Effect of water entrapment by a hydrogel on the microstructural stability of artificial soils with various clay content

Interactions between soil constituents define soil microstructural stability and are enhanced by swellable organic substances (hydrogels) such as extracellular polymeric substances (EPS), root mucilage or synthetic polymers. This study aims to identify the still largely unknown mechanisms behind hydrogel-induced soil microstructural stability. We hypothesized that soil microstructural stability increased with increasing limitation of hydrogel swelling between soil particles. One- and two-dimensional 1H proton nuclear magnetic resonance relaxometry (1H–NMR relaxometry) measurements were performed with untreated and polyacrylic-acid (PAA) treated artificial soils at various clay content and PAA concentrations. The results on the water distribution and water mobility in the artificial soils were related to their microstructural stability as measured by rheology. PAA treatment significantly increased soil microstructural stability up to five times, especially at high clay content. Soil microstructural stability increased with decreasing rotational mobility of water in the PAA-treated artificial. At the two highest PAA concentrations, the microstructural stability was the highest, although the mobility of water molecules was not further restricted. In artificial soils, the viscosity of hydrogel structures between mineral particles and the additional formation of an external network by polymer-clay interactions such as polyvalent cation bridging seem to promote microstructural stability.