Irrigation of farmland consumed approximately 80% of total water usage in California. The costs associated with groundwater monitoring and cleansing have cost the state more than $33 millions of dollars. This research project has significant economic impact to the California agricultural industry and consumers by helping clean up surface and ground water contaminated by pesticides.
Contamination of California ground and surface water by pesticides is an important environmental issue that affects California welfare. It has been reported that a total of 192 million pounds and 186 million pounds of pesticides were used for agricultural practice in California in 2011 and 2012, respectively, among them Propanil is in the top 100 pesticides used list. Because pesticides are relatively mobile and persistent in soil they can penetrate into ground water following their soil application by farmers. As a result, Propanil has been detected in surface and ground water in many areas of California with various concentrations. Crops including vegetables and fruit could be contaminated if the water containing pesticides is reused for irrigation. It is important to remove pesticides from the irrigated water prior to its reuse.
Propanil water samples with concentration at ppm (part per million) level are placed into a beaker containing trace amount of hydrogen peroxide and a glass plate coated with nanoparticles. The samples are then exposed to visible light radiations coming from either regular light bulb or the Sun, and the change of the Propanil concentration was monitored using ultraviolet-visible spectroscopy. Cataphotolysis of Propanil molecules takes place at the surface of nanoparticles, on which the reduction-oxidation reactions occur, leading to oxidative degradation of the Propanil molecules. The Propanil degradation is indicated by the absorbance decrease of the ultraviolet-visible spectrum of Propanil at 248 nm. The product of cataphotolysis is probed using liquid chromatography coupled with mass spectrometry (LC-MS), and the acidity of the water sample was measured using a pH meter. Kinetics information is acquired by monitoring the decay of Propanil as a function of time during its cataphotolysis.
Propanil molecules were found to undergo decomposition during cataphotolysis when the water samples are radiated by visible light either from light bulb or from the Sun. The sunlight was found to increase the degradation rate in comparison to the visible light from the light bulb. The cataphotolytic degradation of Propanil in water follows the first order chemical kinetics, with a rate constant of k = (4.29±0.49) x 10-3 s-1. No detectable organic products were found from cataphotolysis of Propanil in the LC-MS examination of the water samples. The acidity of the water sample increases, suggesting that the Propanil molecules are oxidized into carbon dioxide, which dissolved in water to form carbonic acid, giving rise to lower pH value than the neutral water.