Release Behavior of Pb(II) Ions on the Galena Surface: Dissolution Experiment, DFT Calculation, and MD Simulation

Abstract

In this study, the release behavior of Pb(II) ions from the galena surface and their occurrence forms in the migration process under acid and alkaline conditions were investigated by dissolution experiment, the density functional theory (DFT) calculation, and molecular dynamics (MD) simulation. The dissolution experiments indicated that acidic and high alkaline conditions are more beneficial for the release of Pb(II) rather than neutral and weak alkaline conditions. The quantum chemical calculations indicated that under acidic conditions, H⁺ can destroy the surface structure of galena, leading to the dissolution of Pb²⁺ from the mineral surface into the liquid phase. OH⁻ can also damage the galena surface to a certain extent under alkaline conditions. Additionally, MD simulations were further utilized to study the occurrence forms of Pb(II) ions in alkaline solutions. The results suggested that with a certain concentration of OH⁻, Pb²⁺ ions will form lead hydroxide aggregates, while excessive OH⁻ could lead to the dispersion and dissolution of the lead hydroxo complexes. The surface morphological observation by SEM can well support the calculation and simulation results.

1. Introduction

Mining-associated industrial activities are one of the main sources of worldwide heavy metal pollution [1,2]. Lead (Pb) mineral resources are indispensable resources in the development of the national economy [3]. As one of the most important minerals of Pb [4], galena (PbS) extensively exists in many kinds of mineral deposits, such as lead-zinc mines, coals, and other nonferrous metal minerals [5,6]. Metal leaching and metalliferous drainage are naturally caused when sulfide minerals are exposed to the weathering environment [7,8]. Excessive Pb and other accompanying metal ions (such as Zn, As, and Cd) were released into soils, waters, and sediments, posing a great threat to the ecosystem and causing incalculable harm to human health [9]. For example, Pb is a toxic metal for humans. Its exposure can affect children’s intellectual development [10]. More seriously, high concentration of dissolved Pb(II) resulting from abandoned sulfide mines or tailings poses a serious risk even for a long period of time after the end of mining activities [11]. It is significant to investigate the dissolution and migration behavior of Pb(II) ions from galena for better controlling and preventing Pb pollution.

The pH is an important factor affecting the dissolution and migration behaviors of heavy metals in the sulfide minerals [12,13]. Hsieh and Huang [14] systematically studied the dissolution of PbS in dilute aqueous solutions, and their results showed that the dissolution reaction was pH-dependent. The pH drives the mineral’s dissolution, precipitation-reprecipitation, and sorption-desorption at the mineral surface [15,16]. It is usually considered that the dissolved metal ions on the mineral surface are released into pore water, which becomes mobile in the acid condition while precipitated and immobile in the alkaline condition [17].

Studies on the dissolution of galena under acid and alkaline conditions have been widely carried out [18,19,20]. Chemical and physical changes during galena dissolution were commonly examined by various analytical techniques, such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning tunneling microscopy (STM), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy [5,9,21,22]. However, the details of metal release during the dissolution of galena are still unclear. In recent years, the density functional theory (DFT) has been widely used to study the microcosmic characteristics of mineral surfaces [5,23,24,25]. DFT can be used to calculate and simulate the structure of galena to gain insight into the fundamental aspects of mineral dissolution and metal release behavior at the atomic level [26].

The heavy metal release in the minerals involves two stages: the dissolution stage, where metal ions dissolve from the mineral surface, and the migration stage, where dissolved metal ions enter the solution or form precipitates that adsorb on the mineral surface. In order to explore the release behavior of Pb(II) ions dissolved from the mineral surface in acid and alkaline conditions, dissolution experiments, DFT calculations, and MD simulations were conducted. Specifically, the dissolution experiments of galena were conducted under different pH conditions (in the range of 4–12), and the concentration of released Pb²⁺ was measured by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). For the DFT study, quantum chemistry calculation was used to simulate the release behavior of Pb(II) ions on the mineral surface, as well as the change characteristics of galena surface structure under acid and alkaline conditions. Additionally, Molecular Dynamics (MD) simulations were employed to investigate the forms of Pb(II) ions under alkaline conditions. Finally, the surface morphology of minerals before and after the dissolution was analyzed by scanning electron microscope (SEM) to further validate the simulation results. The use of the DFT calculations and MD simulations provides a comprehensive understanding of the release of metal ions from the mineral surface at the molecular and atomic levels.

