Reevaluation of the contributions of reactive intermediates to the photochemical transformation of 17β-estradiol in sewage effluent

The reverse-reduction effect of dissolved organic matter on the degradation of micropollutants induced by halogen radicals (Cl2•- and Br2•-)

Reactive halogen radicals (e.g., Cl2•- and Br2•-) greatly impact the degradation of micropollutants in natural waters and engineered water treatment systems. The ubiquitous dissolved organic matter (DOM) in real waters is known to greatly inhibit the degradation of micropollutants by reducing micropollutant's intermediate (i.e., TC•+/TC(-H)•), however, such DOM's effects on the halogen-radical-induced system have not been understood yet. The present study focuses on investigating and quantifying such inhibitory effects of DOM during Cl2•-- and Br2•--mediated process. Guanosine (Gs) was selected as a model compound. The transient spectra show that Cl2•- and Br2•- react with Gs generating intermediates (i.e., Gs•+/Gs(-H)•) via single-electron transfer. In the presence of 1.0 mgCL-1 DOM, over 70% of this oxidized Gs was reduced back to Gs. Comparing the extent of reverse-reduction inhibitory among different reaction systems, this inhibitory in Br2•- system was slightly lower than that in Cl2•- and SO4•- system, corresponding the slightly difference of inhibition factor (IF) values as SO4•- < Cl2•- < Br2•-. The reverse-reduction effect of DOM was further quantified for 19 common micropollutants. It varied significantly with IF values of 0.21-1.26 and 0.28-1.40 in Cl2•-- and Br2•--mediated process, respectively. Purines and amines generally exhibited more pronounced inhibition than phenols in both systems. A good correlation of IF values with micropollutant's reduction potential was observed, which can be applied to predict the degradation of more unstudied micropollutants. This study highlights the important role of the reverse-reduction effect of DOM on micropollutant degradation. It can significantly improve the accuracy in predicting degradation rate in advanced oxidation processes for treating water containing halides.

Mechanistic Aspects of Photodegradation of Deoxynucleosides Induced by Triplet State of Effluent Organic Matter

Excited triplet states of wastewater effluent organic matter (3EfOM*) are known as important photo-oxidants in the degradation of extracellular antibiotic resistance genes (eArGs) in sunlit waters. In this work, we further found that 3EfOM* showed highly selective reactivity toward 2'-deoxyguanosine (dG) sites within eArGs in irradiated EfOM solutions at pH 7.0, while it showed no photosensitizing capacity toward 2'-deoxyadenosine, 2'-deoxythymidine, and 2'-deoxycytidine (the basic structures of eArGs). The 3EfOM* contributed to the photooxidation of dG primarily via one-electron transfer mechanism, with second-order reaction rate constants of (1.58-1.74) × 108 M-1 s-1, forming the oxidation intermediates of dG (dG(-H)•). The formed dG(-H)• could play a significant role in hole hopping and damage throughout eArGs. Using the four deoxynucleosides as probes, the upper limit for the reduction potential of 3EfOM* is estimated to be between 1.47 and 1.94 VNHE. Compared to EfOM, the role of the triplet state of terrestrially natural organic matter (3NOM*) in dG photooxidation was minor (∼15%) mainly due to the rapid reverse reactions of dG(-H)• by the antioxidant moieties of NOM. This study advances our understanding of the difference in the photosensitizing capacity and electron donating capacity between NOM and EfOM and the photodegradation mechanism of eArGs induced by 3EfOM*.

Organic peroxyl radicals from biacetyl accelerated the visible-light degradation of steroid estrogens in aqueous solution

Indirect photodegradation is an important pathway for the reduction of steroid estrogens in sunlit surface waters. Nevertheless, the kinetics and mechanisms governing the interaction between coexisting carbonyl compounds and estrogens under visible light (Vis) remain unexplored. This study systematically investigates the Vis-induced photodegradation of 17β-estradiol (E2) in the presence of five specific carbonyl compounds-biacetyl (BD), acetone, glyoxal, pyruvic acid, and benzoquinone. The results demonstrate that, among these compounds, only BD significantly enhanced the photodegradation of E2 under Vis irradiation (λ > 400 nm). The pseudo-first order photodegradation rate constants (k1) of E2 in the Vis/BD system were 0.025 min-1 and 0.076 min-1 in ultrapure water and river water, respectively. The enhancing effect of BD was found to be pH-dependent, increasing the pH from 3.0 to 11.0 resulted in a 76% reduction in the k1 value of E2 in the Vis/BD system. Furthermore, the presence of humic acid, NO3-, or HCO3- led to an increase of more than 35% in the k1 value of E2, while NO2- exerted a pronounced inhibitory effect, resulting in a 92% decrease. Peroxyacetyl and peroxymethyl radicals, derived from BD in a yield ratio of 9, played a crucial role in the degradation of E2. These peroxyl radicals primarily targeted electron-rich hydroxyl sites of E2, initiating hydroxylation and ring-opening reactions that culminated in the formation of acidic byproducts. Notably, toxicity evaluation indicates that these hydroxylated and acidic products exhibited lower toxicity than the parent compound E2. This study highlights the important role of peroxyl radicals in estrogen degradation within aquatic environment, and also helps to design efficient visible light-responsive photo-activators for the treatment of estrogen-contaminated waters.

