Estrogen Nitration Kinetics and Implications for Wastewater Treatment

Understanding estrogen-removal mechanisms in wastewater treatment is imperative, as estrogens have environmental effects at trace concentrations. Previous research investigating co-metabolic degradation of 17α-ethinylestradiol (EE2) by ammonia-oxidizing bacteria (AOB) revealed that, in batch tests where high nitrite-nitrogen (NO₂-N) concentrations occurred as a result of ammonia-nitrogen (NH₄-N) oxidation by AOB, an abiotic estrogen nitration reaction actually was occurring—not co-metabolic degradation. This paper addresses nitration kinetics. A first-order abiotic nitration model was developed that predicts nitration of EE2, 17ß-estradiol (E2), and estrone (E1) as a function of temperature, pH, estrogen (EE2, E2, and E1), and NO₂-N concentration. A contact time of 3.6 to 4.1 days is required for 90% estrogen nitration at 500 mg/L NO₂-N and pH 6.4. At 20°C and pH 6.4, the threshold NO₂-N concentration for nitration to occur is 9 mg/L; therefore, estrogen nitration is not likely in activated sludge treatment of domestic wastewater, but has potential for high-NH₄-N-strength wastewaters.