Oxidative Influence of the Presence of Dopant Atom on the Adsorption of Selenite Oxyanion on Manganese-Doped Hematite

Abstract

Selenium could enter the environment through different anthropogenic sources, posing a potential health risk at higher concentrations. Although dopant atoms incorporated into the Fe(III) oxide lattice play a controlling role in the environmental speciation and fate of various contaminants, their role is poorly understood at the mechanistic level. The present study is an in situ investigation of the surface reactions of selenite oxyanion at the pristine/Mn-doped hematite (Mn-Ht)–water interface to delineate the role of the dopant in interfacial speciation and adsorption characteristics of the selenium oxyanion. Total and species-specific selenium ions present in the supernatant, estimated at the end of the equilibration of selenite ions with an aqueous suspension of the doped hematite, revealed the adsorption of selenite ions followed by their fast surface-induced oxidative transformation to selenate ions. This redox change of selenite ions was further characterized by using cyclic voltammetry and differential pulse voltammetry methods. Electrochemical evidence revealed an increased electron transfer (ET) rate constant (0.023–0.197 s^–1) with Mn doping. The proposed mechanism is related to the suppression of electron–hole pair recombination with Mn doping in the hematite lattice. Due to the dopants, acceptor levels trap the photoelectrons produced in the Mn-Ht lattice; simultaneously, the photoholes collect in the valence band to execute the oxidative transformation of selenite anions to their oxidized species. As selenate exhibits weaker interactions with hematite than selenite ions, it is released into the supernatant phase. Our findings highlight the critical role that a dopant may play in mobilizing contaminants that otherwise will be strongly held by the pristine solid.