Role of bipyridyl in enhancing ferrate oxidation toward micropollutants.

Enhancing Fe(VI) oxidation ability by generating high-valent iron-oxo species (Fe(IV)/Fe(V)) has attracted continuous interest. This work for the first time reports the efficient activation of Fe(VI) by a well-known aza-aromatic chelating agent 2,2'-bipyridyl (BPY) for micropollutant degradation. The presence of BPY increased the degradation constants of six model compounds (i.e., sulfamethoxazole (SMX), diclofenac (DCF), atenolol (ATL), flumequine (FLU), 4-chlorophenol (4-CP), carbamazepine (CBZ)) with Fe(VI) by 2 - 6 folds compared to those by Fe(VI) alone at pH 8.0. Lines of evidence indicated the dominant role of Fe(IV)/Fe(V) intermediates. Density functional theory calculations suggested that the binding of Fe(III) to one or two BPY molecules initiated the oxidation of Fe(III) to Fe(IV) by Fe(VI), while Fe(VI) was reduced to Fe(V). The increased exposures of Fe(IV)/Fe(V) were experimentally verified by the pre-generated Fe(III) complex with BPY and using methyl phenyl sulfoxide as the probe compound. The presence of chloride and bicarbonate slightly affected model compound degradation by Fe(VI) in the presence of BPY, while a negative effect of humic acid was obtained under the same conditions. This work demonstrates the potential of N-donor heterocyclic ligand to activate Fe(VI) for micropollutant degradation, which is instructive for the Fe(VI)-based oxidation processes.

Visible light-driven NH2Cl activation by g-C3N4 photocatalysis producing reactive nitrogen species to degrade bisphenol A.

The photolysis of monochloramine (NH2Cl), a widely used disinfectant, under UVC irradiation produces different radicals for the micropollutant degradation. For the first time, this study demonstrates the degradation of bisphenol A (BPA) via the NH2Cl activation by graphitic carbon nitride (g-C3N4) photocatalysis using visible light-LEDs at 420 nm, termed as the Vis420/g-C3N4/NH2Cl process. The process produces •NH2, •NH2OO, •NO and •NO2 via the eCB-- and O2•--induced activation pathways and •NHCl and NHClOO• via the hVB+-induced activation pathway. The produced reactive nitrogen species (RNS) enhanced 100% of the BPA degradation compared with the Vis420/g-C3N4. Density functional theory calculations confirmed the proposed NH2Cl activation pathways and further demonstrated that eCB-/O2•- and hVB+ induced the cleavage of N-Cl and N-H bonds in NH2Cl, respectively. The process converted 73.5% of the decomposed NH2Cl to nitrogen-containing gas, compared with that of approximately 20% in the UVC/NH2Cl process, leaving much less ammonia, nitrite and nitrate in water. Among different operating conditions and water matrices tested, of particular significance is natural organic matter of 5 mgDOC/L only reduced 13.1% of the BPA degradation compared against that of at least 46% reduction in the UVC/NH2Cl process. Only 0.017-0.161 µg/L of disinfection byproducts were produced, two orders of magnitudes lower than that in the UVC/chlorine and UVC/NH2Cl processes. The combined use of visible light-LEDs, g-C3N4 and NH2Cl significantly improves the micropollutant degradation and reduces the energy consumption and byproduct formation of the NH2Cl-based AOP.

Computational study on persulfate activation by two-dimensional carbon materials with various nitrogen proportions for carbamazepine oxidation in wastewater: The essential role of graphitic N atoms.

Two-dimensional carbon materials with various N atom proportions (2D-CNMs) are constructed to clarify the optimal catalyst for carbamazepine (CBZ) oxidation and the inner mechanism for persulfate-based advanced oxidation processes (P-AOPs). Results show that peroxydisulfate (PDS) can be activated by all 2D-CNMs with the order of C3N > C71N > graphene > C2N > CN, while C3N is the only catalyst for peroxymonosulfate (PMS) activation. The C3N with the maximum graphitic N can activate PDS and PMS in a wide temperature range at any pH, and demonstrates the optimal CBZ oxidation performance. Notably, the graphitic N atoms promote P-AOPs from five aspects: (i) electron structure, (ii) electrical conductivity, (iii) electron transfer from persulfate to catalysts, (iv) electron jump of co-system before and after activation, (v) interaction between catalyst and persulfate. The most vigorous activity of C3N is attributed to the greatest number of graphitic N. This work clarifies the essential role of graphitic N atoms with implications for the catalyst design, and facilitates the environmental applications of P-AOPs for micropollutant abatement.

Superfast degradation of micropollutants in water by reactive species generated from the reaction between chlorine dioxide and sulfite.

