Experimental Study on the Evolution Law of Pb in Soils and Leachate from Rare Earth Mining Areas Under Different Leaching Conditions

Abstract

In the exploitation of ion-adsorption rare earth ores, the environmental effects of leaching agents are key constraints for green mining. Understanding the release behavior of typical heavy metals from soils under leaching conditions is of great significance. Laboratory column leaching experiments were conducted to systematically investigate the effects of three leaching agents—(NH₄)₂SO₄, Al₂(SO₄)₃, and MgSO₄—as well as varying concentrations of Al₂(SO₄)₃ on the release and speciation transformation of heavy metal Pb in mining-affected soils. The results revealed a three-stage pattern in Pb release—characterized by slow release, a sharp increase, and eventual stabilization—with environmental risks predominantly concentrated in the middle to late stages of leaching. Under 3% (NH₄)₂SO₄ and 3% Al₂(SO₄)₃ leaching conditions, Pb concentrations in soil increased significantly, with a higher proportion of labile fractions, indicating pronounced activation and risk accumulation. Due to its relatively weak ion-exchange capacity, MgSO₄ exhibited a lower and more gradual Pb release profile, posing substantially lower pollution risks compared to (NH₄)₂SO₄ and Al₂(SO₄)₃. Pb release under varying Al₂(SO₄)₃ concentrations showed a nonlinear response. At 3% Al₂(SO₄)₃, both the proportion of bioavailable Pb and the Risk Assessment Code (RAC) peaked, while the residual fraction declined sharply, suggesting a threshold effect in risk induction. All three leaching agents promoted the transformation of Pb in soil from stable to more labile forms, including acid-soluble, reducible, and oxidizable fractions, thereby increasing the overall proportion of active Pb (F1 + F2 + F3). A combined analysis of RAC values and the proportion of active Pb provides a comprehensive framework for assessing Pb mobility and ecological risk under different leaching conditions. These findings offer a theoretical basis for the prevention and control of heavy metal risks in the green mining of ion-adsorption rare earth ores.

1. Introduction

Rare earth elements (REEs), as indispensable functional materials in modern industry, are widely used in electronics, advanced materials, energy conservation, environmental protection, aerospace, and national defense. They serve as key resources supporting high-tech development and hold significant strategic importance [1,2,3,4]. Ion-adsorption rare earth ores are particularly rich in medium and heavy REEs, which are mainly adsorbed onto weathered crust minerals in exchangeable ionic forms. These can be extracted through leaching with inorganic salt solutions such as (NH₄)₂SO₄, Al₂(SO₄)₃, and MgSO₄ [5]. Although the in situ leaching method offers advantages such as high efficiency, simple operation, and low cost, long-term use of leaching agents can easily lead to soil structure damage, increased acidification, and an increased risk of heavy metal element migration [6,7]. Typical heavy metals such as Pb are widely present in rare earth mining areas [8,9]. However, the variation in Pb content and its speciation transformation under the influence of different leaching agents remain poorly understood, posing a major challenge for accurate environmental risk assessment and the development of effective remediation strategies in these mining areas [10,11]. Therefore, conducting leaching experiments using different types and concentrations of leaching agents, and analyzing the evolution of Pb content and speciation in both leachates and soils, is essential to provide a theoretical foundation for green mining practices and ecological protection in rare earth mining regions.

Leaching agents influence the release and migration of heavy metals by altering soil pH, disrupting cation exchange equilibrium, and facilitating complexation reactions [12]. For example, when (NH₄)₂SO₄ is used as a leaching agent, the released ammonium ions (NH₄⁺) chemically exchange with heavy metals such as Pb, Cu, Cd, and Zn, promoting their mobilization into acid-extractable forms [13,14]. Li et al. [15] reported that residual NH₄⁺ in mining areas significantly alters the chemical speciation of heavy metals, resulting in combined contamination by ammonium nitrogen and heavy metals. Zhang Jun et al. [16] found that in bare mining areas subjected to (NH₄)₂SO₄ leaching, soil pH dropped to 3.92–5.80, with severe ammonium nitrogen pollution and vegetation degradation, thereby enhancing heavy metal mobility. Through profile analysis, Chen et al. [17] demonstrated that Pb, Zn, and Cu were enriched in the upper layers of ore bodies, often exceeding background standards. Leaching activities facilitated the activation of surface heavy metals and impaired soil ecological functions [18,19]. Tan et al. [20] confirmed via leaching experiments that (NH₄)₂SO₄ reduced the total contents of Cu, Zn, and Cr by 18.34%, 10.96%, and 26.00%, respectively, and facilitated the transformation of heavy metals from non-bioavailable to bioavailable forms. Guo et al. [21] found that MgSO₄ concentration was negatively correlated with Cu, Pb, and Tl contents. Cu, Zn, and Pb exhibited significant vertical migration, while Tl showed minimal loss due to its chemical inertness. Although previous studies have examined the effects of leaching reagents on heavy metals in ion-adsorption rare earth mining areas, the overall research scope remains limited. Systematic investigations into the variations in Pb content and speciation under diverse leaching conditions are still lacking, and the understanding of the underlying mechanisms is fragmented. Therefore, experimental studies addressing the evolution of Pb content and speciation under different leaching reagent types and concentrations are urgently needed.

