New insights into the adsorption of crystal violet dye on functionalized multi-walled carbon nanotubes: Experiments, statistical physics and COSMO-RS models application

Two multi-walled Carbon Nanotubes (MWCNTs) adsorbents, chemically functionalized by either OH (MWCNTs-OH) or COOH (MWCNTs-COOH), were adopted to study the adsorption of Crystal Violet (CV) dye from aqueous solutions. A series of CV dye adsorption isotherms was recorded at different temperatures ranging from 298 to 328 K. Experimental results demonstrated that the adsorbents have similar performances, even if the sample functionalized by COOH (MWCNTs-COOH) shows slightly higher adsorption capacity. In light of the statistical physics, a model was developed and applied to explain the adsorption mechanism and to describe the role of adsorbent functionalization through model parameters. Mathematically, this model was defined by three parameters reflecting the main physicochemical aspects of the adsorption process such as, the number of dye molecules linked per site of adsorbent (n(d)), an energetic coefficient (w) and the number of occupied receptor site (K-m). A very good accordance between the experimental results and the proposed model was observed. The difference in nd values for the two adsorbents can be attributed to the intensity of interactions between dye and the specific functional groups of the adsorbents. An energetic characterization of the systems was carried out by calculating three specific interaction energies of CV dye with the functionalized carbon nanotubes (the electrostatic misfit energy; E-MF, the hydrogen-bonding energy; E-HB and the Van der Waals energy; E-vdw) by application of conductor-like screening model for real solvents (COSMO-RS). The COSMO-RS model clearly indicated that the hydrogen-bonding interaction is the dominant energetic factor in CV dye adsorption. Finally, a good complementarity was retrieved between the results of statistical physics and COSMO-RS models.