During the preparation of single-atom catalysts, Fe aggregation was effectively suppressed through the use of Fe and N precursors with matched high decomposition temperatures, resulting in synthesis of the FePc-gCN catalyst with a high density of single Fe sites (similar to 9.1 wt.%). Compared with catalysts having severe aggregation, FePc-gCN exhibited a 3.45-fold greater degradation rate constant (0.566 min-1) for sulfamethazine in a peroxymonosulfate system. Direct electron transfer played a key role in pollutant removal. High density created short distances between single Fe sites, thereby generating neighbouring synergy that increased the density of states of Fe near Fermi level. This mechanism improved both electron-donating and electron-accepting capabilities of FePc-gCN, which was confirmed in galvanic cell experiments, since the efficient removal of sulfamethazine required FePc-gCN to simultaneously function as a cathode and an anode. In practical applications, FePc-gCN exhibited excellent stability and activity, sustaining a greater than 95% sulfamethazine removal efficiency for more than 460 h in a continuous-flow reactor, and effectively removed trace organic pollutants from real secondary effluents of wastewater treatment plants. This study presents a facile route for preparing high-density single Fe atom catalysts, and high activity of the resulting catalyst is from synergistic effect between neighbouring Fe atoms.

Oxygen vacancies (Ov) on metal oxide surfaces exhibit high catalytic activity for activating peroxymonosulfate (PMS) in wastewater decontamination, yet their in-situ regeneration remains a significant challenge. This study successfully achieves in-situ real-time regeneration of Ov on CuO surfaces through simple alkali etching without interrupting the contaminant removal process. The surface hydroxyl groups introduced by alkali treatment significantly reduce the formation energy of Ov on CuO surfaces from 1.60 eV to 0.38 eV. Both experimental results and density functional theory calculations reveal that the high activity of CuO relies on the synergy of surface hydroxyl groups and Ov. This synergy increases the antibonding states below the Fermi level and the electron spin density of Cu near Ov, thereby promoting electron transfer from CuO to PMS. As a result, by just adding an equimolar amount of alkali relative to PMS in CuO/PMS system, the degradation rate constant of sulfamethoxazole (SMX) greatly increases by 42 times. The primary reactive oxygen species in this system are sulfate radicals and hydroxyl radicals. Furthermore, OH-/CuO/PMS system exhibits a long-term stability (> 300 h) for SMX removal in a real water matrix. This work provides a highly executable method to in-situ real-time regenerate Ov on CuO surfaces, representing significant progress in the critical yet underappreciated field of catalyst regeneration.

To investigate the impact of boron-doped diamond (BDD) electrode aging on the electrosynthesis of sodium peroxydisulfate (Na2S2O8, PDS), BDD electrodes with varying degrees of aging were prepared via electrochemical oxidation. Their surface structure, electrochemical properties and PDS generation mechanism were experimentally and theoretically studied. The ability of BDD electrode to generate & sdot;OH weakened after aging, leading to reduced PDS production and Faraday efficiency, since the generation of PDS need & sdot;OH as the precursor. The aging via electrochemical oxidation did not notably change the crystal structure nor the surface morphology of BDD electrode, but caused an increase in oxygen-containing functional groups on BDD electrode surface. Electrochemical experiments indicate that aging can decrease the electron transfer rate at the BDD electrode-solution interface, primarily affecting the inner-sphere electron transfer rather than the outer-sphere electron transfer. Molecular dynamics simulations and density functional theory calculations suggest that oxygen terminals accumulated on aged BDD electrodes could repel sulfate species and tend to undergo four-electron transfer reaction to produce O2 instead of free-state & sdot;OH.

Developing an effective strategy for the high-value resource utilization of sulfate-rich membrane concentrates (SRMC) generated from wastewater reclamation remains a major challenge. This study demonstrates that the electrolysis of SRMC not only enables complete removal of organic pollutants but also permits the concurrent production of high-value peroxydisulfate (PDS). Using a fluorine-modified boron-doped diamond (BDD-F) electrode for PDS electrosynthesis, a 15 % higher Faraday efficiency was achieved compared to that of the pristine BDD electrode. The key intermediate for PDS formation was identified as HO center dot, and density functional theory calculations revealed that fluorine modification effectively suppresses the oxygen evolution reaction while promoting HO center dot generation. Approximately 230 mM of PDS was produced after 12 h of electrolysis, with the yield increasing linearly over time. When treating actual SRMC, complete organic mineralization was achieved within 5 h, accompanied by the production of similar to 40 mM PDS. Notably, the PDS yield remained stable even after 15 cycles, indicating excellent electrode durability. The PDS derived from SRMC exhibited comparable performance in organic wastewater treatment to commercial one. These results confirm that electrolysis using a fluorine-modified BDD electrode represents a highly effective strategy for sulfate resource recovery from membrane concentrates.

