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 with in 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