Removal of trace organic chemicals in wastewater effluent by UV/H2O2 and UV/PDS

Insight into the effect of UVC-based advanced oxidation processes on the interaction of typical microplastics and their derived disinfection byproducts during disinfection

The 10 µm polystyrene and polyethylene-terephthalate microplastics (MPs), prevalent in finished drink water, were employed to investigate the effect of normal dosage UVC-based advanced-oxidation-processes (UVC-AOPs) on the interaction between MPs and their derived disinfection-byproducts (DBPs) during subsequent chlorination-disinfection, in the presence of Br-, for the first time. The results indicated that UVC/H2O2 caused higher leaching of microplastic-derived dissolved-organic-matter (MP-DOM), with smaller and narrower molecular-weight-distribution than UVC and UVC/peroxymonosulfate (UVC/PMS). The trihalomethanes (as dominant DBPs) molar-formation-potentials (THMs-MFPs) for MP-DOM leached in different UVC-AOPs followed the order of UVC/H2O2>UVC/PMS>UVC. The adsorption of formed THMs, especially Br-THMs, back on MPs was observed in all MPs suspensions with or without UVC-AOPs pre-treatment. The Cl-THMs adsorption by MPs is more sensitive to UVC-AOPs than Br-THMs. The adsorption experiments showed that UVC-AOPs reduce the capacity but increase the rate of THMs adsorption by MPs, suggesting the halogen and hydrogen bonding forces governed the THMs adsorption rate while hydrophobic interaction determines their adsorption capacity. The UVC-AOPs pre-treatment sharply increased the total yield of THMs via both indirectly inducing MP-DOM leaching and directly increasing the THMs-MFPs of MPs by oxidation. 21.36-41.96% of formed THMs adsorbed back on the UVC-AOPs-pretreated MPs, which might increase the toxicity of MPs.

Transformation of metoprolol in UV/PDS process: Role and mechanisms of degradation and polymerization

Advanced oxidation processes for the treatment of organic pollutants in wastewater suffer from difficulties in mineralization, potential risks of dissolved residues, and high oxidant consumption. In this study, radical-initiated polymerization is dominated in an UV/peroxydisulfate (PDS) process to eliminate organic pollutant of pharmaceutical metoprolol (MTP). Compared with an ideal degradation-based UV/PDS process, the present process can save four fifths of PDS consumption at the same dissolved organic carbon removal of 47.3%. Simultaneously, organic carbon can be recovered from aqueous solution by separating solid polymers at a ratio of 50% of the initial chemical oxygen demand. The chemical structure of products was analyzed to infer the transformation pathways of MTP. Unlike previous studies on simple organic pollutants that the polymerization can occur independently, the polymerization of MTP is dependent on the partial degradation of MTP, and the main monomer in polymerization is a dominant degradation product (4-(2-methoxyethyl)-phenol, denoted as DP151). The separated solid polymers are formed by repeated oxidation and coupling of DP151 or its derivatives through a series of intermediate oligomers. This proof-of-concept study demonstrates the advantage of polymerization-dominated mechanism on dealing with large organic molecules with complex structures, as well as the potential of UV/PDS process for simultaneous organic pollution reduction and organic carbon recovery from aqueous solution.

Molecular-level transformations of dissolved black carbon in UV-based advanced oxidation processes

Dissolved black carbon (DBC) released from biochar, is an essential group in the dissolved organic matter (DOM) pool and is widely distributed in aquatic environments. In various advanced oxidation processes (AOPs), DBC exhibits enhanced free radical scavenging compared to typical DOM, attributed to its smaller molecular weight and more compacted aromatic structure; however, the molecular-level transformations of DBC in different AOPs, such as UV/H2O2, UV/PDS, and UV/Chlorine, remain unclear. This study employed a DBC derived from wheat biochar for experimentation. Characterization involved ultraviolet-visible (UV-Vis) spectroscopy and fluorescence excitation-emission-matrix (EEM) spectroscopy, revealing the transformation of DBC through diminished SUVA254 values and reduced intensity of three-dimensional fluorescence peaks. Further insights into the transformation were gained through Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). After each UV-AOP treatment, a conspicuous augmentation in the oxygen content of DBC was observed. The detailed oxygenation processes were elucidated through mass difference analysis, based on 23 types of typical reactions. Results indicated that oxygenation reactions were most frequently detected in all three UV-AOP treatments. Specifically, the hydroxylation (+O) predominated in UV/H2O2, while the di-hydroxylation (+2O) prevailed in UV/PDS. UV/Chlorine treatments commonly exhibited tri-hydroxylation (+3O), with the identification of 1194 Cl-BPs of unknown structures. This study contributes to a comprehensive understanding of the molecular transformations of DBC induced by various free radicals in different UV-AOP processes, leading to a better understanding of the different fates of DBC in UV-AOP processes. In addition, the identification of DBC as a precursor of by-products will also contribute to the understanding of how to inhibit the generation of by-products.

Dissolved Organic Matter-Mediated Photosensitized Activation of Monochloramine for Micropollutant Abatement in Wastewater Effluent

Utilizing solar light and water matrix components in situ to reduce the chemical and energy demands would make treatment technologies more sustainable for micropollutant abatement in wastewater effluents. We herein propose a new strategy for micropollutant abatement through dissolved organic matter (DOM)-mediated photosensitized activation of monochloramine (NH2Cl). Exposing the chlorinated wastewater effluent with residual NH2Cl to solar irradiation (solar/DOM/NH2Cl process) degrades six structurally diverse micropollutants at rate constants 1.26-34.2 times of those by the solar photolysis of the dechlorinated effluent (solar/DOM process). Notably, among the six micropollutants, the degradation rate constants of estradiol, acetaminophen, bisphenol A, and atenolol by the solar/DOM/NH2Cl process are 1.13-4.32 times the summation of those by the solar/DOM and solar/NH2Cl processes. The synergism in micropollutant degradation is attributed to the generation of reactive nitrogen species (RNS) and hydroxyl radicals (HO·) from the photosensitized activation of NH2Cl. Triplet state-excited DOM (3DOM*) dominates the activation of NH2Cl, leading to the generation of RNS, while HO· is produced from the interactions between RNS and other photochemically produced reactive intermediates (e.g., O2·- and DOM·+/·-). The findings advance the knowledge of DOM-mediated photosensitization and offer a sustainable method for micropollutant abatement in wastewater effluents containing residual NH2Cl.

Advanced oxidation processes for water and wastewater treatment - Guidance for systematic future research

Advanced oxidation processes (AOPs) are a growing research field with a large variety of different process variants and materials being tested at laboratory scale. However, despite extensive research in recent years and decades, many variants have not been transitioned to pilot- and full-scale operation. One major concern are the inconsistent experimental approaches applied across different studies that impede identification, comparison, and upscaling of the most promising AOPs. The aim of this tutorial review is to streamline future studies on the development of new solutions and materials for advanced oxidation by providing guidance for comparable and scalable oxidation experiments. We discuss recent developments in catalytic, ozone-based, radiation-driven, and other AOPs, and outline future perspectives and research needs. Since standardized experimental procedures are not available for most AOPs, we propose basic rules and key parameters for lab-scale evaluation of new AOPs including selection of suitable probe compounds and scavengers for the measurement of (major) reactive species. A two-phase approach to assess new AOP concepts is proposed, consisting of (i) basic research and proof-of-concept (technology readiness levels (TRL) 1-3), followed by (ii) process development in the intended water matrix including a cost comparison with an established process, applying comparable and scalable parameters such as UV fluence or ozone consumption (TRL 3-5). Subsequent demonstration of the new process (TRL 6-7) is briefly discussed, too. Finally, we highlight important research tools for a thorough mechanistic process evaluation and risk assessment including screening for transformation products that should be based on chemical logic and combined with complementary tools (mass balance, chemical calculations).

