Mechanistic Study on the Role of Soluble Microbial Products in Sulfate Radical-Mediated Degradation of Pharmaceuticals

Insights into the transformation of natural organic matter during UV/peroxydisulfate treatment by FT-ICR MS and machine learning: Non-negligible formation of organosulfates

Natural organic matter (NOM) is a major sink of radicals in advanced oxidation processes (AOPs) and understanding the transformation of NOM is important in water treatment. By using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in conjunction with machine learning, we comprehensively investigated the reactivity and transformation of NOM, and the formation of organosulfates during the UV/peroxydisulfate (PDS) process. After 60 min UV/PDS treatment, the CHO formula number and dissolved organic carbon concentration significantly decreased by 83.4 % and 74.8 %, respectively. Concurrently, the CHOS formula number increased substantially from 0.7 % to 20.5 %. Machine learning identifies DBE and AImod as the critical characteristics determining the reactivity of NOM during UV/PDS treatment. Furthermore, linkage analysis suggests that decarboxylation and dealkylation reactions are dominant transformation pathways, while the additions of SO3 and SO4 are also non-negligible. According to SHAP analysis, the m/z, number of oxygens, DBE and O/C of NOM were positively correlated with the formation of organosulfates in UV/PDS process. 92 organosulfates were screened out by precursor ion scan of HPLC-MS/MS and verified by UPLC-Q-TOF-MS, among which, 7 organosufates were quantified by authentic standards with the highest concentrations ranging from 2.1 to 203.0 ng L‒1. In addition, the cytotoxicity of NOM to Chinese Hamster Ovary (CHO) cells increased by 13.8 % after 30 min UV/PDS treatment, likely responsible for the formation of organosulfates. This is the first study to employ FT-ICR MS combined with machine learning to identify the dominant NOM properties affecting its reactivity and confirmed the formation of organosulfates from sulfate radical oxidation of NOM.

Direct Determination of Absolute Radical Quantum Yields in Hydroxyl and Sulfate Radical-Based Treatment Processes

The absolute radical quantum yield (Φ ) is a critical parameter to evaluate the efficiency of radical-based processes in engineered water treatment. However, measuring Φ is fraught with challenges, as current quantification methods lack selectivity, specificity, and anti-interference capabilities, resulting in significant error propagation. Herein, we report a direct and reliable time-resolved technique to determine Φ at pH 7.0 for commonly used radical precursors in advanced oxidation processes. For H2O2 and peroxydisulfate (PDS), the values of Φ •OH and Φ SO 4 • - at 266 nm were measured to be 1.10 ± 0.01 and 1.46 ± 0.05, respectively. For peroxymonosulfate (PMS), we developed a new approach to determine Φ • OH PMS with terephthalic acid as a trap-and-trigger probe in the nonsteady state system. For the first time, the Φ • OH PMS value was measured to be 0.56 by the direct method, which is stoichiometrically equal to Φ SO 4 • - PMS (0.57 ± 0.02). Additionally, radical formation mechanisms were elucidated by density functional theory (DFT) calculations. The theoretical results showed that the highest occupied molecular orbitals of the radical precursors are O-O antibonding orbitals, facilitating the destabilization of the peroxy bond for radical formation. Electronic structures of these precursors were compared, aiming to rationalize the tendency of the Φ values we observed. Overall, this time-resolved technique with specific probes can be used as a reliable tool to determine Φ , serving as a scientific basis for the accurate performance evaluation of diverse radical-based treatment processes.

Waste milk humification product can be used as a slow release nano-fertilizer

The demand for milk has increased globally, accompanied by an increase in waste milk. Here, we provide an artificial humification technology to recycle waste milk into an agricultural nano-fertilizer. We use KOH-activated persulfate to convert waste milk into fulvic-like acid and humic-like acid. We mix the product with attapulgite to obtain a slow-release nano fulvic-like acid fertilizer. We apply this nano-fertilizer to chickweeds growing in pots, resulting in improved yield and root elongation. These results indicate that waste milk could be recycled for agricultural purposes, however, this nano-fertilizer needs to be tested further in field experiments.

Factors hindering the degradation of pharmaceuticals from human urine in an iron-activated persulfate system

This study investigated the degradation of clofibric acid (CFA), bezafibrate (BZF), and sulfamethoxazole (SMX) in synthetic human urine using a novel mesoporous iron powder-activated persulfate system (mFe-PS system), and identified the factors limiting their degradation in synthetic human urine. A kinetic model was established to expose the radical production in various reaction conditions, and experiments were conducted to verify the modeling results. In the phosphate-containing mFe-PS system, the 120 min removal efficiency of CFA decreased from 95.1% to 76.6% as the phosphate concentration increased from 0.32 to 6.45 mmol/L, but recovered to 90.5% when phosphate concentration increased to 16.10 mmol/L. Meanwhile, the increased concentration of phosphate from 0.32 to 16.10 mmol/L reduced the BZF degradation efficacy from 91.5% to 79.0%, whereas SMX removal improved from 37.3% to 62.9%. The mFe-PS system containing (bi)carbonate, from 4.20 to 166.70 mmol/L, reduced CFA and BZF removal efficiencies from 100% to 76.8% and 80.4%, respectively, and SMX from 83.5% to 56.7% within a 120-min reaction time. In addition, alkaline conditions (pH ≥ 8.0) inhibited CFA and BZF degradations, while nonacidic pH (pH ≥ 7.0) remarkably inhibited SMX degradation. Results of the kinetic model indicated the formation of phosphate (H2PO4·/HPO4·-) and/or carbonate radicals (CO3·-) could limit pharmaceutical removal. The transformation products (TPs) of the pharmaceuticals revealed more incompletely oxidized TPs occurred in the phosphate- and (bi)carbonate-containing mFe-PS systems, and indicated that H2PO4·/HPO4·- mainly degraded pharmaceuticals via a benzene ring-opening reaction while CO3·- preferentially oxidized pharmaceuticals via a hydroxylation reaction.

Differences in the degradation behavior of disinfection by-products in UV/PDS and UV/H2O2 processes and the effect of their chemical properties

In this work, sixteen typical chlorinated and brominated aromatic disinfection by-products (DBPs) were selected as examples to investigate their different degradation mechanisms initiated by HO• and SO4•-. Addition reactions were the main mode of degradation of DBPs by HO•, while SO4•- dominated H-abstraction reactions and single electron transfer reactions. Chlorinated compounds had higher reactivity than brominated compounds. Furthermore, substituents with stronger electron-donating effects promoted the electrophilic reaction of DBPs with the two radicals. In addition, we developed a model based on the chemical properties LUMO, fmax-, and hardness for predicting the average reaction energy barriers for the initial reactions of DBPs with HO• and SO4•-. The model had good predictive performance for the difficulty of degradation of different DPBs by HO• and SO4•-, with R2 values of 0.85 and 0.87, respectively. Through the degradation efficiency simulation, we found that longer reaction times, higher oxidant concentrations and lower pollutant concentrations were more favorable for the removal of DBPs. The UV/PDS process showed better degradation of DBPs than the UV/H2O2 process. In addition, most degradation products of DBPs exhibited less toxicity to aquatic organisms than their parent compounds. This study provided theoretical guidance for the degradation and removal of other aromatic DBPs at the molecular level.

Importance of Chain Length in Propagation Reaction on •OH Formation during Ozonation of Wastewater Effluent

During the ozonation of wastewater, hydroxyl radicals (•OH) induced by the reactions of ozone (O3) with effluent organic matters (EfOMs) play an essential role in degrading ozone-refractory micropollutants. The •OH yield provides the absolute •OH formation during ozonation. However, the conventional "tert-Butanol (t-BuOH) assay" cannot accurately determine the •OH yield since the propagation reactions are inhibited, and there have been few studies on •OH production induced by EfOM fractions during ozonation. Alternatively, a "competitive method", which added trace amounts of the •OH probe compound to compete with the water matrix and took initiation reactions and propagation reactions into account, was used to determine the actual •OH yields (Φ) compared with that obtained by the "t-BuOH assay" (φ). The Φ were significantly higher than φ, indicating that the propagation reactions played important roles in •OH formation. The chain propagation reactions facilitation of EfOMs and fractions can be expressed by the chain length (n). The study found significant differences in Φ for EfOMs and fractions, precisely because they have different n. The actual •OH yield can be calculated by n and φ as Φ = φ (1 + n)/(nφ + 1), which can be used to accurately predict the removal of micropollutants during ozonation of wastewater.

