[BN+SiO2]-BM treatment of 6:2 FTS, PFOA, and PFOS yielded PFCAs intermediates with peak concentrations higher than SiO2-BM but lower than BN-BM (Figure The BN-BM treatment readily achieved > 98% DeF for 6:2 FTS, PFOA, and PFOS (Figures ; raw data see Table ), while less than 25% DeF was observed in experiments using only SiO2. For the [BN+SiO2]-BM process, the DeF values were much larger than when using SiO2 alone but lower than BN-BM. Moreover, DeF appeared to decrease after reaching peak values, likely because extended milling resulted in F -reacting with SiO2 to form unextractable Si-F species. Overall, these results suggest that BN is largely responsible for the PFAS breakdown to F -. It seems SiO2 does not promote the BN-BM treatment of solid PFAS chemicals. However, as will be revealed below, the promotional role of SiO2 became significant in liquid AFFF treatment.