This study investigates the performance and mechanism of carbon nanotube (CNT)-supported iron phthalocyanine (FePc) as catalyst in peroxymonosulfate-based advanced oxidation processes (PMS-AOPs). The optimal catalyst, FePc-CNTs/300, was prepared by ultrasonically depositing FePc onto CNTs followed by thermal treatment at 300 degrees C. FePc-CNTs/300 performed much better in removing sulfamethazine (SMZ) than the ones with no heat treatment (FePc-CNTs) and 800 degrees C treatment (FePc-CNTs/800), respectively. High removal rate of SMZ (>95 %) can be maintained at least 60 h in continuous-flow reactor with FePc-CNTs/300 as catalyst. The adsorbed PMS dominated electron transfer is responsible for pollutants removal. Density functional theory calculations revealed that CNTs can induce the adsorbed PMS in the electron-defect state, thereby enhancing its electron extraction capability from target pollutants. This mechanism shows that modulating the activity of catalyst through the carrier is an efficient way, which was significant to develop simple and low-cost method to prepare efficient catalyst for PMS-AOPs.

This study demonstrates that direct electron transfer (DET)-dominated peroxymonosulfate (PMS)-based advanced oxidation processes (PMS-AOPs) share a similar oxidation mechanism with laccase, including dual-substrate activation, DET mechanism, polymerization of organic pollutants, and relatively high redox potential at active site. To verify this point, a detachable catalyst (MnPc/CNT) was used. Neither manganese phthalocyanine (MnPc) nor carbon nanotubes (CNT) could efficiently catalyze PMS to remove phenol (PE), whereas MnPc/CNT achieved complete PE elimination primarily through a DET mechanism. In this process, PE was mainly removed via polymerization. Importantly, galvanic cell experiments and density functional theory calculations showed that MnPc/CNT can simultaneously activate both PE and PMS, whereas MnPc or CNT alone cannot. This dual-substrate activation phenomenon was seldom reported in PMS-AOPs. Comparative studies showed that MnPc/CNT, FePc/CNT, and CoPc/CNT exhibited significantly higher PE removal efficiency than NiPc/CNT, CuPc/CNT, and ZnPc/CNT, which correlated with their higher open circuit potentials. Since current catalyst design strategies for PMS-AOPs remain largely empirical and enzyme is highly efficient catalyst, this laccase-like mechanism means that the laccase-mimetic approach will be a promising direction for developing high-performance catalysts.

Developing high-efficiency catalysts for peroxymonosulfate (PMS) activation remains challenging due to sluggish electron transfer and unstable active sites in conventional monometallic-carbon systems. Herein, introducing interfacial metal iron nanodots into corn cores-based biochar (CCBC) loading copper sulfide composites (CuS@Fe-CCBC) achieved 27, 14, 4 and 3-fold improvement in the degradation on sulfamerazine (SMR) than CCBC, Fe-CCBC, CuS, CuS@CCBC with almost complete decomposition of PMS (99.2 %). Based on experiments and theoretical calculations, we confirmed that the CuS@Fe-CCBC catalyst promoted the adsorption and rapid electron transfer of PMS, which relied on the Fe-mediated electron bridge between CuS and CCBC. With S2- species acting as electron donors during the dual redox cycling of Cu+/Cu2+ and Fe2+/Fe3+, the OO bonds and SO bonds of PMS molecules were elongated and then broken. Afterwards, multiple reactive oxygen species (ROSs) were generated, including OH, SO4-, O-1(2), and O-2(-) to contribute 28.1 %, 20.5 %, 46.7 % and 4.8 % on SMR removal. Besides, the degradation efficiency of SMR in the secondary effluent in the corresponding continuous-flow reactor was maintained more than 95 % for 180 h with allowable metal leaching (Cu < 0.5 mg/L, Fe < 0.15 mg/L). This study provides important insights into the practical application of PMS activation process and offers a sustainable solution for the deep purification of wastewater.

