Diketone-mediated photochemical reduction of selenite to elemental selenium: Role of carbon-centered radicals and complexation

Abstract

The reduction of highly toxic selenite (Se(IV)) to insoluble elemental selenium (Se(0)) is an effective strategy to alleviate the toxicity and to recover valuable Se from contaminated waters. Acetylacetone (AcAc) and diacetyl (BD) were found to be able to photoreduce Se(IV) into selenium nanoparticles. However, the kinetics and molecular mechanisms of the diketone-mediated Se(IV) reduction are still open to discussion. Herein, the photo-reduction of Se(IV) was studied with the two diketones and sulfite as reductants, as well as TiO2 as a photocatalyst. The results demonstrate that the two diketones were more effective and robust than sulfite. The reduction rate constants of Se(IV) in the UV/diketone systems were 5.1–6.5 times higher than those of UV/sulfite in a wide pH range (1.0–8.0) and without the need for deoxygenation. Compared with UV/TiO2 reduction, no complicated separation processes were needed after reduction with UV/diketones. After 1 h of irradiation, the recovery rates of Se(0) were 82% and 75% in UV/AcAc and UV/BD, respectively. Interestingly, the reactions that occurred in the UV/AcAc and UV/BD systems were unexpectedly different. On the base of reactive species scavenging experiments and quantum chemical calculations, acetyl radical (CH3C•(O)) and its hydrated product (CH3C•(OH)2) were found to play dominant roles in the UV/BD system, whereas the role of photo-induced complexation between hydrated AcAc and Se(IV) might not be neglected in the UV/AcAc system. Such a difference provides us with some room for choice and adjustment in the further application of UV/diketones for the treatment of Se(IV)-laden wastewater or for the reduction of some other similar metalloids.