LFER Approach to predict the Gibbs free energy of oxoanion Adsorption by the isotype Fe/Al hydroxides goethite and diaspore

在过去的十年里，环境地球化学对高温下含水系统热力学数据的重要性和需求不断增加。这是由于关于地热发电、碳捕获和封存、水力压裂和核废料储存库等潜在地下水污染的激烈辩论。在这些地球工程活动的框架内补给的深层含水层卤水往往含有高浓度的有毒含氧阴离子，如砷酸盐和钒酸盐。然而，控制这些含氧阴离子在地热沃茨中的深度和管道尺度的流动性的因素还不完全清楚。令人关切的是，它们在这些条件下比在周围地表条件下更具移动的性，因此可能在接触后污染较浅的地下水含水层系统。从原理上讲，人们对水-岩相互作用（吸附、沉淀等）了解很多。控制有毒含氧阴离子在室温下阻滞的过程，但控制其固体-水分配的过程沿着温度梯度的过程少得多。该建议的假设是：（i）所有含氧阴离子在高温下的吸附行为可以通过它们的吸附吉布斯自由能和它们的水质子化反应的吉布斯自由能之间的线性自由能关系（LFER）来预测，以及（ii）熵项可以基于表面络合物结构的分子水平信息来预测。PI实验室获得的硒酸盐、亚砷酸盐和草酸的数据不足以验证这些假设。本工作建议的目的是（1）。在较高温度下对溴酸盐-针铁矿（FeOOH）、铬酸盐-针铁矿和砷酸盐-针铁矿这三种二元吸附剂-吸附剂体系进行实验，以用LFER确定的最关键数据点验证这些假设，以及（2.）通过进一步的实验来扩展对二元含氧阴离子-一水硬铝石（AlOOH）体系的研究。后者对于与地球工程活动有关的水-岩相互作用甚至更为重要。其目的是设置LFER和熵的假设在一个可靠的基础上预测的标准摩尔吉布斯自由能吸附的所有其他含氧阴离子，没有实验数据或难以获得。这一目标可以达到只有广泛的实验室工作，改变吸附质/吸附剂的比例，pH值，离子强度，甚至温度在4-D参数空间矩阵适合的平衡热力学建模的表面络合反应。

The importance and demand in environmental geochemistry on thermodynamic data for aqueous systems at elevated temperatures has increased over the last decade. This is due to an intense debate on potential groundwater contamination with regard to geothermal power generation, carbon capture and sequestration, hydrofracking, and nuclear waste repositories. Deep aquifer brines recharged within the framework of these geo-engineering activities often contain elevated concentrations of toxic oxyanions such as arsenate and vanadate. However, the factors controlling the mobility of these oxyanions in geothermal waters at depth and with the pipe scales is not fully known. There is concern that they are more mobile at these than at ambient surface conditions, and may thus contaminate the shallower groundwater aquifer systems upon contact. In principle, much is known about water-rock interaction (adsorption, precipitation, etc.) processes governing retardation of the toxic oxyanions at room temperature, but much less about the processes controlling their solid-water partitioning along the temperature gradient. The hypotheses for this proposal are that (i) the adsorption behavior at elevated temperatures for all oxyanions can be predicted by a linear free energy relationship (LFER) between their Gibbs free energy of adsorption and of their aqueous protonation reaction, and that (ii) the entropy term can be predicted on basis of molecular level information on the surface complex structure. The data yet gained in the PIs lab for selenate, arsenite, and silicic acid do not suffice to validate these hypotheses. The purpose of the current work proposal is (1.) to perform experiments on the three binary adsorbate-adsorbent systems bromate-goethite (FeOOH), chromate-goethite and arsenate-goethite at higher temperatures to verify these hypotheses with the most critical data points identified by the LFER, and (2.) to extent by additional experiments the approach to the binary oxyanion-diaspore (AlOOH) system. The latter is even of greater importance with respect to water-rock interaction related to the geo-engineering activities. The aim is to set the LFER and entropy hypotheses on a reliable basis for prediction of standard molar Gibbs free energies of adsorption of all other oxyanions, for which no experimental data are available or difficult to obtain. This aim can be reached only with extensive experimental laboratory work to vary adsorbate/adsorbent ratio, pH value, ionic strength, and even temperature in a 4-D parameter space matrix suitable for the equilibrium thermodynamic modelling of the surface complexation reactions.