PNNL

Abstract

It is estimated that organic carbon (OC) in soils exceeds the amount of OC in atmosphere and in vegetation pools combined.1 Given the current changes in climate and increasing temperatures in the environment, the fraction of OC sequestered in soils and sediments is more sensitive to decomposition and release as CO2 in the atmosphere. Evidence implies that sorption and association of dissolved OC with inorganic soil components, i.e., soil minerals, is a key mechanism that contributes to stabilization and retention of OC in soil environment2. Studies have shown a direct correlation between sorption of dissolved OC and soil mineral surface properties and their surface area. In addition, properties of organic compounds such as chemical structure and functional groups play a significant role in interaction of dissolved organic species with mineral surfaces. Association of OC with minerals via ligand exchange reactions, polyvalent tertiary bonding, or complexation with metal ions, is found to be preferential and dependent on the organic compounds’ functional groups.3 Here, we present a macroscopic and spectroscopic correlative method that includes nano to macroscale measurements and gives insights into OC interaction with common soil minerals. The data are attained with scanning electron microscopy equipped with energy dispersive X-ray Spectroscopy (SEM/EDX), Infrared Scanning Near-field Optical Microscopy (IR-SNOM), and X-ray Photoelectron Spectroscopy (XPS) (Fig. 1). We investigated interaction of Fe, Al, and Ca - soil minerals with four organic compounds that represent major classes of soil organic matter, namely lipid, protein, carbohydrate, and lignin. The experiments included reaction of organic compounds with mineral powders for investigating sorption specificity in connection with organic chemical structure, and for probing changes in organic species arrangement following the reaction. In addition, experiments with mineral single crystals were performed to investigate organic species interaction and possible their accumulation at specific surface sites. IR-SNOM (Fig. 1) was used for collecting chemical IR maps of mineral powders and single crystals after they were reacted with individual and with the mixture of four organic compounds in correlation with attenuated total reflection infrared spectroscopy. The SEM/EDX and chemical IR correlative maps of mineral powders gave insights into the composition of organic species after reaction, demonstrating that lignin and amino acid were the main components that took part in the organic compounds’ arrangement. Meanwhile XPS and chemical IR maps of mineral single crystals gave insights on localization and spatial distribution of organic compounds at mineral surface sites. The results from this combined approach in correlation with theoretical molecular dynamics (MD) , infer that interaction of organic compounds with mineral surfaces occurred at step and etch voids and consequently at higher energy surface sites. Our results give mechanistic insights onto OC sorption on mineral surfaces. Understanding the pathways of how mineral-organic driven interactions stabilize the organic matter in soil and sediments will contribute in predicting OC residence time and its persistence in environment.