1. Introduction
Red mud (bauxite residue) is a highly alkaline ultrafine-grained waste material generated during the industrial production of aluminum oxide from bauxite [
1,
2,
3]. China is now a world-leading producer of aluminum oxide, accounting for approximately 30% of the total production in the world. In 2019, aluminum oxide production exceeded 70 million tons in China. This production will give rise to the formation of a large quantity of hazardous substances (e.g., heavy metal ions and radioactive elements) in the byproducts (e.g., solid waste materials and leachate), causing serious damage to the environment and human health [
4,
5]. The disposal of red mud requires a large storage space, and the infiltration of various chemical elements into soils may cause soil salinization and groundwater pollution [
6]. Importantly, long-term exposure to these hazardous substances can have devastating consequences on human health. Thus, there is an urgent need for the safe disposal of red mud in China to minimize its negative impacts. Red mud can be utilized for various applications [
7,
8,
9]. Some useful materials, such as valuable metals, can be recovered from red mud that otherwise would be disposed of as waste [
10,
11]. Red mud can also be used as raw materials for the production of bricks [
12,
13], concrete [
14], admixtures for subgrade and high-performance concrete, calcium-silicon compound fertilizer, microporous calcium silicate heat insulation products, ceramic composite materials, thermal insulation and refractory materials [
15].
The main pollutants of red mud include heavy metal ions, fluoride, radioactive substances, and sodium and aluminum ions. The extremely high pH of red mud (>12) can be attributed to the presence of a large quantity of highly alkaline chemicals, making it corrosive to various biological and metallic materials (e.g., aluminum and steel materials) and siliceous materials (e.g., glass) [
14]. There are considerable concerns about the potential leaching of red mud leachate into groundwater, which can cause an increase in pollutants and the pH value of groundwater [
16]. In this regard, it is of practical importance to gain a better understanding of the changes in the physicochemical and mechanical properties of soils subjected to red mud pollution [
4,
17]. Neutralization of red mud with acids (HCl, HNO
3 and H
2SO
4) before storage can be an effective way to make red mud environmentally benign [
18]. Han et al. [
7] investigated the effectiveness of bauxite neutralization using atmospheric CO
2 and whether neutralization could be accelerated when Ca
2+ sources were supplied in solid (flue gas desulfurization gypsum) and aqueous (CaCl
2 solution) forms. Obviously, neutralization of red mud can cause changes in the physicochemical, microstructural, and mechanical properties of red mud. From this perspective, red mud can be utilized to improve the cementation of acid soils, which provides a promising approach to improve their microstructure [
9,
18]. Bai et al. [
19] investigated the effects of flow velocity and the concentration of red mud particles and OH
− ions on the penetration of red mud filtrate with very fine particles in a porous medium. In recent years, red mud has started to be considered as a potential valuable resource instead of waste, and it has been used for the remediation of soils contaminated by high concentrations of metals/metalloids [
9].
In this study, we investigated the changes in the physicochemical and microstructural properties of typical red mud in China subjected to acidic and alkaline treatments, which were compared with those of a typical silty soil. This study can provide some insights into the safe disposal of red mud, which has good engineering significance for soil restoration.
4. Variation in The Main Metal Elements Under the Acidic and Alkaline Conditions
Table 4 indicates the changes in the main metal elements (i.e., Na, Al, Fe, Ca), and Si and Cl in the weight percentage of the solid particles for the Shanxi silty soil, Shanxi red mud and Shandong red mud immersed in a 4 mol/L HCl or NaOH solution for 120 days (28 days only for the Shandong red mud under alkali action). The measured element contents (EDX method) are actually the contents of some elements of the solid particle sample by the analysis of the dispersive X-ray spectra. In fact, after the acid or alkali treatment, the mineral structure of the solid particles changes obviously and generally exists in the form of compounds or ions due to the addition of acid or alkali solution. Some ions are leached in the filtrate, which causes the difference in the various elements for the same soil to some extent. For clarity, the compositions of the solid particle were expressed in the form of element. Overall, the measured results can explain the difference in the main metal elements of solid particles for the different types of soils (e.g., between the Shanxi silty and Shanxi red mud).
Table 4 shows that the Shanxi silty soil initially contained 4.94% Ca (i.e., untreated sample) but almost no Na. The immersion in the NaOH solution results in a 9.42% increase in Na. The Si content in the original Shanxi silty soil accounted for 19.66% of the total weight, which is much higher than that in the Shanxi red mud and Shandong red mud (i.e., 9.09% and 8.24%, respectively). However, immersion in the HCl solution results in almost no increase in Na but a 15.96% increase in Cl for the Shanxi silty soil.
