Effects of Tryptophan and Tyrosine on the Transformation of Monophenols in Chromophoric Dissolved Organic Matter Solutions: Enhance the Forward Transformation and Reduce the Reverse Transformation.

Tryptophan (Trp) and tyrosine (Tyr) are the primary precursors of protein-like components in dissolved organic matter. Phenolic compounds are ubiquitous in aquatic environments and are considered the main electron donor in chromophoric dissolved organic matter (CDOM). Our results showed that Trp and Tyr (50 µM) enhanced the transformation of six monophenols (20 µM) with varying numbers of -CH3 and -OCH3 substituent groups by a factor of 1.0-1.8. The enhancement factor increased with the ratio of Trp (Tyr) to monophenols. In four different CDOM solutions (5 mg C/L, pH 8.0), a maximum enhancement factor of 3.2-6.7 was observed at a Trp/monophenol concentration ratio of 50. Conversely, monophenols greatly inhibited the transformation of Trp or Tyr. The enhancement factor decreased as the initial pH increased from 3.0 to 10.0. Additionally, the enhancement factor was not directly proportional to the oxidation potential of monophenol. We propose that the promotion effects are generated through the direct oxidation of monophenols by Trp (Tyr) radicals as well as through the reaction between Trp (Tyr) radicals and the one-electron reductant of CDOM.

Direct Phototransformation of Sulfamethoxazole Characterized by Four-Dimensional Element Compound Specific Isotope Analysis.

The antibiotic sulfamethoxazole (SMX) undergoes direct phototransformation by sunlight, constituting a notable dissipation process in the environment. SMX exists in both neutral and anionic forms, depending on the pH conditions. To discern the direct photodegradation of SMX at various pH levels and differentiate it from other transformation processes, we conducted phototransformation of SMX under simulated sunlight at pH 7 and 3, employing both transformation product (TP) and compound-specific stable isotope analyses. At pH 7, the primary TPs were sulfanilic acid and 3A5MI, followed by sulfanilamide and (5-methylisoxazol-3-yl)-sulfamate, whereas at pH 3, a photoisomer was the dominant product, followed by sulfanilic acid and 3A5MI. Isotope fractionation patterns revealed normal 13C, 34S, and inverse 15N isotope fractionation, which exhibited significant differences between pH 7 and 3. This indicates a pH-dependent transformation process in SMX direct phototransformation. The hydrogen isotopic composition of SMX remained stable during direct phototransformation at both pH levels. Moreover, there was no variation observed in 33S between the two pH levels, indicating that the 33S mass-independent process remains unaffected by changes in pH. The analysis of main TPs and single-element isotopic fractionation suggests varying combinations of bond cleavages at different pH values, resulting in distinct patterns of isotopic fractionation. Conversely, dual-element isotope values at different pH levels did not significantly differ, indicating cleavage of several bonds in parallel. Hence, prudent interpretation of dual-element isotope analysis in these systems is warranted. These findings highlight the potential of multielement compound-specific isotope analysis in characterizing pH-dependent direct phototransformation of SMX, thereby facilitating the evaluation of its natural attenuation through sunlight photolysis in the environment.

Photochemical Transformation of Dissolved Organic Matter in Surface Water Augmented the Formation of Disinfection Byproducts.

Sunlight may lead to changes in disinfection byproducts (DBPs) formation potentials of source water via transforming dissolved organic matter (DOM); however, the underlying mechanisms behind these changes remain unclear. This work systematically investigated the effect of photochemical transformation of DOM from reservoir water (DOMRe) and micropolluted river water (DOMRi) after 36 h of simulated sunlight irradiation (equivalent to one month under natural sunlight) on DBPs formation. Upon irradiation, high molecular weight (MW) and aromatic molecules tended to be mineralized or converted into low-MW and highly oxidized (O/C > 0.5) ones which might react with chlorine to generate high levels of DBPs, resulting in an elevation in the yields (µg DBP/mg C) of almost all the measured DBPs and the quantities of unknown DBPs in both DOM samples after chlorination. Additionally, DOMRi contained more aromatic molecules susceptible to photooxidation than DOMRe. Consequently, irradiated DOMRi exhibited a greater increase in the formation potentials of haloacetonitriles, halonitromethanes, and specific regulated DBPs, with nitrogenous DBPs being responsible for the overall rise in the calculated cytotoxicity following chlorination. This work emphasized the importance of a comprehensive removal of phototransformation products that may serve as DBPs precursors from source waters, especially from micropolluted source waters.

