Perry Thomas D, Duckworth Owen W, McNamara Christopher J, Martin Scot T, Mitchell Ralph
Division of Engineering and Applied Sciences, Pierce Hall, Harvard University, Cambridge, Massachusetts 02138, USA.
Environ Sci Technol. 2004 Jun 1;38(11):3040-6. doi: 10.1021/es035299a.
Dissolution of carbonate minerals has significant environmental effects. Microorganisms affect carbonate dissolution rates by producing extracellular metabolites, including complex polysaccharides such as alginic acid. Using a combined atomic force microscopy (AFM)/flowthrough reactor apparatus, we investigated the effects of alginic acid on calcite dissolution. Macroscopic dissolution rates, derived from the aqueous metal ion concentrations, are 10(-5.5) mol m(-2) s(-1) for 5 < pH < 12 in the absence of alginic acid compared to 10(-4.8) mol m(-2) s(-1) in its presence. The AFM images demonstrate that alginic acid preferentially attacks the obtuse steps of dissolution pits on the calcite surface. In pure water, the obtuse and acute steps retreat at similar rates, and the pits are nearly isotropic except under highly acidic conditions. In alginic acid, the acute step retreat rate is nearly unchanged in comparison to water, whereas the obtuse step retreat rate increases with decreasing pH values. As a result, the pits remain rhombohedral but propagate faster in the obtuse direction. To explain these observations, we propose that alginic acid preferentially forms dissolution active surface complexes with calcium atoms on the obtuse step, which results in anisotropic ligand-promoted dissolution.
碳酸盐矿物的溶解具有显著的环境影响。微生物通过产生细胞外代谢产物（包括诸如海藻酸等复合多糖）来影响碳酸盐的溶解速率。我们使用组合式原子力显微镜（AFM）/流通式反应器装置，研究了海藻酸对方解石溶解的影响。在不存在海藻酸的情况下，由水溶液中金属离子浓度得出的宏观溶解速率在5 < pH < 12时为10^(-5.5) mol m^(-2) s^(-1)，而在存在海藻酸的情况下为10^(-4.8) mol m^(-2) s^(-1)。AFM图像表明，海藻酸优先侵蚀方解石表面溶解坑的钝角台阶。在纯水中，钝角和锐角台阶以相似的速率后退，并且除了在高酸性条件下，坑几乎是各向同性的。在海藻酸中，与水相比，锐角台阶的后退速率几乎不变，而钝角台阶的后退速率随着pH值的降低而增加。结果，坑保持菱形，但在钝角方向上扩展得更快。为了解释这些观察结果，我们提出海藻酸优先与钝角台阶上的钙原子形成溶解活性表面络合物，这导致了各向异性的配体促进溶解。
