柠檬酸盐对黑云母表面溶解及微结构变化的影响

EFFECTS OF CITRATE DISSOLVING SURFACE AND ALTERING MICROSTRUCTURE OF BIOTITE

采用离位培养法，结合原子力显微镜的接触成像和相位成像模式研究层状硅酸盐含钾矿物黑云母在根际环境条件下的表面溶解现象和矿物微结构转化过程。结果表明，在 pH 4.0 的弱酸水溶液中反应 24 h 时，黑云母（001）面上有微型蚀坑出现，其深度介于 0.1～0.9 nm 之间，表面变得比较粗糙；反应 96 h 时，黑云母（001）面上蚀坑较为明显，但仅占总表面的 4.8%，蚀坑的深度平均为 0.957 nm；反应 140 h 时，黑云母（001）面上有不稳定的覆盖物沉积，阻碍表层溶解过程的持续进行。在 pH 4.0 的柠檬酸溶液中，经过 24 h 培养，（001）面上有大量的蚀坑出现，单层溶解现象明显；48 h 时，表面台阶溶解速率显著提高，溶解面积可达总表面的 48.7%；140 h 时，（001）面上有圆形状胀裂发生，表层（高度，1～2 nm）发生破裂、产生较多的黑云母碎片，溶解速率进一步提高。随着柠檬酸溶液中 Na＋浓度的增加，表层溶解速率增强，（001）面上也有次生覆盖物沉积。同时，界面上 Na^＋-K^＋交换作用加剧，表层结构（高度，2～10 nm）胀裂现象更加明显。随着反应时间的延长，140 h 时，黑云母（001）面的深层结构（深度，～20 nm）亦逐渐隆起并引起周边区域产生裂缝（深度，0.1～1.9 nm），最终导致表层微结构区域水化，形成水化云母（伊利石）。

The ex-situ incubation method coupled with the atomic force microscopy in contacting imaging and phase imaging modes was used to explore how the surface of biotite, a phyllosilicate K-bearing mineral, is eroded or dissolved and its microstructure is altered by citric acid （CA） in simulation of what happens in rhizosphere environment. CA, a kind of low molecular weight organic acid is one of the most important components of root exudate and, therefore, ubiqui- tous in rhizosphere environment. Results show that in weakly acidic aqueous solution, 4.0 in pH, 24 hours of incubation left some tiny etch pits, about 0.1 -0.9 nm in depth, on the surface of biotite （001） , making the surface rough. After 96 h of incubation, pits became more apparent, averaging 0. 957 nm in depth, and more in number, but only covering 4.8% of the surface. After 140 h, an unstable coating precipitated on the surface, hindering the process of dissolution. However, in citric acid solution （CA） 4.0 in pH, after 24 h of incubation, large numbers of etch pits appeared on the surface of biotite （001） and dissolution of the surface layer became apparent. After 48 hr, the surface terrace dissolution rate of the biotite increased significantly, with dissolved area reaching 48.7% of the surface. And 140 hr later, circular spallings were observed on the （001） surface, leading to breakage of the layer （ 1 ~2 nm in thickness） into fragments and promoting further rise in dissolution rate. In CA solution containing Na^＋ , rising Na^＋ concentration increased dissolution rate of the surface and was a secondary coating was formed, too. Meanwhile, K^＋-Na^＋ ion exchange on the solid-fluid in- terface was enhanced and spalling in the （001） surface structure, about 2 - 10 nm in depth became more and more appar- ent. With the incubation going on （140 hr）, biotite expanded in depth （ ~ 20 nm）, causing cracks in the perimeters （0. 1 ~ 1.9 nm in depth） and eventually zonal hydration of the surface microstructure, and formation of hydrous mica （ illite） on the surface of the biotite.