【摘要】 红壤胶体的负电荷较其它类型土壤为少,而正电荷则较多。红壤胶体的腐殖质的“表观负电荷”也较小,但对胶体负电荷的相对贡献却较大。这些特点,砖红壤胶体表现得更为突出。红壤胶体的电荷具有较大的可变性。第四纪红色粘土发育的红壤在pH5以上时产生可变负电荷,在pH6以上时对氯离子为负吸附;砖红壤在低pH时带淨正电荷,pH较高时带淨负电荷,其等电点pH约为4.0。腐殖质使砖红壤的等电点降低,游离氧化铁使等电点升高;花崗岩发育的经壤的負电荷数量介于上两种土壤之间,没有出现等电点。这三种红壤胶体的负电荷有随pH升高而增加的趋势,在pH5—6处有一转折点。由此看来,这三种胶体上可能存在着两种解离程度不同的交换点。红壤胶体的比表面较小,当去除游离氧化铁后,表面积特别是外表面积显著減小。游离氧化铁的“表观比表面”较胶体晶核者大数倍,推想,游离氧化铁可能是以多孔状的聚积体形态,包被在胶体晶核的表面上。根据红壤胶体的淨负电荷和比表面算得的红壤胶体的表面电荷密度,一般多在每平方厘米15—20微库仑之间,某些砖红壤可低到10微库仑左右。去除游离氧化铁后,表面电荷密度显著增大。更多还原

【Abstract】 The charge characteristics of the clay fraction of red soils under present investigation are summarized as follows: (1) The clay fraction of red soils carries a smaller amount of negative charge, but a greater amount of positive charge than that of neutral soils of temperate regions. The humus of the clay fraction of red soils has smaller calculated "apparent negative charge" than that of neutral soils, but it gives a relatively large quantity of negative charge to the entire clay. These characteristics are more significant for certain laterites. (2) The electric charge carried by the clay fraction of red soils varies distinctly at different pH and in different soil types. For a red soil derived from Quaternary red clay in central China, a variable negative charge appears at soil acidity strenger than pH 5, and a negative adsorption of chloride takes place at pH above 6. The laterite carries net negative charge at higher pH and net positive charge at lower pH with an isoelectric point of about pH 4. In the laterite isoelectric point increases after the removal of humus but decreases after the removal of humus and free iron oxides. The variability of electric charge of the red soils derived from granite in southern China lies in between that of the above-mentioned soils. In view of the linear relationship between the negative charge and the pH value and in view of the appearance of an inflection point within the range of pH 5—6, it is inferred that the clay fraction of these red soils possesses two types of negative exchange sites with different degrees of ionization. (3) The specific surface area of the clay fraction of red soils is relatively low. The removal of free iron oxides causes a considerable reduction in specific surface area, especially the external surface area. Since the calculated "apparent specific surface area" of free iron oxides shows a value several times larger than that of the residue, it is supposed that the coatings of free iron oxides might be present on the clay surface in a finely divided and porous state. (4) The surface charge density of red soils obtained by dividing the net negative charge by the specific surface area is generally between 15—20 μ-coulombs per cm~2, or as low as 10 μ-coulombs for some laterites. It increases appreciablely with the rise of pH or the removal of free iron oxides. (5) The experimental results lead to the conclusion that the free iron oxides play an important role in determining the charge characteristics of red soils.更多还原