Reactivity of binary manganese oxide mixtures towards arsenite removal: Evidence of synergistic effects
Introduction
Manganese (Mn) is the third most abundant transition metal, and occurs in various (oxyhydr)oxide forms (hereafter, Mn oxides) in the Earth's crust (Post, 1999). Mn oxides possess large specific surface areas, structural variants (e.g., impurities, defects, and vacancy sites), and multiple oxidation states (Mn(II/III/IV)) within the minerals, as well as an ability to achieve facile interconversions between these different Mn oxidation states (Elzinga, 2011; Ilton et al., 2016; Luo et al., 2018; Peña et al., 2007; Post, 1992, 1999). These unique characteristics make Mn oxides highly reactive towards a wide range of inorganic and organic contaminants (Atique Ullah et al., 2017; Eitel et al., 2018; Hu et al., 2016; Shaughnessy et al., 2003; Taujale et al., 2016; Taujale and Zhang, 2012; Wilk et al., 2005; Yin et al, 2011, 2014; Zhang et al., 2015) and thus they are considered to be influential sorbents and strong oxidants in the environment (Fischel et al., 2015; Lefkowitz and Elzinga, 2015; Nicholson and Eley, 2007; Peacock, 2009; Post, 1999; Simanova et al., 2015; Simanova and Pena, 2015; Taylor and McKenzie, 1966; Villalobos, 2015; Ying et al., 2012).
Arsenic (As) is one of the most frequently studied environmental contaminants, and its concentration in drinking water is enforced at 10 μg per liter (10 μg L−1) by the US Environmental Protection Agency (US EPA) and the World Health Organization (WHO) due to its adverse health effects. A large body of research has demonstrated that Mn oxides effectively oxidize more toxic arsenite (As(III)) to the less toxic arsenate (As(V)) (Barreto et al., 2020; Chiu and Hering, 2000; Fischel et al., 2015; Lafferty et al., 2010a; Manning et al., 2002; Mock et al., 2019; Parikh et al., 2010; Shumlas et al., 2016; Silva et al., 2013; Song et al., 2020; Wu et al., 2018), although the extent of the oxidative and subsequent adsorptive reactions of As with Mn oxides varies by mineral structure (Fischel et al., 2015), structural impurity (Song et al., 2020), pH (Barreto et al., 2020; Lafferty et al., 2010a; Parikh et al., 2008; Shumlas et al., 2016; Silva et al., 2013; Wu et al., 2018), as well as the presence of competing ions (Chiu and Hering, 2000; Lafferty et al., 2011; Mock et al., 2019; Parikh et al., 2010; Wu et al., 2018). In general, these research efforts have employed single mineral systems to evaluate the reactivity of Mn oxides towards As. However, single mineral systems may not closely reflect natural settings, considering that mixed Mn oxide phases are exceedingly common (Post, 1992, 1999; Taujale et al., 2016; Taujale and Zhang, 2012; Zhang et al., 2015) and phase transformations in Mn mineralogy readily occur by mild environmental changes (Elzinga, 2011; Hem and Lind, 1994; Lind, 1988; Luo et al., 2018; Ramstedt and Sjöberg, 2005). Thus, to more accurately understand how As interacts with Mn oxides in the environment, and to better inform potential remediation strategies, analyzing mixed mineral systems with As(III) is required.
While limited, there has been growing scientific and industrial interest in using mixed mineral phases for As removal in waterways. For instance, goethite (α-FeOOH) and birnessite (MnO2) were used in a Donnan reactor to examine the relative extent and magnitude of As adsorption and As(III) oxidation between the two mineral phases, as a representative binary system of natural environments (Ying et al., 2012). Further, the superior efficacy of natural oxide samples that contained mixtures of birnessite, goethite, and hematite (Fe2O3) in As removal from discharge in landfill sites has been demonstrated (Deschamps et al., 2005). Alternatively, synthetic Fe-Mn binary oxides (FeMnOx) have also shown to be effective in As removal from wastewater streams (Zhang et al., 2007; Zhou et al., 2020). In addition, the single and binary oxides of magnetite (Fe3O4) and hausmannite (Mn3O4) were examined for their ability to remove As; where the highest As removal was recorded with the Fe-Mn binary nanomaterials composed of 50% Fe and 50% Mn (Garcia et al., 2014). Yet, no study has evaluated the reactivity of mixed phases in the Mn oxide family towards As(III) removal.
