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Article

Chromium Immobilization as Cr-Spinel by Regulation of Fe(II) and Fe(III) Concentrations

by
Tianci Hua
1,2,
Yanzhang Li
1,2,
Bingxu Hou
1,2,
Yimei Du
1,2,
Anhuai Lu
1,2 and
Yan Li
1,2,*
1
Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China
2
Beijing Key Laboratory of Mineral Environmental Function, School of Earth and Space Sciences, Peking University, Beijing 100871, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(10), 1024; https://doi.org/10.3390/min14101024
Submission received: 18 September 2024 / Revised: 6 October 2024 / Accepted: 11 October 2024 / Published: 13 October 2024
(This article belongs to the Section Environmental Mineralogy and Biogeochemistry)
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Abstract

The complex environmental conditions at Cr-contaminated sites, characterized by uneven ion distribution, oxidants competition, and limited solid-phase mobility, lead to inadequate mixing of Fe-based reducing agents with Cr, posing significant challenges to the effectiveness of Cr remediation through Cr-spinel precipitation. This study investigates the distinct roles of Fe(II), Fe(III), and Cr(III) in Cr-spinel crystallization under ambient temperature and pressure. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray absorption near-edge structure spectroscopy, and Mössbauer spectroscopy were employed to elucidate the phase composition, microstructure, and ion coordination within the precipitates. Our findings indicate that Fe(II) acts as a catalyst in the formation of the spinel phase, occupying octahedral sites within the spinel structure. Under the catalytic influence of Fe(II), Fe(III) transitions into the spinel phase, occupying both the tetrahedral and the remaining octahedral sites. Meanwhile, Cr(III), due to its high octahedral site preference energy, preferentially occupies the octahedral sites. When Fe(II) or Fe(III) is present but does not meet the ideal stoichiometric ratio, a deficiency in Fe(II) leads to low yield and poor crystallinity of Cr-spinel, whereas a deficiency in Fe(III) can completely inhibit its formation. Conversely, when either Fe(II) or Fe(III) is in excess, the formation of Cr-spinel remains feasible. Furthermore, metastable Cr phases can be transformed into stable Cr-spinel by adjusting the Fe(II)/Fe(III)/Cr(III) ratio. These results highlight the broad range of conditions under which Cr-spinel mineralization can occur in environmental settings, enhancing our understanding of the mechanisms driving Cr-spinel formation in Cr-contaminated sites treated with Fe-based reducing agents. This research provides critical insights for optimizing Cr remediation strategies.

1. Introduction

The release of highly mobile, toxic, and carcinogenic hexavalent chromium (Cr(VI)) into the environment through mining, industrial processing, and utilization has led to widespread contamination, severely threatening both human health and ecosystem stability [1,2,3]. One of the key strategies for remediating Cr(VI) contamination involves its reduction to the less toxic and mobile trivalent chromium (Cr(III)) [4,5].
Iron-based reductants, particularly nanoscale zero-valent iron (nZVI) and Fe(II)-containing compounds (e.g., FeS, FeSO4, and Fe3O4), are commonly employed for Cr(VI) reduction. These reductants facilitate the formation of CrxFe1−x(OH)3, CrxFe1−xOOH, or Cr-spinel (Fe[FexCr2−x]O4) precipitates, effectively immobilizing Cr(III) [6,7,8,9]. Among these products, Cr-spinel with an ideal stoichiometric 1:2 ratio of divalent to trivalent metal ions, is particularly noteworthy due to its stability under environmental conditions, making it a highly weathering-resistant and oxidation-resistant phase [10,11].
However, the complex environmental conditions at Cr-contaminated sites—such as uneven ion distribution, competing oxidants, and limited solid-phase mobility—pose significant challenges to achieving complete remediation [12,13,14]. For example, while nZVI rapidly reduces Cr(VI) due to its high reactivity, its tendency to agglomerate limits the distribution and mobility of Fe(0) [15,16,17]. Similarly, Fe(II)-containing minerals release Fe(II) over time [18,19], but the physical barriers within the mineral matrix prevent effective contact between Fe(II) and Cr(VI), resulting in incomplete reduction [20,21]. Moreover, both Fe0 and Fe(II) are often oxidized to Fe(III) before completely reacting with Cr(VI) [22], leading to the formation of metastable phases, like CrxFe1−x(OH)3 and CrxFe1−xOOH [23,24]. These metastable phases are susceptible to reoxidation by environmental oxidants such as O2, manganese oxides, and H2O2, causing the recurring “re-yellowing” effect at remediated sites [25,26,27]. Therefore, promoting Cr-spinel formation is essential for ensuring long-term Cr immobilization, which may be achieved by regulating the stoichiometric balance among Fe(II), Fe(III), and Cr(III).
A critical challenge in sustainable Cr remediation is maximizing the conversion of Cr(III) to stable Cr-spinel under non-ideal stoichiometric conditions. The current lack of detailed understanding of the specific roles played by Fe(II) and Fe(III) in the co-precipitation process hinders the optimization of Cr-spinel formation pathways at Cr-contaminated sites. Furthermore, it remains uncertain whether metastable Cr phases can be transformed into stable Cr-spinel by adjusting the Fe(II)/Fe(III)/Cr(III) ratios during the post-treatment process.
This study systematically explores the formation pathways of Cr-spinel under varying Fe(II), Fe(III), and Cr(III) concentrations and mixing sequences. O2 and Fe(III)-reducing bacterium Shewanella oneidensis MR-1 (S. MR-1, ATCC700550) are utilized as oxidizing and reducing agents, respectively, to regulate the Fe(II)/Fe(III) ratio. The results elucidate the distinct roles of Fe(II), Fe(III), and Cr(III) in Cr-spinel crystallization, offering new insights for enhancing the stability and effectiveness of Cr remediation in heterogeneous contaminated soil and water systems.

