Understanding the role of manganese oxides in retaining harmful metals: Insights into oxidation and adsorption mechanisms at microstructure level

Abstract

The increasing intensity of human activities has led to a critical environmental challenge: widespread metal pollution. Manganese (Mn) oxides have emerged as potentially natural scavengers that perform crucial functions in the biogeochemical cycling of metal elements. Prior reviews have focused on the synthesis, characterization, and adsorption kinetics of Mn oxides, along with the transformation pathways of specific layered Mn oxides. This review conducts a meticulous investigation of the molecular-level adsorption and oxidation mechanisms of Mn oxides on hazardous metals, including adsorption patterns, coordination, adsorption sites, and redox processes. We also provide a comprehensive discussion of both internal factors (surface area, crystallinity, octahedral vacancy content in Mn oxides, and reactant concentration) and external factors (pH, presence of doped or pre-adsorbed metal ions) affecting the adsorption/oxidation of metals by Mn oxides. Additionally, we identify existing gaps in understanding these mechanisms and suggest avenues for future research. Our goal is to enhance knowledge of Mn oxides' regulatory roles in metal element translocation and transformation at the microstructure level, offering a framework for developing effective metal adsorbents and pollution control strategies.

Keywords: Adsorption; Environment; Harmful metals; Manganese oxides; Redox.

Fig. 1

Structural illustrations of common layered manganese oxides. (a) Hexagonal birnessite. H-site represents octahedral vacancy sites; L-site represents particle edge sites. (b) Triclinic birnessite. (c) Lithiophorite. (d) The transformation pathway of buserite. (d) is adapted with permission from the study by Lee and Xu [22]. Copyright 2016, Clay Minerals Society.

Fig. 2

Common tunneled Mn oxides. Mn, manganese.

Fig. 3

Crystal structure of hexagonal birnessite (a) and possible surface complexation species of metals on birnessite (b–e). (b) TCS, triple-corner-sharing complex; (c) DCS, double-corner-sharing complex; (d) DES, double-edge-sharing complex; (e) INC, incorporated inside a Mn vacancy. (a) is adapted with permission from ref. [44]. Copyright 2020, Elsevier.

Fig. 4

Reaction mechanism of oxidative adsorption of cobalt on hexagonal birnessite. The transformation of the octahedral TCS complex into a smaller tetrahedral TCS complex facilitates the migration of Co(II) through the surface oxygen layer and its subsequent occupation of the vacant octahedral Mn(IV) site. Upon occupying the octahedral vacancy, Co(II) undergoes a transition from the high-spin to the low-spin state, accompanied by a significant distortion of the Co(II) octahedron due to the Jahn–Teller effect. Subsequently, an electron exchange reaction occurs between Mn(IV) and Co(II), leading to the formation of a regular low-spin Co(III) octahedron and a Jahn–Teller distorted high-spin Mn(III) octahedron. Adapted with permission from ref. [217]. Copyright 2022, American Chemical Society. TCS, triple-corner-sharing.

Fig. 5

Relationship between pH and surface charge, charge density, and adsorption amount of Pb²⁺ of different MnO₂. (a) Surface negative charge; (b) charge density; (c) birnessite sample (Mn_AOS = 3.92), (d) birnessite sample (Mn_AOS = 3.83); (e) birnessite sample (Mn_AOS = 3.67). AOS is the abbreviation for the average oxidation state of Mn. The dotted line and the solid line represent the Pb²⁺ adsorption amount calculated by the CD-MUSIC-EDL model at 0.001 mol/L and 0.01 mol/L ionic strength, respectively. Data used in (a) and (b) are from the study by Feng et al. [147], and data used in (c), (d), and (e) are from the study by Zhao et al. [177].

Fig. 6

The relationship between Mn AOS and adsorption capacity for metal ions. (a) Three different MnO₂; (b) birnessites with the same structure but different AOS; (c) relative proportions of Mn(II), Mn(III) and Mn(IV) in birnessite with different Mn AOS; (d) adsorption of lead ions by H-birnessite and OH-birnessite with different Mn AOS; (e) Adsorption of lead ions by H-birnessite with different Mn AOS. Data used in (a) are from the study by Feng et al. [147]; data used in (b) are from the study by Wang et al. [62]; data used in (c) are from Wu et al. [123]; data used in (d) are from Zhao et al. [155]; data used in (e) are from the study by Zhao et al. [154]. AOS, average oxidation state.

Fig. 7

The mechanisms and isothermal curves of Pb²⁺ uptake by Co-doped birnessites and Co pre-adsorbed birnessite. (a) Mechanism for Co-doped birnessites; (b) mechanism for Co pre-adsorbed birnessite; (c) isothermal curve for Co-doped birnessites; (d) isothermal curve for Co pre-adsorbed birnessite. In (c), based on the initial molar ratios of Co/Mn as 0, 0.05, 0.10, and 0.20, the products were named HB, Co-doped B5, Co-doped B10, and Co-doped B20. Accordingly, the products were named HB, Co-ads B2, Co-ads B5, Co-ads B10, and Co-ads B20 in (d), respectively. Data used in this figure are from the study by Yin et al. [161] and Yin et al. [250]. The X-ray absorption fine structure (XAFS) spectra of these birnessite samples were measured on the 1W1B beamline at the Beijing Synchrotron Radiation Facility.