Effects of oxalic and citric acids on three clay minerals after incubation

The release of silicon (Si) from and the structural changes to clay minerals (i.e., kaolinite, montmorillonite (Mt), and illite) were investigated after being batch incubated with low-molecular-mass organic acids (i.e., oxalic and citric acids, LMMOA) of varying concentrations (0–0.5 mol L− 1) at 25 °C. Two mineral acids (i.e., HCl and H2SO4) with varying concentrations were also reacted with Ca-montmorillonite (Ca2 +-Mt). With increasing reaction time, the Si released from kaolinite and illite reached its maximum level after incubation with the varying LMMOA concentrations for 15 days. Quantities of Si released from the clay minerals with the LMMOA ranged from 0.22 to 114.01 μg mL− 1 (Si). Quantity of Si released with oxalic acid was higher than that with citric acid. The Si released from the clay minerals increased with increasing organic acid concentrations. More Si was released from Mt than from kaolinite and illite when incubated with the same LMMOA concentration. With increasing organic acid concentrations, the X-ray diffraction (XRD) reflection intensities of the Mt weakened, and this trend was more remarkable in the clay treated with oxalic acid than citric acid. However, no relationship between the XRD reflection intensity and the acid concentrations was noticeable in the incubated kaolinite and illite. While incubating with hydrochloric acid (HCl) and sulfate acid (H2SO4) for 15 day incubation, amounts of Si released from Ca2 +-Mt were generally higher than those reacting with organic acids, and more Si was released in the H2SO4 treatment than that of HCl. But the XRD reflection intensities of the Ca2 +-Mt had no distinctly change compared with those treated with the organic acids.