Sorption characteristics of tetrabromobisphenol A by oxidized and ethylendiamine-functionalized multi-walled carbon nanotubes

Abstract

Two kinds of functional groups, carboxyl, and amidogen were attached to on multi-walled carbon nanotubes (MWCNTs) with mixed acid and ethylenediamine (EDA). Scanning electron microscopy and transmission electron microscopy characterization showed no distinguishable changes in morphology, but found defects on the walls of the modified MWCNTs, and X-ray diffraction characterization indicated that the MWCNTs still retain the graphite structure. The sorption kinetics, sorption isotherms, and thermodynamics of pristine and modified MWCNTs were also investigated. The results showed that the equilibrium sorption could be well modeled by a pseudo-second-order kinetic fit. The uptake of tetrabromobisphenol A (TBBPA) sharply decreased with increasing pH values under alkaline conditions and at higher temperatures. The sorption isotherms of TBBPA on MWCNTs were well described by Langmuir and Freundlich models. Carboxyl- and hydroxyl-functionalized MWCNTs (MWCNTs-O) have higher values of the change in the standard Gibbs free energy (ΔG°) than do the amino-functionalized MWCNTs (MWCNTs-N); this is in agreement with the phenomenon that MWCNTs-O have a slightly higher sorption capacity than MWCNTs-N. The calculated thermodynamic parameters suggest that the sorption of TBBPA onto the modified MWCNTs is a feasible, spontaneous, and exothermic process. It was also proposed that hydrophobic interactions and π–π interactions were dominant in TBBPA sorption onto MWCNTs. The three sorbents retain a high sorption capacity after five sorption and desorption cycles.