The temperature and pH did not affect the adsorption equilibrium time. An obvious increase of adsorption capacity can be observed as the temperature increased from 293 to 303 K (Fig. 3b).The adsorption capacity only presented a small increase with further increasing the temperature from 303 to 323 K. The above results reflected that the adsorption of RhB onto PK-AC was an endothermic process. The high temperature allowed a better mobility of RhB molecules, and was benefit to the adsorption.
From Fig. 3c we can see the adsorption capacity increased from 223.8 to 233.9 mg g−1 with pH increasing from 1.31 to 3.20. The changes of adsorption capacity were small within the pH range of 3.20–10.19. When the pH was changed from 10.19 to 13.56,the adsorption capacity decreased from 232.4 to 217.7 mg g−1. The effect of initial solution pH on RhB adsorption was a complex process. Largura et al. reported that the adsorption of RhB increased as the pH increased due to the existence of electrostatic interaction between negatively charged surface of the adsorbent and RhB[14]. 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 zwitter ionic 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 (Fig. 4) that was close to neutral, indicating the number of the surface acid functional groups and basic functional groups of PK-AC may be approximately equal. The pH had little effect on the adsorption of RhB onto PK-AC. PK-AC showed good adsorption properties both in acidic and alkaline solutions.This result illustrated that PK-AC will be a good adsorbent.