Adsorption of ethinylestradiol (EE2) on polyamide 612: Molecular modeling and effects of water chemistry

This study demonstrates that ethinylestradiol (EE2), a priority estrogenic contaminant in water, can be rapidly and selectively removed from aqueous solutions using industrial-grade polyamide 612 (PA612) particles as adsorbents. Isothermal studies showed that nonporous low surface area (20 m² g^-1) PA612 particles had a maximum adsorption capacity of 25.4 mg g^-1 for EE2 in water, which is higher or comparable to the results obtained with two benchmark activated carbon (AC) adsorbents (10.4-27.6 mg g^-1). The adsorption of EE2 on PA612 followed pseudo-second order kinetics with a high adsorption rate exceeding those of the ACs by 5.3- to 22.4-fold. Computational chemistry calculations and molecular modeling showed that the strong binding affinity between EE2 and PA612 originates from the hydrophobic partitioning of EE2 solutes and hydrogen bonding interactions on PA612 amide groups. PA612 showed high adsorption selectivity for EE2 in water with highly consistent adsorption capacities for EE2 under the influence of a range of water chemistry parameters, including water salinity (NaCl, 1 mM-1 M), metal ions (K(I), Ca(II), and Zn(II); 0.1 M), natural organic matter (humic acids, 0.2-5 mg L^-1), and pH level (4.8-9.1).