Research on Synchronous Synthesis of Schwertmannite for Removal of Pb²⁺ from Acidic Wastewater

Abstract

Schwertmannite (Fe₈O₈(OH)_8−2x(SO₄)_x), an iron oxyhydroxysulfate mineral prevalent in acidic mining environments, demonstrates exceptional heavy metal adsorption capacity owing to its high surface area and abundant functional groups. This study developed a novel one-step synthesis method that simultaneously generates schwertmannite and removes Pb²⁺ from aqueous solutions, contrasting with conventional two-step approaches. Systematic investigation of operational parameters revealed that Pb²⁺ removal efficiency exceeded 98% across concentrations of 0~300 mg·L⁻¹, with optimal performance at n_Sch:n_Pb ratios ≥ 2, pH 3.0~6.0, and 35 °C. Characterization studies identified four primary removal mechanisms: electrostatic adsorption, ion exchange, coordination complexation, and coprecipitation. The in situ method demonstrated significant advantages in processing efficiency, removal stability, and environmental sustainability compared to traditional approaches.

1. Introduction

Acid mine drainage (AMD), generated through the oxidative weathering of sulfide minerals (e.g., pyrite), contains elevated concentrations of iron, sulfate, and various heavy metals [1]. Among these contaminants, lead (Pb) represents a particularly problematic constituent due to its environmental persistence and toxicity [2]. As a Class I priority pollutant, Pb exhibits strong environmental stability and bioaccumulation potential, resisting natural degradation processes. Its mobility through atmospheric deposition, hydrological transport, and soil migration leads to widespread contamination of multiple environmental compartments [3]. The ecological and health impacts of Pb are severe, including growth inhibition in flora and fauna, as well as neurological and renal disorders in humans, particularly children [4].

Schwertmannite (Fe₈O₈(OH)_8−2x(SO₄)_x), a secondary iron oxyhydroxysulfate mineral, forms naturally in AMD-affected environments characterized by low pH, high sulfate, and elevated iron concentrations [5]. This mineral possesses several properties that make it exceptionally effective for heavy metal sequestration, including an extensive specific surface area, abundant iron-hydroxyl functional groups, and structurally incorporated sulfate ions. These characteristics enable schwertmannite to serve as a natural sink for heavy metals through mechanisms dominated by surface complexation and sulfate ion exchange [6,7,8]. Extensive research over the past three decades has documented its effectiveness in removing various aqueous contaminants, including oxyanions (${\mathrm{AsO}}_4^{3-}$, ${\mathrm{CrO}}_4^{2-}$, ${\mathrm{SbO}}_4^{3-}$and ${\mathrm{SeO}}_3^{2-}$) and cations (Cd²⁺, Cu²⁺) [9,10,11,12].

While previous investigations have typically employed a two-step process: First, FeSO₄·7H₂O reacts with H₂O₂ at room temperature and atmospheric pressure to form Schwertmannite. Then, Schwertmannite is used as an adsorbent and added to the aqueous solution containing specific heavy metals to be removed, so as to remove the target heavy metals, which involves separate schwertmannite synthesis and subsequent most metal adsorption on its surface. This study introduces an innovative one-step approach. By generating schwertmannite directly in Pb-contaminated aqueous systems, we better simulate natural AMD remediation processes while enabling more efficient Pb²⁺ removal within its crystal lattice [13,14,15,16]. This methodology not only simplifies treatment operations but also provides new insights into the fundamental interactions between schwertmannite formation and heavy metal immobilization under environmentally relevant conditions [17,18].

2. Experimental

2.1. Chemicals

FeSO₄·7H₂O, H₂O₂, NaOH, HCl, and Pb(NO₃)₂ were purchased from SINOPHARM (Beingjing, China). All chemicals were of analytical reagent (AR) grade. Deionized water was used in all experiments.

2.2. Synthesis of Schwertmannite

Traditional Synthesis Method: A solution was prepared by dissolving FeSO₄·7H₂O (5.56 g L⁻¹) in 250 mL of deionized water, followed by the addition of H₂O₂ (3 mL, 30%). The mixture was adjusted to pH 4 using 0.1 M NaOH or HCl, then agitated in a temperature-controlled orbital shaker (150 rpm, 25 °C) for 24 h. The resulting precipitate was collected by vacuum filtration through a 0.45 µm membrane and air-dried at room temperature.

One-Step Synthesis with Pb²⁺ Removal: The procedure was modified by replacing deionized water with Pb²⁺-containing solutions. All other parameters (reagent concentrations, pH, temperature, and agitation) remained identical to the traditional method. The final product was filtered (0.45 µm) and dried for subsequent analysis.

