| 不同pH值条件下石英溶解的分子机理 |
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| 引用本文： | 张思亭,刘耘.不同pH值条件下石英溶解的分子机理[J].地球化学,2009,38(6):549-557. |
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| 作者姓名： | 张思亭 刘耘 |
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| 作者单位： | 中国科学院,地球化学研究所,矿床地球化学国家重点实验室,贵州,贵阳,550002 |
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| 基金项目： | 国家自然科学基金(40672033,40773005);;中国科学院百人计划项目 |
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| 摘 要： | 对于石英-水反应体系，石英表面的Si-O-Si键的水解断裂是溶解反应过程的关键步骤。采用高级分子轨道从头计算法，研究了不同pH值下石英表面Q1（Si）位置的溶解反应动力学过程。对反应过程的势能面、过渡态、反应路径进行了量子化学计算（包括使用从头计算的分子动力学方法），结果表明：（1）酸性条件下，H^＋连接的是石英表面末端Si-0H上的非桥氧，而非Si-O-Si键上的桥氧，这一链接反而增强了Si—O—Si键；（2）在中性和酸性条件下，石英溶解反应的速度控制步是Q1（Si）—Obr的断裂过程，而碱性条件下反应速率却由水分子靠近Q1（Si）的过程控制，它比Q1（Si）-Obr的断裂过程更慢；（3）活化能计算数据表明，碱性条件下石英溶解速率会随着pH值的增大而迅速增大；相对照的是，在酸性条件下，溶解速率与中性条件时相比变化不大，甚至还略有降低。本文纠正了一些关于石英溶解机理的长期的错误认识，比如前人错误地认为在酸性条件下首先形成Si-O（H^＋）-Si形式的复合物，然后Si-O-Si发生断裂。本研究结果表明，实际上是形成Si—O—Si-OH2^＋…H2O形式的过渡态复合物。然后Si-O-Si发生断裂。前人模型的结果与实验结果分歧，是一个长期遗留的困惑，而本文发现的断键机理模型及其计算结果，可以很好地与不同pH值下石英溶解的实验数据吻合。
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| 关 键 词： | 石英溶解 分子机理 过渡态计算 酸性条件 |
Molecular level dissolution mechanisms of quartz under different pH conditions |
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| ZHANG Si-ting,LIU Yun.Molecular level dissolution mechanisms of quartz under different pH conditions[J].Geochimica,2009,38(6):549-557. |
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| Authors: | ZHANG Si-ting LIU Yun |
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| Institution: | ZHANG Si-ting , LIU Yun State Key Laboratory of Ore Deposit Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550002,China |
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| Abstract: | It is generally believed that the hydrolysis of Si-O-Si bonds is the key step in the dissolution processes of quartz-water system. High level ab initio molecular orbital calculations have been carried out to investigate the molecular level dissolution processes of the Q1 (Si) site of quartz at difference pH conditions. Theoretical methods including potential energy surface calculation, transition states searching, reaction pathway optimization and ab initio molecular dynamics (ADMP) are used in this study. Our results show that under acidic condition, the protonation takes place at the terminal Si-OH unit but not at the bridging oxygen of Si-O-Si linkage. Such way of protonation results in stronger Si-O-Si bonding and makes the dissolution slightly slower than that at neutral condition. The rate-determining steps for both neutral and acidic conditions are the breaking process of Q1 (Si)O-Si bonds. However, under base condition, rate-determining step is the process of one water molecule approaching the Q1 (Si) site. The activation energy results of transition states show that the dissolution rate will increase a lot at the alkalic condition comparing with those at acidic or neutral conditions. The dissolution rate at acidic condition will be slightly lower than the dissolution rate at neutral condition. These findings are consistent with existing experimental evidences and correct a previous theoretical model about quartz dissolution. |
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| Keywords: | dissolution of quartz molecular level mechanism transition sate t heory acidic condition |
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