o-Semiquinone Radical and o-Benzoquinone Selectively Degrade Aniline Contaminants in the Periodate-Mediated Advanced Oxidation Process

New insights into soil active substances enhance the biochar/periodate process for remediation of sulfadiazine: The changes of soil properties and toxicity

In recent years, periodate (SPI)-based advanced oxidation processes have been successfully applied in wastewater treatment. However, their application in soil pollution remediation remains limited. To our knowledge, this study represents the first attempt to utilize SPI catalyzed by the Eichhornia crassipes biochar (EBC) system for the remediation of sulfadiazine (SD)-contaminated soil. In the EBC/SPI system, the degradation performance of SD-spiked soils was significantly improved, achieving complete degradation within 60 min, which indicates a clear synergistic effect between SPI and EBC. Notably, our findings highlighted that active soil constituents play crucial roles in SPI activation. Specifically, free Fe-oxides in soil were essential for SPI activation to form reactive species (RS) compared to amorphous Fe-oxides and dissolved Fe, leading to superior SD degradation. Soil organic matter (SOM) also contributed to RS formation and conversion. Adding Fe3+, Cl-, and humic acid accelerated SD elimination, whereas Mn2+ and HCO3- inhibited it. Quenching experiments and electron paramagnetic resonance spectroscopy confirmed the formation of singlet oxygen, superoxide radicals, and iodate radicals, which actively degraded SD. Analysis of soil properties, including SOM content, total phosphorus, functional groups, crystal structure, and pH value, showed negligible changes after EBC/SPI treatment. Additionally, potential decomposition pathways of SD were proposed based on identified SD intermediates. Ecotoxicity analyses and phytotoxicity tests indicated a marked reduction in the toxicity of these intermediates compared to SD. These findings provide an efficient strategy for soil remediation and offer new insights into the role of inherent substances in the field of contaminated soil remediation.

Overlooked Pathway of UV Filter Degradation in the UV/H2O2: The Important Role of Triplet State UV Filter

The large production and usage of UV filters resulted in pervasive contamination in the aquatic environment. UV/H2O2 is a widely used advanced oxidation process (AOP) to eliminate contaminants including UV filters relying on the generated HO•. In this study, the degradation of UV filters in the UV/H2O2 AOP was investigated by using benzophenone (BP) as a representative. A previously overlooked pathway for BP degradation and HO• formation was newly identified. The triplet state species generated from BP photosensitization (i.e., 3BP*) was found to react with H2O2 to produce HO•, and this pathway contributed to 31% of BP degradation. The second-order rate constant of 3BP* with H2O2 was determined to be 8.7(±1.0) × 107 M-1 s-1 at pH 7.0 by using a laser flash photolysis system. 3BP* acted as a reductant, and it was transformed into a radical cation (BP•+). The further hydrolysis of BP•+ produced hydroxylated BPs as products. This pathway can be barely influenced by pH and inorganic ions in the real water matrix. This work not only reported an unrecognized pathway for pollutant degradation in the UV/H2O2 AOP but also inspired ideas to develop novel technologies to abate pollutants by taking advantage of their triplet states.

Removal of Sulfonamide Antibiotics in Peracetic Acid-Mediated Natural Polyphenol Systems via an Overlooked Polymerization Pathway: Role of ortho-Quinones

Natural polyphenols can be oxidized into reactive quinones, which might play a key role in the removal of specific contaminants in natural polyphenol-related advanced oxidation processes (AOPs). In this study, peracetic acid (PAA) was employed in combination with natural protocatechuic acid (PCA) to remove sulfonamide antibiotics (SAs) from water. More than 95% removal of sulfamethoxazole (SMX) and other SAs was observed in the PCA/PAA system, and neutral pH conditions (5.0-8.0) were more conducive to the removal of SMX. The PCA/PAA system exhibited a great anti-interference ability against complex water matrices. ortho-Quinone, generated from the oxidation of PCA by PAA, played a dominant role in the SMX removal. Electrons tended to transfer from SMX to the generated ortho-quinones and form covalent bonds, resulting in the production of less toxic oligomers via the overlooked polymerization pathway. A reduction in the toxicity of the SMX solution was found following treatment with the PCA/PAA system. More interestingly, several polyphenols structurally related to PCA could also facilitate SMX removal using PAA as the oxidant. Overall, this study proposes a novel strategy for developing reactive quinones dominated AOPs with robust anti-interference performance, as well as enhances the understanding of contaminant removal via an overlooked polymerization pathway in natural polyphenol-related AOPs.

