Laboratory and Semi-Pilot Scale Study on the Electrochemical Treatment of Perfluoroalkyl Acids from Ion Exchange Still Bottoms
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-10-14 |
| Journal | Water |
| Authors | Vanessa Y. Maldonado, Michael Becker, Michael G. Nickelsen, Suzanne E. Witt |
| Institutions | Michigan State University, Fraunhofer USA |
| Citations | 21 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis study validates the electrochemical oxidation (EO) of highly concentrated perfluoroalkyl acids (PFAAs) found in Ion Exchange (IX) still bottoms using Boron-Doped Diamond (BDD) electrodes, assessing performance from laboratory to semi-pilot scale.
- High Destruction Efficiency: Achieved >99% total PFAAs removal in synthetic solutions at the lab scale (50 mA/cm2, 8 h) and 94% removal at the semi-pilot scale, demonstrating effective destruction of concentrated waste.
- Scalability Assessment: The semi-pilot scale (7x increase in anode area) showed 0.8-fold slower pseudo-first-order degradation kinetics (kSA) compared to the lab scale, primarily attributed to increased gas evolution and PFAAs partitioning into foam.
- Energy Optimization (Tandem Approach): The combined IX concentration followed by EO destruction results in massive energy savings, estimating >99.9% reduction in the Electric Energy per Order (EEO) required compared to direct EO treatment of the total volume of raw water.
- Kinetic Performance: The surface area normalized rate constant (kSA) for total PFAAs removal was 1.37 x 10-5 m/s (Lab) and 8.44 x 10-6 m/s (Semi-Pilot) at 50 mA/cm2.
- Key Challenges: Low defluorination percentages were observed due to competitive oxidation of high chloride concentrations (brine), leading to perchlorate (ClO4-) generation, and physical partitioning of hydrophobic PFAAs into electrochemically generated foam.
- Real Waste Performance: Treatment of real still bottoms showed 93% PFAAs removal after 24 h, but kinetics were 7-fold slower than synthetic solutions, highlighting interference from co-contaminants and high organic matter (TOC).
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Anode Material | Boron-Doped Diamond (BDD) | - | Rectangular plate electrodes |
| Optimal Current Density | 50 | mA/cm2 | Used for highest removal efficiency |
| Total PFAAs Removal (Lab) | >99 | % | Synthetic solution, 8 h, 50 mA/cm2 |
| Total PFAAs Removal (Semi-Pilot) | 94 | % | Synthetic solution, 8 h, 50 mA/cm2 |
| kSA (Lab Scale) | 1.37 x 10-5 | m/s | Total PFAAs, 50 mA/cm2 (Synthetic) |
| kSA (Semi-Pilot Scale) | 8.44 x 10-6 | m/s | Total PFAAs, 50 mA/cm2 (Synthetic) |
| EEO (90% Removal, Lab) | 173 | Wh/L | Energy consumption metric |
| EEO (90% Removal, Semi-Pilot) | 194 | Wh/L | Energy consumption metric |
| Energy Reduction (IX/EO Tandem) | >99.9 | % | Compared to direct EO of raw water |
| Synthetic Solution Conductivity | 110 | mS/cm | High brine concentration (4% NaCl) |
| ClO4- Generation (Lab, 8h) | 16.1 | mM | 50 mA/cm2 (1.4% of initial Cl- oxidized) |
| TOC Removal (Lab, 8h) | 67 | % | Synthetic solution, 50 mA/cm2 |
| Defluorination Percentage (Lab, 8h) | 12.6 ± 0.6 | % | Low due to competitive oxidation/foaming |
| Lab Scale Anode Area | 200 | cm2 | 3 anodes, 2 cathodes |
| Semi-Pilot Scale Anode Area | 1400 | cm2 | 6 anodes, 5 cathodes (7x scale-up) |
| Inter-Electrode Gap (Lab) | 3 | mm | - |
| Inter-Electrode Gap (Semi-Pilot) | 2 | mm | - |
| Flow Rate (Semi-Pilot) | 6 | L/min | Equivalent Reynolds number (Re ~2300) |
Key Methodologies
Section titled âKey MethodologiesâThe study utilized galvanostatic electrochemical oxidation (EO) in batch mode using full BDD electrochemical cells to treat synthetic and real IX still bottoms.
