Lackovic Kurt, Johnson Bruce B, Angove Michael J, Wells John D
Colloid and Environmental Chemistry Laboratory, La Trobe University, Bendigo, P.O. Box 199, Bendigo, Victoria 3552, Australia.
J Colloid Interface Sci. 2003 Nov 1;267(1):49-59. doi: 10.1016/s0021-9797(03)00693-3.
The adsorption of citric acid onto goethite, kaolinite, and illite was measured as a function of pH (adsorption edges) and concentration (adsorption isotherms) at 25 degrees C. The greatest adsorption was onto goethite and the least onto illite. Adsorption onto goethite was at a maximum below pH 5 and decreased as the pH was increased to pH 9. For kaolinite, maximum adsorption occurred between pH 4.5 and pH 7, decreasing below and above this pH region, while for illite maximum adsorption occurred between about pH 5 and pH 7, decreasing at both lower and higher pH. ATR-FTIR spectra of citrate adsorbed to goethite at pH 4.6, pH 7.0, and pH 8.8 were compared with those of citrate solutions between pH 3.5 and pH 9.1. While the spectra of adsorbed citrate resembled those of the fully deprotonated solution species, there were significant differences. In particular the C[bond]O symmetric stretching band of the adsorbed species at pH 4.6 and 7.0 changed shape and was shifted to higher wave number. Further spectral analysis suggested that citrate adsorbed as an inner-sphere complex at pH 4.6 and pH 7.0 with coordination to the surface most probably via one or more carboxyl groups. At pH 8.8 the intensity of the adsorbed bands was much smaller but their shape was similar to those from the deprotonated citrate solution species, suggesting outer-sphere adsorption. Insufficient citric acid adsorbed onto illite or kaolinite to provide spectroscopic information about the mode of adsorption onto these minerals. Data from adsorption experiments, and from potentiometric titrations of suspensions of the minerals in the presence of citric acid, were fitted by extended constant-capacitance surface complexation models. On the goethite surface a monodentate inner-sphere complex dominated adsorption below pH 7.9, with a bidentate outer-sphere complex required at higher pH values. On kaolinite, citric acid adsorption was modeled with a bidentate outer-sphere complex at low pH and a monodentate outer-sphere complex at higher pH. There is evidence of dissolution of kaolinite in the presence of citric acid. For illite two bidentate outer-sphere complexes provided a good fit to all data.
在25摄氏度下，测量了柠檬酸在针铁矿、高岭石和伊利石上的吸附量与pH值（吸附边界）和浓度（吸附等温线）的关系。吸附量最大的是针铁矿，最小的是伊利石。针铁矿在pH值低于5时吸附量最大，随着pH值升高到9，吸附量降低。对于高岭石，最大吸附量出现在pH值4.5至7之间，在此pH范围以下和以上吸附量均降低，而对于伊利石，最大吸附量出现在约pH值5至7之间，在较低和较高pH值时均降低。将在pH值4.6、7.0和8.8下吸附到针铁矿上的柠檬酸盐的衰减全反射傅里叶变换红外光谱（ATR - FTIR）与pH值在3.5至9.1之间的柠檬酸盐溶液的光谱进行了比较。虽然吸附的柠檬酸盐的光谱类似于完全去质子化溶液物种的光谱，但存在显著差异。特别是在pH值4.6和7.0时，吸附物种的C = O对称伸缩带形状发生变化并向更高波数移动。进一步的光谱分析表明，在pH值4.6和7.0时，柠檬酸盐以内层配合物形式吸附，最有可能通过一个或多个羧基与表面配位。在pH值8.8时，吸附带的强度小得多，但其形状与去质子化柠檬酸盐溶液物种的相似，表明是外层吸附。吸附到伊利石或高岭石上的柠檬酸不足以提供有关吸附到这些矿物上的模式的光谱信息。吸附实验数据以及在柠檬酸存在下矿物悬浮液的电位滴定数据，通过扩展的恒电容表面络合模型进行拟合。在针铁矿表面，在pH值低于7.9时，单齿内层配合物主导吸附，在较高pH值时需要双齿外层配合物。在高岭石上，柠檬酸吸附在低pH值下用双齿外层配合物建模，在较高pH值下用单齿外层配合物建模。有证据表明在柠檬酸存在下高岭石会溶解。对于伊利石，两个双齿外层配合物能很好地拟合所有数据。
