Enhanced Arsenite Remediation in Synthetic FeS₂/Fe(II)-Containing Arsenic Wastewater via Epigallocatechin Gallate-Initiated Persulfate Activation

Abstract

The Fe­(II)-catalyzed persulfate (PS) activation system demonstrated the ability to remove pollutants, such as As­(III), from groundwater, including FeS₂/Fe­(II)-enriched mine drainage; however, the slow conversion rate of Fe­(III) to Fe­(II) limited the overall efficiency of the process. In this study, natural polyphenols (EGCG) was introduced in the FeS₂/Fe­(II)/PS system to enhance the long-term recycling of Fe­(II), significantly improving the removal of As­(III) within 10 min. The results showed that the addition of 0.025 mM EGCG to the FeS₂/Fe­(II)/PS system, under conditions of 0.1 g/L FeS₂, 0.1 mM Fe­(II), and 0.25 mM PS, remarkably promoted the removal efficiency of As­(III) from 50.7% to 89.3%. EGCG enhanced the reduction of Fe­(III) to Fe­(II) owing to the chelating, reducing, and electron-donating capabilities of ortho- or para-dihydroxybenzene groups in EGCG. The stable Fe­(II) complex formed was continuously oxidized by PS, generating reactive oxygen species (ROS), such as hydroxyl radicals (^•OH), sulfate radicals (SO₄ ^–•), and superoxide radical (O₂ ^•–), which facilitated the oxidation of As­(III). Density functional theory (DFT) calculations, combined with transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analyses, revealed that the resulting As­(V) was primarily adsorbed onto the surfaces of colloid Fe₂O₃ in the form of bidentate binuclear complex, as well as onto FeS₂ surfaces. This study presents an efficient approach to enhancing the FeS₂/Fe­(II)/PS process by incorporating natural polyphenols, offering a promising strategy for the removal of micropollutants in groundwater.

1. Introduction

Arsenic (As) contamination in mine drainage is a critical environmental issue due to its severe toxicity and widespread occurrence. In such environments, arsenic primarily exists as trivalent arsenic (As­(III)) and pentavalent arsenic (As­(V)), with As­(III) being more toxic, mobile, and resistant to conventional treatment methods. , The uncontrolled discharge of As-contaminated mine drainage into aquatic ecosystems has led to severe ecological damage and public health risks, including cancer, skin lesions, and cardiovascular diseases. , Despite various treatment approaches, the efficient and sustainable removal of As­(III) remains a major challenge.

Addressing As contamination is crucial to ensure water safety and environmental sustainability. Given the high toxicity and persistence of As­(III) in water bodies, developing an efficient, cost-effective, and sustainable treatment method is urgently needed. This challenge is relevant to the United Nations Sustainable Development Goals (SDGs) 6 “Clean Water and Sanitation” and SDG 3 “Good Health and Well-being”. Advancing arsenic remediation technologies can reduce the risks associated with mine drainage contamination and contribute to long-term environmental protection.

Persulfate (PS)-based advanced oxidation processes (AOPs) have gained attention for As removal due to their strong oxidizing power and environmental compatibility. , Activated PS generates sulfate radicals (SO₄ ^–•) with a high redox potential (E ⁰ = 2.5–3.1 V), which can effectively oxidize As­(III) to the more easily removable As­(V). , Various PS activation methods, including heat, UV light, and transition metal catalysis, have been explored. Nevertheless, thermal and UV activation usually require high energy consumption, while most homogeneous transition metal catalytic systems suffer from poor stability and secondary metal pollution. Among transition metal-based strategies, Fe­(II)-mediated activation has attracted extensive attention due to its low cost and high activation efficiency. However, conventional single Fe­(II) system suffers from rapid oxidation of Fe­(II) to Fe­(III) and subsequent iron hydroxide precipitation, which remarkably reduces the sustainable catalytic performance. In this context, the FeS₂-based system can continuously supply Fe­(II) through in situ dissolution, effectively alleviate the excessive oxidation and precipitation of Fe­(II), and thus maintain long-term and stable PS activation performance.

Recent studies have shown that natural polyphenols, plant-derived bioactive compounds with multiple phenolic hydroxyl groups, can enhance Fe­(II) stability and redox cycling in iron-based activated persulfate systems. As multifunctional regulators, they act as electron donors (e.g., EGCG, tea polyphenols) to promote Fe­(III) reduction, , as metal chelators (e.g., gallic acid) to stabilize Fe­(II) and accelerate iron cycling and ROS formation, and as free radical regulators to optimize system efficiency, laying a foundation for their application in addressing aqueous As­(III) pollution. Notably, Chen et al. demonstrated that epigallocatechin gallate (EGCG) promoted Fe­(III)/Fe­(II) redox cycling and ^•OH production, thereby significantly boosting coumarin degradation in the Fe­(II)/peracetic acid (PAA) system compared to its EGCG-free counterpart. Similarly, tannin extract, owing to its ortho-diphenol structures similar to EGCG, has also been proven to facilitate Fe­(III)/Fe­(II) redox cycling in Fenton oxidation. While polyphenols have shown effectiveness in enhancing PS activation for organic pollutant degradation, − their role in arsenic remediation remains virtually unexplored.

To bridge this knowledge gaps, this study aims to investigate the enhancement of As­(III) oxidation and adsorption in FeS₂/Fe­(II)-enriched mine drainage using a natural polyphenol-promoted FeS₂/Fe­(II)/PS system. This work also evaluates the synergistic effects of polyphenols and FeS₂/Fe­(II) on PS activation and elucidates the mechanisms underlying As­(III) oxidation and adsorption. Furthermore, this work optimizes key operational parameters to maximize arsenic removal efficiency.

