Multiwalled Carbon Nanotubes for Heterogeneous Nanocatalytic Ozonation

Abstract

Multiwalled carbon nanotubes functionalized by plasma oxygen (CNTs) have been used as heterogeneous catalysts for the ozonation of methyl orange (MO) dye (CI 13025) in aqueous solutions. It was found that the addition of CNTs significantly enhanced the dye decolorization as compared to ozone alone or when activated carbon was used at the same dose as CNTs. Both the initial ozone concentration and catalyst dosage enhanced the removal of MO. However, ozone gas concentrations higher than 6 g/m3 NTP did not further improve the decolorization rates. The removal efficiency of MO increased with pH in the range 2 to 3, while a reverse trend was observed when the pH increased from 3 to 9. The addition of a radical scavenger resulted in only a limited change in the decolorization rates suggesting that molecular ozone was the main pathway by which MO decolorization occurred in solution. However, under favorable conditions for MO attraction to CNT surface (pH = 3), the decolorization rate has significantly increased. At higher pH than the pKa value of MO (3.47) and the point of zero charge of CNT (3.87), a condition that favors the electrostatic repulsion of MO from CNT, the rates were reduced in the presence of CNT as compared to ozone alone possibly due to loss of part of the supplied ozone in un-useful parallel reactions.