Adsorption and Transport of Trace Contaminant Estrone in NF/RO Membranes
Despite of their low concentration, the impact of steroid estrogens such as estrone, 17 α-estradiol, and ethinylestradiol are often more serious than other synthetic endocrine-disrupting chemicals (EDCs) because their endocrine-disrupting potency is much higher. Although their existence in waterways is of increasing concern, nanofiltration (NF) and reverse osmosis (RO) are likely to play an important role in removal of these compounds. This article describes the mechanisms involved in the retention of estrogen estrone using NF and RO. NF/RO membranes may adsorb trace contaminant estrone, which may result in an initially high retention. If the pore size of the membranes is larger than the estrone molecules, breakthrough can be observed when the membrane adsorptive sites are saturated. Although there is negligible effect of ionic strength on estrone adsorption by the membrane, it is strongly influenced by pH near the pKa value of estrone (10.4). Although static adsorption experiments give a linear adsorption isotherm, higher estrone adsorption in the filtration process with a large pore size membrane (TFC-SR2) suggests that adsorption can also occur inside the membrane active layer. Depending on the pore size to molecule size ratio, the transport of trace organics across the membrane active layer can be described as diffusive and/or convective. A modified diffusion model is proposed to assess the degree of diffusive to convective transport of trace contaminant estrone across the membrane.