Products from Photolysis Reactions of Tetracycline Mediated by Clay and Humic Substance Induce Contrasting Expressions of Target Resistance Genes.

Phototransformation is a key process affecting the fate of many antibiotics in the environment, but little is known about whether their photoproducts exert selective pressure on bacteria by inducing antibiotic resistance genes (ARGs). Here, we examined the expression of tetracycline resistance gene tet(M) of a fluorescent Escherichia coli whole-cell bioreporter influenced by the phototransformation of tetracycline. The presence of suspended smectite clay (montmorillonite or hectorite, 1.75 g/L) or dissolved humic substance (Pahokee Peat humic acid or Pahokee peat fulvic acid, 10 mg C/L) in aqueous solutions markedly facilitated the transformation of tetracycline (initially at 400 µg/L) with half-life shortened by 1.4-2.6 times. Despite the similar phototransformation ratios (80-90%) of the total loaded tetracycline after 60 min irradiation, the decreased ratios of cell fluorescence intensity (which was proportional to the expression amount of ARG tet(M)) were much higher with the two clays (94 and 93%) than with the two humic substances (44 and 69%) when compared to the respective dark controls. As illustrated by mass spectroscopic and chemical analyses, tetracycline was proposed to be mainly transformed to amide (ineffective in inducing ARGs) with the presence of clays by reaction with self-photosensitized singlet oxygen (1O2), while the humic substances might catalyze the production of another two demethylated and/or deaminated compounds (still effective in inducing ARGs) in addition to the amide compound via reaction with triplet excited state dissolved organic matter (3DOM*). As clay minerals and humic substances are important soil constituents and ubiquitously present in surface environments, the observed clay and humic-dependent photooxidation pathways of tetracycline and the differing selective pressures of the associated products highlight the need for monitoring the transformation compounds of antibiotics and provide critical insight into the development of antibiotic treatment protocols.

Deciphering the Role of Microbial Extracellular and Intracellular Organic Matter in Antibiotic Photodissipation: Molecular and Fluorescent Profiling under Natural Radiation.

This study addresses existing gaps in understanding the specific involvement of dissolved organic matter (DOM) fractions in antibiotic photolysis, particularly under natural conditions and during DOM photobleaching. Employing fluorescent, chemical, and molecular analysis techniques, it explores the impact of extracellular and intracellular organic matter (EOM and IOM) on the photodissipation of multiclass antibiotics, coupled with DOM photobleaching under natural solar radiation. Key findings underscore the selective photobleaching of DOM fractions, propelled by distinct chemical profiles, influencing DOM-mediated antibiotic photolysis. Notably, lipid-like substances dominate in the IOM, while lignin-like substances prevail in the EOM, each uniquely responding to sunlight and exhibiting selective photobleaching. Sunlight primarily targets fulvic acid-like lignin components in EOM, contrasting the initial changes observed in tryptophan-like lipid substances in IOM. The lower photolability of EOM, attributed to its rich unsaturated compounds, contributes to an enhanced rate of indirect antibiotic photolysis (0.339-1.402 h-1) through reactive intermediates. Conversely, the abundance of aliphatic compounds in IOM, despite it being highly photolabile, exhibits a lower mediation of antibiotic photolysis (0.067-1.111 h-1). The triplet state excited 3DOM* plays a pivotal role in the phototransformation and toxicity decrease of antibiotics, highlighting microbial EOM's essential role as a natural aquatic photosensitizer for water self-purification. These findings enhance our understanding of DOM dynamics in aquatic systems, particularly in mitigating antibiotic risks, and introduce innovative strategies in environmental management and water treatment technologies.

Bromate-induced oxidation of carbamazepine and toxicity assessment of transformation products in the freezing-sunlight process: Effects of trivalent chromium.