From Quencher to Promoter: Revisiting the Role of 2,4,6-Trimethylphenol (TMP) in Triplet-State Photochemistry of Dissolved Organic Matter

Triplet-state dissolved organic matter (3DOM*) plays a crucial role in environmental aquatic photochemistry, with 2,4,6-trimethylphenol (TMP) frequently used as a chemical probe or quencher due to its high reactivity with 3DOM*. However, the influence of TMP-derived oxidation intermediates on the target photochemical reactions has not been comprehensively examined. This study investigated TMP's effect on the photolysis of sulfamethoxazole (SMX), a common antibiotic found in natural waters, in the presence of different DOM sources or model photosensitizer. Contrary to expectation, TMP significantly accelerated SMX photolysis, with the extent of enhancement depending on TMP and DOM concentrations. Laser flash photolysis and kinetic modeling suggested the long-lived TMP-derived reactive species (TMP-RS), including phenoxyl radicals, semiquinone radicals, and quinones, as the key factors in this process. Unlike 3DOM*, TMP-RS may react with SMX with the formation of non-SMX•+ intermediates. This process prevents the reduction of SMX•+ and the subsequent regeneration of SMX. The kinetic model successfully predicts the dynamic contributions of various factors to SMX oxidation during the reaction, highlighting the critical role of TMP-RS. This study advances our understanding of TMP's involvement in triplet-state photochemistry and suggests a reconsideration of the role long-lived organic RSs play in the transformation of environmental micropollutants.

Mechanochemical Thioglycolate Modification of Microscale Zero-Valent Iron for Superior Heavy Metal Removal

Microscale zero-valent iron (mZVI) is widely used for water pollutant control and environmental remediation, yet its reactivity is still constrained by the inert oxide shell. Herein, we demonstrate that mechanochemical thioglycolate (TG) modification can dramatically enhance heavy metal (NiII, CrVI, CdII, PbII, HgII, and SbIII) removal rates of mZVI by times of 16.7 to 88.0. Compared with conventional impregnation (wet chemical process), this dry mechanochemical process could construct more robust covalent bonding between TG and the inert oxide shell of mZVI through its electron-withdrawing carboxylate group to accelerate the electron release from the iron core, and more effectively strengthen the surface heavy metal adsorption through metal(d)-sulfur(p) orbital hybridization between its thiol group and heavy metal ions. Impressively, this mechanochemically TG-modified mZVI exhibited an unprecedented NiII removal capacity of 580.4 mg Ni g-1 Fe, 17.1 and 9.5 times those of mZVI and wet chemically TG-modified mZVI, respectively. Its application potential was further validated by more than 10 days of stable groundwater NiII removal in a column flow reactor. This study offers a promising strategy to enhance the reactivity of mZVI, and also emphasizes the importance of the modification strategy in optimizing its performance for environmental applications.

Picolinic acid-mediated Mn(II) activated periodate for ultrafast and selective degradation of emerging contaminants: Key role of high-valent Mn-oxo species

