Generally, solution pH is recognized as an important parameter that dominates the adsorption process at water–solid interfaces. As listed in Fig. 4, the adsorption capacity kept decreasing with the increasing of solution pH. For AB25, the removal efficiency decreased sharply at the pH of 2–3, then changed little with further increase of pH to 8. For AB93,
ATBA and AGYG, the adsorption capacity remained stable at the pH of 2–7. All the four acid dyes adsorption capacity began to decrease significantly above pH 8. The similar trend of acidity effect can be found in the adsorption on Acid Red 151, using Na-Bent and CTMAB-Bent as adsorbent[39]. The pH of natural wastewater is usually in the range of 6–9,so it does not need to adjust the solution pH in view of practical application