MnFe layered double hydroxides confined MnOx for peroxymonosulfate activation: A novel manner for the selective production of singlet oxygen.

Singlet oxygen (1O2) is a reactive species for the selective degradation of stubborn organic pollutants. Given its resistance to harsh water environment, the effective and exclusive generation of 1O2 is acknowledged as a key strategy to mitigate water production costs and ensure water supply safety. Herein, we synthesized MnOx intercalated MnFe layered double hydroxides (MF-MnOx) to selectively produce 1O2 through the activation of PMS. The distinctive confined structure endowed MF-MnOx with a special pathway for the PMS activation. The direct oxidation of BPA on the intercalated MnOx induced the charge imbalance in the MnFe-LDH layer, resulting in the selective generation of 1O2. Moreover, acceptable activity deterioration of MF-MnOx was observed in a 10 h continuous degradation test in actual water, substantiating the application potential of MF-MnOx. This work presents a novel catalyst for the selective production of 1O2, and evaluates its prospects in the remediation of micro-polluted water.

Degradation of tetracycline by UV/Fe3+/persulfate process: Kinetics, mechanism, DBPs yield, toxicity evaluation and bacterial community analysis.

As a widely produced and used antibiotic, tetracycline (TC) has been frequently found in rivers, soil and drinking water. In this study, the degradation of TC was investigated by UV/Fe3+/persulfate (PS) coupled process. The degradation behavior was well fitted with pseudo-first-order model. Hydroxyl radicals (·OH), sulfate radicals (SO4-·) and superoxide radical (O2-·) were identified as the primary reactive oxygen species (ROS) in UV/Fe3+/PS process, the contribution to TC degradation were found to be 41.94%, 33.94% and 17.44% at pH 3.0, respectively. Fe(IV) generated from the system also played a crucial role in TC removal. The effects of process parameters (PS/Fe3+ dosages, pH, humic acid, Cl-, HCO3-, NO3- and CO32-) on degradation were investigated. It was found that the degradation of TC was highly pH-dependent, and the optimal performance was obtained at pH 3.0. Except for Cl-, the presence of HA, HCO3-, NO3- and CO32- inhibited TC degradation. The possible transformation pathway involving the hydroxylation, N-demethylation, hydrogenation and dehydroxylation was proposed. Furthermore, the toxicity and mutagenicity of TC and transformation products (TPs) were estimated using ECOSAR and TEST softwares, demonstrating that the toxicity level of most TPs was lower/equal to their precursors. The evaluation of DBPs showed that UV/Fe3+/PS process could reduce the potential of DBPs formation, especially for TCAA and TCM. Microbial community composition was analyzed by 16 S rDNA sequencing, and the relative abundance of ARG-carrying opportunistic pathogens was significantly declined after UV/Fe3+/PS treatment. In general, this study provides an economical, efficient and safe strategy for TC removal.

Unraveling spongy Co3O4 mediated activation of peroxymonosulfate: Overlooked involvement of instantaneously produced high-valent-cobalt-oxo.

Peroxymonosulfate (PMS) activation induced by tricobalt tetroxide spinel (Co3O4) has been confirmed as a typical Haber-Weiss reaction, while free radicals were once considered as the dominated reactive species in the previous studies. However, the catalytic mechanism of the spongy Co3O4 driven PMS activation was surprisingly found as a radical/nonradical mixed process rather than a pure radical process in the present work. The important role of sulfate radical (SO4-) was confirmed through the quenching experiments. Despite the inhibition of furfuryl alcohol (FFA) and 1,4-benzoquinone (BQ) on degradation was generally accepted as the evidence to support the existence of 1O2 and O2-, additional experiments using methyl phenyl sulfoxide (PMSO) as the indicator indeed verified high-valent-cobalt-oxo rather than 1O2 and O2- dominated the very early reaction stage. Notably, instead of homogeneous Co3+, heterogeneous Co(IV) = O on catalyst surface was believed to be responsible for the oxidation of organics. Spongy Co3O4 not only possessed stronger catalytic ability than commercial Co3O4 (k[spongy Co3O4] = 0.74 min-1, k[Co3O4] = 0.08 min-1), but also owned preferable stability. The performance of catalytic system was barely affected by the solution pH under the near neutral condition. Besides, little suppression of the widely existing anions on the degradation indicated the potential application of spongy Co3O4/PMS system. This study provides a reliable oxidation technology for the removal of organic pollutants, and sheds new light on the cobalt oxide triggered PMS activation process.

