Degradation of 2,4-dichlorophenoxyacetic acid by UV 253.7 and UV-H2O2: Reaction kinetics and effects of interfering substances

Abstract

This work investigates the degradation of 2,4-dichlorophenoxy acetic acid (2,4-D) using UV irradiation and the UV-H₂O₂ advanced oxidation process (AOP). For UV irradiation at 253.7 nm, ∼66% degradation was observed for a fluence of 20 J cm⁻² and the apparent fluence-based, pseudo-first-order rate constant for 2,4-D was 5.77 (±0.66) × 10⁻⁵ cm² mJ⁻¹. With the UV-H₂O₂ AOP, approximately 97% degradation was observed for a fluence of 700 mJ cm⁻². Due to production of hydroxyl radicals, the apparent fluence-based rate constant was 100 times higher than that for direct UV photolysis. The effects of H₂O₂ dose, initial 2,4-D concentration, and water quality parameters, including pH (4–8), alkalinity (0–5 mM HCO3−), nitrate concentration (0–1 mM as NO3−), and ionic strength (0–17 mM as NaCl), were studied. The observed rate constants were dependent on pH, alkalinity, and nitrate concentration. The degradation of 2,4-D by the UV-H₂O₂ system was also examined in a real surface water. The observed fluence-based rate constant in the surface water matrix was 2.6 (±0.3) × 10⁻³ cm² mJ⁻¹, and this value was similar to a distilled water matrix containing the same alkalinity and pH. In addition, the biodegradability of UV and UV-H₂O₂ treated wastewater increased with irradiation time, suggesting that transformation products can be degraded by biological processes. Based on this study, the UV-H₂O₂ process represents a viable treatment method to transform 2,4-D into benign products.