New insight into the removal process of benzotriazole UV stabilizers by UV/H2O2: Integrating quantum chemical calculation with CFD simulation

Benzotriazole UV stabilizers (BT-UVs) are important UV absorbers. As high-production chemicals and potential hazards, their ubiquitous presence in aquatic environments is of greatly pressing concern. Herein, the removal of six typical BT-UVs by UV/H2O2 was comprehensively investigated by quantum chemistry calculation integrated with CFD simulation. Utilizing such a micro and macro incorporated model in treating contaminants is the first report. From the micro-view, degradation mechanisms of BT-UVs by •OH oxidation were determined, and corresponding rate constants were obtained with values of 109∼1010 M-1s-1. In a macroscopic aspect, combining the established kinetic model and CFD simulation, the effects of UV lamp power (P), volumetric flow rate (Qv), and H2O2 dosage ([H2O2]0) on removal yields of BT-UVs were expounded, increasing P or [H2O2]0 or decreasing Qv are effective in improving removal yields of BT-UVs, but the enhancement was abated when P or [H2O2]0 increased to a certain level. When [H2O2]0 is 5 mg/L and Qv is decreased from 0.1 to 0.05 m3/h, the removal yields of BT-UVs could achieve more than 95% (P = 150 W) and 99% (P = 250 W), respectively. This work provides a new interdisciplinary insight for investigating organic contaminant removal in potential industrial applications of UV/H2O2 systems.

Merits and Limitations of Radical vs. Nonradical Pathways in Persulfate-Based Advanced Oxidation Processes

Urbanization and industrialization have exerted significant adverse effects on water quality, resulting in a growing need for reliable and eco-friendly treatment technologies. Persulfate (PS)-based advanced oxidation processes (AOPs) are emerging as viable technologies to treat challenging industrial wastewaters or remediate groundwater impacted by hazardous wastes. While the generated reactive species can degrade a variety of priority organic contaminants through radical and nonradical pathways, there is a lack of systematic and in-depth comparison of these pathways for practical implementation in different treatment scenarios. Our comparative analysis of reaction rate constants for radical vs. nonradical species indicates that radical-based AOPs may achieve high removal efficiency of organic contaminants with relatively short contact time. Nonradical AOPs feature advantages with minimal water matrix interference for complex wastewater treatments. Nonradical species (e.g., singlet oxygen, high-valent metals, and surface activated PS) preferentially react with contaminants bearing electron-donating groups, allowing enhancement of degradation efficiency of known target contaminants. For byproduct formation, analytical limitations and computational chemistry applications are also considered. Finally, we propose a holistically estimated electrical energy per order of reaction (EE/O) parameter and show significantly higher energy requirements for the nonradical pathways. Overall, these critical comparisons help prioritize basic research on PS-based AOPs and inform the merits and limitations of system-specific applications.

Overlooked Transformation of Nitrated Polycyclic Aromatic Hydrocarbons in Natural Waters: Role of Self-Photosensitization

Photochemical transformation is an important process that involves trace organic contaminants (TrOCs) in sunlit surface waters. However, the environmental implications of their self-photosensitization pathway have been largely overlooked. Here, we selected 1-nitronaphthalene (1NN), a representative nitrated polycyclic aromatic hydrocarbon, to study the self-photosensitization process. We investigated the excited-state properties and relaxation kinetics of 1NN after sunlight absorption. The intrinsic decay rate constants of triplet (³1NN*) and singlet (¹1NN*) excited states were estimated to be 1.5 × 10⁶ and 2.5 × 10⁸ s^-1, respectively. Our results provided quantitative evidence for the environmental relevance of ³1NN* in waters. Possible reactions of ³1NN* with various water components were evaluated. With the reduction and oxidation potentials of -0.37 and 1.95 V, ³1NN* can be either oxidized or reduced by dissolved organic matter isolates and surrogates. We also showed that hydroxyl (^•OH) and sulfate (SO₄^•-) radicals can be generated via the ³1NN*-induced oxidation of inorganic ions (OH^- and SO₄^2-, respectively). We further investigated the reaction kinetics of ³1NN* and OH^- forming ^•OH, an important photoinduced reactive intermediate, through complementary experimental and theoretical approaches. The rate constants for the reactions of ³1NN* with OH^- and 1NN with ^•OH were determined to be 4.22 × 10⁷ and 3.95 ± 0.01 × 10⁹ M^-1 s^-1, respectively. These findings yield new insights into self-photosensitization as a pathway for TrOC attenuation and provide more mechanistic details into their environmental fate.

Trade-off effect of dissolved organic matter on degradation and transformation of micropollutants: A review in water decontamination

The degradation of micropollutants by various treatments is commonly affected by the ubiquitous dissolved organic matter (DOM) in the water environment. To optimize the operating conditions and decomposition efficiency, it is necessary to consider the impacts of DOM. DOM exhibits varied behaviors in diverse treatments, including permanganate oxidation, solar/ultraviolet photolysis, advanced oxidation processes, advanced reduction process, and enzyme biological treatments. Besides, the different sources (i.e., terrestrial and aquatic, etc) of DOM, and operational circumstances (i.e., concentration and pH) fluctuate different transformation efficiency of micropollutants in water. However, so far, systematic explanations and summaries of relevant research and mechanism are rare. This paper reviewed the "trade-off" performances and the corresponding mechanisms of DOM in the elimination of micropollutants, and summarized the similarities and differences for the dual roles of DOM in each of the aforementioned treatments. Inhibition mechanisms typically include radical scavenging, UV attenuation, competition effect, enzyme inactivation, reaction between DOM and micropollutants, and intermediates reduction. Facilitation mechanisms include the generation of reactive species, complexation/stabilization, cross-coupling with pollutants, and electron shuttle. Moreover, electron-drawing groups (i.e., quinones, ketones functional groups) and electron-supplying groups (i.e., phenols) in the DOM are the main contributors to its trade-off effect.

Mechanistic and quantitative profiling of electro-Fenton process for wastewater treatment

Electro-Fenton (EF) process represents an energy-efficient and scalable advanced oxidation technology (AOT) for micropollutants removal in wastewaters. However, mechanistic profiling and quantitation of contribution of each subprocess (i.e., adsorption at electrode, coagulation, radical oxidation, electrode oxidation/reduction, and H₂O₂ oxidation) to the overall degradation are substantially unclear, resulting in difficulty in tunability and optimization for different treatment scenarios. In this study, we investigated degradation kinetics of a target micropollutant in an EF system. The contribution of all possible subprocesses was elucidated by comparing the observed degradation rate in the EF system with the sum of the kinetics in each subprocess. The results indicated that the overall degradation can be attributed to the synergistic action of the above-mentioned subprocesses. The radical oxidation accounts for 87% elimination, followed by electrode reoxidation/reduction of 7.7%. These results not only advance the fundamental understanding of synergistic effect in EF system, but also open new possibilities to optimize these techniques for better scalability. In addition, the methodology in this study could potentially boost the in-depth exploration of subprocess contribution in other Fenton-like systems.

The multiple roles of phenols in the degradation of aniline contaminants by sulfate radicals: A combined study of DFT calculations and experiments

Recent research revealed inhibition or enhancement of dissolved organic matter (DOM) to the degradation of trace organic contaminants (TrOC) in natural and engineered water systems. Phenols containing acetyl, carboxyl, formyl, hydroxy, and methoxy groups were selected as the model DOM to quantitatively study their roles in the degradation of simple anilines, sulfonamide antibiotics, phenylurea pesticides by sulfate radicals (SO₄^•-). Experimental results found that p-methoxyphenol inhibited aniline and sulfamethoxazole degradation by thermally activated peroxydisulfate (TAP), while p-acetylphenol slightly promoted aniline degradation. Quantum chemical calculations were applied to study the microscopic mechanism and kinetics of phenols affecting the degradation of aniline pollutants (AN) in three ways: competitively reacting with SO₄^•-, repairing aniline cationic radicals (AN^•+) and phenylaminyl radicals (AN(-H)^•), and generating phenoxy radicals to degrade anilines. Generally, the degradation of sulfonamides and phenylureas prefer to be inhibited by hydroxy- and methoxy-phenols with low oxidation potential (E_ox), due to their diffusion-limiting reaction with SO₄^•- and rapid back-reduction AN^•+ with the calculated rate constants of (0.02 - 6.38) × 10⁹ M^-1 s^-1. Phenols repairing AN(-H)^• through H abstraction reaction is speculated to possibly dominate the joint degradation of phenols and anilines by TAP, which has a poor correlation with E_ox. This study provides mechanistic insight into the chemical behavior of complex and heterogeneous DOM in complex aqueous environments.