Existing studies have predominantly focused on the release of Pb under a single leaching reagent or a single concentration condition. Systematic investigations comparing the effects of different types of leaching reagents—such as (NH₄)₂SO₄, Al₂(SO₄)₃, and MgSO₄—and their concentration variations on Pb speciation transformation remain limited [22,23]. In particular, the mechanistic pathways of novel alternative leaching reagents, such as Al₂(SO₄)₃ [24], have not yet been clearly elucidated. Moreover, the transformation patterns of Pb in less labile forms—such as the reducible and oxidizable fractions—under different leaching conditions remain insufficiently quantified [25]. These knowledge gaps hinder the robust assessment of heavy metal migration risks in mining areas and the formulation of differentiated remediation strategies. In response, this study employed a self-designed indoor soil column setup to simulate leaching processes. We systematically compared the mechanisms through which different types and concentrations of leaching reagents influence Pb release and chemical speciation dynamics. The study elucidates the concentration-dependent patterns and ion-mediated regulatory mechanisms of Pb transformation, further clarifies the release mechanisms of Pb under varying leaching conditions, and aims to provide a theoretical basis for risk assessment, remediation strategy development, and safe agricultural utilization of contaminated soils in mining regions.

2. Materials and Methods

2.1. Soil Sample Collection

The soil samples used in this study were collected from a rare earth mining area in Fujian Province, China. The geographic location is shown in Figure 1. Sampling was conducted along the ridge line, including sites at the foot, mid-slope, and top of the mountain, as well as surrounding farmland. At each location, 3 to 5 samples were taken to reduce random error. Samples were collected from a depth of 3–18 m. After collection, the moisture content was measured, and the samples were air-dried by spreading them out in a well-ventilated indoor environment, with dead leaves and other debris manually removed. Once air-dried, larger soil clumps were crushed, and the samples were sealed in plastic bags and stored in a dry, ventilated location for future use. To avoid contamination from external metal sources, non-metallic tools were used throughout the sampling process.

2.2. Fundamental Physicochemical Properties of in Situ Ore Soil

• Moisture Content and Particle Size Distribution of In Situ Soil

The in situ soil samples from the mining area were analyzed for moisture content and particle size distribution. Moisture content was measured using the oven-drying method (at 105 °C until constant weight, with a mass variation rate < 0.1%). The results showed moisture content ranging from 14.13% to 16.35% (mean: 15.24% ± 0.72%), indicating relatively uniform spatial distribution of soil moisture. Particle size distribution was determined using a combined dry sieving–sedimentation method. Fine particles (<0.075 mm) accounted for 27.11% ± 1.2%, silt-sized particles (0.25–0.075 mm) accounted for 26.15%, and fine sand (0.5–0.25 mm) made up 14.49%. The particle gradation curve is shown in Figure 2. The calculated coefficient of uniformity (Cu) was 8.75, and the coefficient of curvature (Cc) was 1.18. According to relevant classification standards, the soil was identified as poorly graded. Its fine particle-dominated composition (characterized by low permeability and high specific surface area) significantly influences the adsorption–desorption behavior of heavy metal Pb.

2.

Chemical and Mineralogical Composition of In Situ Soil

The chemical composition of the soil was analyzed using X-ray fluorescence spectroscopy (XRF), and the results are presented in

Table 1. Composition analysis of soil samples [28].

Element	SiO2	Al2O3	K2O	Fe2O3	TiO2	MgO	BaO	MnO	La2O3
Content (%)	55.81	30.09	7.00	5.55	0.41	0.27	0.14	0.12	0.11
Element	Na2O	Nb2O3	P2O5	ZrO2	SO3	Rb2O	Cl	CaO	ZnO
Content (%)	0.08	0.08	0.06	0.06	0.03	0.03	0.02	0.02	0.01

. The soil was primarily composed of SiO₂ and Al₂O₃, indicating that aluminosilicates are the dominant minerals in the clay fraction. Among the rare earth elements, Y₂O₃ was present in relatively high proportions, suggesting that the ore body exhibits characteristics of yttrium-rich heavy rare earth mineralization. The main associated heavy metals were Mn, Pb, and Zn. Pb is typically considered a potential environmental risk factor [26], and its content is closely related to the adsorption and enrichment capacity of clay minerals [27].

