Silica Coating Enhances the PFAS Photocatalytic Properties of Boron Nitride

ABSTRACT Boron nitride (BN) has recently emerged as a promising material capable of breaking down toxic perfluorooctanoic acid (PFOA) and other per‐ and polyfluoroalkyl substances (PFAS) through photocatalysis. However, chemical stability of its powder form has not been addressed, which could limit practical application in water. We synthesized a series of SiO 2 ‐coated BN (SiO 2 ‐BN) with different SiO 2 loadings (3, 6, and 10 wt%) using tetraethyl orthosilicate (TEOS) sol‐gel chemistry, to test the hypothesis that an appropriate silica amount can protect the BN surface from reacting with water. All SiO 2 loadings decreased BN hydrolysis (2.3–4.5× slower hydrolysis rate), by replacing B‐OH sites with B‐O‐Si linkages. The SiO 2 ‐BN materials also had higher PFOA photodegradation rates (∼1.6× faster) and higher extent of defluorination (86%–95% vs. 78%) compared to uncoated BN. Comparative second‐order rate constant analysis showed SiO 2 ‐BN (6 wt%) to be 6.7× more active than BN. We attributed the enhancement to the loss of B‐OH groups that undesirably compete with PFOA for reaction with the photogenerated holes, and to the improved water dispersibility of the material. SiO 2 ‐BN (6 wt%) suggests the ideal structure to be BN platelets that are SiO 2 ‐coated just on the edges. In testing its capabilities for real‐world use, we found that SiO 2 ‐BN photocatalytically degraded 16 other individual PFAS compounds at ppm concentrations and effectively treated real groundwater containing a PFAS mixture at ppb concentrations. The beneficial materials strategy of coating BN at the platelet edges may be generalizable to other two‐dimensional structures for clean water applications.