Fig. 8: Proposed possible degradation pathways of CBZ under the EF treatment using the MOF-derived Fe@PC nanocatalyst.

Each pathway reflects a distinct mechanistic route by which reactive oxygen species, primarily hydroxyl radicals (•OH) and superoxide radicals (•O₂⁻), initiate oxidative transformations of CBZ, leading to its eventual breakdown into smaller, less toxic molecules. Pathway I suggests two sub-routes where free radicals attack CBZ to produce a key intermediate (P9), either directly from CBZ (P1) or from an epoxide intermediate (P8). This is followed by sequential hydroxylation steps forming P11 and ultimately P20. These intermediates suggest gradual functionalization and ring hydroxylation, leading to higher hydrophilicity and facilitating further breakdown. Pathway II involves initial •OH attack on the double bond with the highest electron density in CBZ, forming intermediate P2. This compound undergoes ring-opening to form P3, followed by oxidative transformations that yield P4 or P5. These structures undergo side-chain cleavage to generate P6 or P7, respectively. The transformations in this pathway indicate a rapid deconstruction of the aromatic and heterocyclic rings, which accelerates mineralization. Pathway III describes the formation of an epoxide intermediate (P8) via hydroxyl radical addition, followed by cleavage and oxidation reactions yielding P10, P12, and P13. This pathway is indicative of partial ring retention and slower breakdown steps compared to the more energetically favourable Pathway II. Pathway IV begins with direct •OH addition to the benzene or azepine ring, producing hydroxylated derivatives P17 and P18. These undergo further oxidation and elimination to produce P12 and P13, overlapping with intermediates in Pathway III. The convergence of products suggests multiple routes may lead to common degradation intermediates. Reproduced from ref. ¹⁰⁸. Copyright 2023 Elsevier B.V.