Comparison of ribavirin degradation in the UV/H2O2 and UV/PDS systems: Reaction mechanism, operational parameter and toxicity evaluation

Residues in surface water of ribavirin, which used extensively during the COVID-19 pandemic, have become an emerging issue due to its adverse impact on the environment and human health. UV/H2O2 and UV/peroxydisulfate (PDS) have different degradation effects on ribavirin, and the same operational parameter have different effects on the two processes. In this study, the reaction mechanism and degradation efficiency for ribavirin were studied to compare the differences under UV/H2O2 and UV/PDS processes. We calculated the total rate constants of ribavirin with HO• and SO4 •- in the liquid phase as 2.73 × 108 and 9.39 × 105 M-1s-1. The density functional theory (DFT) calculation results showed that HO• and SO4 •- react more readily with ribavirin via H-abstraction (HAA). The nitrogen-containing heterocyclic ring is difficult to undergo ring-opening degradation. The UV/PDS process was more stable and performed better than the UV/H2O2 for the ribavirin degradation when the same molar oxidant dosage was applied. HO• plays an extremely important role in the degradation of ribavirin by UV/PDS. The reason for this phenomenon is the combination of the higher yield of HO• produced in the UV/PDS process and the faster reaction rate of ribavirin with HO•. The UV/H2O2 process is more sensitive to pH than UV/PDS. Alkaline condition can significantly inhibit the ribavirin degradation. The effects of natural organic matter (NOM) and ribavirin concentration were also compared. Eventually, the toxicity prediction of the product showed that the opening-ring products were more toxic than the parent compound.

Different reaction mechanisms of SO4•- and •OH with organic compound interpreted at molecular orbital level in Co(II)/peroxymonosulfate catalytic activation system

Hydroxyl radical (•OH) and sulfate radical (SO₄•^-) produced in advanced oxidation processes (AOPs) have been widely studied for organic contaminants degradation, however, the different radical characteristics and reaction mechanisms on organics degradation are still needed. In this study, a homogeneous Co(II)/peroxymonosulfate activation system was established for caffeine (CAF) degradation, and pH was controlled to regulate the radicals production. The different attack routes driven by SO₄•^- and •OH were deeply explored by transformation products (TPs) identification and theoretical calculations. Specifically, a method on dynamic electronic structure analysis of reactants (R), transition state (TS) and intermediates (IMs) during reaction was proposed, which was applied to elucidate the underlying mechanism of CAF oxidation by •OH and SO₄•^- at the molecular orbital level. In total, SO₄•^- is kinetically more likely to attack CAF than •OH due to its higher oxidation potential and electrophilicity index. Single electron transfer reaction (SET) is only favorable for SO₄•^-due to its higher electron affinity than •OH, while only •OH can react with CAF via hydrogen atom abstraction (HAA) route. Radical adduct formation (RAF) is the most favorable route for both •OH and SO₄•^- attack according to both kinetics and thermodynamics results. These findings can significantly promote the understanding on the degradation mechanism of organic pollutants driven by •OH and SO₄•^- in AOPs.

Efficient degradation of alkyl imidazole ionic liquids in simulated sunlight irradiated periodate system: Kinetics, reaction mechanisms, and toxicity evolution

As a class of emerging aquatic pollutants, alkylimidazole-based ionic liquids (AM-ILs) have received extensive attention due to the large acute toxicity to aquatic organisms. Therefore, in order to protect both aquatic organisms and human beings, it is necessary to seek an efficient and environmental-friendly technology for removal of AM-ILs from water bodies. In this work, we found that under simulated sunlight (Xe lamp) irradiation, periodate (KIO₄, PI) could efficiently degrade 1-hexyl-2,3-dimethylimidazolium bromide ([HMMIm]Br), a representative AM-ILs with six carbon atoms in the side chain. Kinetics experiments on the degradation of [HMMIm]Br were performed, and the results showed that a high degradation efficiency (≥90.00%) of the cation ([HMMIm]⁺) was still maintained under harsh water conditions of strong acidity/alkaliny or with various non-target inorganic ions. More importantly, the anion of bromide ion (Br^-) was not oxidized to the carcinogenic bromate (BrO₃^-) in current reaction system. The excited stated PI (marked as PI*) was detected by Laser flash photolysis, and it was an important reactive species for [HMMIm]⁺ degradation. As rationalized by theoretical calculations and scavenging experiments, the main oxidation mechanisms of [HMMIm]⁺ were hydroxyl radicals induced substitution reaction, PI* initiated electron and double oxygen transfer, and direct photolysis mediated chemical bond cleavage reaction, which contributed to 73%, 21%, and 6% of [HMMIm]⁺ degradation, respectively. Moreover, toxicity evaluation by ECOSAR software indicated that the oxidation products were generally less toxic to three aquatic organisms (fish, water flea, and green algae) than the target molecule [HMMIm]Br. In conclusion, this work proposed novel oxidation mechanisms of sunlight-activated PI system, and the findings may inspire further researches on the application of photoactivated hypervalent acids in water purification.

Quantification of the diverse inhibitory effects of dissolved organic matter on transformation of micropollutants in UV/persulfate treatment

Dissolved organic matter (DOM), ubiquitous in natural waters, is known to inhibit the degradation of micropollutants in the advanced oxidation processes such as the UV/peroxydisulfate process. However, the quantitative understanding of the inhibitory pathways is missing. In this study, guanosine, aniline and catechol belonging to amines, purines and phenols were first investigated due to their resistance to UV irradiation at 254 nm and similar reactivity with SO₄^•- and HO^•, respectively. The presence of 0.5 mg_C L^-1 Suwannee River NOM (SRNOM) inhibited their degradation rates by 72.9%, 54.5%, and 32.4%, respectively, despite their similar degradation rates in the absence of SRNOM. The results highlight the importance of reverse reduction of oxidation intermediates to the parent compound by antioxidant moieties in SRNOM besides the inner filtering and radical scavenging effects. The three inhibitory pathways were quantified for 34 common micropollutants. In the presence of 0.5 mg_C L^-1 SRNOM, inner filtering effect was found to contribute less than 2.8% of the inhibitory percentages (IP). Radical scavenging effects contribute between 10.7% and 38.9% and compounds having lower reactivity with SO₄^•- (< 4.0 × 10⁹ M^-1 s^-1) tended to be inhibited more strongly. The IP of reverse reduction effects of SRNOM varied significantly from none up to 70.8%. It was linearly related with a micropollutant's reduction potential. Purines and amines generally exhibited more pronounced reverse reduction inhibition than phenols. The results of this study provide guidance on improving the elimination efficiency of micropollutants.

Multiple Roles of Dissolved Organic Matter in Advanced Oxidation Processes

Advanced oxidation processes (AOPs) can degrade a wide range of trace organic contaminants (TrOCs) to improve the quality of potable water or discharged wastewater effluents. Their effectiveness is impacted, however, by the dissolved organic matter (DOM) that is ubiquitous in all water sources. During the application of an AOP, DOM can scavenge radicals and/or block light penetration, therefore impacting their effectiveness toward contaminant transformation. The multiple ways in which different types or sources of DOM can impact oxidative water purification processes are critically reviewed. DOM can inhibit the degradation of TrOCs, but it can also enhance the formation and reactivity of useful radicals for contaminants elimination and alter the transformation pathways of contaminants. An in-depth analysis highlights the inhibitory effect of DOM on the degradation efficiency of TrOCs based on DOM's structure and optical properties and its reactivity toward oxidants as well as the synergistic contribution of DOM to the transformation of TrOCs from the analysis of DOM's redox properties and DOM's transient intermediates. AOPs can alter DOM structure properties as well as and influence types, mechanisms, and extent of oxidation byproducts formation. Research needs are proposed to advance practical understanding of how DOM can be exploited to improve oxidative water purification.

