Clay dispersion and its relation to surface charge in a paddy soil of the Red River Delta, Vietnam

Dispersion is an important issue for clay leaching in soils. In paddy soils of the Red River Delta (RRD), flooding with fresh water and relatively high leaching rates can accelerate dispersion and the translocation of clay. For the clay fraction of the puddled horizon of a typical paddy soil of the RRD, the effect of various cations and anions as well as humic acid (HA) at different pH values on the surface charge (SC) were quantified and the dispersion properties were determined in test tubes and described by the C₅₀ value.In the <2 μm fraction, dominated by illite, the proportion of 2:1 vs. 1:1 clay minerals is 5:1. The organic-C content of the clay fraction is 2.2%. Surface charge was found to be highly pH-dependent. At pH 8 values of -32 and at pH 1 of -8 mmolc kg⁻¹ were obtained. Complete dispersion was observed at pH > 4, where SC is > -18 mmolc kg⁻¹. The flocculation efficiency of Ca strongly depends on the pH. At pH 4, the C₅₀ value is 0.33, 0.66 at pH 5, and 0.90 mmol L⁻¹ at pH 6. At pH 6, close to realistic conditions of paddy soils, the effect of divalent cations on the SC and flocculation decreases in the order: Pb > Cu > Cd > FeII > Zn > Ca > MnII > Mg; FeII was found to have a slightly stronger effect on flocculation than Ca. An increase in concentrations of Ca, MnII, and Mg from 0 to 1 mmol L⁻¹ resulted in a change in SC from -25 to approx. -15 mmolc kg⁻¹. In comparison, the divalent heavy-metal cations Pb, Cu, Cd, and Zn were found to neutralize the SC more effectively. At a Pb concentration of 1 mmol L⁻¹, the SC is -2 mmolc kg⁻¹. From pH 3 to 5, the dispersion of the clay fraction is facilitated rather by SO$ _4^{2-} $ than by Cl⁻, which can be explained by the higher affinity of SO$ _4^{2-} $ to the positively charged sites. With an increase of the amount of HA added, the SC continuously shifts to more negative values, and higher concentrations of cations are needed for flocculation. At pH 3, where flocculation is usually observed, the presence of HA at a concentration of 40 mg L⁻¹ resulted in a dispersion of the clay fraction. While high anion concentrations and the presence of HA counteract flocculation by making the SC more negative, FeII and Ca (C₅₀ at pH 6 = 0.8 and 0.9 mmol L⁻¹, respectively) are the main factors for the flocculation of the clay fraction.For FeII and Ca, the most common cations in soil solution, the C₅₀ values were found to be relatively close together at pH 4, 5, and 6, respectively. Depending on the specific mineralogical composition of the clay fraction, SC is a suitable measure for the determination of dispersion properties and for the development of methods to keep clay particles in the soil in the flocculated state.