Charge state and particle network formation were studied at pH 6-6.5 in model systems containing montmorillonite, humic acid (HA) and calcium ions (Ca2+), which are responsible for microaggregate formation in soils. Since dispersing and aggregation of particles are governed by the composition of the coating layer on particle surfaces, the interfacial equilibria for the same systems were studied in Part I. Here, the composition of the systems was based on the HA and Ca2+ adsorption results using a meaningful HA concentration unit related to the molar amount of its acidic functional groups. Approaching real conditions, the effect of Ca loading was also studied at low and high HA content, representing the colloid fractions of soil poor (∼0.5%) and rich (∼5%) in organic matter. The zeta potential was measured and the structure of dense suspensions was characterized using rheology. HA addition breaks down the shear-tolerant structure of the particle network forming in montmorillonite suspensions at pH ∼6.5. Suspensions become liquefied due to the dispersing effect of the HA. However, a much stronger structure can build up, if both HA and Ca2+ are present in optimal ratio, which can be readily estimated from the charge balance between negative charges (cation exchange sites on montmorillonite and acidic groups of HA) and positive charges of Ca2+. On the other hand, zeta potential determination did not support these changes, except the dispersing effect of HA. The strengthening of the particle network was quantified using the Bingham yield values. With increasing Ca loading, an almost fivefold increase in shear tolerance could be attained in HA-rich suspensions vs. those without HA or with low HA content. The aggregation of mineral particles is enhanced by the joint effect of HA and Ca2+. The higher the OM content, the more the Ca loading is expected to reach optimal aggregation.
ASJC Scopus subject areas
- Geochemistry and Petrology