Proceedings of the Third World Fisheries Congress: Feeding the World with Fish in the Next Millenium—The Balance between Production and Environment
Sepiolite Membrane for Ultrafiltration
Q. K. Wang, Takeshi Matsuura, C. Y. Feng, M. R. Weir, Christian Detellier, E. Rutidinka, R. Le Van Mao
In recent years, a great deal of research has been devoted to the development of a new kind of inorganic membrane that exhibits improved resistance to heat, chemicals, and corrosion. Rapid development and innovation have already been realized in this area (Cot 1998). Clay minerals are a well-known class of naturally occurring inorganic materials with well-known structural adsorption, rheological, and thermal properties (Nagata et al. 1974; Serna et al. 1975; Jones and Galán 1988; Pérez-Rodríguez and Galán 1994). Research on clay as a membrane material has concentrated mainly on pillared clays (Cool et al. 1997; Mishra and Parida 1997). Studies of membranes prepared entirely from clay have begun (Ishiguro et al. 1995; Le Van Mao et al. 1999).
Sepiolite, one of the most important gel-forming clays, can give rise to stable suspensions of high viscosity at relatively low concentrations. It is characteristically fibrous as observed under an electron microscope (Jones and Galán 1988). Its structural and morphological changes that occur on heating can be divided into three phases: low-temperature (<300°C), central (300–600°C), and high-temperature (>600°C) regions. In the high-temperature region, dehydroxylation of the structure takes place at about 800°C, together with a change in entropy due to structural collapse (Jones and Galán 1988). This thermal behavior suggests that there is a limitation in temperature for sintering the membrane.
The objective of the present study was to establish the preparation procedure for pure sepiolite membranes and to investigate their properties and potential for ultrafiltration. The characterization of sepiolite membranes by pore size and pore size distribution is another objective.
Many works have been published on the pore size and pore size distribution of synthetic membranes (Michaels 1980; Zeman and Wales 1981; Singh et al. 1998). One of the methods of determining pore size and pore size distribution is based on the separation of solutes of known sizes. Michaels concluded that the lognormal probability function was generally accounted as means for describing sieving curves for ultrafiltration membranes, and a complete sieving curve could be constructed for a given membrane using only two experimental sieving coefficient values for two different solutes of known Einstein–Stokes radius. On the basis of this observation, Michaels proposed a method to determine pore size and pore size distribution. Singh et al. studied various sulfonated poly(2,6-dimethyl-1,4phenylene oxide) membranes and found that the sieving coefficients and Einstein–Stokes radii were correlated well with the lognormal distribution function. Aimar et al. (1994) described the problems that may be encountered when developing a method for membrane characterization based on solute sieving (e.g., concerning macromolecular transport through capillaries). They also showed the applicability of the lognormal curve to fit the solute separation–solute size correlation.
Scanning electron microscopy (SEM) is a powerful tool for investigating membrane structure. However, because of the low conductivity of the membrane surface, the sample must be coated with a heavy metal, and the coating process may cause some damage to the membrane. Therefore, SEM is not a reliable method for measuring pore size (Hsieh et al. 1979; Aimar et al. 1994). Hence, in this work, membrane pore size and pore size distribution are determined by a method proposed by Michaels (1980).