Environment Assessment of Modified Red Mud Utilized in Roadbed

Abstract

Utilization of red mud in road projects is an effective way to consume large amounts of red mud on a large scale. In order to meet the requirements for road performance, a modified material, Heinchem, has been developed on the basis of extensive experiments, and the long-term environmental risks of red mud modified by this material have been investigated. By collecting and modifying original red mud samples, a series of continuous leaching tank experiments are carried out based on the exposure scenario analysis. According to the leaching content of pollution in the original and modified red mud, the characteristic pollutants are identified. The release mechanism of these characteristic pollutants in the modified red mud is revealed, and the long-term release amount is predicted. Furthermore, in light of the actual road use scenario of the modified red mud, a risk assessment model is established and used to simulate the release, migration, and transformation of characteristic pollutants during the use of modified red mud as roadbed material. The groundwater environmental risk is then assessed. Finally, an acute toxicity test of earthworms and a seed germination test are conducted to investigate the impact of the modified red mud on the farmlands. The results showed that the proposed red mud modified materials have obvious curing effects on V, As, Se, Mo, and F. When the leaching contents of V, Cr⁶⁺, C^r3+, As³⁺, Se⁴⁺, Se⁶⁺, Mo, and F in the modified red mud were lower than 0.15 mg/L, 0.1 mg/L, 0.2 mg/L, 0.012 mg/L, 0.012 mg/L, 0.012 mg/L, 0.075 mg/L, and 1.2 mg/L, respectively, the environmental risk of modified red mud during long-term road use is acceptable. This study provides a new way for the resource utilization of red mud.

1. Introduction

Red mud is a highly alkaline solid waste generated during the process of extracting alumina from bauxite. With the rapid development of the aluminum industry, the accumulation of red mud is becoming an increasingly serious environmental problem [1]. China is the world’s largest producer of alumina [2]. In 2021, China’s alumina production reached 77.475 million tons, accompanied by the production of up to 108.462 million tons of red mud [3]. The huge production of red mud makes red mud treatment a challenge, but only 10% of red mud is reused [4], and the majority of red mud is directly stacked or stored through dam construction [5]. This storage method not only occupies a large amount of land resources but can also potentially cause harm to the surrounding environment due to the high alkalinity, strong corrosiveness, and enrichment of toxic substances such as heavy metals in red mud.

Researchers have conducted extensive research on the resource utilization of red mud, including the preparation of building materials (such as pottery and filling materials), the recovery of valuable metals (such as Fe, Ti, Sr), and environmental remediation materials (such as heavy metal treatment and soil improvement) [6,7,8,9,10]. Due to strong salt alkalinity, special physicochemical properties, and high environmental risks, the utilization of red mud resources is basically limited to the laboratory stage, and there is no economically feasible and risk-controllable large-scale utilization model [11]. With the rapid development of global infrastructure construction, the annual usage of roadbed materials exceeds 3 billion tons. Natural soil and stone materials, as well as cementitious materials, are being attempted to partially or completely replace the raw materials of road bases with industrial solid waste, which has also become an effective way to consume solid waste on a large scale [12,13,14,15].

The application of red mud activated cementitious materials in road engineering can not only consume a large amount of red mud, reduce environmental, economic, and safety issues caused by stacking, but also reduce the large demand for building materials such as cement and lime. It is currently an effective way to solve the large-scale utilization of red mud [16]. For example, in the pilot plant constructed by Shandong Expressway Group in Binzhou Beihai Economic Development Zone, industrial waste red mud is harmlessly processed and used as a new material in highway construction. It was successfully used to replace cement in the paving of the grass-roots level of the ramp at Zhangqiu North Service Area of the Jinan–Gaoqing Expressway Project [17]. The roadbed provides the foundation for the road surface, while the roadbed soil undergoes expansion and contraction. Therefore, materials such as cement and lime are commonly used in engineering to improve the strength of the roadbed. For example, Ahmad et al. [18] explored the use of plastic waste (PW) as aggregates or fibers in cement mortar and concrete manufacturing and found that PW as fibers can enhance mechanical properties; Kakasor Ismael Jaf et al. [19] used a variety of machine-learning models to predict the compressive strength of modified concrete with varying fly ash content at different mix proportions, emphasizing the power of leveraging data-driven methods to improve compressive strength prediction accuracy and reliability. In addition, red mud activated cementitious materials can effectively improve the problems of poor water stability and easy settlement in poor quality roadbed soil [20,21]. Therefore, domestic and foreign scholars have conducted extensive research on red mud activated cementitious roadbed materials.

