Sorption experiments were carried out in batch process for the removal of color methylene blue from its aqueous solution using boiler bottom ash as an adsorbent. The operating variables studied were initial concentration and adsorbent dosage. The effect of external mass transfer coefficient, β, on these operating variables studied were estimated using Furusuwa – Smith model. The calculated β values were then correlated to dimensionless mass transfer numbers (Sh/Sc0.33). The dimensionless mass transfer numbers were found to vary with initial concentration, Co -0.000002 and adsorbent dosage, M0.0085.