低分子量有机酸对可变电荷土壤吸附铝的影响机制

通过吸附性铝的解吸实验研究了低分子量有机酸对三种可变电荷土壤(2种砖红壤和1种赤红壤)吸附铝的影响机制，结果表明，柠檬酸、苹果酸和酒石酸等带有3个及3个以上活性官能团的有机酸在低pH条件下可以通过形成土壤一有机酸一铝三元表面络合物和增加土壤的表面负电荷两种机制显著增加土壤对铝离子的吸附量，但以前一种影响机制为主。乳酸、水杨酸、草酸和丙二酸等带有2个活性官能团的有机酸仅通过改变土壤的表面负电荷影响铝的吸附。土壤氧化铁是土壤吸附有机酸的主要载体，当用化学方法将土壤中的氧化铁除去后，有机酸对铝吸附的影响变小。在pH5．0时有机酸主要通过形成可溶性有机铝络合物减小土壤对铝的吸附，但有机酸的存在增加了Al^3 在吸附性铝中所占的比例，导致铝的解吸率增加。土壤中大量氧化铁的存在使其即使在低pH下也能对铝离子发生专性吸附，导致吸附性铝的解吸率减小。

Effect mechanisms of low-molecular-weight organic acids on aluminum adsorption by variable charge soils

The effect mechanisms of low-molecular-weight organic acids on aluminum adsorption by three variable charge soils (two latosols and one lateritic red soil) were investigated with the desorption experiment of adsorbed aluminum in this study. The results showed that the organic acids with three or more active groups such as citric, malic and tartaric acids can increase Al adsorption obviously through two mechanisms: the formation of soil-organic acid-Al3+ ternary surface complexes and the increase in soil negative surface charge due to specific adsorption of organic acids by the soils. However, the former mechanism is more important than the latter one. The organic acids with two active groups such as oxalic, malonic, lactic and salicylic acids increased the Al adsorption through the second mechanism mentioned above and the effect extent of these acids was smaller than that of citric, malic and tartaric acids. The effect of the organic acids on Al adsorption decreased sharply after the iron oxides were removed from these soils, because the iron oxides in soils were the major absorbents of organic acids. At pH5.0, organic acids decreased the Al adsorption primarily through the formation of soluble organic acid-Al complexes, whereas the presence of organic acids led to the increase in the fraction of Al3+ species in adsorbed Al, which resulted in the increase of desorption rate of Al. The results also showed that the aluminum ions can be adsorbed specifically by these soils even at very low pH (3.9) due to the high content of iron oxides in these soils. The specific adsorption led to the decrease in desorption rate of adsorbed Al.