Study on the Solidification Mechanism of Cr in Ettringite and Friedel’s Salt

Abstract

The solidification of heavy metal Cr has always been a challenge in the treatment of Cr-containing wastes, due to its high mobility in alkaline environments. In addition, the solidification mechanism of Cr has not been fully investigated. In this study, blast furnace slag (BFS)-based cementitious materials were used as binders for the immobilization of heavy metal Cr to investigate the solidification mechanism of Cr in different hydration products. From XRD, FTIR, XPS, and XANES analyses, it could be seen that SO₄²⁻ in ettringite was replaced by Cr in the form of CrO₄²⁻, making SO₄²⁻ re-dissolve in the liquid phase. The SO₄²⁻ in the solution would compete with CrO₄²⁻ ions, leading to the direct influence of SO₄²⁻ content on the solidification efficiency of Cr. In ettringite, Cr mainly existed in the form of Cr⁶⁺, accounting for more than 84% however, Cr was solidified in Friedel’s salt under two coexisting valence states (Cr⁶⁺ and Cr³⁺). This resulted not only from the slow excitation rate of the BFS in the cementitious system that did not contain sulfate, but also from the existence of a certain amount of reducing substances in the BFS, such as Fe²⁺ and S²⁻, which could reduce some of Cr⁶⁺ to Cr³⁺. In Friedel’s salt, the residual Cr⁶⁺ replaced Cl⁻ in the form of CrO₄²⁻, whereas the Cr³⁺ replaced Al³⁺. The binding energies of Cr 2p3/1 and Cr 2p3/2 decreased with the addition of Cr, indicating that the coordination numbers of Cr³⁺ and Cr⁶⁺ increased, and that the binding energies of Cr³⁺ and Cr⁶⁺ decreased after entering the structure of Friedel’s salt.

1. Introduction

Heavy metal Cr accumulates in plants and organisms, and then enters the human body through the food chain, causing skin damage, respiratory or gastrointestinal diseases, and even brain cell damage. The Cr usually exists in the state of Cr³⁺ and Cr⁶⁺. Cr³⁺, an essential trace ion necessary for human body, has weak mobility and low toxicity, whereas Cr⁶⁺ has strong mobility and high toxicity [1,2,3] Therefore, the treatment of Cr-containing waste is quite difficult. The free energy of oxidation of Cr³⁺ to Cr⁶⁺ is −459 kJ under alkaline conditions, and only −22.12 kJ under acidic conditions [4], indicating that an alkaline environment is favorable for the oxidation of Cr³⁺ to Cr⁶⁺ into CrO₄²⁻ or HCrO₄⁻ anions. The Cr⁶⁺ presents a lower adsorption for negatively charged particles and hydration products than Cr³⁺, resulting in its strong solubility and mobility in alkaline environments. Thus, the existence of Cr⁶⁺ becomes a severe threat to the surrounding environment and human health and poses a great challenge to the solidification of Cr-containing wastes. The application of BFS-based material for the solidification of Cr was considered to be an effective method that could significantly reduce the possibility of Cr’s leaching into the environment. This application could be achieved in multiple ways, including physical encapsulation, double-replacement reactions to form precipitates, chemical bond adsorption, and the incorporation of Cr into mineral phase [5,6,7,8,9].

