Fast phase transformation of 2-line ferrihydrite to hematite as induced by the injection of photoelectrons

Abstract

The phase transformation of 2-line ferrihydrite (Fh) to more thermodynamically stable iron (oxyhydr)oxides plays a crucial role in regulating the element cycling and energy transfer in Earth's near-surface environments. Redox transformation has been identified as a significant pathway in the phase transformation of Fh. As typical electron-donors, semiconducting minerals can induce the redox transformation of Fh and also serve as heterogeneous surfaces, influencing the aggregation and crystallization processes. In this work, we investigated the rate and product type of Fh transformation in the presence of nanosized TiO₂, a natural semiconductor, under light irradiation. Our findings demonstrate that light irradiation and the coexistence of both TiO₂ and small organic molecules (methanol) are essential for the rapid phase transformation of Fh (within 3 h), primarily to Hem. Higher methanol content and lower O₂ content facilitate the formation of Hem, as evidenced by the faster transformation rate and the higher Hem content in the products. Analysis of the solid phase during the reaction reveals the presence of Fe(II), indicating the reduction of Fh by photoelectrons from TiO₂. Surprisingly, unlike the commonly recognized transformation of Fh to goethite under reduction conditions at room temperature, Hem is identified as the primary product in this study, regardless of the solution pH (pH 4, 7, and 12). This phenomenon can be attributed to the solid-to-solid electron transfer and the facilitated aggregation of Fh in the presence of TiO₂, both of which promote the structural rearrangement of Fh leading to the formation of hematite while inhibiting its dissolution-recrystallization into lepidocrocite and goethite. This study highlights the role of photoelectrons in accelerating the phase transformation and influencing the transformation pathway of Fh. Considering the widespread occurrence of natural semiconductors and organic chemicals, we propose that photoelectron-induced transformation of Fh is an important yet overlooked process in Earth's near-surface environments.