Micropollutants as internal probe compounds to assess UV fluence and hydroxyl radical exposure in UV/H2O2 treatment

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.

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.

Characterization of UV/chlorine process for micropollutant abatement by probe compound-based kinetic models

Micropollutant (MP) abatement efficiencies are critical information for optimizing water treatment process for cost-effective operations. Nevertheless, due to the vast number of MPs in real water matrices, it is infeasible to measure their abatement efficiencies individually in practical applications. In this study, a probe compound-based kinetic model was developed for generalized prediction of MP abatement in various water matrices by the ultraviolet (UV)/chlorine process. The results show that by measuring the depletion of three probe compounds (ibuprofen, primidone, and dimetridazole) spiked in the water matrix, the exposures of main reactive chlorine species (RCS including chlorine radicals (Cl•), dichloride radicals (Cl₂^-•) and chlorine oxide radicals (ClO•)) and hydroxyl radicals (•OH) during the UV/chlorine process could be calculated using the model. Based on the determined exposures, the abatement efficiencies of various MPs in different water matrices (e.g., surface water, groundwater, and wastewater) could generally be predicted with acceptable accuracy by the model without prior water-specific calibration. In addition, the relative contribution of UV photolysis and oxidation with active chlorine, RCS, and •OH to MP abatement could be quantitatively simulated using the model to clarify the abatement mechanism of MPs during the UV/chlorine process. The probe-based kinetic model can thus offer a useful tool to guide practical water and wastewater treatment for MP abatement and to explore the mechanism of UV/chlorine process.

Advanced oxidation processes for removal of monocyclic aromatic hydrocarbon from water: Effects of O3/H2O2 and UV/H2O2 treatment on product formation and biological post-treatment

Several oxidative treatment technologies, such as ozonation or Fenton reaction, have been studied and applied to remove monocyclic hydroaromatic carbon from water. Despite decades of application, little seems to be known about formation of transformation products while employing different ozone- or ^∙OH-based treatment methods and their fate in biodegradation. In this study, we demonstrate that O₃/H₂O₂ treatment of benzene, toluene, ethylbenzene (BTE), and benzoic acid (BA) leads to less hydroxylated aromatic transformation products compared to UV/H₂O₂ as reference system - this at a similar ^∙OH exposure and parent compound removal efficiency. Aerobic biodegradation tests after oxidation of 0.15 mM BA (12.6 mg C L^-1 theoretical DOC) revealed that a less biodegradable DOC fraction > 4 mg C L^-1 was formed in both oxidative treatments compared to the BA control. No advantage of ozonation over UV/H₂O₂ treatment was observed in terms of mineralization capabilities, however, we detected less transformation products after oxidation and biodegradation using high-resolution mass spectrometry. Biodegradation of BA that was not oxidized was more complete with minimal organic residual. Overall, the study provides new insights into the oxidation of monocyclic aromatics and raises questions regarding the biodegradability of oxidation products, which is relevant for several treatment applications.

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.

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.

Degradation of iopamidol by UV365/NaClO: Roles of reactive species, degradation mechanism, and toxicology

The degradation of iopamidol (IPM) was investigated using a UV₃₆₅/NaClO system. The reactive species (HO·, ClO·, ozone, Cl·, and Cl₂^-·) in the system were identified, and the changing trends of the percentage contributions of these reactive species to IPM removal under various conditions were systematically evaluated. The results showed that ClO· and HO· played the most significant roles in the apparent pseudo-first-order rate constants of IPM degradation (k_obs, min^-1) in the control experiment, and their percentage contributions to k_obs were 41.31% and 34.45%, respectively. In addition, Cl· and Cl₂^-· together contributed 22% to the k_obs. Furthermore, the contribution of ozone to the IPM removal could be neglected. The concentrations of these species increased significantly when the concentration of NaClO was increased from 50 µM to 200 µM, while the percentage contribution of ClO· to k_obs was greatly increased. The concentrations and percentage contributions of HO· and ClO· decreased significantly as the solution pH increased from 5 to 9, with Cl₂^-· playing a greater role in the degradation of IPM under alkaline conditions. While Cl^- or HCO₃^-/CO₃^2- significantly promoted the generation of Cl₂^-· or CO₃^-·, neither had an obvious effect on k_obs, suggesting that Cl₂^-· and CO₃^-· should have a certain reactivity with IPM. Compared with that of Cl₂^-·, the percentage contribution of ClO· and Cl· to k_obs was more likely to be inhibited by NOM. In addition, the organic and inorganic oxidation products of IPM were detected. The oxidation mechanisms of IPM degradation in the UV₃₆₅/NaClO system, such as the H-extraction reaction, deiodination, substitution reaction, amide hydrolysis, and amine oxidation, were proposed according to the obtained 15 organic products. No effect on acute toxicity towards Vibrio fischeri and Photobacterium phosphoreum was detected during the oxidation of IPM by the UV₃₆₅/NaClO system. Furthermore, the engineering feasibility of the oxidation system was demonstrated, by the effective degradation of IPM in actual water. However, HOI rapidly accumulated during the removal of IPM in the UV₃₆₅/NaClO system, which poses certain environmental risks and will needs to be investigated.

