Optimizing boron doping in diamond electrodes for PFOA mineralization: ¹⁴C labeling and DFT studies

Boron-doped diamond (BDD) films were synthesized via microwave plasma-enhanced chemical vapor deposition at five boron-to-carbon ratios to investigate dopant effects on perfluorooctanoic acid (PFOA) electrooxidation and electrode stability. Raman spectroscopy revealed that excessive boron (≥10k ppm) produced sp²-rich regions indicative of structural degradation. Electrodes optimized at ∼10k ppm boron exhibited carrier mobility of ∼98 cm/V^-1s^-1 and achieved threefold higher oxidation rate constants compared to low-doped analogs. PFOA electrooxidation followed first-order kinetics, yielding up to 60 % removal and 50 % mineralization to CO₂, confirmed via ¹⁴C-labeled tracing. Lower-doped electrodes generated more C₄-C₇ perfluorocarboxylic acid intermediates, indicating slower kinetics. Mechanistic studies using methanol as a hydroxyl radical scavenger demonstrated that ^•OH plays a secondary role, with direct electron transfer (DET) dominating PFOA oxidation. A modified validated method for quantifying 11 PFCAs, including those regulated under EU Directive 2020/2184, is reported. Density functional theory simulations supported the hypothesis that boron doping strengthens PFOA-surface covalent-like bonding and facilitates electron transfer. This work establishes optimal boron doping for balancing conductivity, stability, and oxidation performance, and provides design principles for BDD electrodes in advanced electrochemical PFAS treatment systems.