Geochemical and mineralogical constraints in iron ore tailings limit soil formation for direct phytostabilization

Abstract

The present study aimed to characterize key physico-chemical and mineralogical attributes of magnetite iron (Fe) ore tailings to identify potential constraints limiting in situ soil formation and direct phytostabilization. Tailings of different age, together with undisturbed local native soils, were sampled from a magnetite mine in Western Australia. Tailings were extremely alkaline (pH> 9.0), with a lack of water stable aggregate and organic matter, and contained abundant primary minerals including mica (e.g., biotite), with low specific surface area (N-BET around 1.2 m g). These conditions remained relatively unchanged after four years' aging under field conditions. Chemical extraction and spectroscopic analysis [e.g., X-ray diffraction (XRD) and synchrotron-based Fe K edge X-ray absorption fine structure spectroscopy (XAFS) analysis] revealed that the aging process decreased biotite-like minerals, but increased hematite and magnetite in the tailings. However, the aged tailings lacked goethite, a compound abundant in natural soils. Examination using backscattered-scanning electron microscope - energy dispersive X-ray spectrometry (BSE-SEM-EDS) revealed that aged tailings contained discrete sharp edged Fe-bearing minerals that did not physically integrate with other minerals (e.g., Si/Al bearing minerals). In contrast, Fe minerals in native soils appeared randomly distributed and closely amassed with Si/Al rich phyllosilicates, with highly eroded edges. The lack of labile organic matter and the persistence of alkaline-saline conditions may have significantly hindered the bioweathering of Fe-minerals and the biogenic formation of secondary Fe-minerals in tailings. However, there is signature that a native pioneer plant, Maireana brevifolia can facilitate the bioweathering of Fe-bearing minerals in tailings. We propose that eco-engineering inputs like organic carbon accumulation, together with the introduction of functional microbes and pioneer plants, should be adopted to accelerate bioweathering of Fe-bearing minerals as a priority for initiating in situ soil formation in the Fe ore tailings.

Growth of Native Plants on Mined Materials was not Improved by a Commercial Microbial Inoculant

“…Dry-stacked magnetite tailings (hereafter referred to as tailings) were generated as ne-textured (processed by high-pressure grinding to <4 mm) waste materials at the end of magnetite-ore processing. Selected physical and chemical pro les for topsoil and tailings are presented in Cross et al (2021b), with mineralogical characteristics presented in Wu et al (2019).…”

Ecological engineering to accelerate mineral weathering and transformation underpins sustainable tailings rehabilitation

“…In contrast, under arid/semi-arid climatic conditions where Australian Fe-ore mines are located, Febearing minerals in magnetite Fe-ore tailings possess low Gibbs energy and require very long weathering time (e.g. decades) to generate sufficient amounts of secondary Fe-Si mineral gels, which are critical to aggregation of fine particles and development of physical properties in the finely textured and densely compacted magnetite Fe-ore tailings (Table 1) (Wu et al 2019). Crystalline sodalites and other Na-rich alkaline minerals (Gräfe et al 2011;You et al 2019) Strong alkalinity buffered by hydrolysis of sodalites…”

“…biotite, hematite and magnetite) is accelerated by microbial activities upregulated by organic matter in magnetite Fe-ore tailings, leading to the formation of secondary Fe-minerals, essential ingredients for aggregate formation in tailing-soil (Wu et al 2019). These microbial driven geochemical reactions result in the exhaustion of reactive minerals, such as sulfides in sulfidic tailings and sodalites in bauxite residues, leading to long-term hydrogeochemical stabilisation, the foundation for subsequent development of cementation cap in sulfidic and metallic tailings or soil formation in bauxite residues and magnetite Fe-ore tailings (Liu et al 2018;Wu et al 2019;You et al 2019). For example, geochemical reactions enhanced by functional microbes in sulfidic tailings involve coupled acidification, co-dissolution and/or precipitation, and in situ immobilisation/encapsulation of metal(loid)s, leading to the exhaustion of reactive minerals (e.g.…”

“…Extraction of alumina from bauxite has so far generated more than four Gt bauxite residues (or red mud) worldwide, with about 50% located in China (Stanford 2016;Xue et al 2016). In Australia, increasing interests in extracting magnetite-Fe-ore for balancing declining hematite Fe-ore grades have led to the generation of increasing volumes of finely textured and siliceous tailings in semi-arid regions of Western Australia (Wu et al 2019). Unlike natural soil, tailings typically contain various hazardous chemicals and reactive minerals in significant quantities, which are biologically toxic and lack basic physical and chemical properties minimally required to support the colonisation of even extremely tolerant plant species (Huang et al 2014;Li & Huang 2015).…”

“…technosol or ecoengineered soil) in tailings of low pollution risks (e.g. Fe-ore tailings, bauxite residues) (Wu et al 2019; and 2formation of hydrogeochemically stable tailings with high degree of cementation and low risks of acidic/neutral metallic drainages (Liu et al 2018), both of which share a fundamental phase of bioweathering of target minerals in the tailings for relevant purposes (Figure 2). In State 3 ( Figure 2), there are two trajectories, depending on the type of tailings: (3a) formation of functional 'soil' or technosols for tailings of low pollution risks (e.g.…”