Malathion is a widely used insecticide with toxic effects on humans and is considered to be genotoxic and carcinogenic. As the common free radicals in advanced oxidation processes, hydroxyl radical (•OH) and sulfate radical (SO4•-) can efficiently degrade malathion, but highly toxic product malaoxon is prone to produce in this process. In response to this issue, polyethyleneterephthalate (PET) plastics and K2FeO4 were utilized to prepare Fe0-doped porous plastic-derived carbon material (FAC) as peroxymonosulfate (PMS) trigger to perform low toxicity degradation of malathion. FAC (100 mg/L) could trigger 0.5 mM PMS to completely degrade 10 mg/L malathion within 15 min. PMS alone, singlet oxygen (1O2), •OH, and SO4•- contributed to the degradation of malathion, in which 1O2 played the most important role with a contribution of 53.5%. Density functional theory (DFT) was employed to elucidate the reaction site of 1O2 for malathion, further illustrating 1O2 with the product of desmethyl malathion. Based on the DFT program, we calculated the theoretical second-order rate constants, the reactivity of 1O2 with malathion to produce desmethyl malathion, was 1.88 × 1012 M-1 s-1, which was much higher than another reaction pathway with the highly toxic product of malaoxon (1.36 × 107 M-1 s-1). The binding energies of various key proteins of zebrafish and human beings to the degradation products were analyzed by molecular dynamics to characterize their ecological and human toxicity. Surprisingly, in contrast to the highly toxic intermediate malaoxon in the previous studies, desmethyl malathion, the main degradation product in FAC/PMS system, has a significantly low toxicity.

Reverse osmosis concentrate (ROC) from the secondary effluent in municipal wastewater reclamation contains emerging pollutants (EPs) that require further treatment, as these EPs may otherwise pose risks to human health and ecological balance. Since ROC has relatively high salinity, Electrochlorination (E-Cl) could exactly be a promising technology for its EPs removal. However, a comprehensive and systematic investigation has yet to be carried out. In this study, targeted at E-Cl process for the ROC treatment, a series of conscientiously designed experiments were conducted to evaluate the removal performance of carbamazepine (CBZ), investigate the effects of operating conditions and water matrix on the CBZ removal, and reveal its reaction mechanisms. An excellent CBZ removal efficiency of 99.8 % was achieved within 30 min at 2 mA cm-2 in 5 mM NaCl solution. Among water matrix components, SO42-, NO3-, PO4 3- and NH4+ showed minimal effects, whereas HCO3 - and humic acid had significant influences on the CBZ removal. The roles of ClO center dot and Cl2 center dot- radicals in the E-Cl process were clarified through quenching experiments combined with electron paramagnetic resonance and radical probe tests. The steady-state concentrations of ClO center dot (88.95 x 10-14 M) and Cl2 center dot- (493.02 x 10-14 M) were much higher than those of HO center dot (6.91 x 10-14 M). Both ClO center dot and Cl2 center dot- played key roles in CBZ degradation in the E-Cl process. Probable degradation pathways of CBZ were proposed based on the intermediates identification and density function theory (DFT) calculations. This study demonstrates the potential of E-Cl systems for effective EPs removal in ROC, providing mechanistic insights for practical applications.

Coking wastewater, containing numerous toxic organic compounds, presents significant challenges for biological treatment. In this study, we investigated the efficacy of peroxodisulfate (PDS) pretreatment in mitigating its toxicity, with a particular focus on identifying key toxic substances and their interactions. The wastewater was fractionated into four components: hydrophilic substances (HIS), hydrophobic acids (HOA), hydrophobic bases (HOB), and hydrophobic neutrals (HON). The PDS pretreatment achieved a marked reduction in toxicity, as confirmed by toxicological assays. Key toxic substances were comprehensively identified using gas chromatography-mass spectrometry (GC-MS), combined with the Mantel test, Spearman correlation analysis, and interpretable machine learning modeling. In HIS, the key toxic substances were phenol, 4-methylphenol, and 3methylphenol; in HOA, phenol, 4-methylphenol, and 2-methylphenol; in HOB, aniline, quinoline, and indole; and in HON, indole, 6(5H)-phenanthridone, and 2(1H)-quinolinone. Combined toxicity assessments revealed predominantly antagonistic interactions among the identified toxicants, emphasizing the importance of considering their actual concentrations and ratios in treatment design. The major contributors to combined toxicity were identified as phenol in both HIS and HOA components, quinoline in HOB, and indole in HON. Further, toxic thresholds for activated sludge inhibition were determined for phenol (176.79 mg/L) and thiocyanate (SCN- ) (30.32 mg/L), both of which exhibited high concentrations and significant toxicity.

