Congo red adsorption with cellulose-graphene nanoplatelets beads by differential column batch reactor

Abstract

Cellulose beads loaded with graphene nanoplatelets (GNP) were prepared by a physical gelation method, which was feasible under mild conditions. The phases spontaneously separate when the cellulosic solution is dropped into an acid solution, maintaining semi-spherical shapes with diameter between 3.4 and 3.9 mm, which is desirable for continuous-flow systems. Beads were tested for the removal of Congo red dye using a differential column batch reactor. Langmuir isotherm described the adsorption equilibrium, with maximum adsorption capacities of 98.1 and 139.6 mg/g for cellulose and cellulose-GNP beads. GNP increased the number of binding sites in the adsorbent. Removal efficiencies were higher than 90% within a broad range of initial concentration and sorbent loading. Static batch experiments evidenced slow kinetics for both sorbents, reaching equilibrium after 400 min. Hence, mass transfer was enhanced using a differential column batch reactor. The mass transfer coefficient, kL increased from 3.16 × 10−4 to 6.94 × 10−4 L/mg min, reaching equilibrium in half of the static adsorption time. External mass transfer resistance was minimized as turbulent flow is developed around the sorbent particles, allowing to obtain adsorption efficiencies close to 100% in a reasonably low time of 100 min. GNP promoted faster kinetics, as kL was 7.58 × 10−4 L/mg min. A model was developed to describe the adsorption dynamics based on the equilibrium. Although desorption was not favorable, cellulose-GNP beads’ direct disposal is attractive due to the sorbent’s hydrogel-like nature. A future work perspective is to evaluate these adsorbents under continuous fixed-bed operation.