Element Composition of Fractionated Water-Extractable Soil Colloidal Particles Separated by Track-Etched Membranes

Membrane fractionation with track-etched membranes was used to size-profile the microelement composition of water-extractable soil colloids (WESCs). The aim of the study is the element composition of narrow WESC fractions of typical chernozems in the range of 0.01–10 µm. Micro-/ultrafiltration through a cascade of track-etched polycarbonate membrane filters with pore sizes of 5, 2, 1, 0.8, 0.4, 0.2, 0.1, 0.05, 0.03, and 0.01 µm at room temperature was used. ICP–AES using direct spraying of obtained fractions without decomposition was used; Al, Ba, Cd, Cr, Cu, Fe, Mn, Si, Sr, Ti, Zn, Ca, K, Mg, Na, P, and S were found. Narrow WESC fractions differ significantly. For macro- and microelements, maximum amounts of Si, Al, Fe, and Ti and their maximum percentages are observed in fractions with sizes above 1 µm, while Ca, Mg, Mn, Cu, Zn, K, and S are accumulated more in fractions with sizes below 1 µm. The developed approach provides preparative isolation of a detailed set of narrow WESC fractions in the micrometer–nanometer range. This provides element soil profiles that reveal distinct differences and the individual character of each fraction as well as trends in changes in the mineral matrix and microelement composition with fraction size.