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