Photo-regenerable multi-walled carbon nanotube membranes for the removal of pharmaceutical micropollutants from water

Abstract

Pharmaceutical micropollutants fall in the category of “emerging contaminants” in water because of their prevalence and persistence in the aqueous environment, and because of a poor understanding of their low-dose exposure effects on human and animal populations. In this study, photo-regenerable multiwalled carbon nanotube membranes with variable water permeabilities were produced by embedding hierarchical TiO₂ structures (having porous, spherical morphology) onto a pre-deposited bed of multi-walled carbon nanotubes (MWNTs) using a modified sol–gel technique. These MWNT–TiO₂ composites and their constituent materials were characterized by analytical electron microscopy, surface charge measurement, thermogravimetric analysis, and hydrophobicity determination. The adsorption removal potential of MWNT–TiO₂ membranes was demonstrated for three representative pharmaceuticals: acetaminophen, carbamazepine and ibuprofen. The peak initial removal percentages of the pharmaceuticals by the MWNT–TiO₂ membranes were 80%, 45%, and 24% for carbamazepine, ibuprofen, and acetaminophen, respectively. The ability of the membranes to be regenerated, once they were saturated with the pharmaceutical compounds, was verified by repeating the adsorption removal experiment on the same membranes after exposure to UV light at 254 nm. Peak removal efficiencies after regeneration were 55%, 32%, and 19% for carbamazepine, ibuprofen, and acetaminophen, respectively, indicating some loss in sorptive capacity upon regeneration. Furthermore, the effect of pH on adsorption of ibuprofen, the pharmaceutical that attained the highest mass loading on the sorbent at equilibrium saturation, was studied and its mechanism of adsorption was proposed at pH below pK_a.