2. Materials and Methods

2.1. Precipitation Experiments

The chemicals used in this study included ferric chloride hexahydrate (FeCl3·6H2O), ferrous chloride tetrahydrate (FeCl2·4H2O), chromium chloride hexahydrate (CrCl3·6H2O), Luria-Bertani medium (LB medium, containing tryptone 10 g/L, yeast extract 5 g/L, NaCl 10 g/L) [28], sodium hydroxide (NaOH), and sodium acetate (C3H5O3Na), all sourced from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) The pH of the solution was maintained at 9, and the mixture was magnetically stirred at 600 rpm for 12 h to ensure complete reaction. All experiments were performed using deionized water (Millipore, Burlington, MA, USA) at ambient temperature to replicate the natural conditions of the site. Reactions were conducted under an argon atmosphere to prevent premature oxidation of ions, and the resulting products were subsequently exposed to air to simulate the oxidative environment present at the site.
To cover a broad range of Fe(II), Fe(III), and Cr(III) concentrations, in the experiment investigating the role of Fe(II) in Cr-spinel formation, the molar ratios of ion concentrations were Fe(II):Fe(III):Cr(III) = 1:0:0, 0:1.5:0.5, 0:5:0.5, 2:1.5:0.5 and 0.5:1.5:0.5. In the experiment examining the role of Fe(III) in Cr-spinel formation, the molar ratios were Fe(II):Fe(III):Cr(III) = 0:1:0, 1.5:0:0.5, 5:0:0.5, 1:2:1 and 1:0.5:1. For the role of Cr(III) in Cr-spinel formation, the molar ratios were Fe(II):Fe(III):Cr(III) = 0:0:1, 1:1.5:0, 1:1.5:0.25 and 1:1.5:2. To evaluate the effect of precipitation mixing sequences on Cr-spinel formation, the molar ratio of ion concentrations was Fe(II):Fe(III):Cr(III) = 1:1.5:0.5.
Excess FeCl2·4H2O was weighed, dissolved, and co-precipitated with CrCl3·6H2O inside a glove box. O2 was introduced to generate the precipitates. Similarly, excess FeCl3·6H2O was weighed, dissolved, and co-precipitated with CrCl3·6H2O in the glove box. The resulting precipitate was then added to resting cell fluid of S. MR-1 using 80 mmol/L sodium lactate (C3H5O3Na) as the electron donor. The reaction was conducted under sealed anaerobic conditions for two weeks to allow for precipitate formation.
Soluble impurities were removed from the sediment products in an anaerobic environment, after which the samples were freeze-dried for further characterization.

