Weihua Song

Solar-driven activation of peroxymonosulfate (PMS) is a promising advanced oxidation process for water and wastewater treatment; however, the kinetics of emerging contaminant removal in real water matrices remain poorly understood. In this study, we investigated PMS photosensitization in three environmentally relevant water matrices and evaluated its role in the abatement of 42 pharmaceuticals and personal care products (PPCPs). Under simulated solar irradiation, CDOM promoted PMS transformation and generated multiple photochemically produced reactive intermediates (PPRIs). A multiple-probe chemical method was employed to quantify the cumulative exposure doses of HO•, SO4•–, Cl2•–, CO3•–, and 1O2. Across the tested matrices, PPRI exposure doses followed the order CO3•– > 1O2 > Cl2•– ∼ SO4•– > HO•. The measured PPRI exposure doses, together with second-order rate constants, reasonably predicted PPCP removal in the solar/PMS/CDOM system. For several phenolic and aniline-containing PPCPs, DOM-mediated inhibition was observed and was attributed to reductive back-reaction of one-electron oxidation intermediates; incorporating inhibition factors improved the agreement between calculated and experimental removal efficiencies. Furthermore, four commonly detected PPCPs, sucralose, metformin, caffeine, and carbamazepine, were first identified as practical indicator compounds for estimating PPRI exposure without adding external probe chemicals. These findings provide a mechanistic framework for predicting PPCP attenuation during solar/PMS treatment in CDOM-rich waters and highlight the importance of secondary radicals in real aquatic matrices.

Microplastic-derived dissolved organic matter (MP-DOM) is emerging as a non-negligible organic input to marine systems, yet how its interactions with seawater DOM (S-DOM) regulate the photochemical formation of reactive bromine species (RBS) remains unclear. In this study, the RBS formation potential of poly(vinyl chloride) (PVC)-derived MP-DOM, S-DOM, and their mixed systems was systematically investigated under simulated solar irradiation. MP-DOM input exerted a nonlinear ratio-dependent regulatory effect on S-DOM-mediated RBS formation. At relatively low MP-DOM proportions (5–20%), the formation rate constant of 4-bromo-3,5-dimethyl-1H-pyrazole (Br-DMPZ), which represents the RBS formation potential, increased relative to S-DOM alone, reaching a maximum enhancement of 47.4% at 20% MP-DOM under seawater halide ion conditions (540 mM Cl– + 0.8 mM Br–). This enhancement was associated with increased quinone-/carbonyl-related redox-active structures, electron exchange capacity, and tannin-like components, which could facilitate photoinduced charge separation, electron transfer, and Br– oxidation. In contrast, higher MP-DOM proportions (40–50%) promoted hydrophobic lipid-like mixed assemblies, enhancing excited-state quenching and suppressing electron transfer and RBS photogeneration. These findings demonstrate that MP-DOM is not an inert byproduct of microplastic aging but rather an important regulatory factor in S-DOM-mediated halogen photochemistry in plastic-impacted marine waters.

Multiclass cyanotoxins often co-occur in aquatic environments, but their broad structural and polarity ranges make comprehensive analysis difficult using a single chromatographic method. Here, we have developed a single-workflow column-switching liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for simultaneous analysis of 36 cyanotoxins in aqueous samples with minimal sample preparation. The method combines large-volume injection, enrichment on a porous graphitic carbon (PGC) trap column, and valve switching with two complementary separation modes: reversed-phase (RP) chromatography and hydrophilic interaction liquid chromatography (HILIC). The moderately polar cyanotoxins were retained and separated on an RP column, whereas highly hydrophilic paralytic shellfish toxins (PSTs) that were poorly retained by RP separation were captured by the trap cartridge and subsequently transferred to a HILIC column for separation. This configuration substantially improved chromatographic retention and enhanced PST signal-to-noise ratios by up to 40-fold compared with direct RP analysis. Method quantification limits of all target cyanotoxins were ≤ 15 ng L-1 for freshwater matrices and ≤ 30 ng L-1 for seawater matrices. Matrix effects were modest in freshwater for 500 µL injection volume, whereas seawater caused stronger ion suppression that was mitigated by reducing the injection volume to 200 µL. The method accuracies (spike recoveries) of all tested cyanotoxins ranged from 82.5 to 117.0% for freshwater and 79.0 to 119.0% for seawater samples. Overall, this automated RP–PGC–HILIC column-switching platform provides a sensitive and high-throughput strategy for multiclass cyanotoxin monitoring across diverse water matrices.

