Short-Chain Carboxylic Acids Influencing Mineralization Mechanisms of Ferrihydrite Transformation to Hematite and Goethite

Transformation of poorly ordered ferrihydrite (Fh) into crystalline phases controls nutrient and contaminant transport. This process is strongly influenced by organic compounds; however, the mechanism remains unclear due to the structural complexity of organic compounds, particularly carboxylic density. We selected well-defined short-chain organic acids (OAc), i.e., formic acid (FA), oxalic acid (Ox), and citric acid (CA), which commonly occur as components of organic compounds, to examine how carboxylic density influences Fh transformation. Results showed that Fh transformation was mediated by specific organic-mineral/ion interactions. Monocarboxylic FA exhibited minimal binding affinity and thus had negligible influence on Fh transformation. In contrast, dicarboxylic Ox interacted with Fh and dissolved Fe³⁺, accelerating Fh dissolution by destabilizing Fe-O bonds of Fh and lowering aqueous Fe³⁺ activity. During subsequent nucleation, Ox preferentially adsorbed onto nascent hematite (Hm) nuclei via bidentate-mononuclear complexation. This interaction reduced Hm surface energy from 330 to 180 mJ·m^-2, increasing nucleation rate ∼70-fold compared to goethite (Gt). Moreover, Ox likely guided Hm growth via oriented attachment, probably driven by van der Waals forces and/or patch-charge attraction. Conversely, tricarboxylic CA strongly bound to Fh, stabilizing Fh against transformation. This study offers mineralization perspectives for understanding organic-mediated Fh transformation in environmental and engineered systems.