This week our paper about clays and organic acids is out in Langmuir!
Clays – the secondary mineral phases formed during weathering processes – and organic molecules rich in acidic functional groups are abundant in soils and sediments, and interact with each other to form soil aggregates. Such aggregates are vital for nutrient cycling and soil productivity, and once unstable, can bring about soil erosion. Organic acids in soils are products of oxidative decomposition of soil organic matter and, as such, comprise a large group of acidic molecules with different sizes, degrees of polymerization, structures, and functional groups. In our work, we target dicarboxylic acids, which are small molecules with a simple structure terminated with carboxylic groups on two ends. But do clay minerals really interact with such small and soluble molecules?
The answer is: no and yes. In our paper, we focused on abundant basal surfaces of mica clay. These surfaces are negatively charged and so are the organic molecules at most pH solution conditions. As a result clays and deprotonated dicarboxylic should electrostatically repel each other when dispersed in water. One may thus expect the ‘no’ answer: dicarboxylic molecules cannot effectively bind to clays and are rather inefficient in forming organo-mineral soil aggregates.
The missing part of the ‘yes’ answer are inorganic cations. It turns out that some multivalent cations, which densely populate the negatively charged clay surfaces, can act as bridges, bonding the deprotonated carboxyls to the Ca2+-rich clay surface. Clearly, that bridging action is extremely important in the formation of soil aggregates and to soil stability! But soil moisture and fluids percolating through soil horizons contain a multitude of other dissolved ions and species. What happens when the calcium ions are depleted and replaced by ubiquitous sodium?
To find out, and learn more about organic acids trapped between two surfaces, check out our paper!