High-temperature wastewaters can themselves activate peroxydisulfate (PDS) to remove aromatic contaminants via polymerization. This, however, may result in an insufficient carbon source for denitrification during biochemical treatment, and the formed polymers, without a proper reuse method, will be costly to handle as hazardous waste. This study demonstrates that the addition of NaOH can suppress the polymerization of aromatic contaminants, which is observed not only in simulated wastewater but also in actual coking wastewater (ACW). Taking phenol as an example, the formation of phenoxy radical (PhO•) through the reaction between SO4•- and phenol is the crucial step for phenol polymerization. The addition of NaOH can convert sulfate radicals (SO4•-) to hydroxyl radicals (HO•), and simultaneously, HO• can quickly consume PhO•. Both processes contribute to the inhibition of phenol polymerization. After treatment with heat/NaOH/PDS, the biodegradability of ACW is significantly enhanced with a relatively low carbon source loss (around 16%). Moreover, Fourier transform-ion cyclotron resonance mass spectrometry analysis indicates that the transformation of polyphenols to highly unsaturated and phenolic compounds is beneficial for the biodegradability improvement of ACW. Therefore, the NaOH/PDS system is an effective way to utilize waste heat and enhance the biodegradability of wastewater.

Singlet oxygen (O-1(2)) is attractive in water decontamination because of its high selectivity to remove organic pollutants, but its oriented generation in peroxymonosulfate (PMS) activation is still challenging, especially using metal catalysts. In this study, it was found that simple sulfuration of FeCo layered double hydroxide (FeCo-LDH-S-10) can achieve efficient O-1(2) generation in PMS activation, and SO5- is the precursor of O-1(2). Density functional theory calculations revealed that sulfuration of FeCo-LDH positively shifts the d-band center of Co 3d to enhance the interaction between Co site and PMS, thus reducing the free energy barrier for O-1(2) formation from 1.02 eV to 0.61 eV. With norfloxacin (NOR) as the target pollutant, FeCo-LDH-S-10 not only has much higher catalytic activity than the original FeCo-LDH but also most of the reported catalysts. In the continuous-flow reactor containing FeCo-LDH-S-10, a high removal rate of NOR (>90 %) can be kept for more than 132 h. Importantly, the activity of the used FeCo-LDH-S-10 in the continuous-flow reactor can be easily in-situ regenerated through simple sulfuration again. Furthermore, various antibiotics in actual secondary effluent of wastewater treatment plants can be effectively removed by FeCo-LDH-S-10/PMS system, leading to a significant decrease of toxicity. This work emphasizes the feasibility of enhancing actual water decontamination by modulating the O-1(2) formation in PMS-based advanced oxidation processes.

In phenol-rich wastewater, such as coking wastewater, due to the high reactivity of phenol to various reactive oxygen species, it is difficult to selectively oxidize pollutants having lower biodegradability and higher toxicity than phenol. As one kind of such pollutants in coking wastewater, some nitrogenous heterocyclic compounds (NHCs) are more difficult to be removed by SO4•- or HO• than phenol, but this study found that NHCs (quinoline, isoquinoline, and pyridine) can be selectively removed by peroxymonosulfate (PMS) direct oxidation in the presence of 10 mM phenol under thermal condition. The selective oxidation of NHCs needs a suitable pH range (4 < constant pH < 9) because protonated state of NHCs (pH < 4) is unfavorable to their oxidation and high pH would improve the extra PMS consumption by phenol. Under the conditions benefiting the removal of NHCs in heat/PMS system, there was no generation of SO4•- and HO•. Being treated by 60 °C/PMS for 60 min, the biodegradability (BOD5/COD) of real coking wastewater (RCW) was improved from 0.21 to 0.44 with low removal rate of phenols (about 10%). Quinoline and indole, as the two typical NHCs in the studied RCW, their removal rates can be up to 45% and 85%, respectively. Thus, heat/PMS pretreatment is a potential good way to selectively remove high toxic pollutants in phenol-rich wastewater.