Comparative study of the photooxidation of arsenite mediated by dissolved and mineral-associated humic acid under light irradiation

The photochemical processes of dissolved humic acid and its potential contribution to As(III) oxidation in natural water has received considerable attention. However, the role of mineral-humic complexes in As(III) conversion is rarely studied. Herein, two simulated mineral-humic complexes were prepared by coating humic acid on hydrous aluminum oxide, HA@HAO, or montmorillonite, HA@SWy, respectively, and batch experiments at circumneutral pH were performed under light irradiation. Our findings showed that the light-assisted oxidation of As(III) increased with increasing fractions of organic carbon in mineral-humic complexes, and As(III) photooxidation with HA@HAO or HA@SWy was up to 18.2 or 3.5-fold higher compared to that measured in the presence of equivalent amount of free HA, respectively. The reactive triplet state of HA and hydroxyl radicals in HA@HAO and HA@SWy system made a primary contribution to As(III) oxidation under irradiation. The results indicated that mineral-humic complexes have dual roles, an adsorbent and a photosensitizer, to promote As(III) access to reactive intermediates at the particle surfaces. This process was important for As(III) conversion in paddy water as colloidal particles, composed of both minerals and HA, could greatly promote As(III) oxidation and As(V) immobilization. This study provides a previously overlooked, important mechanism of As(III) phototransformation mediated by mineral-associated humic acid in natural environment.

Prediction of free radical reactions toward organic pollutants with easily accessible molecular descriptors

Machine learning (ML) is becoming an efficient tool for predicting the fate of aquatic contaminants owing to the preponderance of big data. However, whether ML can "learn" the differences in reactivity among different free radicals has not yet been tested. In this work, the effectiveness of combining ML algorithms with molecular fingerprints to predict the reactivity of three free radicals was evaluated. First, a dataset containing 211 organic pollutants and their respective rate constants with the carbonate radical (CO3•-) was used to develop predictive models using both linear regression and ML methods. The use of topological atomic alignment information, in the form of the molecular access system (MACCS) and Morgan Fingerprint, and the electronic structure features (energy levels of the lowest unoccupied and highest occupied molecular orbitals, ELUMO and EHOMO, and the energy gap between ELUMO and EHOMO) gave satisfactory predictive performances (ML model with Random Forest algorithm with MACCS: RMSEtest = 0.787; linear regression model with energy levels: RMSEtest = 0.641). Additionally, the model interpretation correctly described that the key reactivity features for CO3•- were relatively close to those for SO4•- rather than those for •OH. These results suggest that combination of ML algorithms with easily accessible molecular fingerprints would be a powerful tool to accurately predict the radical reactions towards organic compounds.

Effect of dissolved organic matter on sulfachloropyridazine photolysis in liquid water and ice

Dissolved organic matter (DOM) is an ubiquitous component of environmental snow and ice, which can absorb light and produce reactive species (RS) and thus is of importance in ice photochemistry. The photodegradation of sulfachloropyridazine (SCP) without and with DOM present in liquid water and ice were investigated in this study. The photodegradation rate constants for SCP without DOM present was enhanced by 52.5 % in ice relative to liquid water, likely due to the enhanced role of SCP self-sensitized RS in ice. DOM significantly promoted SCP photolysis in both liquid water and ice, which was mainly attributed to roles of singlet oxygen (1O2) and triplet excited-state DOM (3DOM*) generated from DOM. 1O2 production from DOM was significantly enhanced in ice relative to liquid water. Hydroxyl radical (•OH) production from DOM in ice was similar to those in liquid water. Enhancement in 3DOM* production in ice was observed at low DOM concentrations. Suwannee River Fulvic Acid (SRFA) and Elliott Soil Humic Acid (ESHA) exhibited differences in RS production in liquid water and ice, as well as in enhancement of 1O2 and 3DOM* produced in ice relative to liquid water. DOM induced reaction pathways of SCP different from those without DOM present, and therefore affected toxicity of SCP photoproducts. There were differences in photodegradation pathways of SCP as well as in toxicity of SCP photoproducts between liquid water and ice.

