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 (Hs) and density (Ds) of the surface active sites on the soil minerals. The acid-base equilibrium constants (pKa) of soil minerals were obtained by extrapolation and the corresponding pHpzc were calculated by the following formula: pHpzc=1/2 (pKa1+ pKa2). The result of calculated value of pHpzc 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.

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红壤变电荷矿物的酸碱缓冲能力及其表面络合模型

氧化铁和高岭石是红土中可变电荷的主要来源。由于在不同酸碱条件下被质子化和去质子化，表面羟基可以缓冲红壤的pH变化。本研究在控制pH=2.9~9.5的条件下，采用标准HCl和NaOH溶液通过自动电位滴定法对氧化铁和高岭石进行滴定，研究土壤矿物质的表面电荷。X 射线衍射 (XRD)、傅里叶变换红外光谱 (FTIR) 和 N2 解吸/吸附等温线 (BET) 用于表征土壤矿物质的晶体结构、表面基团和比表面积。根据表征数据和滴定曲线，利用1-位点/2-pK表面络合模型分析了矿物的酸碱性质。Gran plot 方法通常用于确定等当点，用于计算土壤矿物质表面活性位点的浓度 (Hs) 和密度 (Ds)。通过外推法得到土壤矿物质的酸碱平衡常数（pKa），相应的pHpzc按下式计算：pHpzc=1/2（pKa1+pKa2）。pHpzc的计算值与实验值基本一致，表明该模型计算方法应用于土壤矿物质是可行的。此外，上述参数可以定量解释矿物的酸碱缓冲能力。结果表明，针铁矿和高岭石具有较高的表面活性位点浓度。根据参数，通过Visual Minteq软件利用双层模型（DLM）计算不同pH下矿物的表面化学形态，以解释矿物表面酸碱缓冲行为的机制。最后，还采用酸碱滴定法和模型计算方法对天然红壤样品的酸碱缓冲能力进行了分析。进一步验证了该方法在红壤上的可行性。然后，通过表面复合模型计算了红壤的表面化学物种（≡SOH2、≡SO和≡SOH），以进一步解释其酸碱缓冲机制。还采用酸碱滴定法和模型计算方法分析了天然红壤样品的酸碱缓冲能力。进一步验证了该方法在红壤上的可行性。然后，通过表面复合模型计算了红壤的表面化学物种（≡SOH2、≡SO和≡SOH），以进一步解释其酸碱缓冲机制。还采用酸碱滴定法和模型计算方法对天然红壤样品的酸碱缓冲能力进行了分析。进一步验证了该方法在红壤上的可行性。然后，通过表面复合模型计算了红壤的表面化学物种（≡SOH2、≡SO和≡SOH），以进一步解释其酸碱缓冲机制。