Efficiency of ozonation and sulfate radical - AOP for removal of pharmaceuticals, corrosion inhibitors, x-ray contrast media and perfluorinated compounds from reverse osmosis concentrates

This study investigated the elimination of pharmaceuticals, corrosion inhibitors, x-ray contrast media and perfluorinated compounds from reverse osmosis concentrates during ozonation and UV/persulfate processes. Second-order rate constants for the reactions of candesartan, irbesartan, methyl-benzotriazole, and chloro‑benzotriazole with sulfate radical (SO4·-) were determined for the first time. Experiments were conducted in buffered pure water, in buffered water added with the matrix substituents chloride, carbonate, NOM, and reverse osmosis concentrate with spiked micropollutants (MP). UV/persulfate eliminated all MP to a higher extent than ozonation in RO concentrates due to the higher yield of oxidative species and photolytic degradation. Compounds with electron-rich moieties such as carbamazepine, diclofenac, metoprolol, and sulfamethoxazole were completely eliminated with small ozone doses (< 0.5 mg O3 / mg DOC) and with a small fluence (< 5000 J m-2) in UV/persulfate processes. Photosensitive compounds with high reactivity towards hydroxyl radicals (·OH) and SO4·- like the x-ray contrast media Iopamidol, Iohexol, and Amidotrizoic acid were successfully eliminated with a reasonable fluence in UV/persulfate, whereas these compounds persist in ozonation at common ozone dosages. However, much higher fluences and ozone dosages were required for the least reactive compounds like the class of benzotriazoles. Comparing the application of both oxidative processes to the RO concentrate, ozonation has the disadvantage of forming bromate. The energy input of both processes strongly depends on the target compounds to be eliminated. For the elimination of compounds such as sulfamethoxazole, ozonation is a feasible technique, whereas UV/persulfate is better suited for the elimination of recalcitrant compounds such as x-ray contrast media. In general, oxidative process treatment of RO concentrate could be applied to partly abate micropollutants before discharge.

Solar-driven persulfate degradation of caffeine and cephradine in synthetic human urine

Urine source separation, as an innovative concept for the reuse of microlevel nutrients in human urine, has drawn increasing attention recently. Consequently, removing coexisting pharmaceuticals in urine is necessary for further reuse. This study is the first to apply the solar-driven persulfate process (Solar/PS) to the investigation of cephradine (CFD) and caffeine (CAF) degradation in synthetic human urine. The results showed that significantly more degradation of CFD and CAF occurs with the Solar/PS process than with persulfate oxidation and direct sunlight photolysis, respectively. The generated reactive species ·OH, SO4·-, O2·- and 1O2 were identified in the Solar/PS process. While SO4·- played a dominant role at pH 6, it played a minor role at pH 9 due to the lower amount generated under alkaline conditions. The presence of chloride and ammonia negatively impacted the photodegradation of both compounds. In contrast, bicarbonate exhibited no effect on CAF but enhanced CFD degradation owing to its amino-acid-like structure, which has a higher reactivity toward CO3·-. Although total organic carbon (TOC) was partially mineralized after 6 h of operation, no Microtox® toxicity was observed.

Peracetic acid as an alternative disinfectant for micropollutants degradation and disinfection byproducts control in outdoor swimming pools

Peracetic acid (PAA) has garnered significant interest as a novel alternative to chlorine-based disinfectants for water treatment due to its broad-spectrum antimicrobial activity and its ability of reactive species generation when exposed to UV light. However, limited studies have investigated micropollutant degradation in the presence of PAA under solar irradiation. This is the first study to comprehensively investigate the photodegradation of caffeine (CAF) and 4-methylbenzylidene camphor (4-MBC) and the removal of disinfection byproducts (DBPs) in the presence of PAA under simulated solar light. The study revealed that the photodegradation of CAF and 4-MBC was significantly enhanced in the presence of PAA, following pseudo-first-order kinetics (R2 > 0.98) with reaction rates (kobs) of 0.220 and 0.111 h-1, respectively. In addition, substantial reduction of 21 DBPs, including trihalomethanes, haloacetic acids and haloacetonitriles, and no DBPs formation were observed in the presence of PAA and simulated solar irradiation. The proportion of coexisting H2O2 in the PAA solution considerably influenced target compounds degradation. CAF and 4-MBC were degraded faster under acidic conditions than under alkaline conditions. Hydroxyl radicals (·OH) dominated the degradation of CAF at different pH values, while direct photolysis and other reactive species played a major role in the degradation of 4-MBC.

Elucidating the performance of UV-based photochemical processes for the removal of trace organic contaminants: Degradation and toxicity evaluation

In this study, the performance of standalone ultraviolet (UV) photolysis and UV-based advanced oxidation processes (AOPs), namely, UV/hydrogen peroxide, UV/chlorine, UV/persulphate, and UV/permonosulphate, were investigated for the degradation of 31 trace organic contaminants (TrOCs). Under the tested conditions, standalone UV photolysis did not achieve effective removal of TrOCs. To improve the degradation efficiency of UV photolysis, four different oxidants were added individually to the test solution. The effect of these oxidants in the absence of UV irradiation was also explored and only chlorine showed promising degradation of some contaminants. During the chlorination of 31 investigated TrOCs, only six demonstrated greater than 50% degradation. The combined UV-based AOPs demonstrated much improved degradation (ranging from 65 to 100%) depending on TrOC-structure and oxidant concentration. The UV/hydrogen peroxide process showed similar degradation of TrOCs, irrespective of the functional groups (i.e., electron withdrawing groups, EWGs and electron donating groups, EDGs) present in their structures. Conversely, the UV/sulphate and UV/chlorine based processes achieved better degradation of the TrOCs with EDGs in their structures. TrOCs degradation improved up to 40% when oxidants concentrations were increased from 0.1 to 1 mM, and further increasing the concentration to 2 mM did not improve degradation. Toxicity evaluation using bioluminescence test (BLT assay) demonstrated that except for UV/hydrogen peroxide, all UV-based AOPs increased the toxicity of the treated effluent, indicating generation of toxic by-products. This study elucidates the performance of four different UV-based AOPs for the removal of commonly detected diverse TrOCs for the first time.

Comprehensive Assessment of Reactive Bromine Species in Advanced Oxidation Processes: Differential Roles in Micropollutant Abatement in Bromide-Containing Water

Reactive bromine species (RBS) are gaining increasing attention in natural and engineered aqueous systems containing bromide ions (Br-). However, their roles in the degradation of structurally diverse micropollutants by advanced oxidation processes (AOPs) were not differentiated. In this study, the second-order rate constants (k) of Br•, Br2•-, BrO•, and ClBr•- were collected and evaluated. Br• is the most reactive RBS toward 21 examined micropollutants with k values of 108-1010 M-1 s-1. Br2•-, ClBr•-, and BrO• are selective for electron-rich micropollutants with k values of 106-108 M-1 s-1. The specific roles of RBS in aqueous micropollutant degradation in AOPs were revealed by using simplified models via sensitivity analysis. Generally, RBS play minimal roles in the UV/H2O2 process but are significant in the UV/peroxydisulfate (PDS) and UV/chlorine processes in the presence of trace Br-. In UV/PDS with ≥1 μM Br-, Br• emerges as the major RBS for removing electron-rich micropollutants. In UV/chlorine, BrO• contributes to the degradation of specific electron-rich micropollutants with removal percentages of ≥20% at 1 μM Br-, while the contributions of BrO• and Br• are comparable to those of reactive chlorine species as Br- concentration increases to several μM. In all AOPs, Br2•- and ClBr•- play minor roles at 1-10 μM Br-. Water matrix components such as HCO3-, Cl-, and natural organic matter (NOM) significantly inhibit Br•, while BrO• is less affected, only slightly scavenged by NOM with a k value of 2.1 (mgC/L)-1 s-1. This study sheds light on the differential roles of multiple RBS in micropollutant abatement by AOPs in Br--containing water.

Establishment of microbial model communities capable of removing trace organic chemicals for biotransformation mechanisms research

BACKGROUND

Removal of trace organic chemicals (TOrCs) in aquatic environments has been intensively studied. Some members of natural microbial communities play a vital role in transforming chemical contaminants, however, complex microbial interactions impede us from gaining adequate understanding of TOrC biotransformation mechanisms. To simplify, in this study, we propose a strategy of establishing reduced-richness model communities capable of removing diverse TOrCs via pre-adaptation and dilution-to-extinction.

RESULTS

Microbial communities were adapted from tap water, soil, sand, sediment deep and sediment surface to changing concentrations of 27 TOrCs mixture. After adaptation, the communities were further diluted to reduce diversity into 96 deep well plates for high-throughput cultivation. After characterizing microbial structure and TOrC removal performance, thirty taxonomically non-redundant model communities with different removal abilities were obtained. The pre-adaptation process was found to reduce the microbial richness but to increase the evenness and phylogenetic diversity of resulting model communities. Moreover, phylogenetic diversity showed a positive effect on the number of TOrCs that can be transformed simultaneously. Pre-adaptation also improved the overall TOrC removal rates, which was found to be positively correlated with the growth rates of model communities.