Designing high-performance laccase-mimicking nanozymes could realize effective oxidation of phenolic pollutants, while solve the unstable problem of natural laccase. Cu2O has same Cu active center with natural laccase, but its catalytic activity is not satisfactory. In this work, by simply regulating the exposed facet of Cu2O, we found that its catalytic activity could be greatly improved. Cubic Cu2O (c-Cu2O) with single exposed (100) facet exhibited the highest activity, followed by short hexapods (111), while rhombic dodecahedrons (110) showed the poorest performance. The activity of c-Cu2O was 2.5 times higher than that of natural laccase and exceeded most of other copper-based laccase mimics. c-Cu2O also exhibited superior stability and universal catalytic activity toward several phenolic compounds. Theoretical calculations demonstrate that the d-band center of (100) facet is moderate (-1.74 eV) compared with (110) (-1.71 eV) and (111) (-1.86 eV), which enables balanced substrate adsorption and optimal oxygen activation, thereby resulting in the highest catalytic activity. This study provided a simple approach for realizing highly-effective laccase-like activity and elucidates the atomic mechanism of facet-dependent activity of Cu2O, which provides fundamental insights for designing high-performance nanozymes.

Electrochemical nitrate reduction reaction (NO3RR) presents a promising approach for sustainable water denitrification. Yet its practical implementation is hindered by sluggish reaction kinetics. Herein, we develop a dual-functional catalyst comprising carbon-wrapped Fe3O4 nanoparticles integrated with Fe single atom sites (FeNC-Fe3O4), which demonstrates exceptional activity and stability for NO3RR. The FeNC-Fe3O4 achieves a Faradic efficiency of 95.63 % and an NH3 yield of 2.95 mg cm-2 h-1 at -0.6 V vs. RHE in a neutral electrolyte. Mechanistic investigations, including in situ analysis and density functional theory calculations, reveal that the synergetic interplay between FeNC and Fe3O4 nanoparticles and the enlargement on double-layer capacitance, reduces the energy barrier for key intermediates, and provides abundant active sites for NO3RR. Furthermore, the catalyst exhibits an 85.1 % nitrate conversion efficiency and 87.2 % N2 selectivity at -0.6 V vs. RHE, where Cl- play a crucial role in promoting the N2 generation. This work underscores the potential of integrating Fe3O4 with FeNC to advance the practical utility of single atom catalysts, offering a design paradigm for advanced metal oxide hybrid catalysts.

The combination of membrane filtration and heterogeneous catalytic ozonation (hybrid ozonation membrane filtration (HOMF) process) was receiving increasing attention for water treatment due to its ability to eliminate contaminants and prevent membrane fouling and catalyst loss. Research in HOMF was just at its infant stage and has not yet been summarized. Due to the limited catalytic capacity of pure ceramic membranes, increasing efforts have been devoted to incorporating active catalysts into the ceramic membranes, thereby fabricating dual-functional membranes with both catalytic and anti-fouling properties. Significant improvements in reaction kinetics occurred in HOMF compared to catalytic ozonation, possibly due to nanoconfinement. However, limited evidence existed and debate continued regarding this nanoconfinement. HOMF also showed good performance in mitigating membrane fouling, with the mechanism yet to be studied and summarized. Accordingly, in this paper, the recent development of HOMF was systematically reviewed. The functionalized catalytic ceramic membranes and their active components, critical performance indicators and key challenges were provided. The membrane fouling control and pollutant removal mechanisms in HOMF systems were particularly summarized for the first time, with a key focus on nanoconfinement. Finally, the HOMF process for water treatment was discussed, including engineering applications, conclusions, and perspectives.

