(293d) Adsorption Equilibrium and Kinetics of Nitroaromatic Dye Removal: Comparing Vegetation-Polymer Beads and Activated Carbon

The focus of this research was the assessment of the dye-removal performance of polymer beads impregnated with vegetation as compared to the performance of activated carbon. The adsorption equilibria were determined in batch tests. The kinetic performance was evaluated in batch tests and a batch kinetic model was developed. An assessment of dye removal capabilities in continuous packed bed columns was completed. Powdered grass, wood, compost and sphagnum peat moss were added to several polymers to create biosorbent beads ranging in diameter from 4.0 to 6.5 mm. The polymers included poly (methyl methacrylate), poly (Bisphenol A carbonate), polysulfone and cellulose acetate. The beads adsorption equilibria and kinetic performance were evaluated in the removal of two acid azo dyes from simulated wastewater at a low pH and in the presence of 0.1 M calcium chloride and two basic nitro-aromatic dyes from water. These results were compared with tests performed with plain plastic beads and two size variations of activated carbon, a pellet variety ranging in diameter from 2.5 to 4.5 mm and a granular variety ranging in diameter from 0.5 to 2.5 mm. A total of 22 sorbent materials were evaluated in their uptake of the four dyes. Preliminary scoping tests on the polymer/vegetation beads demonstrated the adsorption of dyes with the possibility to elute the acid dyes from the beads and reuse the beads. The basic dyes demonstrate more colorfastness than the acid dyes and were not shown to elute from the beads. The beads containing vegetation were durable and able to continue dye adsorption after the dye elution and bead regeneration using the acid dyes. After adsorbing acid dyes at low pH in the presence of calcium chloride, the beads were eluted and regenerated by raising the solution pH. The beads post regeneration showed a 30% to 40% decrease in capacity after each elution/regeneration cycle in batch tests and a 50% to 70% decrease in capacity in column tests with the acid dyes. In the study of adsorption equilibria, the Langmuir isotherm was routinely the best fit for isotherm data for the beads and a reasonable fit for the activated carbon. The beads containing powdered grass consistently had the highest adsorption capacities of the beads; with the beads made from poly (methyl methacrylate) performing better on average than beads made from the other polymers. The activated carbon had a significantly higher adsorption capacity than the beads for both the acid and basic dyes. For the purposes of this research a film-solid diffusion model was identified as an effective tool to describe the mass transfer and diffusion processes for the adsorption of these organic dyes. The kinetic model was developed by combining the mass transfer, diffusion and equilibrium equations with the process variables, the equations of continuity and initial and boundary conditions. Although other kinetic models were evaluated, the film-solid diffusion model provided a reasonable fit to both the polymer/vegetation beads and the activated carbon. Test results indicated that mass transfer and solid diffusivity were several orders of magnitude greater in the activated carbon than in the bead matrices. The comparison of the model and experimental data provided insight into the effect of changes in the mass transfer coefficient and the solid diffusivity on each curve generated by the model. For the most part all of the adsorption experimental data demonstrated that mass transfer was a greater influence in the rate of adsorption than solid diffusivity. However, solid diffusivity had a noticeable impact on the specific shape of each curve generated and the curve fit was noticeably closer to the actual data when solid diffusivity was included in the model. Although solid diffusivity seemed to be a more critical parameter for the polymer/vegetation beads, it still had a measurable impact on the activated carbon curve fit as well. In tests using the activated carbon, the solid diffusivity also was demonstrated to decrease as initial concentration of dye increased, further demonstrating the influence of solid diffusivity on the adsorption of these dyes. Overall, the test results for activated carbon showed it had a higher capacity and demonstrated better kinetic performance than the biosorbent beads for these relatively large molecular weight nitroaromatic dyes. The beads show some potential for further development, but may be better suited to adsorption of metals or as support structure for biofilm/bioreactor treatment studies.