CONCLUSIONS

This is the first study that investigated a wide range of TOrC biotransformation based on different model communities derived from varying natural microbial systems. This study provides a standardized workflow of establishing model communities for different metabolic purposes with changeable inoculum and substrates. The obtained model communities can be further used to find the driving agents of TOrC biotransformation at the enzyme/gene level.

ABTS as Both Activator and Electron Shuttle to Activate Persulfate for Diclofenac Degradation: Formation and Contributions of ABTS•+, SO4•-, and •OH

The activation of peroxydisulfate (PDS) by organic compounds has attracted increasing attention. However, some inherent drawbacks including quick activator decomposition and poor anti-interference capacity limited the application of organic compound-activated PDS. It was interestingly found that 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonate) (ABTS) could act as both activator and electron shuttle for PDS activation to enhance diclofenac (DCF) degradation over a pH range of 2.0-11.0. Multiple reactive species of ABTS•+, •OH, and SO4•- were generated in the PDS/ABTS system, while only ABTS•+ and •OH directly contributed to DCF degradation. ABTS•+, generated via the reactions of ABTS with PDS, SO4•-, and •OH, was the dominant reactive species of DCF degradation. No significant decomposition of ABTS was observed in the PDS/ABTS system, and ABTS acted as both activator and electron shuttle. Four possible degradation pathways of DCF were proposed, and the toxicity of DCF decreased after treatment with the PDS/ABTS system. The PDS/ABTS system had good anti-interference capacity to common natural water constituents. Additionally, ABTS was encapsulated into cellulose to obtain ABTS@Ce beads, and the PDS/ABTS@Ce system possessed excellent performance on DCF degradation. This study proposes a new perspective to reconsider the mechanism of activating PDS with organic compounds and highlights the considerable contribution of organic radicals on contaminant removal.

Hydroxylamine enhanced the degradation of diclofenac in Cu(II)/peracetic acid system: Formation and contributions of CH3C(O)O•, CH3C(O)OO•, Cu(III) and •OH

The slow reduction of Cu(II) into Cu(I) through peracetic acid (PAA) heavily limited the widespread application of Cu(II)/PAA system. Herein, hydroxylamine (HA) was proposed to boost the oxidative capacity of Cu(II)/PAA system by facilitating the redox cycle of Cu(I)/Cu(II). HA/Cu(II)/PAA system was quite rapid in the removal of diclofenac within a broad pH range of 4.5-9.5, with a 10-fold increase in the removal rate of diclofenac compared with the Cu(II)/PAA system at an optimal initial pH of 8.5. Results of UV-Vis spectra, electron paramagnetic resonance, and alcohol quenching experiments demonstrated that CH3C(O)O•, CH3C(O)OO•, Cu(III), and •OH were involved in HA/Cu(II)/PAA system, while CH3C(O)OO• was verified as the predominant reactive species of diclofenac elimination. Different from previously reported Cu-catalyzed PAA processes, CH3C(O)OO• mainly generated from the reaction of PAA with Cu(III) rather than CH3C(O)O• and •OH. Four possible elimination pathways for diclofenac were proposed, and the acute toxicity of treated diclofenac solution with HA/Cu(II)/PAA system significantly decreased. Moreover, HA/Cu(II)/PAA system possessed a strong anti-interference ability towards the commonly existent water matrix. This research proposed an effective strategy to boost the oxidative capacity of Cu(II)/PAA system and might promote its potential application, especially in copper-contained wastewater.

Clarification of the role of singlet oxygen for pollutant abatement during persulfate-based advanced oxidation processes: Co3O4@CNTs activated peroxymonosulfate as an example

Singlet oxygen (1O2) has often been identified by the popularly used quenching method as a more important reactive species (RS) than sulfate radicals (SO4•-) and hydroxyl radicals (•OH) for pollutant abatement during persulfate-based advanced oxidation processes (PS-AOPs), especially those activated by carbon-based catalysts. However, latest studies have demonstrated that the quenching method actually can often mislead the interpretations of the role of RS for pollutant abatement during AOPs due to various confounding effects caused by adding high-concentration quenchers in the system. To clarify the role of 1O2 in PS-AOPs, this study developed a probe compound-based experimental and kinetic model to quantify the concentrations and exposures of 1O2, SO4•-, and •OH, as well as their relative contributions to pollutant abatement during a cobalt oxide incorporated carbon nanotubes activated peroxymonosulfate (Co3O4@CNTs/PMS) process. Results show that during the Co3O4@CNTs/PMS process, the exposures and transient concentrations of 1O2 were about 19.6 and 41.3 times higher than those of SO4•- and •OH, respectively. However, the relative contribution of 1O2 to the abatement of most pollutants tested in this study (e.g., sulfisoxazole, sulfamethoxyprazine, trimethoprim, and metoprolol) is generally negligible (f1O2 ≤ 8%) compared to that of SO4•- and •OH ( [Formula: see text]  = 15%-98% and f•OH = 2%-78%) because of the significantly lower reactivity of 1O2 with these compounds than that of SO4•- and •OH. Reasons for misidentifying 1O2 as the dominant RS for pollutant abatement by the quenching method were then analyzed based on reaction kinetics principles. The results of this study highlight that while 1O2 can be generated in significant amounts and be present at higher concentrations than SO4•- and •OH in PS-AOP systems, 1O2 is unlikely to be the dominant RS for the abatement of most pollutants during the PS-AOPs because of its weak and selective oxidation capacity, and caution should be taken when using the quenching method to evaluate the role of RS for pollutant abatement by the PS-AOPs.

Predicting combined antibacterial activity of sulfapyridine and its transformation products during sulfapyridine degradation

Antibiotics have strong antibacterial activity, even trace antibiotics can greatly inhibit the pollutant degradation efficiency. In order to effectively improve the pollutant degradation efficiency, it was hence of great significance to explore sulfapyridine (SPY) degradation and the mechanism of antibacterial activity. This study selected SPY as the research object, of which the trend of SPY concentration through hydrogen peroxide (H₂O₂), potassium peroxydisulfate (PDS) and sodium percarbonate (SPC) and resultant antibacterial activity at pre-oxidation was examined. The combined antibacterial activity (CAA) of SPY and its transformation products (TPs) was further analyzed. The SPY degradation efficiency reached more than 90 %. However, the degradation efficiency of antibacterial activity was between 40-60 %, and the mixture's antibacterial activity was difficult to be removed. The antibacterial activity of TP3, TP6 and TP7 was higher than that of SPY. TP1, and TP8 and TP10 were more prone to synergistic reaction with other TPs. The antibacterial activity of binary mixture gradually changed from synergism to antagonism as binary mixture concentration increased. The results provided a theoretical basis for the efficient degradation of antibacterial activity of the SPY mixture solution.

Treatment of sewage sludge hydrothermal carbonization aqueous phase by Fe(II)/CaO2 system: Oxidation behaviors and mechanism of organic compounds

The Fe(II)/CaO₂ system with a stable oxidant and a low-cost homogeneous activating agent has been considered as a prospective process for the disposal of wastewater. The system was constructed to treat sewage sludge hydrothermal carbonization aqueous phase (HTC-AP) in this study. As the hydrothermal temperature increased, the organics in the HTC-AP were first decomposed and then cyclized, while the Maillard reaction occurs throughout the stage. The oxidation efficiency of the Fe(II)/CaO₂ system was related to the composition of organics in HTC-AP, and the removal of dissolved organic carbon (DOC) by the system was 38.56 % in the HTC-AP obtained by hydrothermal treatment at 220 °C. Redundancy analysis showed that the low molecular weight organics, hydrophobic acids, and hydrophobic neutral components were beneficial to DOC removal, while Maillard products and cyclization products were hard to be oxidized to CO₂ and H₂O. The CN functional group of the protein facilitated DOC removal, and some organics in HTC-AP were oxidized to acids and phenols. The energy input to remove DOC in Fe(II)/CaO₂ system was 27.74 MJ per kg carbon. This study provides a low-energy consumption Fe(II)/CaO₂ system for the post-treatment of HTC-APs and explores the applicability of the system.