Table 4 also shows that there is a significant difference in the chemical composition between Shanxi and Shandong red mud. For instance, the Shandong red mud contains less Ca (not detected due to its small amount) but more Fe (19.30%) than the Shanxi red mud (11.64% and 3.59%, respectively). This conclusion can also be obtained based on the measurement result after the acid treatment (i.e., 4 mol/L HCl solution) for the Shanxi red mud and Shandong red mud (i.e., Ca: 11.75% and 1.1%; Fe: 2.71% and 12.64%, respectively; see the HCl solution column in
Table 4). Hence, the Shandong red mud presents a red-brown color (see
Figure 1b) due to the existence of a large number of Fe
3+ ions. It is important to note that the Al content is still as high as 10.04% and 9.63% for the Shanxi and Shandong red mud after the extraction of aluminum oxide, respectively. Approximately 7.83% and 8.87% of Na was detected in Shanxi red mud and Shandong red mud, respectively, whereas no Na was detected in the Shanxi silty soil (see the untreated column in
Table 4), indicating that the presence of Na in the red mud can be mainly attributed to the addition of alkaline substance NaOH during the production of aluminum oxide.
Table 4 indicates that the Shanxi red mud immersed in the NaOH solution shows an increase in Na content from 7.83% to 14.23% of the untreated sample, whereas that immersed in the HCl solution shows an increase in Cl content to 25.39%, and 75.7% was leached out in the Na
+ content of the untreated sample (i.e., from 7.83% to 1.9%) due to the release of Na
+ that formerly existed in the solid matrix of the red mud into the filtrate. In addition, the Fe content is reduced to 75.5% of the untreated sample (i.e., from 3.59% to 2.71%) and that of other contents (e.g., Al and Si) is also significantly reduced. Similarly, the Shandong red mud immersed in the HCl solution also shows a 10.4% decrease in the Na content (i.e., from 8.87% to 0.92%;
Table 4). Thus, it can be concluded that Na
+ is the predominant exchangeable cation in the fresh red mud residue.
Sahu et al. [
20] investigated the ability of an activated CO
2-neutralized red mud to remove arsenate (As(V)) from aqueous solutions and showed that the sorption capacity of red mud increases after neutralization. However, it has been reported that red mud treated with an HCl solution has a 30% lower capacity to adsorb metals compared with an untreated red mud [
9,
18,
21]. Rai et al. [
18] observed a similar phenomenon by many neutralization methods (e.g., acid neutralization, CO
2 neutralization, sintering, seawater neutralization, and bioremediation), especially for the Na
+ content. Partial dissolution of cancrinite in an acid-treated red mud causes an increase in other phases, e.g., hematite. In particular, there is a higher fraction of water-soluble and exchangeable metals in an acid-treated red mud than in an untreated red mud. Hence, neutralization is a promising method to reduce the adverse environmental impacts resulting from an uncontrolled release from storage, which also has a favorable economy. Obviously, the immersion of red mud in an HCl solution can also have an effect on its physicochemical, microstructural, and mechanical properties.
5. Microstructure of Red Mud under Acidic and Alkaline Conditions
Primary and secondary mineral components of common soils (e.g., silt and clay) are rarely present in red mud.
Figure 4a clearly shows that Shanxi red mud particles are composed of andradite (hydrated, Ca
3(Fe
0.87Al
0.13)
2(SiO
4)
1.65(OH)
5.4, 27.7%), tridymite 2H substructure (SiO
2, 6.0%), coesite (SiO
2, 26.3%), sodium aluminum silicate carbonate (Na
8(AlSiO
4)
6(CO
3)
1.09, 17.6%), calcite (Ca(CO
3), 18.0%) and hematite alpha (Fe
2O
3, 4.4%), as determined by X-ray diffraction (XRD, D8 Advance Mode, Bruker, Karlsruhe, Germany).
Figure 4b shows that the Shandong red mud particles are composed of aluminum titanium oxide (Al
2TiO
5, 17.6%), titanium aluminum oxide (Ti
2.79Al
1.21O
0.024, 2.4%), aluminum iron oxide (FeAlO
3, 17.9%), hydrohematite (Fe
2O
3⋅xH
2O, 23.9%), macaulayite (Fe
24Si
4O
43(OH)
2, 23.2%) and histinerite (Fe
2Si
2O
7⋅xH
2O, 14.9%).
Figure 5,
Figure 6 and
Figure 7 show the changes in microstructure (SEM images) of the Shanxi silty soil, Shanxi red mud and Shandong red mud immersed in a 4 mol/l HCl or NaOH solution for 120 days, including the untreated samples.