The X-ray diffraction (XRD) pattern of the ore soil sample is shown in Figure 3. Phase identification was performed by comparing the diffraction peaks with standard reference patterns in the XRD database. Kaolinite (Al₂O₃·2SiO₂·2H₂O) and quartz (SiO₂) were the dominant mineral phases in the soil, accounting for 72.6% ± 1.8% as corrected by the RIR method. This finding is consistent with the enrichment of Si–Al oxides revealed by XRF analysis, indicating the long-term influence of aluminosilicate weathering and pedogenesis [29]. The detection of secondary minerals such as goethite (α-FeOOH, 3.21%) and illite (KAl₂(AlSi₃O₁₀)(OH)₂, 1.97%) suggests that Fe oxidation–hydration processes and potassium feldspar alteration play roles in regulating the occurrence of heavy metals [30]. In particular, the specific adsorption of Pb²⁺ by goethite may significantly affect the geochemical mobility of Pb [31].

2.3. Experimental Apparatus

A custom-designed soil column leaching device was used to simulate the in situ leaching process. The apparatus consisted of a plexiglass column, a peristaltic pump, infusion tubing, a reagent reservoir, a collection tank, and an iron support frame, as shown in Figure 4. The soil column was made of a plexiglass tube with a height of 100 cm, an outer diameter of 110 mm, and an inner diameter of 100 mm. Three sampling ports were positioned along the column at 15 cm intervals, with the first port located 35 cm from the top of the column (i.e., 15 cm below the top of the soil layer). The soil column was divided into six segments from top to bottom. The uppermost 0–10 cm served as a buffer zone for leaching solution application; the 10–15 cm section was filled with quartz sand (upper layer); the 15–20 cm section comprised surface soil (upper portion); the 20–90 cm section formed the main soil body; the 90–95 cm section consisted of surface soil (bottom portion); and the bottommost 95–100 cm section was filled with quartz sand (lower layer).

The lead (Pb) content in the experimental samples was measured using an inductively coupled plasma optical emission spectrometer (ICP-OES, model 8800, Agilent Technologies, Santa Clara, CA, USA). Inductively coupled plasma mass spectrometry (ICP-MS) is widely applied for elemental and isotopic analysis across various sample types, including environmental, geological, metallic, and chemical materials. It enables qualitative, semi-quantitative, quantitative, and isotopic ratio analyses.

2.4. Experimental Methods

The total digestion of heavy metals was performed using a multi-stage acid dissolution and thermal decomposition system [32]. Hydrofluoric acid (10 mL), perchloric acid (4 mL), and concentrated nitric acid (10 mL × 2) were sequentially added into a PTFE digestion vessel. The sample was digested stepwise at elevated temperatures of 235–240 °C, 265–270 °C, and 235 °C, utilizing oxidation and complexation reactions to break down silicate mineral lattices and decompose organic matter. Temperature was strictly controlled during digestion, maintaining the solution volume at ≥3 mL to prevent drying. In the final stage, ultrapure water (15 mL) and hydrogen peroxide (1 mL) were added to drive off acids until the white fumes dissipated and the solution became clear and viscous. After cooling and rinsing, the digestate was diluted to 50 mL and filtered through a 0.45 μm membrane. Digestion efficiency and data reliability were validated by dynamic temperature monitoring, solution clarity, and replicate experiments with relative standard deviation (RSD) below 5%. The chemical speciation of heavy metals was extracted using the BCR sequential extraction procedure [33,34] applied to soil samples under varying leaching durations and conditions. Detailed extraction parameters are provided in

Table 2. Extraction conditions for the chemical speciation of soil samples.

