Photoinduced disinfection in sunlit natural waters: Measurement of the second order inactivation rate constants between E. coli and photogenerated transient species

Reinvestigation on the Mechanism for Algae Inactivation by the Ultraviolet/Peracetic Acid Process: Role of Reactive Species and Performance in Natural Water

This study provided an in-depth understanding of enhanced algae inactivation by combining ultraviolet and peracetic acid (UV/PAA) and selecting Microcystis aeruginosa as the target algae species. The electron paramagnetic resonance (EPR) tests and scavenging experiments provided direct evidence on the formed reactive species (RSs) and indicated the dominant role of RSs including singlet oxygen (1O2) and hydroxyl (HO•) and organic (RO•) radicals in algae inactivation. Based on the algae inactivation kinetic model and the determined steady-state concentration of RSs, the contribution of RSs was quantitatively assessed with the second-order rate constants for the inactivation of algae by HO•, RO•, and 1O2 of 2.67 × 109, 3.44 × 1010, and 1.72 × 109 M-1 s-1, respectively. Afterward, the coexisting bi/carbonate, acting as a shuttle, that promotes the transformation from HO• to RO• was evidenced to account for the better performance of the UV/PAA system in algae inactivation under the natural water background. Subsequently, along with the evaluation of the UV/PAA preoxidation to modify coagulation-sedimentation, the possible application of the UV/PAA process for algae removal was advanced.

Autochthonous DOM had solar disinfection effect but nitrate counteracted with them

Pathogens in natural water can pose great threat to public health and challenge water quality. In sunlit surface water, dissolved organic matters (DOMs) can inactivate pathogens due to their photochemical activity. However, the photoreactivity of autochthonous DOM derived from different source and their interaction with nitrate on photo-inactivation remained limited understood. In this study, the composition and photoreactivity of DOM extracted from Microcystis (ADOM), submerged aquatic plant (PDOM) and river water (RDOM) were studied. Results revealed that lignin and tannin-like polyphenols and polymeric aromatic compounds negatively correlated with quantum yield of ³DOM*, whilst lignin like molecules positively correlated with •OH generation. ADOM had highest photoinactivation efficiency of E. coli, followed by RDOM and PDOM. Both the photogenerated •OH and low energy ³DOM* could inactivate bacteria damaging cell membrane and causing increase of intracellular reactive species. PDOM with more phenolic or polyphenols compounds not only weaken its photoreactivity, also increase regrowth potential of bacteria after photodisinfection. The presence of nitrate counteracted with autochthonous DOMs on photogeneration of •OH and photodisinfection activity, as well as increased the reactivation rate of PDOM and ADOM, which might be attributed to the increase of survival bacteria and more bioavailable fractions provided in systems.

Triplet Photochemistry of Effluent Organic Matter in Degradation of Extracellular Antibiotic Resistance Genes

Wastewater effluent is a major source of extracellular antibiotic resistance genes (eArGs) in the aquatic environment, a threat to human health and biosecurity. However, little is known about the extent to which organic matter in the wastewater effluent (EfOM) might contribute to photosensitized oxidation of eArGs. Triplet states of EfOM were found to dominate the degradation of eArGs (accounting for up to 85%). Photo-oxidation proceeded mainly via proton-coupled electron transfer reactions. They broke plasmid strands and damaged bases. O₂^•- was also involved, and it coupled with the reactions' intermediate radicals of eArGs. The second-order reaction rates of bla_TEM-1 and tet-A segments (209-216 bps) with the triplet state of 4-carboxybenzophenone were calculated to be (2.61-2.75) × 10⁸ M^-1 s^-1. Besides as photosensitizers, the antioxidant moieties in EfOM also acted as quenchers to revert intermediate radicals back to their original forms, reducing the rate of photodegradation. However, the terrestrial origin natural organic matter was unable to photosensitize because it formed less triplets, especially high-energy triplets, so its inhibitory effects predominated. This study advances our understanding of the role of EfOM in the photo-oxidation of eArGs and the difference between EfOM and terrestrial-origin natural organic matter.

