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Evaporative water concentration takes place in arid or semi-arid environments when stationary water bodies, such as lakes or ponds, prevalently lose water by evaporation, which prevails over outflow or seepage into aquifers. Absence or near-absence of precipitation and elevated temperatures are important prerequisites for the process, which has the potential to deeply affect the photochemical attenuation of pollutants, including contaminants of emerging concern (CECs). Here we show that water evaporation would enhance the phototransformation of many CECs, especially those undergoing degradation mainly through direct photolysis and triplet-sensitized reactions. In contrast, processes induced by hydroxyl and carbonate radicals would be inhibited. Our model results suggest that the photochemical impact of water evaporation might increase in the future in several regions of the world, with no continent likely being unaffected, due to the effects of local precipitation decrease combined with an increase in temperature that facilitates evaporation.

In temperate environments, climate change could affect water pH by inducing enhanced dissolution of CaSO 4 followed by biological sulphate reduction, with the potential to basify water due to H + consumption. At the same time, increased atmospheric CO 2 could enhance weathering of carbonate rocks (e.g., dolomite) and increase the total concentration of dissolved carbonate species. Both processes enhance phototransformation by the carbonate radical (CO 3 •- ), as shown for the non-steroidal anti-inflammatory drug paracetamol, provided that the dissolved organic carbon of water does not undergo important fluctuations. Climate change could also affect hydrology, and prolonged drought periods might considerably decrease flow rates in rivers. This is a substantial problem because wastewater pollutants become less diluted and, as a result, can exert more harmful effects due to increased concentrations. At the same time, in low-flow conditions, water is also shallower and its flow velocity is decreased. Photochemical reactions become faster because shallow water is efficiently illuminated by sunlight, and they also have more time to occur because water takes longer to cover the same river stretch. As a result, photodegradation of contaminants is enhanced, which offsets lower dilution but only at a sufficient distance from the wastewater outlet; this is because photoreactions need time (which translates into space for a flowing river) to attenuate pollution.

Chromophoric dissolved organic matter (CDOM) is the main sunlight absorber in surface waters and a very important photosensitiser towards the generation of photochemically produced reactive intermediates (PPRIs), which take part in pollutant degradation. The absorption spectrum of CDOM ( A CDOM (λ), unitless) can be described by an exponential function that decays with increasing wavelength: A CDOM (λ) = 100 d DOC A o e - S λ , where d [m] is water depth, DOC [mg C L -1 ] is dissolved organic carbon, A o [L mg C -1 cm -1 ] is a pre-exponential factor, and S [nm -1 ] is the spectral slope. Sunlight absorption by CDOM is higher when A o and DOC are higher and S is lower, and vice versa. By the use of models, here we investigate the impact of changes in CDOM spectral parameters ( A o and S ) on the steady-state concentrations of three PPRIs: the hydroxyl radical ( • OH), the carbonate radical (CO 3 •- ), and CDOM excited triplet states ( 3 CDOM*). A first finding is that variations in both A o and S have impacts comparable to DOC variations on the photochemistry of CDOM, when reasonable parameter values are considered. Therefore, natural variability of the spectral parameters or their modifications cannot be neglected. In the natural environment, spectral parameters could, for instance, change because of photobleaching (prolonged exposure of CDOM to sunlight, which decreases A o and increases S ) or of the complex and still poorly predictable effects of climate change. A second finding is that, while the steady-state [ 3 CDOM*] would increase with increasing A CDOM (increasing A o , decreasing S ), the effect of spectral parameters on [ • OH] and [CO 3 •- ] depends on the relative roles of CDOM vs. NO 3 - and NO 2 - as photochemical • OH sources.

Glyphosate is a widely used herbicide, and it is an important environmental pollutant that can have adverse effects on human health. Therefore, remediation and reclamation of contaminated streams and aqueous environments polluted by glyphosate is currently a worldwide priority. Here, we show that the heterogeneous nZVI-Fenton process (nZVI + H 2 O 2 ; nZVI: nanoscale zero-valent iron) can achieve the effective removal of glyphosate under different operational conditions. Removal of glyphosate can also take place in the presence of excess nZVI, without H 2 O 2 , but the high amount of nZVI needed to remove glyphosate from water matrices on its own would make the process very costly. Glyphosate removal via nZVI--Fenton was investigated in the pH range of 3-6, with different H 2 O 2 concentrations and nZVI loadings. We observed significant removal of glyphosate at pH values of 3 and 4; however, due to a loss in efficiency of Fenton systems with increasing pH values, glyphosate removal was no longer effective at pH values of 5 or 6. Glyphosate removal also occurred at pH values of 3 and 4 in tap water, despite the occurrence of several potentially interfering inorganic ions. Relatively low reagent costs, a limited increase in water conductivity (mostly due to pH adjustments before and after treatment), and low iron leaching make nZVI-Fenton treatment at pH 4 a promising technique for eliminating glyphosate from environmental aqueous matrices., Competing Interests: The authors declare no conflict of interest.

