Sequential Fenton reaction and biodegradation for treating decabromodiphenyl ether in soil slurries

Technical Summary: Sequential Fenton Reaction and Biodegradation for Treating Decabromodiphenyl Ether in Soil Slurries

Problem Statement
Decabromodiphenyl ether (BDE-209), a widely used brominated flame retardant, is a persistent organic pollutant (POP) that accumulates in soil due to its hydrophobicity. It poses significant ecological and human health risks, including reproductive toxicity and accumulation in the food chain. While chemical oxidation methods like the Fenton reaction can rapidly degrade such pollutants, they often suffer from limitations in soil environments, including soil acidification, destruction of native microbial communities, and reduced efficiency due to soil buffering capacity and organic matter scavenging. Conversely, bioremediation is environmentally compatible but often requires extended timeframes and struggles with highly recalcitrant compounds like BDE-209. This study addresses the need for an integrated strategy that leverages the rapid degradation capabilities of chemical oxidation to create biodegradable intermediates, which are then further mineralized by microorganisms, thereby overcoming the individual limitations of both approaches.

Methodology
The study utilized soil collected from Taichung, Taiwan (TCS), spiked with 20 mg kg⁻¹ of BDE-209. The research followed a sequential approach involving three main phases:

1. Optimization of Fenton Reaction: The authors optimized three parameters to maximize BDE-209 removal: ferrous ion (Fe²⁺) concentration (50, 500, 5,000 mg L⁻¹), hydrogen peroxide (H₂O₂) concentration (ranging from 0 to 55,420 mg L⁻¹), and reaction time (15 to 120 min). A specific focus was placed on the efficacy of multiple H₂O₂ dosing versus single-dose application.
2. Sequential Treatment (FB) vs. Biodegradation Only (D11):
   • FB Group: Soil underwent the optimized Fenton pretreatment (500 mg L⁻¹ Fe²⁺ and 44,336 mg L⁻¹ H₂O₂, followed by two additional doses of 22,168 mg L⁻¹ H₂O₂ over 30-minute intervals). The residual H₂O₂ was removed via washing before the addition of a mixed bacterial culture (Da-an bacterial community) for a 133-day biodegradation period.
   • D11 Group: Soil was subjected to biodegradation only, starting with a BDE-209 concentration equivalent to the post-Fenton level of the FB group (approx. 11 mg kg⁻¹) to serve as a control for the biodegradation phase.
   • FC Group: An abiotic control was maintained to monitor non-biological changes.
3. Analysis: BDE-209 concentrations were measured via GC/PDECD. Anionic byproducts (bromide, nitrite, nitrate) were analyzed using ion chromatography. Bacterial community dynamics were assessed via 16S rDNA cloning and sequencing. Additionally, ten bacterial strains isolated from the FB group were screened using Biolog MT2 microplates to determine their ability to utilize BDE-209 and various metabolites as sole carbon sources.

Key Results

• Fenton Optimization: The optimal Fenton conditions were identified as 500 mg L⁻¹ Fe²⁺ and 44,336 mg L⁻¹ H₂O₂ applied for 30 minutes with three sequential H₂O₂ doses. Under these conditions, the Fenton reaction alone removed 37.22% of the initial 20 mg kg⁻¹ BDE-209. The study noted that excessive H₂O₂ concentrations (>47,000 mg L⁻¹) or high Fe²⁺ concentrations (>800 mg L⁻¹) led to decreased removal efficiency due to the scavenging of hydroxyl radicals (•OH) by excess H₂O₂.
• Sequential Removal Efficiency: The sequential FB process achieved a total removal of 75.05% (37.22% via Fenton + 60.29% of the remaining contaminant via biodegradation). In contrast, the biodegradation-only group (D11) achieved only 38.35% removal of the same starting concentration.
• Kinetics: The pseudo-first-order kinetic rate constant for the sequential FB process was 0.0069 day⁻¹, significantly higher than the 0.0029 day⁻¹ observed for biodegradation alone.
• Byproducts: Bromide levels increased significantly in the FB group, indicating debromination. Nitrate levels increased only during the biodegradation phase, suggesting nitrification activity.
• Microbial Community Shifts:
  • Initial Community: The starting chemostat was dominated by Acidobacteria sp. (20%), Chlorobium sp. (15%), and Gemmatimonas sp. (15%).
  • Post-Fenton (Day 0): Immediately after Fenton treatment, the community shifted to be dominated by Pseudomonas sp. (50%) and Geobacter sp. (37.5%).
  • Long-term (Day 133): In the FB group, Acidobacteria sp. re-emerged as a dominant genus (21.43%–37.50%), alongside Verrucomicrobia sp. and Chlorobi sp. The D11 group showed a similar trend but with lower relative abundances of Acidobacteria sp. (16.67%–22.73%).
• Isolated Strains: Four strains were identified as capable of utilizing PBDEs and metabolites as sole carbon sources: Brachybacterium sp., Aminobacter sp., Pseudomonas sp., and Brevibacterium sp. Notably, Pseudomonas sp. (strain B2) demonstrated the ability to degrade a wide range of congeners, including BDE-209, BDE-47, and BDE-100.

Significance and Claims
The authors claim that this study provides a scientific basis for developing in situ remediation strategies for BDE-209-contaminated soils. The primary significance lies in demonstrating that the sequential Fenton-biodegradation (FB) process effectively overcomes the limitations of using either method alone. The Fenton pretreatment successfully reduces the recalcitrant BDE-209 concentration and transforms it into more biodegradable intermediates, which subsequently enhances the efficiency and rate of microbial degradation.

The study highlights that while the Fenton reaction alters the soil environment and initial microbial composition, the system recovers and supports a diverse microbial community capable of further degradation. The identification of specific strains (Pseudomonas, Brachybacterium, Aminobacter, and Brevibacterium) that can utilize PBDEs and their metabolites as carbon sources offers potential candidates for bioaugmentation in future field-scale applications. The authors conclude that this integrated approach is a viable strategy for treating persistent organic pollutants in soil/water systems, though they note that further studies using ex situ soils and a broader range of POPs are recommended to broaden the application of this method.