Characterization of nanofiltration membrane using microscopic methods

1. Introduction

The main objectives for this analysis is to compare the surface properties of two nanaofiltration membranes (NF11 and NF12) and to investigate the influence of membrane factor on fouling relationship of the membranes used to treat waste water (sewage water). Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) in-conjunction with image processing softwares (WSxM 5.0 Develop 6.4 and Image J) were used to obtain the following surface parameter of the membranes, pore size distribution and surface roughness parameters (RMS roughness, peak height, skewness and kurtosis). Hwang and Lin [1] used observations made using SEM to qualify the nature of the pores of 3 microfiltration membranes used to treat waster with a cut-off of 0.1m. They also observed the fouling of these membranes after filtration of a solution containing model particles of polymethyl methacrylate (mean diameter = 0.4m). The major disadvantage of this technique is the sample preparation by gold metallization, which entails a less accurate pore size determination [2].

Atomic Force Microscopy (AFM) was first used in 1988 to study the structure of polymeric membranes [3]. This technique can be used in three different modes: contact [4], non-contact [5] and tapping mode [6] and can be applied to all membranes, from microfiltration to reverse osmosis [78], for organic. This technique makes it possible to represent no conducting surfaces with a resolution of the order of the nanometer in either dry or wet environments [910]. Therefore, using AFM makes it possible to avoid drying the sample under vacuum.

The AFM measurements give access to the roughness, pore size, pore density and pore size distribution of a membrane [11]. They can also provide information on the surface electrical properties of a membrane, its fouling potential towards a specific colloid [12] and its filtration performance as a function of its roughness. All this can help to predict fouling without process measurements [13]. Hirose et al. and Warczock et al. [14] studied by AFM the relationship between the skin layer surface structure of NF membranes and their filtration performances. It was shown that the roughest membranes provided the best performances in terms of flux, the flux increases quasi-linearly with the roughness. However, some drawbacks of the AFM technique were pointed at: due to the size of AFM scanning probe tips, there are some limitations to the scanning depth; also, AFM may distort membrane pore size due to rounded corners near pore entrance [15]. Boussu et al. [16] compared the results obtained using contact and non-contact mode AFM.

It was concluded that when comparing surface roughness for different membranes, the same AFM method and the same scan size must be used. However in this work, contact mode was used to analyze surface properties of the two membranes. In non-contact mode, the tip of the cantilever does not contact the surface of the sample therefore the sample does not surfer from sample degradation effects compared to the other modes. In terms of analyzing wet samples, non-contact mode will be more advantageous since it will just oscillate above the adsorbed fluid layer to image both the liquid and surface. Images obtained by AFM were further analyzed using image processing software (WSxM 5.0 Develop 6.4) to obtain the statistical measures (bearing ratio, Power Spectrum Density). The results in the literature shows that the membrane with the highest roughness will perform better in waste water treatment.

-2. Experimental analysis

Analyses of two different nanofiltration membrane materials NF11 and NF12 were performed using Atomic Force Microscope (AFM). The materials were of unused nanofiltration membranes used to treat waste water. The membranes were scanned on two scan sizes (1.0m1.0m and 5m5m). The four topographic images obtained from the microscope were stored in a computer and further analyses using the image processing software WSxM 5.0 Develop 6.4 (see Figure 1 below).2.1. Experimental Set-up

Figure 1: Experimental set up.

2.2. Experimental Procedure

2.2.1. Atomic Force Microscope (AMF)

Atomic force Microscope operated under ambient conditions was used to analyze the surface roughness of two unused nanofiltration membrane materials. The materials were obtained by sawing the membranes with a diamond blade. Images were acquired in taping mode using a comitial AFM probe (Cantilever length of 215um, nominal tip height of 40um, tip radius