2.2. Methods for Sample Characterization

An inductively coupled plasma optical emission spectrometer (ICP-OES) was employed to determine the concentrations of total Fe and Cr in the solution, with accuracy ensured through quality control measures, including duplicate and blank sample analyses. The detection limit for Fe and Cr was 0.05 ppm. Prior to measurement, the samples underwent a secondary filtration. The Fe(III) concentration was calculated by subtracting the Fe(II) concentration from the total Fe concentration. The absence of Cr(VI) in the solution was confirmed using the diphenylcarbazide spectrophotometric method, allowing the Cr(III) concentration to be equated to the total Cr concentration.
To determine the Fe(II) concentration in the solution, 80 µL of supernatant was added to 1.92 mL of phenanthroline reagent containing 5 g/L phenanthroline and 50 mmol/L HEPES at pH 7. After a 15 min period for color development, the absorbance at 562 nm was measured using an Agilent Cary 100 UV-Vis spectrophotometer (Agilent Technologies, Santa Clara, CA, USA).
X-ray diffraction (XRD) analysis was performed to determine the mineral composition of the precipitates and conduct semi-quantitative analysis. Measurements were taken using a SAXSess diffractometer with a Cu target, Kα radiation (λ = 0.15418 nm), operating at 40 kV and 40 mA, with continuous scanning at a rate of 5°/min (Netherlands). Data fitting and analysis were carried out using Highscore Plus software (Version 5.1.0, Malvern Panalytical Inc., Malvern, Britain).
Scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) was utilized to examine the morphology and elemental composition of the precipitates. The analysis was performed using an FEI Quanta 200F SEM under high vacuum conditions at 10 kV, without applying a conductive coating.
High-resolution transmission electron microscopy (TEM) was utilized to elucidate the microstructure of the precipitates. Lattice fringe observations were performed on a JEM-2100F electron microscope (JEOL Ltd., Tokyo, Japan) equipped with a ZrO2/W (100) hot cathode field emission (Schottky type FEG), operating at an acceleration voltage of 200 kV, with a spherical aberration coefficient of 0.5 mm and a chromatic aberration coefficient of 1.1 mm. Images were captured using a Gatan 832 MultiScan CCD camera (Gatan Inc., Pleasanton, CA, USA), and diffraction spots were analyzed through Fourier transform using GMS 3 software (Version 3.5, Gatan Inc., Pleasanton, CA, USA).
X-ray absorption near-edge structure (XANES) spectroscopy was applied to investigate the chemical valence state and electronic structure of Cr in Cr-bearing precipitates. Synchrotron radiation experiments were conducted at the 1W1B-XAFS beamline (Beijing, China), with spectra collected using LabVIEW-developed experimental software (Version 5.0, National Instruments Inc., Austin, TX, USA).
Mössbauer spectroscopy (MS) was used to analyze the valence state and coordination environment of iron-containing minerals in the precipitates. Mössbauer measurements were performed at room temperature with an MFD-500AV instrument, using α-Fe foil as the reference absorber, and data were resolved using MossWin software (Version 4.0, Zoltán Klencsár, http://www.mosswinn.com/).