This work introduces a novel strategy for As­(III) remediation by integrating natural polyphenols into the FeS₂/Fe­(II)/PS system. Unlike conventional Fe­(II)/PS system where polyphenols merely stabilize dissolved Fe­(II), this approach uniquely combines FeS₂’s slow-release Fe­(II) capacity with polyphenols’ dual redox-chelating function to create a self-sustaining system: FeS₂ provides continuous Fe­(II) supply through surface oxidation, while polyphenols not only prevent Fe­(II) precipitation but also regenerate Fe­(II) from Fe­(II), enabling persistent radical generation and enhanced arsenic adsorption/oxidation-a significant improvement over traditional Fe­(II)/PS-polyphenol system that rely on finite dissolved Fe­(II) and exhibit declining performance over time. By leveraging this synergistic mechanism, the study advances PS-based water treatment, offering a more rubust and sustainable solution for arsenic-contaminated mine drainage.

2. Materials and Methods

2.1. Materials

Gallic acid (GA, 98%), tea polyphenols (TP, 98%), procyanidin (PC, 95%), luteolin (LT, 98%), and epigallocatechin gallate (EGCG, 95%) were purchased from Shanghai Titan Technology (China). Their properties are detailed in Table S1 (Supporting Information). To prevent degradation, fresh polyphenol solutions were prepared before each experiment. Standard reagents, including potassium persulfate (PS, 99.5%), potassium borohydride (KBH₄, 96%), sodium arsenite (NaAsO₂, 90%), pyrite (FeS₂), and various other analytical-grade chemicals, were obtained from Sinopharm Chemical Reagent Co., Ltd. (China). Ultrapure water (18.2 MΩ·cm) was used in all As­(III) removal experiments.

2.2. Methods

2.2.1. Batch Experiments

The reaction kinetics of As­(III) in PS-injected FeS₂/Fe­(II)-enriched mine drainage were examined in 500 mL glass vials at 25 °C, with and without polyphenols (GA, TP, PC, LT, and EGCG). An As­(III) stock solution was introduced into FeS₂/Fe­(II)-enriched mine drainage (0.1 mM Fe­(II) and 0.1 g/L FeS₂), achieving an initial As­(III) concentration of 1 mg/L. The reaction was initiated by adding polyphenols (25 μM) and PS (250 μM) to 300 mL of the simulated solution.

At predetermined intervals, 1 mL liquid samples were collected using plastic syringes, filtered through a 0.45 μm membrane, and analyzed. Additional experiments assessed the effects of PS (0.1–1 mM), Fe­(II) (0.05–0.2 mM), EGCG (0.01–0.1 mM), and pH (3–9, adjusted with 0.1 M HCl/NaOH) on As­(III) removal. Given the complexity of real mine drainage, the impact of coexisting ions (Cl^–, HCO₃ ^–, SO₄ ^2–, H₂PO₄ ^– at the varying concentrations from 0.1–10 mM) was also investigated. To evaluate the effects of dissolved oxygen, reactions were conducted with and without nitrogen purging. All experiments were performed in triplicate.

2.2.2. Wet Chemistry Analysis

To identify ROS in the EGCG-promoted FeS₂/Fe­(II)/PS system, quenching experiments were conducted using tert-butanol (TBA) and methanol (MeOH). Prior to the addition of EGCG and PS, 500 mM TBA or MeOH was introduced under identical reaction conditions. Electron paramagnetic resonance (EPR) spectra were recorded using a Bruker EMXplus-6/1 spectrometer (Germany), with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trap for ^•OH and O₂ ^•–.

2.2.3. Analytical Methods

As­(III) concentrations were measured using hydride generation atomic fluorescence spectrometry (HG-AFS, SA-20, Beijing Jitian, China), with a detection limit of 0.09 μg·L^–1 and recovery rates of 95–114%. Fe­(II) and Fe­(III) species in the EGCG-promoted FeS₂/Fe­(II)/PS system were determined via the 1,10-phenanthroline method. PS concentrations were analyzed spectrometrically at 352 nm after complexation with iodide and NaHCO₃.

Pyrite crystallinity was characterized by X-ray diffraction (XRD, Rigaku SmartLab SE, Japan), while morphological changes pre- and postreaction were examined using scanning electron microscopy (SEM, ZEISS Sigma 300, Germany) and transmission electron microscopy (TEM, FEI Talos F200X, USA). Chemical composition was analyzed via X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, USA).

2.2.4. Theoretical Calculations

First-principles density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP), employing plane-wave basis sets and the projector-augmented wave (PAW) method. Exchange–correlation interactions were treated with the Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation (GGA), while van der Waals forces were accounted for using Grimme’s DFT-D3 model. FeS₂ (100) lattice parameters were a = 16.2 Å, b = 16.2 Å, c = 21.7 Å, with an energy cutoff of 450 eV and a 2 × 2 × 1 Gamma k-point mesh. Structural relaxation was performed until atomic forces were <0.02 eV/Å, with an energy convergence criterion of 10^–5 eV.

2.3. Statistical Analysis

To ensure experimental data accuracy, reliability and reproducibility, all batch tests were conducted in triplicate under identical conditions, with results reported as mean values. The coefficient of variation was kept below 5%; samples exceeding this error limit were retested until meeting the acceptable range. Data were statistically analyzed using SPSS 19.0 for Windows. One-way ANOVA was employed to assess treatment differences, and a p-value ≤0.05 was defined as statistically significant.

3. Results and Discussion

3.1. As­(III) Removal in Different Systems

The comparative assessment of five polyphenolsgallic acid (GA), tea polyphenols (TP), procyanidin (PC), luteolin (LT), and epigallocatechin gallate (EGCG)in FeS₂/Fe­(II)/PS systems revealed EGCG as the most effective enhancer of As­(III) removal. As shown in Figure a, EGCG facilitated 89.3% As­(III) removal, markedly surpassing GA (63.5%), TP (74.7%), PC (72.6%), and LT (63.7%). This substantial improvement underscores the role of polyphenol structure in electron transfer dynamics and oxidation-adsorption efficiencies within FeS₂/Fe­(II)/PS systems.

1.

(a) Depletion of As­(III) in different polyphenols-induced FeS₂/Fe­(II)/PS systems, (b) The changes of EGCG, Fe­(II) and Fe­(III) concentrations (b) in EGCG-promoted FeS₂/Fe­(II)/PS systems. Reaction conditions: [polyphenols]₀ = 0.025 mM, [FeS₂]₀ = 0.1 g/L, [Fe­(II)]₀ = 0.1 mM, [PS]₀ = 0.25 mM, [As­(III)]₀ = 1 mg/L.

