Selective Interactions of Soil Organic Matter Compounds with Calcite and the Role of Aqueous Ca

Soil organic matter (SOM) persistence in aridic soils is primarily attributed to SOM stabilization through Ca-complex-ation. In addition, SOM stabilization by calcite, a common mineral in aridic environments, was recently attributed to adsorption and occlusion on calcite voids. We present a systematic study that investigates SOM compounds interaction, individually or as part of a mixture, with calcite using molecular level dynamics and kinetic batch sorption experiments. Four SOM compounds, representing major classes in dissolved SOM, e.g., fatty acids, amino acids, carbohydrates, and lignin, were used in this study. In the single compound system, adsorption to calcite is compound specific as follows: fatty acid >> amino acid congruent to lignin and no adsorption of carbohydrate. Fatty acid selective adsorption is attributed to its carboxyl group strong affinity for Ca-coordinated sites of calcite. Amino acid weaker adsorption is attributed to the combined effects of carboxylate and ammonium multifunctionalities, resulting in frequent liquid partitioning of amino acid. Meanwhile, findings suggest that lignin may deprotonate and subsequently adsorb to calcite. In the competing mixed compounds system, fatty acid adsorbs to calcite similarly to the single system, while carbohydrate yet again does not adsorb. The amino acid and lignin behavior however is driven by calcite dissolution and Ca(aq) release. Evidence suggests the formation of aqueous Ca-organo associations, Ca-complexes, or Ca mixed cross-linkages, which surprisingly are not bound to mineral Ca-surface sites. Although Ca-organo associations are not adsorbed, they have limited mobilities, which dictate their spatial distribution in a water scarce soil system, such as an aridic environment.