Reaction kinetics of dissolved black carbon with hydroxyl radical, sulfate radical and reactive chlorine radicals

As an important component of dissolved organic matter (DOM), dissolved black carbon (DBC) which is characterized of abundant aromatic and oxygen-containing functional groups, is widely distributed in aquatic environments. Its presence may hinder the oxidation of organic micro-pollutants during advanced oxidation processes (AOPs) via free radicals scavenging effect. However, the second-order reaction rate constants of DBC with different free radicals including hydroxyl radical (OH•), sulfate radical (SO₄^•-), reactive chlorine radicals (RCR) are unknown and the relationship between the chemical composition of DBC and the second-order reaction rate constants during different AOPs (UV/H₂O₂, UV/PDS, UV/Chlorine) is also unclear. In this study, a plant-derived DBC was extracted from wheat biochar and fractionated according to molecular weight (i.e., <10 k, <3 k, and < 1 k Da). The second order rate constants of DBC reaction with different free radicals were determined by competitive kinetic method. Besides, the chemical composition of DBC was revealed by ultraviolet-visible (UV-Vis) spectroscopy, fluorescence excitation-emission-matrix (EEM) spectroscopy Fourier Transform Infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) combined with statistical analysis. The results showed that the second-order rate constants decreased as the molecular weight increased. For the <1 k Da DBC, the k_DBC-OH•, k_DBC-SO4•-^-, k_DBC-RCR were (1.83 ± 0.06) × 10⁴, (7.60 ± 0.21) × 10³, and (1.71 ± 0.13) × 10⁴ L·mgC^-1·s^-1, which were 1.98, 2.19, 1.43 times of that for the <10 k Da fraction and 1.38, 1.36, 1.24 times of that for the <3 k Da fraction in UV/H₂O₂, UV/PDS and UV/Chlorine processes. In addition, the results of chemical composition analysis showed that DBC mainly contained humic substances and was rich in O-containing functional groups such as CO. The second order reaction rate constants of DBC with different free radicals decreased with increasing the molecular weight of DBC due to the more aggregated structure of the small molecules that the inner carbon of DBC was not easily exposed to free radicals.

Kinetic study on degradation of micro-organics by different UV-based advanced oxidation processes in EfOM matrix

Effluent organic matter (EfOM) contains a large number of substances that are harmful to both the environment and human health. To avoid the negative effects of organic matter in EfOM, advanced treatment of organic matter is an urgent task. Four typical oxidants (H₂O₂, PS, PMS, NaClO) and UV-combined treatments were used to treat micro-contaminants in the presence or absence of EfOM, because the active radical species produced in these UV-AOPs are highly reactive with organic contaminants. However, the removal efficiency of trace contaminants was greatly affected by the presence of EfOM. The degradation kinetics of two representative micro-contaminants (benzoic acid (BA) and para chlorobenzoic acid (pCBA)) was significantly reduced in the presence of EfOM, compared to the degradation kinetics in its absence. Using the method of competitive kinetics, with BA, pCBA, and 1,4-dimethoxybenzene (DMOB) as probes, the radicals (HO^·, SO₄^-·, ClO^·) proved to be the key to reaction species in advanced oxidation processes. UV irradiation on EfOM was not primarily responsible for the degradation of micro-contaminants. The second-order rate constants of the EfOM with radicals were determined to be (5.027 ± 0.643) × 10² (SO₄^-·), (3.192 ± 0.153) × 10⁴ (HO^·), and 1.35 × 10⁶ (ClO^·) (mg C/L)^-1 s^-1. In addition, this study evaluated the production of three radicals based on the concept of R_ct, which can better analyze its reaction mechanism.

Nonmetal doped carbon nitride nanosheet as photocatalyst for degradation of 4, 5-dichloroguaiacol

Metal-free photocatalysts for high efficient photocatalytic degradation of pollutants have attracted growing concern in recent years. Herein, relying on density functional theory (DFT) calculations, boron and phosphorus doped C₂N layers were explored for the potential of utilization as photocatalysts for 4, 5-dichloroguaiacol (4, 5-DCG) removal. Our computations revealed that the adsorption energy of 4, 5-DCG on B@N-doped C₂N layers were 26.56 kcal mol^-1, and the ΔG^≠ of initial reactions of 4, 5-DCG with OH were also reduced onto the B@N-doped C₂N substrates. The band gap of B@N-doped C₂N was 2.27 eV. The obtained results showed that the doping of boron atom into C₂N layer narrows bandgap, and retains well catalytic performance and adsorption properties. Hence, B@N-doped C₂N layer is a promising photocatalyst for organic pollutants removal. Possible degradation pathways of 4, 5-DCG and aquatic toxicity assessment during degradation were also carried out. Products with higher toxicity would be formed and the transformation products were still toxic to three nutrient levels of aquatic organisms (green algae, fish, and daphnia).

Kinetics and Transformations of Diverse Dissolved Organic Matter Fractions with Sulfate Radicals

Dissolved organic matter (DOM) scavenges sulfate radicals (SO₄^•-), and SO₄^•--induced DOM transformations influence disinfection byproduct (DBP) formation when chlorination follows advanced oxidation processes (AOPs) used for pollutant destruction during water and wastewater treatment. Competition kinetics experiments and transient kinetics experiments were conducted in the presence of 19 DOM fractions. Second-order reaction rate constants for DOM reactions with SO₄^•- (k_DOM,SO4^•-) ranged from (6.38 ± 0.53) × 10⁶ M^-1 s^-1 to (3.68 ± 0.34) × 10⁷ M_C^-1 s^-1. k_DOM,SO4^•- correlated with specific absorbance at 254 nm (SUVA₂₅₄) (R² = 0.78) or total antioxidant capacity (R² = 0.78), suggesting that DOM with more aromatics and antioxidative moieties reacted faster with SO₄^•-. SO₄^•- exposure activated DBP precursors and increased carbonaceous DBP (C-DBP) yields (e.g., trichloromethane, chloral hydrate, and 1,1,1-trichloropropanone) in humic acid and fulvic acid DOM fractions despite the great reduction in their organic carbon, chromophores, and fluorophores. Conversely, SO₄^•--induced reactions reduced nitrogenous DBP yields (e.g., dichloroacetonitrile and trichloronitromethane) in wastewater effluent organic matter and algal organic matter without forming more C-DBP precursors. DBP formation as a function of SO₄^•- exposure (concentration × time) provides guidance on optimization strategies for SO₄^•--based AOPs in realistic water matrices.

Mechanistic Understanding of Superoxide Radical-Mediated Degradation of Perfluorocarboxylic Acids

Perfluorocarboxylic acids (PFCAs) exhibit strong persistence in sunlit surface waters and in radical-based treatment processes, where superoxide radical (O₂^•-) is an important and abundant reactive oxygen species. Given that the role of O₂^•- during the transformation of PFCAs remains largely unknown, we investigated the kinetics and mechanisms of O₂^•--mediated PFCAs attenuation through complementary experimental and theoretical approaches. The aqueous-phase rate constants between O₂^•- and C3-C8 PFCAs were measured using a newly designed in situ spectroscopic system. Mechanistically, bimolecular nucleophilic substitution (S_N2) is most likely to be thermodynamically feasible, as indicated by density functional theory calculations at the CBS-QB3 level of theory. This pathway was then investigated by ab initio molecular dynamics simulation with free-energy samplings. As O₂^•- approaches PFCA, the C-F bond at the alpha carbon is spontaneously stretched, leading to the bond cleavage. The solvation mechanism for O₂^•--mediated PFCA degradation was also elucidated. Our results indicated that although the less polar solvent enhanced the nucleophilicity of O₂^•-, it also decreased the desolvation process of PFCAs, resulting in reduced kinetics. With these quantitative and mechanistic results, we achieved a defined picture of the O₂^•--initiated abatement of PFCAs in natural and engineered waters.

Adsorption of Nd(iii) on a multistage porous imprinted chitosan composite membrane

The separation and recovery of neodymium from industrial pollutants and environmental sewage has become a problem of concern.

Electrocatalytic inactivation of antibiotic resistant bacteria and control of antibiotic resistance dissemination risk

Antibiotic resistance in environmental matrices becomes urgently significant for public health and has been considered as an emerging environmental contaminant. In this work, the ampicillin-resistant Escherichia coli (AR E. coli) and corresponding resistance genes (bla_TEM-1) were effectively eliminated by the electrocatalytic process, and the dissemination risk of antibiotic resistance was also investigated. All the AR E. coli (∼8 log) was inactivated and 8.17 log bla_TEM-1 was degraded by the carbon nanotubes/agarose/titanium (CNTs/AG/Ti) electrode within 30 min. AR E. coli was inactivated mainly attributing to the damage of cell membrane, which was attacked by reactive oxygen species and subsequent leakage of intracellular cytoplasm. The bla_TEM-1 was degraded owing to the strand breaking in the process of electrocatalytic degradation. Furthermore, the dissemination risk of antibiotic resistance was effectively controlled after being electrocatalytic treatment. This study provided an effective electrocatalytic technology for the inactivation of antibiotic resistant bacteria and control of antibiotic resistance dissemination risk in the aqueous environment.

