Unveiling the impact of exposed mineral facets on chromium repartitioning in aging lepidocrocite

As a typical intermediate product of metastable iron oxide phase transformation, lepidocrocite is a critical player in migrating and transforming heavy metals (HMs) in soils and sediments. However, the repartitioning behavior of its associated HMs during the aging of lepidocrocite is not fully understood. We investigated the phase transformation of Cr(VI)-lepidocrocite with different exposed facets at various aging conditions (pH: 7 and 10; temperature: 25 °C and 75 °C). The results indicated that the phase transformation of lepidocrocite is facet-dependent. The rod-like lepidocrocite (R-LEP), characterized by a high ratio of (001)/(010) facets, demonstrated significant structural stability, with only minimal transformation to goethite observed over 9 days. In contrast, the dominant (010) facet in plate-like lepidocrocite (P-LEP) directly underwent the phase transformation from lepidocrocite to hematite. Meanwhile, the coordination of Cr(VI) onto the facets of lepidocrocite notably strengthened the resistance to its phase transformation. Specifically, the interaction between Cr(VI) and the (001) facets was particularly effective in inhibiting the phase transformation of lepidocrocite. In addition, the release behavior of Cr(VI) also showed lepidocrocite facet dependence. For instance, at pH = 7 and 75 °C, the release percentage of Cr(VI) on P-LEP reached up to 81 %, 1.7 times that of the release on R-LEP (46.7 %). Moreover, more non-extractable Cr speciation in P-LEP transformed into aqueous speciation and was partially redistributed on hematite. These findings provide novel insights into the role that the mineral exposed facets play in regulating the environmental behavior of Cr during the iron oxide phase transformation.