Kinetics and Interfacial Thermodynamics of the pH-Related Sorption of Tetrabromobisphenol A onto Multiwalled Carbon Nanotubes

Surface functionalization of multiwalled carbon nanotubes (MWCNTs) was performed using mixed acid and ethylenediamine. The materials were characterized by electron microscope, X-ray diffraction, Raman spectra, Fourier transform infrared, N2 adsorption-desorption, and X-ray photoelectron spectroscopy. The pH-dependent sorption of tetrabromobisphenol A (TBBPA) onto raw and functionalized MWCNTs was investigated. A decrease in TBBPA uptake was found to be dependent on the adsorptive pKa in alkaline conditions. Two types of MWCNTs exhibited rapid binding kinetics for TBBPA sorption within 20 min. The kinetics of TBBPA sorption onto MWCNTs were analyzed using a pseudo-second-order model, an intraparticle diffusion model and Boyd model. The results showed that TBBPA sorption on MWCNTs and N-MWCNTs could be well described by the pseudo-second-order model, and the external diffusion (boundary layer diffusion) was the rate-limiting step. The extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory was applied to calculate interfacial free energies and to explain the sorption characteristics between the sorbent and solute. This analysis revealed that hydrophobic attractive interactions (i.e., interfacial AB interactions) were dominant in TBBPA sorption onto MWCNTs.