Photodegradation of etridiazole by UV radiation during drinking water treatment

The photodegradation of etridiazole (ETZ) in water by UV radiation at 254 nm was investigated. Results indicate that the simulated first-order rate constants decreased with the increase of initial ETZ concentration (i.e., 5, 20 and 30 μM), and did not show any pH dependence within the range from 6.0 to 8.0. The quantum yield was 0.46 ± 0.02 mol E^-1 at pH 7.0. H₂O₂ was generated at trace levels in the range from 0 to 1.0 μM during photodegradation of ETZ. Direct photodegradation was responsible for the decomposition of ETZ in distilled water by UV radiation. Three organic byproducts were identified: 5-ethoxy-3-dichloromethyl-1,2,4-thiadiazole, 5-ethoxy-1,2,4-thiadiazole-3-carboxylic acid and 5-ethoxy-3-hydroxyl-1,2,4-thiadiazole. About 90% of chloro mass in the initial ETZ was released as Cl^- at the end of photodegradation. In contrast, the formation of sulfate and nitrate was insignificant. In general, ETZ decayed more quickly in groundwater than in sand-filtered or surface water. It is reasonably deduced that ETZ may not get removed effectively under a typical UV dose of 40 mJ cm^-2 at most water treatment plants that employ UV radiation for disinfection.