Dark repair of sunlight-inactivated tetracycline-resistant bacteria: Mechanisms and important role of bacteria in viable but non-culturable state

The spread of antibiotic resistant bacteria (ARB) in the environment poses a potential threat to human health, and the reactivation of inactivated ARB accelerated the spread of ARB. However, little is known about the reactivation of sunlight-inactivated ARB in natural waters. In this study, the reactivation of sunlight-inactivated ARB in dark conditions was investigated with tetracycline-resistant E. coli (Tc-AR E. coli) as a representative. Results showed that sunlight-inactivated Tc-AR E. coli underwent dark repair to regain tetracycline resistance with dark repair ratios increasing from (0.124 ± 0.012)‱ within 24 h dark treatment to (0.891 ± 0.033)‱ within 48 h. The presence of Suwannee River fulvic acid (SRFA) promoted the reactivation of sunlight-inactivated Tc-AR E. coli and tetracycline inhibited their reactivation. The reactivation of sunlight-inactivated Tc-AR E. coli is mainly attributed to the repair of the tetracycline-specific efflux pump in the cell membrane. Tc-AR E. coli in a viable but non-culturable (VBNC) state was observed and dominated the reactivation as the inactivated ARB remain present in the dark for more than 20 h. These results explained the reason for distribution difference of Tc-ARB at different depths in natural waters, which are of great significance for understanding the environmental behavior of ARB.

Enhanced solar inactivation of fungal spores by addition of low-dose chlorine: Efficiency and mechanism

This work demonstrated that the solar inactivation of fungal spores was enhanced by addition of low-dose chlorine. Although the effect of low-dose chlorine alone (2.0 mg/L) on culturability of fungal spores was negligible, the solar/chlorine inactivation on fungal spores performed better than solar alone inactivation, with a lower shoulder length and a higher maximum inactivation rate constant. The enhanced inactivation of Aspergillus niger can be ascribed to the membrane oxidation by chlorine, and the enhanced inactivation of Penicillium polonicum can be ascribed to the membrane oxidation by chlorine and ·OH (·OH plays a major role). The oxidization by chlorine and ·OH led to an increase in membrane permeability of fungal spores, which enhanced the solar inactivation, resulting in an increase in intracellular ROS and more serious morphological damage. Due to the presence of background substances such as dissolved organic matter and metal ions (Fe²⁺, Mn²⁺, etc.), the inactivation efficiency in real water matrices was decreased. The main disinfection by-products (DBPs) produced in the inactivation of fungal spores in chlorine alone and solar/chlorine treatments were dichloroacetic acid, trichloroacetic acid, trichloroacetone and trichloromethane. Generally, DBPs formation in solar/chlorine treatment was lower than those in chlorine alone treatment. Moreover, the regrowth potential of the two genera of fungal spores in R2A medium could be inhibited by adding low-dose chlorine.

A model to predict the kinetics of direct (endogenous) virus inactivation by sunlight at different latitudes and seasons, based on the equivalent monochromatic wavelength approach

Sunlight plays an important role in the inactivation of pathogenic microorganisms such as bacteria and viruses in water. Here we present a model that is able to predict the kinetics of direct virus inactivation (i.e. inactivation triggered by sunlight absorption by the virion, without the role played by photochemically produced reactive intermediates generated by water-dissolved photosensitizers) on a global scale (from 60 °S to 60 °N latitude) and for the different months of the year. The model is based on the equivalent monochromatic wavelength (EMW) approach that was introduced recently, and which largely simplifies complex polychromatic calculations by approximating them with a monochromatic equation at the proper wavelength, the EMW. The EMW equation was initially established for mid-July conditions at a mid-latitude, and was then extended to different seasons and to the latitude belt where the day-night cycle is always observed throughout the year. By so doing, the first-order rate constant of direct virus photoinactivation can be predicted on a global scale, with the use of a relatively simple equation plus tables of pre-calculated input data, as a function of latitude, month, and key water parameters. The model was here applied to the virus organism phiX174, a somatic phage that is often used as proxy for pathogenic viruses undergoing fast direct inactivation, and for which a wide array of published inactivation data is available. Model predictions are validated by comparison with field data of inactivation of somatic phages by sunlight.

