Photoelectrochemical reactors for treatment of water and wastewater: a review

The 'water reuse' option is increasingly becoming crucial in the context of global warming and water scarcity. Water reuse consists in the recycling of water toward sectors requiring different water qualities, such as industry, agriculture and human consumption. Water treatment by biological methods is cheap, yet they do not to remove completely recalcitrant pollutants. Alternatively, advanced oxidation processes (AOP) for wastewater treatment are more efficient because a strong oxidizing agent such as the hydroxyl radical ((OH)-O-center dot) is generated under mild conditions. In particular, processes of electrochemical advanced oxidation have recently attracted attention because they allow continuous and in situ electrocatalytic generation of strong oxidizing species under mild conditions, with the advantage of avoiding external addition of chemicals. In addition, energy can be provided by solar light in photochemical electrolysis systems. Here we review photoelectrochemical reactors. We present reactions and synergetic mechanisms. Under optimal conditions, eight (OH)-O-center dot production sites are identified in photoelectro-Fenton combined with photoanodic oxidation and photocatalysis or photoelectrocatalysis. Reactions are controlled by pH, applied wavelength, and competition between reactions requiring the same reagent. Then we discuss reactor design and catalysis. Various reactor configurations are presented: sequential and hybrid reactors, divided and undivided cells, flow cell and stirred tank reactor; and light source positioning can be external, immersed, vertical, horizontal, at the top, at the bottom, or on the side of the reactor. The distance between the light source and the electrode should be minimized to obtain maximal synergy and maximum quantum yields. Hybrid reactors in undivided flow cells are best for the lower footprint, the possibility to regenerate iron catalysts, and the enhancement of mass transfer, compared with stirred tank reactors. Moreover, the interelectrode gap can be easily controlled in flow cell, which allows the optimization of the penetration depth when the light is applied through the reactor.