Removal of 4-chloro-2-methylphenoxyacetic acid from water by sorption on carbon nanotubes and metal oxide nanoparticles

A comparative study was performed on the sorption capacity of the phenoxy acid herbicide (4-chloro-2-methylphenoxyacetic acid), also known as MCPA, on a single-walled carbon nanotube (SWCNT), three multi-walled carbon nanotubes with average outer diameters of 15, 30 and 50 nm (MWCNT15, MWCNT30 and MWCNT50), and nanoscale metal oxides (Al2O3, TiO2 and ZnO). The most efficient sorbent was the SWCNT, followed by the three MWCNTs. The metal oxide nanoparticles showed a much lower sorption capacity. The herbicide bound to the tested carbon nanotubes, by a combination of electron donor–acceptor (π–π EDA) interactions and hydrogen bonds. On the contrary, MCPA bound to the considered metal oxide nanoparticles by electrostatic interactions occurring between the carboxylate anions of the same herbicide and OH2+ of the sorbent. Experiments of cyclic sorption showed the pesticides to be totally removed by only two sorption cycles on the SWCNT and MWCNT15, whereas the same waste water was purified after four cycles on nano Al2O3. Finally, desorption studies carried out in ethanol showed a potential re-use of the SWCNT.