Although both antibiotics have two similar proton-binding sites (carboxyland piperazinyl group) with reported pKa values of 6.22 and 8.51, respectively, there is a difference in pH values which give best adsorbed amounts. This may be confirmed by the fact that the solution pH increased after NOR sorption at pH 4.5–5.5, causing repulsion between adsorbent and adsorbate and the sorption greatly depressed.