Polymeric Adsorption of Estrone from Water and Photocatalytic Degradation by Zinc Oxide

Abstract

Steroid estrogens comprise a class of endocrine-disrupting chemicals that can interfere with the functioning of human and wildlife's endocrine systems at extremely low levels. Surveys in wastewater treatment facilities in North America, Europe, Australia and New Zealand have shown that conventional biological treatment processes such as the activated sludge process are not completely effective in removing steroid hormone contaminants from wastewater. Elevated levels of steroid estrogens have been detected in surface waters and ground waters as a result of inadequate treatment of those compounds in wastewater treatment facilities. The presence of unregulated steroid estrogens in water resources poses direct risks to exposed aquatic species and potential health risks to humans through food and drinking water supply. As one of the most common steroid estrogen contaminant in treated wastewater effluents, estrone was selected as the target contaminant in this study. A three-stage process was designed to eliminate the risk of this contaminant: 1) capture of trace levels of estrone from water via a reversible chemisorption process; 2) desorption of estrone adsorbate in a chemical-aided adsorbent regeneration process; 3) using photocatalytic degradation as an intensive treatment to degrade estrone at relatively high concentrations. Polyamide (Nylon) 6,6 microfiltration membranes were used as a functional polymeric adsorbent to capture low levels of estrone from water. The process is driven by a hydrogen-bonding mechanism between amide groups in polyamide 6,6 and phenolic hydroxyl groups on estrone molecules. It provides instant removal of estrone from water and harvests trace levels of steroid hormones onto polyamide membranes from relatively large amounts of water. Saturated polyamide membranes were regenerated at ambient conditions using pH 12 caustic soda solutions as membrane regenerant. In this step, pre-adsorbed estrone was desorbed from polyamide membranes and became concentrated in regenerant solutions. The regenerated membrane showed highly consistent adsorption capacities for estrone after five cycles of adsorption and regeneration. The higher concentration of estrone after this process allows efficient further treatment by photocatalytic degradation. The photocatalytic degradation treatment involves the use of zinc oxide (ZnO) and titanium dioxide (TiO2) photocatalysts under artificial UVA and solar radiation to degrade estrone in water at near-saturation concentration. Despite its lower specific surface area and inferior dispersion in water, ZnO consistently exhibited superior photocatalytic activity to the benchmark Aeroxide P25® TiO2 photocatalyst, providing up to three times faster photocatalytic degradation for estrone. Complete decomposition of estrone in a 600 μg/l solution was observed after 10 min of exposure under an 18W UVA lamp using 0.5 g/l ZnO photocatalyst. Furthermore, solar irradiation was found to be a highly efficient UV source for estrone degradation using ZnO or P25 TiO2 photocatalysts, with minimal direct photolysis observed. The origin of the superior photocatalytic activity of ZnO was investigated by diffuse reflectance spectroscopy, which showed that ZnO exhibited markedly higher UV absorption than P25 TiO2 in the wavelength range of 320 - 370 nm. This corresponds to its notably superior activity under the weak artificial UVA irradiation. Dissolution of ZnO photocatalyst was observed during photocatalytic degradation processes. This phenomenon was further studied, as dissolution or photodissolution is of concerns for ZnO photocatalysts due to the potential catalyst inactivation and secondary pollution by free zinc ions. For this purpose, ZnO thin films were prepared by magnetron-sputtering and used as the base material for visual inspection of different dissolution patterns of ZnO under various corrosive conditions. It was found that ZnO suffered rapid dissolution in water at pH <=5 or pH >= 11 and in certain ligand solution, i.e. 1 mM ethylenediaminetetraacetic acid (EDTA). The lowest dissolution rate was obtained at pH 10, with 1.2% dissolution after 24 h of exposure. Minimal dissolution was observed on ZnO films in alkalised 1 mM oxalate and acetate solutions. Pitting corrosion was observed on ZnO films after prolonged UV irradiation, which was ascribed to photo-generated holes on surface defect sites. The presence of hole scavengers (Na2SO3) led to significant suppression of ZnO photo-dissolution. This suppression effect remained in place until hole scavengers were completely consumed, from where the photo-dissolution rates accelerated. As an extended study on ZnO, the intrinsic and UV-induced surface wettability of ZnO thin films was studied in relation to their surface morphologies.