滇东高原红壤酸处理后粒度增粗现象及原因探究

Phenomenon and Reason of Grain Size Increasing After Adding HCl in Red Soil in Eastern Yunnan Plateau

近年来常有南方红壤样品在加酸处理后粒度不但没有减小反而增粗的现象报道。为了探究这一现象存在的规律及其原因,本研究在滇东高原选取了西南地区分布最广的3类母岩(碳酸盐岩、玄武岩、泥页岩)发育的红壤开展研究,结果发现:3种母岩发育的红壤加HCl处理后,<2μm的颗粒组分明显减小,中值粒径和平均粒径明显增粗,其中以碳酸盐岩红壤粒度增粗幅度最大。元素和矿物分析结果显示,3种红壤氧化铁含量都超过100 g/kg,碳酸盐岩红壤主要黏土矿物为三水铝石和高岭石,玄武岩红壤为高岭石,泥页岩红壤为三水铝石和蒙脱石。用DCB(citrate-bicarbonate-dithionite)法除去土壤氧化铁,样品再进行加酸消解处理后粒度均变细。结合两性胶体的电荷零点理论,本研究认为加酸促使红壤样品中的氧化铁和三水铝石胶体(两性胶体)带更多正电荷,与带负电荷的黏土矿物胶体之间的黏附作用增强,从而导致粒度变粗。这一结果说明加酸消解前处理方法可能不适用于南方风化程度较高的红壤的激光粒度测试。

In recent years, the size increase of red soil in South China has been often reported after adding acid treatment. In this study 3 red soils derived from three main parent rocks, carbonate rock, basalt and shale in southwest China were selected and collected in eastern Yunnan Plateau in order to explore the regularity and reasons of this phenomenon. The results showed that particle group of 〈2 μm decreased obviously after soil samples were treated with HCl, mid and mean particle sizes of the 3 red soils increased obviously, particularly of red soil derived from carbonate rock. The results of elemental analysis and mineral analysis showed that the contents of iron oxide in 3 red soils were higher than 100 g/kg. The main clay minerals in red soil derived from carbonate rock were gibbsite and kaolinite, was kaolinite in red soil derived from basalt, and were gibbsite and montmorillonite in red soil derived from shale. After iron oxide were removed from soils by DCB （citrate-bicarbonate-dithionite）, particle sizes of the samples were reduced after acid digestion treatment. According to the zero point of charge ampholytoid theory, it is believed that adding acid increased positive charge on iron and gibbsite colloids （ampholytoid） , intensified the aggregation with negatively charged colloids of clay minerals, thus resulted in grain size increase. This result indicates that the pretreatment of acid digestion may not be suitable for laser diffraction method in analyzing particle sizes of red soils highly weathered in south China.