Humic supramolecular structures have polar surfaces and unpolar cores in native soil

•Copper served as tracer to study spatial orientation of humic substances in native soil.•Humic substances were kept fixed in native state by high ionic strength.•Fractionation was facilitated by stepwise reduction of ionic strength at high pH.•Most polar humic fractions bound Cu prior to least polar fractions.•Humic substances are shielded against each other in the order of descending polarity.

It was the aim of our study to prove the hypothesis that humic substances (HS) in native soil are spatially arranged in descending order of polarity, meaning that highly polar supramolecular subunits shield less polar subunits against the free soil solution and form layers of descending polarity. To address this aim, we consecutively extracted humic substances from soil with 8 M (HS1), 4 M (HS2), 2 M (HS3), 1 M (HS4) and 0.5 M LiCl (HS5) solution in 0.2 M LiOH after Cu2+ adsorption in batch soil column experiments. Adsorption was performed for 1, 10 and 60 min with concentrations ranging from 9.5 to 110 mg L−1 Cu2+ in 0.02 M CaCl2 solution. We assumed that high ionic strength facilitates extraction of most polar organic compounds, with polarity of the extracted HS decreasing with decreasing ionic strength, and that Cu extracted together with the successive HS solely formed coordination complexes, facilitating its use as a tracer for organic matter studies. We hypothesized a delayed Cu adsorption on the less polar fractions in case of spatial shielding due to interception on overlying fractions, and a concurrent Cu adsorption in case of random spatial arrangement. It was concluded that humic substances are shielded against each other in the order of descending polarity of the supramolecular subunits (free soil solution | HS1 | HS2 | HS3 | HS4 | HS5).

Graphical abstractDownload high-res image (459KB)Download full-size image