Jain et al. presented a different result, the adsorption of RhB decreased with increasing pH, the explanation of this result was that a low pH increased protonation of adsorbent, this contributed to the diffusion of RhB and generated a great deal of active sites on the surface of adsorbents [15]. Wang and Zhu demonstrated the pH exerted a little effect on RhB adsorption, this phenomenon was caused by the presence of different functional group on RhB [16].In addition, RhB can exhibit different molecular forms in different pH solution, it is a monomeric form below pH = 3.5, a zwitterionic form above pH = 3.5 [17]. H+ion may compete with cation in RhB solution, and OH−may compete with the anion. Besides the form of RhB molecular, pH also influences the surface charges on the active sites of activated carbon. Considering the above-mentioned factors,it is not surprising that the changing tendency of adsorption capacity was irregular. Here, the adsorption capacity changed with the variation of pH, but these changes were small. The pH point of zero charge of PK-AC was 6.48