In recent years, much research has been conducted on the solidification mechanism and the existing state of Cr ions during the hydration reaction of BFS-based cementitious systems. Cr⁶⁺ mainly exists in the form of acid ions such as CrO₄²⁻ and Cr₂O₇²⁻ in hydration products. Owing to the oxidation environment provided by the hydration reaction process of Cr⁶⁺, the solidification efficiency of Cr is only about 91%, which is quite low compared with other heavy metals [10,11]. The research of O. Yamaguch et al., demonstrated that the Cr⁶⁺ in hydration products, such as calcium sulfoaluminate, could be easily reduced to Cr³⁺ and form Cr(OH)₃ precipitates under weak alkaline conditions [12]. The investigation conducted by M.R. Shatat et al. indicated that the Cr solidified as Ca₂CrO₅·3H₂O and Cr doping changed the pore structure and reduced the early strength of the solidified material [13]. The study of M. Wazne et al. indicated that the solidification of Cr was mainly based on the lattice of the AFt phase at pH > 12.5 and adsorption at pH < 8. The Cr was almost completely converted to Cr⁶⁺ at pH < 5 [14]. A comparison between sulfate ettringite and chromate ettringite indicated that the chromate ettringite had a larger lattice and smaller crystalline shape due to the larger radius of CrO₄²⁻ than SO₄²⁻, and displayed poor stability [15]. The solidification mechanism of different heavy metals was investigated, and it was found that the hydration products had a relatively low solidification rate of Cr compared with other heavy metals, at only about 50% [16]. The presence of Cr⁶⁺ would reduce the solidification efficiency of other heavy metal ions, and the Cr⁶⁺ would redissolve once the environmental conditions changed. The study of Zhang et al. demonstrated that Cr⁶⁺ mainly existed as CrO₄-U phase in the hydration products in the absence of a reducing agent. The leaching test showed that the relationship between the leaching concentration of Cr⁶⁺ and total Cr was pH-dependent. The lower the pH of the reaction environment, the higher the reduction rate. The Cr mainly existed in the form of Cr⁶⁺ at pH 8.0~12.7, and Cr³⁺ at pH < 8. Most Cr exists in the form of CrO₄²⁻, replacing the SO₄²⁻ in AFt phase lattice.

In the present study, BFS and flue gas desulfurization gypsum (FGDG) were applied for the solidification of K₂CrO₄. The solidification mechanism in different hydration products was investigated through X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and X-ray absorption near edge structure (XANES) to reveal the valence distribution and binding energy changes of Cr.

2. Experiments

2.1. Materials

The BFS and FGDG were collected from Hebei Xingtai Jintaicheng Environmental Resources Co., Ltd., Xingtai, China. These materials were dried at 35 °C for 72 h in an oven, and then stored separately in a sealed bag for subsequent use. In order to improve reactivity, the FGDG and BFS were ground into a fine powder in a ball mill (5 kg small ball mill, SMΦ500 × 500, Daoxu Donglin Laboratory Instrument Factory, Dongguan, China). K₂CrO₄ was of analytical pure grade.

2.2. Sample Preparation

The sample was prepared by weighing and thoroughly mixing and blending the raw materials and water. The paste mixture obtained after mixing was placed into a 30 × 30 × 50 mm mold, taken out after curing for 24 h, then stored at 35 °C and 90% humidity for further curing for 28 days. Then, the samples were crushed and soaked in anhydrous ethanol for 48 h to stop the hydration reaction, then they were dried in a vacuum oven at 35 °C for 72 h. The specific surface area of raw materials was analyzed using a CZB-9 BET Automatic Specific Surface Area Tester. The samples were finely ground to powder and then characterized by XRF (XRF-1800 sequential X-ray fluorescence spectrometer, Shenzhen, China), XRD (Rigaku Corporation, Shimadzu, Japan, in the range of 10–90°), XPS (Shimadzu-Kratos AXIS SUPRA X-ray photoelectron Spectrometer, Shimadzu, Japan), XANES (BL14W Silicon (111) crystal monochromator for beamline, Easyxafs Company, Washington, IL, USA), and FTIR (Nexus 70 FTIR spectrometer, Xiamen, China). The ion leaching concentration was determined by IC (Thermo Scientific Dionex Aquion IC system, Beijing, China).

2.3. The Solidification Mechanism of Cr

The solidification mechanism and existing state of Cr in ettringite were investigated with different analytical methods. Samples were prepared according to our previous studies (Wang et al., 2021, Wang et al., 2022), by adding CaO as an alkaline activator, with 80% BFS, 10% CaO, and 10% FGDG at a concentration of 80%. The samples were prepared by mixing K₂CrO₄ agents with 2, 4, and 6% of the total cementitious material, then cured for 28 days to investigate the solidification mechanism of Cr in ettringite, as listed in

Table 1. The samples investigated for solidification mechanism of Cr in ettringite.

Samples	BFS (%)	CaO (%)	FGDG (%)	K2CrO4 (%)	Concentration
S0	80	10	10	2	80
S10				4
S20				6

.

The solidification mechanism of Cr in Friedel’s salt was investigated by different analytical methods. The samples were prepared by adding CaCl₂ as the chlorine donor with 80% BFS, 10% CaO, and 10% CaCl₂ at a concentration of 80%. The samples prepared by mixing 2, 4, and 6% K₂CrO₄ agents were compared after 28 days of curing, as listed in

Table 2. The samples investigated for solidification mechanism of Cr in Friedel’s salt.

