Natural sunlight-driven oxidation of Mn2+(aq) and heterogeneous formation of Mn oxides on hematite

Junyeong Choi; Wooyeol Choi; Hoyoung Hwang; Yuanzhi Tang; Haesung Jung

doi:10.1016/j.chemosphere.2023.140734

Natural sunlight-driven oxidation of Mn²⁺(aq) and heterogeneous formation of Mn oxides on hematite

Chemosphere. 2024 Jan:348:140734. doi: 10.1016/j.chemosphere.2023.140734. Epub 2023 Nov 15.

Authors

Junyeong Choi¹, Wooyeol Choi¹, Hoyoung Hwang¹, Yuanzhi Tang², Haesung Jung³

Affiliations

¹ Department of Chemical Engineering, Changwon National University, Changwon, Gyeongsangnam-do, 51140, Republic of Korea.
² School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, 30332, United States. Electronic address: yuanzhi.tang@eas.gatech.edu.
³ Department of Chemical Engineering, Changwon National University, Changwon, Gyeongsangnam-do, 51140, Republic of Korea. Electronic address: haesung.jung@changwon.ac.kr.

PMID: 37977540
DOI: 10.1016/j.chemosphere.2023.140734

Abstract

The oxidation of dissolved Mn²⁺(aq) plays a critical role in driving manganese cycles and regulating the fate of essential elements and contaminants in environmental systems. Based on sluggish oxidation rate, abiotic processes have been considered less effective oxidation pathway for manganese oxidation in environmental systems. Interestingly, a recent study (Jung et al., 2021) has shown that the rapid photochemical oxidation of Mn²⁺(aq) could be a feasible scenario to uncover the potential significance of abiotic Mn²⁺(aq) oxidation. Nevertheless, the significance of photochemical oxidation of Mn²⁺(aq) under natural sunlight exposure remains unclear. Here, we demonstrate the rapid photocatalytic oxidation of Mn²⁺(aq) and the heterogeneous growth of tunnel-structured Mn oxides under simulated freshwater and seawater conditions in the presence of natural sunlight and hematite. The natural sunlight-driven photocatalytic oxidation of Mn²⁺(aq) by hematite showed kinetic constants of 1.02 h^-1 and 0.342 h^-1 under freshwater and seawater conditions, respectively. The natural sunlight-driven photocatalytic oxidation rates are quite comparable to the results obtained from the previous laboratory test using artificial sunlight, which has ∼4.5 times stronger light intensity. It is likely because of ∼5.5 times larger light exposure area in the natural sunlight-driven photocatalytic oxidation than that of the laboratory test using artificial sunlight. We also elucidate the roles of cation species in controlling the oxidation rate of Mn²⁺(aq) and the crystalline structure of Mn oxide products. Specifically, in the presence of large amounts of cations, the oxidation rate of Mn²⁺(aq) was slower likely because of competitive adsorption. Furthermore, our findings highlight that Mg²⁺ contributes significantly to the formation of large-tunneled Mn oxides. These results illuminate the importance of abiotic photocatalytic processes in controlling the redox chemistry of Mn in real environmental aqueous systems on the oxidation of Mn²⁺(aq), and provide an environmentally sustainable approach to effectively remediate water contaminated with Mn²⁺(aq) using natural sunlight.

Keywords: Dissolved Mn(II); Hematite; Heterogeneous nucleation; Mn oxides; Natural sunlight; Photocatalytic oxidation.

MeSH terms

Manganese Compounds / chemistry
Manganese* / chemistry
Oxidation-Reduction
Oxides* / chemistry
Sunlight
Water

Substances

Oxides
ferric oxide
Manganese
Water
Manganese Compounds