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Reported correlations between organic contaminant sorption affinity and soil organic matter (OM) structure vary widely, suggesting the importance of OM physical conformation and accessibility. Batch equilibration experiments were used to examine the sorption affinity of bisphenol A, atrazine, and diuron to five soils of varying OM composition. (13)C cross-polarization magic angle spinning NMR spectroscopy was used to characterize the organic carbon chemistry of the soil samples. High sorption by a soil low in O-alkyl components suggested that these structures may block high affinity sorption sites in soil OM. As such, soil samples were subjected to acid hydrolysis, and NMR results showed a decrease in the O-alkyl carbon signal intensity for all soils. Subsequent sorption experiments revealed that organic carbon-normalized distribution coefficient (K(OC)) values increased for all three contaminants. Before hydrolysis, K(OC) values correlated positively with soil aromatic carbon content and negatively with polar soil O-alkyl carbon content. While these correlations were weaker after hydrolysis, the correlation between K(OC) values and soil alkyl carbon content improved. This study suggests that hydrolyzable O-alkyl soil OM components may block high affinity sorption sites and further highlights the importance of OM physical conformation and accessibility with respect to sorption processes.

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.

Dissolved organic matter (DOM) chemistry and the potential for organic matter (OM) to self-associate with other OM components are important aspects of understanding the mechanisms of DOM sorption to clay surfaces. To investigate this further, we sorbed DOM isolated from peat humic acid onto either kaolinite, montmorillonite and gibbsite via ten sequential batch equilibration sorption experiments. Dissolved organic carbon (DOC) sorption to all minerals increased consistently, suggesting that sorption occurred via mineral-OM interactions at the beginning of the experiment. After six successive DOM loadings, the concentration of DOC sorbed by kaolinite and gibbsite began to plateau, likely due to the saturation of mineral surface sorption sites. Solution-state nuclear magnetic resonance (NMR) analysis of unbound DOM showed that kaolinite and montmorillonite sorbed aliphatic, protein and lignin components initially and primarily aliphatic and aromatic constituents in later sorption experiments, whereas gibbsite sorbed mostly aliphatic compounds during all DOM loadings. Analysis of the organo-clay complexes using 1 H high resolution-magic angle spinning (HR-MAS) NMR confirmed the preferential sorption of aromatic and aliphatic components to all three minerals. Overall, these results suggest that OM-OM interactions may be important mechanisms of DOM sorption to clay mineral surfaces.Soil Syst. 2018, 2, 8 2 of 17 such as van der Waals forces and hydrogen bonding, while aromatic components were sorbed to a lesser extent and primarily by montmorillonite [14][15][16]. Although previous studies have provided molecular-level insight into sorption processes, most have only examined one DOM loading onto the mineral surface [8][9][10][12][13][14][15][16]. In soil, a mineral particle may be coated with several layers of OM to form organo-clay complexes [3,16,17], and the mineral sorption capacity for OM may decrease with successive OM loading as reactive surface functional groups become saturated with OM [18]. Currently, it is unclear whether mineral properties such as SSA and CEC continue to control DOM sorption with increasing OM layering [19][20][21][22], which may limit the amount of OM that can be sequestered by organo-mineral associations [23].Several studies have demonstrated that OM-OM interactions may also stabilize and protect OM from degradation in soil [24][25][26]. For example, protein encapsulation in humic acid prevented hydrolysis of a proteins by strong acid [26], while coating lignin with dodecanoic acid protected the lignin from chemical degradation by NaClO 2 [24]. Similarly, Thevenot et al. [25] observed that lignin and aliphatic soil OM components such as plant and microbial lipids were associated in ligno-aliphatic complexes, which may slow lignin degradation in soil. In another study, the removal of O-alkyl soil OM components by acid hydrolysis increased the sorption capacity of several soils for three organic contaminants [27]. This suggested that high affinity aliphatic and aromatic sorption domains in ...