An innovative, highly stable Ag/ZIF-67@GO nanocomposite with exceptional peroxymonosulfate (PMS) activation efficacy, for the destruction of chemical and microbiological contaminants under visible light

In this work, Ag nanoparticles were loaded on ZIF-67 covered by graphene oxide (Ag/ZIF-67@GO), and its catalytic performance was studied for the heterogeneous activation of peroxymonosulfate (PMS) under visible-light. The catalyst surface morphology and structure were analyzed by FT-IR, XRD, XPS, DRS, FE-SEM, EDX, TEM, BET, ICP-AES and TGA analysis. The efficacy of PMS activation by the Ag/ZIF-67@GO under visible light was assessed by phenol degradation and E. coli inactivation. Phenol was completely degraded within 30 min by HO^•, SO₄^•- and O₂^•- generated through the photocatalytic PMS activation. In addition, total E. coli inactivation was attained in 15 min that confirmed the highly efficient catalytic activation of PMS by the as-made nanocomposite under visible light. The reaction mechanism was elucidated and the importance of the generated reactive species followed the order of: HO^• > SO₄^•- > O₂^•- > h⁺, implying a radical-pathway dominated process.

Treatment of wastewater effluents from Bogotá - Colombia by the photo-electro-Fenton process: Elimination of bacteria and pharmaceutical

In this work, the occurrences of bacteria families and relevant pharmaceuticals in municipal wastewater effluents from Bogotá (Colombia), and their treatment by the photo-electro-Fenton process were studied. Twenty-five representative pharmaceuticals (azithromycin, carbamazepine, ciprofloxacin, clarithromycin, diclofenac, enalapril, gabapentin, iopromide, metoprolol, sulfamethoxazole, trimethoprim, valsartan, clindamycin, erythromycin, levamisole, lincomycin, norfloxacin, oxolinic acid, phenazone, primidone, salbutamol, sulfadiazine, tetracycline, tramadol, and venlafaxine) were quantified in the effluent by LC-MS/MS analysis. Four of these target compounds (azithromycin, diclofenac, trimethoprim, norfloxacin) were found at concentrations that represent an environmental risk. In addition, several bacteria families related to water and foodborne diseases were identified in such effluents (e.g., Pseudomonadaceae, Campylobacteraceae, Aeromonadaceae, Enterobacteriaceae, and Bacteroidaceae), via shotgun-metagenomic technique. Then, a bench-scale photo-electro-Fenton (PEF) system equipped with a DSA anode (Ti/IrO₂-SnO₂) and a GDE cathode was applied to treat such effluents. After 60 min, this treatment led to a decrease in the ratio of the bacterial content in the original samples, ~150 thousand times, and a pondered removal of 66.12% for the pharmaceuticals. The study of the process pathways indicated that the bacteria and pharmaceuticals elimination mainly occurred through attacks of hydroxyl and chlorine radicals. Interestingly, in the case of pharmaceuticals, their environmental risk quotients were diminished after the PEF application. Furthermore, the prolonged action of this electrochemical process induced ~15% of mineralization and a significant reduction of the total DNA (removal >85%). Hence, the photo-electro-Fenton process showed to be a promising alternative to deal with municipal effluents for limiting the waterborne diseases, pollution by pharmaceuticals, and mobility/availability of genetic material coming from microorganisms.

Solid peroxides in Fenton-like reactions at near neutral pHs: Superior performance of MgO2 on the accelerated reduction of ferric species

Fenton-like reactions at near neutral pHs are limited by the slow reduction of ferric species. Enhancing generation of from solid peroxides is a promising strategy to accelerate the rate-limiting step. Herein, the H₂O₂ release and Fenton-like reactions of four solid peroxides, MgO₂, CaO₂, ZnO₂ and urea hydrogen peroxide (UHP), were investigated. Results indicated that UHP can release H₂O₂ instantly and show a similar behavior as H₂O₂ in the Fenton-like reactions. MgO₂ released H₂O₂ quickly in phosphate buffered solutions, which was comparable to CaO₂ but faster than ZnO₂. Metal peroxides induced higher initial phenol degradation rates than UHP and H₂O₂ when the same theoretic H₂O₂ dosages and Fe(III)-EDTA were used. MgO₂ displayed a superior performance for phenol degradation at pH 5, resulting in more than 93% phenol reduction at 1.5 h. According to kinetic analyses, the generation rate of in the MgO₂ system was 18 and 3.4 times higher than those in ZnO₂ and CaO₂ systems, respectively. The addition of MgO₂ significantly promoted H₂O₂ based Fenton-like reactions by increasing production of , and the mixture of MgO₂ and H₂O₂ had an improved utilization efficiency of active oxygen than the MgO₂ system. The findings suggested the critical roles of metal peroxides in favoring Fenton-like reactions and inspired strategies to simultaneously accelerate Fenton-like reactions and improve utilization efficiency of active oxygen.

