Indirect Photochemistry in Sunlit Surface Waters: Photoinduced Production of Reactive Transient Species

Abstract

This paper gives an overview of the main reactive transient species that are produced in surface waters by sunlight illumination of photoactive molecules (photosensitizers), such as nitrate, nitrite, and chromophoric dissolved organic matter (CDOM). The main transients (˙OH, CO3(-˙) , (1)O2, and CDOM triplet states) are involved in the indirect phototransformation of a very wide range of persistent organic pollutants in surface waters.

Solar photo-Fenton and UV/H 2 O 2 processes against the antidepressant Venlafaxine in urban wastewaters and human urine. Intermediates formation and biodegradability assessment

“…However, the direct action of solar light includes (1) the excitation of the organic compound at singletexcited state [30], (2) its intersystem crossing to triplet state and (3) its reaction with oxygen to form singlet oxygen [31]. Afterwards, the micropollutant returns to ground state, but the singlet oxygen created by the reaction with water participates in (i) the superoxide radical anion formation from oxygen and consequently (ii) to the formation of H 2 O 2 from water [31] or (iii) the direct attack to double bonds present in the molecule. Although of lesser importance, these results will play the role of reference when the photo-Fenton process will be described from its parts.…”

“…Venlafaxine degradation is a function of contradicting factors in real effluents. On the antagonists of the process, we can mention the (i) suspended solids, blocking UV and solar light transmission, (ii) the Effluent Organic Matter (EfOM), consisting in still particulate organic matter (POM), biodegradable organic matter, refractory organic matter and other MPs, which all compete for the oxidants generated by AOPs [19] and screen the light [30], (iii) the ROS scavengers, such as (bi)carbonates, nitrate and nitrite [31], and (iv) the microorganisms. On the other hand, the very presence of some substances has been proven to enhance the self-purification capabilities of the effluents, such as (a) the presence of photo-sensitizable organic matter (PhOM), which further produces ROS, and (b) the nitrates and the carbonates, which contribute in producing nitrate radicals, carbonate radicals and ROS, all with mild oxidative potential [19].…”

Pesticide behavior in modified water-sediment systems

“…The former is reactions from excited states of a pesticide molecule after light absorption, while the latter is those with the photo-produced reactive oxygen species (ROS), the transfer of energy mainly from the excited triplet state of colored DOM ( 3 CDOM*) to a ground-state pesticide molecule, or the exchange of an electron or hydrogen between them. 15,27) By the way, the diurnal pH change of water in the field is known to occur under sunlight as a result of photosynthesis by aquatic algae and macrophytes, possibly leading to photosynthetically-driven alkaline hydrolysis of some pesticides. 7,13) There are many kinds of phototrophic bacteria and algae in natural water, and biofilms not only at the water-sediment interface but also on the surface of aquatic macrophytes are known to be productive environments for many types of microbes and algae.…”

Long-term trends of chemical and modelled photochemical parameters in four Alpine lakes

“…The irradiation of the photosensitizers induces the production of transient species, such as the hydroxyl ( • OH) and carbonate (CO 3 −• ) radicals, singlet oxygen ( 1 O 2 ), and CDOM triplet states ( 3 CDOM*) (Pace and Barreca, 2013;Vione et al, 2014). These transient species are reactive and they can be involved in the photochemical transformation of many biorefractory pollutants (industrial chemicals, some pesticides, pharmaceuticals and personal care products) and of naturally occurring DOM (dissolved organic matter; Medeiros et al, 2015;Minella et al, 2015).…”

“…Because CDOM is an important • OH source, but DOM is by far its main sink, the • OH levels can be negatively related with DOC. An even stronger negative relationship is often observed between DOC and CO 3 −• , because DOM both scavenges CO 3 −• and inhibits its formation by consuming • OH (Canonica et al, 2005;Vione et al, 2014). In contrast, • OH and CO 3 −• often positively correlate with nitrate and nitrite, and CO 3 −• with inorganic carbon species as well .…”