Nitrogen dioxide ( • NO 2 ) is produced in sunlit natural surface waters by the direct photolysis of nitrate, together with • OH, and upon the oxidation of nitrite by • OH itself. • NO 2 is mainly scavenged by dissolved organic matter, and here, it is shown that • NO 2 levels in sunlit surface waters are enhanced by high concentrations of nitrate and nitrite, and depressed by high values of the dissolved organic carbon. The dimer of nitrogen dioxide (N 2 O 4 ) is also formed in the pathway of • NO 2 hydrolysis, but with a very low concentration, i.e., several orders of magnitude below • NO 2 , and even below • OH. Therefore, at most, N 2 O 4 would only be involved in the transformation (nitration/nitrosation) of electron-poor compounds, which would not react with • NO 2 . Although it is known that nitrite oxidation by CO 3 • - in high-alkalinity surface waters gives a minor-to-negligible contribution to • NO 2 formation, it is shown here that NO 2 - oxidation by Br 2 • - can be a significant source of • NO 2 in saline waters (saltwater, brackish waters, seawater, and brines), which offsets the scavenging of • OH by bromide. As an example, the anti-oxidant tripeptide glutathione undergoes nitrosation by • NO 2 preferentially in saltwater, thanks to the inhibition of the degradation of glutathione itself by • OH, which is scavenged by bromide in saltwater. The enhancement of • NO 2 reactions in saltwater could explain the literature findings, that several phenolic nitroderivatives are formed in shallow (i.e., thoroughly sunlit) and brackish lagoons in the Rhône river delta (S. France), and that the laboratory irradiation of phenol-spiked seawater yields nitrophenols in a significant amount.

Photochemical degradation plays an important role in the attenuation of many recalcitrant pollutants in surface freshwaters. Photoinduced transformation kinetics are strongly affected by environmental conditions, where sunlight irradiance plays the main role, followed by water depth and dissolved organic carbon (DOC). Apart from poorly predictable weather-related issues, fair-weather irradiance has a seasonal trend that results in the fastest photodegradation in June and the slowest in December (at least in temperate areas of the northern hemisphere). Pollutants that have first-order photochemical lifetimes longer than a week take more than one month to achieve 95% photodegradation. Consequently, they may experience quite different irradiance conditions as their photodegradation goes on. The relevant time trend can be approximated as a series of first-order kinetic tracts, each lasting for one month. The trend considerably departs from an overall exponential decay, if degradation takes long enough to encompass seasonally varying irradiance conditions. For instance, sunlight irradiance is higher in July than in April, but increasing irradiance after April and decreasing irradiance after July ensure that pollutants emitted in either month undergo degradation with very similar time trends in the first 3-4 months after emission. If photodegradation takes longer, pollutants emitted in July experience a considerable slowdown in photoreaction kinetics as winter is approached. Therefore, if pollutants are photostable enough that their photochemical time trend evolves over different seasons, degradation acquires some peculiar features than cannot be easily predicted from a mere analysis of lifetimes in the framework of simple first-order kinetics. Such features are here highlighted with a modelling approach, taking the case of carbamazepine as the main example. This contaminant is almost totally biorecalcitrant, and it is also quite resistant to photodegradation.

Among the advanced oxidation processes (AOPs), the Fenton reaction has attracted much attention in recent years for the treatment of water and wastewater. This review provides insight into a particular variant of the process, where soluble Fe(II) salts are replaced by zero-valent iron (ZVI), and hydrogen peroxide (H 2 O 2 ) is replaced by persulfate (S 2 O 8 2- ). Heterogeneous Fenton with ZVI has the advantage of minimizing a major problem found with homogeneous Fenton. Indeed, the precipitation of Fe(III) at pH > 4 interferes with the recycling of Fe species and inhibits oxidation in homogeneous Fenton; in contrast, suspended ZVI as iron source is less sensitive to the increase of pH. Moreover, persulfate favors the production of sulfate radicals (SO 4 •- ) that are more selective towards pollutant degradation, compared to the hydroxyl radicals ( • OH) produced in classic, H 2 O 2 -based Fenton. Higher selectivity means that degradation of SO 4 •- -reactive contaminants is less affected by interfering agents typically found in wastewater; however, the ability of SO 4 • - to oxidize H 2 O/OH - to • OH makes it difficult to obtain conditions where SO 4 •- is the only reactive species. Research results have shown that ZVI-Fenton with persulfate works best at acidic pH, but it is often possible to get reasonable degradation at pH values that are not too far from neutrality. Moreover, inorganic ions that are very common in water and wastewater (Cl - , HCO 3 - , CO 3 2- , NO 3 - , NO 2 - ) can sometimes inhibit degradation by scavenging SO 4 •- and/or • OH, but in other cases they even enhance the process. Therefore, ZVI-Fenton with persulfate might perform unexpectedly well in some saline waters, although the possible formation of harmful by-products upon oxidation of the anions cannot be ruled out.

