Weathering of scorodite by root exudates: Arsenic dissolution and solid-phase speciation

Scorodite is a ferric arsenate mineral that is often assumed to represent a relatively stable host-phase for As in mining-impacted systems. However, scorodite's long-term stability may be affected by interactions with organic matter and plant root exudates in biologically productive ecosystems. This study investigates the influence of root exudates (from plants employing either Fe acquisition strategies I or II) and humic acid (as a model organic material) on scorodite stability. Wheat (Triticum aestivum) and harsh hakea (Hakea prostrata) roots exuded mostly ascorbic acid, along with some succinic and fumaric acid. The addition of humic acid increased organic acid fluxes from both plant species and modified the composition of root exudates. X-ray absorption near-edge structure (XANES) spectroscopy at the As K-edge revealed that humic acid and root exudates did not change the oxidation state of As. However, As K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and transmission electron microscopy with selected area electron diffraction indicated formation of amorphous ferric arsenate (AFA). The most significant transformation of scorodite to AFA occurred in the harsh hakea-humic acid systems, with up to 20% of scorodite-bound As transforming to AFA-bound As within two weeks. Overall, these findings demonstrate that organic matter and root exudates significantly impact scorodite stability, promoting its partial dissolution and transformation into amorphous ferric arsenate. This suggests that organic-rich environments and biologically active ecosystems may enhance the potential environmental mobility and bioavailability of As in mining-impacted landscapes, with important implications for arsenic management and remediation strategies.