2.3. Experimental Design

2.3.1. Initial Pb²⁺ Concentration Effect

Solutions containing 10~300 mgL⁻¹ Pb²⁺ (prepared from Pb(NO₃)₂) were treated using the one-step method. After a 24 h reaction, suspensions were filtered (0.22 μm) and residual Pb²⁺ concentrations in filtrates were analyzed. Each experiment was performed in triplicate (the same applies below).

2.3.2. Schwertmannite Dosage Effect

Varying quantities of FeSO₄·7H₂O (1.39~22.24 g) and corresponding H₂O₂ volumes (0.75~12 mL) were added to 250 mL of 50 mg L⁻¹ Pb²⁺ solution. The molar ratios (n_sch:n_pb) ranged from 1:1 to 16:1. Post-reaction, samples were filtered (0.22 μm), dried at 80 °C, and stored in desiccators.

2.3.3. pH Dependence Study

The initial pH was systematically varied from 1.0 to 6.0 using HCl/NaOH, while maintaining other standard conditions (5.56 g L⁻¹ FeSO₄·7H₂O, 3 mL H₂O₂, 50 mg L⁻¹ Pb²⁺, 25 °C, 24 h).

2.3.4. Temperature Optimization

Reactions were conducted at 25 °C, 35 °C, and 45 °C (pH 4, 100 mg L⁻¹ Pb²⁺) to evaluate thermal effects. Temperature was controlled within ±0.5 °C using a calibrated water bath.

2.4. Analytical Methods for Lead Concentration

Pb²⁺ concentrations were quantified by flame atomic absorption spectrometry (FAAS; AA6100, Tech comp) with a detection limit of 0.01 mg·L⁻¹. Calibration standards (0.5~10 mg·L⁻¹) were prepared from certified reference materials (1000 mg·L⁻¹ Pb²⁺ stock). Removal efficiency (R) was calculated as Equation(1):

$$R={\displaystyle \frac{C_0-C_{eq}}{C_0}}\times 100\%$$

(1)

R: Removal efficiency (%)

C₀ = initial Pb²⁺ concentration (mg·L⁻¹)

C_eq = equilibrium concentration (mg·L⁻¹)

All experiments were conducted in triplicate, with mean values reported. Relative standard deviations were <5% for all data points.

2.5. Characterization of Adsorbent

The morphological details of the samples were examined with SEM (SU8010, Hitachi, Tokyo, Japan). The Fourier Transform Infrared (FTIR) spectroscopy analysis of Schwertmannite was performed using an FTIR spectrometer (TENSOR II, Bruker Corporation, Karlsruhe, Germany).

3. Results and Discussion

3.1. Effect of Pb Concentration

Pb²⁺ removal efficiency was evaluated across concentrations ranging from 0~300 mg·L⁻¹ at pH 5.68 over 24 h (Figure 1). Remarkably, removal rates consistently exceeded 98% throughout this concentration range, demonstrating superior performance compared to conventional adsorption systems, where efficiency typically declines with increasing contaminant loading.

The observed pH dependence of Pb²⁺ removal by schwertmannite reflects three distinct operational regimes. First, under strongly acidic conditions (pH < 1.5), the high proton concentration inhibits Fe³⁺ hydrolysis through proton competition, restricts the formation of iron polymer species (e.g., [Fe₂(OH)₂]⁴⁺), and disrupts the structural incorporation of${\mathrm{SO}}_4^{2-}$; these combined effects ultimately result in poor crystallinity of the synthesized schwertmannite [19]. Second, within the optimal pH range (pH 1.5~3.0), the solution environment balances Fe³⁺ hydrolysis and polymerization processes, facilitates the effective structural incorporation of${\mathrm{SO}}_4^{2-}$, and achieves both maximum Pb²⁺ removal efficiency (90.57% at pH 3.0) and enhanced mineral purity (schwertmannite content > 95%) [20]. Third, under moderately acidic to neutral conditions (pH > 3.0), the elevated OH⁻ concentration competes with ${\mathrm{SO}}_4^{2-}$ during Fe³⁺ hydrolysis, promoting the formation of iron (oxy)hydroxide impurities; although the Pb²⁺ removal efficiency remains relatively stable, it exhibits a slight reduction to approximately 90% [21].

Four synergistic mechanisms explain the exceptional removal efficiency at elevated concentrations (50~300 mg/L): (i) Strain compensation: Higher Pb²⁺ concentrations provide sufficient driving force to overcome lattice strain through defect stabilization and localized solid solution formation. (ii) Crystallization enhancement: Pb²⁺ acts as a mineralization agent, reducing the activation energy for Fe³⁺-${\mathrm{SO}}_4^{2-}$ binding and accelerating schwertmannite nucleation/growth (Ksp_(PbSO4) = 1.6 × 10⁻⁸). (iii) Co-crystallization: Pb²⁺-${\mathrm{SO}}_4^{2-}$ ion pairs participate directly in the crystallization process, leading to structural encapsulation rather than surface adsorption. (iv) Diffusion kinetics: The increased concentration gradient enhances Pb²⁺ transport to growing crystal surfaces, minimizing diffusion-limited removal.

