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.