“Lost in filtration”—The separation of soil colloids from larger particles

Soil colloid science requires the separation of the colloids from larger particles in suspensions, which is frequently achieved by filtration. However, the results of filtration may be biased due to (i) pore clogging and (ii) the formation of a filter cake. In order to quantify these effects, we filtrated different volumes of soil suspensions containing mainly mineral (M), mainly organic (O) or mineral and organic (MO) colloids through 1.2 μm membranes. Turbidity and the concentrations of colloid-bound C, Si and Al were measured in the filtrates and, as a reference, in centrifugates of the suspensions. To exclude the influence of the filter cake and examine only pore clogging effects, we conducted the same filtration experiment with suspensions which have been pre-treated by a centrifugal elimination of particles >3 μm. Finally, we scanned a membrane after filtration with an electron microscope for the visualisation of possible pore clogging. Turbidity and concentrations of colloid-bound Al and Si in the filtrates of the pre-treated suspensions were one order of magnitude lower than in centrifugates. This discrepancy was most pronounced for M suspensions which indicates that filters preferentially remove mineral colloids. Microscope images revealed no sign for pore clogging and smaller filtrated suspension volumes did not lead to more colloid recovery in pre-treated filtrates. We assume that the colloids are retained within the thick, multilayered structure of the filter without clogging the main pores. When filter cakes are forming (experiment without previous centrifugation), turbidity and concentrations of colloid-bound Al, Si and C decrease with increasing filtration volume. However, the retaining effect of filter cakes seems negligible compared to the retaining effect within the filter. We conclude that the composition of soil colloidal suspensions depends significantly on the technique which is used to remove larger particles. Filtration underestimates the amount of colloids in suspension and centrifugation should be preferred as separation method at least for soils with colloids of similar density, either M or O.