Phenolic compounds are the major source of colour in sugar mill effluent. Details mechanism of diffusional resistances for removal of phenolic compounds namely: phenol, 2-4 dichlrophenol and 2-4 dinitrophenol by adsorption onto activated carbon rice husk (ACRH) was studied. The mechanism studied was based on three diffusional resistance models: external mass transfer, intraparticle diffusion and twin resistance mass transfer. The relationship of mass transfer coefficient versus time shows that the adsorption process follows two regimes started with high mass transfer and then low mass transfer, due to high diffusion from bulk to surface of ACRH particles then slow mass transfer into pores of ACRH. The comparison between the three models was followed up by using the relationship of mass transfer coefficient versus time (kα cn).
It was found that adsorption of phenol in the intraparticle structure is 180% higher than adsorption on the external surface of the ACRH. The value of mass transfer coefficient of the twin resistance model is close to that of external mass transfer model and both are lower than that of intraparticle diffusion model. The experimental results proved that intraparticle diffusion is controlling step. Moreover, three adsorption isotherms were studied: Langmuir, Freundlich and Dubinini -Roduch (D-R) for the three phenolic compounds.