Reactivity and reaction mechanisms of sulfate radicals with lindane: An experimental and theoretical study

Advanced oxidation technologies (AOTs) have been intensely used to eliminate various organic pollutants in engineering waters. In this context, we investigated the kinetics and mechanisms of the sulfate radical (SO₄^-)-mediated degradation of lindane in UV/peroxydisulfate system, and compared results with previous studies on SO₄^--based AOTs for destruction of lindane. The second order rate constant (k) value between SO₄^- and lindane was determined to be (8.95 ± 0.29) × 10⁶ M^-1 s^-1via competition kinetics using p-cyanobenzoic acid as reference compound, which is close to the theoretically calculated value of 4.41 × 10⁷ M^-1 s^-1, that was performed at SMD/M05-2X/6-311++G**//M05-2X/6-31+G** level of theory using density functional theory (DFT) approach. H-atom abstraction pathway was calculated to be thermodynamically favorable and kinetically dominant. In the combined experimental and theoretical study, we aim for a better understanding on the degradation kinetics and mechanisms of lindane, serving as a starting point for more attention to SO₄^--mediated degradation kinetics of cycloaliphatic compounds in future.

Advanced treatment of secondary effluent organic matters (EfOM) from an industrial park wastewater treatment plant by Fenton oxidation combining with biological aerated filter

This study investigated the advanced treatment of secondary effluent organic matters (EfOM) from an industrial park wastewater treatment plant (IPWTP) by Fenton oxidation process and its combination with biological aerated filter (BAF). The constituents of EfOM were characterized by using fluorescence excitation-emission matrix, and the results showed that the major components included aromatic proteins, soluble microbial products, humic and fulvic acid-like substances, and compounds associated with fluorescent region of Ex 250-300 nm/Em 600-700 nm. The EfOM was strongly resistant to biodegradation (biochemical oxygen demand (BOD₅):chemical oxygen demand (COD) ratio at 0.11), resulting in less than 15% dissolved organic carbon (DOC) removal efficiency by the BAF reactor. The advanced treatment of EfOM by Fenton oxidation process led to maximum ~50% mineralization efficiency of EfOM under the optimal conditions of 2.0 mM Fe^II, 10 mM H₂O₂, pH 3.0 and 3.0 h of the reaction time. Particularly, Fenton oxidation treatment effectively improved the biodegradability of EfOM in the IPWTP secondary effluents, e.g., increasing the BOD₅:COD ratio from 0.11 to 0.42. A synergistic combination of Fenton oxidation process with the BAF reactor offered desirable mineralization efficiencies of EfOM (>70%) at lower dosages of Fenton's reagents. The present results suggest that Fenton oxidation process combining with the BAF reactor can be a promising strategy for the advanced treatment of EfOM in IPWTP secondary effluents. This study provides guidance for the characterization and advanced treatment of EfOM in IPWTP secondary effluents for practical purpose.

The application of UV-C laser in persulfate activation for micropollutant removal: Case study with iodinated X-ray contrast medias

A novel light source UV-C laser was applied in persulfate (PS) activation to effectively remove iodinated X-ray contrast medias (ICMs) including iohexol (IOX), iopamidol (IPM) and diatrizoate (DTZ) in this study. Significant ICMs degradation was observed in UV-C laser/PS systems with pseudo first-order rate constants of 0.022-0.067 s^-1. Sulfate radicals (SO₄^•-) were the main active species in the three ICMs degradation, and the steady-state concentrations ([SO₄^•-]_ss) were 3.629 × 10^-11 M (IOX), 1.702 × 10^-11 M (IPM) and 1.148 × 10^-11 M (DTZ), respectively. Under the high intensity of UV-C laser, the optimal reaction efficiency was achieved at pH = 7.0 with PS concentration of 1.0 mM, and the degradation efficiency for IOX reached 93.8% within only 40 s. Both bicarbonate and chloride ions could inhibit the three ICMs degradation and the inhibition rate increased with the increase of ions concentration. The kinetic models were established and the steady-state concentrations of radicals were calculated. Density functional theory (DFT) calculations combined with experiments were used to derive the reaction pathways for three ICMs. Cyclic voltammetry measurements detected a lower redox potential peak in IOX degradation, revealing the existence of electron shuttles under the UV-C laser irradiation to promote the redox reaction. This study is the first report of UV-C laser activation of persulfate. It is a new advanced oxidation process mediated by very effective photolysis and active species formation.

Products distribution and contribution of (de)chlorination, hydroxylation and coupling reactions to 2,4-dichlorophenol removal in seven oxidation systems

We systemically investigated the transformation behavior of 2,4-dichlorophenol (24-DCP) in seven different reaction systems including KMnO₄, heat/PS, O₃, UV, Fenton, NaClO and K₂FeO₄ treatment. The results revealed that complete removal of 24-DCP could be reached in minutes, especially for Fe(VI), KMnO₄, NaClO, Fenton and O₃ system. A total of 41 products were identified by LC-MS, and 10 of them were validated using commercial and self-synthesized standards. Hydroxyl substitution and coupling reactions were commonly observed in the studied systems. Meanwhile, extra routes such as sulfate substitution, (de)chlorination and direct oxidation were also involved for certain oxidation methods. Comparisons showed that a high degree of chlorination (>90%) occurred for NaClO system, while coupling products accounted for ~45% of the removed 24-DCP under PS oxidation. Moreover, low mineralization degree together with high aquatic toxicity was attributed to the occurrence of coupling reaction, which was possibly related to the redox potential of the main oxidative species. Considering the low abundance of coupling products and the gentle reaction condition, UV irradiation is a better option for 24-DCP removal in water and wastewaters. These findings can deepen our understanding on the transformation process of 24-DCP and provide some useful information for the environmental elimination of substituted phenols.

Formation and degradation mechanisms of CX3R-type oxidation by-products during cobalt catalyzed peroxymonosulfate oxidation: The roles of Co3+ and SO4·

Sulfate radical (SO₄^·-)-based advanced oxidation processes (AOPs) attract increasing attention in the control of micropollutants. However, SO₄^·- can react with other chemicals present in water and result in undesired oxidation by-products (OBPs) generation. The formation and degradation mechanisms of CX₃R-type OBPs during cobalt catalyzed peroxymonosulfate (Co²⁺/PMS) oxidation were investigated. In the formation of CX₃R-type OBPs, both Co³⁺ and SO₄^·- could convert chloride to free chlorine that then reacted with natural organic matter, leading to the formation of CX₃R-type OBPs. The concentrations of trichloromethane, chloral hydrate, dichloroacetonitrile, dichloroacetamide and trichloroacetamide after 15 min reaction were 9.8, 3.9, 1.2, 5.9 and 22.3 nM, respectively. Compared to SO₄^·-, Co³⁺ played a more significant role in the CX₃R-type OBP formation and calculated toxicity values of CX₃R-type OBPs. CX₃R-type OBPs could not only be formed but also be degraded at the same time during Co²⁺/PMS oxidation. As for the degradation of CX₃R-type OBPs, both Co³⁺ and SO₄^·- could transform CX₃R-type OBPs to chloride. Compared to Co³⁺, SO₄^·- played a more important role in the degradation of CX₃R-type OBPs and the conversion from chloride to final by-product chlorate. The adverse effects that results from Co³⁺ need more attention in SO₄^·--based AOPs application.

Reactivity of Chlorine Radicals (Cl• and Cl2•-) with Dissolved Organic Matter and the Formation of Chlorinated Byproducts

Chlorine radicals, including Cl^• and Cl₂^•-, can be produced in sunlight waters (rivers, oceans, and lakes) or water treatment processes (e.g., electrochemical and advanced oxidation processes). Dissolved organic matter (DOM) is a major reactant with, or a scavenger of, Cl^• and Cl₂^•- in water, but limited quantitative information exists regarding the influence of DOM structure on its reactivity with Cl^• and Cl₂^•-. This study aimed at quantifying the reaction rates and the formation of chlorinated organic byproducts produced from Cl^• and Cl₂^•- reactions with DOM. Laser flash photolysis experiments were conducted to quantify the second-order reaction rate constants of 19 DOM isolates with Cl^• (k_DOM-Cl•) and Cl₂^•- (k_DOM-Cl2•-), and compare those with the hydroxyl radical rate constants (k_DOM-•OH). The values for k_DOM-Cl• ((3.71 ± 0.34) × 10⁸ to (1.52 ± 1.56) × 10⁹ M_C^-1 s^-1) were orders of magnitude greater than the k_DOM-Cl2•- values ((4.60 ± 0.90) × 10⁶ to (3.57 ± 0.53) × 10⁷ M_C^-1 s^-1). k_DOM-Cl• negatively correlated with the weight-averaged molecular weight (M_W) due to the diffusion-controlled reactions. DOM with high aromaticity and total antioxidant capacity tended to react faster with Cl₂^•-. During the same experiments, we also monitored the formation of chlorinated byproducts through the evolution of total organic chlorine (TOCl) as a function of chlorine radical oxidant exposure (CT value). Maximum TOCl occurred at a CT of 4-8 × 10^-12 M·s for Cl^• and 1.1-2.2 × 10^-10 M·s for Cl₂^•-. These results signify the importance of DOM in scavenging chlorine radicals and the potential risks of producing chlorinated byproducts of unknown toxicity.