3. Results and Discussion

3.1. Fe(II) Contributions to the Formation and Stability of Cr-Spinel

To elucidate the role of Fe(II) in Cr-spinel formation, the concentration of Fe(II) was systematically changed at different Fe(II):Fe(III):Cr(III) molar ratios. When precipitated alone, Fe(II) forms a light green precipitate, while the combined precipitation of Fe(III) and Cr(III) yields a brown precipitate. At Fe(II):Fe(III):Cr(III) ratios of 0.5:1.5:0.5 and 2:1.5:0.5, the co-precipitation of all three ions produces black precipitates in both cases. The complete absence of Fe(II), Fe(III), and Cr(III) in the solution confirms that all ions precipitate at pH 9, due to the solubility product constants (Kθsp) of Fe(OH)2 (1.6 × 10−14), Fe(OH)3 (1.1 × 10−36), and Cr(OH)3 (6.3 × 10−31) [29,30].
Given the interconvertibility between Fe(II) and Fe(III), in the absence of Fe(II), S. MR-1 can be employed to reduce excess Fe(III), thereby generating Fe(II) [31,32]. A co-precipitate with a stoichiometric ratio of Fe(III):Cr(III) = 5:0.5 was added to a suspension containing S. MR-1 (see Figure 1a). After two weeks, the reaction solution exhibited a color change from brownish-red to brownish-black.
XRD analysis was employed to identify the crystalline phases of the precipitates (Figure 1b). The results reveal that the Fe(II) precipitates, upon drying in air, predominantly consist of spinel-type magnetite (JCPDS NO. 89-3854), with major diffraction peaks located at 30.1, 35.5 and 62.6° (2θ). It is accompanied by a minor presence of orthorhombic goethite (JCPDS NO. 81-0464), as identified by diffraction peaks at 21.24, 36.66, and 53.24° (2θ). The observed color change from light green to black, coupled with the XRD results, collectively indicates that Fe(II) undergoes oxidation by O2 during the drying process. Notably, in alkaline environments, amorphous Fe(II) is readily oxidized to Fe(III) by O2 [33]. During this process, a portion of the oxidized Fe(III) interacts with Fe(II) to form magnetite, while the remaining Fe(III) is catalyzed by Fe(II) to form goethite (the yield is approximately 5%, calculated based on the amount of substance).
In contrast, the mixed Fe(III) and Cr(III) precipitate exhibits no crystalline products, existing as CrxFe1−x(OH)3. Upon introducing microorganisms into the Fe(III) and Cr(III) co-precipitation system, the formation of Cr-spinel (JCPDS NO. 89-3855), with featured diffraction peaks at 30.1, 35.5, and 62.6° (2θ) was observed. UV-Vis spectroscopy indicated a gradual increase in Fe(II) concentration over a two-week reaction period (Table 1), suggesting the continuous reduction of Fe(III) by S. MR-1. A portion of the resulting Fe(II) complexes with organic matter, increasing its solubility in alkaline solutions [34]. The remaining Fe(II) participates in the mineralization of Cr-spinel, but the crystallinity of the resulting spinel product is notably poor.
When the molar ratios of Fe(II):Fe(III):Cr(III) are adjusted to 0.5:1.5:0.5 and 2:1.5:0.5, Cr-spinel formation is observed in both resultant products. However, halving the stoichiometric ratio of Fe(II) leads to reduced Cr-spinel crystallinity and a significantly lower relative content compared to the 1:1.5:0.5 ratio. This reduction can be attributed to the insufficient Fe(II) available for integration into the spinel structure. Conversely, when the Fe(II) ratio is doubled, an adequate amount of Fe(II) participates in Cr-spinel formation, with the excess Fe(II) being oxidized by O2 to form magnetite and goethite (approximately 15%). Therefore, Cr-spinel exhibits good crystallinity and is present in sufficient quantity.
These findings underline the crucial role of Fe(II) in Cr-spinel formation, regardless of whether Fe(II) is supplied through chemical reagents or generated via the microbial reduction of Fe(III). Fe(II) not only directly contributes to the synthesis of Cr-spinel but also facilitates the catalytic transformation of CrxFe1−x(OH)3 into the spinel phase. Under Fe(II) deficient conditions, Cr-spinel formation remains possible, though the crystallinity is diminished because Cr-spinel has structural flexibility that permits the accommodation of numerous defects [35].

3.2. The Roles of Fe(III) and Cr(III) in Cr-Spinel Formation

To examine Cr-spinel formation under non-ideal stoichiometric conditions, Fe(III) concentrations were adjusted accordingly. Fe(III) precipitates alone to form a brown product, while the co-precipitation of Fe(II) and Cr(III) produces a green precipitate. Co-precipitation at a molar ratio of Fe(II):Fe(III):Cr(III) = 1:2:1 results in a black precipitate, whereas a ratio of 1:0.5:1 yields a brown product. Similarly, when Fe(III) is absent and Fe(II) is in excess, Fe(II) can be oxidized by O2 to generate Fe(III). Introducing O2 into the co-precipitate with a stoichiometric ratio of Fe(II):Cr(III) = 5:0.5 causes the green precipitate to gradually turn black.
XRD analysis (Figure 2) shows that Fe(III) precipitates alone in a pH 9 solution, forming an amorphous Fe(OH)3 product. The co-precipitate of Fe(II) and Cr(III) forms an unstable structure, with the color change suggesting that even when Fe(II) and Cr(III) co-precipitate, the amorphous Fe(II) is readily oxidized by O2 to Fe(III), leading to the formation of amorphous CrxFe1−x(OH)3. However, when Cr(III) and excess Fe(II) form a co-precipitate that is subsequently oxidized by O2, Cr-spinel is formed. Notably, halving the Fe(III) ratio yields no spinel phase, while doubling the Fe(III) ratio produces stable Cr-spinel with a minor presence of goethite (approximately 10%).
These findings suggest that amorphous Fe(II) alone is insufficient to drive the transformation of Cr(III) into the spinel phase. Instead, the formation of Cr-spinel necessitates an excess of Fe(III) relative to Cr(III). This explains why Cr-spinel fails to form when the co-precipitate of Fe(II) and Cr(III) is oxidized at a ratio of 1:0.5, yet forms when oxidized at 5:0.5. The key factor is that the available Fe(II) in the former ratio does not satisfy the stoichiometric requirements for Fe(II) and Fe(III), thereby preventing the formation of a stable Fe(II) phase and its complete oxidation to Fe(III), leading to the production of amorphous CrxFe1−x(OH)3. Thus, the initiation of the spinel phase transformation is contingent upon the presence of Fe(III), and Cr-spinel only forms when Fe(III) is present in excess relative to Cr(III).
To further explore the formation of Cr-spinel under non-ideal stoichiometric conditions for Cr(III), the concentration of Cr(III) was varied. Cr(III) precipitates alone to form a green precipitate, while the co-precipitation of Fe(II) and Fe(III) yields a black precipitate. Co-precipitation of Fe(II), Fe(III), and Cr(III) at a molar ratio of 1:1.5:0.25 results in a black precipitate, whereas increasing the Cr(III) concentration to 1:1.5:2 leads to a gray-green product.
The XRD results in Figure 3 demonstrate that Cr(III) precipitates in a pH 9 solution as amorphous Cr(OH)3, while Fe(II) and Fe(III) co-precipitate to form magnetite. Notably, when Cr(III) is limited, stable Cr-spinel can form. However, an excess of Cr(III) inhibits the formation of Cr-spinel, consistent with the observations in Figure 2.
These findings suggest that when Fe(II) and Fe(III) co-precipitate at a stoichiometric ratio close to 1:2, spinel structures are favored over goethite. However, the participation of Cr(III) in spinel formation promotes the precipitation of goethite, as evidenced by the XRD patterns in Figure 1, Figure 2 and Figure 3. This implies that Cr(III) can modulate the catalytic activity of amorphous Fe(II), driving the conversion of some Fe(III) into goethite.

