Fig. 2: Photo-assisted technologies for water purification.
From: Photo-assisted technologies for environmental remediation
Mechanisms of photo-assisted technologies for water purification. a, In photo-assisted Fenton oxidation, light can directly photolyse H2O2, or excite catalysts to generate electrons that will decompose H2O2 to generate the hydroxyl radical, •OH. b, In photo-assisted persulfate activation, persulfate is activated by light directly or a photogenerated charge carrier to produce SO4•– and •OH. c, In photo-assisted ozonation, ozone can react with ultraviolet (UV) light or photoexcited electrons to produce •OH and O2•– radicals that react with pollutants in water and eliminate them. Full arrows represent the main reaction route, whereas the dashed arrows indicate secondary reaction routes. d, In photo-assisted electrochemical oxidation, light can excite either the photoanode or photovoltaic system to produce charge carriers to form reactive oxygen species. e, The electrical energy (EEO) needed to degrade the concentration of a pollutant by one order of magnitude in 1 m3 of water varies for different photo-assisted advanced oxidation processes. The EEO values are based on data from ref. 63 and a defined experimental set-up with controlled conditions, including water quality, target contaminants and other relevant process parameters. When kinetic data are available, compounds that are susceptible to direct oxidation (for example, by ozone or UV photolysis) are excluded from consideration. For ozone-based and UV-based processes, the threshold values for rate constants are set at kO3 < 10 M−1 s−1 and kUV < 10−5 m2 J−1. The box plots show quartiles with the centre line indicating the median and the whiskers extending to 1.5× the interquartile range. CB, catalyst conduction band; LMCT, ligand-to-metal charge transfer; PDS, peroxodisulfate; PMS, peroxymonosulfate; VB, catalyst valence band. Panel e adapted with permission from ref. 63, Elsevier.
