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6CCVD Research

On the Role of the Cathode for the Electro-Oxidation of Perfluorooctanoic Acid

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2020-08-08
JournalCatalysts
AuthorsAlicia L. García-Costa, André Savall, Juan A. Zazo, José A. Casas, Karine Groenen Serrano
InstitutionsUniversidad AutĂłnoma de Madrid, Centre National de la Recherche Scientifique
Citations25
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”

This study investigates the critical role of the cathode material in the electro-oxidation and defluorination of Perfluorooctanoic Acid (PFOA) using a Boron-Doped Diamond (BDD) anode under low electrolyte concentration.

  • Cathode Electrocatalysis: Platinum (Pt) was identified as the superior cathode material, significantly enhancing PFOA defluorination compared to BDD, Zirconium (Zr), and Stainless Steel (Steel).
  • Mechanism Confirmation: Pt acts as an electrocatalyst, promoting hydrodefluorination via the reduction reaction of perfluorinated carbonyl intermediates using in situ generated adsorbed atomic hydrogen (Hads).
  • High Efficiency: The BDD-Pt system achieved a high defluorination degree (XF-: 58.6%) and substantial mineralization (XTOC: 76.1%) after 6 hours of electrolysis.
  • Kinetics Improvement: PFOA degradation kinetics were 39% faster with the Pt cathode (11.86 x 10-3 min-1) compared to the other tested materials.
  • Energy Competitiveness: The BDD-Pt system demonstrated higher defluorination efficiency per unit electrical charge than previously reported literature methods, which typically rely on high supporting electrolyte concentrations.
  • Operating Conditions: Effective degradation was achieved at mild conditions: 25 °C, natural pH (pH 4), and low electrolyte concentration (3.5 mM Na2SO4).

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Anode MaterialBoron-Doped Diamond (BDD)N/ACVD on Si substrate
Best Cathode MaterialPlatinum (Pt)N/A5 ”m layer on Titanium substrate
PFOA Initial Concentration100 (0.242)mg/L (mol/m3)Target pollutant concentration
Electrolyte Concentration3.5mMNa2SO4 (standard condition)
Operating Temperature25 ± 1°CMild temperature operation
Applied Current Density (j)7.9mA/cm2Galvanostatic operation (0.5 A total current)
Anode/Cathode Active Area (A)63 x 10-4m2Electrode surface area
Electrolyte Volume (V)1 x 10-3m3Total reactor volume
PFOA Removal (XPFOA)98.1%BDD-Pt system, 6 hours
TOC Removal (XTOC)76.1%BDD-Pt system, 6 hours
Defluorination (XF-)58.6%BDD-Pt system, 6 hours
PFOA Degradation Rate (kPFOA)11.86 x 10-3min-1Pt cathode (39% faster than others)
Limiting Current Density (jlim)0.63A/m2Theoretical mass transport control (n=1)
C-F Bond Dissociation Energy~460kJ/molHigh stability of PFOA molecule

Key Methodologies

Section titled “Key Methodologies”

The electrochemical oxidation system utilized a flow filter-press reactor operated under galvanostatic control with solution recycling.

  1. Electrochemical Setup: A 1-L thermoregulated glass reservoir was connected to a one-compartment flow filter-press reactor. The solution was recycled at a flow rate of 360 L/h.
  2. Electrode Materials: BDD was used exclusively as the anode. Cathodes tested included BDD, Zirconium (Zr), Stainless Steel (Steel), and Platinum (Pt, 5 ”m on Ti substrate).
  3. Electrode Pretreatment: All working electrodes were cleaned by anodic pretreatment (40 mA/cm2 for 30 min in 0.1 M H2SO4) prior to electrolysis.
  4. Reaction Conditions: Experiments were conducted at 25 °C, using 100 mg/L PFOA and 3.5 mM Na2SO4 as the supporting electrolyte, typically starting at the natural pH of 4.
  5. Galvanostatic Control: Current intensity was fixed at 0.5 A, corresponding to a current density of 7.9 mA/cm2 (79 A/m2).
  6. Analytical Techniques:
    • PFOA concentration was monitored using High Performance Liquid Chromatography (HPLC-UV).
    • Total Organic Carbon (TOC) was measured using a TOC analyzer to track mineralization.
    • Fluoride ion (F-) release (defluorination) was quantified using Ion Chromatography (IC).

