Variable charges of a red soil from different depths: Acid-base buffer capacity and surface complexation model

The soil variable charges play a unique role in many soil geochemical processes, particularly soil acidification. However, due to the complicated nature of soil particles, a fundamental understanding of the acid-base buffer capacities and mechanisms is still lacking. In the present study, red soil samples from different depths were examined. The element compositions, crystal structures, and surface groups were characterized by X-ray fluorescence, X-ray diffraction, and Fourier transform infrared spectroscopy, respectively. Thermogravimetric analysis was further conducted to examine the soil compositions and the organic matter content. Kaolinite, quartz, and hematite were identified as the dominant mineral components. As the depth of the soils increased, the contents of hematite and kaolinite increased, while the contents of quartz decreased. The weight loss of the soil samples from 200 to 400 degrees C indicated that the organic matter decreased substantially with increasing soil depth. Based on the potentiometric titration, the pH(pzc), was determined to range from 4.4 to 5.0. The surface complexation model (SCM) was used to further evaluate the acid-base properties of the soils by assuming two pK(a) for one surface site of the bulk soil. The results showed that the values of the model-derived pH(pzc) were well matched with those from the titration experiments; therefore, it is feasible to apply the SCM in examining the variable charges of the bulk soils. The calculated surface site concentration H-s representing the soil buffer capacity, was positively correlated with the contents of the organic matter, implying that the organic matter of the soil plays an important role in the soil acid-base buffer capacity. From the extrapolated pH(pzc), it can be proposed that kaolinite was the major soil mineral controlling the pH(pzc), of soils. This study would provide a quantitative approach for the soil acid-base buffer properties and a fundamental understanding of the underlying mechanisms.