Molecular transformation of dissolved black carbon during photobleaching and its enhanced role in goethite-mediated 17α-ethinylestradiol anaerobic biotransformation.

Dissolved black carbon (DBC) is an important component of dissolved organic matter, which undergoes continuous photobleaching in the sunlit surface water. The resulting photobleached DBC (P-DBC) can subsequently be transported across the sediment-water interface into sediments rich in minerals and microbes, thereby regulating the transformation of organic contaminants. This study investigated photobleaching-induced molecular transformation of DBC and the mechanism of P-DBC in goethite-mediated 17α-ethinylestradiol (EE2) anaerobic biotransformation. The results showed that photobleaching promoted the oxidation and decarbonization of DBC, driving its molecular composition toward a higher oxidation state and lower energy density. Intensity and H content were the key molecular features governing the photochemical reactivity of DBC. P-DBC was enriched in oxygen-containing functional groups that could enhance electron transfer with goethite. Additionally, the higher bioavailability of P‑DBC enriched aromatic‑degrading bacteria. Species associated with iron reduction and organic matter transformation in the presence of goethite were increased. As a result, P-DBC combined with goethite significantly promoted the anaerobic biotransformation of EE2, achieving a removal efficiency of 73.0%. EE2 transformation involved de-ethynylation and decarboxylation to yield more oxygenated products, contrasting with co‑metabolism alone. These findings provide new mechanistic insights into the natural attenuation of estrogens in iron-rich aquatic environments.