The photodegradation of parathion, a US EPA Contaminant Candidate List pesticide, in aqueous solutions by UV and UV/H2O2 processes in batch reactors was evaluated. Direct photolysis of parathion both by LP (low pressure) and MP (medium pressure) lamps at pH 7 were very slow with quantum yields of 6.67 ± 0.33 ×10−4 and 6.00 ± 1.06 ×10−4 mol E−1, respectively. Hydrogen peroxide enhanced the photodegradation of parathion through the reaction between UV generated hydroxyl radical and parathion with a second-order reaction rate constant of 9.70 ± 0.45×109 M−1 s−1. However, addition of hydrogen peroxide did not result in a linear increase in the degradation rate. An optimum molar ratio between hydrogen peroxide and parathion was determined to be between 300 – 400. Photodegradation of parathion yielded several organic byproducts, of which the paraoxon, 4-nitrophenol, O,O,O-triethyl thiophosphate and O,O diethyl-methyl thiophosphate were quantified and their occurrence during UV/H2O2 processes were discussed. NO2−, PO43−, NO3− and SO42− were the major anionic byproducts of parathion photodegradation and their recover ratio were also discussed. A photodegradation mechanism scheme suggesting three simultaneous pathways was proposed in the study.

The phototransformation of two organophosphorus pesticides, parathion and chlorpyrifos, by hydroxyl radicals and carbonate radicals in aqueous solution were studied. The second-order rate constants of parathion and chlorpyrifos with hydroxyl radical were determined to be 9.7 ± 0.5 × 109 and 4.9 ± 0.1 × 109 M−1 s−1, respectively. Addition of hydrogen peroxide increased the UV degradation rates of both pesticides and data were simulated through kinetic modeling. The presence of bicarbonate ions reduced the pesticide degradation rates via scavenging of hydroxyl radical but the formation of carbonate radical during the hydroxyl radical scavenging also contributed to the degradation of the pesticides with second-order reaction rate constants of 2.8 ± 0.2 × 106 and 8.8 ± 0.4 × 106 M−1s−1 for parathion and chlorpyrifos, respectively. The dual roles of bicarbonate ion in UV/H2O2 treatment systems, i.e., scavenging of hydroxyl radicals and formation of carbonate radicals, were examined and discussed using a simulative kinetic model. The transformation of pesticides by carbonate radicals at environmentally relevant bi/carbonate concentrations was shown to be a significant contributor to the environmental fate of the pesticides and it reshaped the general phototransformation kinetics of both pesticides in UV/H2O2 systems.

Metolachlor is one of the most widely used herbicides in the world for controlling weeds. It has been detected in both ground and surface waters in the United States, and there are rising concerns in regard to its health risks and in developing effective treatment processes for its removal from water. Degradation of metolachlor via ultraviolet (UV) photolysis and an UV/hydrogen peroxide advanced oxidation process (AOP) was studied. The quantum yield of metolachlor at 254 nm was found to be 0.302 +/- 0.001 mol E-1 through direct UV photolysis in the range of pH 6-8. The second-order rate constant of the reaction between metolachlor and hydroxyl radical was determined to be 9.07 (+/-0.21) x 10(9) M-1 s-1 by using a competition kinetics model that utilized nitrobenzene as a reference compound. In addition, these parameters were successfully applied in modeling the kinetics of elimination of metolachlor using an UV/H2O2 process in both laboratory and natural waters. The formation of several photolysis byproducts was identified using gas chromatography/mass spectrometry, and a scheme for the metolachlor photodegradation pathway is proposed.

No abstract

BackgroundTo study the influencing factors for different healing patterns of patients with idiopathic macular holes (IMH) after vitrectomy surgery performed with the internal limiting membrane (ILM) flap technique.MethodsThis study was a retrospective, consecutive, observational case series study. We recruited 52 IMH patients who underwent vitrectomy with the ILM flap technique. The participants were divided into 2 groups: group A (25 patients), without significant glial cell proliferation in the macular area on postoperative optical coherence tomography (OCT); and group B (27 patients), with significant glial cell proliferation. The postoperative visual acuity (VA), external limiting membrane (ELM) and ellipsoid zone (EZ) recovery characteristics were compared between the two groups.ResultsThere were statistically significant differences in minimum linear diameter (MLD) of the macular hole and postoperative VA (p = 0.02, 2.81 E-4 respectively) between the two groups. Compared with patients in group A, patients in group B had poorer VA and EZ recovery in the first 12 months after surgery, and a longer ELM recovery period. The OCT results showed that patients in group B had more extensive ILM filling in the macular area after surgery than patients in group A.ConclusionThe presence of aberrant glial cell proliferation was related to a larger MLD of the IMH, and the filling approach for the ILM during the operation was related to the postoperative healing pattern and vision acuity.