Microcystins (MCs) are one group of the most toxic cyanobacterial metabolites in surface waters, yet their selective and efficient removal under environmentally relevant conditions remains a major challenge. Here, we identify the peroxyacetyl radical (PAR) as a previously unrecognized oxidant for MC degradation. We first quantified the bimolecular reaction rate constants of PAR with 10 MC congeners, which fell within a narrow range of (3.94-6.69) × 10⁷ M^-1 s^-1. Our results revealed that PAR selectively attacks the conjugated diene in the Adda side chain, yielding dihydroxylation products while leaving the aromatic ring and Mdha largely unreacted. High-resolution MS/MS confirmed the formation of three dominant dihydroxylated products, accounting for ∼80% of total yield. The environmental relevance of PAR was further demonstrated in solar/peracetic acid (PAA) systems, where MCs were degraded mainly via two parallel pathways: HO^• oxidation and PAR oxidation. Quantitative radical budget analysis revealed that PAR contributed 1.5-3.5 times more to MC degradation than HO^• across varied PAA/H₂O₂ ratios and DOM levels. Overall, this study provides the first mechanistic and kinetic evidence that PAR is a selective oxidant capable of efficiently degrading MCs under sunlight and realistic water matrices, highlighting its potential for next-generation cyanotoxin treatment.

Algal-derived dissolved organic matter (ADOM) released by algal blooms is an important photosensitizer in natural water bodies and participates in a variety of photochemical processes. This study investigates the formation and reactivity of photochemically generated long-lived reactive species (LLRSs, specifically oxidative organic radicals herein) from ADOM and compares it with that from terrestrial dissolved organic matter (SRDOM) under full spectra, ultraviolet (UV) portion (290-420 nm), and visible portion (420-710 nm) of solar irradiation. The LLRSs from ADOM exhibited higher one-electron reduction potentials (1.00-1.06 V in solar full spectrum; 0.91-1.00 V in visible light) than those from SRDOM (0.91-1.00 V in solar full spectrum; 0.85-0.91 V in visible light). The apparent quantum yield of LLRSs (Φ_LLRSs^app) from ADOM was 2.2 × 10^-4 under full spectra of solar irradiation, slightly higher than that of SRDOM (1.8 × 10^-4). Although Φ_LLRSs^app from ADOM under visible light (1.3 × 10^-4) was about five times lower than that under UV irradiation (6.3 × 10^-4), their formation rates were comparable (1.7 × 10^-9 M s^-1 for visible light vs. 2.6 × 10^-9 M s^-1 for UV), owing to the stronger light absorption in the visible range. The formation of LLRSs was classified into dissolved oxygen (DO)-inhibited and DO-promoted pathways, with DO-inhibited processes accounting for 98% and 87% of the total LLRSs formation in SRDOM and ADOM, respectively. The degradation of 27 emerging organic contaminants demonstrated greater LLRSs involvement in ADOM, especially under visible light. These results underscore the key role of LLRSs in contaminant attenuation in bloom-impacted waters and highlight the importance of visible-light-driven photochemistry in enhancing the self-purification potential of ADOM-affected waters.

The peroxyacetyl radical [CH₃C(O)O₂^•, PAR] is a highly selective oxidant that plays a critical role in peracetic acid (PAA)-based advanced oxidation processes (AOPs). The kinetic and mechanisms of PAR reactions with dissolved organic matter (DOM), a major sink and reactive matrix in natural waters, have been poorly understood. In this study, we quantitatively characterized the reactivity of PAR toward diverse DOM isolates using a competitive kinetic method. The second-order rate constants between PAR and DOM isolates (k_PAR,DOM) were determined, ranging from (4.0 ± 0.2) × 10¹ to (4.0 ± 0.2) × 10² L mg_C^-1 s^-1 at pH 7.3. Our results demonstrated that phenolic moieties are the dominant PAR-reactive sites in DOM. The measured k_PAR,DOM values correlated strongly with bulk DOM properties, including the phenolic content, electron-donating capacity (EDC), and optical E₂/E₃, enabling empirical prediction of DOM reactivity toward PAR. Beyond quenching, DOM exhibited dual effects on PAR-mediated contaminant oxidation. Anilines or phenols with an electron-drawing group (e.g., -CN and -Cl) were inhibited through back reduction of their radical cations by DOM, while those with an electron-donating group (e.g., -CH₃) were promoted through secondary oxidation by reactive DOM-derived intermediates (RIs_DOM). These findings establish the first comprehensive kinetic framework for PAR-DOM reactions in aquatic environments.

