Research PaperImpacts of coexisting mineral on crystallinity and stability of Fe(II) oxidation products: Implications for neutralization treatment of acid mine drainage
Graphical Abstract
Introduction
In the environment, acid mine drainage (AMD) occurs during the weathering of rocks and has evolved into a global environmental hazard with increased human activity, such as civil works and mining operations, in and around mining areas (Tong et al., 2021, Lazo, 2020). It is characterized by low pH, high concentrations of Fe2+ and high concentration of toxic substances such as arsenic and selenium, which can harm the soil and depositional environments (Akcil and Koldas, 2006, Tabelin et al., 2020, Sheoran and Sheoran, 2006). The addition of alkalinity-generating substances (e.g. sodium hydroxide) is one of the common AMD treatment techniques that lead to the formation of poorly crystalline iron oxyhydroxides (Akcil and Koldas, 2006). The iron oxyhydroxides such as ferrihydrite and green rust are eventually converted to stable iron oxides such as goethite, lepidocrocite, magnetite, maghemite, hematite, schwertmannite, akaganeite, jarosites and ferric arsenates. The Fe(III) precipitates formed are subject to changes in the physical and chemical conditions of the environment. Previous studies on AMD treatment mainly focused on the influence of pH, organic matter, and co-existing metal ions on AMD waste (Bigham et al., 1996, Gray, 1998, Jönsson et al., 2006, Balintova and Petrilakova, 2011, Yim et al., 2015, Igarashi et al., 2020). Furthermore, the effects of complexing agents, zero-valent metals, and alkaline/industrial materials on AMD formation and treatment sludge/tailings stability were also investigated (Diao et al., 2021, Hu et al., 2018, García-Valero et al., 2020, Singh and Chakraborty, 2021).
In fact, coexisting minerals can also affect the transformation of metastable iron minerals and the crystallization of Fe(III)/Fe(II) coprecipitated products. The transformation of ferrihydrite to crystalline iron oxides (goethite and hematite) was obstructed in the presence of clay minerals and gibbsite (Schwertmann, 1988). Wang et al (Wang et al., 2022). found that when hematite and aluminium co-exist, the admixture of aluminium causes a morphological change in the crystal structure within the hematite from rhombohedral crystals to lamellar disks, leading to the formation of goethite and new hematite crystals. A study by Krishnamurti et al (Krishnamurti, 1998). showed that the rapid oxidation of Fe(II) by montmorillonite hindered the crystallization of lepidocrocite and goethite, but promoted the formation of crystallized ferrihydrite at pH = 6.0. Similarly, under alkaline condition (pH = 8.0), the rapid oxidation of montmorillonite inhibited the formation of magnetite, although its presence delayed the dehydration of green rust (Krishnamurti, 1998).
The transformation of metastable iron oxides was inhibited by the silica or aluminium released from the clay minerals and the slow oxidation rate of Fe(II) (Taylor, 1978). The release of silica from montmorillonite was accelerated with the increase of pH and reached a maximum of 10−2 M at pH ≈ 12.0. While, the concentration of aluminum reached a minimum of 10−6 M at pH ≈ 6.5 as pH increased, and then increased with pH (Baeyens and Bradbury, 1997). During montmorillonite dissolution, aluminium was released at a slower rate compared to silica, due to the relative ratios of basal siloxane and edge surfaces in montmorillonite (Sondi et al., 2008). Under neutral conditions, dissolved silica delayed the oxidation of Fe(II) by reducing the complex formation between inner-sphere Fe(II) and silica-ferrihydrite (Kinsela et al., 2016). Chen et al (Chen and Thompson, 2018). showed that Al(III) promoted the oxidation of Fe(II) and facilitated the formation of disordered goethite and lepidocrocite. During the oxidation of Fe(II), Fe was replaced by aluminium through the incorporation which led to a change in the size and the morphology of the mineral particle. According to the study of Hansel et al (Hansel et al., 2011)., the adsorption or the substitution of aluminium into ferrihydrite can impede its conversion to magnetite and lepidocrocite.
