Villalobos Mario, Trotz Maya A, Leckie James O
Grupo de Biogeoquímica Ambiental, Laboratorio de Análisis Físicos y Químicos del Ambiente (LAFQA), Instituto de Geografía, UNAM, CU, 04510, D.F., Mexico, Mexico.
J Colloid Interface Sci. 2003 Dec 15;268(2):273-87. doi: 10.1016/j.jcis.2003.07.044.
Goethite is a representative iron oxide in natural environments due to its abundance and thermodynamic stability and may be responsible for many surface-mediated processes, including ion retention and mobility in aqueous settings. A large variability in morphologies and specific surface areas of goethite crystals exists but little work has been done to compare surface reactivity between them. The present work offers experimental evidence for the existence of an inverse relationship between sorption capacity for protons and carbonate ions and specific surface area of goethite for three synthetic goethite preparations spanning surface area differences by a factor of 2. An explanation for this was found by assuming a variable reactive site density between preparations in direct relationship to their sorption capacity based on congruency of carbonate sorption computed on a per-site basis. Previous evidence of maximum sorption capacities supports this explanation, and site density ratios between the goethites studied here were obtained. Triple layer surface complexation modeling was successful in describing adsorption data for all goethite preparations using equal stoichiometries. A new formulation of standard state for activities of surface species based on a 1.0 mole fraction of sites on the solid allowed transformation of the conventional molar concentration-based affinity constants to values based on site occupancy. In this fashion, by applying the appropriate site density ratios, a single set of affinity constant values was found that described accurately the adsorption data for all preparations.
针铁矿是自然环境中一种具有代表性的氧化铁，因其含量丰富且具有热力学稳定性，可能参与了许多表面介导的过程，包括水相环境中的离子保留和迁移。针铁矿晶体的形态和比表面积存在很大差异，但在比较它们之间的表面反应性方面所做的工作很少。本研究为三种合成针铁矿制剂提供了实验证据，表明质子和碳酸根离子的吸附容量与针铁矿的比表面积之间存在反比关系，这三种制剂的表面积差异达2倍。通过假设制剂之间的可变反应位点密度与其吸附容量直接相关，基于按每个位点计算的碳酸根吸附一致性，找到了对此现象的一种解释。先前关于最大吸附容量的证据支持了这一解释，并得出了此处研究的针铁矿之间的位点密度比。三层表面络合模型成功地使用相同的化学计量比描述了所有针铁矿制剂的吸附数据。基于固体上1.0摩尔分数的位点对表面物种活度的标准状态进行了新的公式化，从而将基于常规摩尔浓度的亲和常数转换为基于位点占有率的值。通过这种方式，应用适当的位点密度比，发现了一组单一的亲和常数，能够准确描述所有制剂的吸附数据。
