Decolourization of Acid Orange 74 Aqueous Solutions in Presence of Multi-Walled Carbon Nanotubes under Ultrasound Irradiation

Few researches focused on the combination of ultrasound and multi-walled carbon nanotubes (MWCNT) in wastewater treatment. In this study, it was found that the combination of ultrasound and MWCNT was more effective in the removal of Acid orange 74 (AO 74) in comparison to single method. And there was a remarkable synergistic effect in the ultrasound/MWCNT combined system. The adsorption behaviour of AO 74 by the combination of ultrasound and multi-walled carbon nanotubes (MWCNT) was investigated under various experimental conditions. The adsorption performance was slightly inhibited with increasing temperature. This fact showed that the adsorption process was exothermic. The removal ratio was improved with increase in MWCNT dosage, which could be attributed to the increased adsorption sites and increased surface area. The dye removal decreasedfrom 96.0 to 41.5% with an increase in the initial concentration of dye from 15 to 100 mg/L. Lower removal ratios at higher dye concentrations were due to the saturation of adsorption, though the adsorption capacity increased from 14 to 41.5 mg/g. It may be due to the high concentration of dye which can provide a high driving force of concentration gradient to overcome all mass transfer resistance. With the increase in pH, removal ratios of AO 74 decreased gradually from pH 3 to 11. At low pH values, the surface of the adsorbent solids was positively charged and easily adsorbed the AO 74 ions having negative charges. But at high pH values, the surface charge of MWCNT got negatively charged, which did not favour the adsorption of negatively charged dye anions. Experimental isotherm data were represented with Langmuir and Freundlich adsorption models. Langmuir isotherm equation fitted better than Freundlich isotherm equation for this system, which meant adsorption of dye onto MWCNT was monolayer adsorption with a finite number of identical sites. The adsorption of AO 74 followed pseudo second-order kinetics. Kinetics analysis suggested lower temperature was favourable for AO 74 adsorption on MWCNT under ultrasound and justified the exothermic adsorption process.