Samples	BFS (%)	CaO (%)	FGDG (%)	K2CrO4 (%)	Concentration
F10	80	10	10	2	80
F20				4
F30				6

.

3. Results and Discussion

3.1. Raw Materials

The treated BFS and FGDG were analyzed with a CZB-9 BET Automatic Specific Surface Area Analyzer, Beijing, China, and the specific surface areas were 500 and 450 m²/kg, respectively. The particle size of the raw materials was analyzed by a LMS-30 laser diffraction scattering particle size distribution analyzer, Beijing, China. As seen in Figure 1, the volume accumulative distribution (VAD) of 10, 50, and 90% of different raw materials was displayed on the upper left corner of the particle size distribution.

The XRF test results of raw materials were analyzed by a XRF-1800 sequential X-ray fluorescence spectrometer, Shenzhen, China and is shown in

Table 3. XRF results of BFS and FGDG (%).

Material	CaO	Cl	SiO2	MgO	SO3	Al2O3	Fe2O3	K2O	LOI
BFS	40.86	0.03	29.27	9.75	1.48	14.76	1.46	0.46	0.8
FGDG	50.96	0.26	3.85	1.47	40.37	1.37	0.55	0.18	10.7

. BFS was mainly composed of Ca, Si, Mg, and Al elements. As shown in Figure 2a, the XRD pattern of BFS displayed a distinct broad band, indicating the predominant amorphous nature of the BFS. In addition, a small contribution of crystalline CaCO₃ was found. The performance indices of BFS could be calculated from chemical compositions based on Equations (1)–(3), for hydraulic activity, the activity coefficient, and alkalinity coefficient, respectively. The calculated values suggested that the BFS could be considered a high-quality slag with high activity and neutral alkalinity.

Hydraulic activity:

$${\mathrm{K}=\mathrm{w}(\mathrm{CaO}+\mathrm{MgO}+\mathrm{Al}}_2{\mathrm{O}}_3)/\mathrm{w}\left({\mathrm{SiO}}_2\right)=2.23 > 1.6 (\mathrm{High} \mathrm{quality})$$

(1)

Activity coefficient:

$${\mathrm{H}}_0{=\mathrm{w}(\mathrm{Al}}_2{\mathrm{O}}_3)/\mathrm{w}\left({\mathrm{SiO}}_2\right)=0.50 > 0.25 (\mathrm{High} \mathrm{activty})$$

(2)

Alkalinity coefficient:

$${\mathrm{M}}_0=\mathrm{w}(\mathrm{CaO}+\mathrm{MgO})/{\mathrm{w}({\mathrm{SiO}}_2+\mathrm{Al}}_2{\mathrm{O}}_3)=1.15 > 1 (\mathrm{Alkalinity})$$

(3)

where “W” represents the mass fraction.

As the main byproduct of the wet limestone–gypsum desulfurization process, the FGDG was mainly composed of Ca and S elements, with CaSO₄·2H₂O as the mineral phase, as shown in Table 1 and Figure 2b.

3.2. The Solidification Mechanism of Cr in Ettringite

3.2.1. XRD Analysis of Ettringite

In order to determine the solidification mechanism of ettringite on Cr, XRD patterns of three different samples were analyzed to study the effects of K₂CrO₄ on the crystalline shape and lattice of ettringite. As shown in Figure 3, there was no significant difference in the XRD diffraction patterns of different samples, and the main hydration product was ettringite. However, with the increase in K₂CrO₄ content, the diffraction angle of ettringite slightly increased due to the replacement of SO₄²⁻ by CrO₄²⁻ in the liquid phase to form chromate ettringite. The binding ability of CrO₄²⁻ to components, such as Ca²⁺, Al(OH)₄⁻ and OH⁻, was poor, leading to an increase in the phase distance of each product. However, as SO₄²⁻ and CrO₄²⁻ had the same valence state and similar particle sizes (2.3 Å and 2.4 Å) [17], the XRD patterns of each sample did not display many differences. In addition, the intensity of the derivative peaks of CaSO₄ increased with the increase in K₂CrO₄ content, which was generated by the reaction of SO₄²⁻ with Ca²⁺ in the solution. In order to verify the above results, the concentrations of SO₄²⁻ and CrO₄²⁻ in the leachate of different samples, after curing for 28 days, were determined by ion chromatography and spectrophotometry with colorimetric method.