Influence of water matrix components and peroxide sources on the transformation products and toxicity of tebuthiuron under UVC-based advanced oxidation processes

Coupling of UV-C irradiation to different peroxides (H₂O₂, S₂O₈^2- and HSO₅^-) has great potential to degrade persistent organic compounds due to the formation of HO^• or SO₄^•- species. However, an in-depth comparison between the performance of different UV-C/peroxide processes as a function of (i) target compound degradation, (ii) generated transformation products and (iii) lethal/sub lethal toxicity effects has not yet been performed. To this end a comparison study was carried out to evaluate the effectiveness of different UV-C/peroxide processes using the herbicide tebuthiuron (100 or 500 μg L^-1) as a model pollutant. TBH degradation experiments were performed at lab-scale in real municipal wastewater treatment plant effluent and distilled water. Faster degradation occurred by increasing peroxide concentration from 735 to 2206 μmol L^-1 in the municipal wastewater treatment plant effluent, mainly for S₂O₈^2-. Experiments performed in the presence of peroxide trapping agents - HO^• and SO₄^•- (methoxibenzene) or HO^• (2-propanol) - revealed that oxidation in the UV-C/S₂O₈^2- system occurs mainly through SO₄^•-. Lower toxicity for the MWWTP effluent was obtained after oxidative treatments using hydrogen peroxide or monopersulfate as oxidants which react mainly through HO^• radicals. Two mechanistic pathways were proposed for tebuthiuron degradation: (i) hydrogen abstraction by HO^• (H₂O₂ and HSO₅^-) and (ii) electron transfer by SO₄^•- (S₂O₈^2-). In addition, one unprecedented transformation product was identified. In conclusion, results emphasize the relevance of comparing the degradation of toxic compounds in the presence of different peroxide sources and matrices and simultaneouly evaluating responses chemical and biological endpoints.

Influence of water matrix components on the UV/chlorine process and its reactions mechanism

The UV/chlorine system has become an attractive alternative Advanced Oxidation Process (AOP) for the removal of recalcitrant pollutants in the last decade due to the simultaneous formation of chlorine and hydroxyl radicals. However, there is no consensus regarding the results and trends obtained in previous micropollutant removal studies by AOPs, highlighting the complexity of the UV/chlorine process and the need for further research. This study investigates the degradation of acetaminophen (ACTP) by UV/chlorine and the effects of the water matrix in the reaction kinetics. In particular, the effects of natural organic matter (NOM), alkalinity and mineral salts on the kinetics and reactive species were elucidated. The complexity of the system was revealed by the analysis of the radical generation and transformation in different water matrices, applying the kinetic modelling approach to complement the scavenger tests. The higher kinetic rates of ACTP at alkaline pH provided new insights into the chlorine reactions under UV radiation, where secondary and tertiary reactive oxygen species including ozone were proven to play the major role in degradation. On the contrary, at acidic pH, reaction kinetic modelling demonstrated that ClO^• radical occurs at high concentrations in the order of 10^-10 M, being therefore the main oxidant, followed by other chlorine radicals. It is noteworthy that at alkaline pH the presence of typical inorganic ions such as carbonate had little impact on ACTP degradation, contrary to the observed reduction of degradation rates at acidic pH. The expected detrimental effect of the NOM in AOPs was also evidenced, although the use of chlorine as radical source reduces the relevance of the inner filter effect in comparison to UV/H₂O₂.

Reactive chlorine species in the enhanced degradation of UV stabilizers during the sunlight/free chlorine process

Benzotriazole (BT) and 5-methyl-1H-benzotriazole (5-MeBT) are the most commonly used UV stabilizers and recalcitrant contaminants that are widely distributed in aquatic environments. The novelty of this study was to investigate the role of RCSs in the enhanced degradation of BT and 5-MeBT during the sunlight/free chlorine process. The results showed that sunlight/free chlorine could enhance the degradation of BT and 5-MeBT compared with that obtained with sunlight irradiation and chlorination alone, and this process was well described by pseudo-first-order kinetics. The degradation rate constants of BT and 5-MeBT during sunlight/free chlorine treatment at pH 7 were 0.094 ± 0.001 min^-1 and 0.134 ± 0.002 min^-1, respectively. The degradation rates further increased with increases in the chlorine dosage and under alkaline conditions (3.818 ± 0.243 min^-1 for BT and 7.754 ± 0.716 min^-1 for 5-MeBT at pH 9). The enhanced removal obtained during the sunlight/free chlorine process could be attributed to the generation of HO• and reactive chlorine species (RCSs), such as Cl• and ClO•. Under alkaline conditions, RCSs were the dominant reactive species, and their contribution increased from 21.2% to 98.7% with increases in the pH from 7 to 9; this phenomenon was due to changes in free chlorine and BT speciation. Radical scavenging tests further verified that BT was mainly decomposed by ClO•, and ClO• showed high reactivity toward deprotonated BT through second-order rate constant estimation. A byproduct analysis demonstrated that BT underwent hydroxylation and chlorine substitution, and a high yield of 1-chlorobenzotriazole (1-ClBT) formation was observed. Even though the sunlight/free chlorine process resulted in a low level of mineralization, no Microtox® toxicity was detected in the treated solutions. Briefly, the significant contribution of ClO• to BT removal under alkaline conditions implies that sunlight/free chlorine could be utilized in a broader range of treatment conditions.

Impact of water matrices on oxidation effects and mechanisms of pharmaceuticals by ultraviolet-based advanced oxidation technologies: A review

The binding between water components (dissolved organic matters, anions and cations) and pharmaceuticals influences the migration and transformation of pollutants. Herein, the impact of water matrices on drug degradation, as well as the electrical energy demands during UV, UV/catalysts, UV/O₃, UV/H₂O₂-based, UV/persulfate and UV/chlorine processes were systemically evaluated. The enhancement effects of water constituents are due to the powerful reactive species formation, the recombination reduction of electrons and holes of catalyst and the catalyst regeneration; the inhibition results from the light attenuation, quenching effects of the excited states of target pollutants and reactive species, the stable complexations generation and the catalyst deactivation. The transformation pathways of the same pollutant in various AOPs have high similarities. At the same time, each oxidant also can act as a special nucleophile or electrophile, depending on the functional groups of the target compound. The electrical energy per order (EEO) of drugs degradation may follow the order of EEO_UV > EEO_UV/catalyst > EEO_UV/H2O2 > EEO_UV/PS > EEO_UV/chlorine or EEO_UV/O3. Meanwhile, it is crucial to balance the cost-benefit assessment and toxic by-products formation, and the comparison of the contaminant degradation pathways and productions in the presence of different water matrices is still lacking.

Mechanistic Insights into the Markedly Decreased Oxidation Capacity of the Fe(II)/S2O82- Process with Increasing pH

The poor oxidation capacity of the Fe(II)/S₂O₈^2- [Fe(II)/PDS] system at pH > 3.0 has limited its wide application in water treatment. To unravel the underlying mechanism, this study systematically evaluated the possible influencing factors over the pH range of 1.0-8.0 and developed a mathematical model to quantify these effects. Results showed that ∼82% of the generated Fe(IV) could be used for pollutant degradation at pH 1.0, whereas negligible Fe(IV) contribution was observed at pH 7.5. This dramatic decline of Fe(IV) contribution with increasing pH dominantly accounted for the pH-dependent performance of the Fe(II)/PDS process. Unexpectedly, Fe(II) could consume ∼80% of the generated SO₄^•- non-productively under both acidic and near-neutral conditions, while the larger formation of Fe(III) precipitates at high pH inhibited the SO₄^•- contribution mildly. Moreover, the strong Fe(II) scavenging effect was difficult to be compensated for by slowing down the Fe(II) dosing rate. The competition of dissolved oxygen with PDS for Fe(II) was insignificant at pH ≤ 7.5, where the second-order rate constants for reactions of Fe(II) with oxygen were much lower than or comparable to that between Fe(II) and PDS. These findings could advance our understanding of the chemistry and application of the Fe(II)/PDS process.

