The Acid Base Buffer Capacity of Red Soil Variable Charge Minerals and Its Surface Complexation Model

作者：Cheng Pengfei; Wang Ying; Cheng Kuan; Li Fangbai; Qin Haoli; Liu Tongxu^*

来源：Acta Chimica Sinica, 2017, 75(6): 637-644.

DOI：10.6023/A17020056

• iron oxide
• kaolinite
• red soil
• acid-base buffer capacity
• surface complexation model

Iron oxides and kaolinite are the main sources of variable charges in the red soil. As a result of being protonated and deprotonated under different acid-base conditions, the surface hydroxyl groups can buffer the pH changes of red soil. In this study, iron oxide and kaolinite were titrated by the standard HCl and NaOH solution through the auto potentiometric titration under the controlled pH=2.9 similar to 9.5, to study the surface charge of soil minerals. The X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and N-2 desorption/adsorption isotherms (BET) were used to characterize the crystal structures, surface groups and specific surface areas of soil minerals. Based on the characterization data and titration curves, the acid-base properties of the minerals were analyzed by using 1-site/2-pK surface complexation model. The Gran plot method, commonly used to determine the equivalence points, was applied to calculate the concentration (H-s) and density (D-s,) of the surface active sites on the soil minerals. The acid-base equilibrium constants (pK(a)(int)) of soil minerals were obtained by extrapolation and the corresponding pH(pzc) were calculated by the following formula: pH(pzc) = 1/2 (pK(a1)(int)+pK(a2)(int)). The result of calculated value of pH(pzc) was nearly equal with the experimental value, which showed that it is feasible to apply this model calculation method on the soil minerals. In addition, the above parameters can explain the acid-base buffer capacity of the minerals quantitatively. The results show that goethite and kaolinite have the higher surface active site concentration. According to the parameters, the surface chemical speciation of minerals at different pH were calculated by Visual Minteq software with the double layer model (DLM) to explain the mechanism of acid-base buffer behavior on the mineral surfaces. Finally, the acid-base titration method and model calculation approach were also used to analyze the acid-base buffer capacity of the natural red soil samples. The feasibility of this method on the red soil was further verified. Then, the surface chemical species ( SOH2+, SO- and SOH) of the red soil were calculated by surface complex model to further explain their acid-base buffer mechanism.

• 出版日期2017-6-15
• 单位贵州师范大学