Step	Speciation	Extractant	Extraction Time	Temperature
1	Acid-exchangeable	20 mL 0.11 mol/L acetic acid (HAC)	Shaking for 16 h	Room temperature
2	Reducible	20 mL 0.5 mol/L hydroxylamine hydrochloride (NH2OH·HCl, pH = 1.5 a)	Shaking for 16 h	Room temperature
3	Oxidizable	5 mL 30% hydrogen peroxide (H2O2)	Water bath heating 1 h + 1 h	Room temp + 85 °C
		5 mL 30% hydrogen peroxide (H2O2)	Water bath heating 1 h	85 °C
		25 mL 1 mol/L ammonium acetate (NH4Ac, pH = 2 a)	Shaking for 16 h	Room temperature
4	Residual	10 mL hydrofluoric acid (HF)	Digestion 0.5 h	235 °C
		4 mL perchloric acid (HClO4)	Digestion 0.5 h	265 °C
		10 mL nitric acid (HNO3)	Digestion 0.5 h	235 °C
		10 mL nitric acid + 15 mL water + 1 mL hydrogen peroxide (H2O2)	Digestion 0.5 h	235 °C

Note: ^a The pH value was adjusted using HNO₃.

. Lead (Pb) concentrations in both the total digested samples and the BCR sequentially extracted fractions (acid-exchangeable, reducible, oxidizable, and residual) were measured using inductively coupled plasma mass spectrometry (ICP-MS) [35]. The measurements were validated using characteristic spectral lines and an RSD < 3.5% [36]. The Pb spike recovery rates ranged from 95.2% to 103.8%, meeting analytical quality requirements.

3. Variation in Pb Concentration in Leachate

3.1. Effects of Different Types of Leaching Agents

The effects of three leaching agents—(NH₄)₂SO₄, Al₂(SO₄)₃, and MgSO₄—on Pb concentrations in leachate are shown in Figure 5. At a concentration of 3%, both (NH₄)₂SO₄ and Al₂(SO₄)₃ exhibited similar Pb concentration evolution patterns, characterized by three stages: an initial slow-release phase, a mid-term surge, and a late stabilization phase. The concentration values in each stage were highly consistent, indicating that the two agents share a common Pb release mechanism [37]. MgSO₄ exhibited a distinct three-phase pattern of “fluctuation–slow release–surge,” with peak Pb concentrations reduced by 90.7% and 90.3% compared to (NH₄)₂SO₄ and Al₂(SO₄)₃, respectively. This suggests a more environmentally friendly profile; however, improvements in leaching efficiency are required to meet industrial application demands.

Among the three leaching agents, the magnitude of Pb concentration variation followed the order (NH₄)₂SO₄ > Al₂(SO₄)₃ > MgSO₄. This is attributed to the stronger ion-exchange capacity of Al³⁺ and NH₄⁺ compared to Mg²⁺, enabling more effective displacement of Pb²⁺ from the soil. Additionally, the hydrolysis of Al₂(SO₄)₃ produces stronger acidity than that of (NH₄)₂SO₄, significantly enhancing the dissolution of poorly soluble Pb compounds, whereas the hydrolysis of MgSO₄ is weak and exerts minimal influence on pH [38]. Therefore, the trend of Pb concentration change under Al₂(SO₄)₃ leaching is similar to that of (NH₄)₂SO₄, with significantly higher concentrations in the mid to late stages compared to MgSO₄ leaching.

The evolution of Pb concentration in the leachate indicates that the high-risk period for Pb pollution under 3% (NH₄)₂SO₄ and Al₂(SO₄)₃ leaching is concentrated in the later stage of leaching. At this stage, peak Pb concentrations exceeded the limit specified by the Environmental Quality Standards for Surface Water (0.01 mg/L) by more than 400 times, posing significant risks of contaminating nearby water bodies via infiltration or runoff. In comparison, the peak Pb concentration under 3% MgSO₄ leaching was only 0.4355 mg/L (43 times the standard limit). Although Pb activation efficiency under MgSO₄ was lower and the environmental risk relatively controllable, long-term accumulation effects should still be considered. This leaching agent is more suitable for low-intensity mining in ecologically sensitive areas, and the resulting leachate must not be directly discharged into domestic water sources to prevent water pollution.

3.2. Effects of Different Concentrations of Leaching Agent

Under varying concentrations of Al₂(SO₄)₃ (1–7%), the Pb release process exhibited a characteristic three-stage pattern, as illustrated in Figure 6. The initial inhibition phase involves Al³⁺ hydrolysis producing Al(OH)₃ colloids that adsorb Pb²⁺ and PbSO₄ micro-precipitates that suppress Pb release, followed by a mid-term surge characterized by peak dissolution of residual Pb, and finally a late stabilization phase where dissolution–precipitation equilibrium is established. This evolution pattern was consistently observed across all concentration gradients, indicating a concentration-independent mechanism of Al₂(SO₄)₃ on Pb release.

