Sodium Percarbonate Degradation Of Organophosphorus Pesticides

Organophosphorus pesticides (OPPs) are the most worldwide employed pesticides. Their large use, especially for crop protection, may result in the presence of residues of these substances in a wide range of surfaces and ground waters, drinking waters, fruits, vegetables and foodstuff. Pesticides are degraded by chemical and microbiological process. Chemical degradation occurs through reactions such as photolysis, hydrolysis, oxidation and reduction. There is also increasing interest in their transformation products (TPs), because they can present at higher levels in soil than the parent pesticide. Generally, pesticide TPs could show lower toxicity to biota than the parent compounds. In some instances, TPs are more toxic, so they represent a greater risk to the environment than the parent molecules. Comparing to the parents, many TPs have an increase of mobility and even when a TP is less toxic than its parent, it may still have the potential to produce an adverse impact on the environment. As a result, there is a need to consider TPs during the environmental risk-assessment process.Sodium percarbonate (SPC) is a solid compound with exceptional storage stability and no shock sensitivity. It is cheap, non-toxic and readily available, being produced mainly as solid ingredient acting as a source of H₂O₂ in solution for stain bleaching. Due to its storage stability, crystalline nature, ease of handling, and higher hydrogen peroxide contents, it has gained considerable attention in recent years in various chemical transformations. Because of the fast release of hydrogen peroxide from the solid oxidants, SPC has the greatest potential in the practical applications among advanced oxidation technologies and has received more research interest in recent years. So far, few studies were available for the degradation of pesticides using SPC.In this thesis, we first studied the effect of the dry conditions on the stability of the SPC. Then, we investigated the various factors influencing the degradation process and their degradation mechanisms during water treatment by the SPC. The main results were reported as follows:1. Studies were performed on the process parameters for sodium percarbonate synthesis and correlation between characteristic IR Spectra and the active oxygen content. The optimum conditions for the process are as follows :n_Na2CO3:n_H2O2=1:1.44; reaction temperature of 10-15°C; reaction and crystallizingtime of 10-15min; baking temperature below 80°C. The characteristic IR spectra could be used to estimate the active oxygen content. According to the changes of peaks at 954 cm^-1 and 1562 cm^-1, the degree of H₂O₂ decomposition in sodium percarbonate can be evaluated.2. The characters and the influencing factors of the degradation of organophosphorus pesticides by SPC were studied. The result showed that SPC had highly activity in degrading the organophosphorus pesticides. The most efficient degradation of pesticides occurred under base condition and the degradation rates also increased with time extension and temperature enhancement. The degradation was expected to get even better results at a lower initial concentration of pesticides and with more SPC added. The analysis of the by-products suggested that the hydroxyl radical attack might occur in the P=S and P=O function groups. Similar by-products have formed during SPC oxidation of pesticides, which were less toxic to living systems. The experimental results have primarily established an effective and viable method to treat pesticides in wastewater by using oxidation degradation. It has been shown that the application of SPC is useful in environmental degradation studies of pollutants.