Ex-situ characterization techniques

In the case of ex-situ characterization, the phenomenon is not examined where it occurred. These techniques require an isolation or extraction of the sample, which often compromise the experiments. The ex-situ makes complicated the real-time data acquisition.

Scanning electron microscope

The scanning electron microscope produces an image of the sample by scanning with beams of electrons. The incident electrons on the sample generate different kind of signals. The result is 3D image with high resolution that allows to characterize the sample topology. The scanning electron microscope does not allow internal exploration of the sample nor live-data acquisition. Most of the times the sample must be prepared (drying and metal coating) and finally put in an isolated chamber for the observation. For the particular case of filtration cake analysis, the sample could be

74 compromised due to the preparation which involves recuperate the sample from the filtration facility and perform a surface treatment for the observation. This method has been used to observe the cake layer surface and allows to determine the size, shape and the nature of the foulants.

Figure 23. Scanning electron microscopy: schema and principle

Recent applications of the scanning electron microscope in the filtration field aim the characterization of the membrane and the cake layer. Boissier [97] et al. studied the respective impact of wine particles, yeast (Saccharomyces cerevisiae) and fines (lactic bacteria and colloidal aggregates Figure 24), on the performances of cross-flow microfiltration under different permeate flux rate and wall shear stress conditions. The cake properties were assessed by the filtration performance rather than the direct characterization. Alternating the TMP allows to study the cake fouling reversibility by using the SEM to observe the membrane surface. Pure yeast suspension always formed reversible filtration cakes differing from suspensions containing fines, which adhesive properties were observed. This technique did not allow to characterize the cake inner structure.

75 Figure 24. SEM observation of the wine deposit after crossflow microfiltration [97]

Reingruber et al. [98] used environmental SEM (ESEM) for the characterization of microfiltration membranes. This method allows membrane characterization under wet conditions. A qualitative description of the membrane structure is achieved through the observation of the wetting and drying process of the pores. Contrary to the conventional high vacuum mode (HV) the ESEM mode works with pressures in the specimen chamber from 1 – 70 mbar which enables the investigation of wet samples. The number, size and distribution of the pores are determined recording successive images of the drying membrane surface. This SEM mode allows the investigation of samples under conditions that are closer to the “natural” ones with minimum sample preparation [99], however it is under very particular conditions regarding pressure. Also, the characterization is only done at the surface level. Both conditions limit the use of this technique for analyzing the filtration process.

Atomic force microscope

The atomic force microscope (AFM) has been also used for the characterization of filtration cake mechanical properties. This technique generates 3-D high-resolution images, which make possible to characterize the cake surface morphology. After the review of Cappella [100] an intense development has occurred. Other applications are related to the measure of adhesive forces. This technique allows studying local material properties. The force data related to this feature make it valuable in many different fields of research such as surface science, materials engineering, and biology [101].

The principle of this technique is based on the forces experienced between the cantilever’s pointer and the sample surface. The cantilever is displaced as a result of the interaction with the

76 sample. A laser is used to detect the displacement of the cantilever. The laser reflection due to the cantilever displacement allows an estimation of the sample surface. Figure 25 illustrate the principle of the AFM.

Figure 25. Atomic force microscope principle [101]

In the work made by Bowen et al. [102] the AFM was used to measure the adhesion force of activated yeast (mixed with bovine serum albumin BSA, Figure 26) to different type of membranes. For that purpose, a special coated colloid probe and cell probe techniques were used. A single yeast cell was immobilized on tipless cantilever. The adhesion was measured as the required force for detaching the probe coupled at the tip of the AFM cantilever from the membrane surface. The force curves produced by the AFM depicts the following probe-membrane interaction first from an initial state after probe-membrane contact, the probe and membrane move together with no displacement relative to the piezo. Then, there is a stretching causing a relative movement that continues until the contact between the probe and the membrane is broken. The difference in force between the moment when the stretching begins and the contact is broken measures the adhesive force. This way they were able to measure yeast-membrane adhesive forces in the range of 1.4nN to 9.9 nN.

77 Figure 26. SEM image of yeast (Saccharomyces cerevisiae) attached to an AFM cantilever – a cell

probe [102]

The work done by Gong et al. [103] to optimize wastewater treatment plants, used the AFM tool characterize the adsorbed irreversible deposit. The objective was to characterize the organic recovery of the membrane. For a better characterization, dead end filtration experiments were set to characterize the additional resistance due to irreversible fouling. After the three filtration cycles the hydraulic resistance was as high as 20.84 x1010 (m-1) which is 3 times the membrane resistance before fouling.

The AFM is powerful tool that allows not only high resolution imaging, but also to perform a mechanical characterization by force spectroscopy based on the ability to measure forces of a few pN with a vertical and axial resolution of 0.1 and 25 nm respectively [101]. In addition, the AFM allows the investigation without the risk of changing or damaging the sample with an special treatment. By the other hand, the AFM is only able to investigate an open system with limited domains, with a maximum scanning area of 150x150 µm and a maximum height of the sample of 10-20 µm. The acquisition rate of the AFM is also limited, it takes several minutes for scanning an image [104], which make it inadequate for characterizing dynamic processes.