Methods of colloids’ separation. There is a whole range of currently available strategies for separation and analysis of colloids in natural water. An important progress in characterizing the behaviour of TE in aquatic environments has been done thanks to development of high resolution analytical instrumentation and techniques (see Xxxxxxxxxx et al., 2003 for review). Techniques like dialysis, voltammetry, gels diffusion (DET and DGT) are routinely used to obtain in situ information for small colloids, whereas a combination of rapid fractionation procedures, including filtration, field-flow fractionation, split-flow lateral transport thin (XXXXXX) separation cells, and cross-flow ultrafiltration (CFF) can be used for the ex-situ characterization of TE association with colloids. Ultrafiltration is most often used to characterize the proportion of dissolved and colloidal forms in surficial rivers, lakes, groundwater and seawater (Xxxxxxx et al., 1981, 2000; Xxx and Xxxxxxxx, 1996; Xxxxx et al., 1997; Xxxxx et al., 1999; Xxxxxxx and Xxxxxx, 1999; Xxxxxx-Xxxxxxx et al., 1999, 2000; Xxxxx et al., 2000, 2004; Xxxxxxxxx and Xxxxxx, 2002; Xxxxxxxxx et al., 2005a, 2006a, 2006b; Xxx et al., 2000; Xxxxxxx et al., 2007; Xxxxxx et al., 2008). In practice, there are a number of complications associated with concentration polarisation and the formation of a gel layer (the increase in colloid concentration directly above the membrane surface due to the slow back diffusion of colloids compared to their movement, under pressure, towards the membrane) (see Buffle el al., 1992). These effects may result in fractionation occurring in a different size range to that expected from the nominal pore size of the filter. Furthermore, the actual size range separated may alter over the course of the filtration and is dependent on filter manufacturer and surface area (Xxxxxxxx et al., 1996). Choice of appropriate filters and filtration conditions, in particular flow rates, can help to reduce these problems. The separation by membranes may, however, be dependent on molecular structures in NOM as well as the size (Xxxxxx et al., 2004). The separation of molecules according to size and shape by membrane filtration is also influenced by the methodical conditions. When more than one size fraction is required, cascade or parallel filtration may be used to study the colloidal size range in detail. In cascade filtration, the same sample is filtered successively through filters of different pore sizes. In parallel filtration, a...
