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The influence of solute-solute interactions on hormone retention during nanofiltration (NF) was quantified and mechanisms underlying retention identified. A new approach to predict both the mass of hormone sorbed to organic matter and the retention of hormone influenced by solute-solute interactions was applied. Laboratory-scale experiments were carried out in a cross-flow filtration system examining organic matter concentration, solution pH, and hormone type. Solute-solute interactions between HA and estrone improved estrone retention while decreasing estrone adsorption to membranes. HA concentration determined the amount of estrone bound to HA and hence affected estrone retention based on the mechanism of size exclusion. The solution pH influenced both solute-solute as well as solute-membrane interactions. Solute-solute interactions were most important below the pKa of estrone, whereas charge repulsion between estrone and negative functional groups of the membrane dominated estrone retention above the pKa. Of the four hormones studied, progesterone had the greatest affinity for both HA and NF membrane, which was attributed to hydrogen bonding ability. Using partition coefficients KOM from solid -phase microextraction (SPME) resulted in very good agreement between predicted and experimental retention. [ABSTRACT FROM AUTHOR]