Silicic acid as a dispersibility enhancer in a Fe-oxide-rich kaolinitic soil clay

• Silicic acid can sorb on positively charged sites of Fe oxide-rich kaolinitic soil clays.
• Resulting decrease in ζ is a key factor for dispersion of the clay fraction.
• The clearest effect of silicic acid on clay dispersion was seen at pH range from 4 to 6.
• Effect of silicic acid can be obscured in strongly acidic or alkaline conditions.
• Silicic acid may act as an enhancer to transport more Si sink (clays) from terrestrial soils into the oceans.

In soils, dispersion is a crucial step preceding the loss of clay-sized particles by surface run-off and leaching, which in turn results in soil degradation. Despite the fact that silicic acid is common in soil solutions, its effect on aggregation stability of fine sized particles in soils has not been detailed. Here we examined the effect of silicic acid on dispersion of a kaolinitic soil clay fraction rich in Fe-oxides (8.5%) at different pH values by combining dynamic light scattering for particle sizing and test tube experiments for examination of particles in suspension. Adsorption of silicic acid on the clay fraction was characterized by batch adsorption experiments and effects on surface charge by zeta potential (ζ) measurements over a pH range from 2 to 11. We found that silicic acid adsorbed onto clay and co-existing Fe oxides, made ζ more negative, and thereby counteracted aggregation of the clay fraction. Silicic acid showed its most effectivity for maintaining dispersion at a pH range of 4 to 6. Beyond this pH range, dispersion was either strongly favored or prevented, and the effect of silicic acid on dispersion was obscured. Given the ubiquitous presence of silicic acid in soils, our findings regarding its impact on aggregation stability have important implication for conservation of acidic soils.