Antibiotics degradation by UV/chlor(am)ine advanced oxidation processes: A comprehensive review

Antibiotics are emerging contaminants in aquatic environments which pose serious risks to the ecological environment and human health. Advanced oxidation processes (AOPs) based on ultraviolet (UV) light have good application prospects for antibiotic degradation. As new and developing UV-AOPs, UV/chlorine and derived UV/chloramine processes have attracted increasing attention due to the production of highly reactive radicals (e.g., hydroxyl radical, reactive chlorine species, and reactive nitrogen species) and also because they can provide long-lasting disinfection. In this review, the main reaction pathways of radicals formed during the UV/chlor (am)ine process are proposed. The degradation efficiency, influencing factors, generation of disinfection by-products (DBPs), and changes in toxicity that occur during antibiotic degradation by UV/chlor (am)ine are reviewed. Based on the statistics and analysis of published results, the effects caused by energy consumption, defined as electrical energy per order (EE/O), increase in the following order: UV/chlorine < UV/peroxydisulfate (PDS)< UV/H₂O₂ < UV/persulfate (PS) < 265 nm and 285 nm UV-LED/chlorine (EE/O). Some inherent problems that affect the UV/chlor (am)ine processes and prospects for future research are proposed. The use of UV/chlor (am)ine AOPs is a rich field of research and has promising future applications, and this review provides a theoretical basis for that.

Comparative photo-oxidative degradation of etodolac, febuxostat and imatinib mesylate by UV-C/H2O2 and UV-C/S2O82- processes: Modeling, treatment optimization and biodegradability enhancement

The pharmaceutical contamination in aquatic environment has arisen increasing concern due to its potentially chronic toxicity. In recent years, HO° and SO₄°^- based advanced oxidation processes (AOPs) have been widely applied in water and wastewater treatments due to their highly efficiency on contaminant removal. Here, the response surface modeling (RSM) was used to investigate the degradation of three typical pharmaceuticals (i.e., etodolac (ETD), febuxostat (FBU) and imatinib mesylate (IMT)) by UV/H₂O₂ and UV/S₂O₈^2- processes. Based on the multiple regression analysis on full factorial design matrix and calculated reaction rate constants, the RSM was built. The experimental rate constants under optimal conditions were quite close to those obtained from the model, implying the good fit of the RSM. In addition, the RSM results indicated that UV/S₂O₈^2- process was less sensitive to pH in comparison to the UV/H₂O₂ process on target contaminant removal. Finally, it showed that UV/S₂O₈^2- process was superior to the UV/H₂O₂ process to on the enhancement of target contaminant biodegradability.

Influence of chemical structure of organic micropollutants on the degradability with ozonation

The increasing environmental problems due to various organic micropollutants in water cause the search of suitable additional water treatment methods. Gaining experimental data for the large amount and variety of pollutants would consume a lot of time as well as economic and ecologic resources. An alternative approach is predictive quantitative structure-property relationship (QSPR) modeling, which establishes a correlation between the structural properties of a molecules with a biological, physical, or chemical property. Therefore, in this study, QSPR modeling has been conducted using extensive validation techniques and statistical test to investigate the structural influence on the degradability of organic micropollutants with ozonation. In contrast to most of the other studies, the underlying dataset - rate constants for 92 organic molecules - were obtained under standardized conditions with defined experimental parameters. QSPR modeling was executed using a combination of the software PaDEL for descriptor calculation and QSARINS for the modeling process respecting all five OECD-requirements for applicable QSAR/QSPR-models. The final model was selected using a multi-criteria decision-making tool to evaluate the model quality based on all calculated statistical quality parameters. The model included 10 selected descriptors and fingerprints and showed good regression abilities, predictive power, and stability (R² = 0.8221, CCC_tr = 0.9024, Q²_loo = 0.7436, R²_ext = 0.8420, Q²_F1 = 0.8104). The applicability domain of the QSPR model was defined and an interpretation of selected model descriptors has been connected to previous experimental studies. A significant influence of the interpretable descriptors was put into experimental context and compared with previous studies and models. For example, the molar refractivity as a measure of size and polarizability of a molecule and the occurrence of important substructures such as a formamide group seem to decrease the removal rate constant. The contribution of lone electrons entering into resonance as well as the occurrence of fused rings were identified as influences for the increase of the degradability of micropollutants by ozonation.

Assessment of the validity of the quenching method for evaluating the role of reactive species in pollutant abatement during the persulfate-based process

Reactive species such as sulfate radicals (SO₄^•-), hydroxyl radicals (^•OH), and/or singlet oxygen (¹O₂) have often been proposed as the main reactive species for pollutant abatement during the persulfate-based process, and their relative importance is conventionally assessed by the quenching method based on an implicit fundamental assumption that the added high-concentration quenchers (e.g., tert-butanol and methanol) only scavenge their target reactive species, but do not considerably affect the other reaction mechanism of the system. To examine the validity of this assumption, this study evaluated the effects of several commonly used quenchers (tert-butanol, methanol, ethanol, isopropanol, furfuryl alcohol, and L-histidine) on the mechanism of a cobalt mediated peroxymonosulfate (Co(II)/PMS) process. The results demonstrate that besides quenching target reactive species, the added high-concentration quenchers can cause many confounding effects on the Co(II)/PMS process, e.g., accelerating PMS decomposition, interfering reactive species production, and quenching of non-target reactive species. Because of these confounding effects, the quenching method can actually lead to serious misinterpretation of the role of reactive species in pollutant abatement during the persulfate-based process. The findings of this study highlight that the underlying assumption of the quenching method is usually invalid for the persulfate-based process. Therefore, it should be cautious to apply the quenching method to investigate the mechanism of the persulfate-based process, and some debatable conclusions of prior studies obtained with the quenching method may require further verification.

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.

Modification Mechanism of Polyamide Reverse Osmosis Membrane by Persulfate: Roles of Hydroxyl and Sulfate Radicals

Oxidative modification is a facile method to improve the desalination performance of thin-film composite membranes. In this study, we comparatively investigated the modification mechanisms induced by sulfate radical (SO₄^• -) and hydroxyl radical (HO^•) for polyamide reverse osmosis (RO) membrane. The SO₄^• -- and HO^•-based membrane modifications were manipulated by simply adjusting the pH of the thermal-activated persulfate solution. Although both of them improved the water permeability of the RO membrane under certain conditions, the SO₄^• --modified membrane notably prevailed over the HO^•-modified one due to higher permeability, more consistent salt rejection rates over wide pH and salinity ranges, and better stability when exposed to high doses of chlorine. The differences of the membranes modified by the two radical species probably can be related to their distinct surface properties in terms of morphology, hydrophilicity, surface charge, and chemical composition. Further identification of the transformation products of a model polyamide monomer using high-resolution mass spectrometry demonstrated that SO₄^• - initiated polymerization reactions and produced hydroquinone/benzoquinone and polyaromatic structures; whereas the amide group of the monomer was degraded by HO^•, generating hydroxyl, carboxyl, and nitro groups. The results will enlighten effective ways for practical modification of polyamide RO membranes to improve desalination performances and the development of sustainable oxidation-combined membrane processes.

Unravelling the performance of UV/H2O2 on the removal of pharmaceuticals in real industrial, hospital, grey and urban wastewaters

This study provides an integrated assessment of UV/H₂O₂ treatment of different real wastewater matrices: two urban wastewater treatment plants (WWTPs) secondary effluents, greywater, hospital, and pharmaceutical industrial effluents. It considers micropollutant removal (up to 30 pharmaceuticals and 13 transformation products at environmental concentrations), energy efficiency and effluent toxicity. The complexity of the wastewater matrix negatively affected the UV fluence in the photo-reactor, scavenged hydroxyl radicals and hindered a proper H₂O₂ utilization thus reducing the treatment efficiency. At the optimal treatment conditions, overall pharmaceuticals removal was the highest for urban WWTPs effluents (69%-86%), followed by greywater (59%), hospital (36%) and industrial (17%) effluents. The ecotoxicity of the treated samples was reduced around one toxicity unit after the UV/H₂O₂ treatment in all cases except in industrial wastewater. The average observed removal in urban wastewater effluents and greywater for photo-susceptible, moderately photo-susceptible, and most photo-resistant compounds was 93%, 73% and 46% including outliers, respectively. The calculated electrical energy per order (E_EO) values were 0.9-1.5 kWh/(m³·order) for urban WWTP effluents and greywater while for hospital and industrial effluents was much higher (7.3-9.1 kWh/(m³·order)).

