(666d) Adsorption of Organic Micropollutants to Polymer Surfaces Probed By Second Harmonic Scattering Laser Spectroscopy

Modern thin-film composite reverse osmosis membranes suffer from poor rejection of organic micropollutants, including pharmaceuticals, endocrine disruptors, and disinfection byproducts. The physico-chemical interactions between organic solutes and the polymer membrane surfaces are poorly understood, but may play a significant role in determining membrane rejection.

The purpose of this study was to quantitatively measure the adsorption of several organic micropollutants and surrogate molecules to a model polymer surface. To begin developing a quantitative understanding of the interactions between organic micropollutants and polymer membrane surfaces, angle-resolved second harmonic scattering laser spectroscopy was used to probe the adsorption of resonant aqueous cationic and anionic dye molecules to polystyrene surfaces. Calculation of the Gibbs free energy for adsorption of malachite green to negatively charged polystyrene and naphthol yellow S to positively charged polystyrene revealed highly favorable interactions. The Gibbs free energy of adsorption for the adsorption of malachite green to negatively charged polystyrene was in agreement with other independent measurements.

For non-resonant organic solutes, including the micropollutants caffeine and pentoxifylline, a competitive displacement methodology was employed to obtain values for the Gibbs free energy of adsorption. The mechanism of adsorption for these organic micropollutants appears to be dominated by van der Waals interactions, with surface charge playing a relatively minor role, even for ionic adsorbers. Preliminary results on the adsorption of organic solutes to polyamide surfaces will also be presented.

This experimental technique provides a straightforward way of quantifying the adsorption of critical organic micropollutants to polymer membrane surfaces. These measurements could inform the design of membranes with improved rejection of organic micropollutants, reducing the need for costly secondary treatment of purified wastewater.