article · International Journal of Chemical Reactor Engineering
Solar-driven chlorine photolysis provides a synergistic method for breaking down the pollutant 4-nitrophenol. Detailed kinetic modelling accurately maps the chemical pathways driven by reactive oxygen species and reactive chlorine species during this treatment process. Operating at pH 10 with 500 micromolar chlorine under simulated sunlight, the observed degradation rate constant of 2.1 multiplied by 10 to the power of minus 3 reciprocal seconds aligns closely with the kinetic model prediction. Using the Nelder-Mead optimisation algorithm within COPASI, specific reaction rate constants were determined for the primary photolysis steps of hypochlorite and for the degradation of 4-nitrophenol by individual radical species. The resulting profiles and phase diagram illustrate how reactive species distribute across core operational parameters, clarifying the fundamental mechanisms governing pollutant removal in solar advanced oxidation systems.
Understanding how sunlight activates chlorine to destroy toxic organic pollutants helps engineers optimise water treatment systems. By defining the exact chemical rates and radical species involved, this mechanistic modelling clarifies how solar advanced oxidation operates. Such detailed kinetic insight enables more predictable water disinfection and decontamination without relying solely on trial and error.
This fundamental modelling research informs the design and parameter tuning of solar advanced oxidation processes for industrial wastewater treatment. Environmental engineers and water treatment plant designers could use these kinetic data to calibrate treatment protocols for phenolic contaminants. The study represents early-stage kinetic and computational modelling, meaning practical engineering deployment would require pilot testing under real-world water conditions.
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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.
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DOI: 10.1515/ijcre-2026-0057
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