Ion-exchange modeling is one of the most widely used methods to predict ion adsorption data on clay minerals. The model parameters (e.g. number of adsorption sites and the cation adsorption capacity of each site) are optimized normally by curve fitting experimental data, which does not definitively identify the local environment of the adsorption sites. A new approach for constructing an ion-exchange model was pursued, whereby some of the parameters needed were obtained independently, resulting in fewer parameters being based on data-curve fitting. Specifically, a reversed modeling approach was taken in which the number of types of sites used by the model was based on a previous first-principles Density Functional Theory study, and the relative distribution of these sites was based on the clay’s chemical composition. To simplify the ion-exchange reactions involved, montmorillonite was Na-saturated to produce a well-controlled Na-montmorillonite (NaMnt) adsorbent. Ion adsorption data on NaMnt were collected from batch experiments over a wide range of pH, Cs+ concentrations, and in the presence of coexisting cations. Ion-exchange models were developed and optimized to predict these cation adsorption data on NaMnt. The maximum amount of adsorption of monovalent cations on NaMnt was obtained from the plateau of the adsorption envelope data at high pH. The remaining equilibrium constants (pK) were optimized by curve fitting the edges of the adsorption envelope data. The resultant three-site ion-exchange model was able to predict the retention of Li+, Na+, K+, and Cs+ very well as a function of pH. The model was then tested on adsorption envelopes of various combinations of these cations, and on Cs+ adsorption isotherms at three different pH values. The pK values were constant for all assays. The interlayer spacing of NaMnt was also analyzed to investigate its relation with cation adsorption strength. An X-ray diffraction study of the samples showed that the measured d001 values for these cations were consistent with their adsorption pK values. The Cs+ cation showed a strong ability to collapse the interlayer region of montmorillonite. In the presence of multiple competing cations, the broadening and presence of multiple d001 XRD peaks suggested that the cations in the interlayers may be segregated.

中文翻译：

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蒙脱石上一价碱阳离子吸附的离子交换模型

离子交换模型是预测粘土矿物离子吸附数据的最广泛使用的方法之一。模型参数（例如吸附位点的数量和每个位点的阳离子吸附容量）通常通过曲线拟合实验数据进行优化，这并不能明确地确定吸附位点的局部环境。寻求一种构建离子交换模型的新方法，其中一些所需的参数是独立获得的，从而减少了基于数据曲线拟合的参数。具体而言，采用反向建模方法，其中模型使用的站点类型数量基于先前的第一性原理密度泛函理论研究，并且这些站点的相对分布基于粘土的化学成分。为了简化所涉及的离子交换反应，蒙脱石被 Na 饱和以产生控制良好的 Na-蒙脱石 (NaMnt) 吸附剂。NaMnt 上的离子吸附数据是从大范围的 pH、Cs+ 浓度和共存阳离子存在下的批量实验中收集的。开发并优化了离子交换模型以预测这些阳离子对 NaMnt 的吸附数据。单价阳离子在 NaMnt 上的最大吸附量是从高 pH 下吸附包络数据的平台获得的。剩余的平衡常数 (pK) 通过拟合吸附包络数据边缘的曲线进行优化。由此产生的三中心离子交换模型能够很好地预测 Li+、Na+、K+ 和 Cs+ 的保留，作为 pH 的函数。然后在这些阳离子的各种组合的吸附包络上以及在三种不同 pH 值下的 Cs+ 吸附等温线上测试该模型。所有测定的 pK 值都是恒定的。还分析了 NaMnt 的层间距以研究其与阳离子吸附强度的关系。样品的 X 射线衍射研究表明，这些阳离子的测量 d001 值与其吸附 pK 值一致。Cs+阳离子表现出很强的塌陷蒙脱石层间区域的能力。在存在多个竞争阳离子的情况下，多个 d001 XRD 峰的加宽和存在表明夹层中的阳离子可能被隔离。还分析了 NaMnt 的层间距以研究其与阳离子吸附强度的关系。样品的 X 射线衍射研究表明，这些阳离子的测量 d001 值与其吸附 pK 值一致。Cs+阳离子表现出很强的塌陷蒙脱石层间区域的能力。在存在多个竞争阳离子的情况下，多个 d001 XRD 峰的加宽和存在表明夹层中的阳离子可能被隔离。还分析了 NaMnt 的层间距以研究其与阳离子吸附强度的关系。样品的 X 射线衍射研究表明，这些阳离子的测量 d001 值与其吸附 pK 值一致。Cs+阳离子表现出很强的塌陷蒙脱石层间区域的能力。在存在多个竞争阳离子的情况下，多个 d001 XRD 峰的加宽和存在表明夹层中的阳离子可能被隔离。