Figure 7. Mechanism of the adsorption of CR and MB by HKUST-1
(A) and HP-HKUST-1 (B).
In order to determine the diffusion mechanism of CR and MB during
adsorption, the adsorption performance is described by the Weber-Morris
intraparticle diffusion model, as shown in Figure 8 . In
general, three stages are included during the adsorption process by a
porous material: outer diffusion process, inner diffusion process and
intrinsic diffusion process. From Figure 8 , it can be seen that
the adsorption for CR by HKUST-1 or HP-HKUST-1 can be only divided into
two steps owing to the indistinct inner diffusion stage and the
adsorption for MB can be divided into three steps. In other words, CR
can be only adsorbed on the surface of the HKUST-1 or HP-HKUST-1 while
MB can diffuse into the micropores of HKUST-1 or HP-HKUST-1. The
difference of diffusion mechanism for CR and MB adsorption can be
attributed to the different molecular sizes of CR and MB. The molecular
size of CR (2.29×0.82×0.60 nm) is larger than the micro-pore size of
HKUST-1 or HP-HKUST-1 (0.8 nm) 31, 32. Thus, CR cannot
diffuse into the micro-pores of HKUST-1 or HP-HKUST-1. Compared with RB,
the molecular size of MB (1.26 × 0.77 × 0.65 nm) is smaller and CR can
enter the micro-pores of HKUST-1 or HP-HKUST-133. The
introduce of mesopores can provide more unsaturated
Cu2+ and oxygen adsorption sites generated with the
exfoliation of the template agent. Thus, compared with HKUST-1/PES
membrane, the adsorption capacity for MB by HP-HKUST-1/PES membrane can
be improved.