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

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 NO2-, NO3-, 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 (H2O2) 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) × 109 M−1 s−1 and (8.6 ± 0.5) × 109 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, CO32- and HCO3- 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 NO3-, photoreactions produced nitrated OP and IBU. A mechanism for OP photolysis in the presence of nitrate is proposed.

► Bimolecular rate constants kOP and kOP1EC were calculated.
► Estimated half-life of OP in Singapore surface waters ranges from 0.6 to 2.5 d.
► A mechanism for OP photolysis in the presence of nitrate was proposed.
► DOC was predicted to be the most important sensitizer of OH.
► Direct photolysis of OP occurs with an insignificant quantum yield.