A key determinant of EGCG’s superior performance is its distinct molecular structure. Unlike other polyphenols, EGCG contains six ortho-phenolic hydroxyl groups (Table S1), which significantly enhance its redox activity. These ortho- and para-dihydroxybenzene functionalities accelerate Fe­(III)/Fe­(II) cycling by promoting electron shuttling, thereby enhancing persulfate activation and radical generation. Mechanistically, EGCG undergoes oxidation to quinones via sequential one-electron transfer steps, forming highly reactive semiquinone radicals (SQ^•) that further intensify oxidative transformation processes.

This critical analysis highlights the structural advantages of EGCG over other polyphenols in FeS₂/Fe­(II)/PS systems. While GA and LT possess a limited number of hydroxyl groups that restrict their redox potential, TP and PC, despite their polyphenolic nature, exhibit steric hindrance that reduces their interaction efficiency with FeS₂/Fe­(II). Thus, EGCG’s ability to act as a potent electron mediator and radical generator positions it as a promising natural enhancer for As­(III) remediation in mine drainage treatment.

The introduction of natural polyphenols significantly enhanced As­(III) removal under identical reaction conditions compared to the control experiment (50.7%) (Figure a), highlighting their catalytic role in persulfate (PS)-based in situ chemical oxidation (ISCO) for groundwater remediation. Among the tested polyphenols, EGCG demonstrated the highest efficiency, facilitating 89.3% As­(III) removal, far exceeding GA (63.5%), TP (74.7%), PC (72.6%), and LT (63.7%) (Figure a). The enhancement was particularly rapid, suggesting that polyphenols can effectively accelerate oxidation-adsorption processes within FeS₂/Fe­(II)/PS systems, thereby improving the viability of ISCO applications in environmental remediation.

In the EGCG-promoted FeS₂/Fe­(II)/PS system, the concentrations of EGCG, Fe­(II), and PS declined rapidly (Figures b and S1), while Fe­(III) accumulated as a byproduct of the As­(III) oxidation process. This transformation underscores EGCG’s dual function as both a chelating and reducing agent. By facilitating Fe­(III) reduction back to Fe­(II), EGCG sustains a stable Fe­(II) concentration in solution through the formation of a Fe­(III)/Fe­(II)-EGCG complex. , This redox cycling mechanism is critical, as it ensures continuous Fe­(II) availability, thereby sustaining PS activation and promoting reactive oxygen species (ROS) generation, which drives As­(III) oxidation.

3.2. Proposed Mechanisms of EGCG Promoting Reaction

3.2.1. Oxidation Mechanisms

It was hypothesized that multiple ROS, including SO₄ ^–•, ^•OH, and O₂ ^•–, were generated in the EGCG-promoted FeS₂/Fe­(II)/PS system. , To elucidate the dominant radical species responsible for As­(III) removal, free-radical quenching experiments and EPR spectroscopy were conducted. As shown in Figure , both TBA and MeOH inhibited As­(III) removal, but MeOH exhibited a significantly stronger inhibitory effect than TBA. Specifically, increasing the TBA concentration from 0 to 100 mM led to a reduction in As­(III) removal from 89.3% to 76.1%, while the same concentration range of MeOH reduced As­(III) removal to 66.1% (Figure a). The findings indicate that SO₄ ^–• and ^•OH were the dominant oxidative species responsible for As­(III) transformation in the EGCG-promoted system.

2.

Effect of different radical scavengers TBA (a) and MeOH (b) on As­(III) removal. Reaction conditions: [EGCG]₀ = 0.025 mM, [FeS₂]₀ = 0.1 g/L, [Fe­(II)]₀ = 0.1 mM, [PS]₀ = 0.25 mM, [As­(III)]₀ = 1 mg/L.

This analysis underscores EGCG’s pivotal role in facilitating PS activation via Fe­(III)/Fe­(II) cycling while simultaneously enhancing ROS-mediated As­(III) oxidation. The superior efficiency of EGCG over other polyphenols can be attributed to its unique ortho-phenolic hydroxyl groups, which promote electron transfer and radical generation. These insights offer promising implications for advancing PS-based ISCO strategies by leveraging naturally derived polyphenols to enhance oxidative remediation efficiency in FeS₂/Fe­(II)-rich environments.

To elucidate the dominant reactive species in the EGCG-promoted FeS₂/Fe­(II)/PS system, EPR spectroscopy was applied using DMPO as a spin trap agent. As shown in Figure a, two characteristic spectra corresponding to DMPO–OH (1:2:2:1 intensity ratio) and DMPO–SO₄ adducts were detected. Notably, the DMPO–OH signal was significantly more intense than that of DMPO–SO₄. This disparity is attributed to the rapid transformation of DMPO–SO₄ (rate constant k = 9.1 × 10⁵ M^–1·s^–1) into DMPO–OH (rate constant k = 6.0 × 10⁸ M^–1·s^–1). , These results align with the radical quenching experiments, confirming that EGCG plays a crucial role in enhancing the generation of SO₄ ^–• and ^•OH, which are the primary oxidants driving As­(III) removal.

3.

EPR spectra of DMPO–OH (a) and DMPO–SO₄ ^– and (b) DMPO-O₂ ^•– obtained at 1, 5, and 10 min in EGCG-promoted FeS₂/Fe­(II)/PS or PS system.

Beyond sulfate and hydroxyl radicals, the role of molecular oxygen (O₂) in the reaction system was critically assessed. Previous studies have highlighted the indispensable function of oxygen in Fenton-like reactions, particularly in facilitating pollutant oxidation through superoxide radicals (O₂ ^•–). , The experimental results reinforce this perspective, as shown in Figure . Under air-saturated conditions, As­(III) removal reached 98.5% within 10 min, whereas in a nitrogen-purged environment, the removal efficiency dropped to 87.4%. This reduction indicates that O₂ actively participates in the reaction, further enhancing As­(III) oxidation.

4.