As shown in Table 4, as K₂CrO₄ content increased, the concentration of CrO₄²⁻ in the leachate increased at a slow rate, indicating that ettringite displayed an excellent solidification effect on CrO₄²⁻ and reduced the content of soluble Cr. Meanwhile, the concentration of SO₄²⁻ in the leachate gradually increased, consistent with the addition of CrO₄²⁻, revealing that the replacement of SO₄²⁻ in ettringite by CrO₄²⁻ could redissolve SO₄²⁻ into the liquid phase. As the equilibrium constants of chromate ettringite (${10}^{-44.9\pm 0.3}$) were more than 3000 times higher than those of sulfate ettringite (${10}^{-44.9\pm 0.3}$), the chromate ettringite lattice displayed a low binding energy between components and poor binding capacity [18].

$$\begin{array}{cc}{\mathrm{Ca}}^{2+}+& {\mathrm{Al}\left(\mathrm{OH}\right)}_{4^{-}}+{\mathrm{CrO}}_4^{2-}+{\mathrm{SO}}_4^{2-}+{\mathrm{H}}_2\mathrm{O}\\ & \to \mathrm{CaO}·{\mathrm{Al}}_2{\mathrm{O}}_3·{(3 - \mathrm{X})\mathrm{CaSO}}_4·{32\mathrm{H}}_2\mathrm{O}+\mathrm{CaO}·{\mathrm{Al}}_2{\mathrm{O}}_3·{\mathrm{XCaCrO}}_4·{32\mathrm{H}}_2\mathrm{O}\end{array}$$

(4)

Note: 0 < X < 3.

Table 4. Leaching concentrations of SO₄²⁻ and CrO₄²⁻ in the samples (ppm).

Samples	SO42−	CrO42−
S0	7.38	ND
S10	15.19	3.54
S30	25.33	8.77

Note: ND = Not detected.

Table 4. Leaching concentrations of SO₄²⁻ and CrO₄²⁻ in the samples (ppm).

Samples	SO42−	CrO42−
S0	7.38	ND
S10	15.19	3.54
S30	25.33	8.77

Note: ND = Not detected.

As shown in Equation (4), when the solution contained a large amount of SO₄²⁻ and no CrO₄²⁻, the sulfate ettringite could stably exist with only a small amount of Ca²⁺, Al(OH)₃⁻, and OH⁻ components dissolved. With the incorporation of CrO₄²⁻, the reaction of CrO₄²⁻ with Ca²⁺, Al(OH)₄⁻, and OH⁻ in the solution caused a decrease in the saturation of the liquid phase, resulting in the further decomposition of the sulfate ettringite [19]. This decomposition and generation was continuous and directly related with the content of SO₄²⁻ and CrO₄²⁻ in the solution.

Therefore, the solidification effect of the samples on CrO₄²⁻ was investigated by adding different amounts of FGDG, as listed in

Table 5. Leaching concentration of CrO₄²⁻ at different levels of FGDG doping.

Samples	BFS (%)	CaO (%)	FGDG (%)	K2CrO4 (%)	CrO42− (ppm)
S0	80	10	6	4	4.23
S10			10		8.77
S30			14		34.91

. The concentration of CrO₄²⁻ in the leachate significantly increased with the increase in FGDG. This was because when SO₄²⁻ content in the solution was sufficiently high, it would compete with CrO₄²⁻, leading to the release of a small number of components in the liquid phase and limiting CrO₄²⁻ from participating in the reaction [20]. Therefore, the content of SO₄²⁻ exerted a direct influence on the efficiency of Cr solidification.

3.2.2. FTIR Analysis of Ettringite

The effects of different amounts of K₂CrO₄ on the chemical bonding and molecular structure of the ettringite were investigated by FTIR analysis. As shown in Figure 4, with an increase in K₂CrO₄ content, the absorption bands centered around 3452 cm⁻¹ and 1639 cm⁻¹ were reduced, and the stretching vibrations of the S-O bond in ettringite near 1116 cm⁻¹ significantly decreased.

