Figure 6 . Adsorption property of HP-HKUST-1/PES membrane (A, C: the adsorption curves and the adsorption kinetic models of powders and membrane adsober for CR adsorption; B, D: the adsorption curves and the adsorption kinetic models of powders and membrane adsorber for MB adsorption; concentration and volume of CR and MB solution: 200 mg L-1, 80 mL, mass of the HKUST-1 and HP-HKUST-1 powders: 0.015 g; 20°C).
The increase of the adsorption rate can be attributed to the increase diffusion of the CR and MB to the adsorption active sites in HP-HKUST-1. It has been reported that the hierarchically porous structure facilitates the diffusion of the guest molecules within the pores and has a smaller diffusion limit30. In other words, mesoporous adsorption provides a larger concentration gradient for secondary multilayer adsorption and provides conditions for capillary coalescence, with mass transfer enhanced. Compared with the powdered HKUST-1, CR and MB can diffuse in the mesopores of powdered HP-HKUST-1 and the required time for CR and MB moved to the adsorption active sites can be shortened, leading to the increase of the adsorption rate.
From Figure 6 , it can be also seen that the adsorption rate by HP-HKUST-1/PES membrane is faster than that by the HP-HKUST-1 powder while the increase of adsorption capacity is not obviously. Specifically, the adsorption capacity of HP-HKUST-1/PES membrane for CR with 474 mg g-1 and for MB with 272 mg g-1 can be achieved within 20 min. It can be deduced that the adsorption rate constant can be increased about three times for CR adsorption and five times for MB adsorption, after the HP-HKUST powder is immobilized in membrane pores under the condition of adsorption carried out in the flow-through mode. Enhanced contact and mass transfer are the two contributors for the faster adsorption by membrane adsorber, as described in our previous research16-22. The aggregation of the nano HP-HKUST-1 powder can be prevented beyond the scale of membrane pore size. Therefore, the active sites for adsorption on the surface of the nano HP-HKUST-1 powder can be exposed more, leading to the contact between the CR or MB and the HP-HKUST-1 enhanced. However, the quantity of active sites (on surface and at the inner) for adsorption cannot be increased just by avoid of aggregation. So, the adsorption capacity of the membrane adsorber can be hardly increased compared with the powder. During the process of adsorption by membrane adsorber with flow-through mode, the mass transfer is performed in the membrane pores and the distance of the mass transfer can be reduced to nano scale. Compared with the adsorption by powder, the resistance of the mass transfer in membrane pores can be reduced significantly. Therefore, the adsorption rate can be improved significantly by membrane adsorber.