Effect of dissolved oxygen on As­(III) removal in EGCG-promoted FeS₂/Fe­(II)/PS system. Reaction conditions: [EGCG]₀ = 0.025 mM, [FeS₂]₀ = 0.1 g/L, [Fe­(II)]₀ = 0.1 mM, [PS]₀ = 0.25 mM, [As­(III)]₀ = 1 mg/L.

The presence of O₂ ^•– adducts at different reaction time intervals (Figure b) further validates this oxygen-dependent mechanism. The formation of these radicals can be attributed to two concurrent pathways: (i) the reduction of O₂ by Fe­(II), which subsequently donates an electron to persulfate (PS) to generate SO₄ ^–•; and (ii) the electron transfer from semiquinone radicals (SQ^•–) within EGCG to O₂, leading to hydrogen peroxide (H₂O₂) formation. Notably, the generated H₂O₂ can be further activated by SQ^•–, thereby sustaining radical production and prolonging the oxidative capacity of the system.

The findings collectively establish that SO₄ ^–•, ^•OH, and O₂ ^•– are the key ROS driving As­(III) oxidation in the EGCG-promoted FeS₂/Fe­(II)/PS system. Moreover, the interplay between Fe­(II)/Fe­(III) cycling, EGCG-mediated electron transfer, and O₂ participation underscores the system’s robustness in sustaining radical-driven remediation. These mechanistic insights pave the way for optimizing natural polyphenol-assisted persulfate activation strategies for improved contaminant removal in real-world mine drainage scenarios.

3.2.2. Adsorption Mechanisms

The enhanced As­(III) removal efficiency and the involvement of reactive radicals underscore the pivotal role of EGCG in the FeS₂/Fe­(II)/PS system. To gain deeper insights into the mechanistic pathways, we employed SEM-EDS and XRD analyses to examine the structural and compositional evolution of pyrite before and after reaction with As­(III) (Figures and S2). Despite the preservation of crystallization properties and overall morphology, significant alterations in elemental composition were observed, providing evidence for the transformation processes governing As­(III) remediation.

5.

SEM and EDS images of FeS₂ before (a,c) and after (b,d) reaction.

A key finding from the SEM–EDS analysis was a 10-fold increase in arsenic content (w/w %) in the spent material compared to fresh pyrite, confirming the effective removal and immobilization of As­(III) onto the solid phase. This accumulation strongly suggests that As­(III) adsorption onto FeS₂ surfaces is a dominant pathway facilitated by EGCG. However, beyond mere adsorption, notable shifts in oxygen, sulfur, and iron composition indicate more intricate chemical interactions at play. Given the established role of surface hydroxyl groups in arsenic complexation, it is plausible that As­(III) removal is driven by both surface complexation and oxidative conversion mechanisms.

Moreover, during the adsorption-oxidation process in the pyrite-activated PS system, Fe­(II) dissolution from FeS₂ played a crucial role in activating persulfate to generate reactive oxygen species (ROS). This Fe­(II)-mediated PS activation was further amplified by the presence of sulfur species within pyrite, which not only influenced the redox cycling between Fe­(III)/Fe­(II) but also contributed to the stabilization of reaction intermediates. , This interplay between FeS₂ surface chemistry, ROS formation, and EGCG-promoted electron transfer highlights a synergistic mechanism that accelerates As­(III) oxidation and adsorption.

Taken together, these findings illustrate that EGCG enhances As­(III) removal through a dual mechanism: (1) adsorption onto FeS₂ surfaces, facilitated by elemental rearrangement and surface complexation, and (2) oxidation via persulfate activation, driven by Fe­(II) cycling and sulfur interactions. This mechanistic understanding provides a strong foundation for optimizing FeS₂-based persulfate activation strategies in arsenic remediation in complex mine drainage environments where multiple reactive species interact dynamically.

To assess the contributions of sulfur and iron species in the EGCG-promoted FeS₂/Fe­(II)/PS system for As­(III) remediation, we conducted XPS analysis before and after reaction with As. Notably, the presence of colloidal material was observed on the pyrite surface and in suspension (Figure S3), implying its role in arsenic removal. To differentiate between the adsorption and transformation mechanisms, XPS characterization was performed separately on pyrite with the attached flocculated material (FeS₂ + colloid) and on pyrite alone.

As illustrated in Figure , the As 3d signal intensity in FeS₂ + colloid was significantly higher than that in FeS₂ alone, indicating that the majority of arsenic removal was mediated by the colloidal fraction rather than direct adsorption onto pyrite surfaces. Transmission electron microscopy (TEM) analysis further confirmed that the colloidal material exhibited Fe₂O₃ lattice fringes (Figure S4), reinforcing its role as a dominant sorbent in arsenic sequestration. This observation is critical, as it underscores a secondary phase transformation mechanism in which Fe­(II) oxidation leads to the formation of Fe₂O₃, enhancing arsenic immobilization.

6.

XPS spectra of S 2p (a), Fe 2p (b), and As 3d (c) of FeS₂ before and after reacting with As­(III).

To further probe the nature of arsenic binding on FeS₂ and Fe₂O₃ surfaces, we conducted density functional theory (DFT) calculations to evaluate adsorption energy (E _ads) and Gibbs free energy (ΔG) for different arsenic species. As shown in Figure , the E _ads values for As­(III) and As­(V) on FeS₂ in the bidentate binuclear configuration were −1.432 eV and −1.427 eV, respectively, indicating moderate adsorption affinity. However, on Fe₂O₃, the monodentate mononuclear complex exhibited a more favorable E _ads value of −1.894 eV, while the bidentate binuclear complex had an E _ads of −1.421 eV. This suggests that As­(III) adsorption onto Fe₂O₃ is energetically more favorable in a monodentate mononuclear configuration than that onto FeS₂.

7.

Schematic diagram of oxidation and adsorption capacity of FeS₂ and Fe₂O₃ on As­(III).

