Heat induced superfast diclofenac removal in Cu(II)-activated peracetic acid system: Mediation from non-radical to radical pathway

N-Doped Carbon Nanotubes as Metal-Free Catalysts for PAA Activation to degrade emerging pollutants: Exploration of Reaction Mechanisms and Prediction of Active Sites

In this study, we utilized urea as a nitrogen precursor and synthesized a series of nitrogen-doped carbon nanotubes with varying catalytic activities for PAA by adjusting the mass ratio of urea to carbon nanotubes (ranging from 0.01:1 to 2:1) and the preparation temperature (between 400°C and 1000°C). The activation mechanism was thoroughly examined through extensive characterization and calculations. During the activation process with PAA, we observed that the removal of contaminants was linearly correlated with the extent of graphitization (R2=0.873) and the degree of nitrogen doping (R2=0.951). These findings were further corroborated by density functional theory (DFT) calculations. Different types of nitrogen atoms can reduce the peroxide-breaking energy barrier in PAA to varying degrees, thereby facilitating the conversion of NCNT-PAA* complexes into adsorbed hydroxyl radicals. The system achieves an impressive 100% oxidative removal of 20 μM micropollutants (e.g., bisphenol A) within 60 minutes, thanks to the synergistic effects of electron transfer and radical adsorption. Furthermore, it maintains a remarkable micropollutant removal rate of nearly 80% after five consecutive uses. Additionally, carbon materials can be effectively integrated with membrane filtration, which not only facilitates the recycling of carbon materials in practical applications but also enhances the catalytic efficiency of nitrogen-doped multi-walled carbon nanotubes (NCNTs) while ensuring the safety of the effluent. These results underscore the extensive application prospects and research potential of carbon-based materials, while also providing a novel approach for the advanced oxidation technology of PAA.

The mechanism of alkali to inhibit the organics polymerization in improving the biodegradability of wastewater treated by heat/peroxydisulfate

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.

Overlooked Role of Coexistent Hydrogen Peroxide in Activated Peracetic Acid by Cu(II) for Enhanced Oxidation of Organic Contaminants

Cu(II)-catalyzed peracetic acid (PAA) processes have shown significant potential to remove contaminants in water treatment. Nevertheless, the role of coexistent H2O2 in the transformation from Cu(II) to Cu(I) remained contentious. Herein, with the Cu(II)/PAA process as an example, the respective roles of PAA and H2O2 on the Cu(II)/Cu(I) cycling were comprehensively investigated over the pH range of 7.0-10.5. Contrary to previous studies, it was surprisingly found that the coexistent deprotonated H2O2 (HO2-), instead of PAA, was crucial for accelerating the transformation from Cu(II) to Cu(I) (kHO2-/Cu(II) = (0.17-1) × 106 M-1 s-1, kPAA/Cu(II) < 2.33 ± 0.3 M-1 s-1). Subsequently, the formed Cu(I) preferentially reacted with PAA (kPAA/Cu(I) = (5.84 ± 0.17) × 102 M-1 s-1), rather than H2O2 (kH2O2/Cu(I) = (5.00 ± 0.2) × 101 M-1 s-1), generating reactive species to oxidize organic contaminants. With naproxen as the target pollutant, the proposed synergistic role of H2O2 and PAA was found to be highly dependent on the solution pH with weakly alkaline conditions being more conducive to naproxen degradation. Overall, this study systematically investigated the overlooked but crucial role of coexistent H2O2 in the Cu(II)/PAA process, which might provide valuable insights for better understanding the underlying mechanism in Cu-catalyzed PAA processes.

Enhanced Degradation of Micropollutants in a Peracetic Acid/Mn(II) System with EDDS: An Investigation of the Role of Mn Species

Extensive research has been conducted on the utilization of a metal-based catalyst to activate peracetic acid (PAA) for the degradation of micropollutants (MPs) in water. Mn(II) is a commonly employed catalyst for homogeneous advanced oxidation processes (AOPs), but its catalytic performance with PAA is poor. This study showed that the environmentally friendly chelator ethylenediamine-N,N'-disuccinic acid (EDDS) could greatly facilitate the activation of Mn(II) in PAA for complete atrazine (ATZ) degradation. In this process, the EDDS enhanced the catalytic activity of manganese (Mn) and prevented disproportionation of transient Mn species, thus facilitating the decay of PAA and mineralization of ATZ. By employing electron spin resonance detection, quenching and probe tests, and 18O isotope-tracing experiments, the significance of high-valent Mn-oxo species (Mn(V)) in the Mn(II)-EDDS/PAA system was revealed. In particular, the involvement of the Mn(III) species was essential for the formation of Mn(V). Mn(III) species, along with singlet oxygen (1O2) and acetyl(per)oxyl radicals (CH3C(O)O•/CH3C(O)OO•), also contributed partially to ATZ degradation. Mass spectrometry and density functional theory methods were used to study the transformation pathway and mechanism of ATZ. The toxicity assessment of the oxidative products indicated that the toxicity of ATZ decreased after the degradation reaction. Moreover, the system exhibited excellent interference resistance toward various anions and humid acid (HA), and it could selectively degrade multiple MPs.

Doped Cu0 and sulfidation induced transition from R-O• to •OH in peracetic acid activation by sulfidated nano zero-valent iron-copper

Peracetic acid (PAA) has emerged as a new effective oxidant for various contaminants degradation through advanced oxidation process (AOP). In this study, sulfidated nano zero-valent iron-copper (S-nZVIC) with low Cu doping and sulfidation was synthesized for PAA activation, resulting in more efficient degradation of sulfamethoxazole (SMX, 20 μM) and other contaminants using a low dose of catalyst (0.05 g/L) and oxidant (100 μM). The characterization results suggested that S-nZVIC presented a more uniform size and distribution with fewer metal oxides, as the agglomeration and oxidation were inhibited. More significantly, doped Cu0 and sulfidation significantly enhanced the generation and contribution of •OH but decreased that of R-O• in S-nZVIC/PAA/SMX system compared with that of nZVIC and S-nZVI, accounting for the relatively high degradation efficiency of 97.7% in S-nZVIC/PAA/SMX system compared with 85.7% and 78.9% in nZVIC/PAA/SMX and S-nZVI/PAA/SMX system, respectively. The mechanisms underlying these changes were that (i) doped Cu° could promote the regeneration of Fe(Ⅱ) for strengthened PAA activation through mediating Fe(Ⅱ)/Fe(Ⅲ) cycle by Cu(Ⅰ)/Cu(Ⅱ) cycle; (ii) S species might consume part of R-O•, resulting in a decreased contribution of R-O• in SMX degradation; (iii) sulfidation increased the electrical conductivity, thus facilitating the electron transfer from S-nZVIC to PAA. Consequently, the dominant reactive oxygen species transited from R-O• to •OH to degrade SMX more efficiently. The degradation pathways, intermediate products and toxicity were further analyzed through density functional theory (DFT) calculations, liquid chromatography-mass spectrometry (LC-MS) and T.E.S.T software analysis, which proved the environmental friendliness of this process. In addition, S-nZVIC exhibited high stability, recyclability and degradation efficiency over a wide pH range (3.0∼9.0). This work provides a new insight into the rational design and modification of nano zero-valent metals for efficient wastewater treatment through adjusting the dominant reactive oxygen species (ROS) into the more active free radicals.