Crop Residue Ashes as Adsorbents for Basic Dye (Methylene Blue) Removal: Adsorption Kinetics and Dynamics

In the quest for identifying low-cost, locally available and effective adsorbents, pigeon pea (Cajanus cajan L.) residue ash (PRA) and sunflower (Helianthus annuus L.) residue ash (SRA) were evaluated as adsorbents for sequestering a basic dye, methylene blue (MB), from aqueous solution. Effects of contact time, initial solution pH and adsorbent dose on MB adsorption were investigated. MB adsorption onto PRA and SRA was dependent on initial solution pH, with adsorption being greater at higher pH. Percent MB removal increased with increase in adsorbent's dose. Adsorption followed pseudo-second order kinetics. Intraparticle diffusion was not the sole rate-controlling step. Both film diffusion and intraparticle diffusion controlled the adsorption process. The equilibrium data fitted well to Langmuir isotherm as compared to Freundlich isotherm, on the basis of coefficient of determination and error analysis. The maximum MB adsorption capacities of PRA and SRA were found as 58.8 and 62.5mgg(-1), respectively. The free energy change and dimensionless equilibrium parameter revealed spontaneous and favorable nature of MB adsorption onto crop residue ashes. Results demonstrate that the PRA and SRA have high adsorptive potential as compared to many previously reported ash based adsorbents, and hence can be used for treating water contaminated with cationic dyes.