James et al. [22] studied the mechanical properties of the Bayer method red mud lime solidified expansive soil roadbed in response to the problem of slow strength development and delayed construction period of traditional roadbed materials. They found that the compressive strength of lime expansive soil roadbed developed faster under the action of Bayer method red mud, which can effectively reduce the required construction period for waiting for strength growth. Cui et al. [23] used the Bayer method of red mud to excite cementitious materials to improve silty clay to solve engineering problems such as insufficient bearing capacity and road swelling in silty clay roadbeds. The results showed that under the same dosage and curing period, the 7-day UCS of the red mud improvement group increased by 0.3 to 1.7 MPa compared to cement soil. Sun et al. [24] prepared a high fluidity, lightweight cementitious pouring material by mixing sintered red mud with cement, fly ash, and water reducing agent. It can be injected into sections with limited construction space through pouring, and the CBR (California bearing batio) value is between 16–24%. It has been applied in the backfilling of retaining walls in highway expansion projects. Compared to traditional roadbed materials that can be obtained locally, red-mud-activated cementitious materials used in roadbeds do not have the cost advantage of large-scale replacement of traditional roadbed materials, and their application range is small. They can only be used as low-quality roadbed soil improvement materials, resulting in a lower consumption of red mud (<10%) [22].

Red mud contains heavy metals such as Cu, Pb, As, Ba, etc., which vary depending on the source of bauxite and the production process [25]. Li et al. found that after one day of leaching red mud, heavy metals such as Pb, Cr, Cd, As, Hg, Ba, etc. in the leaching solution exceeded the standard for Class IV water in the Environmental Quality Standards for Surface Water. When applying red-mud-activated cementitious materials to road engineering, it is necessary to consider the risk of heavy metal leaching from the cementitious materials during the road use process [26]. Chen et al. [27] used 15% red-mud-modified loess to prepare road base materials. The concentration of heavy metals in the leaching solution of the modified loess is very low, meeting the Class IV water quality standard for groundwater. In an alkaline environment, Cr³⁺ and Cd²⁺ generate hydroxide precipitation with very low solubility, which is consolidated in the gel generated during hydration or alkali activation of red mud activated cementitious materials [28,29]. Jiminez et al. [30] believe that during the alkaline excitation process of red mud, the silicon oxide tetrahedra and aluminum oxide octahedra gradually break apart and a large number of anions appear inside. Metal ions can compete to replace the cation sites of Na⁺ and Ca²⁺ in the spatial structure to balance the electricity price. As, Cs, Sr, etc., are adsorbed by the cementitious material in the form of chemical bonds and encapsulated inside the cementitious material. AFt in hydration products has a curing effect on Pb and Zn. In the hydration reaction, when the environmental humidity is less than 85%, and SO₄⁻ is almost completely consumed. The SO₄²⁻ is insufficient to continue synthesizing ettringite with Ca²⁺ and AlO²⁻. At this time, appropriate gypsum supplementation is necessary. Under the action of SO₄²⁻, it is beneficial for Pb²⁺, Zn²⁺, and Cd²⁺ to be directly captured by ettringite in a dissolved state, making the heavy metals in the red mud synchronized and stable, thereby reducing the risk of heavy metal leaching from the red mud activated cementitious material [31,32,33]. During the formation of hydration products of red-mud-activated cementitious materials, heavy metals can be solidified by physical wrapping, chemical adsorption, gel, and other methods [34]. However, under complex road environmental conditions, the solidification effect of hydration on heavy metals is not stable, and the solidification/release process of heavy metals may affect the surrounding environment and groundwater.

At present, most of the studies on the utilization of red mud in road engineering focus on the selection of red mud curing materials, the design of mixing ratios, and the enhancement of mechanical properties, but there are fewer studies on the leaching of pollutants and the assessment of environmental risks. To this end, this study aims at evaluating the long-term environmental risk of modified red mud when it is used as a roadbed material based on laboratory tests and theory analysis. The structure of the present paper is as follows: first, the tested materials and methodology are introduced. Then, the identification of characteristic pollutants in modified red mud is presented. On this basis, the environmental risk assessment of modified red mud is elaborated, including the release patterns of characteristic pollutants in modified red mud, the environmental risk assessment under the actual road use scenario, and the impact on farmland. Finally, some conclusions are drawn.

2. Materials and Methods

2.1. Sample and Specimen Preparation

Seven original red mud samples were collected from the red mud storage yard of Chiping Xinfa in Shandong, China, with a generation time span from 2010 to 2019. The sample information is detailed in

Table 1. List of red mud and modified red mud sample information.