Commercial Applications

Section titled “Commercial Applications”

The findings directly support the development of highly efficient and cost-effective electrochemical systems for environmental remediation, particularly targeting persistent pollutants.

  • PFAS Contaminant Destruction: Implementation of BDD/Pt electrochemical reactors for the complete destruction (mineralization and defluorination) of PFOA and related perfluoroalkyl substances (PFAS) in industrial effluent.
  • Wastewater Treatment (AOPs): Integration of the BDD-Pt system into Advanced Oxidation Processes (AOPs) for treating complex industrial wastewater, leveraging the high efficiency of BDD anodes and the catalytic reduction capabilities of the Pt cathode.
  • Groundwater Remediation: Deployment of electrochemical cells for treating contaminated groundwater, especially where low salt concentrations are required for subsequent discharge or reuse.
  • Electrocatalyst Design: Utilization of Platinum as a benchmark electrocatalyst for hydrodehalogenation reactions in aqueous phase, informing the design of next-generation cathode materials for reductive pollutant degradation.
  • Preconcentration Integration: The high efficiency at low concentrations suggests potential commercial viability when coupled with preconcentration steps (e.g., adsorption or filtration) to treat highly dilute streams economically.
View Original Abstract

Perfluorooctanoic acid (PFOA), C7F15COOH, has been widely employed over the past fifty years, causing an environmental problem because of its dispersion and low biodegradability. Furthermore, the high stability of this molecule, conferred by the high strength of the C-F bond makes it very difficult to remove. In this work, electrochemical techniques are applied for PFOA degradation in order to study the influence of the cathode on defluorination. For this purpose, boron-doped diamond (BDD), Pt, Zr, and stainless steel have been tested as cathodes working with BDD anode at low electrolyte concentration (3.5 mM) to degrade PFOA at 100 mg/L. Among these cathodic materials, Pt improves the defluorination reaction. The electro-degradation of a PFOA molecule starts by a direct exchange of one electron at the anode and then follows a complex mechanism involving reaction with hydroxyl radicals and adsorbed hydrogen on the cathode. It is assumed that Pt acts as an electrocatalyst, enhancing PFOA defluorination by the reduction reaction of perfluorinated carbonyl intermediates on the cathode. The defluorinated intermediates are then more easily oxidized by HO‱ radicals. Hence, high mineralization (xTOC: 76.1%) and defluorination degrees (xF−: 58.6%) were reached with Pt working at current density j = 7.9 mA/cm2. This BDD-Pt system reaches a higher efficiency in terms of defluorination for a given electrical charge than previous works reported in literature. Influence of the electrolyte composition and initial pH are also explored.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2020 - Environmental Contamination and Human Exposure to PFASs Near a Fluorochemical Production Plant: Review of Historic and Current PFOA and GenX Contamination in the Netherlands [Crossref]
  2. 2019 - Characteristic and Human Exposure Risk Assessment of Per- and Polyfluoroalkyl Substances: A study Based on Indoor Dust and Drinking Water in China [Crossref]
  3. 2004 - Decomposition of Environmentally Persistent Perfluorooctanoic Acid in Water by Photochemical Approaches [Crossref]
  4. 2001 - Accumulation of Perfluorooctane Sulfonate in Marine Mammals [Crossref]
  5. 2019 - Occurrence and Distribution of Perfluorooctane Sulfonate and Perfluorooctanoic Acid in Three Major Rivers of Xinjiang, China [Crossref]
  6. 2012 - Occurrence of Perfluorinated Compounds in Water and Sediment of L’Albufera Natural Park (Valencia, Spain) [Crossref]
  7. 2019 - Human Exposure to Per-And Polyfluoroalkyl Substances (PFAS) through Drinking Water: A Review of the Recent Scientific Literature [Crossref]
  8. 2020 - Understanding the Effect of Carbon Surface Chemistry on Adsorption of Perfluorinated Alkyl Substances [Crossref]
  9. 2011 - Adsorption of Perfluorooctanesulfonate (PFOS) and Perfluorooctanoate (PFOA) on Alumina: Influence of Solution pH and Cations [Crossref]
  10. 2018 - Competitive Adsorption of Perfluoroalkyl Substances on Anion Exchange Resins in Simulated AFFF-Impacted Groundwater [Crossref]

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