红壤可变电荷矿物的酸碱缓冲能力及表面络合模型

doi:10.6023/A17020056

摘要/Abstract

氧化铁和高岭石是红壤中可变电荷的主要来源，对红壤的酸碱变化起到缓冲作用.本研究基于红壤矿物的表征和酸碱滴定实验结果，采用1-site/2-pK表面络合模型获得了其表面活性位点浓度H_s、密度D_s、酸碱平衡常数pK_a^int以及电荷零点pH_pzc等相关参数，定量解析了氧化铁和高岭土的酸碱缓冲能力.结果表明：该模型能较好地适用于分析针铁矿、赤铁矿及高岭石的表面酸碱性质；针铁矿、高岭石表面活性位点浓度H_s较高，说明其对酸具有较好的缓冲效果.根据上述酸碱性质参数，模拟计算了不同pH下的矿物表面化学物种，揭示了矿物表面反应缓冲土壤酸碱变化的机制.采用上述酸碱滴定方法及模型计算方法，分析实际林地红壤样品的酸碱缓冲能力，并采用表面络合模型计算了其表面化学物种，验证了该方法用于林地红壤酸碱缓冲能力分析的可行性.

关键词: 氧化铁, 高岭石, 红壤, 酸碱缓冲能力, 表面络合模型

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=2.9~9.5, to study the surface charge of soil minerals. The X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and N₂ 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.

Key words: iron oxide, kaolinite, red soil, acid-base buffer capacity, surface complexation model

引用此文

程鹏飞, 王莹, 程宽, 李芳柏, 秦好丽, 刘同旭. 红壤可变电荷矿物的酸碱缓冲能力及表面络合模型[J]. 化学学报, 2017, 75(6): 637-644.

Cheng Pengfei, Wang Ying, Cheng Kuan, Li Fangbai, Qin Haoli, Liu Tongxu. The Acid-Base Buffer Capacity of Red Soil Variable Charge Minerals and Its Surface Complexation Model[J]. Acta Chim. Sinica, 2017, 75(6): 637-644.

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