Sample Number of Original Red Mud	Generation Time	Sample Number of Modified Red Mud
		With 8% Heinchem	With 12% Heinchem
XFCN1	2009	XFCN1-8	XFCN1-12
XFCN2	2010	XFCN2-8	XFCN2-12
XFCN3	2014	XFCN3-8	XFCN3-12
XFCN4	2016	XFCN4-8	XFCN4-12
XFCN5	2017	XFCN5-8	XFCN5-12
XFCN6	2018	XFCN6-8	XFCN6-12
XFCN7	2019	XFCN7-8	XFCN7-12

. The physical and mechanical properties of sample XFCN1 are listed in

Table 2. Comparison of physical and mechanical properties between natural and modified red mud.

Physical and Mechanical Index Parameters	Original Red Mud (XFCN1)	Modified Red Mud (XFCN1-12)	Remarks
Optimal moisture content (%)	28.5	26.8	Compaction test
Maximum dry density (g/cm3)	1.74	1.71
Liquid limit (%)	45.4	36.4	Atterberg limits test
Plastic limit (%)	29.6	21.5
Plasticity index	15.8	14.9
Internal friction angle (°)	24.2	36.8	Undrained triaxial test
Cohesion force (kPa)	31.0	143.2
7-day non-immersion strength (kPa)	506	3080.6	Uniaxial compression test
7-day immersion strength (kPa)	0 (disintegration)	3047.1
CBR2.5 (%)	2.8	94	CBR test
Compression coefficient a1-2 (MPa-1)	0.19	0.07	Oedometer test
Free swelling rate (%)	17	5	Soil swelling test
Swelling ratio without load (%)	0.05	0

. The mechanical indexes are measured under the condition of 96% compaction degree. The chemical composition of sample XFCN1 is listed in

Table 3. Chemical composition of original red mud.

Sample of Original Red Mud	Chemical Composition
	SiO2	Fe2O3	Al2O3	CaO	MgO	TiO2	Na2O	K2O	Loss
XFCN1	28.09	25.54	19.81	1.71	0.29	2.15	10.17	0.31	11.43

. The results for the other samples are similar to the ones for XFCN1 and are not listed for simplicity’s sake. It can be seen that the mechanical properties of the original red mud are poor. For example, the California bearing ratio value at 2.5 mm penetration (CBR2.5) is 2.8%, which cannot meet the requirements of roadbed performance according to China’s specifications for the design of highway subgrades (JTG D30-2015) [35]. To this end, a polymer composite material, named Heinchem, is proposed to treat the original red mud. Heinchem is composed of 55.0% 425R ordinary Portland cement, 41.4% phosphogypsum, 2.3% ferrous sulfate, 1.2% silica, and 0.1% polyacrylamide. The physical and mechanical properties of the modified red mud, corresponding to the original sample XFCN1, with 12% Heinchem (XFCN1-12, see Table 1) are also listed in Table 2. The mechanical performances of the modified red mud, including strength and deformation characteristics, are significantly better than those of the original red mud, and meet the requirements of roadbed performance.

Two types of modified red mud samples were prepared by adding 8% and 12% Heinchem material to the seven original red mud samples. The raw and modified red mud powder samples were dried naturally at room temperature in the laboratory, the dried samples were sieved and set aside. Five specimens were prepared according to the predicted optimum moisture content and simmered for one day and night for compaction experiments. The electric compactor was set to compact 27 times, and 400–500 g masses of the modified red mud were poured into the cylinder 5 times. After being compacted 5 times, the instrument was turned off, and the specimen was slightly higher than the surface of the cylinder but did not exceed the top surface of the cylinder by 6 mm. The center of the specimen was put into a small aluminum box for weighing and was then dried it to find the mass of the dry soil. The dry density of the specimen was found by calculating the water content based on the mass of the wet soil and the dry soil and repeating the above experimental steps. The dry density of the specimen was calculated, repeating the above experimental steps according to the dry density and moisture content of the five compacted specimens. A curve was drawn between the dry density and moisture content to find the maximum dry density of the specimen and the best moisture content. In this way, the specimen with a diameter of 100 mm, a height of 100 mm, and a compaction degree of 96% was prepared. After the preparation of the specimens, they were cured at constant temperature and humidity (temperature 20 ± 2 °C, relative humidity 95%) for 7 days.

2.2. Exposure Scenario and Experimental Method

The road structure design with the proposed modified red mud is shown in Figure 1, where the modified red mud is designed to partially replace the cement stabilized macadam base and completely replace the lime soil roadbeds. The main exposure pathway of modified red mud roads during operation is that the pollutants in modified red mud are released and migrated to groundwater owing to rainwater infiltrating into the road structure. Therefore, the influence of modified red mud on the groundwater environment is focused on long-term risk assessment.

