Formation of nitrated naphthol under freezing conditions

As global warming intensifies freeze-thaw cycles, the understanding of environmental chemistry processes in ice and their potential impact on global water quality becomes increasingly important. Herein, we report the nitrite (NO ₂ ⁻ )-facilitated transformation of 1-naphthol to the nitrated derivatives within ice matrix. Experimental results show that under freezing conditions with 10 µM NO ₂ ⁻ , 8.87 µM of the initial 10 µM 1-naphthol was transformed, yielding 1.62 µM nitro-naphthols within 2 hours. In contrast, no such transformation was observed in liquid water at 4°C. This transformation is attributed to the freeze-concentration effect, which leads to the accumulation of protons, NO ₂ ⁻ , and 1-naphthol within the liquid-like grain boundary region in the ice. In this concentrated brine microenvironment, NO ₂ ⁻ underwent protonation to form reactive nitrous acid (HNO ₂ ) and nitrosonium ions (NO ⁺ ), thereby driving nitration, nitrosation, and oxidative coupling reactions. Natural organic matter (NOM) inhibited both 1-naphthol transformation and nitro-naphthol formation by competing for reactive nitrogen species. These findings advance the understanding of NO ₂ ⁻ chemistry and the behavior of hydroxylated polycyclic aromatic hydrocarbons in frigid environments.