3.4 Enhanced repeatability
The repeatability of HP-HKUST-1/PES membrane is shown in Figure 10 . The adsorption performance of the membrane adsorber is stable. It can be calculated that the adsorption capacity for CR and MB by the HP-HKUST-1/PES membrane are only decreased by 36% and 31% after seven cycles of adsorption-desorption experiments. The adsorption of CR and MB by HP-HKUST-1/PES membrane owing to the physical adsorption such as van der Waals forces, electrostatic forces and π-π bonds can be eluted by dissolving membrane adsorber in methanol for desorption, leading to higher adsorption capacity after several adsorption cycles. However, the adsorption of CR and MB by HP-HKUST-1/PES membrane owing to the chemical adsorption such as coordination interaction cannot be eluted by the desorption process. In the case of CR adsorption, the unsaturated Cu2+ on the HP-HKUST-1 can be chelated with the -N=N- group and -NH2 group in CR34. In the case of MB adsorption, the unsaturated Cu2+ forms bonds by coordination with sulfur and nitrogen atoms in MB was the chemical adsorption. Besides, π orbital electrons of the aromatic ring in MB enter the 4s vacant orbital of Cu2+ to form σ bonds, while electrons of the 3d orbital of Cu2+ enter the anti-π orbital (π*) of the aromatic ring to form d-π* antibonding35, 36. The high stabilization energy of the antibonding also increase the difficulty of MB desorption.
From Figure 10 , it can be also seen that the decrease tread of adsorption capacity for CR and MB by HKUST-1/PES membrane is not as obvious as that of HP-HKUST-1/PES membrane, since the adsorption active sites provided for chemical adsorption is much less than that of HP-HKUST-1. Besides, the adsorption active sites provided for physical adsorption owing to the larger surface of the HP-HKUST-1 leads to the adsorption for CR and MB by HP-HKUST-1/PES membrane still much higher than that by HKUST-1/PES membrane. During the process of repeating adsorption-desorption experiments, it was found that there is no HP-HKUST-1 leaching from the membrane pores under the inertial and van der Waals forces as interfacial interactions between the nanoparticles and the membrane. Thus, it can be deduced that the decrease of adsorption capacity for CR and MB after several adsorption-desorption experiments is only due to the decrease of adsorption active sites provided for chemical adsorption and the capacity owing to the physical adsorption can be kept stably for a long time