Accurate Removal of Toxic Organic Pollutants from Complex Water Matrices

Characteristic emerging pollutants at low concentration have raised much attention for causing a bottleneck in water remediation, especially in complex water matrices where high concentration of interferents coexist. In the future, tailored treatment methods are therefore of increasing significance for accurate removal of target pollutants in different water matrices. This critical review focuses on the overall strategies for accurately removing highly toxic emerging pollutants in the presence of typical interferents. The main difficulties hindering the improvement of selectivity in complex matrices are analyzed, implying that it is difficult to adopt a universal approach for multiple targets and water substrates. Selective methods based on assorted principles are proposed aiming to improve the anti-interference ability. Thus, typical approaches and fundamentals to achieve selectivity are subsequently summarized including their mechanism, superiority and inferior position, application scope, improvement method and the bottlenecks. The results show that different methods may be applicable to certain conditions and target pollutants. To better understand the mechanism of each selective method and further select the appropriate method, advanced methods for qualitative and quantitative characterization of selectivity are presented. The processes of adsorption, interaction, electron transfer, and bond breaking are discussed. Some comparable selective quantitative methods are helpful for promoting the development of related fields. The research framework of selectivity removal and its fundamentals are established. Presently, although continuous advances and remarkable achievements have been attained in the selective removal of characteristic organic pollutants, there are still various substantial challenges and opportunities. It is hopeful to inspire the researches on the new generation of water and wastewater treatment technology, which can selectively and preferentially treat characteristic pollutants, and establish a reliable research framework to lead the direction of environmental science.

Role of Antioxidant Moieties in the Quenching of a Purine Radical by Dissolved Organic Matter

Dissolved organic matter (DOM) has been known to inhibit the degradation of trace organic contaminants (TrOCs) in advanced oxidation processes but quantitative understanding is lacking. Adenine (ADN) was selected as a model TrOC due to the wide occurrence of purine groups in TrOCs and the well-documented transient spectra of its intermediate radicals. ADN degradation in the presence of DOM during UV/peroxydisulfate treatment was quantified using steady-state photochemical experiments, time-resolved spectroscopy, and kinetic modeling. The inhibitory effects of DOM were found to include competing for photons, scavenging SO₄^•- and HO^•, and also converting intermediate ADN radicals (ADN(-H)^•) back into ADN. Half of the ADN(-H)^• were reduced back to ADN in the presence of about 0.2 mg_C L^-1 of DOM. The quenching rate constants of ADN(-H)^• by the 10 tested DOM isolates were in the range of (0.39-1.18) × 10⁷ M_C^-1 s^-1. They showed a positive linear relationship with the total antioxidant capacity of DOM. The laser flash photolysis results of the low-molecular-weight analogues of redox-active moieties further supported the dominant role of antioxidant moieties in DOM in the quenching of ADN(-H)^•. The diverse roles of DOM should be considered in predicting the abatement of TrOCs in advanced oxidation processes.

Removal of recalcitrant organics in reverse osmosis concentrate from coal chemical industry by UV/H2O2 and UV/PDS: Efficiency and kinetic modeling

The lack of stability in catalytic ozonation treatment of reverse osmosis (RO) concentrate from coal chemical industry calls for new advanced oxidation processes. Herein, UV/H₂O₂ and UV/PDS were employed to remove the bulk recalcitrant organics in the RO concentrate with a focus on the process efficiency and kinetic modeling. Results show that UV/H₂O₂ overmatched UV/PDS in reducing the COD and DOC of the wastewater and the advantage became more evident in aspects of biodegradability improvement and energy cost. Specifically, the COD and DOC were removed by 62.0% and 55.5% with UV/H₂O₂ (6 mM) while the BOD₅/COD was elevated to 0.54 at a specific energy consumption of 0.83 kWh g^-1 (lab-scale). The UV/H₂O₂ process also exhibited a good adaptability to the fluctuation of wastewater quality. Afterwards, the reaction rate constants of the bulk organics upon UV photolysis and HO^• oxidation were calculated based on pseudo-first-order kinetics and radical steady-state approximation of DOC removal in the bench-scale UV/H₂O₂ reactor. A computational fluid dynamics model was then developed for the analysis of distributions of flow, radiation and chemicals in flow-through reactors which facilitated the practical process efficiency assessment. This work demonstrates the applicability of UV/H₂O₂ in removing recalcitrant organics in the RO concentrate and presents an approach from bench-scale experiments to flow-through system evaluation.

ABTS as an electron shuttle to accelerate the degradation of diclofenac with horseradish peroxidase-catalyzed hydrogen peroxide oxidation

Horseradish peroxidase (HRP)-catalyzed hydrogen peroxide (H₂O₂) oxidation could degrade a variety of organic pollutants, but the intrinsic drawback of slow degradation rate limited its widespread application. In this study, 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) was introduced into HRP/H₂O₂ system as an electron shuttle to enhance diclofenac degradation under neutral pH conditions. The green-colored ABTS radical (ABTS^•+), generated by the oxidation of ABTS with HRP-catalyzed H₂O₂ oxidation, was proved to be the main reactive species for the rapid degradation of diclofenac in HRP/H₂O₂/ABTS system. There was no destruction of ABTS/ABTS^•+ in HRP/H₂O₂/ABTS system, and ABTS was verified as an ideal electron shuttle. The reaction conditions including solution pH (4.5-10.5), HRP concentration (0-8 units mL^-1) and H₂O₂ concentration (0-500 μM) would impact the formation of ABTS^•+, and affect the degradation of diclofenac in HRP/H₂O₂/ABTS system. Moreover, compared with Fenton and hydroxylamine/Fenton systems, HRP/H₂O₂/ABTS system had better diclofenac degradation efficiency, higher H₂O₂ utilization efficiency and stronger anti-interference capacity in actual waters. Overall, the present study provided a meaningful and promising way to enhance the degradation of organic pollutants in water with HRP-catalyzed H₂O₂ oxidation.

Redox-Active Moieties in Dissolved Organic Matter Accelerate the Degradation of Nitroimidazoles in SO4•--Based Oxidation

The presence of dissolved organic matter (DOM) is known to inhibit the degradation of trace organic contaminants (TrOCs) in SO₄^•--based advanced oxidation processes (AOPs) due to filtering of the photochemically active light and radical scavenging effects. This study revealed an unexpected contribution for DOM in the degradation of nitroimidazoles (NZs) in the UV/persulfate AOP. The apparent second-order rate constants of NZs with SO₄^•- increased by 2.05 to 4.77 times in the presence of different DOMs. The increments were linearly related to the total electron capacity of DOM. Quinone and polyphenol moieties were found to play a dominant role. The reactive species generated from SO₄^•-'s oxidation of DOM, including semiquinone radical (SQ^•-) and superoxide (O₂^•-), were found to react with NZs via Michael addition and O₂^•- addition. The second-order rate constants of tinidazole with SQ^•- is determined to be (5.69 ± 0.59) × 10⁶ M^-1 s^-1 by laser flash photolysis. Reactive species potentially generated from DOM may be considered in designing processes for the abatement of different types of TrOCs.

The graceful art, significant function and wide application behavior of ultrasound research and understanding in carbamazepine (CBZ) enhanced removal and degradation by Fe0/PDS/US

Carbamazepine (CBZ) antibiotic organic contamination wastewater poses a huge threat to environmental safety. An advanced oxidation technology (Fe⁰/PDS/US) of using ultrasound (US) enhanced zero-valent iron/potassium persulfate (Fe⁰/PDS) can remove CBZ effectively. The optimal reaction conditions were determined by exploring the effect of single-factor experimental conditions such as ultrasonic power, ultrasonic frequency, CBZ concentration, solution pH, PDS dosage, and Fe⁰ dosage on the removal of CBZ. In addition, we also investigated into the effect of background ions (PO₄^3-, HCO₃^-, Cl^- and HA) on Fe⁰/PDS/US and analyzed the related results. The mechanism of CBZ removal in Fe⁰/PDS/US were explored by analyzing CBZ removal efficiency and reaction rates, the ion concentration of S₂O₈^2-, SO₄^2-, Fe²⁺ and Fe³⁺, pH and the active radicals. The result indicates that US can improve the efficiency of activated PDS and expand the pH range of Fe⁰/PDS. It has prominent performance in catalytically degrading CBZ when the pH is 10.0. SO₄^•-, ^•OH and O₂^•- all coexist in the Fe⁰/PDS/US and make contribution to CBZ removal, whereas the SO₄^•- plays a key role. US can greatly promotes the degradation of target pollutant CBZ by speeding up the dissolution of the outer portion of iron powder, producing sufficient amount of Fe²⁺ with a continuous and stable way, and better activating S₂O₈^2- to generate sufficient SO₄^•- radicals. The degradation of CBZ may embrace three reaction processes, in which organic intermediate products with low molecular weight and biological toxicity is produced, boosting further mineralization and biodegradation of products. The Fe⁰/PDS/US is of great potential application value in removal of organic pollution and environmental purification.

