3.3 Effect of parameters on adsorption
During the process of membrane adsorber for CR and MB, the adsorption performance would be affected by some key parameters such as the concentration of CR and MB and the flowrate of the liquid flowing through the membrane. The effect of CR and MB concentration on adsorption is illustrated in Figure. 9(A, B) . It can be seen that higher capacity can be achieved under the condition of higher concentration of CR and MB. Besides, it can be also seen that the mesopores introduction would promote adsorption more obviously under the higher concentration of CR and MB. For example, the capacity of CR can be increased from 207 mg g-1 by HKUST-1 powder to 523 mg g-1 by HP-HKUST-1 powder under the condition of CR concentration being 250 mg L-1. On the contrary, the capacity can be only increased from 97 mg g-1 by HKUST-1 powder to 291 mg g-1 by HP-HKUST-1 powder under the condition of CR concentration being 50 mg L-1. It can be deduced that the increased capacity being 316 mg g-1 can be achieved under the condition of higher CR concentration but the increased value of only about 194 mg g-1 is obtained under the condition of lower CR condition.
The effect of flowrate and mass of HP-HKUST-1 on the adsorption of CR and MB by HP-HKUST-1/PES membrane is presented in Figure 9(C, D ) and Figure S3 . It can be seen that the adsorption rate constant can be increased significantly with the increase of flowrate while the capacity can be hardly affected by the flowrate. It can be calculated that the adsorption rate constant for CR by HP-HKUST-1/PES membrane can be increased from 0.0045 g mg-1h-1 under the condition of flowrate of being 20 mL min-1 to 0.0774 g mg-1h-1 under the condition of flowrate of being 60 mL min-1. The corresponding value for MB adsorption can be increased from 0.0196 g mg-1 h-1to 0.114 g mg-1 h-1. High adsorption rate obtained by flowrate increase can be attributed to the enhanced mass transfer rate in the micro or nano scale membrane pores. The rate of mass transfer may be relatively lower and the CR and MB molecules cannot be replenished in time due to the relatively higher intrinsic adsorption rate, if the flowrate is low. Thus, a thicker boundary layer around the adsorbate nanoparticles may be formed and the concentration of the CR and MB on the surface of HP-HKUST-1 would be lower than that of the bulk solution, leading to the adsorption rate limited. The mass transfer would be enhanced with the boundary layer being thinner under the condition of higher flowrate. In this case, the fresh CR and MB molecules transferred to the surface of the HP-HKUST-1 could be promoted, leading to the adsorption rate increased.