This concentration-independent high removal efficiency represents a significant advancement over traditional adsorption approaches, demonstrating the method’s robustness for treating wastewater across a wide range of Pb²⁺ contamination levels.

3.2. Effect of Schwertmannite Mass

The relationship between schwertmannite quantity and Pb²⁺ removal efficiency was investigated using an initial Pb²⁺ concentration of 50 mg·L⁻¹ (pH 5.68) with varying nSch:nPb molar ratios (1:1 to 16:1). As shown in Figure 2, removal efficiency increased from 83.68% to 99.01% as the ratio increased from 1:1 to 2:1, reaching near-quantitative removal (≈100%) at ratios ≥ 4:1.

This dose-dependent behavior stems from two synergistic removal mechanisms. First, in terms of the structural basis for Pb²⁺ binding, schwertmannite possesses a unique crystal structure (with the chemical formula Fe₈O₈(OH)₆(SO₄)·nH₂O), which provides multiple binding sites for Pb²⁺ through three key structural units: Fe³⁺ octahedral coordination frameworks, interlayer ${\mathrm{SO}}_4^{2-}$ anions stabilized by hydrogen bonding, and structural flexibility that enables ion substitution [22]. Second, regarding the primary removal mechanisms, despite the significant ionic radius mismatch between Pb²⁺ and Fe³⁺ (119 pm for Pb²⁺ vs. 64.5 pm for Fe³⁺), Pb²⁺ can still partially replace Fe³⁺. This replacement process achieves charge compensation via proton adjustment and is accompanied by competitive coordination with interlayer ${\mathrm{SO}}_4^{2-}$. Meanwhile, during the crystal growth of schwertmannite, Pb²⁺ can be encapsulated in interlayer spaces, incorporated into pore structures during crystal aggregation, and adsorbed on the surface of developing crystals [23]. Furthermore, a synergistic enhancement effect further improves the removal efficiency: as the generation of schwertmannite increases, its binding sites exhibit a geometric expansion (with the expansion rate proportional to the cube of the generation amount, ∝ n³), the cumulative specific surface area increases significantly, the pore connectivity is improved, and the availability of crystal defects is notably enhanced [24]. The synergistic effect of the aforementioned chemical and physical mechanisms collectively explains the observed efficiency plateau under high-dose conditions, which signifies that the sequestration capacity of schwertmannite for Pb²⁺ has reached saturation, i.e., the complete sequestration of Pb²⁺ is achieved.

3.3. Effect of pH

The pH dependence of Pb²⁺ removal was investigated across six pH levels (1.0~6.0) using 50 mg·L⁻¹ Pb²⁺ solutions (Figure 3). Schwertmannite, naturally occurring in acidic mine drainage (pH 2~4), demonstrated optimal performance in this acidic range. Removal efficiency increased from 79.30% at pH 1.0 to a maximum of 90.57% at pH 3.0, maintaining approximately 90% efficiency up to pH 6.0.

The observed pH dependence of Pb²⁺ removal by schwertmannite reflects three distinct operational regimes. First, under strongly acidic conditions (pH < 1.5), the high proton concentration inhibits Fe³⁺ hydrolysis through proton competition, restricts the formation of iron polymer species (e.g., [Fe₂(OH)₂]⁴⁺), and disrupts the structural incorporation of ${\mathrm{SO}}_4^{2-}$; these combined effects ultimately result in poor crystallinity of the synthesized schwertmannite [22]. Second, within the optimal pH range (pH 1.5~3.0), the solution environment balances Fe³⁺ hydrolysis and polymerization processes, facilitates the effective structural incorporation of ${\mathrm{SO}}_4^{2-}$, and achieves both maximum Pb²⁺ removal efficiency (90.57% at pH 3.0) and enhanced mineral purity (schwertmannite content > 95%) [23]. Third, under moderately acidic to neutral conditions (pH > 3.0), the elevated OH⁻ concentration competes with ${\mathrm{SO}}_4^{2-}$ during Fe³⁺ hydrolysis, promoting the formation of iron (oxy)hydroxide impurities; although the Pb²⁺ removal efficiency remains relatively stable, it exhibits a slight reduction to approximately 90% [24].

So the pH 2.0 condition proved particularly advantageous for yielding well-crystallized schwertmannite, consistent production yields, minimal competing phases, and excellent Pb²⁺ incorporation [25]. This pH-dependent behavior aligns with schwertmannite’s natural formation conditions and confirms the importance of maintaining proper acidity for both mineral synthesis and heavy metal removal applications.

