative in complex wastewater. rGO embedded g-C₃N₄ achieved good photomineralization of BPA at 80% in 4 h compared to 40% of bare g-C₃N₄. This study sheds light on the photocatalysis mechanism of BPA with a metal-free, promising rGO/g-C₃N₄ photocatalyst.

Highly efficient photoelectrochemical removal of atrazine and the mechanism investigation: Bias potential effect and reactive species

In this work, efficient photoelectrochemical (PEC) removal of atrazine, one of the most widely used chemical herbicides in the world, was obtained by adjusting the bias potential applied on the photo-anode, and the optimal atrazine removal efficiency reached 96.8% at the potential of 0.2 V vs. SCE in 2 h with the reaction rate constant of 1.72 h^-1. The results indicated at the optimal potential, the separation efficiency of photo-generated holes and electrons was the highest with the lowest electron transfer resistance. Mechanism investigation revealed that superoxide radicals, hydroxyl radicals and holes all contributed to atrazine degradation, and the bias potential on the photo-anode could influence atrazine removal efficiency by changing the generation amount and distribution of the reactive oxygen species (ROS). It was presumed the nucleophilicity of superoxide radical played an important role in atrazine dechlorination, leading to the enhanced removal efficiency. However, the bias potential did not show obvious influence on the degradation intermediates of atrazine in the PEC system compared with that in photocatalytic oxidation, since it was actually an electro-assisted photocatalytic process in the potential range investigated. The work will provide fundamental basis for establishing efficient PEC system for pollutant remediation experimentally and theoretically.