Unravelling molecular transformation of dissolved effluent organic matter in UV/H2O2, UV/persulfate, and UV/chlorine processes based on FT-ICR-MS analysis

Ultraviolet-based advanced oxidation processes (UV-AOPs) are very promising in advanced treatment of municipal secondary effluents. However, the transformation of dissolved effluent organic matter (dEfOM) in advanced treatment of real wastewater, particularly at molecular level, remains unclear. In this study, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) coupled with multiple statistical analysis were performed to better understand the transformation of dEfOM in UV/H₂O₂, UV/persulfate (UV/PS), and UV/chlorine treatments. An obvious increase in oxygen content of dEfOM was observed after every UV-AOPs treatment, and the detailed oxygenation processes were further uncovered by mass difference analysis based on 24 types of typical reactions. Generally, UV/H₂O₂ process was subjected to the most oxygenation reactions with the typical tri-hydroxylation one (+3O), whereas di-hydroxylation reaction (+H₂O₂) was dominant in UV/PS and UV/chlorine processes. Additionally, the three UV-AOPs shared the majority of precursors, and more proportions of unique products were identified for each process. The precursors with lower H/C and higher aromaticity were readily degraded by UV/chlorine over UV/H₂O₂ and UV/PS, with the products featuring lower molecular weight. Moreover, dEfOM of high aromaticity tended to produce chlorinated byproducts through addition reactions in chlorination and UV/chlorine processes. Among these UV-AOPs, the highest reduction of both acute toxicity and specific UV absorbance at 254 nm (SUVA₂₅₄) was observed for UV/chlorine, implying the potential for UV/chlorine process in advanced treatment of wastewater. In addition, acute toxicity was highly correlated with SUVA₂₅₄ and CHOS compounds. This study is believed to help better understand the different fates of dEfOM in real wastewater during UV-AOPs treatment.

Iodometric spectrophotometric determination of peroxydisulfate in hydroxylamine-involved AOPs: 15 min or 15 s for oxidative coloration?

In this study, iodometric spectrophotometry, the most-used method for detecting peroxydisulfate (PDS), was modified by increasing the concentration of potassium iodide (KI) for realizing the immediate PDS determination and avoiding the interference of hydroxylamine. Kinetic studies showed that the reaction between PDS and I^- to generate the yellow-colored I₃^- followed the kinetic equation as [Formula: see text] . Detection time of the iodometric spectrophotometry was shortened from 15 min to 15 s when KI concentration was increased from 0.6 M to 4.8 M. Different with the previous iodometric spectrophotometry, the modified method using 4.8 M KI as the indicator was well tolerable to the interference of hydroxylamine at acidic pH conditions. The calibration curve of the modified method showed a well linear relationship (R² = 0.999) between the absorbance of I₃^- at 352 nm and PDS concentration in the range of 0-80 μM. The modified method was highly sensitive with the absorptivity of 2.5 × 10⁴ M^-1 cm^-1 and the limit of detection of 0.11 μM. Moreover, the modified method was successfully applied for monitoring the change of PDS concentration during the degradation of diclofenac with four different PDS-based AOPs, the calculated reaction stoichiometric efficiency (RSE(%)=DiclofenacdegradedPDSconsumed×100%) followed the order as heat/PDS system > hydroxylamine/Fe²⁺/PDS system > hydroxylamine/Cu²⁺/PDS system > Fe²⁺/PDS system.

Enhancement of Iron-Based Photo-Driven Processes by the Presence of Catechol Moieties

Photo-induced Advanced Oxidation Processes (AOPs) using H2O2 or S2O82− as radical precursors were assessed for the abatement of six different contaminants of emerging concern (CECs). In order to increase the efficiency of these AOPs at a wider pH range, the catechol organic functional compound was studied as a potential assistant in photo-driven iron-based processes. Different salinity regimes were also studied (in terms of Cl− concentration), namely low salt water (1 g·L−1) or a salt–water (30 g·L−1) matrix. Results obtained revealed that the presence of catechol could efficiently assist the photo-Fenton system and partly promote the photo-induced S2O82− system, which was highly dependent on salinity. Regarding the behavior of individual CECs, the photo-Fenton reaction was able to enhance the degradation of all six CECs, meanwhile the S2O82−-based process showed a moderate enhancement for acetaminophen, amoxicillin or clofibric acid. Finally, a response-surface methodology was employed to determine the effect of pH and catechol concentration on the different photo-driven processes. Catechol was removed during the degradation process. According to the results obtained, the presence of catechol in organic macromolecules can bring some advantages in water treatment for either freshwater (wastewater) or seawater (maritime or aquaculture industry).

Degradation of nitrogen-containing hazardous compounds using advanced oxidation processes: A review on aliphatic and aromatic amines, dyes, and pesticides

Nitrogen-containing amino and azo compounds are widely used in textile, agricultural and chemical industries. Most of these compounds have been demonstrated to be resistant to conventional degradation processes. Advanced oxidation processes can be effective to mineralize nitrogen-containing compounds and improve the efficacy of overall treatment schemes. Due to a global concern for the occurrence of toxic and hazardous amino-compounds and their harmful degradation products in water, it is important to develop technologies that focus on all the aspects of their degradation. Our focus is to present a state-of-the-art review on the degradation of several amine- and azo-based compounds using advanced oxidation processes. The categories reviewed are aromatic amines, aliphatic amines, N-containing dyes and N-containing pesticides. Data has been compiled for degradation efficiencies of each process, reaction mechanisms focusing on specific attack of oxidants on N atoms, the effect of process parameters like pH, initial concentration, time of treatment, etc. and identification of intermediates. Several AOPs have been compared to provide a systematic overview of available literature that will drive essential aspects of future research on amine-based compounds. Ozone is observed to be highly reactive to most amines, dyes and pesticides, followed by Fenton processes. Degradation of amines is highly sensitive to pH and mechanisms differ at different pH values. Cavitation is a promising alternative pre-treatment method for cost reduction. Hybrid methods under optimized conditions are demonstrated to give synergistic effects and must be tailored for specific effluents in question. In conclusion, even though nitrogen-containing compounds are recalcitrant in nature, the use of advanced oxidation processes at carefully established optimum conditions can yield highly efficient degradation of the compounds.

Peroxymonosulfate/solar radiation process for the removal of aqueous microcontaminants. Kinetic modeling, influence of variables and matrix constituents

New technologies to address the presence of pharmaceutical and personal care products (PPCPs) in wastewater are needed, especially in those cases in which water will be reused. In this work, the activation of peroxymonosulfate (PMS) with simulated solar radiation has been applied to the oxidation of a mixture of six PPCPs, i.e. caffeine, primidone, N,N-diethyl-3-methylbenzamide (DEET), methylparaben, clofibric acid and ibuprofen. The sole application of solar radiation, i.e. solar photolysis, only led to the oxidation of clofibric acid (complete degradation in 90 min). The combination of PMS and solar radiation resulted in the degradation of all target micropollutants. The complete degradation of this mixture at initial 100 ppb was achieved with 0.5 mM of initial PMS after 90 min. A kinetic study that acceptably simulates the experimental data under different conditions has been proposed. The effects of initial PPCP concentration (1 mg L^-1-100 μg L^-1), PMS dose (0.1-5 mM), and pH (3-9) were tested and kinetically simulated. Finally, the PPCPs removal study was carried out in two real water matrices (river and a secondary effluent of an urban wastewater treatment plant). A higher dose of PMS, ten times higher, was required to achieve complete degradation of the micropollutants if compared to ultrapure water.

