Isomer-Specific Biodegradation and Chemical Oxidation of Nonylphenol
- Author(s): Lu, Zhijiang
- Advisor(s): Gan, Jay
- et al.
Nonylphenol (NP), a well-known environmental estrogen with numerous isomers, is commonly treated as a single compound in the evaluation of its environmental occurrence, fate and transport, treatment removal and toxicity. Recent studies showed that NP isomers exhibited different estrogenicity and biodegradability. However, at present little systematic information is available on its isomer-specific biodegradation and chemical oxidation under natural and engineered conditions.
We comprehensively evaluated isomer selectivity in biodegradation in river sediments and during secondary wastewater treatment processes. In a well aerated river sediment, half-lives of NP isomers ranged from 0.9 to 13.2 d. The overall removal of tNP was efficient during the wastewater treatment processes simulated in a laboratory-scale conventional activated sludge bioreactor, ranging from 90 to 99%. Isomers with short side chain and/or bulky α-substituents were found to be more recalcitrant to degradation and followed the order of dimethyl >= ethylmethyl > methylpropyl >= iso-propylmethyl. Moreover, steric effect index and the degree of branching as quantified by IDWbar were identified to correlate closely with isomer biodegradability.
We also considered isomer selectivity during oxidation of NP by potassium permanganate, a promising treatment for NP-containing effluent, drinking water and ground water. The removal of NP isomers by KMnO4 was efficient during pH range 5-7. At pH 7 with 10 mg/L of KMnO4 and 50 µg/L tNP, the half-lives of 19 isomers varied from 4.8 to 6.3 min. In general, the reactivity followed the order: α-dimethyl > α-ethylmethyl ≈ α-methylpropyl ≈ α-iso-propylmethyl.
We further considered the role of MnO2 oxidation in the natural attenuation of NP and its structure analogues in soils. At pH 5.5 and 100 mg/L δ-MnO2, 92, 84 and 76% of 4-n-NP, 4-tert-octylphenol and tNP were transformed in 90 min, respectively. Multiple reaction products, including hydroquinone, hydroxylated products, dimers and trimers were identified, allowing the construction of transformation pathways.
These results together suggest that isomer selectivity commonly occurs before and after the release of NP to the environment, and that knowledge of isomer-specific behavior of NP contributes to improved understanding and management of NP as an important environmental contaminant and endocrine disruptor.