Mechanisms of selenium removal by partially oxidized magnetite nanoparticles for wastewater remediation

Highlights

• Oxidized magnetite can effectively remediate selenite contaminated wastewaters.

• No reduction processes are required to remove large amounts of Se oxyanions.

• Selenite immobilization is driven by formation of inner-sphere adsorption complexes.

• Partially oxidized magnetite nanoparticles behave like maghemite phases.

Abstract

Magnetite nanoparticles are a promising cost-effective material for the remediation of polluted wastewaters. Due to their magnetic properties and their high adsorption and reduction potential, they are particularly suitable for the decontamination of oxyanion-forming contaminants, including the highly mobile selenium oxyanions selenite and selenate. However, little is known how the remediation efficiency of magnetite nanoparticles in field applications is affected by partial oxidation and the formation of magnetite/maghemite phases. Here we characterize the retention mechanisms and capacity of partially oxidized nanoparticulate magnetite for selenite and selenate in an oxic system at different pH conditions and ionic strengths. Data from adsorption experiments showed that retention of selenate is extremely limited except for acidic conditions and strongly influenced by competing chloride anions, indicating outer-sphere adsorption. By contrast, although selenite adsorption capacity of oxidized magnetite is also adversely affected by increasing pH, considerable selenite quantities are retained even at alkaline conditions. Using spectroscopic analyses (XPS, XAFS), both mononuclear edge-sharing (²E) and binuclear corner-sharing (²C) inner-sphere selenite surface complexes were detected, while reduction to Se(0) or Se(–II) species could be excluded. Under favourable adsorption conditions, up to ~pH 8, the affinity of selenite to form ²C surface complexes is higher, whereas at alkaline pH values and less favourable adsorption conditions ²E complexes become more dominant. Our results demonstrate that magnetite can be used as a suitable reactant for the immobilization of selenite in remediation applications, even under (sub)oxic conditions and without the involvement of reduction processes.

Introduction

Selenium (Se) pollution of soils, sediments and waters is a global phenomenon. Although Se is a naturally occurring trace element, the major cause of Se release and contamination are human activities such as coal production and combustion, phosphate and sulphide-ore mining, metal processing, oil refining, waste disposal or agricultural irrigation (Dhillon and Dhillon, 2003; Lemly, 2004; Tan et al., 2016). Additionally, Se occurs in vitrified high-level nuclear waste (HLW) in the form of the long-lived, harmful radionuclide ⁷⁹Se and plays a major role in the long-term safety assessment of HLW repositories (Bingham et al., 2011; De Cannière et al., 2010; Frechou et al., 2007).

In aquatic systems, Se levels can rapidly become toxic endangering not only the health of aquatic organisms but also of other beings, including humans, due to bioaccumulation in the food chain (Lenz and Lens, 2009; Munier-Lamy et al., 2007). Mobility and bioavailability of Se in water is mainly controlled by its solubility and retention through biogeochemical processes (Dhillon and Dhillon, 2003; Fernández-Martínez and Charlet, 2009), which are largely determined by the prevailing Se oxidation state and speciation (Fig. 1a). Under reducing conditions, Se forms sparingly soluble minerals and compounds, primarily elemental selenium (Se⁰) and metal selenides (Se(–I) and Se(–II)) (Fig. S1). Under (sub)oxic conditions, however, inorganic Se occurs in the oxidation state Se(IV) and Se(VI) as highly soluble and mobile oxyanions selenite (SeO₃^2-) and selenate (SeO₄^2-). Both selenite and selenate can appear in protonated or deprotonated form, depending on the solution pH (Fig. 1b).

A key factor controlling the fate of dissolved Se oxyanions in aquatic environments is sorption on geological materials. Of special relevance in this context are metal (hydr)oxides and particularly iron (hydr)oxides due to their high affinity towards oxyanion-forming pollutants such as Se, As, Sb, V, Mo or Cr (Börsig et al., 2017; Chan et al., 2009; Nakamaru and Altansuvd, 2014; Weidner and Ciesielczyk, 2019). Besides adsorption, reduction of oxyanions to less soluble compounds is an important abiotic immobilization process (Kirsch et al., 2008; Scheinost et al., 2008; Wilkin et al., 2005), driven by Fe⁰ or by Fe(II)-bearing iron (hydr)oxides, yet depends strongly on the prevailing redox conditions.