Chlorine dioxide (ClO2) is used as an oxidant or disinfectant in (waste)water treatment, whereas sulfite is a prevalent reducing agent to quench the excess ClO2. This study demonstrated that seven micropollutants with structural diversity could be rapidly degraded in the reaction between ClO2 and sulfite under environmentally relevant conditions in synthetic and real drinking water. For example, carbamazepine, which is recalcitrant to standalone ClO2 or sulfite, was degraded by 55%-80% in 10 s in the ClO2/sulfite process at 30-µM ClO2 and 30-µM sulfite concentrations within a pH range of 6.0-11.0. Results from experiments and a kinetic model supported that chlorine monoxide (ClO·) and sulfate radicals (SO4·-) were generated in the ClO2/sulfite process, while hydroxyl radical generation was insignificant. Apart from radicals, dichlorine trioxide (Cl2O3) was generated and largely contributed to micropollutant degradation, supported by experimental results using stopped-flow spectrometry and quantum chemical calculations. The impacts of pH, sulfite dosage, and water matrix components (chloride, bicarbonate, and natural organic matter) on micropollutant abatement in the ClO2/sulfite process were evaluated and discussed. When treating the real potable water, the concentrations of organic (five regulated disinfection byproducts) and inorganic byproducts (chlorite and chlorate) formed in the ClO2/sulfite process were all below the drinking water standards. This study disclosed fundamental knowledge advancements relevant to the reaction mechanisms between ClO2 and sulfite, and highlighed a novel process to abate micropollutants in water and wastewater.

Protrudent Iron Single-Atom Accelerated Interfacial Piezoelectric Polarization for Self-Powered Water Motion Triggered Fenton-Like Reaction.

Water in motion presented in natural systems contains a rich source of renewable mechanical energy. Harvesting this water energy to trigger the generation of reactive oxygen species (ROS) for water purification is a desirable yet underexplored solution. Herein, the authors report a self-powered water motion triggered Fenton-like reaction system for wastewater treatment through the piezo-activation of peroxymonosulfate (PMS). Isolated protrudent Fe single atomic sites are immobilized on the surface of molybdenum disulfide (MoS2 ) nanosheet to improve piezoelectric polarization of MoS2 , to accelerate piezoelectric charge separation, and to enhance PMS activation for water purification. ROS (• OH, SO4•- , O2•- , and 1 O2 ) generation for PMS piezo-activation are observed, and different water contaminants, including antibiotic, industrial chemicals, and dyes are efficiently removed under the natural water fluid. Aimed at solving concurrent issues of environmental pollution and energy crisis, this study provides a pathway for single atomic-mediated piezo-activation of Fenton-like reactions through ambient self-powered water motion for water purification.

Synchronous removal of emulsions and soluble organic contaminants via a microalgae-based membrane system: performance and mechanisms.

In this study, we applied a flexible strategy to manufacture a microalgal biochar-based membrane (MBCM). Due to the hierarchical surface topography on a micro-nano scale, the MBCM was found to have both underwater superoleophobic and underoil superhydrophobic properties. Combining an underoil superhydrophobic oil-containing region (OCR) with an underwater superoleophobic water-containing region (WCR) achieved the successive filtration of multiphase emulsions. The MBCM also served as a high-performance carbocatalyst for advanced oxidation processes (AOPs), due to the N functionalities (5.08%) of the graphene-like structure. This was caused by the high-temperature pyrolysis of rich proteins and alkaline salts in the algal residue. As a result, the MBCM/AOPs system achieved greater than 99.5% emulsions separation efficiency in different emulsion mixtures, while also achieving an outstanding degradation rate (99.8%) of soluble organic contaminants (SOCs). This in-depth exploration resulted in a low-cost and green strategy for developing multifunctional membranes to treat complex wastewater. The paper explains the mechanisms used by MBCM to synchronously remove emulsions and SOCs from wastewater.

First-Principles Evaluation of Volatile Organic Compounds Degradation in Z-Scheme Photocatalytic Systems: MXene and Graphitic-CN Heterostructures.

It is a formidable challenge to use the traditional trial-and-error method to identify suitable catalysts for the photocatalytic degradation of volatile organic compounds (VOCs). In this work, by performing density functional theory calculations, we designed three Z-scheme g-CN/M2CO2 (M = Hf, Zr, and Sc) heterostructures, which not only exhibit favorable structure stability but also show promising ability for photocatalytic degradation of VOCs. The enhancement of the photocatalytic activity of these three Z-scheme systems can be ascribed to the low recombination rate of electron-hole pairs because photoelectrons migrated from the g-CN layer to the M2CO2 layer as well as the internal electric fields in the Z-scheme heterojunction. Among the three heterostructures, only g-CN/Zr2CO2 presents favorable spectra utilization under photoirradiation as well as the direct band gap. As a result, in the Z-scheme g-CN/Zr2CO2 heterostructure, the electrons in the conduction band of g-CN migrate to the holes in the valence band of the Zr2CO2 layer, which improves extraction and utilization of photogenerated electrons in the g-CN sheet. Moreover, the Z-scheme g-CN/Zr2CO2 system shows superior performance for photocatalytic VOC degradation in comparison with individual g-CN and Zr2CO2, which can be attributed to the enhanced VOC adsorption capacity as well as excellent ability to photoactivate O2 and H2O into •O2- and •OH radicals. Our findings pave a new promising way to facilitate the application of MXene-based materials for VOC photocatalytic degradation.