Micropollutant degradation by UV/H2O2 in drinking water: Facilitated prediction through combination of model simulation and portable measurement

Ultraviolet-based advanced oxidation processes (UV-AOPs) are highly effective for micropollutant degradation. However, it is considerably time and labor consuming to evaluate the practical performance of UV-AOPs. This study developed a novel method, through combination of model simulation with portable measurement (MS-PM), to facilitate prediction of the photon fluence-based rate constant of micropollutant degradation (k'_p,MP) by UV/H₂O₂, a commercially available UV-AOP. Model simulation was performed with photochemical, hydroxyl radical (HO^•) concentration steady-state approximation, and quantitative structure-activity relationship models; and portable measurement was conducted on a mini-fluidic photoreaction system to quantify the HO^• scavenging capacity (HRSC) of a water matrix. The method was established and further verified experimentally in seven test waters by taking sulfamethazine (SMN) as a model micropollutant. A lower k'_p,SMN was predicted in a water matrix with a higher HRSC, for example, 57.5 and 347.8 m² einstein^-1 in the raw water (HRSC = 5.91 × 10⁵ s^-1) and sand-filtered effluent (HRSC = 5.25 × 10⁴ s^-1) of a drinking water treatment plant at an H₂O₂ dose of 25 mg L^-1, respectively. The predicted values agreed generally well with the experimental ones. The MS-PM method has advantages of high efficiency and convenience, low cost, and acceptable accuracy, which will significantly facilitate the design and field assessment of UV-AOPs for micropollutant removal from drinking water.

A novel diagnostic method for distinguishing between Fe(IV) and •OH by using atrazine as a probe: Clarifying the nature of reactive intermediates formed by nitrilotriacetic acid assisted Fenton-like reaction

In this work, we found that the distribution of two specific atrazine (ATZ) oxidation products (desethyl-atrazine (DEA) and desisopropyl-atrazine (DIA)) was different in oxidation processes involving aqueous ferryl ion (Fe(IV)) species and ^•OH. Specifically, the molar ratio of produced DEA to DIA (i.e., [DEA]/[DIA]) increased from 7.5 to 13 with increasing pH from 3 to 6 when ATZ was oxidized by Fe(IV), while the treatment of ATZ by ^•OH led to the [DEA]/[DIA] value of 2 which was independent of pH. Moreover, ATZ showed high reactivity towards Fe(IV) over a wide pH range, especially at near-neutral pH, at which ATZ oxidation in Fe(II)/peroxydisulfate system was even much faster than another well-defined Fe(IV) scavenger, the sulfoxides. By using this approach, it was obtained that the [DEA]/[DIA] value remained at 2 during ATZ transformation by the nitrilotriacetic acid (NTA) assisted Fenton-like (Fe(III)/H₂O₂) system, which was independent of solution pH and reactants dosage. This result clarified that ^•OH was the primary reactive intermediate formed in the NTA assisted Fe(III)/H₂O₂ system. This study not only developed a novel sensitive diagnostic tool for distinguishing Fe(IV) from ^•OH, but also provided more credible evidence to the nature of reactive intermediate in a commonly controversial system.