Elucidating the mass transfer mechanism of CrVI adsorption by encapsulated chitosan-carbon nanotubes-iron beads in packed-bed columns

Abstract

This study evaluates the CrVI breakthrough behaviors of a continuous-flow column packed with composite chitosan-MWCNTs‑iron beads under various operating conditions. Under the tested range of experimental parameters, a maximum of 54% of CrVI removal was achieved at: water flow rate, 1 mL/min, feed CrVI concentration, 30 mg/L; and packed bed height 8 cm. A homogeneous surface diffusion model (HSDM) involving convection-dispersion and diffusion equations was formulated, numerically solved, and experimentally validated. The high degree of conformity between the calculated and experimental breakthrough curves facilitated the determination of mass transfer parameters including axial dispersion DL, (1.30 × 10−8 to 1.56 × 10−7 m2/s) and surface diffusion Ds, (7.28 × 10−11 to 1.80 × 10−10 m2/s) via an error minimizing approach. The DL value was a function of molecular diffusion, which varied with the flow velocity, mass loading, and bed height. The Ds values were slightly higher than those previously reported, owing to the heterogeneous nature of the adsorbent. The external film diffusion coefficient (kf) was also determined by using Wilson-Geankoplis empirical correlation, which was recognized as a rate-controlling factor during CrVI mass transfer to the adsorbent beads. Furthermore, sensitivity analysis showed that a transition from diffuse- to shock-front occurred with a decreasing DL, while a decrease in the slope of breakthrough curves was observed by decreasing Ds and bed porosity, and increasing kf and Langmuir parameters. Lastly, the study demonstrated a high correlation between utilization of fractional bed capacity and axial Peclet number. This method is suitable for optimizing operating conditions to maximize the utilization of a fixed-bed columns.