Influence of organic matter on the photocatalytic degradation of steroid hormones by TiO2-coated polyethersulfone microfiltration membrane

Water treatment in photocatalytic membrane reactors (PMR) holds great promise for removing micropollutants from aquatic environments. Organic matter (OM) that is present in any water matrix may significantly interfere with the degradation of steroid hormone (SH) micropollutants in PMRs. In this study, the interference of various OM types, humic acid (HA), Australian natural organic matter (AUS), worm farm extract (WF), tannic acid (TA), and gallic acid (GA) with the SH degradation at its environmentally relevant concentration (100 ng/L) in a flow-through PMR equipped with a polyethersulphone-titanium dioxide (PES-TiO2) membrane operated under UV light (365 nm) was investigated. Results of this study showed that OM effects are complex and depend on OM type and concentration. The removal of β-estradiol (E2) was enhanced by HA at its levels below 5 mgC/L while the enhancement was abated at higher HA concentrations. The E2 removal was inhibited by TA, and GA, while no significant interference observed for AUS, and WF. The data demonstrated diverse roles of OM that acts in PMRs as a light screening agent, a photoreactive species scavenger, an adsorption alteration trigger, and a photosensitizer. The time-resolved fluorescence measurement showed that HA, acting as a photosensitizer, promoted the sensitization of TiO2 by absorbing light energy and transferring energy/electron to the TiO2 substrate. This pathway dominated the mechanism of the enhanced E2 degradation by HA. The favorable effect of HA was augmented as increasing the light intensity from 0.5 to 10 mW/cm2 and was weakened at higher light intensities due to the increased scavenging reactions and the limited amount of HA. This work clarifies the underlying mechanism of the OM interference on photocatalytic degradation of E2 by the PES-TiO2 PMR.

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

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

Induced selectivity in the photochemistry of estrone derivatives in sustainable and micellar environment: preparative and mechanistic studies

In this study, we carried out preparative and mechanistic studies on the photochemical reaction of a series of 3-acylestrone derivatives in confined and sustainable micellar environment under steady-state conditions and the results were compared with those obtained in cyclohexane solution. The aim of this work is mainly focused to show whether the nature of the surfactant (cationic, neutral and anionic) leads to noticeable selectivity in the photoproduct formation. The 3-acylestrone derivatives underwent the photo-Fries rearrangement, with concomitant homolytic fragmentation of the ester group and [1;3]-acyl migration. This pathway afforded the ortho-acyl estrone derivatives, the main photoproducts together with estrone. However, epimerization of the ortho regioisomer 2-acetylestrone and estrone through Norrish Type I photoreaction occurred involving the fragmentation of the C-α at the carbonyl group (C-17) of the steroid. UV-visible and 2D-NMR (NOESY) spectroscopies have been employed to measure the binding constant K_b and the location of the steroids within the hydrophobic core of the micelle.

Accurate Removal of Trace 17β-Estradiol and Estrogenic Activity in Blended Systems under a Photoelectrocatalytic Circulating Flow

Trace 17β-estradiol (E2) is persistent against advanced treatment when blended with higher concentrations of low-toxicity organics, thus wasting energy. A circulating-flow selective photoelectrocatalysis (CF-SPEC) system is established with a selective E2-TiO₂-NR photoanode, accurately reducing 1 μg L^-1 E2 to less than 0.1 ng L^-1 along with eliminating estrogenic activity even when blended with natural organic matter (NOM) at a thousand times higher concentration. Such high efficiency is derived from the augmented selectivity and activity of E2-TiO₂-NRs toward E2 during CF-SPEC. Under a flow, the difference in adsorption capacity between NOM and E2 is further amplified 5.6-fold. Furthermore, the higher initial ^•OH concentration and faster mass transfer jointly endow CF-SPEC with a stronger oxidation capacity. As a result, the removal of E2 increases by 58.7%, and the elimination of estrogenic activity increases 5.8-fold. In addition, deeper mineralization and less homo- and heterocoupling under CF-SPEC are observed, leading to more thorough estrogenic activity removal. Although additional energy is needed to maintain the flow, there is a 55% decrease in energy consumption due to the accurate removal capacity. This work suggests a combination of flow degradation and surface engineering that can be expanded for the selective removal of toxic trace pollutants in blended systems.

Enhanced and selective phototransformation of chlorophene on aluminum hydroxide-humic complexes

Mineral-humic complexes, known as mineral-associated organic matter (MAOM), are ubiquitous in natural waters. However, the interaction between organic pollutants and MAOM remains elusive, which may affect their degradation process. In this study, photochemical transformation of chlorophene (CP) in the presence of MAOM, prepared by coating aluminum hydroxide with humic acid (HA-HAO), was investigated. Our results showed that the degradation of CP was significantly enhanced in the presence of HA-HAO, and the degradation rate constant was ~5 times as that with HA only. It was because the adsorption of CP to HA-HAO particles was greatly enhanced, and concentration of reactive oxygen species (ROS) was increased on HA-HAO surfaces, which further promoted the reactions between CP and ROS. The quenching experiments combined with EPR technology confirmed that superoxide anion (O₂^·-) was the primary reactive radical on CP photodegradation. More importantly, the degradation of CP with HA-HAO followed a hydroxylation process, rather than the oligomerization reaction with HA only. Spectroscopic analysis provided direct evidence for the formation of hydrogen bonding between CP phenolic hydroxyl group and surface oxygen of HAO, which would suppress the reactivity of phenolic hydroxyl group, consequently the ortho- and meta-positions of CP became more facile for the hydroxylation reaction. This study shows the importance of MAOM in altering the photochemical behavior and transformation pathway of organic contaminants.