Methyl group donor‑resolved isotopic diagnostics reveal mechanistic diversity in UV‑driven methylation of Hg(II)

Photochemical methylation of mercury (Hg) is difficult to isolate in nature. Here we show that short-wavelength UV-C (254 nm) selectively drives aqueous Hg(II)→MeHg photomethylation in the presence of low-molecular-weight organic compounds with different methyl group (acetone, acetic acid, acetaldehyde), whereas no MeHg forms under visible/UV-A/UV-B light. Gross 4-h yields reach 59-84 % (acetone), 25-40 % (acetic acid), and 38-66 % (acetaldehyde). Coupled concentration-isotope time series resolve large odd-mass-independent fractionation (odd-MIF; Δ¹⁹⁹Hg up to 25 ‰) with donor-specific Δ¹⁹⁹Hg/Δ²⁰¹Hg slopes. Acetone and acetic acid experiments display odd-only MIF with slopes of 0.9-1.0 and 1.1-1.3, consistent with magnetic isotope effects and a dominant role of radical-pair chemistry. By contrast, acetaldehyde experiment exhibits steeper slopes (1.8-2.0) accompanied by small negative Δ²⁰⁰Hg (down to -0.4 ‰), indicating complex mechanisms possibly related to multi-step electron transfer/heterogeneous processes. A reversible two‑pool isotopologue model recovers apparent forward and reverse rates and MDF enrichment factors, and reveals contrasting MDF partitioning: demethylation dominates in acetone, whereas methylation dominates in acetic acid. These methyl group donor-resolved isotope fingerprints identify a short wavelength photochemical route to MeHg and represent actional diagnostics for evaluating whether similar processes occur in atmospheric waters exposed to short wavelength radiation.