3.3. Cr-Spinel Formation from Different Mixing Orders of Fe(II), Fe(III), and Cr(III) Precipitates

To assess whether metastable mineral phases can be converted into stable Cr-spinel by adjusting the Fe(II):Fe(III):Cr(III) ratio, a third ion precipitate was introduced into the co-precipitate of the other two ions, achieving a 1:2 stoichiometric ratio of divalent to trivalent metal ions. Upon introducing Fe(II) precipitate to the Fe(III) and Cr(III) co-precipitate, the light green precipitate vanishes, and the mixture turns black. Similarly, when Fe(III) precipitate is added to the co-precipitate of Fe(II) and Cr(III), the green precipitate dissipates, yielding a black precipitate. Additionally, adding Cr(III) precipitate to the Fe(II) and Fe(III) co-precipitate also causes the green precipitate to disappear, resulting in a black precipitate.
Figure 4 illustrates that Cr-spinel precipitates can form under all three mixing conditions, though the crystallinity and product composition vary depending on the mixing sequence. The (FeIII + CrIII)s + (FeII)s group shows the highest crystallinity and produces a small amount of goethite (approximately 10%) as a byproduct. The (FeII + FeIII)s + (CrIII)s group exhibits moderate crystallinity without goethite formation. In contrast, the (FeII + CrIII)s + (FeIII)s group shows the lowest crystallinity, with a relatively higher content of goethite (approximately 30%) byproduct.
The microstructure and semi-quantitative elemental analysis of the precipitates are presented in Figure 5. The analysis reveals that all three mixed precipitates aggregate into micrometer-sized particles, comprising elements such as Fe, Cr, and O. The high-resolution lattice fringes and diffraction spots indicate a cubic crystal structure with an approximate size of 30 nm. The observed interplanar spacings of 2.53 Å, 2.10 Å, 4.84 Å, and 2.53 Å correspond to the (311), (400), (1 1 ¯ 1 ¯ ), and ( 3 ¯ 11) crystal planes, respectively. These findings further confirm the formation of Cr-spinel.
Further characterization of the precipitates obtained from the three different precipitation sequences was performed. XANES analysis of the Cr K-edge shows characteristic features of Cr-spinel, including a pre-edge peak at 5990 eV, a white line (WL) peak at 6009 eV, and a post-WL shoulder at 6021 eV. These observations confirm that all Cr(III) in the products from the three different precipitation sequences is incorporated into the spinel structure (Figure 6). The ratio of WL peak intensity to post-WL shoulder intensity indicates particle size, with a larger ratio corresponding to a larger particle size (i.e., higher degree of crystallinity) [36]. The (FeIII + CrIII)s + (FeII)s group exhibits the largest average particle size, followed by the (FeII + FeIII)s + (CrIII)s group, with the (FeII + CrIII)s + (FeIII)s group having the smallest average particle size (Table 2). These results are consistent with the crystallinity observed in the XRD analysis.
The MS of the (FeII + CrIII)s + (FeIII)s precipitate (Figure 7) reveals a doublet with an isomer shift (I.S.) of 0.34 and quadrupole splitting (Q.S.) of −0.22, along with a sextet at I.S. = 0.31 and Q.S. = −0.01, corresponding to Fe(III) in tetrahedral coordination within ferrite and spinel structures, respectively. Another sextet, with I.S. = 0.61 and Q.S. = −0.06, is attributed to Fe(II) and Fe(III) in octahedral coordination within the spinel structure [37].
These findings suggest that the crystal structure of Cr-spinel can be described as (FeIII)tet[FeIICrIIIxFeIII1−x]octO4, where Cr(III) and Fe(II) occupy octahedral sites, and Fe(III) is distributed between the tetrahedral and the remaining octahedral sites. This distribution aligns with the octahedral site preference energy (OSPE) ranking of the three ions: Cr(III) (37.7 kcal/mol) > Fe(II) (4.0 kcal/mol) > Fe(III) (0 kcal/mol) [38]. This observation explains why Cr-spinel can form even under Fe(II)-deficient conditions, while its formation is impeded when Fe(III) is insufficient relative to Cr(III). In the spinel structure, Fe(III) occupies tetrahedral and octahedral sites, while Fe(II) and Cr(III), with their higher OSPE, are accommodated in the octahedral positions. Any excess Fe(III) is expelled from these sites. Although Cr-spinel may exhibit structural defects when Fe(II) is insufficient, it can maintain stability. However, when Fe(III) is insufficient, the absence of adequate octahedral sites for Fe(II) and Cr(III) causes the spinel structure to collapse.

