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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。
Red soil colloids negative charge less than other types of soil, while the positive charge is more. The “apparent negative charge” of humus in red soil colloids is also small, but the relative contribution to the colloidal negative charge is greater. These characteristics, brick red soil colloidal performance is more prominent. Red soil colloidal charge has a greater variability. Red soil with Quaternary red clay had a variable negative charge above pH 5 and negatively charged chloride above pH 6. Brick red soil had a net positive charge at low pH and a net negative charge at higher pH The isoelectric point is about pH 4.0. Humus reduced the isoelectric point of red soils and the free iron oxide increased the isoelectric point. The negatively charged amount of granulated soil was between the upper two soils, and no isoelectric point appeared. The negative charge of these three red colloids tends to increase with increasing pH, with a turning point at pH5-6. In view of this, there may be two kinds of colloids on the dissociation of two different points of exchange. The red soil colloids have a small specific surface, and the surface area, especially the external surface area, is significantly reduced when the free iron oxide is removed. The “apparent specific surface” of free iron oxide is several times larger than colloidal nuclei, suggesting that the free iron oxide may be in the form of a porous aggregate that is coated on the surface of colloidal nuclei. According to the net negative charge of the red soil colloids and the specific surface area, the surface charge density of the red soil colloids is generally more than 15-20 microcoulombs per square centimeter, while some red soil bricks can be as low as 10 microcoulombs. After the removal of free iron oxide, the surface charge density is significantly increased.