Deciphering radical pathways (ROS and RCS) in solar-driven chlorine photolysis: a mechanistic kinetic modeling study of 4-nitrophenol degradation

Abstract

This paper explores the role of reactive oxygen species (ROS) and reactive chlorine species (RCS) in the degradation process using detailed kinetic modeling of the solar-activated chlorine degradation of 4-nitrophenol (4-NP). The rate constants for the radical and non-radical routes of hypochlorite and the oxidation of 4-NP by free radicals were determined thanks to the kinetic modeling. Results indicate that the combined action of chlorine and solar illumination in the SunTest model results in a fully synergistic treatment. The observed 4-NP degradation rate constant from the experimental profiles in the control run (pH 10, 500 µM chlorine) is k _obs  = 2.1 × 10 ⁻³ s ⁻¹ ( R ²  = 0.993). The predicted values from the modeling profiles closely matched the experimental ones, with 2 × 10 ⁻³ s ⁻¹ ( R ²  = 0.997). To estimate the kinetic constants of the main photolysis reactions [k ₂ for R ₂ : ClO ⁻  + hν → O ^•−  + Cl ^• , k ₃ for R ₃ : ClO ⁻  + hν → O( ³ P) + Cl ⁻ and k ₄ for ClO ⁻  + hν→ O( ¹ D) + Cl ⁻ ], several optimization algorithms available in COPASI were tested, but the Nelder–Mead one was ultimately selected because of the stability of its results and its robustness in the optimization of nonlinear parameters. Additionally, the rate constants for the reactions of 4-NP with O ^•− , Cl ^• , ClO ^• , Cl ₂ ^•− , HOCl ^•− , HO ₂ ^•− , O ₂ ^•− and O( ³ P) were predicted to be k _R127  = 4.999 × 10 ⁹  M ⁻¹ s ⁻¹ , k _R129  = 2.44 × 10 ⁹  M ⁻¹ s ⁻¹ , k _R130  = 3.59 × 10 ⁵  M ⁻¹ s ⁻¹ , k ₁₃₁  = 3.44 × 10 ⁷  M ⁻¹ s ⁻¹ , k ₁₃₂  = 4.19 × 10 ⁴  M ⁻¹ s ⁻¹ , k ₁₃₃  = 1.75 × 10 ⁵  M ⁻¹ s ⁻¹ , k _R134  = 4.099 × 10 ⁴  M ⁻¹ s ⁻¹ , and k _R135  = 0.51 M ⁻¹ s ⁻¹ respectively. At pH 10, the rate constants for the photolysis of ClO ⁻ are k _R2  = 1.27 × 10 ⁻³ s ⁻ , k _R3  = 1.68 × 10 ⁻⁴ s ⁻¹ , and k _R4  = 0 s ⁻¹ . Additionally, profiles of ROS and RCS, along with detailed analyses of radical distribution during 4-NP degradation, were generated and examined under key operating conditions. This enabled the construction of a phase diagram describing radical distribution as a function of the main operational parameters.