Photochemically produced reactive intermediates (PPRIs), including HO^•, ¹O₂, triplet chromophoric dissolved organic matter (³CDOM*), CO₃^•-, and long-lived oxidative organic radicals (LOORs), play crucial roles in photochemical transformations in sunlit waters. Despite the existence of well-established probe methods for HO^•, ¹O₂, and ³CDOM*, reliable techniques for quantification of the CO₃^•- and LOORs remain lacking. Here, we developed a multiple-probe chemical method capable of quantifying all five key PPRIs under solar irradiation. The method integrates two complementary probe sets: (i) a high-level probe set (5 μM terephthalic acid (TA), furfuryl alcohol (FFA), 2,4,6-trimethylphenol (TMP), and 3,4-dimethoxyphenol (DMOP)) to determine ³CDOM* while suppressing the CO₃^•- and LOOR contribution, and (ii) a low-level probe set (0.1 μM 4-methylphenol, 0.05 μM TMP and DMOP, 5 μM TA and FFA) to quantify CO₃^•- and LOORs with minimal probe-probe interference. Using this framework, the five PPRIs were determined in both simulated and natural waters. Kinetic modeling revealed that at Br^- < 10 μM, halogen radicals minimally affected the method accuracy, confirming its applicability to most freshwaters and wastewater effluents. Overall, this multiple PC method provides a practical, sensitive, and accessible tool for the simultaneous determination of five major PPRIs in sunlit waters.

In UV-based advanced oxidation processes (AOPs), chemical probes and scavengers are widely used to identify radical species responsible for organic contaminant removal due to their simplicity and selectivity. A prevailing assumption is that these reagents selectively target radicals without interacting with triplet excited-state contaminants generated under UV irradiation. Here, we challenge this assumption by demonstrating that triplet-state contaminants can directly interact with commonly used probes and scavengers, revealing an overlooked transformation pathway that biases radical identification and mechanistic interpretation. Using naproxen as a model compound, we show that seven representative reagents (isopropanol, p-benzoquinone, furfuryl alcohol, tert-butanol, methanol, ethanol, and HCO₃^-) efficiently quench triplet-state naproxen, with bimolecular rate constants ranging from 1.0 × 10⁶ to 7.1 × 10⁸ M^-1 s^-1. The quenching process followed saturation kinetics and can be well described by the Hanes-Woolf model. Transient absorption spectroscopy and quantum chemical calculations reveal feasible electron transfer and proton-coupled electron transfer pathways. Further, in a UV/H₂O₂ system, we found that isopropanol promotes naproxen degradation with triplet-state interactions, leading to significant underestimation of ^•OH contribution. These results highlight a critical interference pathway in AOPs and underscore the need for caution when using probes and scavengers for kinetic and mechanistic evaluations.

Peracetic acid (PAA) has been extensively investigated as an alternative disinfectant for water and wastewater treatments. However, the photochemical transformation of PAA in sunlit surface waters has not been previously investigated. For the first time, the photosensitized transformation of PAA was here observed in chromophoric dissolved organic matter (CDOM)-enriched solutions under simulated solar irradiation. Triplet CDOM (³CDOM^*) is here proposed to play a key role in the phototransformation processes. Using triplet model compounds, the reaction rate constant of ³CDOM^* with PAA can be estimated to around 1.1 × 10⁸ M^-1 s^-1. The reaction mechanism of triplet with PAA involves both energy- and electron-transfer. CH₃COO^• was generated from the reaction between PAA and excited triplets. Furthermore, HO^• and organic reactive species (i.e., CH₃COOO^• and CH₃COO^•) are involved in the photochemical transformation of PAA in sunlit CDOM-enriched solutions. These reactive species (including HO^•, CH₃COOO^•, and CH₃COO^•) also play a role in the removal of contaminants of emerging concerns (CECs). Overall, the current study provided new insights into the photochemical transformation of PAA in CDOM-enriched solutions. The solar irradiation of wastewater with PAA enhancement could be a useful and economically beneficial advanced oxidation process for CEC abatement.