Sorption of steroid estrogens to soils and sediments

Steroid estrogens at sub-micrograms per liter levels are frequently detected in surface water, and increasingly cause public concern of their potential impacts on ecosystems and human health. Assessing the environmental fate and risks of steroid estrogens requires accurate characterization of various physicochemical and biological processes involving these chemicals in aquatic systems. This paper reports sorption of three estrogens, 17beta-estradiol (estradiol), estrone, and 17alpha-ethinyl estradiol (EE2), by seven soil and sediment samples at both equilibrium and rate-limiting conditions. The results indicated that attainment of sorption equilibrium needs about 2 d when aqueous estrogen concentrations (C(t)s) are 25 to 50% of their solubility limits (S(W)S), but equilibrium requires 10 to 14 d when the Ct is 20 times lower than the S(W). The measured sorption isotherms are all nonlinear, with the Freundlich model parameter n ranging from 0.475 to 0.893. The observed isotherm nonlinearity correlates to a gradual increase of single-point organic carbon-normalized sorption distribution coefficient (capacity) (K(OC)) as the equilibrium estrogen concentration (Ce) decreases. At Ce = 0.5S(W), all three estrogens have log K(OC) values of 3.14 to 3.49, whereas at Ce = 0.02S(W), the log K(OC) values for estrone, EE2, and estradiol are within ranges of 3.40 to 3.81, 3.45 to 3.85, and 3.71 to 4.12, respectively. This study suggests that, when at sub-micrograms per liter levels, these estrogenic chemicals may exhibit even slower rates and greater capacities of sorption by soils and sediments.