- Electrochemical Setup: Experiments were conducted in two custom-built systems: a Laboratory scale (2 L volume) and a Semi-Pilot scale (14 L volume). The scale-up maintained a constant area-to-volume (A/V) ratio of 10 m-1 and an equivalent Reynolds number (Re ~2300) for hydrodynamic similarity.
- Electrode Configuration: Both setups used BDD rectangular plate electrodes. The semi-pilot scale reduced the inter-electrode gap from 3 mm (Lab) to 2 mm to enhance mass transfer.
- Operating Conditions: Galvanostatic control was applied. The optimal current density of 50 mA/cm2 (determined from lab optimization runs at 10, 25, and 50 mA/cm2) was used for all semi-pilot and real still bottoms experiments.
- Feedstock: Synthetic still bottoms contained high concentrations of PFAAs (PFBA, PFOA, PFHxS, PFOS) in a brine solution (4% NaCl) with 10,000 mg/L methanol. Real still bottoms contained PFAAs, PFAA precursors (FTS, N-EtFOSAA, N-MeFOSAA), and higher TOC (14,050 mg/L).
- Kinetic Analysis: PFAAs degradation followed pseudo-first-order kinetics. Surface area normalized rate constants (kSA) were calculated to compare efficiency across scales.
- Efficiency Metrics: Coulombic Efficiency (CE) and Electric Energy per Order (EEO) were calculated based on fluoride (F-) generation and PFAAs removal, respectively, to quantify treatment cost and current utilization.
- Foam Analysis: Foam generated during the electrochemical process was collected separately and analyzed for PFAAs content to quantify partitioning, confirming that highly hydrophobic PFAAs (e.g., PFOS) accumulated significantly in the foam layer.
Commercial Applications
Section titled âCommercial ApplicationsâThe findings support the implementation of BDD-based Electrochemical Oxidation (EO) as a robust destruction technology for highly concentrated PFAS waste streams, particularly when integrated into a tandem treatment train.
- Concentrated Waste Destruction: EO is ideal for treating low-volume, high-concentration liquid wastes, such as still bottoms generated from the regeneration or distillation of Ion Exchange (IX) resins or Reverse Osmosis (RO) concentrates used in PFAS remediation.
- Aqueous Film-Forming Foam (AFFF) Remediation: Applicable for the final destruction of concentrated PFAS residuals derived from treating AFFF-impacted water or soil.
- Landfill Leachate Management: Used as a polishing or destruction step for concentrated PFAS fractions extracted from landfill leachates, preventing environmental migration.
- Tandem Treatment Systems (IX/EO): Commercial systems benefit from combining IX (for concentration and volume reduction) with EO (for destruction), achieving high removal rates while drastically minimizing the overall energy footprint and eliminating the need for off-site disposal (e.g., incineration).
- Industrial Effluent Mineralization: BDD anodes provide high oxidizing power necessary for the complete mineralization of persistent organic pollutants (POPs) and complex organic matrices (high TOC) found in specialized industrial wastewater.
View Original Abstract
The ubiquitous presence of perfluoroalkyl acids (PFAAs) in the environment remains a serious environmental concern. In this study, the electrochemical oxidation (EO) of PFAAs from the waste of ion exchange (IX) still bottoms was assessed at the laboratory and semi-pilot scales, using full boron-doped diamond (BDD) electrochemical cells. Multiple current densities were evaluated at the laboratory scale and the optimum current density was used at the semi-pilot scale. The results at the laboratory scale showed >99% removal of total PFAAs with 50 mA/cm2 after 8 h of treatment. PFAAs treatment at the semi-pilot scale showed 0.8-fold slower pseudo-first-order degradation kinetics for total PFAAs removal compared to at the laboratory scale, and allowed for >94% PFAAs removal. Defluorination values, perchlorate (ClO4â) generation, coulombic efficiency (CE), and energy consumption were also assessed for both scales. Overall, the results of this study highlight the benefits of a tandem concentration/destruction (IX/EO) treatment approach and implications for the scalability of EO to treat high concentrations of PFAAs.
Tech Support
Section titled âTech SupportâOriginal Source
Section titled âOriginal SourceâReferences
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