ZIF-8 assisted synthesis of magnetic core-shell Fe3O4@CuS nanoparticles for efficient sulfadiazine degradation via H2O2 activation: Performance and mechanism.

A novel magnetic core-shell Fe3O4@CuS have been successfully synthesized by chemical etching and cation exchange method using Zeolitic imidazolate frameworks (ZIF) as the template. The morphology and microstructural properties characterization indicated that Fe3O4@CuS nanoparticles were rhombic dodecahedral shape, highly stable, and magnetic with a large specific surface area (772.20 m2/g). The catalytic activity of Fe3O4@CuS was assessed on sulfadiazine (SDZ) degradation by H2O2 activation. Multi-factors affecting the SDZ removal was adequately investigated. Approximately 93.2% SDZ (50 µM) was removed with 0.2 g/L Fe3O4@CuS and 5 mM H2O2 in 90 min. In particular, Fe3O4@CuS exhibited a quality catalytic performance within a wide pH range of 3.0-11.0. Radical scavenger tests and electron paramagnetic resonance (EPR) analysis confirmed that •O2-, •OH, and 1O2 all contributed to the SDZ degradation, and •OH played the dominant role. Meanwhile, mechanism investigation suggested that the effective catalytic activity of Fe3O4@CuS could be ascribed to the sulphur-enhanced copper-based Fenton reaction on the CuS shell, sulphur-enhanced iron-based Fenton reaction on the Fe3O4 core, and the effective electron transfer between the shell and core. Finally, the possible SDZ degradation pathways were further proposed on the basis of the intermediates identification. This work put forward a new strategy to synthesize magnetic core-shell Fe3O4@CuS using ZIF-8 as the template with outstanding performance for H2O2 activation to degrade SDZ.

Development of oxygen vacancies enriched CoAl hydroxide@hydroxysulfide hollow flowers for peroxymonosulfate activation: A highly efficient singlet oxygen-dominated oxidation process for sulfamethoxazole degradation.

In this study, oxygen vacancies enriched cobalt aluminum hydroxide@hydroxysulfide (CoAl-LDH@CoSx) hollow flowers was synthesized by in-situ etching of CoAl-LDH using sodium sulfide solution. The analysis of SEM, EDS, XRD, and XPS were used to characterize the samples. The as-synthesized 0.2CoAl-LDH@CoSx displayed higher catalysis performance of sulfamethoxazole (SMX) degradation via the activation of PMS than the pristine CoAl-LDH. 98.5 % of SMX (40 µM) was eliminated with 0.1 g/L 0.2CoAl-LDH@CoSx and 0.3 mM PMS at pH 6.0 in 4 min. The degradation fitted with the pseudo-first-order reaction kinetics well with rate constant of 0.89 min-1 for 0.2CoAl-LDH@CoSx/PMS system and 0.55 min-1 for CoAl-LDH/PMS system. Singlet oxygen (1O2) was verified as dominant reactive oxygen species responsible for SMX degradation via quenching tests. Mechanism investigation suggested that the oxygen vacancies, redox cycles of Co(II)/Co(III) and S22-/(S2- and sulfate species) on the surface of 0.2CoAl-LDH@CoSx were crucial for PMS activation. In addition, the plausible degradation pathways of SMX were proposed by analysis of the SMX degradation intermediates. This study not only reveals that 0.2CoAl-LDH@CoSx is an efficient catalyst to activate PMS for SMX degradation, but also shed a novel insight into development of heterogeneous catalysts with oxygen vacancies.

Can Cu2ZnSnS4 nanoparticles be used as heterogeneous catalysts for sulfadiazine degradation?

As a quaternary copper-based semiconductor, Cu2ZnSnS4 (CZTS) is drawing growing attention and is anticipated as a promising photocatalyst, thanks to its large absorption coefficient, exceptional photostability, and theoretical power conversion efficiency. However, CZTS has never been used as an activator of H2O2 for the degradation of refractory organic pollutants. In this study, the synthesis of CZTS nanoparticles obtained with diverse morphologies and crystallinities using solvents of deionized water (CZTS-W) and ethylene glycol (CZTS-EG) was examined in the activation of H2O2 to degrade sulfadiazine (SDZ). The results revealed that CZTS coupled with H2O2 could be an effective system for the degradation of SDZ. Compared to CZTS-EG, CZTS-W presented higher reusability in consecutive cycles with negligible leaching of copper. Reactive oxygen species quenching tests and electron paramagnetic resonance analyses illustrated that •O2-, •OH, and 1O2 contributed to the degradation of SDZ, and 1O2 prevailed over •O2- and •OH. The mechanistic investigation showed that efficient degradation could be associated to the effective recycling of Cu(II)/Cu(I) and low-valent/high-valent sulfur. Also, the degradation pathways of SDZ have been proposed through the detection of intermediate products. This study manifests that CZTS synthesized using deionized water is encouraging for the elimination of organic pollutants.