The compacted modified red mud base can be regarded as a compact block structure. Therefore, the recommended method in the Dutch standard “container experiment for testing the diffusion leaching behavior of inorganic components in block waste or building materials” (EA NEN 7375-2004) [36] is used to understand the leaching characteristics of pollutants, as shown in Figure 2. Under the actual roadbed application conditions of modified red mud, the groundwater environment is influenced by the rainfall, the pH value of rainwater, and the release rate of pollutants. According to these factors, the specific simulation parameters, including the type and pH value of leaching agent and the liquid (volume)–solid (mass) ratio, can be determined. In this study, pure water is used as the leaching agent considering the strong alkalinity of red mud. The dimensions of the specimens are 100 mm in height and 100 mm in diameter. The liquid–solid ratio is 10 L/kg. In addition, the continuous leaching method is adopted according to the least favorable principle, in which the leaching agent is replaced 0.25, 1, 2.25, 4, 9,16,36, and 64 days after the start of the experiments.

Red mud is a strongly alkaline solid waste. In addition to its high pH, the fluorine and metal elements in it are also environmental issues worthy of concern. In this study, the PH value of the leaching solution is measured according to the Chinese standard “Glass Electrode Method for the Determination of Corrosivity of Solid Waste” (GB/T15555.12-1995) [37]. The measurement method of heavy metal content and leaching content in red mud and modified red mud refers to the Chinese standard “Determination of Metal Elements in Solid Waste—Inductively Coupled Plasma Mass Spectrometry” (HJ766-2015) [38]. The content of fluoride is measured by ion chromatography according to the Chinese standard “Identification of Leaching Toxicity of Hazardous Waste” (5085.3–2007) [39].

The calculation of heavy metal release in each leaching stage i:

$$E_i=C_{i}V/fA,$$

(1)

where $E_i$ is the heavy metal leaching amount in leaching stage i, mg/m²; $C_i$ is the concentration of heavy metals in leaching stage i, μg/L; $V$ is the volume of the leaching solution, L; $A$ is the surface area of the test block, m²; $f$ is the conversion coefficient, and is equal to 1000. The cumulative release amount at the nth stage $U_n$ can be calculated using the following:

$$U_n={\sum }_{i=1}^n{E}_i,$$

(2)

2.3. Risk Assessment Methods

Based on the exposure scenarios analysis, the environmental impact of modified red mud used as road filling material on groundwater is evaluated using the Industrial Waste Evaluation Management (IWEM) developed by the US EPA. This model is commonly used in U.S. beneficial use determinations to estimate the concentration of leached constituents at 1, 5, 10, and 50 m from a roadway that uses modified red mud as a roadbed fill. The model uses site-specific parameters such as hydrogeological conditions as well as material-specific properties to calculate predicted constituent concentrations. In this exercise, the precipitation at the Chiping Xinfa red mud dump in Liaocheng City was chosen as the infiltration input and had a rate of 0.57 m/year. The IWEM assumes that the entire infiltration rate will penetrate the stockpile, which may not be accurate in practice due to the varying site conditions and the varying hydraulic conductivity of the modified red mud roads themselves. To estimate the fate and transport of stormwater infiltration, the rate was modeled as 10%, 25%, 50%, and 100% of its original value, with default values for the other parameters.

According to Figure 1, the road cross-section is defined as shown in Figure 3a. The section is divided into six stripes. Starting from the left, they are a ditch, an embankment, two travel lanes, two more travel lanes, another embankment, and another ditch. In this study, the ditches and embankments are a single layer, but the travel lanes have five layers, as shown in Figure 3a. The thickness of the subgrade is 0.5 m, and the thickness of the other layers refers to Figure 1. The hydraulic conductivities for the pavement, cement-stabilized macadam base, modified red mud base, sub-base, and subgrade are 0.085, 0.051, 0.03, 0.02, and 0.02 m/year, respectively. The corresponding dry bulk densities are 2.35, 2.25, 1.88, 1.85, and 1.85 g/cm³, respectively. The hydraulic conductivity and dry bulk density of embankment material are equal to those for the subbase. The manning’s coefficient, slope, and maximum water depth of the ditch are 0.016, 2 × 10⁻⁴, and 0.2 m. Moreover, the condition shown in Figure 3b is investigated, in which angle α between the groundwater flow direction and the road is at an acute angle, and the groundwater well is in the same direction as the groundwater flow. The roadway geometry parameters include the road segment length L_AB, the angle α, the shortest distance D between the road edge and the well, and the distance L along the road from the point at which distance measurement was made to the midpoint of the road segment. In this study, the cases listed in

Table 4. Simulated cases using IWEM with different roadway geometry.