The high-risk period for each leaching concentration consistently occurred during the mid-phase of the leaching process, during which the Pb release rate peaked, with an average daily increase exceeding 70%, warranting intensified monitoring and control efforts [39]. The peak concentration of Pb significantly exceeds the Chinese national Class III standard limit of 0.01 mg/L, and the associated environmental risk index increased exponentially with rising leaching agent concentrations. The peak Pb concentrations and their exceedance multiples are summarized in

Table 3. Peak Pb concentration in the leachate and corresponding exceedance multiple.

Al2(SO4)3 Concentration	Peak Pb Concentration (mg/L)	Exceedance Multiple
1%	2.529	252 times
3%	4.505	450 times
5%	18.24	1824 times
7%	20.17	2017 times

.

Experimental results revealed a nonlinear relationship between Al₂(SO₄)₃ concentration and Pb activation efficiency: when the concentration increased from 5% to 7%, the peak Pb concentration rose by only 1.93%, indicating that Pb activation efficiency under Al₂(SO₄)₃ leaching conditions approached saturation. In the low-concentration group (1–3%), Pb was primarily released from oxide-bound forms, and mineral dissociation was limited, resulting in a “high” environmental risk level (252–450 times above the standard), necessitating interception and stabilization measures. In contrast, the high-concentration group (5–7%) dissolved weak acid-extractable Pb, breaching the mineral stability barrier, significantly enhancing activation efficiency and escalating the environmental risk to an “extremely high” level (1824–2017 times above the standard), posing a potential risk of regional ecological imbalance. Notably, in the later stages, the Pb concentration in the low-concentration group declined sharply, suggesting that the chemical inertness of residual Pb became a critical limiting factor. Meanwhile, the high-concentration group maintained a relatively high activation efficiency, indicating that part of the activation threshold of weak acid-extractable Pb had already been exceeded.

4. Variation in Pb Content in Soil

4.1. Types of Leaching Agents

The dynamic variations in soil Pb content under the application of three leaching agents—(NH₄)₂SO₄, Al₂(SO₄)₃, and MgSO₄—are shown in Figure 7. All treatments exhibited a typical three-stage pattern: rapid increase, oscillatory adjustment, and final stabilization.

However, soil Pb pollution was most severe under Al₂(SO₄)₃ leaching. The trends observed under (NH₄)₂SO₄ and Al₂(SO₄)₃ were highly similar, which can be attributed to their shared mechanisms of hydrolytic acidification, enhanced ion-exchange capacity, and mineral interactions. Specifically, hydrolysis led to soil acidification, increased ionic strength, efficient ion exchange involving NH₄⁺ and Al³⁺, and chemical interactions between hydrolysis products and soil minerals. In contrast, MgSO₄ exhibited the weakest driving effect on Pb release due to its limited hydrolysis and low ion-exchange capacity. The temporal profile of Pb release under MgSO₄ leaching differed markedly from those of (NH₄)₂SO₄ and Al₂(SO₄)₃, due to chemical differences among the leaching agents and heterogeneous interaction mechanisms with soil [40]. Hydrolysis of (NH₄)₂SO₄ and Al₂(SO₄)₃ released H⁺, substantially lowering pH and enhancing Pb solubilization. MgSO₄ exhibited minimal acidification and was less effective in dissolving Pb-bearing minerals, resulting in a significantly slower release rate.

A comparative analysis of the activation efficiency of three leaching agents—(NH₄)₂SO₄, Al₂(SO₄)₃, and MgSO₄—on soil Pb was conducted, as shown in

Table 4. Activation efficiency and dynamic variation characteristics.

Leaching Agent	Activation Efficiency	Dynamic Behavior Characteristics
MgSO4	Moderate (peak value: 69.39 mg/kg)	Early-stage ion-exchange accumulation, mid-stage fluctuation, and late-stage secondary activation
Al2(SO4)3	High (peak value: 70.19 mg/kg)	Rapid activation in the early stage, desorption-dominated mid-stage, and colloidal re-adsorption in the late stage
(NH4)2SO4	Moderate (peak value: 67.79 mg/kg)	Rapid adsorption in the early stage, mid-stage f fluctuation, and slow desorption in the late stage

, revealing significant differences in dynamic behavior and activation intensity among the leaching agents.

Analysis reveals significant differences in the Pb activation mechanisms of the three sulfates: Al₂(SO₄)₃ exhibits the highest initial activation efficiency, as its strong acidity rapidly dissolves carbonate- and oxide-bound Pb; however, in the later stages, the formation of Al(OH)₃ colloids inhibits Pb release, causing notable fluctuations in concentration. (NH₄)₂SO₄ achieves moderate activation efficiency via NH₄⁺ ion exchange with controllable environmental risk [41]. MgSO₄ gradually releases residual Pb through Mg²⁺ ion exchange and weak acid effects, with secondary activation occurring in later stages due to mineral structure degradation. Comparative analysis of the effects of the three leaching agents on Pb concentrations in leachate and soil indicates significant differences in their environmental risk impact and ecological compatibility, as summarized in

Table 5. Environmental risk and ecological compatibility.