Efficient sulfadiazine degradation via in-situ epitaxial grow of Graphitic Carbon Nitride (g-C3N4) on carbon dots heterostructures under visible light irradiation: Synthesis, mechanisms and toxicity evaluation.

The synthesis of environmental-friendly metal-free photocatalysts has great significance in photocatalytic technology. In this work, we firstly report the successful synthesis of in situ epitaxial growth of g-C3N4 on carbon dots through a facile thermal polymerization technique. Characterization and density functional theory (DFT) calculations were conducted to clarify the structure engineering and the electronic/chemical properties of the in-plane interconnected carbon dots/g-C3N4 (C-CN) heterostructures. With the optimal carbon dots content, the C-CN exhibited 3.2 times higher degradation rate for sulfadiazine (SDZ) than that of g-C3N4. Besides, the C-CN heterostructures displayed excellent stability and reusability in five consecutive cycles. The enhanced photocatalytic activity was related to the narrowed band gap and the local electronic density of valance band and conduction band orbitals of the unique plane heterostructures, corroborated by the spectroscopic characterizations and theoretical calculations. Photogenerated holes dominated the degradation of SDZ, while OH showed a negligible contribution. Moreover, DFT calculation succeeded to predict that the atoms with high Fukin index (f0) on SDZ molecule were more vulnerable to radicals attack. SDZ degradation pathway mainly included smiles-type rearrangement, SO2 extrusion, ring hydroxylation and SN bond cleavage processes. The eco-toxicity assessment revealed the generation of less toxic intermediates after photocatalysis. Our findings not only afford a new technique for constructing g-C3N4-based in-plane heterostructures with high and stable photocatalytic efficiency, but also highlight the feasible application of metal-free photocatalysts in environmental remediation.

Heterogeneous activation of hydrogen peroxide by cysteine intercalated layered double hydroxide for degradation of organic pollutants: Performance and mechanism.

A novel cysteine intercalated copper aluminum layered double hydroxide (CuAl-Cys-LDH) was synthesized and applied as heterogeneous catalyst for activating hydrogen peroxide (H2O2) to degrade rhodamine B (RhB) and 4-Nitrophenol (4-NP). The effects of initial pH, CuAl-Cys-LDH dosage, and H2O2 concentration on RhB and 4-NP removal were comprehensively investigated. The results indicated the intercalation of cysteine into the interlayer of LDH greatly enhanced its catalytic activity and stability. With 0.2 g/L CuAl-LDH and 50 mM H2O2, 93.7% of RhB and 80.2% of 4-NP could be removed in the CuAl-Cys-LDH activated H2O2 system. While the CuAl- LDH activated H2O2 system could only degrade 51.2% of RhB and 46.8% of 4-NP under the identical experimental conditions. Significantly, the CuAl-Cys-LDH catalyzed H2O2 system exhibited high degradation efficiency within a wide pH range from 4.0 to 10.0. Based on the electron paramagnetic resonance (EPR) tests and radical quenching experiments, it was inferred that •OH radical was the dominant species responsible for organic contaminants degradation. Mechanism study revealed that the intercalated cysteine in the interlayer of LDH strongly accelerated the rate-determining conversion of Cu(II) to Cu(I) by oxidation itself to cystine, thus enhanced the catalytic efficiency for H2O2 activation to produce •OH radicals. The findings of this work indicated that CuAl-Cys-LDH is a conveniently prepared and highly efficient and stable catalyst for the degradation of organic contaminants in environmental remediation.

In situ growth of Ag-SnO2 quantum dots on silver phosphate for photocatalytic degradation of carbamazepine: Performance, mechanism and intermediates toxicity assessment.