Furthermore, thermodynamic analysis revealed that the oxidation of FeS₂–As­(III) and Fe₂O₃–As­(III) to FeS₂–As­(V) and Fe₂O₃–As­(V) was spontaneous, with ΔG values of −1.126 eV and −2.256 eV, respectively. Notably, the larger negative ΔG for Fe₂O₃–As­(III) oxidation indicates that the transformation of As­(III) to As­(V) occurs more readily on Fe₂O₃ surfaces than on FeS₂. These findings align with the XPS results, confirming that As­(V) is predominantly adsorbed onto Fe₂O₃ rather than FeS₂.

With respect to the critical implications for As­(III) removal, the combination of XPS, TEM, and DFT analyses reveals a dual-phase arsenic removal mechanism in the EGCG-promoted FeS₂/Fe­(II)/PS system. Fe­(II) oxidation leads to Fe₂O₃ colloid formation, which serves as the primary adsorption site for arsenic sequestration. As­(III) adsorption is more thermodynamically favorable on Fe₂O₃ surfaces, primarily in a monodentate mononuclear configuration. The oxidation of As­(III) to As­(V) occurs more efficiently on Fe₂O₃ than on FeS₂, reinforcing Fe₂O₃’s role as the dominant arsenic-binding phase.

Overall, the findings challenge the conventional assumption that pyrite alone is the primary reactive site for arsenic removal. Instead, Fe₂O₃ formation via Fe­(II) oxidation plays a crucial role in facilitating arsenic sequestration, highlighting the necessity of considering secondary phase transformations in FeS₂-based remediation strategies.

Building on the observed experimental findings, a comprehensive removal mechanism for As­(III) in the EGCG-enhanced FeS₂/Fe­(II)/PS system is proposed (Figure ). The remediation process involves a synergistic interaction between Fe­(II), FeS₂, persulfate (PS), and EGCG, wherein both oxidation and adsorption mechanisms contribute to arsenic sequestration.

8.

Oxidation and adsorption mechanisms of As­(III) in EGCG-promoted FeS₂/Fe­(II)/PS system.

The coexistence and leaching of Fe­(II) from FeS₂ play a critical role in activating PS to generate ROS. Specifically, oxygen reduction by Fe­(II) leads to the formation of O₂ ^•–, which in turn accelerates PS activation to produce SO₄ ^–•. These radicals, along with ^•OH, serve as potent oxidizing agents, facilitating the rapid transformation of As­(III) to As­(V), which is less toxic and more readily adsorbed.

A key challenge in this system is the progressive oxidation of Fe­(II) to Fe­(III), which reduces the efficiency of PS activation, as Fe­(III) is significantly less effective in generating ROS. However, EGCG plays a crucial role in sustaining Fe­(II) levels by chelating Fe­(III) and reducing it back to Fe­(II). This Fe­(III)/Fe­(II) cycling mechanism enhances the overall efficiency of PS activation and ROS production, ensuring sustained oxidation of As­(III).

Beyond oxidation, arsenic removal is further facilitated by the formation of iron-based colloids, particularly Fe₂O₃, which arise from interactions between FeS₂, Fe­(II), and EGCG. These colloidal Fe₂O₃ particles act as highly effective adsorption sites, capturing As­(V) through surface complexation and precipitation processes. The coexistence of FeS₂ and Fe­(II) enhances the formation of Fe₂O₃, thereby increasing the system’s adsorption capacity for arsenic species.

The proposed mechanism highlights several key insights about the dual role of EGCG. EGCG not only stabilizes Fe­(II) concentrations through Fe­(III) reduction but also modifies Fe­(III) speciation, thereby preventing passivation and enhancing PS activation. This extends the efficiency and lifespan of Fe-based Fenton-like processes. While ROS-mediated oxidation effectively transforms As­(III) into As­(V), the subsequent adsorption onto Fe₂O₃ surfaces ensures stable immobilization, reducing the risk of arsenic leaching. The regeneration of Fe­(II) in situ mitigates the deactivation of PS activation, addressing a major limitation in conventional Fenton-like systems.

These findings emphasize that EGCG-promoted FeS₂/Fe­(II)/PS systems represent a highly efficient, dual-function arsenic remediation strategy by integrating redox transformations with colloidal adsorption mechanisms. Such an approach holds great potential for real-world applications, particularly in mine drainage and groundwater treatment, where Fe–S minerals and natural organic ligands are abundant.

3.3. Effect of PS, Fe­(II) and EGCG Dosage

The relationship between persulfate (PS) dosage and As­(III) removal efficiency in the EGCG-promoted FeS₂/Fe­(II)/PS system is depicted in Figure a. As expected, an increase in PS concentration generally promotes the generation of ROS, enhancing As­(III) oxidation and subsequent removal. However, a critical evaluation of the data reveals an important limitation: beyond a certain threshold, increasing PS dosage does not proportionally enhance arsenic removal.

9.

Effect of PS (a), Fe­(II) (b), and EGCG (c), coexisting anion (d) concentrations on As­(III) removal in EGCG-promoted FeS₂/Fe­(II)/PS. Reaction conditions: [EGCG]₀ = 0.025 mM, [FeS₂]₀ = 0.1 g/L, [Fe­(II)]₀ = 0.1 mM, [PS]₀ = 0.25 mM, [As­(III)]₀ = 1 mg/L.

While a higher PS concentration theoretically results in more SO₄ ^–• and ^•OH, facilitating a stronger oxidation environment, the observed removal efficiency only slightly decreased from 86.82% to 78.3% when PS dosage was increased from 0.1 to 1 mM. Accordingly, the first-order rate constants for the removal of As­(III) in EGCG-promoted FeS₂/Fe­(II)/PS varied from 1.46 min^–1 to 0.93 min^–1 (Figure S5a). This minimal variation suggests that the system reaches a radical saturation point, beyond which additional PS does not translate into improved As­(III) oxidation. Instead, excessive PS can engage in self-quenching reactions, where excessive SO₄ ^–• undergo recombination or react with nontarget species, reducing their effective participation in arsenic oxidation.

This phenomenon aligns with findings from other PS-driven remediation processes, such as the ultrasound-activated PS degradation of 1,1,1-trichloroethane and the Fe­(II)-activated PS breakdown of carbamazepine. , In both cases, an optimal PS dosage exists beyond which radical scavenging and competitive side reactions reduce degradation efficiency. The similarity across these systems reinforces the notion that careful optimization of PS concentration is crucial for maximizing contaminant removal while avoiding radical waste and secondary interference reactions.

