Elsevier

Chemical Geology

Volume 558, 30 December 2020, 119862
Chemical Geology

Effects of Ni incorporation on the reactivity and stability of hausmannite (Mn3O4): Environmental implications for Mn, Ni, and As solubility and cycling

https://doi.org/10.1016/j.chemgeo.2020.119862Get rights and content

Abstract

Trace metal structural impurities are common in Mn (II/III) oxides, yet their effects on the oxides' reactivity and stability have not been experimentally assessed. The present investigation quantifies such effects for the first time by measuring the extent of mineral dissolution of pristine and Ni-substituted hausmannite (MnIIMnIII2O4) (at 1 and 2 wt% Ni) in 8-h batch reactions at pH 5 with/without arsenite (As(III)). Ni substitution occurred at Mn(III) octahedral sites, causing noticeable structural modification in lattice parameters with a decrease in Jahn-Teller distortion, particularly at 2 wt%. In both acidic and reductive dissolution (with As(III)), the Ni-substituted hausmannite exhibited enhanced Mn release relative to the pristine mineral, with concurrent release of structural Ni increasing with substitution percentage. When As(V) release was normalized by surface area, Ni-substituted hausmannite showed a higher As(III) oxidation percentage than the pristine phase. Further, higher ratios of Mn(II):As(V) were observed in Ni-substituted hausmannite. As K-edge X-ray absorption spectroscopy and attenuated total reflectance-Fourier transform infrared spectroscopy analyses indicated that As(III) oxidation lead to the formation of binuclear bidentate As(V) surface complexes. Enhanced reactivity of Ni-substituted hausmannite may be attributed to lowered mineral stability, which promotes accelerated mineral dissolution and increased structural Mn release, resulting in formation/exposure of highly reactive sites. Thus, structural impurities dictate the properties, reactivity, and stability of the Mn(II/III) oxides, affecting the level of dissolution and the extent of redox reactions, which together impact the fate and cycling of transition metals and metalloids in surface environments.

Introduction

Manganese (Mn) oxides are ubiquitous in nature, and found in various geological settings, ranging from mining areas and ore deposits to sediments in freshwater and marine environments (Chapnick et al., 1982; Chukhrov, 2006; Haack and Warren, 2003; Lee and Xu, 2016; Maynard, 2010; Murray et al., 1984; Post, 1999). They are also among the most reactive mineral phases for adsorption and oxidation reactions, playing critical roles in regulating the speciation and distribution of trace metals and metalloids, as well as the cycling of essential nutrients in natural environments (Chapnick et al., 1982; Greene and Madgwick, 1991; Haack and Warren, 2003; Manceau et al., 2007; Maynard, 2010; Mock et al., 2019; Post, 1999; Taylor, 1968; Tebo et al., 2005; Vodyanitskii, 2009; Wang et al., 2019; Wu et al., 2018). In these natural Mn oxides structural impurities are commonly observed (Chao, 1976; Maynard, 2010; Post, 1999). Structural incorporation of Ni has been noted in oceanic Mn nodules and crusts (Burns, 1993; Manceau et al., 2007; Peacock and Sherman, 2007), and also in terrestrial Mn-containing mineral and rock samples (Taylor et al., 1964; Taylor and McKenzie, 1966). Therefore, knowledge of the effects of metal impurities on the reactivity and stability of Mn oxides is essential to better understand the geochemical behavior of natural Mn oxides and the fate and mobility of associated transition metal(loid)s.

To date, effects of trace metal impurities on Mn oxide reactivity have been examined primarily for birnessite, the most common natural Mn(II/III/IV) oxide. For example, a recent study by Wang et al. (2019) showed that divalent cations (Zn, Mg and Ca) substituted in the birnessite lattice affected the mineral's ability to adsorb and oxidize fulvic acid, and modified its transformation processes to varying degrees. In addition, Zn appeared to influence the kinetics of the mineral's reductive transformation processes by Mn(II), where Zn-coprecipitated birnessite exhibited a faster conversion rate from birnessite into feitknechtite (β-MnOOH), a metastable Mn-(hydr)oxide, than the pristine phase, but slower conversion of feitknechtite into more stable manganite (γ-MnOOH) due to the lowered Mn(II) concentration (Zhao et al., 2018). In another study, the structural incorporation of Co(III) in birnessite was observed to inhibit the transformation process into todorokite, producing mineral mixtures of phyllomanganates and todorokite-like tectomanganates (Wu et al., 2019). Transition metals that are originally adsorbed on the surface of birnessite have also been shown to become structurally incorporated during the mineralogical transformation of the adsorbent. For instance, Zn(II)-adsorbed birnessite is converted into a Zn-substituted hausmannite (Zn(II)1-xMn(II)xMn(III)2O4) through reductive transformation by Mn(II) at circumneutral pH (Lefkowitz and Elzinga, 2015). Similarly, the mineralogical transformation of Ni(II)-sorbed birnessite by Mn(II) results in the formation of Ni-substituted feitknechtite (Lefkowitz and Elzinga, 2017).