Magnetite [Fe₃O₄] is one of the most widespread iron (hydr)oxide minerals in nature and is also known as a sink for dissolved oxyanions due to its high adsorption and reduction potential. Due to these properties and its magnetic character, the use of magnetite is considered in various in situ and ex situ environmental remediation approaches, including wastewater treatment or soil remediation (Kuppusamy et al., 2016; Usman et al., 2018). Particularly for decontamination of wastewaters based on removal of hazardous oxyanions by nanoremediation, nanoparticulate magnetite is a promising cost-effective material (Chowdhury and Yanful, 2010; Horst et al., 2015; Li et al., 2017), as shown by several previous studies (Jordan et al., 2009; Loyo et al., 2008; Martínez et al., 2006; Missana et al., 2009; Scheinost and Charlet, 2008; Usman et al., 2018).

For field applications, however, it must be taken into account that reactivity and capacity of a remediation material is highly dependent on its stability under various environmental conditions. This applies in particularly for redox- and pH-sensitive minerals such as magnetite and other reduced or mixed-valent iron minerals like zerovalent iron (ZVI) and green rust (Börsig et al., 2018; Génin et al., 2006; Liu et al., 2014; Mu et al., 2017). A common phenomenon for magnetite is thereby the transformation into maghemite [γ-Fe₂O₃] in (sub)oxic environments (Fig. S1) due to partial or complete oxidation (Iyengar et al., 2014; Li et al., 2019; Rebodos and Vikesland, 2010).

$$4F{e}^{II}F{e}_2^{III}{O}_4+O_2\to 6\gamma ‐F{e}_2^{III}{O}_3$$

Maghemite is the Fe(II)-free oxidation product of magnetite and since it has the same spinel crystal structure as magnetite, both minerals represent the end members of a solid solution series (Gorski and Scherer, 2010; Iwatsuki and Fukasawa, 1993). Oxidation of magnetite generally results in the formation of a maghemite surface layer. However, while this maghemite layer can protect the bulk of the underlying magnetite from further oxidation for larger sized (non-nano) particles, nanoparticulate magnetite is more vulnerable to oxidation (He and Traina, 2005; Khan et al., 2015; Rebodos and Vikesland, 2010). In this case, oxidation can lead to significant amounts of maghemite in the near-surface region (core-shell structure) or even to complete transformation (Kuhn et al., 2002; Sharifi Dehsari et al., 2018; Signorini et al., 2003).

In order to assess the efficiency of magnetite for remediation measures, it is therefore important to know to what extent oxidation and formation of a mixed magnetite/maghemite phase (non-stoichiometric magnetite) affects the retention of pollutants in (sub)oxic systems, particularly in terms of the specific mechanisms. The aim of this paper was therefore to characterize the retention efficiency of partially oxidized magnetite with respect to dissolved Se oxyanions under oxic conditions. Although a respectable number of studies have dealt with the capacity of Se oxyanion retention by magnetite in the context of wastewater treatment or HLW disposal (Gonzalez et al., 2010; Jordan et al., 2009; S. S. Kim et al., 2012; Martínez et al., 2006; Verbinnen et al., 2013; Wei et al., 2012), studies investigating the sorption mechanisms at the molecular level are rather rare (Loyo et al., 2008; Missana et al., 2009). Besides, there is to our knowledge no publication dealing with the retention of selenium oxyanions specifically on partially oxidized magnetite nanoparticles as they would occur in (sub)oxic wastewater treatment applications. In this study, we examined the interaction of selenite and selenate with pre-oxidized magnetite in adsorption experiments at various hydrochemical conditions, including pH and ionic strength. By using hydrochemical data and a combination of solids analyses, we were able to determine the Se retention capacity and stability as well as the involved immobilization mechanisms.