Coupled Manganese Redox Cycling and Organic Carbon Degradation on Mineral Surfaces

Dong Ma, Juan Wu, Peng Yang, Mengqiang Zhu

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69 Scopus citations

Abstract

Minerals, natural organic matter (NOM), and divalent manganese (Mn(II)) often coexist in suboxic/oxic environment. Multiple adsorption and oxidation processes occur in this ternary system, which are coupled to affect the fate of both OM and Mn therein and alter their chemical reactivity toward metals and other pollutants. However, the details about the coupling are poorly known although much has been gained for the binary systems. We determined the mutual influence of surface-catalyzed Mn(II) oxidation and humic acid (HA) adsorption and oxidation in a Fe(III) oxide (goethite)-HA-Mn(II) system at pH 5-8. The presence of Mn(II) substantially increased HA adsorption whereas HA greatly impaired the extent and rate of Mn(II) oxidation by O2 on goethite surfaces. The impacts were more pronounced at higher pH. Mn(II) oxidation produced β-MnOOH, γ-MnOOH, and Mn3O4 which in turn oxidized HA, producing small organic acids. The presence of HA markedly altered the composition of Mn(II) oxidation products by inhibiting the formation of β-MnOOH while favoring the production of γ-MnOOH and Mn(II) adsorbed on the HA-mineral assemblage. Nonconducting γ-Al2O3 exhibited similar but weaker effects than semiconducting goethite in the above processes. Our results suggest that similar to Mn-oxidizing microorganisms, mineral surfaces can drive the coupling of the Mn redox cycle with NOM oxidative degradation under suboxic/oxic and circumneutral/alkaline conditions.

Original languageEnglish
Pages (from-to)8801-8810
Number of pages10
JournalEnvironmental Science and Technology
Volume54
Issue number14
DOIs
StatePublished - Jul 21 2020
Externally publishedYes

Funding

This work was supported by the U.S. National Science Foundation under Grant EAR-1529937. We are grateful to Dr. Philip A. Moore at the USDA-Agriculture Research Service in Fayetteville, AR for providing the soils for humic acid extraction. The authors thank Janet Dewey at Wyoming Aqueous Analytical Laboratory for her assistance in solution sample characterization. We acknowledge beamline scientists Ryan Davis, Matthew J. Latimer, and Erik J. Nelson for their assistance in XAS data collection at beamline 4-1 at SSRL. Use of SSRL, SLAC National Accelerator Laboratory, was supported by the U.S. DOE, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-76SF00515.

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