Composition-Dependent Sorptive Fractionation of Anthropogenic Organic Matter on Fe-Rich Clay Minerals.

"Stabilized" soil organic matter has important roles in terrestrial carbon cycling, soil water retention, providing nutrients for plant growth, and contaminant sequestration. Numerous studies have suggested that the amount of organic matter which a soil can stabilize is limited by its ability to form mineral-organic associations. In recent years, advanced analytical techniques, including synchrotron-based spectroscopy, have illustrated the heterogeneity and complexity of mineral-organic matter associations. In addition, it has been suggested that organic matter "self-assembles" on mineral surfaces in discrete zones produced by multiple and various chemical interactions. In this context, the occurrence and extent of preferential sorption remains unclear. We conducted batch-equilibrium sorption experiments with Fe-enriched montmorillonite and structural fractions of organic matter derived from composted biosolids. At the end of the sorption experiments, the sorbed organic matter was extracted with 0.1 M NaOH, and compared to the original organic matter using total organic carbon (TOC) analysis and negative ion mode electrospray ionization (ESI-) Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS). Overall, 45% to 64% of the original organic carbon was sorbed. Excluding the hydrophilic neutral (HiN) structural fraction (87%), more than 98% of the formulas assigned to peaks in the original solutions were also detected in the sorbed organic matter. This indicated that most of the organic matter components were sorbed to the Fe-enriched montmorillonite, at least to some extent. Nevertheless, every sample contained aromatic and highly oxygenated formulas that were preferentially sorbed. In addition, the HiN fraction included a high abundance of nitrogen-containing formulas that preferentially remained in solution. Overall, the batch-equilibrium experiments demonstrated that sorptive fractionation is affected by organic matter composition, at least under the studied conditions.