Distance LAB	Angle α	Distance D	Distance L
0.5, 1.0 km	10°, 15°, 30°, 60°	1, 5, 10, 50 m	1, 5, 10, 50 m

with different layouts of roadway are simulated. According to the meteorological data of Liaocheng City, where the red mud yard of Chiping Xinfa is located, the precipitation and evaporation rates are 0.57 and 1.8 m/year, respectively. The constituents and corresponding leachate concentrations and total leachable material concentrations are determined based on the continuous leaching tank experiment.

Moreover, since there are often farmlands on both sides of the road, the acute toxicity test of earthworms and seed germination test were conducted to investigate the impact of the modified red mud on the farmlands. The soil sample was collected from the farmland about 2 km away from the red mud storage yard of Chiping Xinfa. For the acute toxicity test of earthworms, 100 mL leaching solution collected from the continuous leaching tank experiment was added into 400 g soil sample. After 24 h of resting, 10 earthworms (Eisenia foefide) were added to the soil sample. The survival rate was then examined after 7 and 14 days. For the seed germination test, 25 mL leaching solution was added into 100 g soil sample. Then, 10 Chinese cabbage seeds were added into the soil sample after 24 h. The germination percentage was then measured after 4 days. It is noted that for the two tests, a blank test, in which the leaching solution was replaced by deionized water, was performed for comparison. The toxicity level can be evaluated by

Table 5. Classification criteria for biological toxicity levels.

Toxicity Level	Seed Germination Rate (%)	Acute Mortality Rate of Earthworms (%)
Very toxic	≥80	≥80
High-toxic	80~50	80~50
Medium-toxic	50~30	50~30
Low-toxic	30~10	30~10
Non-toxic	≤10	≤10

, as recommended by the International Organization for Standardization (ISO).

3. Identification of Characteristic Pollutants in Modified Red Mud

The quality of groundwater is divided into five levels in the Chinese standard “Groundwater Quality Standard” (GB/T14848-2017) [40], in which the higher the level, the worse the quality. Groundwater at the third level is mainly suitable for centralized drinking water sources and industrial and agricultural water. According to the results of continuous leaching experiments, it was found that the leaching content of most metals in the original and modified red mud samples was lower than the limit values for the third level, except those listed in

Table 6. Metal leaching content for original and modified red mud samples.

Sample Number	Al (μg/L)	Cr (μg/L)	As (μg/L)	Se (μg/L)	Mo (μg/L)	V (μg/L)
XFCN1	15,432.5	18	217.5	280.9	470.8	1468
XFCN1-8	58,925	446.4	107.8	96.1	226.8	928.1
XFCN1-12	59,825	893.5	69.8	79.7	219.5	545.1
XFCN2	12,212.5	70.65	46.39	118.79	55.46	734.3
XFCN2-8	32,700	331.2	45.2	86.9	97.3	626.8
XFCN2-12	50,850	787.1	34.4	106.6	142.7	291.4
XFCN3	55,025	140.3	951.1	478.7	789.4	5704
XFCN3-8	38,275	186	32.5	9.0	30.4	971.9
XFCN3-12	45,625	171.5	27.0	7.9	27.2	834.4
XFCN4	19,670	180	275.1	451.5	492.6	1912
XFCN4-8	48,525	295	106.2	132.2	169.8	1235
XFCN4-12	71,300	366	64.5	125.9	156.3	687.1
XFCN5	39,250	53.34	326.9	178	263.9	2915
XFCN5-8	60,650	171	87.3	47.9	94.9	1718
XFCN5-12	52,175	202	58.8	43.4	87.9	947.5
XFCN6	37,900	236.6	981.1	271.7	403.9	7069
XFCN6-8	110,575	254.8	34.3	50.7	128.8	420.1
XFCN6-12	110,150	302.9	31.2	52.3	136.9	253.9
XFCN7	7,802.5	516.6	163.3	260.5	274.5	1542
XFCN7-8	66,500	166.7	82.63	98.4	262.1	1184
XFCN7-12	80,525	147.6	57.6	69.8	191.9	697.6
Limit value	200	5	10	10	70	50

