Laboratory and Semi-Pilot Scale Study on the Electrochemical Treatment of Perfluoroalkyl Acids from Ion Exchange Still Bottoms

At a Glance

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

Technical Documentation & Analysis: Electrochemical Treatment of PFAAs using BDD Electrodes

Executive Summary

This analysis focuses on the successful application of Boron-Doped Diamond (BDD) electrodes for the electrochemical oxidation (EO) of highly concentrated Perfluoroalkyl Acids (PFAAs) found in Ion Exchange (IX) still bottoms. The study validates BDD as the material of choice for destructive PFAS treatment and provides critical scale-up data.

High Destruction Efficiency: Achieved >99% total PFAAs removal (including >95% PFBA and >99% PFOA/PFOS/PFHxS) at the laboratory scale using 50 mA/cm² BDD anodes over 8 hours.
Scale-Up Validation: Successfully scaled the EO process by a factor of 7 (from 200 cm² to 1400 cm² anode area), achieving 94% PFAAs removal at the semi-pilot scale.
Tandem Process Optimization: The combination of IX concentration followed by BDD EO destruction resulted in >99.9% energy savings compared to direct EO of dilute water, confirming the economic viability of the tandem approach.
Material Requirement: The high efficiency relies entirely on the use of full BDD electrodes, leveraging their high oxygen evolution overpotential for effective PFAA mineralization.
Scale-Up Challenges Identified: Slower kinetics at the semi-pilot scale were attributed to non-uniform current density distribution and increased gas evolution (foaming), suggesting the need for smaller, modular BDD cells.
Key Kinetic Data: Surface area normalized pseudo-first-order degradation rate constant (k_sa) for synthetic still bottoms was 1.37 x 10^-5 m/s (Lab) and 8.44 x 10^-6 m/s (Semi-Pilot) at 50 mA/cm².

Technical Specifications

Parameter	Value	Unit	Context
Anode Material	Boron-Doped Diamond (BDD)	N/A	Rectangular plate electrodes
Optimum Current Density	50	mA/cm²	Synthetic still bottoms solution
Total PFAAs Removal (Lab)	>99	%	50 mA/cm², 8 h treatment
Total PFAAs Removal (Semi-Pilot)	94	%	50 mA/cm², 8 h treatment
Real Still Bottoms Removal (Lab)	93	%	50 mA/cm², 24 h treatment
k_sa (Lab Scale, Synthetic)	1.37 x 10^-5	m/s	Pseudo-first-order degradation rate constant
k_sa (Semi-Pilot Scale, Synthetic)	8.44 x 10^-6	m/s	Pseudo-first-order degradation rate constant
Electric Energy per Order (EEO)	173	Wh/L	90% removal, Laboratory scale
Electric Energy per Order (EEO)	194	Wh/L	90% removal, Semi-Pilot scale
Anode Area (Lab Scale)	200	cm²	3 anodes
Anode Area (Semi-Pilot Scale)	1400	cm²	6 anodes (7x scale-up)
Inter-electrode Gap (Lab / Semi-Pilot)	3 / 2	mm	Optimized for mass transfer
Initial Chloride Concentration	41,670	mg/L	Synthetic solution (4% NaCl)
pH Range (Lab Scale)	7.7 ± 0.1	N/A	Initial pH

Key Methodologies

The electrochemical treatment relied on precise control of material properties and operating conditions to achieve high PFAA destruction rates.

Electrode Material: Full Boron-Doped Diamond (BDD) rectangular-plate electrodes were used as the anode material, providing the necessary high overpotential for direct anodic oxidation of PFAAs.
Operating Conditions: Experiments were performed in duplicate, batch mode, under galvanostatic control (constant current density).
Current Density Optimization: Laboratory scale experiments evaluated current densities of 10, 25, and 50 mA/cm², confirming 50 mA/cm² as the optimum for maximum PFAA removal.
Scale-Up Design: The semi-pilot scale setup was scaled by a factor of 7 (1400 cm² anode area, 14 L volume) while maintaining a constant Area-to-Volume (A/V) ratio of 10 m^-1.
Hydrodynamic Control: Flow rates (2 L/min for Lab, 6 L/min for Semi-Pilot) were set to maintain an equivalent Reynolds number (Re ≈ 2300) between the two scales to ensure comparable mass transfer conditions. The inter-electrode gap was reduced from 3 mm (Lab) to 2 mm (Semi-Pilot) to enhance mass transfer (k_m).
Analytical Monitoring: Key parameters including PFAAs, TOC, F^-, and ClO₄^- were monitored over time. PFAS analysis followed a modified EPA 537 method using LC-MS/MS.

6CCVD Solutions & Capabilities

The research highlights the critical role of high-quality BDD electrodes and precise cell engineering in scaling up PFAS destruction technology. 6CCVD is uniquely positioned to supply the advanced diamond materials and customization services required to replicate and advance this research toward commercial deployment.

Research Requirement	6CCVD Solution & Capability	Technical Advantage for Engineers
High-Purity BDD Anodes	Heavy Boron-Doped PCD Wafers/Plates. We specialize in MPCVD growth of highly conductive BDD films, essential for achieving the high oxygen evolution overpotential (>2.3 V) needed for PFAA mineralization.	Ensures maximum current efficiency for destructive oxidation, minimizing competitive side reactions (e.g., Cl^- oxidation to ClO₄^-).
Custom Electrode Dimensions	Custom Dimensions up to 125mm (PCD). The study utilized custom rectangular plates (200 cm² and 1400 cm²). We provide custom laser cutting and shaping services for plates up to 125mm diameter.	Allows for precise replication of laboratory cell geometries and efficient design of larger, non-standard industrial reactors.
Modular Scale-Up Optimization	Precision Laser Cutting for Modular Cells. The paper recommends increasing the number of smaller modules rather than increasing electrode size to improve current distribution [36]. 6CCVD provides BDD electrodes cut to tight tolerances for modular stack designs.	Optimizes current density uniformity across the anode surface, mitigating the scale-up kinetic slowdown observed in the semi-pilot system (k_sa,lab / k_sa,sp ratio of 1.6).
Inter-Electrode Gap Control	Custom Substrate Thickness (0.1µm - 10mm). We supply BDD films on substrates or as freestanding plates with thicknesses matching the required inter-electrode gaps (e.g., 2 mm or 3 mm).	Enables engineers to precisely control the gap, enhancing mass transfer (k_m) and reducing ohmic losses, leading to lower Electric Energy per Order (EEO).
Metalization for Contact	In-House Metalization Services (Au, Pt, Ti, W, Cu). The BDD electrodes require robust electrical contacts. We offer custom metalization stacks tailored for harsh electrochemical environments.	Ensures low-resistance electrical connection and long-term stability of the anode assembly under high current density operation.
Surface Finish Requirements	Polishing Services (Ra < 5nm for Inch-size PCD). While EO typically uses rougher surfaces, we offer polishing capabilities for applications requiring low surface roughness (Ra < 5nm).	Provides flexibility for researchers exploring the impact of surface morphology on mass transfer and reaction kinetics.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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 (ClO4−) 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

Original Source

DOI: https://doi.org/10.3390/w13202873

References

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