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

Abstract

This work reports a new bi-metal oxide composite of MnO 2 -Bi 2 O 3 as the peroxymonosulfate (PMS) activator. The reaction system based on MnO 2 -Bi 2 O 3 activated PMS exhibits an excellent mineralization and detoxicity ability towards diclofenac (DCF) in water. The main reaction mechanism is summarized as: (1) surface Mn(II)/(III)/(IV) transformation and PMS activation are interacted to produce active species including 1 O 2 , SO 4 · - , and · OH; (2) the electrons produced by surface Mn(II)/(III)/(IV) transformation is also able to reduce dissolved oxygen for · O 2 – species production. Under the attack of · O 2 –, 1 O 2 , SO 4 · - , and · OH, DCF is quickly decomposed into some non/low harmful products. This work 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. • A new bi-metal oxide composite of MnO 2- Bi 2 O 3 was synthesized. • MnO 2 -Bi 2 O 3 activated PMS achieved efficient degradation and toxicity decline of diclofenac. • DFT calculations and experiments demonstrate the superiority of MnO 2 -Bi 2 O 3 in activating PMS. • Mn(II)/(III)/(IV) redox cycle dominates in the activation of PMS. 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 MnO 2 -Bi 2 O 3 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 MnO 2 -Bi 2 O 3 , 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 MnO 2 -Bi 2 O 3 could serve as a reliable PMS activator. Density functional theory calculations together with related characterizations and experiments illustrated why MnO 2 -Bi 2 O 3 exhibits excellent PMS activation for DCF degradation and de-toxicity: (1) MnO 2 -Bi 2 O 3 possesses higher adsorption energy for PMS adsorption and lower energy barrier for PMS activation as a result of enhanced charge transfer between MnO 2 -Bi 2 O 3 and PMS as compared to the seperate tetragonal MnO 2 or monoclinic Bi 2 O 3 controls; and (2) the Mn(II)/(III)/(IV) and Bi(III)/(V) redox cycles at the surface of MnO 2 -Bi 2 O 3 promote PMS activation and thus produce strong oxidizing agents: · O 2 – and 1 O 2 . Overall, our study not only demonstrates MnO 2 -Bi 2 O 3 as a novel oxidation system based on PMS activation, but also provides a general idea for designing highly efficient and robust PMS activator.