2. Materials and Methods

2.1. Dissolution Experiments

Pure minerals were used as the research object to study the release behavior of heavy metal ions. The pure galena samples used in the experiment were separated from the raw mineral. The raw mineral was crushed, and the gangue minerals were removed. It was then ground in the agate mortar and sieved through 400 mesh sieves. The obtained galena powder was washed with ultrapure water in the Ultrasonic Cleaner (SB-5200D, Scientz Co., Ningbo, China) for 4 min, dried in a vacuum, and then kept in the vacuum condition to prevent oxidation for further analysis and experiments. X-ray diffraction (XRD DX-2700, Dandong Fangyuan Instrument Co., Dandong, China) and chemical analysis were utilized to characterize the galena samples. The X-ray diffraction analysis (Figure 1) showed the clear characteristic peaks of PbS, indicating that the purity of the galena sample was extremely high and almost no impurities coexisted. The chemical analyses were conducted to determine the proportion of PbS in galena. The galena samples were digested by using mixed acids (hydrochloric acid, nitric acid, and perchloric acid), and then the concentration of Pb in the solution was analyzed by the Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES PQ9000, Analytik Jena Co., Jena, Germany). The galena sample was burned at around 1300 °C, and generated sulfur dioxide was absorbed by an iodine-containing standard solution. Starch was used as an indicator of the endpoint, and the sulfur content in the sample was determined based on the amount of iodine standard solution consumed. The chemical analyses revealed that the proportion of PbS in galena was 99.48%. Therefore, the purity of mineral samples meets the requirements for single mineral experiments.

In the dissolution experiments, about 5.0 g of the galena samples were mixed with 100 mL of a leaching solution adjusted to the appropriate pH (4, 6, 8, 10, and 12) with HCl (0.2 and 2 M, Prepared by HCl, GR, 37%) or NaOH (0.2 and 2 M, Prepared by NaOH, GR, ≥97%). The mixture was put into the 250 mL Erlenmeyer flask with a stopper and shaken well at 180 rpm and 25 °C for 16 h. After shaking, the obtained supernatant of each sample was collected, filtered with 0.45 μm cellulose filter, and preserved under refrigeration at 4 °C before analysis. The remaining solids were collected by filtration, dried using the vacuum freeze-drying method, and stored in a sealed bottle for further analysis.

The pH values of the solution were determined using a pH electrode (PHS-3E, Rex Electric Chemical Co., Shanghai, China). Concentrations of Pb²⁺ in liquid samples were determined by ICP-OES (PQ9000, Analytik Jena Co., Jena, Germany) with a detection limit of 0.08 ppm. The morphology and structure of minerals before and after treatment were analyzed by using the Scanning Electron Microscope (SEM MLA650F, FEI Co., Hillsboro, OR, USA).

2.2. Computational Details

All computational simulations were performed remotely using Materials Studio 2019 (MS 2019, Accelrys Co., San Diego, CA, USA) at the Beijing Super Cloud Computing Center. The quantum chemical calculations and molecular dynamics simulations were carried out using the Cambridge Serial Total Energy Package (CASTEP) and Forcite modules, respectively [23,27].