Kinetic and mechanistic insights into the abatement of clofibric acid by integrated UV/ozone/peroxydisulfate process: A modeling and theoretical study

The feasibility of integrated UV/ozone (O₃)/peroxydisulfate (PDS) process for abatement of clofibric acid (CA) was systematically explored in this study with focus on the kinetic simulation and oxidation mechanisms. The results indicated the UV/O₃/PDS process was of prominent treatment capability with pseudo-first-order rate constant of CA degradation increased by 65.9% and 86.0% compared to UV/O₃ and UV/PDS processes, respectively. A chemical kinetic model was developed and successfully employed to predict CA elimination as well as the specific contributions of UV, hydroxyl radical (^•OH) and sulfate radical (SO₄^•-) under different PDS dosage, pH, natural organic matters, bicarbonate and chloride conditions in UV/O₃/PDS process. According to quantum chemical calculation, radical addition on ortho site of isopropoxy substituent and single electron transfer were corroborated to be the dominant reaction channels for the oxidation of CA by ^•OH and SO₄^•-, respectively. Additionally, the reactive sites and transformation pathways of CA were proposed via Fukui function calculation and UPLC-Q-TOF-MS analysis. Moreover, the performance of UV/O₃/PDS process was further evaluated with regard to the energy demand and bromate formation. This study first proposed a kinetic model in UV/O₃/PDS process and elucidated the regioselectivity and products distribution of CA during oxidative treatment.

Experimental and theoretical study of kinetic and mechanism of hydroxyl radical-mediated degradation of sulfamethazine

Hydroxyl radical (^•OH)-based advanced oxidation technologies (AOTs) is an effective and clean way to remove sulfonamide antibiotics in water at ambient temperature and pressure. In this study, we systematically investigated the degradation kinetics of sulfamethazine (SMT) by ^•OH with a combination of experimental and theoretical approaches. The second-order rate constant (k) of SMT with ^•OH was experimentally determined to be 5.27 ± 0.06 × 10⁹ M^-1 s^-1 at pH 4.5. We also calculated the thermodynamic and kinetic behaviors for the reactions by density functional theory (DFT) using the B3LYP/6-31G*. The results revealed that ^•OH addition pathways at the methylene (C4) site on the pyridine ring and the ortho sites (C12 and C14) of the amino group on the benzene ring dominate the reaction, especially C14 site on the benzene ring accounted for 43.95% of SMT degradation kinetics. The theoretical k value which was calculated by conventional transition state theory is 3.96 × 10⁹ M^-1 s^-1, indicating that experimental observation (5.27 ± 0.06 × 10⁹) is correct. These results could further help AOTs design in treating sulfonamide during wastewater treatment processes.

Enhanced oxidative degradation of decabromodiphenyl ether in soil by coupling Fenton-persulfate processes: Insights into degradation products and reaction mechanisms

Decabromodiphenyl ether (BDE-209) has extreme hydrophobicity, which results in its significant accumulation in soil, sediments and other solid materials. In this work, an oxidation method coupling Fenton with persulfate (PS) was proposed for the effective degradation of BDE-209 adsorbed on solid surfaces. After adding 0.1 M PS to the Fenton system at 1.0 h, the removal rate of BDE-209 was significantly increased from 62.2% to 94.0%. The degradation of BDE-209 in various soil samples was also investigated by the coupling Fenton-PS method. Removal efficiency of 73.4-95.8% was obtained, suggesting that this coupling method was feasible in real application. According to the radical scavenging experiments, •OH dominated the overall reaction of BDE-209 in the coupling system. Meanwhile, the enhanced removal was attributed to the generation of SO₄^•- from the catalytic decomposition of PS. The calculated energy barriers for SO₄^•- attacking on the carbons were smaller than •OH initiated reactions, which further confirmed that SO₄^•- plays a role in the accelerated removal of BDE-209. The initial attack of BDE-209 by SO₄^•- generated the SO₄^•- adducts, which may undergo debromination or CO bond cleavage reaction together with subsequent hydroxyl substitution to form the primary product OH-Nona-BDEs and pentabromophenol. Under the successive attack of radicals, these primary products were further transformed into lower-brominated hydroxylation products and bromophenols via direct debromination and hydroxyl substitution reaction. This work provides an economical and effective method for treating BDE-209 contaminated soils and samples.

Enhancing the production of biogas through anaerobic co-digestion of agricultural waste and chemical pre-treatments

Large amounts of agricultural solid wastes (ASWs) and animal dung are produced annually causing serious environmental problem that requires proper treatment. The present study proposes a strategy for optimizing the anaerobic co-digestion of ASWs and cow dung (CD), identifies the key factors governing the co-digestion performance and evaluates the effect of NaHCO₃ alkalinity treatment on improving the economy and performance of anaerobic digestion (AD). The results revealed that the highest cumulative methane production (CMP) of 297.99 NL/kgVS can be generated by co-digestion of ASWs and CD at a ratio of 60:40. Further improvement was achieved via alkalinity treatment with 1.0 g of NaHCO₃/gVS leading to decrease in lignin, cellulose, and hemicellulose contents of feedstock by 3.5%, 10.5% and 15.9%, respectively, converting them to soluble fractions and improving the CMP by 11.2-29.7% based on substrate quality. The improved CMP in the chemically treated substrates reflects a 19% increase in the generated revenue. The kinetics of the AD process was successfully fitted to modified Gompertz model with very low standard deviation residuals (SDR) ≤ 5.21 and R² ≥ 0.979. Results confirm that the proposed strategy is an effective method for producing biogas from co-digestion of ASWs and CD.

A novel Fe2+/persulfate/tannic acid process with strengthened efficacy on enhancing waste activated sludge dewaterability and mechanism insight

As an essential section before final sludge disposal, sludge dewatering has currently been one of the focus issues. In this study, an innovative Fe²⁺/persulfate/tannic acid (TA) process was verified to further strengthen systemic efficacy on enhancing sludge dewaterability, compared with the conventional Fe²⁺/persulfate process. With the efficient TA/Fe²⁺ (molar ratio) of 0.25 added in Fe²⁺ (0.3 mmol/gTS (total solid))/persulfate (0.6 mmol/gTS) process, sludge dewaterability was enhanced remarkably. Capillary suction time, specific resistance to filtration, and water content of dewatered sludge cake were further reduced by 61.5%, 35.3%, and 6.4% than these in Fe²⁺/persulfate. Sludge supernatant viscosity was further reduced by 86.7% due to the more removal of extracellular polymeric substances (EPS). The secondary structure of EPS protein changed apparently and fluorescent components of EPS decreased distinctly. Sludge functional group contents were observed to be lower. TA effectually increased sludge particle size and heightened sludge flocculability, rendering the large and compact aggregations. Moreover, TA accelerated the recovery of Fe²⁺, facilitating persulfate activation to generate more SO₄^·- and ^·OH for EPS disruption and cell lysis in the conditioning system. These findings provided a novel approach based on the Fe²⁺/persulfate process in sludge treatment for desirable dewaterability.

Environment-friendly PCN derivatives design and environmental behavior simulation based on a multi-activity 3D-QSAR model and molecular dynamics

A multi-activity three-dimensional quantitative structure-activity relationship (3D-QSAR) model was established based on the comprehensive evaluation index (CEI) of polychlorinated naphthalenes (PCNs). The CEI values were calculated using the vector analysis method in combination with the following parameters: biological toxicity (predicted by logEC₅₀), bioconcentration (predicted by logK_ow), long-distance migration (predicted by logP_L), and biodegradation (predicted by total-score). Additionally, sixty-four CN-70 derivatives with lower CEI values were designed, among which three derivatives with reduced CEI values were selected for verification based on an evaluation of their persistent organic pollutant properties and practicability. Finally, an environmental behavior simulation was conducted via molecular dynamics simulation aided by the Taguchi experimental design by considering the degradation characteristics of the three aforementioned CN-70 derivatives as an example. Only two of the selected CN-70 derivatives were observed to be more easily degraded when compared with the CN-70 molecule (ascending range: 11.57 %-13.57 %) in a real-world setting, which was consistent with the biodegradability prediction results (ascending range: 14.94 %-22.49 %) obtained through the molecular docking studies. The multi-activity 3D-QSAR model established in this study overcame the limitations of generating molecular designs based on single-effect models from the source because it focused on the multiple effects of the pollutants.

