The environmental degradation of naphthalimide, rhodamine and BODIPY fluorophores by hydroxyl radicals: a theoretical insight

TL;DR

This study uses theoretical methods to explore how three common fluorophores degrade in the environment due to hydroxyl radicals and their potential ecological risks.

Contribution

The paper provides a novel theoretical insight into the environmental degradation and ecological risks of RDA, NPA, and BOD fluorophores by hydroxyl radicals.

Findings

RDA shows strong pH-dependent reactivity with a wide range of lifetimes in natural waters.

NPA degrades more slowly and is more environmentally persistent compared to RDA and BOD.

BOD reacts rapidly under all conditions and may be less ecologically harmful than RDA and NPA.

Abstract

Rhodamine (RDA), naphthalimide (NPA), and BODIPY (BOD) are widely used fluorophores whose environmental fate is hitherto unexplored. In this study, quantum chemical calculations were employed to investigate the HO˙-driven degradation processes of these fluorophores in aqueous and organic media, focusing on the mechanism, kinetics, thermodynamics, and ecological risks. The results suggest that all three fluorophores undergo reactions with HO˙, but their behaviors differ markedly. RDA displays strong pH-dependent reactivity, with overall rate constants ranging from 8.76 × 108 to 4.02 × 1010 M−1 s−1 depending on protonation state, and lifetimes spanning from hours to years in natural waters. NPA degrades more slowly, with rate constants of 7.06 × 107 M−1 s−1 (neutral form) and 8.20 × 108 M−1 s−1 (anion), resulting in greater environmental persistence. BOD reacts rapidly across all conditions (4.39 × 109 M−1 s−1) consistently via RAF mechanism. Temperature and solvent polarity also influence degradation: higher temperatures accelerate all reactions, and methanol enhances reactivity, while lipid-like media reduce degradation rates for RDA and NPA but not for BOD. NPA, RAD and these intermediates are predicted to exhibit high ecological toxicity (log LC50/EC50/ChV < 2), poor biodegradability, and high bioaccumulation potential, while BOD and its products may be comparatively benign. Overall, the hazard ranking is predicted to be RDA ≈ NPA > BOD, with RDA posing the highest long-term ecological risk. These findings highlight the need to evaluate not only degradation efficiency but also the toxicity and environmental fate of intermediates when applying advanced oxidation processes for fluorophore removal.

In the environment, all three fluorophores undergo reactions with HO˙, but their behaviors differ markedly.

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