This critical analysis highlights a key consideration for real-world applications. While higher PS concentrations theoretically improve oxidation, excessive dosages can lead to radical self-quenching and unintended side reactions, necessitating a balance between effective oxidation and reagent efficiency. The interaction between excessive PS and radical scavengers (dissolved organic matter, excess Fe­(II), or even reaction byproducts) can lead to inefficient PS utilization, requiring system-specific adjustments. Given the diminishing returns at higher PS dosages, an optimal concentration range must be identified to ensure cost-effective and environmentally sustainable arsenic remediation strategies.

The findings underscore the nonlinear response of As­(III) removal to increasing PS dosages in EGCG-promoted FeS₂/Fe­(II)/PS systems. While radical-driven oxidation is essential for arsenic transformation, excessive PS can lead to counterproductive radical interactions, ultimately reducing removal efficiency. Future studies should explore real-world wastewater matrices, competitive ion effects, and long-term system stability to refine dosage strategies for practical arsenic remediation applications.

The effectiveness of arsenic (As­(III)) removal in the EGCG-promoted FeS₂/Fe­(II)/PS system is significantly influenced by Fe­(II) concentration, EGCG dosage, and the presence of coexisting anions. A critical analysis of these factors provides deeper insights into the reaction mechanisms, optimal conditions, and potential limitations for practical applications.

Figure b illustrates the impact of Fe­(II) concentration on As­(III) removal, revealing a nonlinear relationship between Fe­(II) dosage and treatment efficiency. Increasing Fe­(II) from 0.05 mM to 0.1 mM led to a substantial enhancement in As­(III) removal, rising from 85.1% to 99.4%, and the first-order rate constants increased from 1.15 min^–1 to 1.53 min^–1 (Figure S5b). This increase can be attributed to the higher availability of Fe­(II) to activate PS, generating more ROS such as SO₄ ^–• and ^•OH, which accelerate As­(III) oxidation.

However, a further increase in Fe­(II) concentration beyond 0.1 mM resulted in a sharp decline in As­(III) removal efficiency from 99.4% to 67.1%, and in the first-order rate constants decreased from 1.53 min^–1 to 1.01 min^–1 (Figure S5b), indicating a threshold beyond which excess Fe­(II) leads to radical quenching reactions (eqs and ). , In such cases, Fe­(II) competes with As­(III) for ROS, reducing the effective oxidation of As­(III) and potentially forming Fe­(III) precipitates that scavenge radicals. This phenomenon is consistent with previous studies in advanced oxidation processes (AOPs), where excessive Fe­(II) disrupts radical-mediated degradation pathways.

Thus, the selection of 0.1 mM Fe­(II) as the optimal concentration is not only justified by its high removal efficiency but also by its cost-effectiveness and prevention of radical scavenging side effects, ensuring sustainable treatment performance.

$$\mathrm{F}\mathrm{e}(\mathrm{I}\mathrm{I})+\mathrm{S}{\mathrm{O}}_4^{·-}\to \mathrm{F}\mathrm{e}(\mathrm{I}\mathrm{I}\mathrm{I})+\mathrm{S}{\mathrm{O}}_4^{2-}$$ 1

$$\mathrm{F}\mathrm{e}(\mathrm{I}\mathrm{I})+{}^{·}\mathrm{O}\mathrm{H}\to \mathrm{F}\mathrm{e}(\mathrm{I}\mathrm{I}\mathrm{I})+\mathrm{O}{\mathrm{H}}^{-}$$ 2

The role of EGCG as a Fe­(III)/Fe­(II) redox cycling promoter is crucial in this system, yet its concentration must be carefully optimized. As depicted in Figure c, increasing EGCG dosage from 0.01 mM to 0.025 mM enhanced As­(III) removal from 57.0% to 86.4%, and the first-order rate constants from 0.81 min^–1 to 1.51 min^–1 (Figure S5c), highlighting its ability to stabilize Fe­(II), prevent Fe­(III) passivation, and facilitate continuous PS activation.

However, a further increase in EGCG to 0.1 mM led to a drop in As­(III) removal to 71.4%, and the first-order rate constants from 1.51 min^–1 to 1.01 min^–1 (Figure S5c), suggesting that EGCG’s dual role as both a Fe­(III) chelator and a radical scavenger introduces trade-offs in performance. While EGCG enhances Fe­(II) regeneration, it also acts as a ROS scavenger by reacting with SO₄ ^–• and ^•OH, thereby limiting their ability to oxidize As­(III). This behavior is consistent with EGCG’s well-documented antioxidant properties in biological systems, where it serves as a protective agent against oxidative stress but, in this case, counteracts the desired oxidation processes.

From a practical perspective, selecting 0.025 mM EGCG as the optimal dosage ensures a balance between its Fe­(III) reduction capacity and its ROS-scavenging tendency, maximizing As­(III) oxidation while maintaining reagent efficiency. The presence of coexisting anions in mine drainage introduces an additional layer of complexity in As­(III) remediation, as these species can interfere with radical-driven oxidation and adsorption processes. Previous studies have established that anions such as Cl^–, HCO₃ ^–, SO₄ ^2–, and H₂PO₄ ^– can participate in radical quenching reactions, thereby altering system performance. ,

Figure d highlights that while Cl^– and SO₄ ^2– had negligible effects on As­(III) removal efficiency, the presence of HCO₃ ^– and H₂PO₄ ^– led to a significant decline, with removal efficiency dropping from 93.1% to 19.6% and 93.1% to 19.2%, respectively. This drastic reduction can be explained by two primary mechanisms radical scavenging effects and formation of Fe-precipitates. HCO₃ ^– and H₂PO₄ ^– act as scavengers for SO₄ ^–• and ^•OH radicals, converting them into less reactive species such as CO₃ ^–• (E ₀ = 1.65 V) and HPO₄ ^–•, which have significantly lower oxidation potentials than SO₄ ^–• (E ₀ = 2.5–3.1 V) and ^•OH (E ₀ = 2.8 V). This transformation reduces the oxidative capacity of the system, leading to lower As­(III) oxidation rates.

