Reductive transformation of birnessite and the mobility of co-associated antimony

Highlights

• Sb(V)-birnessite underwent no detectable transformation in the absence of Mn(II).

• Mn(II) caused transformation of Sb(V)-birnessite to manganite and hausmannite at pH 7.5.

• Transformation of birnessite immobilised Sb at pH 5.5 and 7.5.

• Sb(V) was the only detectable solid-phase Sb species for all experimental treatments.

Abstract

Manganese (Mn) oxide minerals, such as birnessite, are thought to play an important role in affecting the mobility and fate of antimony (Sb) in the environment. In this study, we investigate Sb partitioning and speciation during anoxic incubation of Sb(V)-coprecipitated birnessite in the presence and absence of Mn(II)_aq at pH 5.5 and 7.5. Antimony K-edge XANES spectroscopy revealed that Sb(V) persisted as the only solid-phase Sb species for all experimental treatments. Manganese K-edge EXAFS and XRD results showed that, in the absence of Mn(II), the Sb(V)-bearing birnessite underwent no detectable mineralogical transformation during 7 days. In contrast, the addition of 10 mM Mn(II) at pH 7.5 induced relatively rapid (within 24 h) transformation of birnessite to manganite (~93%) and hausmannite (~7%). Importantly, no detectable Sb was measured in the aqueous phase for this treatment (compared with up to ∼90 μmol L⁻¹ Sb in the corresponding Mn(II)-free treatment). At pH 5.5 , birnessite reacted with 10 mM Mn(II)_aq displayed no detectable mineralogical transformation, yet had substantially increased Sb retention in the solid phase, relative to the corresponding Mn(II)-free treatment. These findings suggest that the Mn(II)-induced transformation and recrystallization of birnessite can exert an important control on the mobility of co-associated Sb.

Introduction

Antimony (Sb) is a carcinogenic environmental pollutant that is also of significant industrial demand and economic importance (Herath et al., 2017, Klochko, 2017). Antimony is widely used in the production of plastics, lead-acid batteries and flame retardant formulations (U.S. Geological Survery, 2018). It is also being increasingly utilized in a wide range of emerging new technologies (He et al., 2019, Klochko, 2017).

Dramatic increases in the production of Sb over recent decades, along with mineral processing and historic mining, has led to Sb release into the environment. This has left a legacy of Sb contamination in soil, sediment and groundwater in many areas globally (Borčinová Radková et al., 2020, Johnston et al., 2020, Majzlan et al., 2011, Mudd et al., 2018, Sun et al., 2019a).

In natural systems, Sb exists predominantly as the tri- (Sb(III)) and pentavalent (Sb(V)) oxidation states (Filella et al., 2002: Wilson et al., 2010). The oxidation state significantly influences the toxicity and mobility of Sb with lower toxicity, but higher mobility reported for Sb(V) compared with Sb(III) (Filella et al., 2002, Filella et al., 2009, Mitsunobu et al., 2010). Therefore, understanding environmentally relevant redox processes that influence Sb speciation is essential for accurate risk assessment of Sb-contaminated environments.

Sequestration of aqueous Sb species by mineral-water interactions is an important phenomenon that regulates Sb mobility in environmental systems (Burton et al., 2019, Herath et al., 2017, Wilson et al., 2010). Previous studies demonstrate that Sb sorbs strongly to Mn oxide minerals, such as birnessites (δ-MnO₂). For this reason, Mn oxides are believed to play an important role regulating Sb fate in many oxic to moderately reducing soils and sediments (Belzile et al., 2001, Du et al., 2020; Sun et al., 2019a, Sun et al., 2018, Wang et al., 2012a).

In soils, sediments and groundwater systems, fluctuating redox conditions stimulate biogeochemical Mn redox cycling that can modify the crystal structure or surface chemical characteristics of Mn-oxides (Blöthe et al., 2015; Hens et al., 2019; Lovely, 2000, Villalobos et al., 2006). In Sb-contaminated environments, such redox-induced transformation of Mn-oxide mineral phases may potentially influence the speciation and mobility of co-associated Sb.

It is now well-established that interaction of aqueous Mn(II) with metastable Mn(III/IV) minerals (e.g. birnessite group minerals) can induce the mineralogical transformation of these minerals to more thermodynamically stable phases (Hens et al., 2019; Elzinga, 2011, Elzinga and Kustka, 2015, Lefkowitz et al., 2013, Perez-Benito, 2002). For example, the reaction of birnessite with aqueous Mn(II) has been shown to induce the formation of a wide range of secondary Mn oxides such as, feitknechtite (β-Mn^IIIOOH), manganite (γ-Mn^IIIOOH), groutite (α-Mn^IIIOOH), hausmannite (Mn^IIMn^III₂O₄) and nsutite (γ-Mn^IV,III(O,OH)₂) (Elzinga, 2011, Lefkowitz et al., 2013; Lefkowitz and Elzinga, 2015; Lefkowitz and Elzinga, 2017; Zhao et al., 2016).

Several factors, including the abundance of Mn(II)_aq (i.e., ratio of Mn(II)_aq to structural Mn(IV)) and the solution pH, have been reported to influence both the magnitude of Mn(II)-induced birnessite transformation and the mineralogy of transformation products (Elzinga, 2011, Lefkowitz et al., 2013). For example, Lefkowitz et al. (2013) demonstrated that feitknechtite was a major transformation product at low Mn(II) levels (i.e., low Mn(II)/Mn(IV) ratios), while manganite and hausmannite instead formed in the presence of higher aqueous Mn(II) concentrations at pH ~7.5–8.5. It has also been observed that reaction of Mn(II) with birnessite under acidic conditions (pH 2.4–6) encouraged the formation of a suite of secondary Mn oxides including nsutite, ramsdellite (MnO₂) and groutite (Lefkowitz et al., 2013, Mandernack et al., 1995, Tu et al., 1994).

The effect of pH and Mn(II)_aq concentrations on the extent and mechanisms of the reductive transformation of pure birnessite minerals has been studied previously (e.g., Elzinga, 2011, Lefkowitz et al., 2013). Likewise, the speciation and sorption mechanisms of Sb on pure birnessite has also been studied previously (Sun et al., 2019a, Sun et al., 2018, Wang et al., 2012a). However, to date, there have been no investigations into the reductive transformation of Sb(V)-coprecipitated birnessite. Hence, the secondary mineralogical products of this transformation and the consequences for the mobility and speciation of Sb remain unclear. Since the structural properties (e.g. degree of crystallinity, the number of vacant sites and amount of structural Mn(III)) of the initial Sb-coprecipitated birnessite may be different from those of the pure birnessite phase (Essington and Vergeer, 2015, Ling et al., 2015, Wang et al., 2012b), prevailing transformation pathways and end products are also likely to differ from what is expected for pure birnessite.

To close this gap in current knowledge, we investigate Sb behaviour during Mn(II)-induced alteration of Sb(V)-bearing birnessite in a laboratory-based experiment. This experiment examined the effects of Mn(II)_aq at pH 5.5 and 7 on Mn oxide transformations and explored the associated changes in Sb partitioning and speciation during 7 days of anoxic incubation. The results provide new insights into processes which control Sb mobility and speciation in Mn-reducing environments such as seasonally waterlogged soils and sediments and groundwater systems. The outcomes of this study expand our understanding of the geochemical co-cycling of Sb and Mn in contaminated environments subject to Mn redox transformations.