. From Table 6, it can be seen that the types of metals in original red mud whose leaching contents exceed the standard include aluminum (Al), chromium (Cr), arsenic (As), selenium (Se), and molybdenum (Mo). Due to the lack of limit values for vanadium (V) in GB/T14848-2017, the limit value of 50 μg/L suggested in the Chinese standard “Environmental Quality Standard for Surface Water” (GB3838-2002) [41] is adopted for comparison. It is clear that the leaching content of V far exceeds the limit value. As shown in Figure 4, by comparing the results between original and modified red mud, it can be found that although the leaching contents of V, As, Se, and Mo in modified red mud samples still exceed the limit values, they are significantly lower than that in the original red mud samples. This is due to the better adsorption properties of the modified material Heinchem, which adsorbs heavy metal ions and other pollutants in water. For example, calcium hydroxide (Ca (OH)₂) in cement forms insoluble calcium arsenate and calcium selenate with As and Se, thus reducing the leaching of these elements. The cement and phosphogypsum in the modified materials will adsorb Al and Cr in the soil through ion exchange and then release them during hydration, which will lead to an increase in their leaching rate [42]. The leaching contents of Al, and Cr in modified red mud are significantly higher than in the original red mud sample. Moreover, the difference in leaching contents of V, As, Se, Mo, Al, Cr between the modified red mud with 8% and 12% modified materials is not significant. It can be concluded that the modified material Heinchem has a significant solidification effect on V, As, Se, and Mo in red mud.

The leaching content of fluoride (F) in the original and modified red mud samples is shown in Figure 5. Except for the value of 4.42 mg/L measured in sample XFCN2, the leaching contents of F in the other original red mud samples are larger than 10 mg/L and range from 15.5 to 42.9 mg/L. The leaching content of F in the original red mud is significantly larger than the limit value of 1.0 mg/L corresponding to the third quality level in GB/T14848-2017. Due to the addition of modified material Heinchem, the leaching contents of Heinchem in the modified red mud samples are significantly lower than those for the original red mud but are still higher than 1.0 mg/L. For the modified red mud with 8% and 12% Heinchem, the ranges of the leaching contents of F are 1.39–4.78 mg/L and 1.15–4.61 mg/L, respectively. The solidification effect for the modified red mud with 12% Heinchem is slightly better than that with 8% Heinchem. Overall, the modified Heinchem has a significant solidification effect on F in red mud.

The reason for the excessive leaching content of Al in the modified red mud is due to the presence of Al in the added modified materials. The excessive Al in water would influence the taste, but it does not endanger people’s health. For example, the Chinese standard for the use of food additives (GB2760-2014) [43] allows for a residue limit of 100 mg/kg of Al in food. Therefore, the impact of Al in modified red mud on the groundwater environment will be ignored. The pollutants that need to be mainly considered are V, Cr, As, Se, Mo, and F.

4. Environmental Risk Assessment of Modified Red Mud

4.1. Release Patterns of Characteristic Pollutants in Modified Red Mud

Based on the results of the continuous leaching tank experiment, the cumulative release amount was calculated by Equations (1) and (2). The cumulative leaching curves for the considered pollutants are plotted in Figure 6. It can be seen that as the leaching time increases, the accumulated release amounts of the considered pollutants gradually increase, with faster release rate in the initial stages and slightly slower release rate after five leaching stages (9 days).

In order to predict the amounts of released pollutants in modified red mud for road use, a pollutant release model is essential. In this study, the model proposed by Crane [44] is adopted, which is a one-dimensional diffusion model based on Fick’s second law of diffusion. It can predict the diffusion rate and concentration distribution of substances under different conditions, which is instructive for experimental design and process control and is widely used to predict the release of inorganic substances. The release model is shown in Equation (3).

$$M_{area}=2\rho C_0{\left(D_{obs}t/\pi \right)}^{1/2},$$

(3)

where $M_{area}$ stands for the accumulated release of heavy metals per unit area within leaching time $t$, mg/m²; $C_0$ is the effective contents of heavy metals, mg/kg; and $\rho$ is the density of unburned brick test block, kg/m³. $D_{obs}$ denotes average diffusion coefficient throughout the entire leaching cycles, m²/s, and is determined by:

$$D_{obs}=\frac{1}{n}{\sum }_{i=1}^n{D}_{obs,i},$$

(4)

where $D_{obs,i}$ is the diffusion coefficient of pollutants at the stage $i$ and can be calculated by:

$$D_{obs,i}=\pi {\left[\frac{M_i}{2\rho C_0\left(\sqrt{t_i}-\sqrt{t_{i-1}}\right)}\right]}^2,$$

(5)

where $M_i$ denotes the leaching amount of the $i$-th leaching stage of the pollutant, mg/m²; $t_i$ represents the time at the end of leaching stage i, s.

Based on the results of the continuous leaching tank experiment, the parameters $C_0$ and $D_{obs}$ can be determined. Then, the long-term release amounts of characteristic pollutants in the modified red mud can be predicted by Equations (3) and (5). The release curve is shown in Figure 7. It can be seen that the time required for the complete release of characteristic pollutants in different modified red mud is different.

