## 1 Introduction  
The supported liquid membrane (SLM) technique is a promising liquid membrane technique which allows extraction/separation/removal of metal ions for various wastewater treatments [1; 2; 3; 4; 5; 6]. The advantages of SLMs over traditional separation technologies are the low inventory of the organic phase used, lower capital and operating costs, and low energy consumption for mass transfer by combining extraction and stripping in one single stage. Furthermore, with proper selection of impregnated extractant, one can relatively easily modify the SLM with selectivity toward the target metal ions [7; 8].  
The most important factors that influence the performance of supported liquid membrane (SLM) system are physical stability of the impregnated carrier and the rate of the mass transfer through the membrane. The structural characteristics of membrane support directly affect the performance of carrier facilitated transport membrane [9]. Many studies have focused on the search of new membrane support. The strategies for the developments of new membrane support included creating dense interface gel layers inside the pores [10], coating with surface layer [11], functionalization of membrane support [12], formation of composite membranes [13; 14] and improvement of pore structure of membrane [15]. A suitable membrane support should have pore structure characteristics of high porosity, pore size distribution, and proper tortuosity [16].  
The porous PolyHIPE materials have high porosity provided by large amounts of spherical voids highly connected with small window throats [17]. With high porosity, PolyHIPE can incorporate substantial amounts of metal ion extractant into the membrane for metal ion extraction. In addition, a porous membrane with high interconnectivity can have high transport efficiency [18]. Furthermore, the small window throats may also be responsible for higher extractant stability, due to the capillary effect. Therefore, in our previous study we have demonstrated that PolyHIPE with highly interconnected macroporous structure can be advantageous for membrane applications [15]. The remarkable combination of high flux and high stability of Aliquat 336/PolyHIPE membrane has prompted us to further investigate the parameters and mechanisms of carrier facilitated transport relating to the pore structure of PolyHIPE membrane.  
Facilitated transport through membranes is a complex and coupled process. It is important to understand and quantify the coupling to describe the transport in membranes. To understand the facilitated transport of ions through PolyHIPE membrane, Cr(VI) transport through PolyHIPE membrane impregnated with Aliquat 336 is used as a model system because of the following reasons. Cr(VI) is very important in many industry applications, such as electroplating, leather tanning, and industrial metal alloys. Cr(VI) is toxic compound, and hazardous to the environment. Therefore, the study of recovery of Cr(VI) is interesting from both environment and resource recycling points of view. Furthermore, numerous studies of Cr(VI) separation provide abundant comparisons with the system in this study.  
It was a concept common to many models that aim at relating macroscopic transport properties of porous materials to their microstructure [19]. In this study, the pore structure of PolyHIPE substrate was characterized based on the pore size distribution, porosity, tortuosity, and constrictivity, to evaluate membrane properties. The transport behavior of Cr(VI) was examined in order to elucidate the mechanism for the facilitated transport of Cr(VI) ions through Aliquat 336/PolyHIPE membrane. The feasibility for practical application of the PolyHIPE membrane was demonstrated by the long-term performance of the membrane.