Photodegradation kinetics of p- tert-octylphenol, 4- tert-octylphenoxy-acetic acid and ibuprofen under simulated solar conditions in surface water

Abstract

Direct and indirect (sensitized) photolysis of p- tert-octylphenol (OP), 4-octylphenoxy-acetic acid (OP1EC), and ibuprofen (IBU) were investigated in laboratory water and surface water under simulated and natural sunlight conditions. Photodegradation obeyed apparent-first order kinetics with rates increasing in the presence of NO 2 - , NO 3 - , and humic acid (HAC). The bimolecular rate constants, k″, were determined for the reactions of OP and OP1EC with hydroxyl radical ( OH) using photolyzed hydrogen peroxide (H 2O 2) as the hydroxyl radical ( OH) and IBU as the reference compound. The k″ values for OP and OP1EC were (average and standard deviation) (10.9 ± 0.5) × 10 9 M −1 s −1 and (8.6 ± 0.5) × 10 9 M −1 s −1, respectively. Direct photolysis of OP is small with a quantum yield of 0.015 in the range of 285–295 nm. Based on laboratory and average solar intensity data, the estimated half-life of OP in different Singapore surface waters was estimated to range from 0.6 to 2.5 d. The steady state hydroxyl radical concentration ([ OH] ss) was estimated using a kinetic model that considered dissolved organic carbon compounds (DOC), nitrate, and nitrite as OH sources, and DOC, CO 3 2 - and HCO 3 - as scavengers. In surface waters containing DOC 2.3–6.5 mg L −1, nitrate 0–3.2 mg L −1, and nitrite 0–2.5 mg L −1, the calculated [ OH] ss ranged from 5.2 × 10 −15 to 9.6 × 10 −15 M. Half-lives calculations based on this model underestimated the measured half-life by a factor of approximately 4.2 to 1.1. DOC was predicted to be the most important sensitizer except in a sample that contained relatively high nitrate and nitrite. In the presence of NO 3 - , photoreactions produced nitrated OP and IBU. A mechanism for OP photolysis in the presence of nitrate is proposed.