3.4. Remediation Strategies of Cr-Contaminated Sites through Cr-Spinel Precipitation

Analysis of the phase structure and atomic coordination of the precipitates reveals that Fe(II) acts as a catalyst in the formation of the spinel phase, occupying the octahedral sites within the spinel structure. Under the catalytic influence of Fe(II), Fe(III) transitions into the spinel phase, occupying both the tetrahedral and the remaining octahedral sites. Cr(III), due to its high OSPE, preferentially occupies the octahedral sites. Consequently, effective remediation of Cr-contaminated sites requires maintaining the Fe(II) concentration near the ideal stoichiometric ratio without excess, while ensuring that the Fe(III) concentration exceeds Cr(III) to promote the formation of stable Cr-spinel.
Prolonged exposure of Cr-contaminated sites to air typically results in the predominant presence of Fe as Fe(III) and Cr as Cr(VI) [39]. In untreated sites, the native Fe(III) content is often insufficient to facilitate effective Cr stabilization. Thus, the first step in remediation involves adding Fe(II) to reduce Cr(VI) according to the following reaction:
3Fe(II) + Cr(VI) → 3Fe(III) + Cr(III)
After complete reduction of Cr(VI), additional Fe(II) is introduced to achieve the desired stoichiometric ratio, followed by the addition of Fe(III) in proportion to the total Cr concentration to promote Cr-spinel formation. In contrast, for sites that have already undergone chemical remediation and possess excess Fe(III), only the addition of Fe(II) is required to form stable Cr-spinel.
Experimental results show that mixing Fe(II) and Cr(III) precipitates with Fe(III) leads to a decrease in spinel content and crystallinity, while increasing the formation of goethite. This effect is likely due to the formation of an unstable intermediate during the initial interaction between Cr(III) and Fe(II), which promotes the transformation of more Fe(III) into goethite, reducing the availability of Fe(III) for Cr-spinel formation. As a result, both the yield and stability of the spinel phase decrease. Fortunately, this scenario is less likely to occur under typical soil and aqueous conditions at Cr-contaminated sites.
Considering the Kθsp of Fe(OH)2, Cr(OH)3, and Fe(OH)3, Fe(III) will precipitate completely at a pH of approximately 3.7, followed by the precipitation of Cr(III). Fe(II), however, begins to precipitate only as the solution approaches neutrality. Consequently, Cr(III) and Fe(III) first form a co-precipitate, which is subsequently combined with the Fe(II) precipitate to produce the final product, resulting in stable, high-yield Cr-spinel.
In soil environments, Fe(III) typically appears first, followed by Cr(III) through the reduction of Cr(VI), and finally Fe(II), facilitating the formation of stable Cr-spinel. If external disturbances mix the three ions, the final product depends on the mixing order. When Fe(III) and Fe(II) are combined first, a stable spinel structure forms, with Cr(III) subsequently occupying octahedral sites. At this stage, Fe(II) is incorporated as a crystalline phase and cannot catalyze the transformation of excess Fe(III) into goethite, thereby preventing the formation of secondary crystalline phases. Conversely, when Fe(II) mixes first with Cr(III), an unstable phase forms. If Fe(II) is in excess (as illustrated in Figure 2), it undergoes oxidation by O2, leading to the formation of Cr-spinel. When Fe(II) is limited, it is instead oxidized into CrxFe1−x(OH)3. Upon subsequent mixing with Fe(II), this mixture can still yield Cr-spinel with favorable crystallinity.
Thus, even under non-ideal stoichiometric ratios and varying precipitation sequences of Fe(II), Fe(III), and Cr(III), stable Cr-spinel can form at Cr-contaminated sites. Furthermore, by adjusting the stoichiometric balance in sites treated with Fe-based reducing agents, metastable mineral phases can be converted into stable Cr-spinel. Given the importance of cost-effectiveness, the introduction of Fe(III)-reducing bacteria may provide a viable strategy to modulate the stoichiometric balance of Fe(II) and Fe(III) at the site, thereby enhancing the stability and efficacy of Cr-spinel formation. As demonstrated in Table 1, Fe(II) produced through microbial reduction of Fe(III) exhibits enhanced solubility and mobility, effectively addressing challenges related to uneven element distribution and limited sediment migration in Cr-contaminated sites. Furthermore, the ongoing metabolic activity of microorganisms contributes to the maintenance of a reducing environment at the site, which is advantageous for the remediation of non-point Cr pollution.