Heat treatment is commonly applied to observe the influence of coexisting minerals on the final products, which can shorten the aging time of the minerals (Martínez and McBride, 1998, Sørensen et al., 2000, Wang et al., 2015). At room temperature, maghemite, akageneite, goethite, and ferrihydrite were transformed to hematite whereas lepidocrocite and magnetite were transformed to maghemite (Strangway et al., 1968, Schwertmann et al., 2000, Torrent et al., 2006, El Hajj et al., 2013, Chen et al., 2015). At temperatures from 100 to 500 ℃, ferrihydrite, akaganeite, and goethite were transformed to hematite (Johnston and Lewis, 1983, Vempati et al., 1990, Yee et al., 2006, Glotch and Kraft, 2008). Magnetite and lepidocrocite were converted to maghemite around 200 ℃, and further to hematite from 200 to 700 ℃ (Schwertmann and Fechter, 1994, Özdemir and Dunlop, 2010, Chen and Grassian, 2013). Therefore it is expected that, the coexisting minerals can influence the type of iron oxide coprecipitates formed from Fe(II) oxidation and thereby the subsequent environmental processes. For example, the adsorption capacity of different iron oxides for metal ions may vary. Furthermore, the coexisting minerals can also impact the aging of the Fe(II) oxidation coprecipitates. However, the impact of the concentration of coexisting minerals on the type of Fe(II) oxidation coprecipitates, their aging, stability and properties are unknown. Lack of broad knowledge on the influence of coexisting minerals on Fe(II) oxidation coprecipitates would make it difficult to explore the optimum conditions for AMD treatment in a natural environment.
In this study, we investigated the effect of five coexisting minerals (Kaolin, Montmorillonite K10, SiO2, Al2O3, and CaCO3) on the Fe(II) oxidation coprecipitates formed using two iron salts (Cl- and SO42-). The heated coprecipitates were obtained by heating the fresh coprecipitates at 450 ℃ for 2 h. Both fresh and heated coprecipitates were analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), hysteresis loop, Fourier Transform Infrared Spectroscopy (FTIR), Brunauer-Emmett-Teller (BET), and acid dissolution experiments.
Section snippets
Chemicals
All chemicals (FeSO4·7 H2O, FeCl2·xH2O, NaOH, Kaolin, Montmorillonite K10, SiO2 (amorphous), γ-Al2O3, CaCO3, CaO, Na2CO3, HCl (36%), Acetic Acid (99%), Sodium acetate (≥99%), 1.10-Phenanthroline (99%), and NH4F) were analytical grade and used without further purification. Ultrapure water was used for all experiments.
Preparation of Fe(II) oxidation (co)precipitates
The Fe(II) oxidation coprecipitates were prepared by following the chemical co-precipitation method by changing the ratio of coexisting minerals and using FeSO4 and FeCl2 as an iron
Effect of SO42- and Cl- on Fe(II) oxidation coprecipitates
The XRD spectra of fresh and heated coprecipitate from the oxidation of FeSO4 and FeCl2 are shown in Figs. 1a and 1b. The fresh coprecipitates of the FeSO4 system mainly included lepidocrocite (JCPDS 76–2301, 2θ ≈ 14°, 27°, 36°, 47°), and goethite (JCPDS 81–0464, 2θ ≈ 21°, 33°, 36°, 40°, 41°, 53°, 61°, 59°) (Fig. 1a). While in the FeCl2 system, the main minerals in the fresh coprecipitates were akaganeite (JCPDS 34–1266, 2θ ≈ 12°, 17°, 56°), maghemite (JCPDS 39–1346, 2θ ≈ 43°), magnetite (JCPDS
Conclusion
In soil, various minerals coexist and these coexisting minerals can have a significant impact on the formation and transformation of iron oxides. The results of this study showed SO42- promoted the formation of goethite and lepidocrocite, while Cl- inhibited the Fe(II) oxidation and promoted the formation of magnetite. Both excessive montmorillonite and kaolin in FeSO4 system suppressed the formation of lepidocrocite. However, during Fe(II) oxidation, montmorillonite facilitated the formation
Environmental implications
The neutralization treatment of acid mine drainage involves the oxidation of Fe(II) and formation of iron oxides, however, little is known about the effect of co-existing minerals on those processes. Different formed minerals with varied stability and affinity may further affect the biogeochemical fates of hazardous contaminants. Our work is of significance and novelty because it systematically investigates the effects of five co-existing minerals on transformation of fresh and aged Fe(II)
CRediT authorship contribution statement
Qingya Fan: Investigation, Methodology, Formal analysis, Writing – original draft. Lingli Wang: Validation, Formal analysis, Writing – review & editing.Yu Fu: Visualization, Writing – review & editing. Zhaohui Wang: Conceptualization, Writing – review & editing, Funding acquisition, Supervision.
Declaration of Competing Interest
There are no conflicts of interest to declare. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 41977313), Fundamental Research Funds for Central Universities and Open Foundation of State Key Laboratory of Mineral Processing (BGRIMM-KJSKL-2021–17).
Declarations of interest
There are no conflicts of interest to declare.
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