The utilization of periodate (PI, IO4-) in metal-based advanced oxidation processes (AOPs) for the elimination of emerging contaminants (ECs) have garnered significant attention. However, the commonly used homogeneous metal catalyst Mn(II) performs inadequately in activating PI. Herein, we exploited a novel AOP technology by employing the complex of Mn(II) with the biodegradable picolinic acid (PICA) to activate PI for the degradation of electron-rich pollutants. The performance of the Mn(II)-PICA complex surpassed that of ligand-free Mn(II) and other Mn(II) complexes with common aminopolycarboxylate ligands. Through scavenger, sulfoxide-probe transformation, and 18O isotope-labeling experiments, we confirmed that the dominant reactive oxidant generated in the Mn(II)-PICA/PI system was high-valent manganese-oxo species (Mn(V)=O). Due to its reliance on Mn(V)=O, the Mn(II)-PICA/PI process exhibited remarkable selectivity and strong anti-interference during EC oxidation in complex water matrices. Nine structurally diverse pollutants were selected for evaluation, and their lnkobs values in the Mn(II)-PICA/PI system correlated well with their electrophilic/nucleophilic indexes, EHOMO, and vertical IP (R2 = 0.79-0.94). Additionally, IO4- was converted into non-toxic iodate (IO3-) without producing undesired iodine species such as HOI, I2, and I3-. This study provides a novel protocol for metal-based AOPs using PI in combination with chelating agents and high-valent metal-oxo species formation during water purification.

Halogenation of Anilines: Formation of Haloacetonitriles and Large-Molecule Disinfection Byproducts

Aniline-related structures are common in anthropogenic chemicals, such as pharmaceuticals and pesticides. Compared with the widely studied phenolic compounds, anilines have received far less assessment of their disinfection byproduct (DBP) formation potential, even though anilines and phenols likely exhibit similar reactivities on their respective aromatic rings. In this study, a suite of 19 aniline compounds with varying N- and ring-substitutions were evaluated for their formation potentials of haloacetonitriles and trihalomethanes under free chlorination and free bromination conditions. Eight of the aniline compounds formed dichloroacetonitrile at yields above 0.50%; the highest yields were observed for 4-nitroaniline, 3-chloroaniline, and 4-(methylsulfonyl)aniline (1.6-2.3%). Free bromination generally resulted in greater haloacetonitrile yields with the highest yield observed for 2-ethylaniline (6.5%). The trihalomethane yields of anilines correlated with their haloacetonitrile yields. Product analysis of aniline chlorination by liquid chromatography-high-resolution mass spectrometry revealed several large-molecule DBPs, including chloroanilines, (chloro)hydroxyanilines, (chloro)benzoquinone imines, and ring-cleavage products. The product time profiles suggested that the reaction pathways include initial ring chlorination and hydroxylation, followed by the formation of benzoquinone imines that eventually led to ring cleavage. This work revealed the potential of aniline-related moieties in micropollutants as potent precursors to haloacetonitriles and other emerging large-molecule DBPs with the expected toxicity.

Photochemical Transformation of Monochloramine Induced by Triplet State Dissolved Organic Matter

The photoexcited dissolved organic matter (DOM) could produce reactive intermediates, affecting chemical oxidant transformation in UV based advanced oxidation processes (AOPs). This study confirmed the critical role of triplet state DOM (3DOM*), generated from DOM photoexcitation, in the transformation of monochloramine (NH2Cl), a commonly used chemical oxidant and disinfectant in water treatment. NH2Cl (42.25 μM, as Cl2) was decayed by 17.4-73.4 % within 60 min, primarily due to 3DOM* , in DOM (2-30 mgC L-1) solutions irradiated by 365 nm, where NH2Cl has no absorption. The second-order quenching rate constants of triplet state model photosensitizers by NH2Cl were determined to be 0.95(± 0.04)-4.49(± 0.04)× 108 M-1 s-1 by using laser flash photolysis. As a reductant, 3DOM* reacted with NH2Cl through one-transfer mechanism, leading to amino radical (NH2•) generation, which then transferred to ammonia (NH4+, pKa 9.25) through H-abstraction by the phenolic moieties in DOM. Additionally, the intermediate product of 3DOM* oxidized by NH2Cl or those triplet state quinones can hydrolyze to form phenolic moieties, elevating NH4+ yield to higher than 99% upon 365 nm irradiation. These findings suggest that the widespread DOM can be applied to convert NH2Cl via 3DOM* with minimal toxic risks.