Quantum chemical calculations were performed using the CASTEP module to investigate the release behavior of lead ions on the surface of PbS under acidic and alkaline conditions, as well as the changes in mineral surface structures. Galena (PbS) belongs to the isometric crystal system with a NaCl structure, space group Fm3m, and lattice parameters of a = b = c = 5.93 Å. Based on parameters from the literature, the original cell model for PbS was constructed and structurally optimized [5]. The original cell model of galena is shown in Figure 2a. After the mineral crystal structure was optimized, the crystal was cleaved to obtain the cleavage plane model of galena. In this study, optimized unit cells of galena were cleaved along the PbS (100) direction. To ensure better interactions between the mineral surfaces and acidic/alkaline ions, a 3 × 3 × 1 supercell treatment was applied to the cleaved surface of the galena. After surface relaxation, the slab model with both surface and crystal internal characteristics was obtained (Figure 2b). The calculation parameters used for surface relaxation are shown in

Table 1. The calculation parameters used for surface relaxation.

Geometric Optimization Parameters
Module	CASTEP
Functional	GGA-PW91
Energy Cutoff	350 eV
Pseudopotentials K-point set	Ultrasoft 3 × 3 × 1
Convergence tolerance
Energy	1.0 × 10−5 eV/atom
Max. force	0.03 eV/Å
Max. displacement	0.001 Å
Max. stress	0.05 GPa

. The subsequent simulation of the surface structures in the direction of galena (100) under acidic or alkaline conditions is completed under the slab model.

The MD simulation was conducted in the Forcite module to analyze the agglomeration or dispersion of released Pb(II) ions in the acidic and alkaline environment. The relative strength of acidity and alkalinity in the aqueous environment was realized by controlling the ratio of hydrogen ion or hydroxide ion to metal ion in the system. The ratio of metal ion to hydrogen ion or hydroxide ion ranged from 1: 2 to 1: 10. The basic parameters of molecular dynamics simulation are shown in

Table 2. MD calculating parameter.

Dynamics	System Models
Module	Forcite
Forcefield	COMPASS
Ensemble	NPT
Thermostat	Nose
Barostat	Berendsen
Temperature	298 K
Pressure	1.0 × 10−5 Gpa
Dynamics	System models
Module	Forcite
Time step	1.0 Fs
Number of steps	3.0 × 106
Total time	3000 ps

. The data of 1ns simulation time after the dynamic equilibrium of the molecular dynamics calculation system were selected as the final simulation results.

3. Results

3.1. Pure Mineral Dissolution Experiments

Dissolution tests of galena under acid and alkaline conditions were carried out. Dissolved concentrations of Pb²⁺ in different pH (4, 6, 8, 10, and 12) are shown in Figure 3. With the increase in pH, the concentration of Pb²⁺ from the galena surface decreased first and then increased as the pH was above 8.

Generally, the weathering dissolution of metal sulfides is an electrochemical process [28]. In accordance with the electrochemistry theories [9,20,29,30], the electrochemical reactions for galena can be given as reactions (1–5) [9,20,28]. In both acidic and alkaline solutions, galena undergoes anodic oxidation reaction on the mineral surface according to reaction formula (1), releasing metal Pb(II) ions and generating hydrophobicity S°. However, the corresponding cathodic reduction reaction varies under acidic and alkaline environmental conditions. Under acidic conditions, O₂ undergoes a cathodic reduction reaction with H⁺ to generate H₂O (reaction 2), and higher acidity will facilitate the reaction. When galena is in alkaline solutions, O₂ is reduced via reaction (3), but high concentrations of OH⁻ will hinder the reaction. The presence of OH⁻ can result in the hydroxylation of Pb²⁺. In a weak alkaline solution with a relatively low concentration of OH⁻, the dissolved Pb²⁺ from galena will form Pb(OH)₂ precipitation (reaction 4). With the increase of the content of OH⁻, reaction (5) is promoted, leading to the poorly soluble Pb(OH)₂ changes into soluble Na₂[Pb(OH)₄] at a high alkaline solution. This is consistent with the dissolution results in Figure 3; the acidic condition is conducive to the release of Pb²⁺ on the galena surface, while in the near neutral environments, the formation of Pb(OH)₂ precipitation leads to a decrease in the concentration of Pb²⁺ ions in the solution [31]. When the pH value was higher than 8, the release of Pb²⁺ was increased due to the conversion of the lead hydroxide precipitates into soluble multi-hydroxide ion complexes in the high-alkaline solution.