HCO₃ ^– and H₂PO₄ ^– can complex with Fe­(II) and Fe­(III), leading to the formation of insoluble Fe-carbonate and Fe-phosphate precipitates. This reduces the availability of Fe­(II) in solution, thereby limiting its role in PS activation and radical generation, further diminishing As­(III) removal efficiency.

These findings emphasize that the EGCG-promoted FeS₂/Fe­(II)/PS system is highly sensitive to water chemistry, particularly the presence of inorganic anions that influence radical reactions and Fe­(II) solubility. Several key implications arise for real-world applications. When fine-tuning Fe­(II) and EGCG dosages, the optimal Fe­(II) and EGCG concentrations (0.1 mM and 0.025 mM, respectively) must be carefully maintained to prevent radical scavenging and Fe­(III) precipitation, ensuring cost-effective arsenic removal.

If bicarbonate or phosphate concentrations are high, pretreatment strategies such as pH adjustment or selective ion removal should be considered to minimize radical quenching and Fe­(II) complexation effects. For site-specific remediation strategies, the significant variation in As­(III) removal efficiency due to coexisting anions suggests that field-scale applications should incorporate comprehensive water chemistry analysis to design tailored treatment protocols.

A critical assessment of the EGCG-promoted FeS₂/Fe­(II)/PS system underscores the importance of optimizing key parameters to balance oxidation, radical generation, and adsorption efficiency. While Fe­(II) and EGCG serve as crucial activators for PS, their excessive concentrations can lead to radical scavenging effects, reducing system performance. Similarly, coexisting anions play a decisive role in influencing As­(III) remediation efficiency, necessitating targeted strategies to mitigate their adverse effects. Future research should focus on developing adaptive treatment methods that account for diverse mine water compositions and evaluating the long-term stability of FeS₂-mediated arsenic removal under variable environmental conditions.

$$\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}+\mathrm{S}{\mathrm{O}}_4^{·-}\to \mathrm{C}{\mathrm{O}}_3^{·-}+{\mathrm{H}}^{+}+\mathrm{S}{\mathrm{O}}_4^{2-}$$ 3

$$\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}+{}^{·}\mathrm{O}\mathrm{H}\to \mathrm{C}{\mathrm{O}}_3^{·-}+{\mathrm{H}}_2\mathrm{O}$$ 4

$${\mathrm{H}}_2\mathrm{P}{\mathrm{O}}_4^{-}+\mathrm{S}{\mathrm{O}}_4^{·-}\to \mathrm{H}\mathrm{P}{\mathrm{O}}_4^{·-}+\mathrm{H}\mathrm{S}{\mathrm{O}}_4^{-}$$ 5

$${\mathrm{H}}_2\mathrm{P}{\mathrm{O}}_4^{-}+{}^{·}\mathrm{O}\mathrm{H}\to \mathrm{H}\mathrm{P}{\mathrm{O}}_4^{·-}+{\mathrm{H}}_2\mathrm{O}$$ 6

3.4. Effect of Initial pH

Figure illustrates the influence of initial pH on As­(III) removal in the EGCG-enhanced FeS₂/Fe­(II)/PS system. As the pH increased from 3 to 7, the removal efficiency of As­(III) significantly improved, rising from 43.71% to 100%. However, when the pH was further elevated to 9, the removal efficiency declined to 63.6%. This pH-dependent trend aligns with observations from studies on ibuprofen (IBP) degradation via piezo-catalytic activation of PS, where the reaction rate initially increased and then decreased within a pH range of 3–10.5, peaking at pH 7. The observed behavior is attributed to variations in the concentrations of SO₄ ^–• and ^•OH radicals under different pH conditions. Under alkaline conditions (pH > 7), these radicals are quenched by OH^– ions, leading to a reduction in As­(III) removal efficiency.

10.

Effect of initial pH on As­(III) removal in EGCG-promoted FeS₂/Fe­(II)/PS system. Reaction conditions: [EGCG]₀ = 0.025 mM, [FeS₂]₀ = 0.1 g/L, [Fe­(II)]₀ = 0.1 mM, [PS]₀ = 0.25 mM, [As­(III)]₀ = 1 mg/L.

In terms of iron species behavior, Fe­(III) typically precipitates as ferric oxyhydroxide at pH values above 3. , However, the introduction of EGCG modifies this process. EGCG acts as a chelating agent for Fe­(III), promoting the Fe­(II)/Fe­(III) redox cycle and extending the effective pH range for As­(III) removal up to pH 7. At pH values beyond 7, EGCG predominantly exists in its anionic form, rendering it more vulnerable to oxidation by dissolved oxygen. This oxidation disrupts the Fe­(II) regeneration pathway, further compromising the system’s efficiency. Additionally, the generation of O₂ ^•– radicals is significantly suppressed under alkaline conditions, contributing to the observed decline in As­(III) removal at higher pH levels. These findings underscore the critical role of pH and EGCG in modulating radical generation and iron species dynamics, ultimately influencing the efficacy of As­(III) remediation in the FeS₂/Fe­(II)/PS system.

3.5. Limitations of This Work and Mitigation Strategies

The study presents a novel approach to arsenic remediation, but it has several limitations that could hinder its practical application. The system exhibits optimal As­(III) removal efficiency only within a narrow pH range (3–7). At alkaline conditions (pH > 7), the removal efficiency drops significantly due to the quenching of SO₄ ^–• and ^•OH radicals by OH^– ions and the oxidation of EGCG, which disrupts the Fe­(II)/Fe­(III) redox cycle. Therefore, it is crucial to incorporate pH-buffering agents or modify the system to stabilize radical generation across a broader pH range. Alternatively, preadjust the pH of the mine drainage to fall within the optimal range before treatment.

EGCG, a key component for enhancing the Fe­(II)/Fe­(III) redox cycle, is susceptible to oxidation under alkaline conditions, limiting its effectiveness at higher pH values. It is necessary to explore alternative chelating agents or antioxidants that are more stable under alkaline conditions. Alternatively, encapsulate EGCG in protective matrices to enhance its stability.