Bromate ()-induced pharmaceutical and personal care products (PPCPs) oxidation is enhanced in freezing systems. Reduced forms of metals are widely present, often coexisting with various contaminants. However, their effects on the interaction of PPCPs with in ice in cold regions may have been overlooked. Herein we investigated the effects of representative reducing metal Cr(III) on the interaction between the representative PPCP carbamazepine (CBZ) and in the freezing system. Our findings demonstrated that the degradation rate constants of CBZ by and Cr(III) were 29.4%-60.3% lower than those by in ice, revealing the inhibition of Cr(III) on CBZ degradation by in ice. In /freezing/sunlight system, contributed 62.8% to CBZ degradation. In /Cr(III)/freezing/sunlight system, Cr(III) promoted the generation of hydroxyl radical (·OH), leading to 51% contribution of ·OH to CBZ degradation. Oxidants were consumed by Cr(III) to form Cr(VI) rather than reacting with CBZ, thereby decreasing CBZ degradation by in ice. Due to sunlight-induced Cr(VI) reduction in ice, only 0.3% of Cr(III) was converted to Cr(VI) in /Cr(III)/freezing/sunlight system. -induced CBZ degradation rate in ice decreased in order of Fe(II), Cr(III), and Mn(II), which was due to the different reducing capabilities. An effective reduction in comprehensive toxicity of systems followed the freezing-sunlight process, even in the presence of Cr(III). This work sheds new light on the environmental behaviors and fate of PPCPs, brominated disinfection by-products, and reducing metals during seasonal freezing.

Photo-transformation of ofloxacin in natural aquatic conditions: Impact of indirect photolysis on the product profile and transformation mechanism.

The natural phototransformation of organic pollutants in the environment depends on several water constituents, including inorganic ions, humic substances, and pH. However, the literature information concerning the influence of various water components on the amount of phototransformation and their impact on the development of various transformation products (TPs) is minimal. This study investigated the phototransformation of ofloxacin (OFL), a fluoroquinolone antibiotic, in the presence of various water components such as cations (K+, Na+, Ca2+, NH4+, Mg2+), anions (NO3-, SO42-, HCO3-, CO32-, PO43-), pH, and humic substances when exposed to natural sunlight. The study reveals that neutral pH levels (0.39374 min⁻1) enhance the phototransformation of OFL in aquatic environments. Carbonate, among anions, shows the highest rate constant (2.89966 min⁻1), significantly influencing OFL phototransformation, while all anions exhibit a notable impact. In aquatic environments, indirect phototransformation of OFL, driven by increased reactive oxygen species, expedites light-induced reactions, potentially enhancing OFL phototransformation. A clear difference was visible in the type of transformation products (TPs) formed during direct and indirect photolysis. The impact of indirect photolysis in the product profile was evaluated by examining the unique properties of TPs in direct and indirect photolysis. The primary transformation products were generated by oxidation and cleavage processes directed towards the ofloxacin piperazinyl, oxazine, and carboxyl groups. The toxicity assessment of TPs derived from OFL revealed that among the 26 identified TPs, TP3 (demethylated product), TP7 and TP8 (decarboxylated products), and TP15 (piperazine ring cleaved product) could potentially have some toxicological effects. These findings suggest that the phototransformation of OFL in the presence of various water components is necessary when assessing this antibiotic's environmental fate.

Photo-transformation of graphene oxide in synthetic and natural waters.

Graphene oxide (GO) is widely employed due to its outstanding properties, leading to an increasing release into the environment and natural waters. Although some studies have reported on the photo-transformation of GO, its behavior in complex natural waters remains inadequately explored. This study demonstrates that different types of ions may promote the photoreduction of GO in the order of Ca2+ > K+ > NO3- > Na+ by interacting with the functional groups on the surface of GO, and the photoreduction is enhanced with increasing ion concentrations. Additionally, natural organic matter (NOM) can inhibit the photoreduction of GO by scavenging reactive oxygen species. However, with increasing NOM concentrations (≥ 5 mgC/L), more NOM adsorb onto the surface of GO through hydrogen bonding, Lewis acid-base interactions, and π-π interactions, thereby enhancing the photoreduction of GO. On this basis, our results further indicate that the combined effects of different ions, such as Ca2+, Mg2+, NOM, and other complex hydrochemical conditions in different natural waters can promote the photoreduction of GO, resulting in a reduction in oxygen functional groups and the formation of defects. This study provides a theoretical basis for assessing the long-term transformation and fate of GO in natural waters.

Photochemical processes transform dissolved organic matter differently depending on its initial composition.

