As shown in
Fig. 3, the removal rate of Cr(VI) increases
first with the increase
of pH and then decreases. However, it is obvious to see that the
optimum pH values at which the Cr(VI) adsorptions reaches their
peak values are very sensitive to the initial concentrations. For the
solutions with initial Cr(VI) concentrations of 35 and 70 mg/L, the
optimum pH for Cr(VI) removal is around 3. This is well consistent
with the previous reports in the similar range of Cr(VI)
concentrations [31]. Increasing the initial Cr(VI) concentration
moves the optimum pH for Cr(VI) adsorption to lower values,
which are 2.7 and 2.5 for the solutions with initial Cr(VI)
concentrations of 105 and 140 mg/L, respectively. This is