Is bicarbonate stable in and on the calcite surface?
Is bicarbonate stable in and on the calcite surface?
复制标题
碳酸氢盐在方解石表面内和表面上稳定吗？
DOI：
10.1016/j.gca.2015.12.016
复制
发表时间：
2016
影响因子：
5
通讯作者：
Andersson M
中科院分区：
文献类型：
--
作者：
Andersson M
我们利用COSMO-RS隐式溶剂模型的密度泛函理论，预测了碳酸氢盐脱质子生成碳酸盐（即HCO3−<=> CO32−+ H+）的pka，当HCO3−被包含在方解石表面并被吸附时。我们使用簇模型（80-100个原子）来表示平面{10.4}表面、锐步、钝角步、锐步上的两种扭结和钝角步上的两种扭结。根据预测的pkk值，根据表面位置的不同，其范围从- 6.0到2.4，我们得出结论，当碳酸氢盐是方解石时，即使溶液中的pH很低，碳酸氢盐也会脱质子为碳酸盐。所有的表面位点都是如此，即使在2.4 < pH < 6.35的溶液中，h2co30是主要的溶解物质。当碳酸氢盐吸附在方解石上时，预测的pKafor去质子化为7.5，比水溶液中的pH值10.35低~ 3个单位。这意味着即使溶解的CO32−浓度低几个数量级，吸附的碳酸盐也是稳定的。这对表面电荷和方解石表面的行为有显著的影响。我们的研究结果有助于解释方解石-水体系中碳酸盐物种的潜在决定行为，特别是在水中碳酸氢盐物种占主导地位的pH范围内和碳酸盐物种在表面占主导地位的pH范围内，即当7.5 < pH < 10.35时。我们对各种方解石表面位置的原子尺度数据提供了所需的输入，以改进和约束表面络合建模，并且对于预测在实验困难或不可能的系统中的行为特别有用，例如在高温和高压下。
We have used density functional theory with the COSMO-RS implicit solvent model to predict the pKafor the deprotonation of bicarbonate to carbonate, i.e. HCO3−<=> CO32−+ H+, when HCO3−is included in, and adsorbed on, a calcite surface. We have used cluster models (80–100 atoms) to represent the flat {10.4} surface, acute steps, obtuse steps, two types of kinks on the acute step and two types of kinks on the obtuse steps. Based on the predicted pKavalues, which range from −6.0 to 2.4 depending on the surface site, we conclude that bicarbonate deprotonates to carbonate when it isincalcite even when pH in solution is very low. This is true for all surface sites, even for solutions where 2.4 < pH < 6.35, where H2CO30is the dominant dissolved species. When bicarbonate is adsorbedoncalcite, the predicted pKafor deprotonation is 7.5, which is ∼3 pH units lower than in aqueous solution, 10.35. This means that adsorbed carbonate is stable even when the concentration of dissolved CO32−is several orders of magnitude lower. This has a significant effect on surface charge and thus the behaviour of the calcite surface. Our results help explain the potential determining behaviour of the carbonate species in calcite–water systems, particularly in the pH range where the bicarbonate species dominates in water and where the carbonate species dominates at the surface, i.e. when 7.5 < pH < 10.35. Our atomic scale data for the various calcite surface sites provide the needed input to improve and constrain surface complexation modelling and are especially useful for predicting behaviour in systems where experiments are difficult or impossible, such as at high temperature and pressure.