Molecular fractionation and transformation of dissolved black carbon on hematite facets: implications for sunlight-driven photodegradation of antibiotics in aquatic environments.

Dissolved black carbon (DBC), the mobile and reactive fraction released from biochar, can adsorb onto iron oxides and alter their photochemical reactivity, thereby influencing the photochemical transformation of coexisting organic pollutants. However, how DBC molecular composition, which varies with pyrolysis temperature, governs its interactions with different hematite facets, and how these interactions in turn affect organic pollutant photodegradation remains unclear. Herein, hematite nanocrystals exposing {001}, {100}, and {012} facets, and two DBCs pyrolyzed at 300 ℃ and 500 ℃ (DBC300 and DBC500), were employed to investigate DBC molecular fractionation and transformation, and their impacts on sunlight-driven tetracycline hydrochloride (TCH) photodegradation. FT-ICR MS analysis showed that the intrinsic molecular composition of DBC primarily determined its molecular fractionation and transformation, whereas hematite crystal facets mainly regulated the extent of these processes. DBC300 underwent adsorption coupled with more extensive molecular transformation, whereas DBC500 was dominated by adsorption-driven molecular fractionation. Distinct facet-dependent behaviors were observed, with {001}-dominated hematite nano-plates favoring adsorption-driven molecular fractionation, {100}-dominated hematite nano-rods exhibiting the highest surface area-normalized adsorption density, and {012}-exposed hematite nano-cubes showing the greatest potential for molecular transformation. Photodegradation experiments showed that DBC adsorption substantially inhibited TCH photodegradation on all hematite nanocrystals. Although the retained low-molecular-weight aromatic fractions slightly enhanced light harvesting, adsorbed high-MW and oxygen-rich DBC fractions primarily inhibited hematite photochemical reactivity by suppressing interfacial charge transfer and altering reactive intermediate generation. These findings highlight the role of mineral-organic interactions in regulating contaminant fate in iron-rich aquatic environments and reveal the emerging challenges associated with biochar applications.