Incorporation of single-walled carbon nanotubes in ultrafiltration support structure for the removal of steroid hormone micropollutants

Abstract

Ultrafiltration (UF) is an established water treatment solution that operates at relatively low pressures, but cannot remove pollutants with sub-nanometre sizes such as steroid hormones. To tackle this challenge while maintaining a fast flow of water through the membrane, UF can be coupled with a nanoparticle layer that removes micropollutants via adsorption. In this study, a mechanical method of incorporating single-walled carbon nanotubes (SWCNTs) in the UF structure is demonstrated. Such an SWCNT–UF composite with a SWCNT loading of 2 g/m2 offered the high UF-range permeability, and partial removal (50–75%) of hormones from a 100 ng/L feed solution at neutral pH. A full breakthrough was not achieved in the three-hour experiments. The diffusion of hormones to the highly accessible SWCNT surface area was a limiting factor. Nevertheless, adsorption kinetics were very fast, and adsorption was independent of residence time in the range of 0.08–7.1 s. Since dynamic adsorption is kinetic-dependent, water quality parameters such as pH could affect the adsorptive interactions. For example, adsorption was poor at pH 11–12. Although more work is needed to accomplish the drinking water guideline of 1 ng/L with such a composite, the adsorbent–membrane concept is promising, because the adsorption efficiency can be further enhanced by increasing the particle loading and/or choosing adsorbents with higher surface areas.