Solution-state NMR investigation of the sorptive fractionation of dissolved organic matter by alkaline mineral soils

Abstract

Environmental context Dissolved organic matter plays a key role in global carbon cycling and environmental contaminant transport. We use one- and two-dimensional solution-state nuclear magnetic resonance spectroscopy to characterise dissolved organic matter before and after binding to alkaline subsoils with low organic carbon content. The results show that the dissolved organic matter is selectively fractionated through preferential binding of specific organic carbon functional groups. Abstract Sorption to clay minerals is a prominent fate of dissolved organic matter (DOM) in terrestrial environments. Previous studies have observed that DOM is selectively fractionated by interactions with both pure clay minerals and acidic mineral soils. However, the specific DOM functional groups that preferentially sorb to mineral surfaces in alkaline soils require further examination because higher basicity could change the nature of these sorptive interactions. Biosolids-derived DOM was characterised using one- and two-dimensional solution-state NMR spectroscopy before and after sorption to three alkaline subsurface mineral soils with varying mineralogy. Carboxylic DOM components sorbed preferentially to all soils, likely due to cation bridging and ligand exchange mechanisms. Aliphatic constituents were selectively retained only by a soil with high clay mineral content, possibly by van der Waals interactions with montmorillonite surfaces. Polar carbohydrate and peptide components of the DOM did not exhibit preferential sorption and may remain mobile in the soil solution and potentially stimulate microbial activity. A relatively low signal from aromatic DOM components prevented a full assessment of their sorption behaviour. The results suggest that DOM is selectively fractionated by similar interactions in both acidic and alkaline soils that may play a key role in the chemical and biochemical processes of subsurface environments.