Development of a VUV-UVC/peroxymonosulfate, continuous-flow Advanced Oxidation Process for surface water disinfection and Natural Organic Matter elimination: Application and mechanistic aspects

Surface waters are often charged with high amounts of natural organic matter (NOM), organic contaminants and pathogens. In this work, a Vacuum UV/PMS process (VUV-UVC/PMS) was employed for treating river water, assessing the simultaneous NOM mineralization and bacterial disinfection. The VUV-UVC process (without PMS) decreased TOC concentration from 3.83 to 0.15 mg/L within 20 min, achieving complete disinfection. Adding 5 mg/L PMS increased the rate of TOC removal by 80%; complete removal of TOC was achieved in 15 min and disinfection was attained twice as fast. The mechanism of NOM mineralization was scrutinized; aeration played a considerable role due to oxygen supply, mixing, and inducing in-situ H₂O₂ production. HO^• and SO₄^•- were the main radical species involved, alongside an important contribution of the matrix; sulfate enhanced TOC removal, due to the formation of additional radicals, underlining its importance. Furthermore, over 99% TOC reduction and complete disinfection was achieved in the VUV-UVC/PMS process operated under continuous-flow mode with a 2-min hydraulic retention time. Finally, the use of Atrazine (ATZ) as a probe compound and a series of scavenging tests led to an integrated proposal for the mineralization of NOM. Accordingly, the VUV-UVC/PMS process is evaluated as an efficient and promising technology for surface water treatment.

Enhancing solar disinfection (SODIS) with the photo-Fenton or the Fe2+/peroxymonosulfate-activation process in large-scale plastic bottles leads to toxicologically safe drinking water

Solar disinfection (SODIS) in 2-L bottles is a well-established drinking water treatment technique, suitable for rural, peri‑urban, or isolated communities in tropical or sub-tropical climates. In this work, we assess the enlargement of the treatment volume by using cheap, large scale plastic vessels. The bactericidal performance of SODIS and two solar-Fe²⁺ based enhancements, namely photo-Fenton (light/H₂O₂/Fe²⁺) and peroxymonosulfate activation (light/PMS/Fe²⁺) were assessed in 19-L polycarbonate (PC) and 25-L polyethylene terephthalate (PET) bottles, in ultrapure and real water matrices (tap water, lake Geneva water). Although SODIS always reached total (5-logU) inactivation, under solar light, enhancement by or both Fe²⁺/H₂O₂ or Fe²⁺/PMS was always beneficial and led to an increase in bacterial elimination kinetics, as high as 2-fold in PC and PET bottles with tap water for light/H₂O₂/Fe²⁺, and 8-fold in PET bottles with Lake Geneva water. The toxicological safety of the enhancements and their effects on the plastic container materials was assessed using the E-screen assay and the Ames test, after 1-day or 1-week exposure to SODIS, photo-Fenton and persulfate activation. Although the production of estrogenic compounds was observed, we report that no treatment method, duration of exposure or material resulted in estrogenicity risk for humans, and similarly, no mutagenicity risk was measured. In summary, we suggest that SODIS enhancement by either HO^•- or SO₄^•--based advanced oxidation process is a suitable enhancement of bacterial inactivation in large scale plastic bottles, without any associated toxicity risks.

Simulated sunlight-induced inactivation of tetracycline resistant bacteria and effects of dissolved organic matter

The transmission of antibiotic resistance in surface water has attracted much attention due to its increasing threat to human health. The role of sunlight irradiation and the effect of dissolved organic matter (DOM) on the transmission of antibiotic resistance are still unclear. In this study, photo-inactivation of antibiotic resistant bacteria (ARB) was investigated using antibiotic resistant E. coli (AR E. coli) that contained the tetracycline resistance gene (Tc-ARG) as a representative. The results showed that AR E. coli underwent significant photo-inactivation due to the membrane damage induced by direct irradiation and by the generated reactive oxygen species. Simulated sunlight irradiation specifically suppressed the expression of tetracycline resistance, which is attributed to the destruction of tetracycline-specific efflux pump. Tetracycline inhibited the photo-inactivation of AR E. coli due to its selective pressure on tetracycline resistant E. coli and competitive light absorption effect. Suwannee River fulvic acid (SRFA), a representative DOM, promoted the inactivation of AR E. coli and further inhibited the expression of tetracycline resistance gene due to the generation of its excited triplet state, singlet oxygen, and hydroxyl radical. The extracellular Tc-ARG also underwent fast photodegradation under light irradiation and in the presence of SRFA, which leads to the decrease of its transformation efficiency. This study provided insight into the sunlight-induced inactivation of ARB, which is of significance for understanding the transmission of tetracycline resistance in surface water.