Iron/zinc and phosphoric acid modified sludge biochar as an efficient adsorbent for fluoroquinolones antibiotics removal

Iron/zinc (Fe/Zn), phosphoric acid (H₃PO₄) or in combination (Fe/Zn + H₃PO₄) modified sludge biochar (SBC) were prepared and tested in this study to adsorb fluoroquinolones antibiotics including ciprofloxacin (CIP), norfloxacin (NOR) and ofloxacin (OFL) from water. Fe/Zn + H₃PO₄-SBC had an increased surface area (S_BET), total pore volume (V_tot), mesoporous volume (V_mes), pore diameter (D_p) and oxygen-containing functional groups. It exhibited superior adsorption performance for CIP, NOR and OFL with the maximum adsorption amount of 83.7, 39.3, 25.4 mg g^-1, respectively. Pseudo-second kinetic and Freundlich isotherm model presented the better fitting. The results of models and characterization analysis in combination indicated that physisorption and chemisorption, including pore filling, hydrogen bonding, π-π interaction, electrostatic interaction and functional groups complexation on a heterogeneous surface were the dominant process and mechanism. Liquid film diffusion was the main rate-limiting step. The adsorption process of CIP, NOR and OFL onto Fe/Zn + H₃PO₄-SBC were a spontaneous endothermic process. This study demonstrated that Fe/Zn + H₃PO₄ modified SBC exhibited high adsorption capacity, which was a promising adsorbent for fluoroquinolones as well as for other antibiotics effective removal from waters.

Carbon nanotube supported sludge biochar as an efficient adsorbent for low concentrations of sulfamethoxazole removal

A novel adsorbent of sludge biochar (SBC) and multi-walled carbon nanotube (CNT) composite was synthesized (CNT-SBC) to remove low concentrations of sulfamethoxazole (SMX) from water. The key factors of dose, contact time, pH and temperature were investigated. Higher dose of adsorbents provided more active sites for SMX adsorption. The effect of pH was due to the electrostatic interaction. Increasing the temperature was beneficial to SMX adsorption, which was a spontaneous endothermic process and the strength of the spontaneous increased with CNT supporting. As pseudo-second-order, Elovich, Langmuir and Freundlich models fitted the experimental data better, this suggested that both physisorption and chemisorption played vital roles during the adsorption process. In addition, liquid film diffusion was the main rate-limiting step of adsorption. Compared with SBC (5.43 × 10³ μg g^-1), CNT-SBC-1 (CNT:SBC = 1:2), CNT-SBC-2 (CNT:SBC = 1:4) and CNT-SBC-3 (CNT:SBC = 1:6) exhibited better adsorption performance with up to 2.35 × 10⁴, 1.49 × 10⁴ and 1.22 × 10⁴ μg g^-1 at 25 °C, respectively. The characterization analysis demonstrated that the stronger adsorption capacity of CNT-SBC was mainly attributed to the pore filling, functional groups complexation and π-π interaction. In summary, as an efficient and environment-friendly adsorbent, CNT-SBC has promising potential for low concentrations of SMX and other emerging contaminants removal from water.

QSAR modeling for the ozonation of diverse organic compounds in water

The ozonation-based advanced oxidation process is a promising treatment technology for wastewater with micropollutants. The second-order reaction rate constant (k_O3) of ozone (O₃) with organic compounds is an important index for estimating removal efficiency of organic pollutants in engineered treatment; however, the experimental k_O3 values are currently only available for hundreds of chemicals. In this study, two quantitative-structure activity relationship (QSAR) models were developed to predict k_O3 of various organic chemicals with multiple linear regression (MLR) and support vector machine (SVM) methods. The built QSAR models cover a large dataset (136 chemicals) and more structurally diverse chemicals as compared to the existing models. The MLR model possesses satisfactory goodness-of-fit (R²_tr = 0.734), robustness (Q²_LOO = 0.700, Q²_BOOT = 0.772) and predictive ability (R²_ext = 0.797, Q²_ext = 0.794), and the SVM model also has good fitness (R²_tr = 0.862) and predictability (R²_ext = 0.782, Q²_ext = 0.775). The applicability domain of the models has been extended and includes chemicals (especially some emerging pollutants) that are rarely covered in many previous models. The underlying molecular structural factors influencing ozonation are revealed. The energy of the highest occupied molecular orbital (E_HOMO) and the phenol/enol/carboxyl OH group (O-057) are the two most important molecular structural factors governing the reactivity of organic compounds with ozone. The developed models can serve as a prescreening tool for the removal prediction of organic pollutants by ozone.

Electrochemical removal of levofloxacin using conductive graphene/polyurethane particle electrodes in a three-dimensional reactor

The conductive polyurethane/polypyrrole/graphene (CPU/PPy/Gr) particle electrode was prepared by an in-situ oxidative polymerization method and used as particle electrodes to degrade levofloxacin (LEV) in a three-dimensional electrode reactor. The prepared CPU/PPy/Gr electrode was characterized systematically and the effects of initial pH, initial LEV concentration, aeration volume, voltage, and electrolyte concentration on the degradation efficiency were investigated. Results showed that more than 90% LEV was degraded and the energy consumption was 20.12 kWh/g LEV under conditions of pH 7, 6 V voltage, 2.0 L/min aeration volume, 20 mg/L initial LEV concentration, and 7 mM concentration of electrolyte (Na₂SO₄). A possible electrochemical oxidation pathway of LEV by the CPU/PPy/Gr electrode was proposed. In addition, the biotoxicity of LEV and its oxidation products was calculated using ECOSAR (Ecological Structure Activity Relationships) program in EPISuite. Toxicity evaluation using luminescent bacteria showed that the toxicities of some intermediates were higher than the parent compound. But the toxicity of degradation processes for LEV was effective decreasing. A possible reactive mechanism in the three-dimensional reactor was also recommended. In brief, the prepared CPU/PPy/Gr particle electrode constitutes an insight into the promising practical application in the wastewater treatment.

Elucidating sulfate radical-mediated disinfection profiles and mechanisms of Escherichia coli and Enterococcus faecalis in municipal wastewater

Practical applications of disinfection technologies for engineered waters require an in‒depth understanding of disinfection profiles and mechanisms of pathogenic bacteria in a complex matrix. This study investigated the inactivation of E. coli and E. faecalis by SO₄^•-, an emerging advanced disinfectant, in ultrapure water (UPW) and wastewater effluent (WE). Based on the bacterial inactivation kinetics in UPW in a zerovalent iron/peroxydisulfate system, the second order rate constants (k) for SO₄^•- reacting with E. coli and E. faecalis were measured to be (1.39 ± 0.1) × 10⁹ M^-1 s^-1 and (6.71 ± 0.1) × 10⁹ M^-1 s^-1, respectively. The morphological images of both bacteria by the scanning electron microscope indicated that SO₄^•- initiates oxidative reactions on the wall/membranes, causing their irreversible damage, ultimately affecting membrane permeability and physiological functions. To profile the inactivation kinetics of two strains of bacteria in WE matrix, a mechanistic process‒based model with the obtained k values was developed. Sensitivity and uncertainty analyses indicated that the key parameters for the model predictions were the concentrations of halide ions (i.e., Br^- and Cl^-) in WE and their k values reacting with SO₄^•- accounting for >80% of uncertainty or variance expected in predicted bacterial inactivation. This model allows precise estimation of required disinfectant dose even in complex water matrices, shedding lights on the extension of application of SO₄^•-‒based technology in wastewater treatments.

Experimental and theoretical insights into kinetics and mechanisms of hydroxyl and sulfate radicals-mediated degradation of sulfamethoxazole: Similarities and differences

Hydroxyl radical (^•OH)- and sulfate radical ()-based advanced oxidation technologies (AOTs) have been proven an effective method to remove antibiotics in wastewater treatment plants (WWTPs). This study aims to gain insights into kinetics and mechanisms of neutral sulfamethoxazole (SMX) degradation, a representative antibiotic, by ^•OH and using an experimental and theoretical approach. First, the second-order rate constants (k) of SMX with ^•OH and were determined to be (7.27 ± 0.43) × 10⁹ and (2.98 ± 0.32) × 10⁹ M^-1 s^-1 in UV/H₂O₂ and UV/persulfate (UV/PS) systems, respectively. The following theoretical calculations at the M06-2X level of theory revealed that addition of radicals to the benzene ring is the most favorable first-step reaction for both ^•OH and , but that exhibits higher energy barriers and selectivity than ^•OH due to steric hindrance. We further analyzed subsequent reactions and, interestingly, our findings closely corroborated HOMO/LUMO distributions of SMX to the oxidation pathways. Finally, the estimation of energy consumption for UV alone, ^•OH-, and -mediated oxidation processes was compared. These comparative results, for the first time, provide insights into the similarities and differences of degradation of SMX by ^•OH/ at the molecular level and can help improve antibiotics removal using radical based AOTs in WWTPs.