$$\mathrm{PbS}\to {\mathrm{Pb}}^{2+}+{\mathrm{S}}^0+2{\mathrm{e}}^{-},$$

(1)

$$\mathrm{Acid}: {\mathrm{O}}_2+4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}\to 2{\mathrm{H}}_2\mathrm{O},$$

(2)

$$\mathrm{Alkaline}: {\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}\to 4{\mathrm{OH}}^{-},$$

(3)

$${\mathrm{Pb}}^{2+}+2{\mathrm{OH}}^{-}=\mathrm{Pb}{\left(\mathrm{OH}\right)}_2\downarrow ,$$

(4)

$$\mathrm{Pb}{\left(\mathrm{OH}\right)}_2+2{\mathrm{OH}}^{-}=\mathrm{Pb}{{\left(\mathrm{OH}\right)}_4}^{2-},$$

(5)

3.2. DFT Calculation

To further investigate the complex release process of Pb(II) on the mineral surface in the acid and alkaline solutions at an atomic level, quantum chemical calculation was used to study the release characteristics and change in occurrence form of Pb(II) on the mineral surfaces. In accordance with reactions (1–5), the oxygen participates in the reaction in the solution rather than directly reacts with PbS. Given that the computational models in the quantum chemical calculation were established to study on the reactions at the interface of PbS, oxygen was not introduced into the models. The slab model of the galena (100) surface was obtained and calculated with the DFT method, as described in Section 2.2. The models under acidic and alkaline conditions were obtained by introducing water molecules (H₂O) and hydrogen ions (H⁺) or molecules (H₂O) and hydroxyl ions (OH⁻) into the vacuum layer above the slab. The adsorption models of H⁺ on galena (100) surface under acidic conditions are shown in Figure 4a,b). After geometric optimization, the galena (100) surface undergoes a considerable relaxation of surface atoms. As a result, the S atoms move outward, bind with H⁺, and break with the connected Pb atoms. The release of S atoms and the fracture between S and Pb atoms further induce the release of Pb.

The adsorption models of OH⁻ ions on the galena (100) surface under alkaline conditions are shown in Figure 4c,d. After geometric optimization, the distance between OH⁻ and Pb atoms, as well as that between OH⁻ and S atoms, is shortened, resulting in the bonding of OH⁻ ions with Pb and S atoms. At the same time, the outermost Pb atoms exhibit a certain degree of relaxation and a tendency to move towards vacuum but do not completely break with other surrounding atoms. The Pb atoms on the galena surface are more likely to combine with OH⁻ ions to form hydroxides rather than release directly.

Based on the above calculation results, it can be seen that the Pb atoms on the surface of galena have a tendency of releasing into the liquid phase under both acidic or alkaline conditions, but are more easily released in acidic solution than the alkaline solution. The simulation results are consistent with that of the dissolution experiments.

3.3. MD Simulation

The released Pb(II) mainly exists in the form of free ions under acidic conditions and in various forms of hydroxyl complexes under alkaline conditions [32]. In an alkaline environment, the Pb(II) ions are first released from the mineral surface and dissolved into the solution; the Pb(II) ions will combine with a certain number of OH⁻ to form hydroxyl complexes. The relationship between the state of hydroxyl complexes and the ratio of metals to hydroxyl groups is not yet very clear. Therefore, molecular dynamics (MD) simulation was further carried out to explore the occurrence forms of Pb(II) ions in the aqueous solution.

