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.