Selective removal of phenanthrene for the recovery of sodium dodecyl sulfate by UV-C and UV-C/PDS processes: Performance, mechanism and soil washing recycling

Soil washing is commonly used to remediate PAHs contaminated sites. However, the effluent after washing containing PAHs and surfactant may cause secondary pollution and remediation cost is still high, unless PAHs are selectively removed from the effluent and the surfactant is recovered and recycled. Herein, ultraviolet irradiation (254 nm, UV-C) and its combination with peroxydisulfate (UV-C/PDS) were applied to selectively degrade PHE in the synthetic soil washing effluent. At natural pH of 8.6, 98.2 % of PHE was removed within 30 min under 6 W UV-C irradiation. After adding 2 mM PDS, the time was shortened to 8 min but still achieving 98.7 % PHE removal and less toxic treated effluent than UV-C alone. The ¹O₂ was the main oxidizing species in UV-C alone system, while ¹O₂ as well OH and SO₄^- were responsible for PHE removal in the UV-C/PDS system. The possible intermediates of PHE degradation were recognized using liquid chromatography-mass spectrometry technique and the degradation pathways in both systems were proposed. Soil washing recycling experiments verified the recovered SDS could be reused directly without surfactant supplement and the soil washing efficiency changed insignificantly during three cycles. It indicates UV-C/PDS coupled with soil washing is a promising remediation technology.

A critical review on advanced oxidation processes for the removal of trace organic contaminants: A voyage from individual to integrated processes

Advanced oxidation processes (AOPs), such as photolysis, photocatalysis, ozonation, Fenton process, anodic oxidation, sonolysis, and wet air oxidation, have been investigated extensively for the removal of a wide range of trace organic contaminants (TrOCs). A standalone AOP may not achieve complete removal of a broad group of TrOCs. When combined, AOPs produce more hydroxyl radicals, thus performing better degradation of the TrOCs. A number of studies have reported significant improvement in TrOC degradation efficiency by using a combination of AOPs. This review briefly discusses the individual AOPs and their limitations towards the degradation of TrOCs containing different functional groups. It also classifies integrated AOPs and comprehensively explains their effectiveness for the degradation of a wide range of TrOCs. Integrated AOPs are categorized as UV irradiation based AOPs, ozonation/Fenton process-based AOPs, and electrochemical AOPs. Under appropriate conditions, combined AOPs not only initiate degradation but may also lead to complete mineralization. Various factors can affect the efficiency of integrated processes including water chemistry, the molecular structure of TrCOs, and ions co-occurring in water. For example, the presence of organic ions (e.g., humic acid and fulvic acid) and inorganic ions (e.g., halide, carbonate, and nitrate ions) in water can have a significant impact. In general, these ions either convert to high redox potential radicals upon collision with other reactive species and increase the reaction rates, or may act as radical scavengers and decrease the process efficiency.

Hydroxyl and sulfate radical-based oxidation of RhB dye in UV/H2O2 and UV/persulfate systems: Kinetics, mechanisms, and comparison

The degradation kinetics and mechanisms of Rhodamine B (RhB) dye by ^•OH and SO₄^•- based advanced oxidation processes were investigated. The ^•OH and SO₄^•- radicals were generated by UV photolysis of hydrogen peroxide and persulfate (i.e., UV/H₂O₂ and UV/PS), respectively. The effects of initial solution pH, RhB concentration, oxidant dosage, Fe²⁺ concentration, and water matrices were examined. The results showed that the degradation of RhB followed pseudo-first-order kinetics in both processes, with the UV/H₂O₂ process exhibiting better performance than that of the UV/PS process. Acidic conditions were favorable to the degradation of RhB in both systems. Increasing the oxidant dosage or decreasing the contaminant concentration could enhance the degradation of RhB. Photo-Fenton-like processes accelerated the performance when Fe²⁺ was added into both systems. The removal efficiency of RhB was inhibited upon the addition of humic substances. The addition of Cl^- displayed no significant effect and promoted RhB degradation in UV/H₂O₂ and UV/PS systems, respectively. The presence of NO₃^- promoted RhB degradation, while H₂PO₄^- and C₂O₄^2- showed an inhibitory effect on both UV/H₂O₂ and UV/PS processes. Radical scavenging tests revealed the dominant role of SO₄^•- radicals in the UV/PS system. Furthermore, the evolution of low molecular weight organic acids and NH₄⁺ during the degradation of RhB in these two processes were compared. Both UV/H₂O₂ and UV/PS systems led to similar formation trends of NH₄⁺ and some ring-opening products (e.g., formic acid, acetic acid, and oxalic acid), suggesting some analogies in the decay pathways of RhB by ^•OH and SO₄^•--induced oxidation processes.

Vacuum ultraviolet irradiation for mitigating dissolved organic nitrogen and formation of haloacetonitriles

The main objective of this work was to investigate the feasibility of using vacuum ultraviolet (VUV, 185 + 254 nm) and ultraviolet (UV, 254 nm) for the reduction of dissolved organic nitrogen (DON) and haloacetonitrile formation potential (HANFP) of surface water and treated effluent wastewater samples. The results showed that the reduction of dissolved organic carbon (DOC), DON, hydrophobicity (HPO), absorbance at 254 nm (UV₂₅₄), and fluorescence excitation-emission matrix (FEEM) of both water samples by VUV was higher compared to using UV. The addition of H₂O₂ remarkably improved the performances of VUV and UV. VUV/H₂O₂ exhibited the highest removal efficiency for DOC and DON. Even though HANFP increased at the early stage, its concentration decreased (19-72%) at the end of treatment (60 min). Decreases in DON (30-41%) and DOC (51-57%) led to HANFP reduction (53-72%). Moreover, FEEM revealed that substantial reduction in soluble microbial product-like compounds (nitrogen-rich organic) had a strong correlation with HANFP reduction, implying that this group of compounds act as a main precursor of HANs. The VUV/H₂O₂ system significantly reduced HANFP more than UV/H₂O₂ and therefore is suitable for controlling HAN precursors and HAN formation in drinking water and reclaimed wastewater.

Comparison of ozonation and UV based oxidation as pre-treatment process for ultrafiltration in wastewater reuse: Simultaneous water risks reduction and membrane fouling mitigation

Wastewater reuse risk and membrane fouling are two concerns in ultrafiltration (UF) of secondary effluent (SE) for wastewater reuse. In this work, several wastewater reuse risk issues, such as dissolved effluent organic matters (dEfOM), organic micro-pollutants (OMPs) and bio-toxicity of SE, as well as membrane fouling were comprehensively investigated when ozonation, UV/H₂O₂ and UV/persulfate (UV/PS) were used as the pre-treatments for UF process. To be specific, individual UF could remove DOC and UV₂₅₄ by only 7.5% and 19.8%, respectively, however, humics were largely degraded during the pre-oxidation processes revealed by molecular weight and fluorescence analysis. UF and ozonation showed limited removal of OMPs, however, UV/H₂O₂ and UV/PS dramatically degraded all the OMPs by more than 80%. Genotoxicity were not detected after the oxidation treatment. Membrane fouling may result from the collaborative effect of organic components, such as humic and protein like substances. Fourier transform infrared spectra of the fouled membranes showed that aromatic CC group and polysaccharides group in dEfOM were largely reduced after the oxidation pre-treatments, resulting in the improved membrane flux sustaining. Increased roughness of the membranes in the combined process supported that the less organics content after the oxidation pre-treatment contributed to improve the performance of the UF process. For the excellent organics degradation in UV/PS pre-treatment process, membrane fouling of subsequent UF process showed maximum mitigation.

Oxidative removal of benzotriazole using peroxymonosulfate/ozone/ultrasound: Synergy, optimization, degradation intermediates and utilizing for real wastewater

This study investigated the degradation efficiency of BTA using combination of ultrasound (US), peroxymonosulfate (PMS), and ozone. The effects of different operational parameters were investigated to optimize the process performance. The results showed that the highest removal efficiency was reached at neutral pH, ozone = 6.8 mg/L, PMS = 1.5 mM and US power = 200 W. Under these conditions, 40 mg/L of BTA was completely degraded within 60 min leading to the almost 85% of chemical oxygen demand removal, 75% of total organic carbon removal and 73.3% of organic nitrogen removal. Based on the scavenging tests, it was found that hydroxyl radical was the main oxidizing agent in the oxidation of BTA by PMS/ozone/US process. The inhibitive effect of anions on BTA removal was under this order NO₂^- > HCO₃^- > Cl^- > NO₃^- > SO₄^2-. Degradation intermediates of BTA were identified and oxidation pathway was proposed. Finally, real samples of saline water, petrochemical wastewater and secondary effluent matrices were investigated for the performance of PMS/ozone/US process and it was found that 54%, 72.3% and 94.6% BTA removal efficiency were reached, respectively. PMS/ozone/US process was compared to US/peroxone (ozone + H₂O₂) and the results showed importance of US irradiation in both systems. Accordingly, PMS/ozone/US process could be considered as an efficient and promising process for BTA degradation in various wastewaters.