Enhanced adsorption of carbon sphere by doping with titania nanotubes for crystal violet removal: isotherm, kinetics, and thermodynamic studies

Abstract

In this study, the carbon sphere (Cs) has been prepared and modified by titania nanotubes (TNTs) to be utilized as an adsorbent toward crystal violet (CV) dye as a model for cationic dyes from aqueous solution. The prepared TNTs@Cs composites has been characterized by various techniques such as XRD, SEM, and TEM analysis. The adsorption analysis displayed that the adsorption capacity of CV dye onto the modified Cs with TNTs is 92.5 mg g⁻¹, which is much higher than that achieved by pristine Cs (12.5 mg g⁻¹). Various factors that influence the overall adsorption processes, such as pH, contact time, initial CV dye concentration, adsorbent weight, and temperature, were studied. The TNTs@Cs_76.7 composite showed the highest removal percentage of 99.00% at optimum conditions. The adsorption isotherm analysis showed that the experimental data of adsorption CV dye fitted better with the Langmuir isotherm model with R² of 0.999, and the estimated maximum adsorption capacity was 84.7 mg g⁻¹. On the other hand, the adsorption kinetic study showed that the adsorption of CV follows the pseudo-second order kinetic model with an equilibrium adsorption capacity (q_e) of 10.66, 18.622, 47.61, and 48.31 mg g⁻¹ for Cs, TNTs@Cs₉₃, TNTs@Cs_86.8, and TNTs@Cs_76.7 composites, respectively. The thermodynamic analysis showed negative free energy (ΔG) values, this indicates that the adsorption of CV is a spontaneous and feasible process. Furthermore, the ΔH and ΔS are positive values that indicate an endothermic adsorption process. Furthermore, the prepared TNTs@Cs_76.7 composite displayed remarkable adsorption stability and the removal efficiency of CV remains at 96.3% after five cycles.