Elsevier

Environmental Pollution

Volume 157, Issue 2, February 2009, Pages 698-703
Environmental Pollution

Sorption of alkylphenols on Ebro River sediments: Comparing isotherms with field observations in river water and sediments

https://doi.org/10.1016/j.envpol.2008.08.007Get rights and content

Abstract

This study reports sorption isotherms of the endocrine disruptors nonylphenol (NP) and octylphenol (OP) in three sediment samples from the Ebro River basin (NE Spain), with organic carbon fractions (fOC) ranging from 0.0035 to 0.082 gOC g−1. All isotherms were fitted to the Freundlich model with slightly nonlinear exponents ranging from 0.80 to 0.94. The solubility of the compounds as well as the organic carbon (OC) content had the strongest influences on the sorption behavior of these compounds. Comparison of the laboratory-spiked samples with the native contamination of NP of 45 water and concurrent sediment samples resulted in reasonable matches between both data sets, even though the lowest concentrations in the field were not completely reached in laboratory tests. This good agreement indicates that sorption laboratory data can be extrapolated to environmental levels and therefore the distribution of nonylphenol between sediments and water can be predicted with a precision of one order of magnitude. Furthermore, laboratory experiments with simultaneous loading of NP and OP revealed negligible competition for sorption sites at low concentrations.

Introduction

Alkylphenol polyethoxylates are non-ionic surfactants used in a large variety of industrial and domestic applications (i.e. pesticide formulations and plastic additives) (Maguire, 1999, Hou et al., 2006) that can degrade to alkylphenols (APs). The environmental occurrence of APs such as nonylphenol (NP) and octylphenol (OP) has been investigated since the late 1970s (Sheldon and Hites, 1978, During et al., 2002) and their pervasive use, persistency and accumulation properties have led to a widespread distribution in various matrices (Dachs et al., 1999, Petrovic et al., 2002, Vazquez-Duhalt et al., 2006, Correa-Reyes et al., 2007). NP is present in the environment as a mixture of various 4-nonylphenol isomers (Isobe et al., 2001) whereas OP is found dominantly in the form of 4-tert-octylphenol. In the early 1990s, these compounds were shown to cause estrogenic activities (Moeder et al., 2006).

Most commonly APs reach the environment through the discharge of municipal and industrial wastewater treatment plants (WWTP) (United States Environmental Protection Agency, Office of Water, 2005). In most receiving aqueous systems this leads to a net accumulation, particularly in sediments (Petrovic et al., 2002). For instance, data from 2003 reported NP and OP in Ebro River sediments (Spain) at levels between 0.03 and 2.33 mg kg−1 (dry weight) (Petrovic et al., 2002). This survey led to a more detailed monitoring campaign between 2004 and 2006 (unpublished data), during which NP and OP were detected in various sampling points along the whole Ebro River and some of its tributaries. In these surveys, NP reached concentrations of 0.025 mg L−1 in water and 6 mg kg−1 in sediment (dry weight). Moreover, OP was also identified at usually ∼10 times lower concentrations than NP.

The above field monitoring data of the Ebro offer an ideal background to further investigate the environmental fate of NP and OP and helped to elucidate their sorption characteristics in sediments that can be transferred to other river basins. This study is timely, because sorption of APs has not been well investigated in sediments. In addition laboratory sorption data obtained under well-controlled boundary conditions have to our knowledge never been linked with field data from monitoring campaigns. Sorption—and thus the fate of APs—depends on several environmental parameters such as organic carbon (OC) content. Flow dynamics of the water may also control equilibrium conditions, and thus sorption kinetics. Considering a mobile system, pore water plays an important role, as sorption equilibrium is more easily achieved between it and the sediment matrix (Kraaij et al., 2003). With most of these factors being variable, the extrapolation of laboratory experiments to field conditions remains challenging. In addition, sorption experiments are often conducted at concentration levels that are orders of magnitudes higher than those occurring in the environment and thus the extrapolation of sorption isotherm data to field concentrations produce a source of uncertainty in risk assessment. This is due to the possible different sorption behavior of a compound at high and low concentrations. When solution concentration increases, nonlinearity should be expected (Weber et al., 1992) as it occurs because the affinity for solute decreases progressively with increasing solute concentration sites become filled (Xing et al., 1996). Sorption capacity of the various constituents of the organic matter also needs to be taken into account. For instance, black carbon is considered to have 10–1000 times more sorption strength than other forms of OC (Cornelissen and Gustafsson, 2006) and therefore is believed to be responsible for a large part of the sorption of some hydrophobic organic compounds in soils and sediments (Gustafsson et al., 1997, Accardi-Dey and Gschwend, 2002). Furthermore, competitive sorption with co-occurring contaminants may also affect sorptive behavior, leading to over- or under-estimation of the sediment capacity to retain APs (Pignatello, 1998), due to the fact that competition is the result of overlap in the set of sites that can be occupied by non-identical solutes (Xing et al., 1996).

The primary objective of this study was therefore to investigate the equilibrium sorption behavior of NP and OP in sediments from the Ebro River. The role of organic carbon content on sorption of NP was also considered and samples that represented the range of organic carbon concentrations in Ebro sediments were chosen. With this, equilibrium constants (KOC values) derived from these laboratory experiments were compared with those from field data. In our study we extended sorption isotherms towards low concentration ranges that reach environmentally occurring maximum loadings. Furthermore, as OP and NP occur mostly together in the environment, the effect of simultaneous and potentially competitive sorption of these two compounds was investigated in this study.

Section snippets

Sorbents and chemicals

Sediments were sampled along the Ebro River Basin (NE Spain) in October 2006 (third sampling campaign). Sediments were collected using a Van Veen drag from the middle of the river, in order to obtain a representative sample. All sediments were characterized for their OC content and a suite of organic pollutants. Out of the 22 sediments sampled, three were selected for sorption studies: (i) from the Ebro Spring in Miranda de Ebro (R0), (ii) from the Araquil, a tributary of the Ebro (T8), and

Sorption isotherms

The Freundlich parameters were calculated by regression analyses using the equation CS=KFCW1/n, where CS is the concentration of a chemical adsorbed in the solid phase (mg kg−1); CW is the equilibrium solution concentration (mg L−1); KF is the Freundlich coefficient ((mg kg−1)/(mg L−1)1/n) and 1/n the Freundlich exponent, and are summarized in Table 1. This approach fitted the experimental data well and produced squared correlation coefficients (R2) higher than 0.97. The isotherms were found to be

Conclusions

Overall, our results indicate a high linearity of alkylphenol sorption in sediments. Variations in the data sets are primarily related to the solubilities of the compounds and to the OC content of the sorbents, as could be shown by much reduced variability after normalization to these factors. Competitive sorption between OP and NP was tested but was shown to cause only a reduced effect for OP on concurrent environmental behavior. We also found consistency with other data on sorption from both

Acknowledgements

We are indebted to Prasesh Sharma for performing DOC analysis and to Bernice Nisch and Renate Seelig for technical support. This research project was funded by the European Union under the Global Change and Ecosystems (FP6) Water Cycle and Soil Related Aspects (AquaTerra, Project number 505428 GOCE) and financial support from the Spanish Ministry of Education and Science, project number CTM2005-25168-E. A.N. agrees the support by two grants of the Departament d'Universitats, Recerca i Societat

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