Biotransformation of nonylphenol polyethoxylate surfactants by estuarine mixed bacterial cultures

Abstract

Biotransformation of nonylphenol polyethoxylates (NPnEOs), a major class of non-ionic surfactants, was studied under laboratory conditions by using a static die-away method. The experiments were performed with autochthonous bacterial cultures that originated from two essentially different estuarine compartments, i.e., from the brackish water and saline water layers of the highly stratified Krka River estuary, Croatia. Highly specific HPLC methods were applied to study biotransformation kinetics of individual NPnEO oligomers in the range of 1–16 EO units. Mixed bacterial cultures from the brackish water layer exhibited a significantly greater ability to transform NPnEO than that from the saline water layer. The results suggest, however, that this difference should not be interpreted as a salinity effect, but rather as a consequence of a better preadaptation of the brackish water bacteria to NPnEOs in their natural habitat. The biodegradation rates, calculated by presuming first-order kinetics, showed a strong temperature dependence. The estimated overall half-lives of NPnEOs under winter temperature conditions (13°C) ranged from 23–69 days, while the half-lives determined under summer temperature conditions (22.5°C) ranged from 2.5–35 days. Transformation kinetics of larger, more hydrophilic, oligomers of NPnEOs was faster than that of the lower ones. In fact, the elimination of higher NPnEOs was followed by a significant formation of the short-chain NPnEOs (nEO = 1–4). The main lipophilic intermediate formed during the experiments was nonylphenol diethoxylate (NP2EO), whose concentration in the medium increased by a factor of 10–20. Its relatively fast accumulation was followed by a rather slow degradation. Consequently, residual concentrations of NP2EO were detected in the medium even after 30 days.