Photo-Fenton degradation of a herbicide (2,4-D) in groundwater for conditions of natural pH and presence of inorganic anions

Highlights: • The effect of inorganic anions in the photo-Fenton degradation of 2,4-D is studied. • A kinetic model is proposed using the ferrioxalate as iron source for pH = 5. • Ferrioxalate improved the photochemical activation of the process. • Kinetic parameters associated with the anions influence were estimated. • A good agreement between the kinetic model and experimental data was observed. -- Abstract: The effects of four inorganic anions (Cl⁻, SO₄²−, HCO₃⁻, NO₃⁻) usually present in groundwater were investigated on the photo-Fenton degradation of 2,4-dichlorophenoxyacetic acid (2,4-D). A kinetic model derived from a reaction sequence is proposed using the ferrioxalate complex as iron source at pH close to natural conditions (pH = 5). It was demonstrated that oxalate not only maintained iron in solution for the natural groundwater system, but also increased the photochemical activation of the process. Results showed that the minimum conversion of 2,4-D for the simulated groundwater after 180 min was 63.80%. This value was only 14.1% lower than the conversion achieved without anions. However, with all anions together, the consumption of hydrogen peroxide (HP) per mole of herbicide showed an increase with respect to the test without anions. Only one kinetic parameter was estimated for each anion applying a nonlinear regression method. Subsequently, these optimized kinetic constants were used to simulate the system behaviour, considering the influence of all the studied anions together. A good agreement between kinetic model predictions and experimental data was observed, with the following errors: RMSE_2,4-D = 3.98 × 10⁻³ mM, RMSE_HP = 1.83 × 10⁻¹ mM, RMSE_OX = 1.39 × 10⁻² mM, and RMSE_2,4-DCP = 5.59 × 10⁻³ mM.