As one of the attractive phosphate adsorption materials, layered double hydroxide (LDH) is frequently used in the form of nanomaterials, which makes LDH suffer from weak recyclability and agglomeration. Powder immobilization is applicable for separation, though usually sacrifices the adsorption capacity. In this investigation, Two types of rare earth-based layered double hydroxide/chitosan (CS) hydrogel beads were fabricated, namely LaCa-LDH/CS and CeCa-LDH/CS. The results demonstrate that the combination of CS and LDH exhibits superior adsorption performance than individual LDH, primarily due to the incorporation of -NH2 and the uniform dispersion of LDH in CS. The maximum adsorption capacities for phosphate on LaCa-LDH/CS and CeCa-LDH/CS are 149.5 and 174.6 mg P/g at 200 mg P/L and pH 5, respectively, surpassing the powder form of LaCa-LDH (107.9 mg P/g), CeCa-LDH (105.0 mg P/g), and pure CS beads (10.2 mg P/g). Furthermore, both LaCa-LDH/CS and CeCa-LDH/CS exhibit steady phosphate removal performance at a wide pH range, and the adsorption capacities only experience a decrease of 20.0 % and 28.4 % from pH 3–7, outperforming previous reports. The co-existing anions effect experiments proved excellent selectivity of LDH/CS to phosphate. The stable binding of phosphate on LDH/CS is confirmed through a long-term adsorption stability test lasting for 10 days. Moreover, two LDH/CS could remove 96 % phosphate from natural water. After five cycles, the adsorption capacities of two LDH/CS were maintained at over 95 %. The primary adsorption mechanism involves electrostatic attraction as pH < pHpzc and ligand exchange as pH > pHpzc. Ion exchange and hydrogen bonding also contribute to a certain extent.

The spontaneous sorption and release of moisture by sorbents is proposed to be an energy-saving strategy for air humidity control. However, rational design of sorbents with humidity control ability and wide applicability still faces challenges. This study demonstrates quaternary ammonium-functionalized organosilica capable of controlling humidity intelligently and tuning humidity range as required. The prepared organosilica exhibited step-shaped water sorption isotherm due to its highly ordered pore structure, which was capable of precisely maintaining humidity within a narrow range. Importantly, the step position could be broadly tuned by easily changing the pore size during synthesis. Series organosilica with different pore sizes could realize humidity maintenance in 20%-40%, 40%-60% and 55%-80%, respectively, making it suitable for diverse humidity control applications. Grand Canonical Monte Carlo simulations elucidated the underlying mechanism of different water sorption behaviors in different pore structures. Quaternary ammonium groups act as the initial nucleation sites for water vapor, followed by distinct water condensation mechanisms in micropores (pore filling) and mesopores (capillary condensation), respectively. Moreover, organosilica exhibits efficient antibacterial activity and self-cleaning capability due to the function of quaternary ammonium groups. The organosilica with tunable pore structure and antibacterial activity provides a new model for intelligent and low-energy humidity control.

Mimicking efficient biocatalytic cascades using nanozymes has gained enormous attention in catalytic chemistry, but it remains challenging to develop a nanozyme-based cascade system to sequentially perform the desired reactions. Particularly, the integration of sequential hydrolysis and oxidation reactions into nanozyme-based cascade systems has not yet been achieved, despite their significant roles in various domains. Herein, a self-cascade Ce-MOF-818 nanozyme for sequential hydrolysis and oxidation reactions is developed. Ce-MOF-818 is the first Ce(IV)-based heterometallic metal-organic framework constructed through the coordination of Ce and Cu to distinct groups. It is successfully synthesized using an improved solvothermal method, overcoming the challenge posed by the significant difference in the binding speeds of Ce and Cu to ligands. With excellent organophosphate hydrolase-like (Km = 42.3 µM, Kcat = 0.0208 min-1 ) and catechol oxidase-like (Km = 2589 µM, Kcat = 1.25 s-1 ) activities attributed to its bimetallic active centers, Ce-MOF-818 serves as a promising self-cascade platform for sequential hydrolysis and oxidation. Notably, its catalytic efficiency surpasses that of physically mixed nanozymes by approximately fourfold, owning to the close integration of active sites. The developed hydrolysis-oxidation self-cascade nanozyme has promising potential applications in catalytic chemistry and provides valuable insights into the rational design of nanozyme-based cascade systems.

Layered double hydroxide (LDH) is frequently used for phosphate removal in water, while a desirable LDH adsorbent should have sufficiently high adsorption capacity and selectivity.