The degradation of 2, 4-Dichlorophenoxyacetic acid herbicide by advanced oxidation system

Abstract

2,4-Dichlorophenoxyacetic acid is a common herbicide used widely in agriculture and gardening. It is a concerned environmental pollutant because of its high solubility in water so that it will not be adsorbed on the topsoil but may leach down into ground water and then contaminate the natural water resources. The biodegradability of this pollutant in natural environment is very low. There are many remediation methods for the removal of 2,4-D in aqueous solutions. UV/Fenton is one of the effective processes but there is still room for improvement. The aim of this project is to examine the interactions of Fe2+ and Fe3+ on the iron-catalyzed oxidation. This study is environmentally significant because soils have a high content of iron which is necessary to initiate a series of photochemical reactions upon sunlight irradiation. Carboxylic acids, commonly found in soils, such as oxalate combine with Fe2+ and Fe3+ to form photosensitive organometallic complexes which can further enhance the oxidative reaction. The transformation of 2,4-D by various iron-mediated processes were conducted and compared. When the reaction was initiated by ferrous oxalate or ferrioxalate instead of Fe2+ or Fe3+ ions, the rates were significantly improved, because of the higher light sensitivity of the organometallic complexes. The wavelength effect on the iron catalyzed oxidation was also investigated. The initial decay rates and the total removal percentages of 2,4-D generally followed the order: UV253.7 > UV300> UV350> UV419. Among all combinations, the Fe2+/oxalate/H2O2/UV system was the most effective and efficient treatment process for 2,4-D; therefore, the kinetics dependence of pH, oxalate, and hydrogen peroxide concentrations on the degradation performance of the 2,4-D was studied. A rate enhancement of about 2.9 times was found when 1.2 mM of oxalate was added to the conventional Fe2+/H2O2/UV process. However, excess oxalate suppressed the reaction due to the scavenging and light attenuation effects. The intermediates such as 2,4-dichlorophenol, 2-chloro-4-hydroxyphenoxyacetic acid, 4-chloro-2-hydroxy-phenoxyacetic acid, 5-hydroxy-2,4-dichlorophenoxyacetic acid, and 6-hydroxy-2,4-dichlorophenoxyacetic acid had been identified and confirmed by LC-MS. In order to find another metal complex comparative to ferrous oxalate, 2,4-D was degraded by ferrous-mediated process in the presence of different ligands such as acetate, formate, citrate, malelate, and EDTA. Hydrogen peroxide was found to be photo-produced by the photolysis of ferrous oxalate or ferrous citrate. As such, the transformation of 2,4-D by the Fe2+/oxalate/H2O2/UV system could be operated in two steps: the photolysis of ferrous oxalate first, followed by adding the spiked H2O2 sometime after the commencement of the reaction. A new approach combining sorption and an advanced oxidation process had been developed for wastewater treatment. Ferric ions were incorporated into the anion-exchange resin with oxalate to synthesize a ferrioxalate-exchanged resin (FOR). The optimal result was obtained in the treatment of 2,4-D with FOR when the system was irradiated at 350 nm with 1 mM H2O2, at which 80 % of the 2,4-D disappeared after an operation time of 60 minutes. Sorption and radical oxidation were found to be the major reaction pathways for the removal of 2,4-D. Similarly, ferrous ions were immobilized on the cation-exchange resin, CERF, to catalyze the photochemical oxidation of 2,4-D. The co-existence of UV light and oxalate was the key to initiating the attack by hydroxyl radicals of 2,4-D in the presence of CERF and H2O2. Besides, citrate and EDTA were shown to drive the CERF-induced photooxidation that was more favorable in the acidic environment. The solid catalyst could easily be separated from the solution by filtration after the reaction, and then recycled. The effect of oxalate concentration on the recycling efficiency of the catalyst was studied. The removal rate was strongly dependent on the concentration of the oxalate.