Environ. Sci. Technol. 2006, 40, 2225-2234
Sorption and Remobilization Behavior of 4-tert-Octylphenol in Aquatic Systems J. L. ZHOU* Department of Biology and Environmental Science, School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QJ, U.K.
The sorption and desorption of 4-tert-octylphenol in aquatic systems were studied to unravel the underlying controls. The sorption process was relatively slow, reaching a final sorption equilibrium in 10 d. The sorption process was interpreted as consisting of two stages: an initial rapid adsorption on particle surface followed by a slow intraparticle diffusion. The key parameter affecting the sorption of 4-tert-octylphenol by sediment is the occurrence of colloids, which in turn explains the so-called sediment concentration (SC) effect. This was confirmed by the increasing amount of colloids with increasing SC, and the relative constancy of intrinsic partition coefficient of 4-tertoctylphenol between sediment and water (Kp) and between colloids and water (Kc). Further evidence was from the nonoccurrence of SC effect from the addition of the same amount of colloids in different SC. The adsorption equilibrium is best described by the Freundlich model at low equilibrium concentrations. The adsorption of 4-tertoctylphenol was enhanced in the presence of salts, due to the salting out effect, and a salting constant of 1.3 L/mol was obtained. Desorption experiments showed that the release of 4-tert-octylphenol from contaminated sediments was highly dependent on the “age” of sediments, with kinetics of desorption being much faster in fresh sediments than in “aged” sediments.
Introduction Endocrine disrupting chemicals (EDCs) are currently a priority in environmental research, due to their long-term impacts, wide range of biological effects, and diversity. Of the list of EDCs, 4-tert-octylphenol is receiving increasing attention due to its widespread occurrence in the aquatic environment as a result of the discharge of its parent compounds, alkylphenol polyethoxylates (1-2). In the U.K. it is estimated that around 6500 tons of alkylphenol polyethoxylates are discharged into its aquatic systems every year (3). As a result of the disposal of such large quantities, the concentration of 4-tert-octylphenol at some sites may reach levels that can cause estrogenic effects in fish. In addition, 4-tert-octylphenol has been shown to be the most estrogenic of the breakdown products of alkylphenol polyethoxylates (4), and is up to 5 times more estrogenic than nonylphenol (5). Finally, primary biodegradation of alkylphenol polyethoxylates results in the formation of more persistent and toxic metabolites such as alkylphenols,; thus they pose a long-term threat to the aquatic ecosystems and human health. * Phone: 44 1273 877318; fax: 44 1273 677196; e-mail: j.zhou@ sussex.ac.uk. 10.1021/es052002v CCC: $33.50 Published on Web 02/08/2006
2006 American Chemical Society
With a relatively high octanol-water partition coefficient (log Kow ) 4.12), 4-tert-octylphenol is expected to behave as most other hydrophobic organic contaminants in terms of interactions with sediments and persistence (6, 7). In a comprehensive survey, Ahel et al. (2) found high concentrations of nonylphenol and its derivatives in sediments (up to 25 µg/g) from some of the most polluted sites in the River Glatt of Switzerland. They also found significantly higher concentrations of these compounds in muddy sediments rich in organic matter than in sand at the same location, and attributed this to the importance of physicochemical processes such as sorption to sediments. In studying the partition of 4-tert-octylphenol under laboratory conditions, Johnson et al. (6) obtained Kp in the range 6-700 L/kg, and organic carbon normalized partition coefficient (Koc) in the range 3500-18 000 L/kg. Although they observed a strong relationship between Kp and organic carbon content of sediment, significantly higher Koc (82 000-390 000 L/kg) values were obtained for suspended particles of the same location. They tentatively attributed such enhancement in sorption to the fact that suspended particles were mainly composed of organic aggregates. However, such inference is subject to verification and further research is needed to understand the underlying controls of the sorption of 4-tert-octylphenol. Recent research also showed that aquatic colloids played a key role in the environmental behavior of organic micropollutants such as phthalate esters (8), polycyclic aromatic hydrocarbons (PAHs; 9), and EDCs such as estrone and 17βestradiol (10-12). Due to limited data available for assessing the distribution and movement of alkylphenols such as 4-tertoctylphenol in aquatic systems, there is an urgent need to perform detailed investigations of the sorption of 4-tertoctylphenol in aquatic environments. This study therefore aims to focus on the interactions of 4-tert-octylphenol with aquatic sediments, by studying the kinetics of the interactions, extent of its sorption in sediments, effects of environmental conditions (e.g., colloids, SC) on its sorption, and desorption potential.
Materials and Methods Reagents and Chemicals. Pure 4-tert-octylphenol standard was purchased from Aldrich, U.K. A stock solution was prepared by dissolving 0.1 g of 4-tert-octylphenol in methanol (0.1 L) to obtain a final concentration of 1000 mg/L. The stock solution was kept in a freezer unless in use. Glassdistilled solvent methanol from Rathburn Chemicals Ltd, Peeblesshire, Scotland, was used in the extraction of 4-tertoctylphenol from sediment particles and glass containers. Ultrapure water was obtained from a Maxima Unit from USF Elga, U.K. Water and Sediment Samples. Sediment and water samples were collected from River Ouse and Southwick Beach, England, and used within 2 weeks. The river and marine end-members were collected in glass bottles (2.5 L), and sediments were collected in pre-ashed glass jars. After returning to the laboratory, water samples were filtered through muffled (450 °C, 4 h), 47-mm GF/F filter papers and stored at 4 °C. The sediment samples were wet-sieved using filtered river water to obtain particles
