Diuron or 3-(3, 4-dichlorophenyl)-1, 1-dimethyl urea is one of the most common herbicides that are used to eliminate and control unwanted weed. It is classified to be a carcinogenic and genotoxic compound. Because of its potent toxicity, great chemical stability, and long half-life over 300 days in soil and water in nature, it can slowly penetrate through soil and contaminate both surface and underground water. Contamination of diuron in environment becomes very serious problem, especially in agricultural country.
Our previous work attempted to apply corona discharge to a microreactor as a portable treatment unit for degradation of residual diuron in water. By applying very low turn-on voltage of approximately 1.7 V, corona discharge can be generated between two conductive electrodes within the microchannel. Consequently, reactive hydroxyl radicals, which are used to oxidize diuron, are formed by dissociation of water induced by the corona discharge. To the best of our knowledge, no other work has used the corona discharge in aqueous solution for diuron degradation. In this work, effect of ion contamination on the degradation of diuron via the corona discharge in a microreactor is investigated. Many kinds of ions reside in natural water. However, based on preliminary studies and reports on influence toward the degradation, nitrate, nitrite, sulfate and persulfate ions were chosen to be investigated in this work.
The microreactor used was a 10 mm wide, 21 mm long and 250 µm thick microchannel, formed between a stainless steel emission plate and a graphite collector. A 10 ppm aqueous solution of diuron containing aforementioned ion in the concentration of 0.005 to 0.05 M was supplied into the reactor at controlled speed via a syringe pump. The resident time was varied in the range of 25 to 100 s. The reaction was initiated by supplying direct current across the electrodes to create the corona discharge. The current used as 1 mA. After reaching steady-state, the solution coming out of the reactor was collected and analyzed for the extent of diuron degradation by a high-performance liquid chromatography (HPLC), for total organic carbon (TOC) by a TOC analyzer, and for reaction intermediates by a liquid chromatography equipped with mass spectroscopy (LC-MS/MS).
As expected, greater extent of diuron degradation is achieved by increasing the mean resident time because diuron is exposed to hydroxyl radicals for longer period of time. The formation of the hydroxyl radicals was suggested from hydrogen peroxide detected, which is a product from the recombination of hydroxyl radicals. Addition of anion was found to affect the degradation. In the presence of persulfate ion, electrons in the corona discharge not only induce the dissociation of water into hydroxyl radical, but they also induce the formation of sulfate radical from persulfate ion. The sulfate radical is a strong radical comparable to the hydroxyl radical. The synergy of the sulfate and hydroxyl radicals promotes the degradation of diuron. Diuron was totally degraded within 25 s of resident time if 0.05 M of persulfate ion is presented in the solution, while only 60% degradation was achieved without the persulfate ion. Furthermore, persulfate ion is the only ion investigate that not only enhances diuron degradation, but also improves TOC removal efficiency.
For nitrate, nitrite and sulfate ions, although their effects are not as prominent as that observed from persulfate ion, the efficiency of diuron degradation is still affected. In fact, the diuron degradation is slightly retarded in the presence of nitrate or sulfate ion because of the decrease in electrical field strength by the increased solution conductivity. On the other hand, the degradation of diuron is significantly lowered when nitrite ion is added to the solution because nitrite ion acts as a scavenger for electron and hydroxyl radical.
In addition to the investigation of the effects of anions on the disappearance of diuron, their effect on the degradation pathway was also studied. In addition to common intermediates observed from diuron degradation, 2 new nitrated intermediates were identified when either nitrate or nitrite ion was added to the solution. It is suggested that these intermediates are formed via nitration or nitrosation of diuron by nitrogen dioxide radical or peroxynitrite ion generated by interaction between electron and nitrate or nitrite ion, respectively. It should be noted both nitrogen dioxide radical and peroxynitrite ion are directly formed by fast reaction from nitrite ion, while they are formed via several slow steps from nitrate ion. Therefore, higher concentration of the new nitrated intermediates was observed in the solution added with nitrite ion.
For sulfate and persulfate ions, it is interesting to find that these ions and/or radicals created from these ions do not directly interact with diuron. No intermediate associating with either sulfate or persulfate groups was detected, although enhanced degradation of diuron is achieved in the presence of persulfate ion. It is therefore suggested that sulfate radical interacts with diuron via electron transfer instead of direct substitution or direct addition.
Although nitrate, nitrite, sulfate or persulfate ion is present in the solution, the main pathways of diuron degradation are not altered. In addition to hydroxylation and demethylation, direct reduction of diuron by electron, which is a unique degradation pathway taking place only in corona discharge, was observed.