Enhanced removal of antimony by acid birnessite with doped iron ions: Companied by the structural transformation
Graphical abstract
Introduction
Antimony is a priority control pollutant that more complex to control than arsenic. Excessive exposure to antimony through digestive and respiratory activities can lead to deleterious health concerns, and the toxicity of Sb(III) is 10 times than that of the Sb(V) (Shtangeeva et al., 2011; He and Wan, 2004; Nakamaru and Altansuvd, 2014). In most cases, the concentration of antimony is less than 1 mg/L in natural water (Filella et al., 2002a, b), but due to human activities, the antimony concentrations in some contaminated water are 100–7000 mg/kg (Guo et al., 2009; Hammel et al., 2000).
Manganese oxide, which shows strong oxidation reactivity, can effectively oxidize Sb(III) into less toxic and mobile Sb(V) (Liu et al., 2015). But it is poor to adsorb anions such as arsenate (Gude et al., 2017), chromate (Gheju et al., 2016), and antimonite (Luo et al., 2017b) due to a large amount of negative charges exists on the surface of manganese oxide. In order to improve the adsorption of anionic heavy metals, iron-manganese composite was developed (Zhu et al., 2015; Zhang et al., 2007, 2009). This composite consists of amorphous iron hydroxide and manganese hydroxide, the latter of which is used for oxidation while the former is used for adsorption. However, due to the amorphous structure of the iron-manganese composite, its structure and the influence of material phase transition during the adsorption process are difficult to study, to say nothing of control, which poses difficulties for further material modification to improve the oxidation and adsorption capabilities.
AB is widely present in nature. It is a kind of layered manganese oxide minerals, whose layer is connected by [Mn(IV)O6] or [Mn(III)O6] octahedrons. The presence of [Mn(III)O6] octahedron or octahedral vacancies caused by octahedral center Mn defects creates a large amount of negative charges (Silvester et al., 1997). During recent years, the researches on the adsorption coordination mechanisms of metal ions by AB have made great progress, such as Cu2+ (Kwon et al., 2013; Peña et al., 2015), Zn2+ (Kwon et al., 2013), Cd2+ (Van Genuchten and Pena, 2016), Cr(III) (Landrot et al., 2012a). As for arsenic, among all kinds of manganese oxide, AB has the highest adsorption capacity (Feng et al., 2006). In general, the adsorption of heavy metals by birnessite is always related to the structure, octahedral vacancy, manganese oxidation degree, morphology and surface area, as well as the distribution of various cations.
On the other hand, the structure transformation during the treatment of heavy metals has an adverse effect on the removal capability of heavy metals. It was found that hexagonal layer birnessite (HexLayBir) transformed to orthogonal layer birnessite (OrthLayBir) at aqueous low Mn(II)/Mn (in HexLayBir) molar ratios and pH ≥ 8 (Zhao et al., 2016). HexLayBir commonly shows a stronger adsorption and oxidation reactivity than OrthLayBir (Tang et al., 2014; Zhao et al., 2009). At addition, the hexagonal layer birnessite could converted to triclinic birnessite (β-MnIIIOOH and γ-MnIIIOOH), which reduced the adsorption capacity of heavy metals (Landrot et al., 2012a).
The modification of birnessite by doping heavy metal ions such as Co2+ (Yin et al., 2011), nickel (Yin et al., 2014), Fe3+ (Yin et al., 2013) and Cu2+ (Eren and Gumus, 2014) to improve adsorption capacities for heavy metals had been researched, but these studies did not consider the structure changes and its effects on the adsorption performance.
In this work, iron ions are incorporated into the crystal AB, to combine the oxidation reactivity of manganese oxide with the adsorption capacity of iron for antimony removal. Subsequently, by studying on the dynamics and thermodynamics of Sb(V) and Sb(III), as well as the solid phase transformation, the mechanisms of adsorption and oxidation of Sb(III) by Fe-doped acid birnessites and the effect of the solid phase transformation on the removal of heavy metals are revealed.
Section snippets
Materials and methods
All chemicals used in this study were analytic grade purchased from Sinopharm Chemical Reagent Shanghai Co. Ltd.
Powder XRD
The crystal structures of doped samples were measured by XRD. As shown in Fig. 1a four broad peaks at 12.16°, 24.71°, 36.81° and 66.12° 2θ Cu-K(alpha) are corresponding to the (001), (002), (−111), and (114) planes ascribed to that of AB (JCPDS No.00-013-0105) with poor crystallinity. The broadening of diffraction peaks at 2.42 Å and 1.41 Å are due to the overlapping of the polycrystalline diffraction peaks. The XRD patterns of Fe-doped samples also remained the characteristic peaks of AB at
Conclusion
The adsorption capacities of Sb(III) by AB, 5% Fe, and 10% Fe were 524 mg/g, 674 mg/g, and 759 mg/g, respectively, far higher than amorphous iron and manganese oxide. The maximum adsorption capacities of Sb(V) by 5% Fe and 10% Fe were determined to 47.2 mg/g and 52.3 mg/g, respectively, while the undoped AB hardly adsorbed Sb(V). It could draw a conclusion that doping iron could greatly improve the adsorption capacities for both Sb(V) and Sb(III).
The kinetics of Sb(III) removal by Fe-doped
Acknowledgments
The authors acknowledge the financial support from National Key Research and Development Program of China (2016YFA0203101, 2017YFA0207204), the National Natural Science Foundation of China (Grant No. 21876190 and 21836002), the Key Research and Development Program of Ningxia (2017BY064), and the “One Hundred Talents Program” in Chinese Academy of Sciences.
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