Chemical Force Spectroscopy Evidence Supporting the Layer-By-Layer Model of Organic Matter Binding to Iron (oxy)Hydroxide Mineral Surfaces.

The adsorption of dissolved organic matter (DOM) to metal (oxy)hydroxide mineral surfaces is a critical step for C sequestration in soils.  Although equilibrium studies have described some of the factors controlling this process, the molecular-scale characterization of the adsorption process has been limited.  Chemical force spectroscopy with iron (oxy)hydroxide-coated atomic force microscope tips revealed differing adhesion strengths to DOM extracted from three soils and a reference peat material.  The DOM was characterized using ultrahigh resolution negative ion mode electrospray ionization Fourier Transform ion cyclotron resonance mass spectrometry.  The results indicate that carboxyl-rich aromatic and N-containing aliphatic molecules of DOM are correlated with high adhesion forces.  Increasing molecular mass was shown to decrease the adhesion force between the mineral surface and the DOM.  Kendrick mass defect analysis suggests that mechanisms involving two carboxyl groups results in the most stable bond.  We conceptualize these results using a layer-by-layer “onion” model of organic matter stabilization on soil mineral surfaces.