Comment on "Enthalpy of Uranium Adsorption onto Hematite".
Comment on "Enthalpy of Uranium Adsorption onto Hematite".
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对“铀在赤铁矿上的吸附焓”的评论
DOI：
10.1021/acs.est.0c07856
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发表时间：
2021
影响因子：
11.4
通讯作者：
Kersten
中科院分区：
文献类型：
--
作者：
Kersten
Estes和Powell(2020) 1描述了氧化铁在高温下对U （VI）的吸附。根据他们的原始吸附数据集和表面络合模型（SCM）拟合，他们得出结论，吸附过程是吸热的，并且在pH 4的温度下，赤铁矿上的吸附会增加。他们的方法是使用Van， t Hoff方程进行严格的热力学数据评估，与经常使用但不正确的由经验Freundlich或Langmuir吸附参数得出的焓相比，这是向前迈出的一步。2然而，该结果并不一定能为环境（环中性）pH下地下水中U （VI）迁移的环境问题提供相关信息。U （VI）在铁（氢）氧化物上的吸附在很大程度上取决于水的化学性质，特别是pH和溶解无机碳（DIC）的浓度。目前尚不清楚为什么作者将他们的实验限制在酸性pH范围内（pH< 6）。事实上，他们只考虑了一半的事实，即上升的吸附边，而不是经常报道的U （VI）吸附膜的下降吸附边。后者在环境CO2条件下出现在环中性至微碱性pH值。3Estes和Powell使用了扩散层SCM (2-pK DLM)，需要温度相关反应和介电常数输入。我们使用Estes和Powell提供的SCM常数，尝试使用Visual MINTEQ 5代码将他们的模型预测扩展到碱性pH范围。然而，由于DIC是地下水的重要组成部分，CO2气体溶解度的温度依赖性也被visualminteq代码中的五阶多项式视为默认值。一旦模型系统被设定在与环境大气CO2分压的平衡状态，溶液中就会形成铀酰三碳酸配合物UO2 (CO3) 34 -。它们在环中性到碱性的pH范围内与表面络合作用竞争，导致众所周知的铀酰吸附边下降。因此，在大气CO2中，当pH值为bbbb8时，吸附量减少，而当pH值为bbbb9时，吸附量几乎为零（图1）。将反应温度从15℃提高到80℃，U （VI）吸附边明显向更高的ph方向移动。这是因为三碳酸配合物的形成是一个放热反应（ΔfH= - 39.2 kJ mol - 1）， 5，因此在升高时减少
Estes and Powell (2020) 1 described adsorption of U (VI) by an iron oxide at elevated temperatures. On the basis of their pristine adsorption data sets and surface complexation model (SCM) fitting, they concluded that the adsorption process is endothermic and that adsorption onto hematite at pH 4 will increase at elevated temperatures. Their approach in terms of a rigorous thermodynamic data evaluation using the Van, t Hoff equation is a step forward compared to the often used but incorrect enthalpies derived from empirical Freundlich or Langmuir adsorption parameters. 2 However, the result does not necessarily provide relevant information to the environmental concerns of U (VI) mobility in groundwater at ambient (circumneutral) pH. U (VI) adsorption onto Fe (hydr) oxides depends strongly on water chemistry, especially the pH and concentration of dissolved inorganic carbon (DIC). It is unclear why the authors limited their experiments to the acidic pH range (pH< 6). In fact, they considered thereby half of the truth only, that is, the ascending adsorption edge, but not the often reported descending adsorption edge of the U (VI) adsorption envelopes. The latter appears at circumneutral to slightly alkaline pH values under ambient CO2 conditions. 3Estes and Powell used the diffuse layer SCM (2-pK DLM) requiring both temperature dependent reaction and dielectric constants entry. 4 We used the SCM constants provided by Estes and Powell to try and extend their model predictions to the alkaline pH range using the Visual MINTEQ 5 code. However, since DIC is an essential component of groundwater, the temperature dependence of the solubility of the CO2 gas was also considered by a fifth order polynomial as default in the Visual MINTEQ code. Once the model system is thereby set at equilibrium with the ambient atmospheric partial pressure of CO2, uranyl triscarbonato complexes UO2 (CO3) 3 4− are formed in solution. They compete with surface complexation in the circumneutral to alkaline pH range, leading to the well-known descending uranyl adsorption edge. 3 As a result, at atmospheric CO2, the adsorption decreased at pH values of> 8 and became nearly zero at pH> 9 (Figure 1). Increasing the reaction temperature from 15 to 80 C drives a noticeable shift in this U (VI) adsorption edge toward higher pH. This is because formation of the triscarbonato complex is an exothermic reaction (ΔfH=− 39.2 kJ mol− 1), 5 and is therefore decreasing at elevated