Wavelength-Dependent Photoreactivity of Root Exudates from Aquatic Plants under UV-LED Irradiation

Abstract

Aquatic plants and their root exudates play important roles in the removal of trace organic contaminants in aquatic environments. However, the wavelength-dependent photogeneration of reactive species (RS) and photodegradation of organic contaminants by root exudates are poorly understood. This paper presents the first evidence that the photogeneration of 3DOM*, 1O2, and ·OH by root exudates varied under UV light-emitting diode (UV-LED) irradiation in the wavelength range of 265–400 nm. The quantum yields of 3DOM* and 1O2 of the four plants’ root exudates, including Scindapsus aureus, Pistia stratiotes, Eichhornia crassipes, and Nymphaea tetragona, reached the highest value at an irradiation wavelength of 275 nm and further decreased with increasing wavelength, while the ·OH quantum yield decreased gradually from 3.2–5.5 × 10–4 at 265 nm to 1.4–2.6 × 10–6 at 400 nm. The root exudate of Pistia stratiotes showed higher quantum yields of 3DOM* and 1O2 than that of the other three plants, while the highest ·OH quantum yield was observed in that of Eichhornia crassipes. The excitation wavelength affected the indirect degradation of organic contaminants by altering the photogeneration of RS in root exudate solutions. These findings highlight the wavelength-dependent photoreactivity of root exudates and improve the understanding of their photochemical processes in aquatic environments.