Hydroxyl complexes of Pb(II) usually exist in multiple forms [6,33]. In the simulation process, the proportion of Pb²⁺ to OH⁻ ions is determined according to the possible forms of the complexes to ensure that the model can provide enough ligands and metal ions that form corresponding complexes. When the ratio of Pb²⁺ and OH⁻ is 1:2 (Figure 5a), after MD simulation, the Pb²⁺ ions in the random thermal motion state in the original model ultimately gather together, and almost all the OH⁻ ligands in the system coordinate with Pb²⁺ ions, which is essentially the formation of Pb(OH)₂ precipitation. When the ratio of Pb²⁺ and OH⁻ was 1:3 (Figure 5b), the simulation result is similar to the above findings, except that the form of lead hydroxide precipitation is between Pb(OH)₂ and Pb(OH)_1–3, and Pb²⁺:OH⁻ = 16:41 in the hydroxide complex. This kind of lead hydroxide agglomerates still belongs to the precipitation since Pb²⁺ and OH⁻ are assembled in very small areas of space. When the ratio of Pb²⁺ ions and OH⁻ was 1:10 (Figure 5c), a highly alkaline solution environment with excessive hydroxyl, the lead hydroxyl complex still shows a tendency to agglomerate, but the aggregation becomes loose and dispersed. The proportion of Pb²⁺ ions and OH⁻ ions in these loose aggregates is uncertain. Unlike the precipitates shown in the above two situations, the system is actually in a state between a colloid and a solution. These findings are well consistent with the dissolution results shown in Figure 3 and the reaction (4–5).

3.4. SEM Analysis of Minerals

To verify the MD simulation results, the surface morphology of the galena samples after the dissolution was analyzed by SEM (Figure 6). It is observed that before dissolution, the galena samples had a dense surface and sharp edges with some fines generated during fragmentation (Figure 6a). After dissolution, the surface structure of the galena was eroded to a certain extent, and the edges became blunt significantly in both acidic and high alkaline solutions. A large number of precipitates were found on the galena surface after dissolution in the alkaline solution at pH = 10 (Figure 6c), while they did not appear in the acidic situation (Figure 6b). With the pH increasing to 12, the amounts of precipitate particles were reduced (Figure 6d), which was attributed to the fact that the Pb(OH)₂ precipitates were dissolved due to the formation of soluble lead hydroxyl complexes at high concentrations of OH⁻. These findings are well consistent with the simulation results.

4. Conclusions

In this work, dissolution experiments and DFT calculations and MD simulations were performed to understand the release behaviors of Pb(II) ions from the galena surface under acidic and alkaline conditions. The main conclusions are as follows:

(1)

Dissolution experiments showed that the release of Pb²⁺ from the galena surface is closely related to the pH of the extract solutions, and acidic or highly alkaline environments are conducive to the dissolution of Pb(II) ions.

(2)

The results of DFT calculations indicated that Pb atoms on the surface of galena have a tendency to release into the liquid phase under both acidic or alkaline conditions but are more easily released in an acidic solution than the alkaline solution.

(3)

The MD simulations suggested that Pb ions form hydroxide aggregates with a certain concentration of OH⁻ under alkaline conditions, while excessive OH⁻ could lead to the dispersion and dissolution of hydroxide aggregates. That is to say, the formation of lead hydroxyl complexes does not always result in the formation of precipitates. Only when hydroxyl complex clusters are formed, and the particle size of the clusters reaches a certain value can precipitation be formed. If the content of hydroxide ions in the solution is too high, it may lead to the re-dispersion of lead hydroxide complex clusters, resulting in the precipitation and dissolution of hydroxides. The ratio of cations to anions in clusters is not a constant value but varies within a range, which is significantly different from the ratio described by stoichiometry in traditional solution chemistry.

(4)

The surface morphological observation by SEM can well support the calculation and simulation results.

According to the results of this study, to minimize release of Pb from sulfide mineral, the pH must be maintained in the circumneutral region. In the future, further research can be conducted on the enrichment and inhibition mechanisms of metal ions such as Pb, to explore the adsorption behaviors of metal ions on mineral surfaces and adsorption materials under different pH environments.