The results show that the two red muds are honeycombs with particles (
Figure 6 and
Figure 7) that are much smaller (<10 μm) than that of silty soil (
Figure 5). For silty soil, even if an acid (HCl) or alkali (NaOH) solution is added, the crystal structures of their mineral particles are still relatively complete. Red mud particles are poorly crystallized and contain amorphous substances in dispersed and disordered states. The fine particles of red mud form a solid mass with very few large pores and low hydraulic conductivity. Obviously, very few plants can survive on red mud due to its low infiltration rate and high water-holding capacity. Red mud is generated after a series of complex physical and chemical processes, especially after dissolution in a highly alkaline NaOH solution. Thus, the structures of primary and secondary mineral components in red mud can be greatly destroyed, indicating that red mud no longer has the physicochemical and mechanical characteristics of common silty soil or clay soil. For this reason, the interactions between the solid particles and pore water cannot be directly explained by the exchange between the bound water and free water. Thus, physical parameters such as the plasticity index for the classification of silty soil and clay soil no longer apply to red mud, and it is necessary to establish more relevant standards for the classification of red mud based on its particular physicochemical and microstructural properties.
Figure 5 shows that there is a dramatic change in the microstructure of the Shanxi silty soil immersed in the HCl or NaOH solution. According to the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory [
22], the addition of acidic substances into soils makes it easier for fine solid particles in soils to form a loose suspended state (
Figure 5a), while addition of alkaline substances makes it easier for finer solid particles to be attached to the surface of larger particles (
Figure 5c). Under alkaline conditions, the zeta potentials of some materials, such as quartz sands and kaolinite particles, tend to increase with increasing pH value [
23]. The potential energy is the sum of the repulsion and the attraction components, indicating an increase in the thickness of the adsorption layer [
24]. This increase can be attributed to the increase or decrease in the repulsive force between the solid matrix and fine suspended particles under the actions of the HCl or NaOH solution [
25,
26]. In general, the solid particles of silty soil (
Figure 5b) are larger than those of red mud (
Figure 6 and
Figure 7), indicating the presence of a small percentage of fine particles between larger particles.
As mentioned in
Section 3.2, there will be an obvious change in the specific surface area of the Shanxi silty soil when it is immersed in the HCl or NaOH solution. This outcome can be attributed to the formation of amorphous substances due to the chemical reaction of silt particles in the HCl, but it is possible that a porous product is simply formed, which are present in a suspended state (
Figure 5a). On the contrary, it is obvious that the amorphous substance is recrystallized into well-crystallized compounds in alkali (i.e., NaOH solution;
Figure 5c), which is obvious in the Shandong red mud in
Figure 7c.
A comparison between
Figure 6 and
Figure 7 shows a significant difference in microstructures between the Shanxi red mud and Shandong red mud. For the Shandong red mud, the solid particles tend to be smaller and more uniform, whereas those of the Shanxi red mud are slightly larger and filled with finer solid particles. However, solid particles immersed in a 4 mol/L HCl solution become more cloudy compared with those immersed in a 4 mol/L NaOH solution, and the formation of red mud agglomerates under acid neutralization can also be seen from the SEM images (
Figure 6 and
Figure 7). In fact, the morphological structure of untreated red mud indicates scattered fine particles. The HCl solution results in the occurrence of chemical reactions among the solid particles and between the solid particles and pore water in the red mud and consequently the formation of new amorphous chemical substances and an increase in muddiness.
Overall, acid neutralization results in an increase in the aggregation and uniform distribution of red mud particles, which promote the formation of macroaggregates [
20]. This result will increase the water permeability of the acid treated soil layer and improve the growth ability of plants.
6. Conclusions
Typical red mud in China has a high cation exchange capacity and active physicochemical properties, which can be closely related to its extremely high alkalinity and complex microstructure. The neutralization of red mud with the HCl solution results in the release of Na+ from red mud particles into the leachate, which can effectively decrease the pH value in the filtrate. On the other hand, the migration of the red mud filtrate also causes alkalization damage to common soil (e.g., silty clay).
Red mud generally has a higher liquid limit and plastic limit but a lower plasticity index (IP = 4.8–18.7) than silt (IP = 4.5–31.3) under either acidic or alkaline conditions. The acid neutralization of red mud can also result in a significant decrease in the liquid limit, plastic limit and plasticity index, whereas the opposite is observed for the addition of the NaOH solution. In this sense, the neutralization of acid can significantly improve the cementation property of red mud. In addition, the specific surface area of red mud immersed in the NaOH solution decreases, whereas that immersed in the HCl solution increases. For instance, the specific surface area of the Shandong red mud immersed in the HCl solution is increased by 2.64 times, whereas that of the Shanxi red mud is increased by 1.55 times. That is, neutralization can also improve the growth ability of plants, which has good engineering significance for soil restoration.
It is no longer acceptable to classify red mud according to the standards obtained based on common soils with low ion concentrations and neutral pH values, and it is necessary to establish more appropriate standards for the classification of red mud based on its particular physicochemical and microstructural properties.