Despite accumulating evidence for the importance of metal substituents in affecting Mn oxide reactivity and transformation (Elzinga, 2011; Green et al., 2004; Greene and Madgwick, 1991; Lefkowitz et al., 2013; Lefkowitz and Elzinga, 2015; Manceau et al., 1992; Tebo et al., 2005; Vodyanitskii, 2009; Wang et al., 2018), no studies exist for Mn(III) oxides. Of particular importance is hausmannite (MnIIMnIII2O4), which is a common metastable Mn(II/III) oxide found in various geological settings in both bulk and nano-scale mineral phases (Elzinga, 2011; Green et al., 2004; Greene and Madgwick, 1991; Hem, 1978; Lefkowitz et al., 2013; Lefkowitz and Elzinga, 2015; Pardee, 1927; Tebo et al., 2005; Vodyanitskii, 2009; Wang et al., 2015). It is also one of the secondary Mn(III)-containing mineral phases produced upon reductive transformation of birnessite by mild changes in environmental conditions (Lefkowitz et al., 2013). Further, hausmannite is the most widely distributed spinel structured Mn oxide, with Mn(II) occupying the tetrahedral sites, and Mn(III) in the octahedral sites (O'Neill and Navrotsky, 1983; Post, 1999, Post, 1992). These octahedral Mn(III) sites present structural distortions due to the Jahn-Teller effect. In general, minerals with the spinel structure are capable of housing a broad range of chemical substituents (Dong et al., 2013; Hirai et al., 2016; Li et al., 2014). Hausmannite is no exception, and may accommodate structural incorporation of transition metals such as Ni, Co, and Zn, due to the similar radii of the metal substituents to Mn(II/III) and their close geological association (Green et al., 2004; Lefkowitz and Elzinga, 2015; Shacat et al., 2004). In particular, the dissolution reactions of hausmannite has been related to the pronounced release of Ni in natural aquatic systems, such as Lake Vanda and Lake Joyce (Green et al., 2004; Shacat et al., 2004), yet the fate and geochemical reactivity of Ni-containing hausmannite has been rarely investigated.

In the present study we systematically investigated the effects of Ni impurities on the mineral structure and characteristics, as well as the mineral reactivity and stability of hausmannite by using both pristine and Ni-substituted phases. Specifically, we measured the extent and rate of acidic and reductive mineral dissolution at pH 5 with/without arsenite (As(III)), using these as a means of assessing the mineral's reactivity and stability. Ni is redox-inactive, and hence, does not involve with the As(III) oxidation reaction, which allows us to solely detect changes made in the mineral's reactivity and stability by substitution. Furthermore, we employed a combination of techniques including powder X-ray diffraction (PXRD), synchrotron-based X-ray absorption spectroscopy (XAS), and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) for mineral solids characterization before and after batch reactions and to study the reaction in situ. Based on these complementary measurements, we demonstrate that Ni substitution into the hausmannite structure significantly changes the mineral's reactivity toward acidity and/or As(III) oxidation, highlighting the importance of considering the effect of impurities.

Section snippets

Materials

All chemical agents and reference materials for the present study were of analytical grade or better, including manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O, 99+%, Acros Organics), nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O, 99+%, Acros Organics), nickel chloride hexahydrate (NiCl2·6H2O, 98+%, Acros Organics), acetone (CH3COOH, ≥99.5%, Thermofisher Chemicals), ethyl alcohol (C2H5OH, 200 proof, Pharmco-Aaper), sodium (meta)arsenite (NaAsO2, ≥90%, Aldrich Chemistry), sodium arsenate

Effects of Ni substitution on hausmannite structure and characteristics

The effects of Ni substitution on the hausmannite structure and characteristics were systematically examined by comparing Ni-substituted hausmannite to pristine hausmannite. Of interest were changes in the crystalline structure, particle size, and surface area induced by Ni substitution into the hausmannite structure.

First, PXRD patterns of Ni-substituted hausmannite minerals matched that of the pristine and the reference material (Mn3O4), indicating that no other mineral phases were present in

Conclusion and environmental implications

Natural Mn oxides commonly have structural impurities; however, the impacts of metal substituents on the geochemical behaviors of Mn(II/III) oxides have never been experimentally assessed. The present study investigated hausmannite (MnIIMnIII2O4), the most widely distributed spinel structured Mn oxide in the environment, and demonstrates for the first time that the incorporation of trace metal Ni(II) strongly affect the stability and redox reactivity of this mineral. This suggests that the

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The present study used 6BM and 7BM beamlines of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. We thank the 6 BM and 7 BM beamline scientists, Bruce Ravel (6 BM), and Syed Khalid and Steven Ehrlich (7 BM), for their support on XAS data acquisition and analysis. We also acknowledge financial support from the National Science

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