(128a) Solar Photocatalytic Degradation of 2,4-D at Semi-Pilot Scale in a CPC Reactor Using TiO2

The 2,4-dichlorophenoxyacetic acid (2,4-D) is a broad-spectrum herbicide of chlorophenoxy type, and is used worldwide on a large scale for general weed control on grain crops, pastures, and non-crop areas. It is a pollutant of great environmental concern because of its relatively high water solubility and mobility in aquatic systems. In order to avoid dangerous accumulation, the removal of this kind of herbicides from aqueous solutions has been investigated using powerful oxidation methods to achieve the degradation and mineralization of these compounds.

In recent years, advanced oxidation processes (AOPs), which are all characterized by the production of ^â?¢OH radicals, have received considerable attention for herbicides degradation because they are able to oxidize and mineralize almost any organic molecule, yielding CO₂ and inorganic ions.

Among the AOPs, heterogeneous photocatalysis is currently recognized as an effective technique to treat recalcitrant pollutants, such as 2,4-D in aqueous environments. Heterogeneous photocatalysis is based on direct or indirect absorption of radiant energy (visible or UV) by a solid (photocatalyst), which is typically a wide band gap semiconductor. Anatase-TiO₂ is the most studied catalyst and it is capable of producing electronic transitions by absorption of light in the near ultraviolet region (UV).

The use of solar irradiation to power this process is environmentally attractive, and has a potential to reduce the cost of this technology. In addition to small scale studies, several pilot plant experiments have been performed in order to test the applicability of solar photocatalysis for wastewater treatment. One of the best options of reactors for photocatalytic applications using sunlight is the compound parabolic collector (CPC) reactors. The advantages of CPC reactors are: the possibility of using solar UV radiation coming from all directions in the sky (global UV radiation), simplicity of construction and operation, turbulent flow regime (which improves mass transfer), and high reduction of the vaporization of volatile pollutants.

In this research, the solar photocatalytic activity of Degussa P25 TiO₂ was investigated in the degradation of 2,4-D herbicide as a commercial formulation Diamond® using a solar CPC reactor. The photocatalytic tests were performed in 30 L of water containing 50 mg/L of herbicide concentration. In order to obtain optimized conditions for the photocatalytic degradation, a fractional factorial experimental design was conducted, the investigated variables were catalyst dosage (0.3, 0.6, and 0.9 g/L), and flow in the reactor (2, 3, and 5 gal/L). The decay in herbicide concentration was followed by reversed-phase chromatography and the mineralization was followed by measurement of the total organic carbon (TOC). The best conditions were 0.6 g/L of catalyst loading and flow of 2 gal/L achieving almost complete degradation (98.68%) and 80% of mineralization of commercial 2,4-D with an accumulated radiation of 120 kJ/m². These results demonstrate the effectiveness of treatments environmentally friendly in order to degrade recalcitrant compounds in wastewaters.