Kinetics and equilibrium of dissolved oxygen adsorption on activated carbon

The macroscopic adsorption behavior of dissolved oxygen on a coconut shell-derived granular activated carbon has been studied in batch mode at 301 and 313 K for initial dissolved oxygen concentrations of 10–30 mg/l and oxygen/carbon ratios of 2–180 mg/g. BET (Brunauer, Emmett, and Teller) surface area, micropore volume, and pore size distribution were determined from N2 isotherm data for fresh and used samples of carbon. The surface groups were characterized using Boehm titrations, potentiometric titrations, and FTIR study. The material is characterized by its high specific surface area (1307m2/g), microporocity (micropore volume 0.54cm3/g), its basic character (0.57meq/g total basic groups) and its high iron content (15,480 ppm Fe). BET n  -layer isotherm describes adsorption equilibrium suggesting cooperative adsorption and important adsorbate–adsorbate interactions. Kinetic data suggest a process dependent on surface coverage. At low coverage a Fickian, intraparticle diffusion rate model assuming a local equilibrium isotherm (oxygen dissociation reaction) adequately describes the process. The calculated diffusion coefficients (D)(D) vary between (4.7–8.2)×10-9m2/min and (3.5–5.3)×10-9m2/min for initial oxygen concentration of 10 and 20 mg/l, respectively. Sensitivity analysis shows that the oxygen dissociation equilibrium constant determines the equilibrium concentration, whereas the diffusion coefficient controls the kinetic rate of the adsorption process having no effect at the final equilibrium concentration. A combined kinetic mass transfer model with concentration-dependent diffusion (parabolic form) has been developed and successfully applied on the dissolved oxygen adsorption system at high surface coverage. For equilibrium uptake of 0.08mg/m2 the estimated mean mass transfer coefficient and adsorption rate constant are 3.38×10-5m/min and 1.0×10-2l/(m2min), respectively.