Aromatic nitroderivatives are compounds of considerable environmental concern, because some of them are phytotoxic (especially the nitrophenols, and particularly 2,4-dinitrophenol), others are mutagenic and potentially carcinogenic (e.g., the nitroderivatives of polycyclic aromatic hydrocarbons, such as 1-nitropyrene), and all of them absorb sunlight as components of the brown carbon. The latter has the potential to affect the climatic feedback of atmospheric aerosols. Most nitroderivatives are secondarily formed in the environment and, among their possible formation processes, photonitration upon irradiation of nitrate or nitrite is an important pathway that has periodically gained considerable attention. However, photonitration triggered by nitrate and nitrite is a very complex process, because the two ionic species under irradiation produce a wide range of nitrating agents (such as • NO 2 , HNO 2 , HOONO, and H 2 OONO + ), which are affected by pH and the presence of organic compounds and, in turn, deeply affect the nitration of aromatic precursors. Moreover, aromatic substrates can highly differ in their reactivity towards the various photogenerated species, thereby providing different behaviours towards photonitration. Despite the high complexity, it is possible to rationalise the different photonitration pathways in a coherent framework. In this context, this review paper has the goal of providing the reader with a guide on what to expect from the photonitration process under different conditions, how to study it, and how to determine which pathway(s) are prevailing in the formation of the observed nitroderivatives.

Water browning in lakes (progressive increase of the content of chromophoric dissolved organic matter, CDOM) has the potential to deeply alter the photodegradation kinetics of pollutants during summer stratification. Browning, which takes place as a consequence of climate change in several Nordic environments, causes the thermocline to be shallower, because higher CDOM decreases the penetration of sunlight inside the water column. Using a model approach, it is shown in this paper that pollutants occurring in the epilimnion would be affected differently depending on their main photodegradation pathway(s): almost no change for the direct photolysis, slight decrease in the degradation kinetics by the hydroxyl radicals ( • OH, but the resulting degradation would be too slow for the process to be effective during summer stratification), considerable decrease for the carbonate radicals (CO 3 •- ), increase for the excited triplet states of CDOM ( 3 CDOM*) and singlet oxygen ( 1 O 2 ). Because it is difficult to find compounds that are highly reactive with CO 3 •- and poorly reactive with 3 CDOM*, the degradation rate constant of many phenols and anilines would show a minimum with increasing dissolved organic carbon (DOC), because of the combination of decreasing CO 3 •- and increasing 3 CDOM* photodegradation. In contrast, overall photodegradation would always be inhibited by browning when the whole water column (epilimnion + hypolimnion) is considered, either because of slower degradation kinetics in the whole water volume, or even at unchanged overall kinetics, because of unbalanced distribution of photoreactivity within the water column.

The abiotic photochemical reactions that take place naturally in sunlit surface waters can degrade many contaminants that pose concern to water bodies for their potentially toxic and long-term effects. This works aims at assessing the ability of European rivers to photoproduce reactive transient intermediates, such as HO • radicals and the excited triplet states of chromophoric dissolved organic matter ( 3 CDOM*), involved in pollutant degradation. A photochemical mapping of the steady-state concentrations of these transients was carried out by means of a suitable modeling tool, in the latitude belt between 40 and 50°N. Such a map allowed for the prediction of the photochemical lifetimes of the phenylurea herbicide isoproturon (mostly undergoing photodegradation upon reaction with HO • and especially 3 CDOM*) across different European countries. For some rivers, a more extensive dataset was available spanning the years 1990-2002, which allowed for the computation of the steady-state concentration of the carbonate radicals (CO 3 • - ). With these data, it was possible to assess the time trends of the photochemical half-lives of further contaminants (atrazine, ibuprofen, carbamazepine, and clofibric acid). The calculated lifetimes were in the range of days to weeks, which might or might not allow for efficient depollution depending on the river-water flow velocity., Competing Interests: The authors declare no conflict of interest.