4.2. Risk Assessment of Characteristic Pollutants

According to the above analysis, the input constituents are V, Cr, As, Se, Mo, and F. The simulation cases listed in Table 4 are performed, and some representative results are displayed in

Table 7. Exposure concentration of pollutants in the modified red mud during road use.

Types of Pollutants	D = 1 m, L = 1 m, α = 10°			D = 5 m, L = 5 m, α = 10°			D = 10 m, L = 10 m, α = 10°
	LC (mg/L)	LAB 0.5 km	LAB 1 km	LC (mg/L)	LAB 0.5 km	LAB 1 km	LC (mg/L)	LAB 0.5 km	LAB 1 km
		EC (mg/L)	EC (mg/L)		EC (mg/L)	EC (mg/L)		EC (mg/L)	EC (mg/L)
V	0.16	0.0495	0.0494	0.15	0.0413	0.0409	0.18	0.0475	0.0352
As3+	0.014	0.0096	0.01	0.012	0.0078	0.0078	0.015	0.0094	0.0068
As5+	200	0.0037	0.0036	100	0.00028	0.00033	200	0.0021	0.0017
Cr6+	0.1	0.0499	0.0486	0.1	0.0446	0.0458	0.12	0.0499	0.0363
Cr3+	0.2	0.0296	0.0292	0.2	0.0263	0.0273	0.4	0.0944	0.0367
Se4+	0.014	0.0094	0.0095	0.012	0.0077	0.0074	0.016	0.0099	0.0075
Se6+	0.014	0.0094	0.0096	0.012	0.0077	0.0077	0.016	0.0099	0.0074
Mo	0.08	0.0538	0.0533	0.075	0.0494	0.0486	0.08	0.0494	0.0476
F	1.4	0.9325	0.9578	1.2	0.7719	0.7787	1.4	0.8318	0.6627

Remark: LC stands for leaching concentration; EC stands for exposure concentration.

. The case with D = 5 m, L = 5 m, and α = 10° is the most unfavorable scenario. In this case, when the leaching contents of V, Cr⁶⁺, Cr³⁺, As³⁺, Se⁴⁺, Se⁶⁺, Mo, and F in the modified red mud are lower than 0.15 mg/L, 0.1 mg/L, 0.2 mg/L, 0.012 mg/L, 0.012 mg/L, 0.012 mg/L, 0.075 mg/L, and 1.2 mg/L, respectively, the environmental risk of the modified red mud during long-term road use is acceptable. There is no limit requirement for As⁵⁺ due to the fact that when the allowable leaching content (LC) of As⁵⁺ is 200 mg/L, the exposure concentrations (EC) calculated through IWEM are far lower than the third level standard of groundwater. Moreover, the highest leaching content of As in the 14 modified red mud samples evaluated is 0.108 mg/L. Therefore, it can be considered that as long as the leaching content of As³⁺ is lower than 0.012 mg/L, the exposure concentration is lower than the third level standard of groundwater and the environmental risk of As can be ignored.

4.3. Impact on Farmland

The pH of the leaching solution at different leaching experimental stages is exhibited in Figure 8. It should be noted that due to the relatively high value of PH of the leaching solution (between 11.95 and 12.81) after the end of Stage 8, the leaching time is extended to 92 days. It can be seen that the changing trend of PH value for the samples with 8% and 12% modified materials is similar—that is, a peak appears on the first day at around 11.0–12.06; subsequently, the pH immediately decreases and then continues to rise and reaches its peak again at 36 days; finally, the pH shows a slow decreasing trend. At the 7th leaching stage (cumulative leaching time of 36 days) where the pH reaches the second peak, the values of PH for most of the samples are all higher than 12.3, except the sample XFCN7-8 (red mud was produced in May 2019, with 8% modified material added), which had a PH of 11.79. Overall, regardless of whether 8% or 12% modified materials were added, the pH of the leaching solution shows a slow downward trend after 36 days.

Due to the high content of alkaline substances in red mud, this section mainly discusses the impact of the use of modified red mud on farmland. Tests with earthworms and seeds yielded more realistic results than the use of traditional road construction materials for investigating the pH of leachate and its effect on soil organisms.

Table 8. Effect of pH of leaching solution on the survival rate of earthworms in soil.

