The objective of the present work is to determine the effect of exposure time on functionalization through characterization of activated carbon. Pristine activated carbon has been functionalized with mixture of acids (sulphuric acid, nitric acid, acetic acid) in 3:2:1 ratio at different exposure time. FTIR and SEM analysis have also revealed that longer exposure time did not help in introduction of oxygen containing functional groups onto the surface of adsorbent rather cut the functionalization due to water cluster formation effect. These acidic groups block the reactive adsorption sites and ultimately reduce the adsorption capacity. © 2016 The Authors.