Mechanism and toxicity assessment of carbofuran degradation by persulfate-based advanced oxidation process

Abstract

The advanced oxidation process based on persulfate has been proven to be a promising method for degrading the highly toxic carbamate pesticide carbofuran (CBF). However, the mechanism of CBF degradation by sulfate radicals (SO₄·^-) and hydroxyl radicals (·OH) is still unclear and requires further research and discussion. This study investigated the mechanism and toxicity assessment of CBF degradation using density functional theory (DFT) theory calculation methods. The results indicated that SO₄·^- and ·OH can undergo addition and abstraction reactions with CBF. Thermodynamic and kinetic analysis showed that the abstraction reaction between SO₄·^- and the secondary H atom is the optimal reaction pathway, exhibiting the highest branching ratio (Γ = 41.84%). The rate constants for the reactions of CBF with SO₄·^- and ·OH at room temperature were found to be 3.66 × 10⁹ and 8.96 × 10⁸ M^-1 s^-1, respectively, which are consistent with experimental data reported in previous studies. The acute and chronic toxicity of CBF and its degradation products to aquatic organisms was predicted through an ecological toxicity assessment model. The toxicity of the degradation products was lower than that of the parent CBF, confirming the viability of using persulfate-based advanced oxidation processes for water treatment.

Fig. 1. (a) The structure of CBF. (b) Molecular surface ESP distribution diagram of CBF.

Fig. 2. The Gibbs free energy barrier ΔG^≠ (kcal mol⁻¹) and the Gibbs free energy change Δ_rG (kcal mol⁻¹) at 298 K for the addition pathways of CBF with SO₄·⁻.

Fig. 3. The Gibbs free energy barrier ΔG^≠ (kcal mol⁻¹) and the Gibbs free energy change Δ_rG (kcal mol⁻¹) at 298 K for the addition pathways of CBF with ·OH.

Fig. 4. The Gibbs free energy barrier ΔG^≠ (kcal mol⁻¹) and the Gibbs free energy change Δ_rG (kcal mol⁻¹) at 298 K for the abstraction reactions of CBF with SO₄·⁻.

Fig. 5. The Gibbs free energy barrier ΔG^≠ (kcal mol⁻¹) and the Gibbs free energy change Δ_rG (kcal mol⁻¹) at 298 K for the abstraction reactions of CBF with ·OH.

Fig. 6. Subsequent reactions of IM1-3(OH) and IM1-3(SO₄).

Fig. 7. Subsequent reactions of IM1-6(OH) and IM1-6(SO₄).

Fig. 8. Subsequent reactions of IM1-5(SO₄) and IM1-15.

Fig. 9. Classification of acute toxicity of CBF and transformation products to aquatic organisms. (a) The acute toxicity index: LC₅₀/EC₅₀ and (b) the chronic toxicity index: ChV (unit: mg L⁻¹).