Equilibrium and Kinetic Studies of Liquid Phase Adsorption of Methylene Blue Onto Phosphoric Acid Modified Bambaranut Shell

Abstract In view of the global need to curb the effect of contaminants in waste water on our environment, the adsorption potentials of modified carbon from bambaranut (Vigna subterranean) shell was investigated for its efficiency in the removal of methylene blue from waste water. The adsorbent morphology and surface chemistry were established by Brunauer-Emmett-Teller (BET) determination and Scanning Electron Microscopy (SEM), as well as other standard laboratory procedures. The prepared material was used for the uptake of MB from aqueous solution in a batch process, using UV spectrophotometer Model 752 at 620nm to analyze for the residual dye concentration. The effect of operational parameters such as contact time, adsorbent dosage, initial dye concentration and pH were analyzed to determine the factors controlling the rate of adsorption. Results from the study showed that the active carbon prepared was a porous material, with surface area of 193 m2/g, average pore size of about 10.98nm, and pore volume of 0.530cm3/g. With increase in initial dye concentration from 15mg/l to 75mg/l, a decrease in percent adsorption from 95.4% to 72.19% was observed. Increase in adsorbent dosage (from 0.1g to 0.5g), contact time (from 5 min to 40 min) and pH from 2 to 10 resulted in increase in percent adsorption from 84.03% to 98.83%, 54.24% to 84% and 48.17% to 84.03% respectively. About 98.83% removal of MB dye was achieved after 20 min, at pH of 6, temperature of 27±2oC, 0.5g weight of adsorbent and initial concentration of 60mg/l of 50ml MB dye solution. Langmuir isotherm best fits the equilibrium adsorption data with R2 = 0.996; the adsorption intensity obtained from Freundlich model (n>1) and the energy of adsorption obtained from the D-R model (< 8kJ/mol) suggested that physisorption dominates the adsorption of methylene blue onto the prepared activated carbon. Adsorption kinetic data was best described using Pseudo second order kinetic model (R2 = 0.996), giving equilibrium rate constant (k2) of 7690g mg-1 min-1. The characteristic results showed that bambaranut shell can be employed as an alternative to commercial adsorbents in the removal of methylene blue dye from aqueous solutions and waste water.