Additionally, Fe­(III) tends to precipitate as ferric oxyhydroxide at pH > 3, reducing the availability of active iron species for the redox cycle. Therefore, it is crucial to use stabilizing agents or nanoparticles to prevent iron precipitation and maintain the availability of Fe­(II)/Fe­(III) species across a wider pH range.

The generation of O₂ ^•– is inhibited under alkaline conditions, reducing the system’s overall oxidative capacity. To mitigate this limitation, it is vital to introduce additional radical-generating pathways or catalysts that are effective under alkaline conditions, such as transition metal oxides or carbon-based materials.

The use of EGCG, a natural polyphenol, may be cost-prohibitive for large-scale applications. Additionally, the system’s reliance on PS activation could increase operational costs. To address this limitation, it is necessary to optimize the dosage of EGCG and PS to minimize costs. Investigate the use of cheaper, synthetic analogs of EGCG or alternative green catalysts for PS activation.

The use of PS and iron species could lead to secondary pollution, such as residual sulfate ions or iron sludge, which may require additional treatment steps. It is vital to implement post-treatment processes, such as sulfate ion removal or iron sludge recovery, to mitigate secondary pollution. Alternatively, design a closed-loop system to recycle iron species and minimize waste.

The study may lack validation under real-world mine drainage conditions, which often contain complex matrices of competing ions and organic matter that could interfere with As­(III) removal. It is crucial to conduct pilot-scale studies using actual mine drainage to evaluate the system’s performance under realistic conditions. Optimize the system to account for interference from coexisting contaminants.

The reaction kinetics and thermodynamics of the system may not be fully optimized, potentially limiting its efficiency under varying environmental conditions. To tackle this limitation, it is necessary to perform detailed kinetic and thermodynamic studies to identify rate-limiting steps and optimize reaction conditions (e.g., temperature, reactant concentrations).

While the EGCG-promoted FeS₂/Fe­(II)/PS system offers a promising approach for As­(III) remediation, addressing these limitations through targeted mitigation strategies will be crucial for its practical application. Future research should focus on improving pH stability, reducing costs, and validating the system under real-world conditions to enhance its scalability and effectiveness.

3.6. The Way Forward

The study presents a promising approach for arsenic remediation, but there are several avenues for further research and development to enhance its practicality, efficiency, and scalability. For optimizing reaction conditions, the pH range needs to be expanded beyond 3–7, such as incorporating pH-buffering agents or modifying the system to stabilize radical generation under alkaline conditions, while conducting systematic studies to optimize temperature, EGCG concentration, PS dosage, and FeS₂/Fe­(II) ratios for maximum As­(III) removal efficiency. For kinetic and thermodynamic studies, further work needs to perform detailed kinetic and thermodynamic analyses to understand the rate-limiting steps and improve the overall reaction efficiency.

For the development of stable chelating agents, further work needs to explore alternative chelating agents or antioxidants that are more stable under alkaline conditions and can effectively promote the Fe­(II)/Fe­(III) redox cycle. Meanwhile, for EGCG stabilization, it is crucial to develop methods to stabilize EGCG, such as encapsulation in protective matrices or chemical modification, to prevent its oxidation at higher pH levels.

To enhance radical generation through catalyst modification, further work needs to incorporate additional catalysts (e.g., transition metal oxides, carbon-based materials, or bimetallic systems) to enhance PS activation and radical generation, especially under alkaline conditions. For hybrid systems, it is necessary to combine the FeS₂/Fe­(II)/PS system with other advanced oxidation processes (e.g., photocatalysis or electrocatalysis) to boost radical production and improve As­(III) oxidation efficiency.

To mitigate secondary pollution due to iron sludge, further work needs to develop strategies to recover and recycle iron species from the system to minimize sludge generation and reduce secondary pollution. Meanwhile, for sulfate ion removal, it is necessary to implement post-treatment processes to remove residual sulfate ions, such as precipitation or adsorption techniques, to ensure compliance with environmental regulations.

It is also necessary to integrate this system with other remediation technologies. For hybrid remediation systems, further work needs to integrate the EGCG-promoted FeS₂/Fe­(II)/PS system with other remediation technologies, such as membrane filtration, phytoremediation, or bioremediation, to achieve comprehensive arsenic removal. Meanwhile, for sequential treatment, further test needs to design sequential treatment processes where the system is used as a pretreatment step to oxidize As­(III) to As­(V), followed by adsorption or coagulation–flocculation for final removal.

4. Conclusions

This study demonstrates that epigallocatechin gallate (EGCG) enhances As­(III) remediation in FeS₂/Fe­(II)-rich simulated mine drainage via persulfate (PS) activation: adding 25 μM EGCG significantly improved As­(III) oxidation and adsorption (93.5% removal within 60 min under optimized conditions, 2.4-fold higher oxidation efficiency than the polyphenol-free system), ^•OH and O₂ ^•– were dominant radicals for As­(III) transformation, Cl^– and HCO₃ ^– inhibited removal while SO₄ ^2– and H₂PO₄ ^– had negligible effects, and EPR and DFT calculations revealed EGCG promotes electron transfer and radical generation via Fe­(III)-EGCG complexes. Environmentally, EGCG’s natural, nontoxic, biodegradable properties and the system’s mild conditions support its practical potential in mine waters, though limitations include EGCG’s light/high-temperature sensitivity and the need to adjust dosages for different water matrices. These findings highlight natural polyphenol-driven PS activation as a sustainable arsenic remediation approach, with future research focusing on improving EGCG stability and optimizing parameters for real mine drainage application.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c03017.

• Concentration of remaining PS in EGCG-induced FeS₂/Fe­(II)/PS systems, XRD patterns of FeS₂ before and after reaction, photo taken after the reaction, TEM images of the formed colloid substance and the spent FeS₂, k _obs of different parameters in EGCG-promoted FeS₂/Fe­(II)/PS, basic characteristics of typical natural polyphenols and corresponding As­(III) efficiency (PDF)

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Y.W. and H.L. contributed equally to this work and share the first coauthorship.

The authors declare no competing financial interest.