Dissolved organic matter (DOM) is one of the most important fluxes in the global carbon cycle but its response to light exposure remains unclear at a molecular-level. The chemical response of DOM to light should vary with its molecular composition and environmental conditions while some basic hypotheses are still unclear, such as the balance between photobleaching and photo-humification and the question of oxidative properties. Here we exposed aquatic DOM from diverse freshwaters impacted by different levels of anthropogenic activity and algal exudates to environmentally-realistic light conditions. We found that photobleaching occurred in DOM with relatively high initial humic content producing low H/C molecules, whereas DOM with low initial humic content was humified. DOM pools with relatively high initial saturation and low aromaticity were prone to transform towards more unsaturated molecular formulae and high H/C molecules with a distinct decrease of bioavailability. Photo-transformation was mainly influenced by reactive intermediates, with reactive oxygen species (ROS) playing a dominant role in humification when the initial humus content of DOM was high. In contrast, for algal DOM with high protein content, it was likely that the autoxidation of excited state DOM was more important than indirect oxidation involving ROS. Our results reveal how photo-transformation patterns depend on the initial composition of DOM and provide new insights into the role of photochemical processes in biogeochemical cycling of DOM.

Aqueous fate of furaltadone: Kinetics, high-resolution mass spectrometry - based elucidation and toxicity assessment of photoproducts.

Furaltadone (FTD) is an antibiotic belonging to the nitrofurans group. It has been broadly used in livestock and aquaculture for therapeutic purposes, as well as for stimulating promotion. Although the European Union has imposed restrictions on the use of FTD since 1995 due to concerns regarding its toxicity, in many cases FTD has been excessively and/or illegally applied in productive animals in developing countries, because of its high efficacy and low-cost. Unlike other nitrofuran compounds, the hydrolytic and photolytic behavior of FTD in natural aquatic systems has not been thoroughly investigated. To this end, hydrolysis in different pH values and photolysis in aquatic environment, including lake, river and sea water have been both examined. Hydrolysis was found to have an insignificant impact on degradation of FTD in the aquatic environment relevant pH values, whereas indirect photolysis proved to be the main route of its elimination. The identification of tentative photoproducts (PPs) was performed using ultra high performance liquid chromatography coupled to hybrid LTQ/Orbitrap high resolution mass spectrometry. A possible pathway for photolytic transformation of FTD was proposed. Additionally, in silico simulations were used to evaluate the toxicity such as the mutagenicity of FTD and PPs. Complementary to the low-cost and time-limited simulations, an in vitro method (Vibrio Fischeri bioluminescence) was also used to assess ecotoxicity.

UV-induced reactive species dynamics and product formation by chlorite.

Chlorite (ClO2-) is a regulated byproduct of chlorine dioxide water treatment processes. The transformation of chlorite under UV irradiation into chloride (Cl-) and chlorate (ClO3-) involves reactive species chain reactions that could enhance chlorine dioxide water treatment efficiency while reducing residual chlorite levels. This study conducted a mechanistic investigation of chlorite phototransformation by analyzing reaction intermediates and stable end products, including chlorine dioxide (ClO2), free chlorine (HOCl/OCl-), hydroxyl­radical (•OH), Cl-, and ClO3- through combined experimental and modeling approaches. Experiments were performed at UV254 irradiation in pure buffered water within the pH range of 6 to 8. Results indicated that the apparent quantum yields for chlorite phototransformation increased from 0.86 to 1.45, and steady-state •OH concentrations at 1 mM initial chlorite concentration rose from 8.16 × 10-14 M - 16.1 × 10-14 M with decreasing pH values. It was observed that under UV irradiation, chlorite acts as both a significant producer and consumer of reactive species through three distinct reaction pathways. The developed kinetic model, which incorporates optimized intrinsic chlorite quantum yields Φchloritein ranging from 0.33 to 0.39, effectively simulated the loss of oxidants and the formation of major products. It also accurately predicted steady-state concentrations of various species, including •OH, •ClO, Cl• and O3. For the first time, this study provides a comprehensive transformation pathway scheme for chlorite phototransformation. The findings offer important insights into the mechanistic aspects of product and oxidizing species formation during chlorite phototransformation.

Disclosing phototransformation mechanisms of decabromodiphenyl ether (BDE-209) in different media by simulated sunlight: Implication by compound-specific stable isotope analysis.

