The influence of citrate on surface dissolution and alteration of the micro- and nano-structure of biotite

Unraveling the kinetics and mechanisms of K-bearing mineral dissolution in the presence of organic acids on the nanometer scale is important for understanding the effectiveness of organic acids present in most soil environments. Herein, batch experiments were used coupled with atomic force microscopy (AFM) to image the surface dissolution and alteration of the micro- and nano-structure of biotite in pH 4.0 aqueous (H2O), citric acid (CA), and citric acid with sodium chloride (CA + NaCl) solutions. We directly measured the release of potassium (K), aluminum (Al), and silicon (Si) from biotite into solution and observed the alteration of biotite at room temperature (25 °C) during long reaction times (0–168 h). In the acidic solution, biotite dissolved slowly, but the K, Si and Al release rates could be increased by adding citric acid and salt. The AFM observation indicated that the alteration of biotite was a coupled dissolution–precipitation process. An amorphous Si-bearing coating was deposited on the (001) surface or edge sites in the acidic environment after long time dissolution. Bumps or bulges on the (001) surface accelerated the stripping of the biotite segment from the surface after 96 h of reaction in citrate solutions. Na+–K+ ion exchange occurred in the biotite interlayer resulting in swellings and cracks on the biotite (001) surface and eventually forming Na-bearing hydrated mica. These observations may contribute to resolving the previously unrecognized interaction of organic acids and K-bearing minerals in a more complex soil system.