Highly efficient and rapid removal of a toxic dye: Adsorption kinetics, isotherm, and mechanism studies on functionalized multiwalled carbon nanotubes

In the present work, multiwalled carbon nanotubes functionalized with L-tyrosine (CNT-TYR) was used as a nanoadsorbent to remove a toxic dye, methylene blue (MB). To understand the adsorption behaviour of the MB dye on the nanoadsorbent, the adsorption mechanism, kinetics and isotherm were studied in detail. The factors that influence the adsorption process such as stirring speed (200-800 rpm), contact time (0-150 min), amount of nanoadsorbent (5-25 mg), pH (2-10), temperature (25-65 degrees C), and initial dye concentration (10-300 mg L-1) were optimized. It was observed that > 95% adsorption occurred at pH 6, 25 degrees C temperature, and 7 min of contact time. The maximum adsorption capacity attained at the optimum conditions was 440 mg g(-1), which is higher in comparison to other reported nanoadsorbents. The kinetics study was done using pseudo first order, pseudo second order, intraparticle diffusion, and fractional power model, and the results indicated that the pseudo second order model was followed for the adsorption of MB dye on CNT-TYR. The study of adsorption isotherms was conducted for the Langmuir, Freundlich, Temkin, Elovich, and Redlich-Peterson models, and the results revealed that the Langmuir model was better fitted to the obtained data (R-2 >0.99) than the other models, which indicated monolayer adsorption via chemisorption. The reason for the high adsorption capacity of CNT-TYR was explained by the different types of interactions such as electrostatic, pi-pi interaction, and H-bonding, which explain the adsorption mechanism and were further confirmed by the FTIR spectrum of MB saturated nanoadsorbent. The results indicate that CNT-TYR has high efficiency for the removal of dyes and various other toxic contaminants present in water.