4. Conclusions

This study systematically explores the effects of varying ion concentration ratios and precipitation sequences on Cr-spinel mineralization under ambient conditions. Our findings reveal that the absence of any one of the essential ions—Fe(II), Fe(III), or Cr(III)—prevents the formation of Cr-spinel. However, when the missing ion is reintroduced, Cr-spinel successfully forms. If the stoichiometric ratio of Fe(II) is insufficient, Cr-spinel still forms but with reduced yield and poor crystallinity. Conversely, a deficiency in Fe(III) entirely inhibits Cr-spinel formation. In cases with excess Fe(II) and no Fe(III), the oxidation of Fe(II) to Fe(III) can facilitate Cr-spinel formation. Likewise, in the presence of excess Fe(III) and the absence of Fe(II), the post-reduction of Fe(III) to Fe(II) ultimately leads to Cr-spinel formation.
Our research clarifies how ion concentration ratios influences Cr-spinel formation at ambient temperature and pressure, highlighting the extensive range of possible mineralization pathways. These findings enhance the understanding of Cr-spinel formation mechanisms in Cr-contaminated sites treated with Fe-based reducing agents and improve remediation strategies employing Cr-spinel.

Author Contributions

Y.L. (Yan Li): conceptualization, methodology, writing, and editing. T.H.: data processing, experimental analysis, and writing. Y.L. (Yanzhang Li): review and data processing. B.H.: review and data processing. Y.D.: data processing. A.L.: review and funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

Financial support has been provided by the National Natural Science Foundation of China (Nos. 92351302, 42192502) and the National Key Research and Development Program of China (No. 2019YFC1805900).

Data Availability Statement

The data used to support the findings of this study are included within the article.