3.4. Effect of Temperature

The Pb²⁺ removal process was evaluated across three temperature conditions (25 °C, 35 °C, and 45 °C) to assess thermodynamic influences on schwertmannite formation and heavy metal sequestration. As shown in Figure 4, the system exhibited remarkable temperature stability, with less than 2% variation in removal efficiency across the tested range. This minimal temperature dependence suggests the process is both thermodynamically favorable and exothermic in nature. Within the temperature interval of 20 °C to 40 °C, the copper removal efficiency rose from 84.96% to 88.67%. It was thus confirmed that temperature exerts a relatively slight impact on the copper removal process, which is similar to this research [18].

At 25 °C, several limiting factors were observed: Kinetic constraints slowed Fe³⁺ hydrolysis and polymerization, resulting in incomplete schwertmannite formation; Reduced molecular mobility led to disordered crystal growth and amorphous character; The prolonged reaction duration resulted in suboptimal Pb²⁺ incorporation efficiency.

The 35 °C condition demonstrated optimal performance characteristics: Reaction rates increased by 30~50% compared to 25 °C; Crystal growth proceeded with improved ordering and phase purity (>90% schwertmannite); ${\mathrm{SO}}_4^{2-}$ incorporation remained stable within the mineral structure; Pb²⁺ removal efficiency reached maximum values.

Elevated temperature (45 °C) introduced competing reaction pathways: Accelerated Fe³⁺ hydrolysis favored amorphous Fe(OH)₃ formation; rapid precipitation caused particle agglomeration defects, lattice disordering, and reduced schwertmannite content (<70% phase purity). Thermal destabilization of structural ${\mathrm{SO}}_4^{2-}$, occurring through hydrogen bond weakening, altered Fe-O-SO₄ coordination geometry, and competing iron oxyhydroxide phases (particularly γ-FeOOH), emerged [17,18].

This temperature optimization has important implications for scaling the technology, as it requires only moderate heating while delivering significantly improved performance over room temperature operation. The fundamental understanding of these temperature effects also informs potential applications in varying climatic conditions [26].

3.5. Microstructural Characterization of Schwertmannite

The traditional schwertmannite (Figure 5a) appears as loose blocky material covered with fine aggregated spherical particles on the surface. In contrast, lead-loaded schwertmannite (Figure 5b) exhibits compact irregular blocky material, spherical granular particles, and a small amount of flaky crystalline particles. This structural transformation may result from the coprecipitation and isomorphous substitution of the novel schwertmannite, which encapsulates heavy metal ions within its lattice, thereby altering the mineral’s structure from loose to compact blocky particles [27]. The emergence of flaky crystalline particles is attributed to the coordination complexation reaction between hydroxyl groups in the novel schwertmannite and lead ions in the solution, leading to the adsorption and fixation of lead on the mineral surface [17,18]. The infrared spectra of schwertmannite are shown in Figure 6.

As shown in Figure 6, both traditional schwertmannite and lead-loaded schwertmannite exhibit characteristic absorption peaks at wavenumbers of 3359 cm⁻¹, 1614 cm⁻¹, 1006 cm⁻¹, and 489 cm⁻¹. The characteristic peak at 3359 cm⁻¹ is assigned to the stretching vibration of hydroxyl groups (-OH); the peak at 1614 cm⁻¹ is attributed to the deformation and bending vibration of -OH in adsorbed water; and the peaks at 1006 cm⁻¹ and 489 cm⁻¹ are the characteristic absorption peaks of sulfate ions (${\mathrm{SO}}_4^{2-}$), which is consistent with the typical infrared spectral behavior of ${\mathrm{SO}}_4^{2-}$ in sulfate-containing minerals. Compared with traditional schwertmannite, the transmittance of the hydroxyl vibration peak (3359 cm⁻¹) of lead-loaded schwertmannite increased by 20.60%, indicating that lead ions (Pb²⁺) in the solution underwent coordination complexation with -OH on the schwertmannite surface, leading to decreased stability of hydroxyl groups and, thus, an increase in the transmittance of the corresponding characteristic peak. In addition, there was no significant change in the transmittance of the characteristic peaks corresponding to H-O-H (from adsorbed water) and ${\mathrm{SO}}_4^{2-}$, suggesting that the removal of lead ions by schwertmannite mainly occurs through two mechanisms: isomorphic substitution and coordination complexation [17,18,28,29,30].

4. Conclusions

This study demonstrates a novel in situ synthesis approach for simultaneous schwertmannite formation and Pb²⁺ removal, offering significant advantages over conventional two-step adsorption methods. It establishes schwertmannite-mediated in situ treatment as a technically superior and economically viable solution for Pb²⁺ contamination, with potential applications extending to other hazardous metal pollutants. Future research should focus on pilot-scale validation and life cycle assessment to facilitate full-scale implementation.