The asymmetric stretching vibration peaks of H₂O in chromate ettringite appeared around 3400 cm⁻¹ and 1620 cm⁻¹, indicating that SO₄²⁻ in ettringite was replaced by CrO₄²⁻, and that the quantity and structure of crystal water in ettringite changed. Meanwhile, the crystallinity of ettringite was poor due to the relatively weak bonding strength of Cr-O. With an increase in CrO₄²⁻ content, the intensity of the bending vibration peak located near 617 cm⁻¹ in the spectrum of CaSO₄ greatly increased, which verified the replacement of SO₄²⁻ in ettringite by CrO₄²⁻ and the generation of CaSO₄ with Ca²⁺ in solution. The absorption band near 1426 cm⁻¹ was ascribed to CO₃²⁻ after the carbonization of samples, which remained unchanged with varying Cr content.

3.2.3. XPS Analysis of Ettringite

The valence distribution and binding energy change of Cr under different K₂CrO₄ doping contents were investigated by XPS analysis. Figure 5 presents the XPS curve-fitting of Cr. With an increase in Cr content, the main peaks of the binding energy of Cr 2p3/1 and Cr 2p3/2 increased and binding energy decreased, suggesting that the bond energy strength of Cr⁶⁺ became weakened by the doping of Cr. The main binding energy peaks of K₂CrO₄ were around 589.2 eV [21]. However, in the presence of Cr⁶⁺ in ettringite, its main binding energy peaks were located at 586.2 eV and 577.4 eV, respectively [22]. As shown in Figure 5, the main binding energy peaks of Cr in different samples ranged from 585 to 587.4 eV and 577.3 to 578 eV, indicating that Cr in different samples was dominated by Cr⁶⁺, accounting for more than 84%. These results further verified that the Cr of K₂CrO₄ entered the ettringite to generate chromate ettringite.

3.3. Study of the Solidification Mechanism of Cr in Friedel’s Salt

3.3.1. XRD Analysis of Friedel’s Salt

The XRD patterns of samples F10, F20, and F30 were tested to reveal the effect of K₂CrO₄ doping on the crystalline shape and lattice of Friedel’s salt, verifying its solidification mechanism of Cr. As shown in Figure 6, there was no distinct difference in the XRD patterns of different samples. With an increase in K₂CrO₄ content, the diffraction peak position of Friedel’s salt changed to a certain extent, but the intensity seemed to be unchanged, indicating that once the Cr entered the crystal of Friedel’s salt to make varied forms, the total amount was unchanged. The leaching concentrations of Cl⁻ and CrO₄²⁻ in the leachate of different samples were investigated by the horizontal vibration method, and the results are shown in

Table 6. Leaching concentrations of Cl⁻ and CrO₄²⁻ in the samples (ppm).

Samples	Cl−	CrO42−
S0	32.19	ND
S10	35.28	2.13
S30	39.67	7.62

Note: ND = Not detected.

. The concentration of CrO₄²⁻ in the leachate slightly increased with the addition of K₂CrO₄, suggesting that part of the CrO₄²⁻ entered the Friedel’s salt crystals for solidification and that the content of soluble Cr was reduced. Meanwhile, the Cl⁻ concentration also slightly increased, but with no apparent regularity. These results suggest that an increase in CrO₄²⁻ content led to the dissolution of partial Cl⁻ from Friedel’s salt. This can be due to two reasons: (1) the Cl⁻ was replaced and released into the solution, thus generating Friedel’s salt with more chlorine as Ca₄Al₂O₆Cl₂∙10H₂O; (2) the Cr in Friedel’s salt were not only replacing Cl⁻ with anions for solidification but were also reduced from Cr⁶⁺ to Cr³⁺ and solidified as cations.

3.3.2. FTIR Analysis of Friedel’s Salt

The effect of K₂CrO₄ addition on the chemical bonding and molecular structure changes of Friedel’s salt was investigated by FTIR analysis. As shown in Figure 7, as K₂CrO₄ content increased, the FTIR spectra of different samples hardly changed, and the absorption bands centered around 3450 cm⁻¹ and 1637 cm⁻¹ were basically unchanged with a slight reduction in the transmittance. This indicated that the doping of CrO₄²⁻ exerted a slight effect on the generated species of Friedel’s salt, but the total amount of Friedel’s salt was basically unchanged, which was demonstrated in Section 3.3.1. This was due to the generation of Friedel’s salt with different atomic compositions. The weak absorption at 1112 cm⁻¹ indicated that almost no ettringite was formed. In addition, there was no significant difference in the number and intensity of vibration peaks in the absorption bands of Friedel’s salt around 538 cm⁻¹ and 425 cm⁻¹. The weak absorption band located near 1488 cm⁻¹ was also caused by the carbonization of CO₂, and remained unchanged with the addition of Cr.

