醋酸-醋酸铵缓冲体系中柠檬酸、草酸和苹果酸对高岭石的溶解特征

采用间歇法(batch method)模拟研究醋酸-醋酸铵缓冲体系中柠檬酸、草酸和苹果酸三种低分子量有机酸对高岭石的溶解特征。结果表明:柠檬酸、草酸和苹果酸三种有机酸均能显著促进高岭石的溶解,溶解能力都是随其浓度和酸度的升高而增强;当有机酸浓度为1mmol/L,pH3.5时,草酸>柠檬酸>苹果酸;pH5.5和pH4.5时,柠檬酸>草酸>苹果酸;而当有机酸浓度≥5mmol/L时,草酸>柠檬酸>苹果酸,且对高岭石的溶解能力都大于无机酸。当柠檬酸、草酸和苹果酸浓度为1mmol/L时,反应级数(nHL)分别为0.09、0.27和0.18,速率常数(kHL)分别为4.03×10-13、2.62×10-12和4.73×10-13;当其浓度为5mmol/L时,其反应级数(nHL)分别为0.16、0.37和0.16,速率常数(kHL)分别为1.38×10-12、2.32×10-11和4.97×10-13;其浓度为10mmol/L时,反应级数(nHL)分别为0.18、0.34和0.16,速率常数(kHL)分别为2.17×10-12、2.60×10-11和6.05×10-13。对于柠檬酸和草酸而言,在促进高岭石溶解的作用上,相对于质子,配体的贡献是主要的;而对于苹果酸而言,配体的贡献是次要的。配体促进溶解速率(RL)可以用配体浓度的指数形式来表示。对柠檬酸、草酸和苹果酸而言,pH5.5时,分别为RL=10-13.01配体]0.23、RL=10-13.28配体]0.70和RL=10-13.72配体]0.38;pH4.5时,分别为RL=10-13.00配体]0.51、RL=10-13.03配体]1.05和RL=10-14.07配体]0.77;pH3.5时,分别为RL=10-12.99配体]0.64、RL=10-12.72配体]0.89和RL=10-14.61配体]1.69。

Dissolution of Kaolinite Induced by Citric Acids, Oxalic Acids and Malic Acids in HAC-NH4AC Buffer Solution

Experiments were conducted to investigate the dissolution characteristics of kaolinite induced by three kinds of low-molecular-weight organic acids(LOAs)(citric acids,oxalic acids and malic acids) with the batch method in HAC-NH4AC buffer solution.The results showed that LOAs enhanced significantly the solubility of kaolinite,The solubility of kaolinite was enhanced with an increase in the concentration and acidity of citric acids,oxalic acids and malic acids.When the concentrations of LOAs were 1 mmol/L,LOAs would promote the kaolinite solubility in the order of oxalic acid > citric acid > malic aicd(pH=3.5) and citric acid > oxalic acid > malic acid(pH=4.5 and pH=5.5);However,when the concentrations of LOAs were high or higher than 5 mmol/L,the order would be oxalic acid >citric acid >malic aicd.Moreover,in general,the solubility of LOAs on kaolinite was higher than that of proton(HAC).When the concentrations of citric acids,oxalic acids and malic acids were 1 mmol/L,the reaction orders(nHL) were 0.09,0.27 and 0.18,respectively,and the rate constants(kHL) were 4.03×10-13,2.62×10-12 and 4.73×10-13 respectively;when the concentrations were 5 mmol/L,the reaction orders(nHL) were 0.16,0.37 and 0.16,respectively,and the rate constants(kHL) were 1.38×10-12,2.32×10-11 and 4.97×10-13,respectively;when the concentrations were 10 mmol/L,the reaction orders(nHL) were 0.18,0.34 and 0.16,respectively,and the rate constants(kHL) were 2.17×10-12,2.60×10-11 and 6.05×10-13,respectively.With relation to citric acids and oxalic acids,the contributions of ligand-promoted kaolinite dissolution compared to proton-promoted kaolinite dissolution were highest,in relation to malic acids,the contributions of ligand-promoted kaolinite dissolution were lowest.The log of ligand-promoted kaolinite dissolution rates could express a function of log ligand concentration.With relation to citric acids,oxalic acids and malic acids,when the solution pH was 5.5,the relation of ligand-promoted dissolution rates and ligand concentrations respectively showed: RL=10-13.01citrate]0.23,RL=10-13.28oxalate]0.70 and RL=10-13.72malate]0.38;when the solution pH was 4.5,RL=10-13.00citrate]0.51,RL=10-13.03oxalate]1.05 and RL=10-14.07malate]0.77;when the solution pH was 3.5,RL=10-12.99citrate]0.64,RL=10-12.72oxalate]0.89 and RL=10-14.61malate]1.69.