Boron Nitride Based Photocatalysts for Efficient PFAS Degradation

Journal Title
Journal ISSN
Volume Title

The strong persistence, pervasiveness, and toxicity of per- and polyfluorinated substances (PFAS) make them recalcitrant contaminants of emerging concern. The stability of C-F bonds and the surfactant properties of PFAS lead to incomplete removal by standard water treatment methods, and ppb (μg/L) levels of PFAS have been detected in human tissues and blood serum. Exposure to parts-per-quadrillion of PFAS in water has negative health effects, including kidney, liver, and reproductive problems in adults, suppressed immune response to vaccines, and neurological/behavioral issues in children. The U.S. EPA published a proposed rule in 2023 establishing Federal Maximum Contaminant Levels (MCLs) for 6 PFAS in drinking water. Current water treatment technologies only physically remove, but not destroy PFAS, and also general PFAS contaminated waste. Photocatalysis has been proved as an efficient approach for degrading PFAS in water with air as the oxidant, and light as the energy source, but identifying photocatalysts is still challenging. The primary goal of this work is to present that boron nitride-based materials are excellent catalysts for the PFAS remediation in water, in which three boron nitride-based photocatalysts were prepared and demonstrated their efficiency for PFAS destruction under UV light or even solar illumination. The corresponding mechanism has also been investigated. These findings present fresh opportunities for materials design and for the re-evaluation of other wide band gap semiconductors for PFAS photocatalytic degradation. The insights also serve as rationally-guided design principles for improved heterogeneous photocatalysts. Boron nitride (BN) was discovered to degrade PFOA upon irradiation with 254 nm (ultraviolet C, UVC) light. The ability of BN to degrade PFOA photocatalytically has previously been unreported and is unexpected, because its band gap is too large for light absorption. Radical scavenging experiments suggest that PFOA degrades in the presence of BN via a hole-initiated reaction pathway similar to the TiO2 case and involves superoxide/hydroperoxyl and hydroxyl radicals. Surface defects were surmised to allow BN to absorb in the UVC range and to photogenerate reactive oxygen species. Sealed batch studies indicated BN was ∼2 and ∼4 times more active than TiO2, before and after ball milling the material, respectively. BN can be reused, showing no decrease in activity over three cycles. BN was active for the photocatalytic degradation of GenX, another PFAS of concern. We then investigated why BN exhibited superior photoreactivity towards PFAS degradation. The role of BN’s surface hydrophobicity on PFOA photocatalysis was investigated by comparing BN (hexagonal phase) with TiO2 (anatase phase) under reaction conditions in which the exposed surface areas were the same. BN exhibited a ~2.5× faster PFOA degradation rate compared to TiO2, with both materials photo-generating holes at nearly identical rates (23 μM EDTA/min for BN vs. 22 μM EDTA/min for TiO2) as quantified through hole scavenging experiments. Sorption experiments indicated that PFOA surface coverage was ~2× higher on BN than on TiO2. Though surface coverage correlates with PFOA activity, it does not completely explain it. Density functional theory calculations highlight the importance of BN hydrophobicity in inhibiting the competing hole reaction with surface-hydroxyls. These insights serve as rationally-guided design principles for improved heterogeneous photocatalysts. Boron nitride (BN) destroys PFAS under UVC irradiation, but is ineffective at longer wavelengths, though. By simple calcination of BN and UVA active titanium oxide (TiO2), it creates a BN/TiO2 composite that is more photocatalytically active than BN or TiO2 under UVA for PFOA. Under UVA, BN/TiO2 degraded PFOA ~15× faster than TiO2, while BN was inactive. Band diagram analysis and photocurrent response measurements indicated that BN/TiO2 is a type-II heterojunction semiconductor, facilitating charge carrier separation. Additional experiments confirmed the importance of photogenerated holes for degrading PFOA. Outdoor experimentation under natural sunlight found BN/TiO2 to degrade PFOA in deionized water and salt-containing water with a half-life of 1.7 h and 4.5 h, respectively. These identified photocatalytic properties of BN/TiO2 highlight the potential for the light-driven destruction of other PFAS. Lastly, we found that commercially available defective BN can produce ammonia due to its hydrolytic instability. We hypothesize that BN hydrolysis can be inhibited by preventing the exposure of edge defect sites to water. In this study, we prepared SiO2-coated BN ("SiO2-BN") with varying amounts of SiO2 through tetraethyl orthosilicate (TEOS) hydrolysis and condensation. We then tested these materials for PFOA photodegradation under UVC illumination. Unexpectedly, the SiO2 coating promoted PFOA sorption and degradation, with 6 wt% SiO2-BN exhibiting a 1.6× faster PFOA photodegradation rate than BN and achieving near complete (95%) defluorination after 24 hours of UVC irradiation. While some enhancement in PFOA degradation is likely due to the increased PFOA sorption, the SiO2 coating also functionally removes B-OH edge sites, thus inhibiting the competing reaction of photogenerated holes with surface hydroxyls. Furthermore, the SiO2 coating stabilizes BN, as evidenced by 6 wt% SiO2-BN exhibiting an approximately 4× lower formation rate of N-containing byproducts compared to BN. These findings provide valuable insights and directions for the design of stable and effective catalysts for PFAS photocatalysis.

Doctor of Philosophy
Boron Nitride, PFAS, Photocatalysis

Wang, Bo. "Boron Nitride Based Photocatalysts for Efficient PFAS Degradation." (2023) Diss., Rice University.

Has part(s)
Forms part of
Published Version
Copyright is held by the author, unless otherwise indicated. Permission to reuse, publish, or reproduce the work beyond the bounds of fair use or other exemptions to copyright law must be obtained from the copyright holder.
Link to license
Citable link to this page