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 Cr
3+ and Cr
6+. Cr
3+, an essential trace ion necessary for human body, has weak mobility and low toxicity, whereas Cr
6+ 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 Cr
3+ to Cr
6+ 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 Cr
3+ to Cr
6+ into CrO
42− or HCrO
4− anions. The Cr
6+ presents a lower adsorption for negatively charged particles and hydration products than Cr
3+, resulting in its strong solubility and mobility in alkaline environments. Thus, the existence of Cr
6+ 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. Cr
6+ mainly exists in the form of acid ions such as CrO
42− and Cr
2O
72− in hydration products. Owing to the oxidation environment provided by the hydration reaction process of Cr
6+, 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 Cr
6+ in hydration products, such as calcium sulfoaluminate, could be easily reduced to Cr
3+ 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 Ca
2CrO
5·3H
2O 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 Cr
6+ 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 CrO
42− than SO
42−, 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 Cr
6+ would reduce the solidification efficiency of other heavy metal ions, and the Cr
6+ would redissolve once the environmental conditions changed. The study of Zhang et al. demonstrated that Cr
6+ mainly existed as CrO
4-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 Cr
6+ 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 Cr
6+ at pH 8.0~12.7, and Cr
3+ at pH < 8. Most Cr exists in the form of CrO
42−, replacing the SO
42− 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.