Using FLOWFFF and HPSEC to determine trace metal–colloid associations in wetland runoff

Natural organic matter (NOM) and iron colloids can coexist in surface water. These colloids might exhibit different affinities to metals and metalloids. Previously it has been shown, that organic and inorganic colloids in the low nanometer range can be fractionated using Flow Field-Flow Fractionation analyzes (FlowFFF), but it is not yet understood how the presence of inorganic colloids influences results obtained by High Performance Size Exclusion Chromatography (HPSEC). Studies that compare the use of these size-separation techniques for the analyzes of organic and inorganic colloids and associated elements are needed in order to interpret results obtained by either of these methods. Therefore, associations between colloids from a small stream draining a wetland area and a selected range of elements (Fe, Al, Ti, Pb, Cu, Ni, As, U, and Rare Earth Elements (REE)) have been investigated. FlowFFF analyzes and HPSEC analyzes were combined with ultrafiltration, functional group titration and arsenic speciation analysis.NOM and, in a sample with a pH > 5.2, slightly larger iron organo-mineral colloids, were present in the <0.2 μm fraction in the surface water. Both exhibited notably different affinities for trace elements. Cu, Ni, Al, and the REE all showed similar modes (i.e. peak maxima) and size distributions to the NOM, while Pb and As showed a preferential association with iron organo-mineral colloids. It was not possible to differentiate between NOM and iron-organo mineral colloids with HPSEC. The differences in the results regarding the apparent molecular mass distributions obtained by FlowFFF and HPSEC are discussed.

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► Three size-separation techniques were compared and differences interpreted.
► Colloids below 25 nm are present in wetland runoff and control metal and metalloid speciation.
► NOM and iron organo-mineral colloids compete with each other for binding and complexation.
► HPSEC does not allow the differentiation between NOM and iron-organo-mineral colloids.