Colloids The Use of Membrane Filtration to separate Particulate from Dissolved Matter

10 Colloids The Use of (Membrane) Filtration to separate Particulate from Dissolved Matter 

In natural waters and soil and sediment systems one needs to distinguish analytically between dissolved and particulate material. Fig. 7.1 classifies various types of particulate and dissolved materials. Obviously, operational distinguishing (e.g., based on filtration or centrifugation) between dissolved and particulate matter merely by filtration is often not able to discriminate between particles and solutes, because size distribution of aquatic components vary in a continuous matter from Angstroms to microns. 

The use of filters and membranes of different pore size to accomplish a sequential size fractionation is in principle, and under certain circumstances, possible it was proposed (for literature see Buffle, 1988 and 1991) to estimate the size of the various colloids and macromolecules and to determine to which extent trace elements (particularly metals) are associated with various size categories of colloids and macromolecules. Such sequential size fractionation techniques need to be applied with extreme caution we list some of the reasons why these techniques may yield errorous results (for details consult Buffle, 1991)  

1) Although most particles, larger than a given pore size, are normally retained, many smaller particles (sometimes 10 - 1000 times smaller than the pore size) may also be retained. If above and within a pore depth filtration occurs (see Chapter 7.6 and Fig. 7.12) colloidal particles smaller than the pore size become attached to the larger particles. Furthermore the pore size distribution of membrane filters is often non-narrow. 

3) The interaction of solutes (especially adsorption) with the filter material and the retained particles. The problem is especially serious with trace elements (heavy metals) that adsorb especially at pH values of 7 and above on filter walls and on the filter materials (glass, acrylic copolymers, cellulose esters, polycarbonates etc.). One also needs to consider that 

---