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., Competing Interests: The author declares no conflict of interest.

Heterogeneous Fenton processes with solid catalysts have gained much attention for water and wastewater treatment in recent years. In the field of solid catalysts, zero valent iron (ZVI) is among the most applicable due to its stability, activity, pollutant degradation properties and environmental friendliness. The main limitation in the use of ZVI in heterogeneous Fenton systems is due to its deactivation in neutral and alkaline conditions, and Fenton-like processes have been developed to overcome this difficulty. In this review, the effect of solution pH on the ZVI-Fenton performance is discussed. In addition, the pH trend of ZVI efficiency towards contaminants removal is also considered in oxic solutions (i.e., in the presence of dissolved O₂ but without H₂O₂), as well as in magnetic-field assisted Fenton, sono-Fenton, photo-Fenton and microwave-Fenton processes at different pH values. The comparison of the effect of pH on ZVI performance, taking into account both heterogeneous Fenton and different Fenton-like processes, can guide future studies for developing ZVI applications in water and wastewater treatment.

This study shows that methyl 2-aminobenzoate (also known as methyl anthranilate, hereafter MA) undergoes direct photolysis under UVC and UVB irradiation and that its photodegradation is further accelerated in the presence of H₂O₂. Hydrogen peroxide acts as a source of hydroxyl radicals (·OH) under photochemical conditions and yields MA hydroxyderivatives. The trend of MA photodegradation rate vs. H₂O₂ concentration reaches a plateau because of the combined effects of H₂O₂ absorption saturation and ·OH scavenging by H₂O₂. The addition of chloride ions causes scavenging of ·OH, yielding Cl₂· - as the most likely reactive species, and it increases the MA photodegradation rate at high H₂O₂ concentration values. The reaction between Cl₂· - and MA, which has second-order rate constant k C l 2 • - + M A = (4.0 ± 0.3) × 10⁸ M -1 ·s -1 (determined by laser flash photolysis), appears to be more selective than the ·OH process in the presence of H₂O₂, because Cl₂· - undergoes more limited scavenging by H₂O₂ compared to ·OH. While the addition of carbonate causes ·OH scavenging to produce CO₃· - ( k C O 3 • - + M A = (3.1 ± 0.2) × 10⁸ M -1 ·s -1 ), carbonate considerably inhibits the photodegradation of MA. A possible explanation is that the elevated pH values of the carbonate solutions make H₂O₂ to partially occur as HO₂ - , which reacts very quickly with either ·OH or CO₃· - to produce O₂· - . The superoxide anion could reduce partially oxidised MA back to the initial substrate, with consequent inhibition of MA photodegradation. Fast MA photodegradation is also observed in the presence of persulphate/UV, which yields SO₄· - that reacts effectively with MA ( k S O 4 • - + M A = (5.6 ± 0.4) × 10⁸ M -1 ·s -1 ). Irradiated H₂O₂ is effective in photodegrading MA, but the resulting MA hydroxyderivatives are predicted to be about as toxic as the parent compound for aquatic organisms (most notably, fish and crustaceans).

The APEX (Aqueous Photochemistry of Environmentally-occurring Xenobiotics) software previously developed by one of us was used to model the photochemistry of As(III) in paddy-field water, allowing a comparison with biotic processes. The model included key paddy-water variables, such as the shielding effect of the rice canopy on incident sunlight and its monthly variations, water pH, and the photochemical parameters of the chromophoric dissolved organic matter (CDOM) occurring in paddy fields. The half-life times ( t 1/2 ) of As(III) photooxidation to As(V) would be ~20-30 days in May. In contrast, the photochemical oxidation of As(III) would be much slower in June and July due to rice-canopy shading of radiation because of plant growth, despite higher sunlight irradiance. At pH < 8 the photooxidation of As(III) would mainly be accounted for by reaction with transient species produced by irradiated CDOM (here represented by the excited triplet states ³CDOM * , neglecting the possibly more important reactions with poorly known species such as the phenoxy radicals) and, to a lesser extent, with the hydroxyl radicals (HO • ). However, the carbonate radicals (CO₃ •- ) could be key photooxidants at pH > 8.5 provided that the paddy-water ³CDOM * is sufficiently reactive toward the oxidation of CO₃ 2- . In particular, if paddy-water ³CDOM * oxidizes the carbonate anion with a second-order reaction rate constant near (or higher than) 10⁶ M -1 ·s -1 , the photooxidation of As(III) could be quite fast at pH > 8.5. Such pH conditions can be produced by elevated photosynthetic activity that consumes dissolved CO₂.