In the present study we aim to address this critical knowledge gap by investigating a mixed mineral system of Mn oxides, namely hausmannite (Mn3O4) and manganite (γ-MnOOH), and quantifying the adsorptive and oxidative capacity first of the single Mn oxides (i.e., single system, hausmannite only and manganite only) and then of Mn oxide mixtures (i.e., binary system, both minerals with varying ratios) at two different pHs, using As(III) as a probe. Hausmannite and manganite are the most common Mn(III)-containing oxides and are often found as mixtures in natural settings, as they can readily undergo transformation processes by mild environmental changes (Elzinga, 2011; Lefkowitz et al., 2013; Lind, 1988; Lind and Hem, 1993; Luo et al., 2018; Peña et al., 2007). Both minerals have also shown to be effective at oxidizing As(III) to As(V) (Barreto et al., 2020; Chiu and Hering, 2000; Guo et al., 2015; Silva et al., 2013; Song et al., 2020). Thus, binary systems composed of hausmannite and manganite may address the complexity and heterogeneity present in both natural and engineered systems, adequately serving as a model to assess changes in the environmental reactivity of Mn oxide mixtures toward As.
Specifically, three mineral mixtures were created for the binary system to depict scenarios where the minerals were present in equal weight percent (wt.%) (e.g., Haus 50 wt% and Mang 50 wt%, hereafter HM11) or where one dominated (e.g., Haus 20 wt% and Mang 80 wt% (hereafter, HM14), or Haus 80 wt% and Mang 20 wt% (hereafter, HM 41)). Then, the As(III) removal (i.e., sum of As(V) produced and As adsorbed), Mn release, Mn(II):As(V) ratio, as well as the rate of As(V) or Mn(II) production, were measured and compared between the single and binary mineral phases. In order to determine As speciation on the mineral surfaces, X-ray photoelectron spectroscopy (XPS) and quick X-ray absorption spectroscopy (QXAS) analyses were used. High resolution transmission electron microscopy (HRTEM) was also employed to (1) identify any changes in mineral morphology as a result of As(III) oxidation, (2) visually confirm the formation of other Mn oxide phases induced by As(III) oxidation, and finally (3) investigate the aggregation structure and morphology of mineral mixtures in the binary system. Thus, the results of this study provide mechanistic understanding of the reactivity of different mixed mineral systems toward As oxidation and adsorption and how they diverge from single mineral systems and hence, potentially aid our ability to better predict As removal processes in both natural and industrial settings.
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), manganese chloride (MnCl2·4H2O, 99+%, Acros Organics), acetone (CH3COOH, ≥99.5%, Thermo Fisher Chemicals), ethyl alcohol (C2H5OH, 200 proof, Pharmco-Aaper), potassium persulfate (K2S2O8, ≥99%, Fisher Chemical), sodium (meta)arsenite (NaAsO2, ≥90%, Aldrich Chemistry), sodium arsenate dibasic heptahydrate (Na2HAsO4
Mineral identification and characterization
Freshly synthesized minerals of Haus and Mang were analyzed for crystal structure, SA, size, as well as morphology, prior to their use in a series of batch reactions. First, the PXRD patterns of Haus and Mang well-matched those of the reference materials, hausmannite (AMCSD 0002024) and manganite (AMCSD 0010565), respectively, indicating that no other Mn mineral phases were present in the samples (See supporting information (SI) 1.1, Fig. S1). Secondly, the BET SA of Haus, Mang, and HM11 was
Conclusions
The present study systematically compared and contrasted single and binary Mn oxide systems for As(III) removal to better characterize the complexity and heterogeneity of natural environments, where single mineral phases of Mn oxides rarely exist, but mixed phases dominate. While the single Haus system was more effective at removing As(III), either by oxidation and/or adsorption, than Mang, in the binary mineral system the efficacy of As(III) removal increased as the Mang proportion in 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.
Acknowledgments
This research was supported by the National Science Foundation under Grant No. 2003866. The authors acknowledge the support by the 7-BM beamline scientists, Syed Khalid and Steven Ehrlich, for QXAS data acquisition and analysis. The authors also acknowledge the use of 7-BM beamline 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.
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