A Critical View of the Application of the APEX Software (Aqueous Photochemistry of Environmentally-Occurring Xenobiotics) to Predict Photoreaction Kinetics in Surface Freshwaters

The APEX (aqueous photochemistry of environmentally occurring xenobiotics) software computes the phototransformation kinetics of compounds that occur in sunlit surface waters. It is free software based on Octave, and was originally released in 2014. Since then, APEX has proven to be a remarkably flexible platform, allowing for the addressing of several environmental problems. However, considering APEX as a stand-alone software is not conducive to exploiting its full potentialities. Rather, it is part of a whole ecosystem that encompasses both the software and the laboratory protocols that allow for the measurement of substrate photoreactivity parameters. Coherently with this viewpoint, the present paper shows both how to use APEX, and how to experimentally derive or approximately assess the needed input data. Attention is also given to some issues that might provide obstacles to users, including the extension of APEX beyond the simple systems for which it was initially conceived. In particular, we show how to use APEX to deal with compounds that undergo acid–base equilibria, and with the photochemistry of systems such as stratified lakes, lakes undergoing evaporation, and rivers. Hopefully, this work will provide a reference for the smooth use of one of the most powerful instruments for the modeling of photochemical processes in freshwater environments. All authors have read and agreed to the published version of the manuscript.

Kinetic modeling of lag times during photo-induced inactivation of E. coli in sunlit surface waters: Unraveling the pathways of exogenous action

This work presents a kinetic analysis of the exogenous photo-induced disinfection of E. coli in natural waters. Herein, the inactivation of bacteria by light and photo-generated transient species, i.e., hydroxyl radical (HO^•), excited triplet states of organic matter (³CDOM*) and singlet oxygen (¹O₂), was studied. It was found that the exogenous disinfection of E. coli proceeds through a lag time, followed by an exponential phase triggered by photo-generated HO^•, ¹O₂ and ³CDOM*. Also, we report that the concentration increased of transient species (and especially HO^•) precursors decreased the lag times of bacteria inactivation. Due to the limitations of the competition kinetics methodology to include the lag phase, an alternative strategy to study the interaction between E. coli and photo-generated transient species was proposed, considering the log-linear pseudo-first order rate constants and lag-times. On this basis and by using APEX software, a full kinetic analysis of exogenous bacterial inactivation, taking into account both lag-time and exponential decay, was developed. This approach provided insights into the conditions that could make exogenous inactivation competitive with the endogenous process for the E. coli inactivation in natural sunlit waters. Hence, this research contributes to the understanding of fundamental kinetic aspects of photoinduced bacterial inactivation, which is the basis for light-assisted processes such as the solar disinfection (SODIS).

Photochemistry of Surface Fresh Waters in the Framework of Climate Change

Photochemical processes taking place in surface fresh waters play an important role in the transformation of biorecalcitrant pollutants and some natural compounds and in the inactivation of microorganisms. Such processes are divided into direct photolysis, where a molecule is transformed following sunlight absorption, and indirect photochemistry, where naturally occurring photosensitizers absorb sunlight and produce a range of transient species that can transform dissolved molecules (or inactivate microorganisms). Photochemistry is usually favored in thoroughly illuminated shallow waters, while the dissolved organic carbon (DOC) acts as a switch between different photochemical pathways (direct photolysis, and indirect photochemistry triggered by different transient species). Various phenomena connected with climate change (water browning, changing precipitations) may affect water DOC and water depth, with implications for the kinetics of photoreactions and the associated transformation pathways. The latter are important because they often produce peculiar intermediates, with particular health and environmental impacts. Further climate-induced effects with photochemical implications are shorter ice-cover seasons and enhanced duration of summer stratification in lakes, as well as changes in the flow velocity of rivers that affect the photodegradation time scale. This contribution aims at showing how the different climate-related phenomena can affect photoreactions and which approaches can be followed to quantitatively describe these variations.