The occurrence of carbamazepine (CBZ) in environments poses a potential risk to aquatic life and exhibits growth inhibition of human embryonic cells. In this work, for the first time a series of Ag-SnO2 quantum dots (QDs)/silver phosphate (AgSn/AgP) composites were synthesized and used as photocatalysts for CBZ degradation. The obtained AgSn/AgP composites showed superior photodegradation efficiency for CBZ removal. The degradation rate constant of 10AgSn/AgP (with 10 wt% of Ag-SnO2 QDs) was almost 8.5, 5.7, 5.7, and 1.9 times as that of Ag-SnO2 QDs, Ag3PO4, Ag/Ag3PO4, and SnO2 QDs/Ag3PO4, respectively. The improved photocatalytic activity could be primarily ascribed to the improved charge separation through a collaborative effect of Ag-SnO2 QDs and Ag3PO4, and in situ photoreduced metallic silver. Electron spin resonance (ESR) measurement and radical trapping experiments suggested that holes (h+), (superoxideradical) ·O2- and (hydroxylradical) ·OH corporately participated in the decomposition of CBZ. Moreover, a reasonable mechanism for photocatalytic degradation of CBZ over 10AgSn/AgP composites was tentatively proposed. Additionally, eight degradation intermediates were determined by liquid chromatography-mass spectrometry (LC-MS). Toxicity evaluation using the Ecological Structure Activity Relationships (ECOSAR) program revealed that the toxicity of most photodegradation intermediates were much lower than that of the parent compound CBZ. This work not only provides a new technique for preparing Ag3PO4-based photocatalysts with high activity, but also demonstrates that 10AgSn/AgP could be a promising photocatalyst for treating water and wastewaters containing CBZ.

Sodium dodecyl sulfate intercalated and acrylamide anchored layered double hydroxides: A multifunctional adsorbent for highly efficient removal of Congo red.

In this study, a novel adsorbent was designed and synthesized via intercalation of sodium dodecyl sulfate (SDS) in the interlayer, and subsequent grafting of acrylamide (AM) on the layer of layered double hydroxides (LDH) (named as AM/SDS-LDH), and applied for congo red (CR) removal. The morphological structure, wettability and physicochemical properties of the adsorbents were thoroughly characterized using SEM, EDS, N2-adsorption/desorption isotherm, static contact angle, XRD and FTIR. Results elucidated that SDS and AM were successfully introduced into the interlayer and onto the layer of LDH, respectively. Adsorption experimental results suggested that the maximum adsorption capacities of CR on SDS-LDH and AM/SDS-LDH at pH 5.0 and 293 K were 714.29 and 1118.78 mg/g, respectively, which were much higher than that of CR on LDH (588.24 mg/g). Based on the BET, XRD and FTIR analysis, the higher adsorption capacity of AM/SDS-LDH was mainly attributed to high surface area, large basal spacing as well as the abundant NH2 groups. The experimental data can be well fitted by pseudo-second-order kinetic and Langmuir isotherm models. Thermodynamic parameters indicated the adsorption process was favorable under the higher temperature condition. The synergistic effect existed during the adsorption process of CR onto AM/SDS-LDH, including the electrostatic interactions, anion exchange and hydrophobic-hydrophobic interactions. Overall, this study provided a strategy for design and fabrication of highly efficient adsorbents.

Degradation of dyes by peroxymonosulfate activated by ternary CoFeNi-layered double hydroxide: Catalytic performance, mechanism and kinetic modeling.

Ternary CoFeNi-layered double hydroxide (CoFeNi-LDH) was synthesized and initially applied to activate peroxymonosulfate (PMS) for the degradation of Congo red (CR) and Rhodamine B (RhB). The results show that the CoFeNi-LDH/PMS system can efficiently degrade nearly 100% of 20 mg/L CR or 20 mg/L RhB within 6- and 10-min reaction times, respectively. And the catalyst exhibits higher degradation efficiency on CR than on RhB under identical conditions, which is confirmed by electron clouds of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) performed by DFT calculations. Quenching tests reveal that SO4- is the dominant active species participating in the degradation process. Mechanism investigation demonstrates that Co(II)-Co(III)-Co(II) cycle is responsible for activating PMS to generate radicals for dyes degradation. A dynamic kinetic model is successfully developed to simulate the concentration profiles of CR and RhB degradation in CoFeNi-LDH/PMS system. The empirical second order rate constants between SO4- and CR (kSO4-/CR), HO and CR (kOH/CR), SO4- and RhB (kSO4-/RhB), HO and RhB (kHO/RhB) are determined to be 2.47 × 107, 3.44 × 106, 8.39 × 106 and 2.62 × 107 M-1s-1, respectively. In addition, toxic assessment using ECOSAR program suggests that the overall toxicity of CR and RhB decreased after treatment with CoFeNi-LDH/PMS system. Repeating tests and application of CoFeNi-LDH in different water sources give us adequate confidence that the as-synthesized CoFeNi-LDH is favorable for the purification of dye-contaminanted waters in practical.