Indicator	(NH4)2SO4	Al2(SO4)3	MgSO4
Peak Soil Pb Concentration (mg/kg)	67.79 (near risk screening threshold)	70.19 (exceeds risk screening threshold)	69.39 (near risk screening threshold)
Leachate Risk	Medium (peak 4.68 mg/L)	High (peak 4.505 mg/L)	Low (peak 0.4355 mg/L)
Ecological Compatibility	Medium (acidification risk)	Low (Al3+ toxicity, soil degradation)	High (non-toxic, minimal structural damage)

.

The assessment indicates that Al₂(SO₄)₃ exhibits the highest leaching efficiency but also poses the greatest environmental risk, necessitating the implementation of interception and passivation measures; MgSO₄ causes minimal environmental disturbance but requires the establishment of a long-term dynamic monitoring system to mitigate secondary activation risks; comparatively, (NH₄)₂SO₄ offers the best balance between overall effectiveness and risk controllability, resulting in a lower risk associated with Pb release.

4.2. Effects of Different Concentrations of Leaching Agents

The three-stage dynamic pattern of soil Pb content under leaching with varying concentrations of Al₂(SO₄)₃ (1–7%) is illustrated in Figure 8. The stages include initial activation (release of reducible Pb via Al³⁺ ion exchange), mid-term fluctuations (competition between colloidal adsorption–desorption and mineral dissolution), and late-stage desorption (Pb release driven by colloid aging and common ion effects). This pattern is universally observed across all concentration gradients, indicating that variations in Al₂(SO₄)₃ concentration regulate only the reaction rate without altering the fundamental pathway of Pb release.

The high-risk periods for Pb release at different leaching concentrations are concentrated in the mid-to-late stages, during which the Pb release rate peaks (daily increase > 70%) accompanied by the maximum accumulation, necessitating focused control measures. Soil Pb accumulation increases with the concentration gradient of the leaching agent, reaching or exceeding the risk-screening value for agricultural land (70 mg/kg) demonstrating a significant dose–response relationship; the peak Pb content and corresponding risk levels in the soil are summarized in

Table 6. Peak concentration of soil heavy metal Pb and corresponding risk levels.

Al2(SO4)3 Concentration	Peak Pb Content (mg/kg)	Risk Level
1%	64.42	Moderate
3%	70.19	High (Exceeding Risk-Screening Value)
5%	66.73	High (Approaching Risk-Screening Value)
7%	74.96	High (Exceeding Risk-Screening Value)

.

Studies indicate a dual effect of Al₂(SO₄)₃ concentration gradients on Pb activation mechanisms: the low-concentration group (1–3%) activates Pb in oxide-bound forms via Al³⁺ ion exchange, yet initial release is significantly suppressed by colloidal adsorption, concentrating risks in the later stage (peak contents near or exceeding risk-screening values). This necessitates combined interception and passivation measures, with ecological risks remaining manageable, making it suitable for short-term or sensitive-area mining, albeit requiring dynamic monitoring. In contrast, the high concentration group (5–7%) induces direct dissolution of silicate minerals through strong acid hydrolysis, releasing residual Pb and surpassing Pb stability barriers. Concurrently, SO₄²⁻ complexation enhances Pb mobility, resulting in a high-risk profile throughout the entire cycle (peak content reaching risk-screening values), with irreversible risks especially during the later desorption phase, potentially triggering regional ecological imbalance.

5. Analysis of Pb Chemical Speciation

5.1. Different Types of Leaching Agents

During ionic rare earth mining, cation exchange reactions induced by leaching solutions alter the chemical speciation of heavy metals, thereby driving their migration and release. Based on the BCR sequential extraction method, the environmental risk levels of heavy metal chemical forms are classified as follows:

• Acid-extractable fraction (F1): water-soluble/exchangeable form, exhibiting high mobility and bioavailability, capable of plant uptake and food chain contamination (high risk);
• Reducible fraction (F2): bound to iron and manganese oxides, stable under oxidizing conditions but released under anoxic conditions (conditional risk);
• Oxidizable fraction (F3): associated with organic matter and sulfides, released upon oxidative decomposition (potential secondary risk);
• Residual fraction (F4): fixed within mineral lattices, chemically inert, with low bioavailability and minimal environmental risk.

