The effects of microwave regeneration on adsorptive performance of functionalized carbon nanotubes Shahab Karimifard and Mohammad Reza Alavi Moghaddam ABSTRACT In this study, the microwave regeneration method was applied to investigate the properties and adsorptive performance of functionalized carbon nanotubes (f-CNTs) in different cycles of regeneration/reuse. For this purpose, an organic and hazardous dye (Reactive Blue 19) was chosen as a widely used pollutant. N 2 adsorption/desorption isotherms, scanning electron microscopy and Fourier transform infrared spectroscopy were used to characterize f-CNTs during the regeneration/ reuse procedure. The morphology, specific surface area and pore volume of f-CNT samples were not significantly altered. However, the functional groups present on the f-CNTs’ surface were gradually removed after successive cycles of regeneration/reuse. A sudden decrease of adsorption capacity (about 20%) after the first cycle of regeneration/reuse was attributed to the elimination of functional groups interacting with the dye molecules because of the molecular-level heating. Relatively high regeneration efficiencies (73.30 to 80.16%) proved that the microwave regeneration method was successful. Very high step stripping efficiencies (80.16 to 98.02%) in four cycles of regeneration/reuse demonstrated that the microwave regeneration method could be utilized in consecutive cycles. After four cycles of regeneration/reuse, the CNTs could not be considered as functionalized. Shahab Karimifard Mohammad Reza Alavi Moghaddam (corresponding author) Department of Civil and Environmental Engineering, Amirkabir University of Technology (Tehran Polytechnic), Hafez St., Tehran 15875-4413, Iran E-mail: Alavim@yahoo.com; alavi@aut.ac.ir Key words | adsorption, functionalized carbon nanotubes, microwave, regeneration, reuse INTRODUCTION Since their discovery in 1991 by Sumio Ijima (Ijima ), carbon nanotubes (CNTs) have been used extensively in different branches of science due to their excellent chemi- cal/physical characteristics (Zhan et al. ). Their large surface area, very small size, mechanical strength and elec- trical properties have demonstrated CNTs as superb candidates to adsorb a vast variety of pollutants from water and wastewater (Szlachta & Wojtowicz ; Wang et al. ; Yu et al. ). In addition, functionalization of CNTs could modify their chemical/physical properties lead- ing to better performance in removal of pollutants; therefore several studies used functionalized CNTs (f-CNTs) to remove different types of pollutants from aqueous solution (Mishra et al. ; Xu et al. ; Ghaedi et al. ; Karimi- fard & Alavi Moghaddam a). The applicability of CNTs for removal of contaminants from wastewater depends not only on the adsorption capacity, but also on regeneration possibility and reusability (Wang et al. ). Because of the high costs of production and some concerns about the environmental impacts of CNTs (Gupta et al. ), their regeneration after the adsorp- tion procedure should be considered by researchers. The progressive accumulation of pollutants on the surface of CNTs during the adsorption process leads to a gradual reduction of the CNTs’ adsorption capacity (Salvador et al. ). Therefore, a suitable and efficient process must be utilized to regenerate the exhausted CNTs. For carbonaceous materials exhausted with organic pollutants, thermal regeneration procedures are the most widely used techniques among numerous methods (Ania et al. ; Salvador et al. ). However, during conven- tional thermal processing, energy is transferred to material through radiation, convection and conduction of heat from the surface, which could lead to overconsumption of energy, excessive loss of material and reduction of the adsorption capacity (Kuo ). In recent years, micro- wave regeneration has been investigated as a novel thermal method because of its energy saving, small space requirement, and capability for molecular-level heating which leads to fast and homogenous regeneration of 2638 © IWA Publishing 2016 Water Science & Technology | 73.11 | 2016 doi: 10.2166/wst.2016.117 Downloaded from https://iwaponline.com/wst/article-pdf/73/11/2638/460323/wst073112638.pdf by guest on 15 June 2020