Nitric acid (HNO3) modification of activated carbon (AC) prepared from waste tea with the assistance of microwave activation was studied. Unmodified and modified AC samples were characterised in terms of structural, morphological and chemical properties. Methylene blue (MB) and phenol adsorption from aqueous solution were investigated to determine the effect of modification on the adsorption performances. Modification process increased the oxygen containing groups on the surface of the AC. The adsorption of MB and phenol on the modified AC was quite different. It is reported that the adsorption of these contaminants highly depends on electrostatic and dispersion interaction between the contaminant and the surface property of the AC. Formation of carboxylic groups causes two different effects on MB adsorption. One involves the weakening of dispersion forces which reduces the adsorption. The other involves the enhancement of electrostatic force and lead to higher adsorption capacity. For phenol adsorption, it is proposed that absence of basic surface functional groups and existence of carboxylic groups have predominant factors which hinder the adsorption. (C) 2014 Elsevier B.V. All rights reserved.