The mobility, bioavailability, environmental impact range, and impact intensity of heavy metals in each chemical fraction are summarized in

Table 7. Characteristics of different chemical forms of heavy metals.

Chemical Fraction	Mobility	Bioavailability	Environmental Impact Range	Environmental Impact Intensity
F1 (Acid-extractable)	High	High	Large	High
F2 (Reducible)	Moderate	Moderate	Moderate	Moderate
F3 (Oxidizable)	Low	Slightly below moderate	Very small	Slightly above moderate
F4 (Residual)	Very low	Very low	Minimal	Low

. Analysis indicates that fractions F1, F2, and F3 exhibit significantly higher mobility and bioavailability compared to F4 (mobility coefficient > 1.8, bioavailability index ≥ 0.65). Their combined environmental effects (diffusion radius > 50 m, pollution equivalent ×3.2) are markedly greater than the residual fraction, thus they are defined as active components.

The Risk Assessment Code (RAC) is a morphology-based method for evaluating the environmental risk of heavy metals. It is founded on the principle that a higher proportion of acid-extractable fraction (F1) indicates greater environmental risk. The method uses the percentage of F1 in the total concentration as the risk index, with the classification as follows: <1% (no risk), 1–10% (low risk), 10–30% (moderate risk), 30–50% (high risk), and >50% (very high risk). Based on this framework, the effects of different leaching agents on the chemical speciation of Pb and associated environmental risks were compared. The distribution of Pb chemical species under different leaching agents is shown in Figure 9.

Analysis indicated that Pb in the raw ore soil was primarily present in the residual fraction (F4), accounting for 62.6%, while the labile fractions (F1 + F2 + F3) made up 37.4%, with a corresponding RAC value of 16.6%, indicating moderate environmental risk. After leaching with (NH₄)₂SO₄, the proportion of labile Pb increased to 53.2%, and the RAC value rose to 28.4%, approaching the threshold of high risk. Following Al₂(SO₄)₃ leaching, the labile fraction reached 52.8%, slightly lower than that under (NH₄)₂SO₄ treatment, while the RAC increased to 29.6%, indicating a stronger promotion of short-term Pb mobility [42]. In the MgSO₄ leaching group, the labile fraction was 51.7%, with an RAC value of 28.6%, slightly lower than that observed with Al₂(SO₄)₃. The significant activation of Pb under Al₂(SO₄)₃ treatment was attributed to the strong acidification induced by Al³⁺ hydrolysis, resulting in the most pronounced increase in Pb mobility risk. MgSO₄ facilitated Pb desorption through ionic competition with Mg²⁺, leading to a moderate increase in risk. In contrast, the limited complexation capacity of NH₄⁺ in (NH₄)₂SO₄ resulted in a comparatively lower disruption of stable Pb forms. These results suggest that the cationic characteristics of the leaching agents (Al³⁺/Mg²⁺/NH₄⁺) are critical determinants in regulating the environmental behavior of heavy metals.

Leaching with all three extractants significantly increased the proportion of labile Pb, but differences in the Risk Assessment Code (RAC) values were observed. This discrepancy arises because the RAC is based solely on the proportion of the acid-extractable fraction (F1), reflecting immediate bioavailability, whereas the labile fraction also includes reducible (F2) and oxidizable (F3) forms, which indicate potential release under reductive or oxidative conditions. In the MgSO₄ leaching treatment, the contents of F2 and F3 were relatively high, leading to an increase in the overall labile Pb proportion; however, the low F1 content resulted in a limited increase in RAC. By contrast, Al₂(SO₄)₃ leaching produced the highest F1 proportion and the highest RAC value, indicating its greater capacity to activate rapid Pb mobilization. RAC and the labile fraction index reflect Pb environmental behavior from distinct perspectives; their combined application provides a more comprehensive evaluation of Pb mobility and ecological risk under different leaching conditions.

5.2. Effect of Leaching Agent Concentration

As shown in Figure 10, leaching with varying concentrations of Al₂(SO₄)₃ markedly affected the speciation of Pb in soil. Under 1% Al₂(SO₄)₃ treatment, the residual fraction (F4) decreased to 51.2%, and the RAC value reached 26.8%, indicating that Al₂(SO₄)₃ promotes the transformation of stable Pb into more labile forms, thereby enhancing its mobility and bioavailability. When the concentration increased to 3%, the total proportion of labile Pb reached 52.8%, the residual fraction dropped to 47.2%, and the RAC value rose to 29.6%, the highest among all treatments, suggesting the strongest activation effect and a significant disruption of speciation equilibrium, posing the greatest environmental risk. Further increasing Al₂(SO₄)₃ concentration to 5% and 7% led to a slight decrease in F1, fluctuations in F2 and F3, and a slower reduction in the residual fraction. The RAC values were 26.6% and 27.3%, respectively, slightly lower than that of the 3% treatment, indicating a stabilization of the activation effect and an approach toward dynamic equilibrium. Notably, under 7% Al₂(SO₄)₃ treatment, neither the labile Pb proportion nor the RAC value showed further increases, suggesting that higher concentrations do not necessarily result in a linear enhancement of Pb activation.