As one of the typical brominated flame retardants, decabromodiphenyl ether (BDE-209) has been widely detected in environment. However, scarce information was available on BDE-209 phototransformation mechanisms in various media. In this study, compound-specific stable isotope analysis was first applied to investigate BDE-209 phototransformation in n-hexane, MeOH:H2O (v:v, 8:2), and simulated seawater by simulated sunlight. BDE-209 transformation followed pseudo-first-order kinetic, with degradation rate in the following of n-hexane (2.66 × 10-3 min-1) > simulated seawater (1.83 × 10-3 min-1) > MeOH:H2O (1.41 × 10-3 min-1). Pronounced carbon isotope fractionation was first observed for BDE-209 phototransformation, with carbon isotope enrichment factors (εC) of -1.01 ± 0.14‰, -1.77 ± 0.26‰, -2.94 ± 0.38‰ in n-hexane, MeOH:H2O and simulated seawater, respectively. Combination analysis of products and stable carbon isotope, debromination with cleavage of C-Br bonds as rate-limiting step was the main mechanism for BDE-209 phototransformation in n-hexane, debromination and hydroxylation with cleavage of C-Br bonds as rate-limiting steps in MeOH:H2O, and debromination, hydroxylation and chlorination in simulated seawater. This present study confirmed that stable carbon isotope analysis was a robust method to discovery the underlying phototransformation mechanisms of BDE-209 in various solutions.

Monitoring iohexol and its transformation products as evidence of reclaimed water irrigation input to contiguous waterbodies.

Irrigation with reclaimed water alleviates water supply shortages, but excess application often results in impairment of contiguous waterbodies. This project investigated the potential use of iohexol, an iodinated contrast media used in medical imaging, together with its bio- and phototransformation products as unique reconnaissance markers of reclaimed water irrigation intrusion at three golf courses within the state of Florida. Inter-facility iohexol concentrations measured in reclaimed waters ranged over ~2 orders of magnitude while observed intra-facility seasonal differences were ≤1 order of magnitude. A ~50 % reduction in iohexol was observed post-disinfection for reclaimed water facilities utilizing UV light while none was observed with use of chlorine. Iohexol biotransformation products were observed to decline or shift to lower molecular weight compounds when exposed to UV light but not during disinfection using chlorine. Iohexol biotransformation products were observed in most of the samples but were more prevalent in samples collected during the dry season. Much fewer iohexol phototransformation products were observed in chlorinated reclaimed water, and they were only observed in UV light irradiated reclaimed water when the pre-disinfectant iohexol concentration was ≥5000 ng/L or from solar exposure of reclaimed water spiked with 10 µM of iohexol. For the Hillsborough golf course overlaying an aquifer, the groundwater did not contain iohexol or phototransformation products but did contain biotransformation products. It is not known if these biotransformation products are from active or historical intrusion. The additional presence of sucralose in the aquifer suggests that intrusion has occurred within the past 3 years. This study demonstrates three crucial points in attempting to utilize iohexol to denote reclaimed water intrusion from irrigation overapplication: (1) interpretable results are obtained when iohexol concentrations in the reclaimed water employed for irrigation are ≥1000 ng/L, with higher concentrations in the range of ≥5000 ng/L better able to meet analytical sensitivity requirements after further dilution or degradation in the environment; (2) it is beneficial to assess iohexol transformation products in tandem with iohexol monitoring to account for environmental transformations of iohexol during storage and transport to the receiving water of concern; and (3) inclusion of monitoring for sucralose, an artificial sweetener ubiquitous in wastewater sources that is comparatively stable in the environment, can aid in interpretating whether reclaimed water intrusion based on identification of iohexol and transformation products in the receiving water is attributable to historic or ongoing irrigation overapplications.

Insights into sunlight-driven transformation of tetracycline by iron (hydr)oxides: The dominating role of self-generated hydrogen peroxide.

Iron (hydr)oxides are abundant in surface environment, and actively participate in the transformation of organic pollutants due to their large specific surface areas and redox activity. This work investigated the transformation of tetracycline (TC) in the presence of three common iron (hydr)oxides, hematite (Hem), goethite (Goe), and ferrihydrite (Fh), under simulated sunlight irradiation. These iron (hydr)oxides exhibited photoactivity and facilitated the transformation of TC with the initial phototransformation rates decreasing in the order of: Hem > Fh > Goe. The linear correlation between TC removal efficiency and the yield of HO• suggests that HO• dominated TC transformation. The HO• was produced by UV-induced decomposition of self-generated H2O2 and surface Fe2+-triggered photo-Fenton reaction. The experimental results indicate that the generation of HO• was controlled by H2O2, while surface Fe2+ was in excess. Sunlight-driven H2O2 production in the presence of the highly crystalline Hem and Goe occurred through a one-step two-electron reduction pathway, while the process was contributed by both O2-induced Fe2+ oxidation and direct reduction of O2 by electrons on the conduction band in the presence of the poorly crystalline Fh. These findings demonstrate that sunlight may significantly accelerate the degradation of organic pollutants in the presence of iron (hydr)oxides.