Adsorption of estrone in microfiltration membrane filters

Adsorption of estrone was identified in microfiltration membrane filters with 0.1–0.45 μm nominal pore sizes. This phenomenon is of concern as it may lead to distortion when such filters are used to process estrone solutions prior to chromatography analysis, such as in slurry-type photocataltytic degradation studies. The adsorption of estrone was comparatively assessed in a variety of membrane filters, and considerable adsorption was observed in nylon (NYL), polypropylene (PP), polytetrafluoroethylene (PTFE) and cellulose acetate (CA) filters. While estrone adsorption gradually approached equilibrium in most filters, it maintained at near 100% in NYL filters during most of the filtration process and remained substantial (42.2%) when the filter capacity was reached. This rather unusual phenomenon was investigated, and ascribed to the hydrogen bonding between estrone and NYL membranes as demonstrated by Fourier transform infrared spectroscopy. Further studies showed that estrone adsorption in NYL membranes could be substantially reduced by increasing the pH to facilitate the deprotonation of estrone molecules. Similar improvements were obtained in PP and PTFE filters by pre-dosing estrone solutions with anhydrous ethanol (1:1, vol:vol). The only filter that showed consistently low estrone adsorption (<2.3%) was the 0.45-μm glass microfibre (GMF) filter.GMF and PTFE filters were further selected to process estrone solutions in a slurry-type photocatalytic degradation system using P25® TiO2 photocatalysts. Both types of filters showed satisfactory performance in terms of estrone solute recovery, as verified by samples simultaneously processed by high-speed centrifuging. It is suggested that caution should be taken when using MF membrane filters to process estrone aqueous solutions. Although GMF and PTFE filters (with solvent pre-dosing) may be appropriate to use, verifications on solute recovery are highly recommended.