Photochemical reactions and the self-depuration of sunlit freshwaters

Abstract

Reactions induced by sunlight (direct photolysis and indirect photochemistry) are important ecosystem services that aid freshwater bodies in removing contaminants, although they may also exacerbate pollution in some cases. Without photoinduced reactions, pollution problems would be considerably worse overall. The photochemical reaction rates depend on seasonality, depth, water chemistry (which also significantly affects the reaction pathways), and pollutant photoreactivity. They are also deeply impacted by less studied factors, including hydrology, water dynamics, and precipitation regimes, which are key to understanding the main impacts of climate change on surface-water photochemistry. Climate change is often expected to both exacerbate freshwater pollution, and enhance photochemical decontamination. Therefore, photochemical knowledge will be essential to understand the future evolution of freshwater environments.

Photochemical Implications of Changes in the Spectral Properties of Chromophoric Dissolved Organic Matter: A Model Assessment for Surface Waters

“…Because the three photosensitisers compete for sunlight irradiance, high CDOM absorbance would decrease the production of • OH by both nitrate and nitrite, while favouring • OH photoproduction by CDOM itself. The opposite happens when the absorbance of CDOM is low [ 1 , 2 , 11 , 12 ].…”

“…In these conditions, CDOM is the main • OH source, and a further increase in A CDOM ( A o increase, S decrease) mainly enhances CDOM photochemistry (inhibition of nitrate/nitrite photolysis has minor role) and leads to higher [ • OH] and, therefore, [CO 3 •− ]. Discussion here only focuses on • OH (and CO 3 •− ) generation because scavenging of • OH and CO 3 •− would not change at constant DOC, HCO 3 − , and CO 3 2− [ 11 , 12 ].…”

“…Figure 5 reports the calculated steady-state [ • OH] ( Figure 5 a) and [CO 3 •− ] ( Figure 5 b) in the presence of 10 −6 M NO 3 − and 10 −8 M NO 2 − , as a function of S and for different values of A o . The concentration values of NO 3 − and NO 2 − are 100 times lower than before, and they ensure that the photochemistry of nitrate and nitrite plays a minor role when compared to CDOM [ 11 , 12 , 30 ]. As shown in Figure 5 , [ • OH] and [CO 3 •− ] decrease with increasing S and they are higher as A o is higher.…”

“…In the case of indirect photochemistry, sunlight is absorbed by natural compounds called photosensitisers (most notably, nitrate, nitrite, and chromophoric dissolved organic matter, CDOM) that produce reactive transient species (photochemically produced reactive intermediates, or PPRIs). CECs degradation by indirect photochemistry takes place upon reaction with PPRIs, the main ones being the hydroxyl ( • OH) and carbonate (CO 3 •− ) radicals, the excited triplet states of CDOM ( 3 CDOM*), and singlet oxygen ( 1 O 2 ) [ 11 , 12 ].…”

Constraining the yields and pathways of long-lived photo-oxidants: Suggestion of a formation process not involving organic matter triplet states

“…The chromophoric dissolved organic matter is the main surface-water sunlight absorber below 500 nm, and its ability to absorb the most energetic component of sun's radiation makes it the main photosensitizer (Vione 2023). Absorption generates transients that are involved in the indirect photodegradation of natural compounds and anthropogenic pollutants (Remucal 2014;Yan et al 2019).…”