As shown in the figure, the F1 content follows the order: 3% > 1% > 7% > 5% > raw soil, further confirming that 3% is the optimal activation concentration, potentially due to enhanced Al³⁺ reactivity, improved Pb release kinetics, and synergistic interactions with soil components. Differences among other concentration groups are primarily attributed to changes in ionic strength affecting reaction equilibrium. The variation in residual Pb proportion follows the order: 3% > 7% > 5% > 1%, indicating that 3% leaching induces the most pronounced alteration in Pb speciation, with the highest RAC value and greatest environmental risk. The raw soil exhibited the highest proportion of residual Pb, followed by 1% > 5% > 7% > 3%, further suggesting that the 1% concentration had minimal impact, while 5% and 7% treatments demonstrated a gradual increase in risk. The concentration of Al₂(SO₄)₃ exhibited a nonlinear response in terms of Pb speciation transformation and RAC values. The 3% treatment significantly increased both the proportion of labile Pb and the RAC value, indicating the highest ecological risk at this concentration.

6. Conclusions

(1) (NH₄)₂SO₄, Al₂(SO₄)₃, and MgSO₄ all enhanced Pb release and mobilization. Pb under (NH₄)₂SO₄ and Al₂(SO₄)₃ exhibited comparable dynamics—gradual early release, mid-phase intensification, and late stabilization—concentrating risk in the final stage. In contrast, MgSO₄ induced a “fluctuation–gradual–increase” trajectory with lower peaks, indicating relatively contained environmental risk.

(2) The release of Pb under different concentrations of Al₂(SO₄)₃ exhibited a nonlinear response. Increasing concentrations markedly elevated Pb peak levels. At low concentrations (1% and 3%), Al₂(SO₄) primarily mobilized oxide-bound Pb, posing considerable drainage risks. At higher concentrations (5% and 7%), it further activated weak acid–extractable Pb, raising drainage risks to a critical level and potentially disrupting ecological balance.

(3) (NH₄)₂SO₄, Al₂(SO₄)₃, and MgSO₄ showed distinct differences in Pb mobilization efficiency and environmental risks. Al₂(SO₄)₃ was the most efficient, rapidly dissolving carbonate- and oxide-bound Pb, but carried the highest risk. MgSO₄ was the least effective, inducing minimal disturbance and suitable for sensitive ecosystems, while (NH₄)₂SO₄ demonstrated moderate efficiency and risk with favorable ecological compatibility.

(4) Under varying Al₂(SO₄)₃ concentrations, Pb accumulation increased substantially with higher concentrations, in some cases exceeding agricultural risk-screening thresholds. At low concentrations (1% and 3%), Al₂(SO₄)₃ mobilized oxide-bound Pb via Al³⁺ exchange with manageable ecological risks, whereas at higher concentrations (5% and 7%), it activated residual Pb through strong acid dissolution and complexation, markedly elevating risks and potentially causing irreversible ecological imbalance.

(5) (NH₄)₂SO₄, Al₂(SO₄)₃, and MgSO₄ all facilitated the transformation of Pb from residual to more labile forms, thereby increasing its ecological mobility. Al₂(SO₄)₃ exerted the strongest effect through acidification caused by Al³⁺ hydrolysis, resulting in the highest F1 fraction and RAC values. MgSO₄ was less effective due to weaker ion-exchange capacity, whereas (NH₄)₂SO₄ showed moderate activation. Although RAC captures the immediate risk associated with F1, integrating F2 and F3 as potential release fractions yields a more comprehensive evaluation of Pb environmental risk.

(6) Al₂(SO₄)₃ concentration had a nonlinear effect on Pb speciation and RAC. The 3% treatment exhibited the strongest activation and mobility, with higher F1 and RAC values and a pronounced decline in the residual fraction, resulting in the greatest ecological risk. In contrast, although the 5% and 7% groups showed increased labile fractions, their RAC gains were limited and the system approached stabilization, suggesting that increasing concentration does not necessarily translate into proportionally greater risk.