Poly(γ‑Glutamic Acid) Enhances Fe‑Pd Nanoparticle‑Catalyzed Dechlorination of p‑Chlorophenol Under Alkaline Conditions

Abstract

Nanoscale zero‑valent iron (nZVI) is a leading nanomaterial for detoxifying chlorinated organics, yet its rapid aggregation and surface passivation limit field deployment. We report palladium‑doped Fe‑nZVI nanoparticles stabilized with poly(γ‑glutamic acid) (Fe‑Pd@PGA NPs). The particles are ~100 nm, uniformly dispersed, and remain colloidally stable for >3 h in weakly alkaline solution (pH ≈ 9). Using p‑chlorophenol (p‑CP) as a model, Fe‑Pd@PGA NPs achieve a maximum pseudo‑first‑order rate constant of 0.331 min⁻¹ at an Fe/p‑CP molar ratio of 100. This rate exceeds that of bare Fe‑Pd NPs by more than tenfold, underscoring PGA’s role as a stabilizer and activity promoter. The system tolerates common groundwater anions (Cl⁻, H₂PO₄⁻, humic acid) with negligible activity loss, while HPO₄²⁻ and HCO₃⁻ slightly inhibit dechlorination. These findings position Fe‑Pd@PGA NPs as a robust, scalable platform for in‑situ remediation of chlorinated pollutants.

Background

Water contamination by chlorinated organics remains a global challenge. nZVI offers a powerful reduction pathway, converting toxic species into benign products, but suffers from aggregation and surface oxidation, especially under neutral to alkaline pH. Bimetallic Fe‑Pd nanoparticles have improved degradation kinetics, yet their practical application is hampered by poor colloidal stability. Polymer surface stabilizers—such as polysaccharides, polyelectrolytes, and surfactants—have been explored to provide electrostatic and steric repulsion. Poly(γ‑glutamic acid) (PGA), a biodegradable anionic polypeptide rich in carboxyl groups, offers strong metal‑binding and steric hindrance, making it an attractive candidate for nanoparticle stabilization.

Methods

Materials

FeSO₄·7H₂O, K₂PdCl₄, KBH₄, p‑CP, and PGA (MW ≈ 100 kDa) were purchased from commercial suppliers. All reagents were used as received; deionized water (18.2 MΩ·cm) was used throughout.

Preparation of Fe‑Pd@PGA NPs

FeSO₄·7H₂O (250 mg) was dissolved in 50 mL PGA solution (10–70 mg) to form a Fe²⁺–PGA complex under argon. KBH₄ (50 mg mL⁻¹, 3 : 1 BH₄⁻ : Fe²⁺) reduced Fe²⁺ to Fe⁰, generating Fe@PGA NPs. Palladium loading (0.1–0.8 wt % Pd/Fe) was introduced via K₂PdCl₄ addition, followed by continued argon purging until H₂ evolution ceased. The resulting Fe‑Pd@PGA NPs were cooled to 25 °C and used immediately.

Stability Test

Nanoparticle suspensions were placed in 50 mL glass tubes, shaken gently, and allowed to settle. Photographs were taken at regular intervals to monitor sedimentation.

Batch Dechlorination Experiments

A 550 mg L⁻¹ p‑CP stock solution was diluted to 20 mg L⁻¹ (Fe/p‑CP = 100). 2 mL of this solution was added to 98 mL Fe‑Pd@PGA suspension, and the reaction proceeded under continuous argon purge at 25 °C. Samples were withdrawn, filtered (0.22 µm), and analyzed by HPLC. Experiments were conducted in triplicate.

Simulated Groundwater Tests

Groundwater simulants contained 1 mM of Cl⁻, H₂PO₄⁻, HPO₄²⁻, or HCO₃⁻, and 5 mg L⁻¹ humic acid. p‑CP concentration and pH were monitored over time.

Characterization

SEM, TEM, XRD, and FTIR characterized morphology, crystal structure, and surface chemistry. Stability was assessed by sedimentation analysis.

Results & Discussion

Synthesis and Characterization

Fe‑Pd@PGA NPs exhibit spherical morphology (50–200 nm) with minimal aggregation, as confirmed by TEM (Fig. 1). SEM images show smooth surfaces versus oxidized bare Fe‑Pd NPs (Fig. 2). XRD patterns confirm the presence of metallic Fe⁰ (α‑Fe) and minor Fe oxides, indicating successful synthesis. FTIR spectra reveal characteristic carboxyl peaks (3440 cm⁻¹, 1631 cm⁻¹) and shifts upon nanoparticle binding, evidencing Fe‑PGA interaction.

Colloidal Stability

Fe‑Pd@PGA suspensions remain clear for >180 min, whereas bare Fe‑Pd NPs settle within 10 min (Fig. 3). This stability is attributed to PGA’s electrostatic repulsion and steric hindrance.

Dechlorination Performance

Fe‑Pd@PGA NPs outperform bare Fe‑Pd NPs by >10×. With 25 mg PGA and 0.8 wt % Pd, >90 % p‑CP removal occurs within 30 min; 50 mg PGA achieves complete removal in the same time (Fig. 4). The optimal PGA loading is ~50 mg; higher amounts induce aggregation and reduce activity (Fig. 5). The pseudo‑first‑order rate constant k reaches 0.331 min⁻¹ at pH 9.0, exceeding literature values by one to two orders of magnitude (Table 1).

Effect of Anions and Humic Acid

Cl⁻, H₂PO₄⁻, and humic acid have negligible impact on k, whereas HPO₄²⁻ and HCO₃⁻ reduce k to ~0.06–0.07 min⁻¹ (Fig. 6). pH remains largely unchanged during reactions, indicating that inhibition is not pH‑driven but likely due to competitive adsorption or surface blocking.

Proposed Mechanism

Fe‑Pd@PGA NPs form a positively charged Fe‑oxide shell that binds PGA’s carboxylate groups, preventing aggregation and providing a high surface area scaffold. PGA chelates Fe²⁺/Fe³⁺ released during Fe corrosion, suppressing hydroxide precipitation and preserving active sites. Palladium facilitates hydrogen spillover, enhancing the reduction of p‑chlorophenol to phenol. The combined electrostatic‑steric stabilization and metal‑binding capacity of PGA underpin the observed activity boost.

Conclusions

Poly(γ‑glutamic acid) effectively stabilizes Fe‑Pd nanoparticles, yielding colloidally stable, highly active dechlorination catalysts. The Fe‑Pd@PGA system achieves a rate constant of 0.331 min⁻¹ under weakly alkaline conditions, outperforming previous reports while tolerating typical groundwater constituents. These findings demonstrate the promise of PGA‑coated Fe‑Pd NPs for in‑situ remediation of chlorinated organic pollutants.

Abbreviations

• Fe‑Pd@PGA NPs: Palladium‑doped zero‑valent iron nanoparticles involving poly(γ‑glutamic acid)
• nZVI: Nanoscale zero‑valent iron
• Fe‑Pd: Bimetallic iron–palladium nanoparticles
• PGA: Poly(γ‑glutamic acid)
• p‑CP: p‑Chlorophenol
• HPLC: High‑performance liquid chromatography
• FTIR: Fourier transform infrared spectroscopy
• SEM: Scanning electron microscopy
• TEM: Transmission electron microscopy
• XRD: X‑ray diffraction