3.3.3. XPS Analysis of Friedel’s Salt

The valence distribution and binding energy changes of Cr in Friedel’s salt at different levels of K₂CrO₄ doping were investigated by XPS analysis. The XPS curve-fittings of Cr for the split peaks are shown in Figure 8. The Cr in Friedel’s salt crystals coexisted in two valence states, and the binding energy peaks of both Cr⁶⁺ and Cr³⁺ increased with an increase in Cr content, indicating that the Cr content of both valence states increased. According to the calculation of the peak areas, the contents of Cr³⁺ and Cr⁶⁺ in Friedel’s salt were almost equal when the cementitious system was mixed with 2 and 4% of K₂CrO₄. This was because the potential activity of BFS displayed a slow excitation rate with the absence of SO₄²⁻ in the cementitious system and reducing substances such as Fe²⁺ and S²⁻ in the BFS could reduce some amount of Cr⁶⁺ to Cr³⁺. The residual Cr⁶⁺ would replace the Cl⁻ in Friedel’s salt in the form of CrO₄²⁻ to enter Friedel’s salt crystals, whereas the Cr³⁺ replaced the Al³⁺ to enter Friedel’s salt crystals. The reaction equation is shown as in Equation (5). The Cr³⁺ and Cr⁶⁺ in sample F30 with 6% K₂CrO₄ doping accounted for 39.2 and 60.8% of the total Cr, respectively. The percentage of Cr⁶⁺ significantly increased, whereas the percentage of Cr³⁺ showed a downward tendency. This was probably due to the depletion of almost all the reducing substances in the BFS and the increased proportion of solidified Cr in the form of CrO₄²⁻ in Friedel’s salt.

It can be seen from Figure 8 that the binding energies of Cr 2p3/1 and Cr 2p3/2 decreased with an increase in K₂CrO₄ content. As Cr was incorporated into the system, the binding energies of Cr³⁺ and Cr⁶⁺ became weakened after the partial reduction of Cr⁶⁺, and the bonds energy strengths of Cr³⁺ and Cr⁶⁺ became weaker. The above conclusion validated the results in Section 3.3.1, suggesting that the Cr in Friedel’s salt was solidified by replacing Al³⁺ and Cl⁻, and that Cr entered the crystal in the form of Cr³⁺ and CrO₄²⁻, which was different from the solidification mechanism of Cr in ettringite.

$${\mathrm{Ca}}^{2+}+{\mathrm{Al}\left(\mathrm{OH}\right)}_{4^{-}}+{\mathrm{Cr}\left(\mathrm{OH}\right)}_{4^{-}}+{\mathrm{CrO}}_4^{2-}+{\mathrm{Cl}}^{-}+{\mathrm{H}}_2\mathrm{O}\to 3\mathrm{CaO}·{\mathrm{M}}_2{\mathrm{O}}_3·\mathrm{CaX}·{10\mathrm{H}}_2\mathrm{O}$$

(5)

Note: M = Al³⁺ or Cr³⁺; X = 2Cl⁻ or CrO₄²⁻.

