Efficient diclofenac removal by superoxide radical and singlet oxygen generated in surface Mn(II)/(III)/(IV) cycle dominated peroxymonosulfate activation system: Mechanism and product toxicity

Concerns about the ubiquitous occurrence of diclofenac (DCF, a pain killer) in the natural environment are unprecedently rising because of its toxicity and poor treatability. Herein, we develop and utilize a new bi-metal oxide composite of MnO2-Bi2O3 as the peroxymonosulfate (PMS) activator to construct an efficient oxidation system for DCF degradation and de-toxicity. Under optimal reaction conditions of 3.75 g/L for MnO2-Bi2O3, 0.125 g/L for PMS and 5.38 for pH, DCF can be almost totally degraded within 10 mins of reaction (99.5% of degradation efficiency), and the mineralization efficiency reaches up to 70.1%, which are respectively 3.1- and 4.5-fold higher than those of using the PMS alone. Moreover, the toxicity of the degraded products is reduced comparing to the DCF itself as verified by the Toxicity Estimation Software Tool (T.E.S.T.). Six continuous runs for PMS activation and XRD/XPS characterizations confirmed that MnO2-Bi2O3 could serve as a reliable PMS activator. Density functional theory calculations together with related characterizations and experiments illustrated why MnO2-Bi2O3 exhibits excellent PMS activation for DCF degradation and de-toxicity: (1) MnO2-Bi2O3 possesses higher adsorption energy for PMS adsorption and lower energy barrier for PMS activation as a result of enhanced charge transfer between MnO2-Bi2O3 and PMS as compared to the seperate tetragonal MnO2 or monoclinic Bi2O3 controls; and (2) the Mn(II)/(III)/(IV) and Bi(III)/(V) redox cycles at the surface of MnO2-Bi2O3 promote PMS activation and thus produce strong oxidizing agents: .O-2(-) and O-1(2). Overall, our study not only demonstrates MnO2-Bi2O3 as a novel oxidation system based on PMS activation, but also provides a general idea for designing highly efficient and robust PMS activator.