低分子量有机酸对高岭石中铝释放的影响

选择了几种土壤中可能存在的低分子量脂肪羧酸 ,研究了它们对高岭石中铝释放的影响。结果表明 ,有机酸可以通过络合作用促进高岭石中铝的释放。几种有机酸对体系中可溶性铝影响的大小顺序为 :草酸 >柠檬酸 >丙二酸 >苹果酸 >乳酸。草酸、柠檬酸和乳酸对可溶性铝释放的促进作用随体系pH的升高而减小 ,其中草酸体系中可溶性铝随pH的变化幅度最大。在苹果酸体系中 ,可溶性铝随pH的升高而稍有增加。体系中的可溶性铝随有机酸浓度的增加而增加 ,而交换性铝随柠檬酸浓度的增加而减少 ,先随苹果酸浓度增加而增加 ,然后又逐渐减少。与对照相比 ,柠檬酸和草酸使交换性铝的量减小 ,苹果酸和乳酸在低pH下使交换性铝明显增加 ,而苹果酸在较高pH下使交换性铝减少。有机酸影响释放出的铝在固液相间的分配比 ,苹果酸在低pH下使体系中释放出的大部分铝以交换性形态存在 ,而在较高pH下 ,大部分铝以可溶形态存在。在草酸体系中 ,释放出铝的大部分都以可溶形态存在。不同有机酸的不同表现与体系中铝的溶解平衡、铝的吸附 -解吸平衡、有机酸的吸附 -解吸平衡、有机酸的离解平衡和铝与有机酸的络合 -离解平衡有关。

EFFECT OF LOW-MOLECULAR-WEIGHT ORGANIC ACIDS ON ALUMINUM RELEASE FROM KAOLINITE

Five kinds of low molecular weight organic acids were chosen for investigating the effect of organic acids on Al release from kaolinite.The results showed that organic acids could accelerate the release of Al from kaolinite due to formation of Al organic acid complexes.Organic acids increased the soluble Al in the order oxalic acid>citric acid>malonic acid>malic acid>lactic acid.The ability of releasing soluble Al of oxalic acid,citric acid and lactic acid decreased with the increase of pH,and the effect of pH on the release of soluble Al in oxalic acid system was the most remarkable,whereas the soluble Al increased with pH in malic acid system.The enhanced release of soluble Al by organic acids increased with the increase in concentration of the organic acids.The amount of exchangeable Al in the systems of malic acid and lactic acid at lower pH(pH 3.5 and pH 4.0)was larger while that at higher pH(pH 4.5 and pH 5.0)was smaller than that in the control system.Citric acid decreased the exchangeable Al in the pH range of 3.5 to 5.0,and oxalic acid did in the range of 3.5 to 4.5.In the citric acid system at pH 4.0,exchangeable Al decreased with the increase in the concentration of citric acid.Whereas the exchangeable Al increased with the increase in the concentration of the organic acid at first and then decreased gradually in malic acid system at the same pH.Organic acids can also influence the distribution of Al released from kaolinite between kaolinite surface and solution.In the malic acid system,most of Al existed as exchangeable Al on kaolinite surface at lower pH,while most of Al existed as soluble Al in solution at higher pH.In oxalic acid system,most of Al existed as soluble Al in solution and the proportion of exchangeable Al to total Al released was small in the range of studied pH.The different effects of organic acids on Al release were caused by chemical equilibria in these systems,including dissolution equilibrium of kaolinite,adsorption and desorption of Al,adsorption and desorption of organic acids,dissociation equilibria of organic acids and complexation equilibrium between Al and organic acids