Diuron [3-(3, 4-dichlorophenyl)-1, 1-dimethyl urea] is a herbicide widely used for weed elimination in Thailand. It is classified to be a carcinogenic and genotoxic compound. Diuron is a bio-recalcitrant compound with great chemical stability. Hence, it can slowly dissolve in water and can penetrate through soil to contaminate underground water as well as surface water. Due to its persistence with half-life over 300 days in nature and its potent toxicity, diuron contamination becomes very serious environmental problem.
Many methods for the removal of residual contaminant in water have been proposed. Among them, heterogeneous photocatalytic degradation is one of the promising technologies because it is inexpensive, environmental friendly and sustainable. The process relies on oxidation of the pollutants by highly active hydroxyl radicals produced from the semiconductor catalyst that is activated by light. It can use sunlight or UV light which is available in abundance as the energy source. The final products of the treatment are usually harmless compounds such as carbon dioxide, water and inorganic ions. Nevertheless, limitations of the conventional photocatalytic process include high mass transfer resistance for the diffusion of the contaminant to the surface of the catalyst and low light penetration throughout the reactor.
Microreactor is one alternative that can be used to solve such problems. Unique features of the microreactor include short molecular diffusion distance, large surface-to-volume ratio, and high spatial illumination homogeneity with excellent light penetration throughout the reactor if it is applied for the photocatalytic reaction. In this work, the photocatalytic degradation of diuron in a microreactor is investigated. Not only the effects of common variables such as resident time and thickness of the micro-channel on kinetics of the degradation are studied, but the reaction pathway is also reported. More importantly, to the best of our knowledge, this is the first report on involvement of light on the degradation pathway. Degradation intermediates formed under exposure of light and those formed without the need of light are systematically investigated.
The microreaction system in this work is fabricated in plate-like manner. A piece of soda-lime glass that was previously deposited with P25 titanium dioxide particles via spin coating technique was put together with another piece of glass that was drilled for inlet and outlet streams. A layer of Teflon sheet with 0.8 cm by 4.8 cm opening was placed in between the glass plates, hence forming a channel. The thickness of the channel was determined by the thickness of the Teflon sheet, which was varied in the range of 200 to 750 μm. A 10 ppm diuron aqueous solution was constantly supplied into the reactor via a syringe pump. Flow rate of the solution was varied in from 0.4 to 5.8 ml/h, which was corresponding to resident time of 15 to 1 min. At the first 1 h, the reactor was kept in the dark so that adsorption of diuron onto the catalyst was complete. Then, the photocatalytic reaction was initiated by exposing the reactor to light from a UV-A lamp. The product coming out of the reactor was periodically collected and analyzed for diuron concentration by a reverse phase high performance liquid chromatography (HPLC). The degradation intermediates were also identified using a liquid chromatography equipped with mass spectroscopy (LC-MS/MS). For an in-depth investigation on the involvement of light on the type of intermediates formed, strips of black tape were placed on top of the reactor to control light exposure. Three configurations, i.e., ON-OFF-ON, ON-ON-OFF, and ON-OFF-ON-OFF-ON were used. Then, the intermediates were analyzed and compared.
Due to chemical stability of diuron, degradation via photolysis is negligible. On the other hand, when the photocatalyst was present in the system, significant degradation was observed. The extent of diuron degradation is increased as the mean resident time in the reactor is prolonged. It was found that diuron was degraded by 63% after being in the reactor for 15 min. Although the pseudo first order model could well represent the degradation kinetic, the results were better fitted when all transport phenomena was included in the model. Nevertheless, by analyzing the degradation results from varying the thickness of the micro-channel, it was found that mass transfer resistance in the reactor could be neglected as long as the thickness of the channel was smaller than 500 mm.
Several kinds or degradation intermediates were detected by LC-MS/MS. Most intermediate products were associating with the attack by hydroxyl radical at various functional groups in the diuron molecule. By analyzing structures of the intermediates, the degradation pathway was proposed. At the first stage of the degradation, the pathway is consisted of three main routes, i.e., substitution of a chlorine atom on the aromatic ring by hydroxyl group, oxidation and/or demethylation of the alkyl side of diuron molecule, and simultaneous substitution of a chlorine atom and cleavage of amino group in the alkyl chain. The following steps involve further hydroxylation and ring opening reactions.
It was interesting to observe that new intermediates were formed when the center part of the reactor was masked. Without the light, hydroxyl radicals are not generated. Therefore, the results suggest that the intermediates firstly formed by the photocatalytic degradation of diuron in the first section of the reactor, where it was exposed to light, can interact further with each other, with water, or with hydroxyl ion to form other intermediates without the assistance of light. The reactions that can take place without the light include elimination of methyl group from the aliphatic part of the molecule, hydroxylation of the aliphatic functional groups and conjugation of the intermediates to form larger molecule. The later has never been reported elsewhere.