Conditions	PH	Survival Rate of Earthworms		Standard Error
		7d	14d	7d	14d
Modified red mud leaching solution	12.63	80.0%	75.0%	14%	21%
	12.62	100.0%	86.7%	0%	15%
	12.84	100.0%	90.0%	0%	17%
	12.74	93.3%	93.3%	12%	12%
	12.74	100.0%	93.3%	0%	6%
	13.00	96.7%	96.7%	6%	6%
	12.39	96.7%	93.3%	6%	12%
	12.63	90.0%	90.0%	10%	10%
	12.42	90.0%	83.3%	10%	21%
	12.56	93.3%	93.3%	12%	12%
	12.64	96.7%	80.0%	6%	10%
	12.89	96.7%	93.3%	6%	17%
	11.79	96.7%	93.3%	6%	6%
	12.48	96.7%	90.0%	6%	17%
Blank	8.6	90%	90%	10%	10%

shows the result of the acute toxicity test of earthworms. It is clear that after 7 days of cultivation, there is only one test in which the survival rate of earthworms is lower than 80%. After 14 days of cultivation, there are four tests in which the survival rates of earthworms are lower than 90% (75.0%, 86.7%, 83.3%, and 80.0%). For the cases where the PH value of the leaching solution is higher than 12.8, the survival rates of earthworms are all higher than 90%. By the regression trend analysis between PH and the survival rate of earthworms at 14 days, it is shown that there is no significant correlation between pH and the survival rate of earthworms. According to the classification criteria for biological toxicity levels, as shown in Table 5, the effect of modified red mud on earthworms in the tested soil is mostly slightly toxic or non-toxic, with a few showing low toxicity.

Table 9. Effect of pH of leaching solution on the growth of Chinese cabbage.

Leaching Solution pH	Germination Ratio	Standard Error	Root Length	Standard Error	Inhibition Ratio
12.06	100%	0.00	6.91	1.38	15.1%
12.06	96.7%	0.06	7.01	2.08	16.8%
12.06	96.7%	0.06	7.36	1.66	9.8%
12.04	96.7%	0.06	6.54	2.01	16.7%
12.06	93.3%	0.06	7.69	1.93	22.7%
12.35	96.7%	5.8%	7.86	2.02	−6.6%
12.31	96.7%	5.8%	7.95	2.21	−5.5%
12.35	96.7%	5.8%	7.97	1.41	−5.2%
12.29	93.3%	11.5%	7.75	1.28	−7.8%
Blank 1	93.3%	0.12	6.0	1.64	-
Blank 2	95.0%	7.1%	8.41	2.01	-

shows the results of the Chinese cabbage seed germination test, including the germination ratio, root length, and inhibition ratio. For the blank test where pure water is added into the soil, the germination rate is 93.3–95%. When the leaching solution of the modified red mud samples (pH ≥ 12) is added to the soil, the germination ratios are all higher than 90%. The root length range is 6.91–7.97 cm, some of which are inhibited, but does not exceed 10%. It can be concluded that the modified red mud has little effect on the germination rate and root length elongation of Chinese cabbage.

5. Conclusions

(1)

Although the modified red mud has a significant solidification effect on F, V, As, Se, and Mo, the leaching contents of F, V, Cr, As, Se, and Mo in the modified red mud still cannot meet the requirement of groundwater quality, which should be considered in the groundwater environmental risk assessment when the modified red mud is used as a roadbed material. Moreover, the difference in leaching contents of F, V, Cr, As, Se, and Mo between the modified red mud with 8% and 12% modified materials is small.

(2)

When the leaching contents of V, Cr⁶⁺, Cr³⁺, As³⁺, Se⁴⁺, Se⁶⁺, Mo, and F in the modified red mud are lower than 0.15 mg/L, 0.1 mg/L, 0.2 mg/L, 0.012 mg/L, 0.012 mg/L, 0.012 mg/L, 0.075 mg/L, and 1.2 mg/L, respectively, the environmental risk of the modified red mud during long-term road use is acceptable.

(3)

The addition of a leaching solution of the modified red mud into the soil will slightly increase the soil pH value. The pH of the leaching solution of the modified red mud has little inhibitory effect on the growth of earthworms and Chinese cabbage.

(4)

The modified red mud for road use has little influence on the groundwater, this study provides a new way for the resource utilization of red mud. The replacement of traditional road materials by modified red mud has the potential to reduce material costs and improve road performance while conserving land resources. However, the technology of modified red mud is still in the developmental stage, and the composition of red mud may vary greatly depending on the source, requiring further research and standardization to ensure its reliability and consistency under different environmental and engineering conditions.

6. Prospects and Limitations

In this paper, the environmental impact of treating red mud with the modified material Heinchem and applying it to road base materials has been studied to some extent. However, the variability of the material as well as the diversity of the environment can be an important factor in the variation of the results. Therefore, it is suggested that future research is necessary to gain a deeper understanding of the environmental impacts of other modifiers applied to red mud and to reveal their practical engineering applications in other areas. Additionally, when assessing environmental risk, models are limited in that their results may be biased by assumptions about data distribution, assumptions about site-specific data, and assumptions about pollutant behavior.