Recently, He Hongping, a researcher at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, used the experimental study of microbial dissolution of granite to reveal the influence of microorganisms on the activation and differentiation of rare earth elements during granite weathering. Relevant research results are published online in Geochimica et Cosmochemica Acta.

Ion adsorption type rare earth deposits are mainly developed in weathered crust of granite rich in rare earth elements. The activation, migration, and re enrichment of rare earth elements during weathering are crucial to the formation of such deposits. Although more and more studies have recognized that microorganisms and other geochemical factors jointly control the geochemical behavior of rare earth elements during weathering, the specific microbial effects and mechanisms of action remain unclear.

In order to explore the impact of microorganisms on the activation and differentiation of rare earth elements during the weathering process of granite, and to understand the potential contribution of microorganisms to the mineralization of ion adsorption type rare earth deposits, He Hongping's team conducted an experimental study of microbial dissolution of granite using the granite bedrock of the Dabu ion adsorption type rare earth deposit in Jiangxi Province and wild microbial strains in the weathering crust of the deposit. Their research found that experimental strains can significantly promote the activation of rare earth elements in granite under normal temperature, pressure, and oligotrophic conditions.

At the end of the 30-day reaction, the microorganisms increased the apparent dissolution of total rare earth elements by about 4-21 times. In the actual reaction process, partially dissolved rare earth elements will be adsorbed and fixed again by microbial cells and extracellular metabolites, which reduces the apparent dissolution rate of rare earth elements by about 25% - 82%. Microbial growth and metabolism lead to solution acidification and secretion of abundant small molecular organic acids, which have a positive impact on granite dissolution. During the dissolution of granite by the experimental strain, the pH of the solution is in the range of weak acidity to near neutrality. Under this condition, the complexation of organic acid ligands is the dominant mechanism for microorganisms to promote the release of rare earth elements.

The differentiation of rare earth elements in the dissolution process of granite is mainly restricted by the physical and chemical properties of rare earth element hosting minerals, but also affected by microbial action. In the initial stage of granite dissolution, bastnasite and bastnasite with weak weathering resistance are preferentially dissolved, resulting in higher dissolution rates of light and medium rare earth elements under different reaction conditions. The difference in the complexing stability of small molecular organic acids secreted by microorganisms with different rare earth elements is also an important factor affecting the differentiation of rare earth elements. The abundant organic acids secreted by the two strains of Bacillus used in the experiment, especially malic acid and tartaric acid, may be the key to the preferential release of rare earth and heavy rare earth elements during the dissolution of granite.

The above understanding provides a new perspective for understanding the ore-forming biogeochemical processes of ion adsorption type rare earth deposits.