<p>Herbicide can cause pollution for the aquatic environment and limit the potential of using this source of water for drinking supply water. Thus, removal of this type of toxicant from water source is crucial in an integrated treatment system for drinking supply water from polluted surface water source. This paper aims to remove 2,4-dichlorophenoxyacetic acid (2,4-D), one type of herbicide, from water by a combined two-step Fenton reaction and ozonation in advanced oxidation process (AOP). Affecting factors of Fenton reaction (pH, contact time, dose of H₂O₂, dose of Fe) and ozone process (pH, dose of ozone, contact time) to 2,4-D removal efficiencies were investigated. The highest 2,4-D removal efficiency, achieving 100% for an initial concentration of 10&#xa0;mg/L, was obtained under optimal conditions using the combined Fenton/ozone process. The conditions included (i) Fenton process, pH 3, H₂O₂ dose of 0.3&#xa0;mmol/L&#xa0;(mM/L), Fe²⁺ dose of 0.03&#xa0;mM/L, and a contact time of 1&#xa0;h and (ii) ozone process, pH 8, ozone dose of 571.43&#xa0;mg O₃/L, and a contact time of 1&#xa0;h. The molar ratio of chemicals (H2O2 and Fe2+) to 2,4-D concentration; carbon content - C of 2,4D; and chemical oxygen demand (COD) were investigated to find out the most suitable conditions of Fenton/ozone reactions for 2,4-D reduction. Reduction mechanism of 2,4-D by two-step Fenton and ozone reaction was proposed through analyzing metabolites of degradation process. The Fenton process is responsible for organic chlorine radical reduction, while ozonation is for mineralization of metabolite degradation products.</p> Graphical Abstract <p></p>