Manganese and its compounds have been extensively studied because of their far-reaching roles in a wide range of biogeochemical processes in natural systems. The (ad)sorption behavior of Mn(II), however, is poorly understood despite its important role as the primary reaction step for surface-catalyzed Mn(II) oxidation that is the principal abiotic process forming various Mn (oxyhydr)oxides in nature. Here, we systematically examined Mn(II) (ad)sorption to one of the most common natural sorbents, goethite, in oxygen- and carbonate-free systems. Traditional sorption edge and isotherm experiments were conducted by varying sorbate-to-sorbent ratio (0.027–15 μmol∙m−2) and solution pH (pH 5.0–9.0). The effects of dissolved carbonates on Mn(II) sorption were also assessed in a range of naturally prevalent concentrations (0.5–10 mM NaHCO3). The Mn(II) uptake on goethite followed a typical Langmuir isotherm in the absence of dissolved carbonates, with increasing maximum adsorption capacities (Γmax) from 0.19 at pH 6.5 to 3.4 μmol∙m−2 at pH 9.0. The presence of dissolved carbonates raised the extent of Mn(II) adsorption, which appeared to be directly correspondent to that of the adsorption of dissolved carbonates. Extended X-ray absorption fine structure (EXAFS) analysis indicated that Mn(II) predominantly formed inner-sphere binuclear bidentate surface complexes. Mn(II) uptake became deviated from the Langmuir model and showed a clear indication of surface precipitation when the Mn(II) sorption density (Γ) exceeded a threshold value in a given solution composition. This secondary Mn(II) phase was identified as rhodochrosite using X-ray diffraction (XRD) and transmission electron microscope (TEM). Additionally, it would be plausible that a minor fraction of adsorbed Mn(II) coexisted with the secondary rhodochrosite according to a linear combination fitting (LCF) of the X-ray absorption near edge structure (XANES) spectra of Mn reference and samples. These systematic investigations of the macroscopic and microscopic behaviors of Mn(II) (ad)sorption to goethite provide a critical avenue for disentangling surface-catalytic Mn(II) oxidation processes, which ultimately lead to the formation of diverse Mn (oxyhydr)oxides in the environment.
Bibliographical noteFunding Information:
This research was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Government ( NRF-2017R1A2B4011773 ; NRF-2020R1A2C2010089 ) to GL, Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant Number JP18K19045 ) to KS, and U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division to SSL under Contract DE-AC02-06CH11357 to UChicago Argonne, LLC, as operator of Argonne National Laboratory (“Argonne”), a U.S. Department of Energy Office of Science laboratory. The authors are grateful for the comments of four anonymous reviewers, which substantially improved the manuscript. XAFS data were collected at BL06 of the Kyushu Synchrotron Light Research Center (SAGA-LS, Kyushu, Japan) under the proposal No. 2018IK004 and at the beamline 12-BM-B of Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne under Contract No. DE-AC02-06CH11357. We thank Y Lee for the assistance of HR-XRD analysis of our solid samples at 3D XRS beamline of Pohang Accelerator Laboratory (PAL) and J Choi for helping us visualize the Mn(II) adsorption mode using the CrystalMaker® software. We are also grateful that the Hydrogeology Laboratory and the Institute for High-pressure Mineral Physics and Chemistry at Yonsei University provided ICP-OES, XRD, and BET facilities for our sample analyses.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology