Highly efficient removal of DEET by UV-LED irradiation in the presence of iron-containing coagulant

N,N-Diethyl-3-methyl benzoyl amide (DEET) has been detected as an emerging pollutant in various water bodies because of its widespread use as an insect repellent. In this study, the combination of UV-LED 275 and iron-containing coagulant (FeCl 3 ) was used for the elimination of DEET in water. It was found that UV-LED 275 /FeCl 3 (98 %) system presented a favorable removal of DEET compared with UV 254 /FeCl 3 (59 %) and UV-LED 275 /Fe 2 (SO 4 ) 3 (81 %) processes at initial pH 3.5. DEET degradation by both UV-LED 275 /FeCl 3 and UV-LED 275 /Fe 2 (SO 4 ) 3 processes followed pseudo-first-order kinetics with the calculated pseudo-first-order rate constants ( k obs ) of 0.0105 and 0.0046 cm 2  mJ −1 , respectively. The results of ESR analysis and radicals quenching experiments indicated that hydroxyl radicals ( OH) and superoxide radicals (O 2 − ) were responsible for DEET degradation in UV-LED 275 /FeCl 3 process, and the former played the major role. An increase in FeCl 3 dosage was beneficial to the degradation. In the UV-LED 275 /FeCl 3 process, DEET degradation increased with a decrease in pH from 3.5 to 3.0, whereas it was almost completely suppressed with an increase in pH from 4.3 to 6.3. DEET degradation was almost unchanged after the introduction of NO 3 − , and it impeded after the addition of humic acid (HA), HCO 3 − , and SO 4 2− . The plausible degradation pathway mainly involved hydroxylation, cleavage of the C–N bond, acetylation , and dealkylation. Among the disinfection by-products (DBPs) evaluated, UV-LED 275 /FeCl 3 pretreatment generally increased the generation of trichloromethane, chloral hydrate, dichloroacetic acid, and trichloroacetic acid, which implied that further assessment of environmental risk was needed during its practical applications.