Abstract 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 °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 p K 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.

中文翻译：

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不同深度红土的可变电荷：酸碱缓冲能力和表面络合模型

摘要 土壤可变电荷在许多土壤地球化学过程，特别是土壤酸化过程中起着独特的作用。然而，由于土壤颗粒的复杂性，对酸碱缓冲能力和机制仍缺乏基本的了解。在本研究中，研究了来自不同深度的红土样品。元素组成、晶体结构和表面基团分别通过 X 射线荧光、X 射线衍射和傅里叶变换红外光谱表征。进一步进行热重分析以检查土壤成分和有机质含量。高岭石、石英和赤铁矿被确定为主要矿物成分。随着土壤深度的增加，赤铁矿和高岭石的含量增加，而石英的含量下降。土壤样品从 200 到 400 °C 的重量损失表明有机质随着土壤深度的增加而显着减少。基于电位滴定，pH pzc 确定为 4.4 至 5.0。表面复合模型 (SCM) 用于通过假设大块土壤的一个表面位点有两个 p K a 来进一步评估土壤的酸碱特性。结果表明，模型导出的 pH pzc 值与滴定实验的值匹配良好；因此，将 SCM 应用于检查大块土壤的可变电荷是可行的。计算的表层浓度 H s 代表土壤缓冲能力，与有机质含量呈正相关，说明土壤有机质在土壤酸碱缓冲能力中起重要作用。从外推的pH pzc ，可以提出高岭石是控制土壤pH pzc 的主要土壤矿物。这项研究将为土壤酸碱缓冲特性提供一种定量方法，并对潜在机制有基本的了解。