Stability of naturally occurring AMD–schwertmannite in the presence of arsenic and reducing agents

Highlights

• Secondary iron precipitates from AMD are efficient scavengers of metal(loids).

• Ion exchange was found to be a key mechanism in the accumulation of metal(loids).

• Metal(oids) stabilized secondary iron minerals, delaying the remobilization processes.

• The use of XRD and XAS was crucial to understand mineral transformation processes.

• pH, redox and metal(loids) are key factors in the long-term stability of AMD minerals.

Abstract

Secondary iron oxides formed in acid mine drainage, such as schwertmannite, are scavengers for metal(loid)s in mining environments. Increasing the understanding of the geochemical transformations of these minerals, as well as knowing how metal(loid)s affect these transformations, is crucial to ultimately predict the fate of these trace elements in acidic mine drainage and to minimize the potential environmental risk. In this study, transformation experiments have been conducted with a schwertmannite-rich sediment collected from a mining area and with synthesized schwertmannite as a reference material. The transformation of schwertmannite into goethite was studied as a function of the presence of arsenic, pH value, and redox conditions. Arsenic delayed the mineral transformation from pseudo-stable amorphous phases to more stable crystalline forms, especially at higher arsenic loadings and more acidic pH. Experiments in the presence of Fe(II) and ascorbic acid have proven that both components promote the mineral transformation or reductive dissolution of schwertmannite under anoxic conditions. The presence of arsenic reduced the catalytic effect of Fe(II), stabilizing the schwertmannite particles. On the other hand, arsenic had no effect on the reductive dissolution at these conditions when ascorbic acid was used as a reducing agent.

Introduction

Contamination of soils and aqueous systems affected by acid mine drainage (AMD) is a serious threat that demands the attention of the scientific community (Moodley et al. 2018). Among the environmental risks derived from AMD, the extremely low pH of affected waters, the high concentration of sulphate and heavy metals either dissolved or accumulated in adjacent soils or the impact on the local flora and fauna, can be mentioned. In anoxic environments, Fe-S biogeochemistry can control trace metals and metal(loid)s bioavailability and acidity generation. However, sulphide minerals undergo oxidation upon exposure to air following natural weathering processes and anthropogenic activities (Karimian et al. 2018). This oxidation process leads to the formation of large amounts of iron and aluminium secondary minerals, which determines the mobility of trace elements in AMD systems. Although iron (hydr)oxides have been part of the problem, due to their natural presence and transformation in mining areas generating extreme acidity in soils and streams upon sulphides oxidation, they are also known to be part of the solution due to their implication in natural attenuation processes (Asta et al. 2010; Carlson et al. 2003). This natural ability of iron minerals has been considered in the elaboration of some recent remediation techniques, such as Technosols, which may contain acid mine water or mining sediments in their composition along with other agro-industrial wastes (Arán et al. 2018). The accumulation of trace elements by iron and aluminium secondary precipitates have thus the advantage that toxic elements are immobilised in the sediments and therefore the risk associated to the spreading of these elements into the environment is reduced. Another advantage of the accumulation of trace elements by secondary precipitates could be the possibility to recover valuable elements, such as Rear Earth Elements (REE) which are defined as critical raw materials. REE are present in AMD sites in several orders of magnitude higher than in natural environments due to their affinity to schwertmannite and basaluminite (Lozano et al. 2019; Lozano et al. 2020).

In this sense, the role of iron precipitates formed in AMD has been studied from the microscopic and macroscopic points of view (Antelo et al. 2013; Asta et al. 2010; Baleeiro et al. 2018; Jönsson et al. 2006) to assess their potential use by exploiting their sequestration or immobilisation properties. The pH, along with time, temperature, composition of the solid and aqueous phases and oxidizing capacity of the medium, are key factors that determine the type of iron mineral that is formed in AMD affected areas (Nordstrom and Alpers 1999; Bigham and Nordstrom, 2000). Whereas amorphous pseudo-stable phases, schwertmannite and ferrihydrite, precipitate in the lower pH range, 2.8–6.5, crystalline, thermodynamically stable phases such as goethite are formed at pH > 6.5.

Schwertmannite, typically formed in AMD environments, can remain stable for years under acidic conditions (Regenspurg et al. 2004), although it tends to transform into goethite depending on the environmental conditions, being the pH an important factor (Antelo et al. 2013; Kumpulainen et al. 2008; Parviainen et al. 2015; Regenspurg et al. 2004). Considering the ability of schwertmannite to remove As and other potentially toxic species from solution (Carlson et al. 2003; Regenspurg and Peiffer 2005), it is common in AMD environments to find schwertmannite enriched with these species in its structure. It has been proven that the presence of Cu(II) in the schwertmannite structure does not only decrease the transformation rate to goethite, but also increases its capacity for As retention by a factor of ~25% (Antelo et al. 2013). The sorption capacity of schwertmannite drastically decreases during the transformation towards more stable and crystalline forms (Antelo et al. 2013; Baleeiro et al. 2018; Burton and Johnston 2012), which emphasizes the importance of preventing schwertmannite from ageing to naturally mitigate the mobility of trace metals and metal(loid)s in AMD or acidic landscapes. Burton et al. (2010) and Fan et al. (2019a) also reported a delay in the schwertmannite-goethite transformation under Fe(II)-rich reducing conditions in the presence of arsenate or chromate ions. Therefore, the presence of structural or adsorbed oxyanions produce an inhibitory effect on the transformation of schwertmannite towards more stable mineral phases (Regenspurg and Peiffer 2005; Schoepfer et al. 2017).

The aim of the present study was to analyse the effect that the incorporation of arsenic and the presence of reducing agents such as Fe(II) or ascorbic acid have on the stability and adsorption behaviour of iron precipitates collected from an AMD affected area as well as on its synthetic analogue, schwertmannite. Arsenic and other metal(loid)s can be found associated with sulphide minerals and their oxidation products in mining environments. The formation of schwertmannite as a consequence of the sulphide minerals oxidation in AMD areas provides large surface areas and functional groups that allow the adsorption of pollutants, predominantly metals and metal(loid)s. We intend to contribute to a better understanding of the processes and factors that can prevent schwertmannite from ageing and thus pollutants from being remobilised into the soil solution or aqueous systems.