Acknowledgments

The XAFS beam time is granted by the 1W1B endstation of Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Synthesis of Cr-spinel with Fe(II) at non-ideal stoichiometric ratios. (a) Comparison of the reduction processes of Fe(III) and Cr(III) co-precipitate by S. MR-1. (b) XRD patterns of Fe(II) precipitate, co-precipitate of Fe(III) and Cr(III), reduction of Fe(III) and Cr(III) co-precipitate by S. MR-1, and the co-precipitation of Fe(II), Fe(III), and Cr(III) at stoichiometric ratios of 2:1.5:0.5 and 0.5:1.5:0.5, respectively. The symbols ♥ and ♦ represent spinel and goethite, respectively.
Figure 1. Synthesis of Cr-spinel with Fe(II) at non-ideal stoichiometric ratios. (a) Comparison of the reduction processes of Fe(III) and Cr(III) co-precipitate by S. MR-1. (b) XRD patterns of Fe(II) precipitate, co-precipitate of Fe(III) and Cr(III), reduction of Fe(III) and Cr(III) co-precipitate by S. MR-1, and the co-precipitation of Fe(II), Fe(III), and Cr(III) at stoichiometric ratios of 2:1.5:0.5 and 0.5:1.5:0.5, respectively. The symbols ♥ and ♦ represent spinel and goethite, respectively.
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Figure 2. XRD patterns of Fe(III) precipitate, co-precipitate of Fe(II) and Cr(III), the oxidation of Fe(II) and Cr(III) co-precipitate by O2, and the co-precipitation of Fe(II), Fe(III), and Cr(III) at stoichiometric ratios of 1:2:1 and 1:0.5:1, respectively. The symbols ♥ and ♦ represent spinel and goethite, respectively.
Figure 2. XRD patterns of Fe(III) precipitate, co-precipitate of Fe(II) and Cr(III), the oxidation of Fe(II) and Cr(III) co-precipitate by O2, and the co-precipitation of Fe(II), Fe(III), and Cr(III) at stoichiometric ratios of 1:2:1 and 1:0.5:1, respectively. The symbols ♥ and ♦ represent spinel and goethite, respectively.
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Figure 3. XRD patterns of Cr(III) precipitate, co-precipitate of Fe(II) and Fe(III), and the co-precipitation of Fe(II), Fe(III), and Cr(III) at stoichiometric ratios of 1:1.5:0.25 and 1:1.5:2, respectively. The symbols ♥ and ♦ represent spinel and goethite, respectively.
Figure 3. XRD patterns of Cr(III) precipitate, co-precipitate of Fe(II) and Fe(III), and the co-precipitation of Fe(II), Fe(III), and Cr(III) at stoichiometric ratios of 1:1.5:0.25 and 1:1.5:2, respectively. The symbols ♥ and ♦ represent spinel and goethite, respectively.
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Figure 4. XRD patterns of precipitates formed from various mixing sequences of ion precipitates. The symbols ♥ and ♦ represent spinel and goethite, respectively.
Figure 4. XRD patterns of precipitates formed from various mixing sequences of ion precipitates. The symbols ♥ and ♦ represent spinel and goethite, respectively.
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Figure 5. SEM-EDS and TEM patterns of (a) the mixed precipitate of Fe(III) and Cr(III) co-precipitate with Fe(II) precipitate, (b) the mixed precipitate of Fe(II) and Cr(III) co-precipitate with Fe(III) precipitate, and (c) the mixed precipitate of Fe(II) and Fe(III) co-precipitate with Cr(III) precipitate. The red plus sign indicates the detection position of EDS.
Figure 5. SEM-EDS and TEM patterns of (a) the mixed precipitate of Fe(III) and Cr(III) co-precipitate with Fe(II) precipitate, (b) the mixed precipitate of Fe(II) and Cr(III) co-precipitate with Fe(III) precipitate, and (c) the mixed precipitate of Fe(II) and Fe(III) co-precipitate with Cr(III) precipitate. The red plus sign indicates the detection position of EDS.
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Figure 6. Cr K-edge XANES of the precipitates.
Figure 6. Cr K-edge XANES of the precipitates.
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Figure 7. Mössbauer spectrum analysis of the mixed precipitate of Fe(II) and Cr(III) co-precipitate with Fe(III) precipitate.
Figure 7. Mössbauer spectrum analysis of the mixed precipitate of Fe(II) and Cr(III) co-precipitate with Fe(III) precipitate.
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Table 1. The temporal variation in Fe(II) concentration.
Table 1. The temporal variation in Fe(II) concentration.
Reaction Time
(Day)		02468101214
Fe(II)
(mg/L)		0.078.5100.5135.0152.0174.0198.5223.0
Table 2. The relative strength between WL peak and post-WL shoulder.
Table 2. The relative strength between WL peak and post-WL shoulder.
WL PeakPost-WL ShoulderWL/Post-WL
(FeIII + CrIII)s + (FeII)s2.0451.4381.422
(FeII + FeIII)s + (CrIII)s1.6631.1921.395
(FeII + CrIII)s + (FeIII)s1.5991.1811.354
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Hua, T.; Li, Y.; Hou, B.; Du, Y.; Lu, A.; Li, Y. Chromium Immobilization as Cr-Spinel by Regulation of Fe(II) and Fe(III) Concentrations. Minerals 2024, 14, 1024. https://doi.org/10.3390/min14101024

AMA Style

Hua T, Li Y, Hou B, Du Y, Lu A, Li Y. Chromium Immobilization as Cr-Spinel by Regulation of Fe(II) and Fe(III) Concentrations. Minerals. 2024; 14(10):1024. https://doi.org/10.3390/min14101024

Chicago/Turabian Style

Hua, Tianci, Yanzhang Li, Bingxu Hou, Yimei Du, Anhuai Lu, and Yan Li. 2024. "Chromium Immobilization as Cr-Spinel by Regulation of Fe(II) and Fe(III) Concentrations" Minerals 14, no. 10: 1024. https://doi.org/10.3390/min14101024

APA Style

Hua, T., Li, Y., Hou, B., Du, Y., Lu, A., & Li, Y. (2024). Chromium Immobilization as Cr-Spinel by Regulation of Fe(II) and Fe(III) Concentrations. Minerals, 14(10), 1024. https://doi.org/10.3390/min14101024

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