3.4. XANES Analysis of Solidification Cr of Hydration Products

The Lm-edge XANES spectrum of Cr is quite sensitive to the atomic near-neighbor structure, and the information of near-neighbor structure of the Cr ions could be obtained by comparison of spectral lines. The more vacant orbitals there were that were suitable for leapfrogging, the stronger the characteristic peak of the Lm absorption edge with the electron transition to the vacant orbitals. Therefore, the energy calibration of the sample was carried out through a standard chromium foil, and the standard chromium foil was measured as a reference. The Cr-edge XANES spectra were mainly marked by three main peaks, namely leading edge, white line, and trailing edge. The XANES spectra of samples S0, S10, and S20 are shown in Figure 9a. The Cr spectrum displayed the characteristic peak of Cr³⁺ at 5993 eV, and Cr⁶⁺ at 6010 and 6025 eV [23,24]. Three samples showed similar patterns without a leading edge peak, suggesting their similar coordination environments and the absence of Cr³⁺ in ettringite. The high similarity of XANES spectra among the different samples and K₂CrO₄ reference indicated that the Cr in different samples basically existed in the state of Cr⁶⁺ in ettringite. The XANES spectra of samples F10, F20, and F30 are shown in Figure 9b. Compared with the spectra shown in Figure 9a, a front edge peak appeared in the spectrum of Friedel’s salt, which was very similar to the spectrum of Cr₂O₃ reference samples. In addition, three main peaks marked in the plots showed obvious characteristics, indicating that Cr³⁺ and Cr⁶⁺ coexisted in Friedel’s salt.

4. Discussion

In general, the solidification of Cr by the hydration products generated by slag-based cementitious materials was mainly in the following ways: (1) Cr⁶⁺ enters hydration product crystals to form chromate-bearing products by replacing SO₄²⁻ in the form of CrO₄²⁻; (2) Cr⁶⁺ is reduced to Cr³⁺ by Fe²⁺ and S²⁻ in BFS during hydration reactions, and then Cr³⁺ replaces Al³⁺ to enter hydration product crystals as cations; (3) after reacting with free Ca²⁺ in the solution to form CaCrO₄ precipitate, CrO₄²⁻ is attached to the surface of hydration products; (4) Cr forms CrO₄²⁻ and is then physically adsorbed onto the surface of the hydration products by van der Waals forces [25].

Combined with the results of our previous studies, the pH of the leachate of the paste bulk was between 11 and 12. The CaCrO₄ precipitation could not stably exist in the strongly alkaline environment, and the OH⁻ provided by the alkaline material was the most competitive for the adsorption sites, which meant that the CrO₄²⁻ released by the decomposition of CaCrO₄ could not be solidified via adsorption; thus, it would re-dissolve into the liquid phase.

5. Conclusions

In this work, the solidification mechanism and final state of Cr in different hydration products were investigated by XRD, XPS, FTIR, and XANES. The ettringite demonstrated the excellent solidification effect on CrO₄²⁻, which reduced the content of soluble Cr. The replacement of SO₄²⁻ by CrO₄²⁻ in the ettringite crystals made SO₄²⁻ become soluble in the liquid phase. When there was sufficient SO₄²⁻ content in the solution, it would directly affect the solidification efficiency of Cr. The XPS pattern of Cr in ettringite showed that the main binding energy peaks of Cr in the different samples ranged from 585 to 587.4 eV and 577.3 to 578 eV, verifying that the major form of Cr was Cr⁶⁺, with a percentage above 84%.

The Cr in two valence states coexisted in Friedel’s salt, due to the slow excitation rate of the potential activity of BFS in the absence of SO₄²⁻ in the system. Reducing substances, such as Fe²⁺ and S²⁻, in the BFS could reduce some amount of Cr⁶⁺ to Cr³⁺. Then, Cr³⁺ replaced Al³⁺, and the residual Cr⁶⁺ replaced Cl⁻ in Friedel’s salt as CrO₄²⁻ in the crystal. When Cr³⁺ and Cr⁶⁺ entered Friedel’s salt crystals, the coordination number increased and bond energy strength weakened with the decrease in the binding energies of Cr 2p3/1 and Cr 2p3/2.

The Cr XANES spectra of ettringite indicated that the coordination environments of the samples were similar without a leading edge peak, suggesting that Cr³⁺ was absent in ettringite and that the Cr in ettringite was basically in the state of Cr⁶⁺. The XANES spectrum of Cr in Friedel’s salt showed a leading edge peak similar to the Cr₂O₃ reference sample, and the three main peaks showed obvious characteristics that confirmed the existence of both Cr³⁺ and Cr⁶⁺ in cementitious system.

This study provides new ideas for the recovery and utilization of solid waste BFS and FGDG, as well as valuable solutions for solving the contamination problems of stockpiles and treating solid waste containing Cr. This research offers important guidelines for low cost and safe disposal of hazardous waste containing other heavy metals. Nevertheless, this study only focused on exploring the solidification mechanism of Cr in solid waste. Further investigation is needed for the solidification mechanism of Cr in wastewater.