An experimental and theoretical study on the degradation of clonidine by hydroxyl and sulfate radicals

Emerging contaminants such as pharmaceuticals that cannot be completely removed by traditional biological treatments are ubiquitously present in water bodies with detected concentrations ranging from ng L^-1 to mg L^-1. Advanced oxidation technologies (AOTs) are promising, efficient, and environmentally friendly for the removal of these pharmaceuticals. In this study, we investigated the degradation kinetics of a model pharmaceutical, clonidine (CLD), via hydroxyl radical (OH) in UV/H₂O₂ and sulfate radical (SO₄^•-) in UV/peroxydisulfate (PS) systems for the first time. The second-order rate constants (k) of protonated cationic CLD with OH and SO₄^•- were measured to be (2.15 ± 0.07) × 10⁹ M^-1 s^-1 and (1.12 ± 0.03) × 10⁹ M^-1 s^-1, respectively. We also calculated the pK_a value of CLD and thermodynamic behaviors for reactions of CLD/HCLD⁺ with OH and SO₄^•- at M05-2X/6-311++G**//M05-2X/6-31+G** level with SMD solvation model. The pK_a value was calculated to be 8.14, confirming the literature value. H atom abstraction pathway was the most favorable pathway for both OH and SO₄^•-, while single electron transfer pathway was thermodynamically feasible only for SO₄^•- for CLD but not for HCLD⁺. In addition, the reactivities of both tautomeric forms of CLD (i.e., amino and imino CLD) with both radicals were also investigated. This study contributed to a better understanding on the degradation mechanisms of CLD and proposed the possibilities of the elimination of pharmaceuticals by applying AOTs during wastewater treatment processes.

Characterizing property and treatability of dissolved effluent organic matter using size exclusion chromatography with an array of absorbance, fluorescence, organic nitrogen and organic carbon detectors

In this study, size exclusion chromatography with an array of absorbance, fluorescence, organic nitrogen and organic carbon detectors was used for characterizing property and treatability of effluent organic matter (EfOM) from 12 wastewater treatment plants. According to their apparent molecular weight (AMW), EfOM fractions were assigned to biopolymers (>20 kDa), humic substances that comprise sub-fractions of humic-like acids (HA-I & HA-II, 2.3-7.0 kDa) and fulvic-like acids (FA, 1.5-2.3 kDa), building blocks (0.55-1.5 kDa) and low molecular weight neutral substances (<550 Da). The fractions of biopolymers and low molecular weight neutral substances didn't show humic-like fluorescence, while the fractions of HA-II, FA and building blocks usually had signatures of both humic-like and protein-like fluorescence. Humic substances generally contributed the largest proportion of dissolved organic carbon and nitrogen (DOC & DON) in effluents. Coagulation removed EfOM fractions following the order of biopolymers > HA subfraction > FA subfraction > building blocks, while little removal of protein-like fluorescence in HA-II and FA subfractions was detected. Anion exchange treatment could effectively reduce DOC and DON concentrations; the sequence of the treatment efficiency was humic substances > biopolymers > building blocks. Increasing O₃ doses caused DOC and DON of EfOM to be gradually transformed from large AMW fractions into small AMW fractions, while chromophores and fluorophores in HA subfractions were relatively more refractory than those in the other fractions. Size exclusion chromatography with multiple detectors are suggested to be an informative technique for estimating treatability of EfOM by advanced wastewater treatment processes.

Visible and UV photocatalysis of aqueous perfluorooctanoic acid by TiO2 and peroxymonosulfate: Process kinetics and mechanistic insights

The global occurrence and adverse environmental impacts of perfluorooctanoic acid (PFOA) have attracted wide attention. This study focused on the PFOA photodegradation by using photocatalyst TiO₂ with peroxymonosulfate (PMS) activation. Aqueous PFOA (50 mg L^-1) at the pH 3 was treated by TiO2/PMS under 300 W visible light (400-770 nm) or 32 W UV light (254 nm and 185 nm). The addition of PMS induced a significant degradation of PFOA under powerful visible light compared with sole TiO₂. Under visible light, 0.25 g L^-1 TiO₂ and 0.75 g L^-1 PMS in the solution with the initial pH 3 provided optimum condition which achieved 100% PFOA removal within 8 h. Under UV light irradiation at 254 nm and 185 nm wavelength, TiO₂/PMS presented excellent performance of almost 100% removal of PFOA within 1.5 h, attributed to the high UV absorbance by the photocatalyst. The intermediates analysis showed that PFOA was degraded from a long carbon chain PFOA to shorter chain intermediates in a stepwise manner. Furthermore, scavenger experiments indicated that SO₄^•-radicals from PMS and photogenerated holes from TiO₂ played an essential role in degrading PFOA. The presence of organic compounds in real wastewater reduced the degradation efficacy of PFOA by 18-35% in visible/TiO₂/PMS system. In general, TiO₂/PMS could be an ideal and effective photocatalysis system for the degradation of PFOA from wastewater using either visible or UV light source.

Insights into degradation pathways and toxicity changes during electro-catalytic degradation of tetracycline hydrochloride

The removal of antibiotics has attracted much attention due to their extremely high adverse impacts on the environment. However, the potential risks of degradation intermediates are seldom reported. In this work, the influence of different factors on the electro-catalytic degradation efficiency of tetracycline hydrochloride (TCH) by the prepared carbon nanotubes/agarose/indium tin oxide (CNTs/AG/ITO) electrode was investigated. Under optimal conditions (10 wt% CNTs dosage, pH = 7), the maximum degradation efficiency for TCH (10 mg L^-1) reached up to 96% within 30 min treatment with 4 V potential. Superoxide anions (•O₂^-) played an important role in the electro-catalytic degradation. Totally 10 degradation intermediates were identified using HPLC-MS/MS, and the degradation pathway was proposed. Toxicities of the parent antibiotic and the identified intermediates were calculated using the ECOSAR (Ecological Structure Activity Relationship) program in EPISuite, and results showed that more toxic intermediates were generated. The maximal chronic toxicity for green algae of the intermediate increased 1439.92 times. Furthermore, antimicrobial activity was further verified by disk agar biocidal tests with Escherichia coli ATCC25922 and higher biotoxicity intermediates compared with parent compounds were confirmed to be formed. Therefore, more attention should be paid on the potential risk of degradation intermediates in the treatment of wastewater containing antibiotics.

Experimental and theoretical insight into hydroxyl and sulfate radicals-mediated degradation of carbamazepine

Carbamazepine (CBZ), a widely detected pharmaceutical in wastewaters, cannot currently be treated by conventional activated sludge technologies, as it is highly resistant to biodegradation. In this study, the degradation kinetics and reaction mechanisms of CBZ by hydroxyl radical (OH) and sulfate radical ()-based advanced oxidation processes (AOPs) were investigated with a combined experimental/theoretical approach. We first measured the UV absorption spectrum of CBZ and compared it to the theoretical spectrum. The agreement of two spectra reveals an extended π-conjugation system on CBZ molecular structure. The second-order rate constants of OH and with CBZ, measured by competition kinetics method, were (4.63 ± 0.01) × 10⁹ M^-1 s^-1 and (8.27 ± 0.01) × 10⁸ M^-1 s^-1, respectively at pH 3. The energetics of the initial steps of CBZ reaction with OH and were also calculated by density functional theory (DFT) at SMD/M05-2X/6-311++G**//M05-2X/6-31 + G**level. Our results reveal that radical addition is the dominant pathway for both OH and . Further, compared to the positive ΔG_R⁰ value for the single electron transfer (SET) reaction pathway between CBZ and OH, the ΔG_R⁰ value for SET reaction between CBZ and is negative, showing that this reaction route is thermodynamically favorable. Our results demonstrated the remarkable advantages of AOPs for the removal of refractory organic contaminants during wastewater treatment processes. The elucidation of the pathways for the reaction of OH and with CBZ are beneficial to predict byproducts formation and assess associated ecotoxicity, providing an evaluation mean for the feasibility of AOPs application.