Near-infrared light to heat conversion in peroxydisulfate activation with MoS2: A new photo-activation process for water treatment.

The advantage of light-to-heat conversion can be employed as an optical alternative for environmental remediation. As a proof of concept, for the first time we introduce the light-to-heat conversion application in peroxydisulfate (PDS) activation by molybdenum disulphide (MoS2) under near infrared (NIR) light irradiation. Theoretical kinetics analysis suggests that the reaction rates of PDS activation is increased up to 9.2 times when increasing from room temperature to 50 °C. MoS2 has the capability to quickly convert NIR light to heat energy (~45°C), thereby being able to activate PDS to generate hydroxyl and sulfate radicals. The observed reaction rate of carbamazepine degradation by NIR/MoS2/PDS process is 6.5 times of that in MoS2/PDS and even 2.6 times higher than the sum of those in NIR/MoS2, MoS2/PDS and NIR/PDS processes. Combining with theoretical calculation and oxidation species analysis, a new photo-activation PDS mechanism is proposed, in which MoS2 absorbs the energy of light to generate heat energy for overcoming the energy barrier of PDS activation. By loading MoS2 on carbon cloths, a flexible photothermal membrane is designed for practical application of sunlight-to-heat conversion to activate PDS with high efficiency, stability, and recycling. The present results demonstrate the potential of applying light-to-heat conversion in Fenton-like processes in pollution control, which opens new avenues towards utilization of inexhaustible solar energy and novel approaches for environmental remediation.

Mechanism Insight into enhanced photodegradation of pharmaceuticals and personal care products in natural water matrix over crystalline graphitic carbon nitrides.

Pharmaceuticals and personal care products (PPCPs), an emerging class of highly recalcitrant water contaminants, have raised considerable concerns in environment community. Graphitic carbon nitride (CN) has shown a great potential towards the photodegradation of water contaminants under visible light irradiation. However, the conventional bulk CN (BCN) presents the amorphous structure, resulting in an inefficient yield of hydroxyl radicals (•OH) for the complete mineralization of PPCPs. This study provides fundamental insights into significantly enhancing the hydroxyl radical generation via improving the crystallinity of the pristine CN materials. Experimental measurements and accompanying density functional theory (DFT) computational analysis suggest that the crystalline carbon nitride (CCN) exhibited an enhanced adsorption ability towards the dissolved O2. Upon the light irradiation, the adsorbed O2 molecules readily undergo a direct two-electron reduction reaction on the CCN surface, instead of the conventional two successive single-electron reduction reactions on the BCN surface, to produce H2O2 subsequently converting into •OH radicals. Along with the improved charge separation efficiency and electron transfer ability, CCN-based materials show superior photocatalytic activity towards PPCPs-type pollutants, compared with the pristine BCN catalysts. Importantly, the catalyst show excellent photodegradation activities under natural sunlight irradiation, at low PPCPs concentration (20 µg/L), in the mixed PPCPs solution or in the real wastewater/water samples, indicating the potential of CCN to enable practical ex situ destructive treatment of PPCPs-contaminated groundwater.

Integrating nitrogen vacancies into crystalline graphitic carbon nitride for enhanced photocatalytic hydrogen production.

We report a facile alkali-assisted salt molten method to construct crystalline carbon nitride with rich nitrogen vacancies. Experimental and computational results show that the crystalline structure allows efficient charge transfer and the nitrogen vacancies provide more active sites, resulting in the enhanced photocatalytic hydrogen production.

Novel carbon and defects co-modified g-C3N4 for highly efficient photocatalytic degradation of bisphenol A under visible light.

Graphite carbon nitride (g-C3N4, CN) is considered as a promising semiconductor for environmental catalysis. However, pure CN can not meet the requirements for actual applications due to its high recombination rate of photogenerated electron-hole pairs and a relatively large band gap preventing full utilization of solar energy. In this work, we report synthesis of a novel carbon and defects co-modified g-C3N4 (CxCN) by calcination of melamine activated by oxalic. This new catalyst CxCN has porous structure with much higher surface areas compared with pristine CN. UV-vis analysis and DFT calculations show that CxCN has a lower bandgap for enhancing visible light adsorption compared with CN. Photoluminescence (PL) and photoelectrochemical analyses show that CxCN has a low recombination rate of photogenerated electron-hole pairs, which improves the utilization of solar energy. As a result, CxCN samples show high efficiency for the degradation of bisphenol A (BPA) under visible light irradiation, where the best catalyst of CxCN (C1.0CN) samples shows about 22 times higher photocatalytic degradation rate than that of CN. Moreover, C1.0CN shows high mineralization rate and can degrade BPA into CO2 and H2O by the generated active species, like superoxide radicals (O2-) and holes (h+).