FILTRATION OF INORGANIC COLLOIDS AND ORGANICS

Aquatic suspended particles are usually characterized by a continuous particle size distribution. The distinction between particulate and dissolved compounds, conventionally made in the past by membrane filtration, does not consider organic and inorganic colloids appropriately. Colloids of iron(IIl) and manganese(III,IV) oxides, sulfur, and sulfides are often present as submicron particles that may not be retained by membrane filters (e.g., Buffle et al., 1992). Recent measurements in the ocean led to the conclusion that a significant portion of the operationally defined dissolved organic carbon may in fact be present in the form of colloid particles. 

The analysis of cow s milk shows it to be a mixture consisting of 78 per cent water and 12 per cent solid substances in solution or colloidal dispersion. Of this 12 per cent casein, the most important, belongs to a complex group of organic compounds called proteins a sugar (lactose), butter-fat, and small amounts of inorganic salts comprise the remainder. By addition of a small amount of acetic acid the dispersed protein is precipitated. By evaporation of the filtrate the crude sugar is obtained. 

In summary, although the MF of coUoids is generally well understood, the literature is somewhat limited in the areas of filtration of colloids much smaller than the membrane pore size, and in systems where aggregation occurs. Systems are, in this regard, often poorly characterised, especially in the presence of humic substances. As shown in Chapter 2 organics stabilise inorganic colloids at sizes much smaller than pores, and their behaviour in MF or surface waters is largely unknown. 

In aquatic systems, phosphorus occurs in a wide variety of inorganic and organic forms (Figure 8.1) [3]. While these may exist in either the dissolved, colloidal, or particulate forms, the predominant species is orthophosphate in either the mono- or diprotonated forms (HP04, H2P04 ). The dissolved component is operationally defined by filtration, and for this reason, the term filterable is used in preference to either dissolved or soluble, both of which are used extensively and interchangeably in the literature. 

Colloids can be organic or inorganic. Even if they are not separated from the dissolved load by classical filtration, colloids have the physicochemical properties of a solid. Colloids are finely divided amorphous substances or sohds with very high specific surface areas and strong adsorption capacities. It is shown by Ferret et al. (1994) for the Rhine River that the colloids contribute less than 2% of the total particle volume and mass, but represent a dominant proportion of the available surface area for adsorption of pollutants. The abundance of colloids, their fate, through coagulation and sedimentation processes in natural waters therefore control the abundance of a number of elements. 

Natural waters always contain suspended particles. These are inorganic substances like alumosilicates, carbonates, quartz as well as organic colloidal matter. Their concentration varies in a large interval 10-100 mg/dm for river water 10-20 mg/ dm for coastal waters > 1 mg/dm for deep sea water and the open ocean. Suspended particles should be isolated from the water after sampling otherwise processes of adsorption, desorption etc. may take place which lead to change in the original content of the sample. There are three ways to isolate suspended matter sedimentation, filtration and centrifugation. 

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