Investigation of time-dependent reactions of H+ ions with variable and constant charge soils: a comparative study

A sampling-separation method and a dynamic monitoring method were used to investigate the time-dependent reactions of H⁺ ions with two contrasting types of soil, variable charge soils (VCS) and constant charge soils (CCS), by directly evaluating H⁺ ion consumption and other relevant consequences. The results for both CCS and VCS show that H⁺ ion consumption, increase in positive surface charge and increase in soluble Al are all characterized by a rapid step followed by a slow one. The higher the content of free Fe oxides in the soil, the larger the increase in positive surface charge and in H⁺ ion consumption in the initial rapid step. This is due mainly to protonation on external surfaces. The gradual increase in positive surface charge in the slow step for the 3 VCSs is a result of H⁺ ion diffusion to the reactive sites of Fe–OH on internal surfaces. The very low content of free Fe oxides on internal surfaces of the 2 CCSs render a negligible increase in positive surface charge in the slow step. For the 3 VCSs, the gradual consumption of H⁺ ions in the slow process is the result of protonation, Al dissolution and/or transformation into exchangeable acidity. For the 2 CCSs, however, the gradual consumption is mainly the result of Al dissolution and/or transformation into exchangeable acidity. The time-dependent Al dissolution from both VCS and CCS is influenced by several factors such as mineral components, solubility and dissolution rates of the soils, and H⁺ ion concentration in soil suspensions.