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

Area of Science:

• Environmental Geochemistry
• Biogeochemistry
• Mineralogy

Background:

• Scorodite (ferric arsenate) is considered a stable arsenic host in mining-impacted systems.
• Its long-term stability may be compromised by organic matter and plant root exudates in ecosystems.

Purpose of the Study:

• To investigate the impact of root exudates and humic acid on scorodite stability.
• To understand the transformation pathways of scorodite under organic influence.

Main Methods:

• Utilized wheat and harsh hakea plants to collect root exudates.
• Applied X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy.
• Employed transmission electron microscopy with selected area electron diffraction.

Main Results:

• Root exudates and humic acid did not alter the oxidation state of arsenic (As).
• Formation of amorphous ferric arsenate (AFA) was observed, indicating scorodite transformation.
• Significant scorodite to AFA transformation (up to 20%) occurred in harsh hakea-humic acid systems within two weeks.

Conclusions:

• Organic matter and root exudates significantly affect scorodite stability, promoting transformation to AFA.
• Biologically active, organic-rich environments may increase arsenic mobility and bioavailability.
• Findings have implications for arsenic management and remediation in mining-impacted landscapes.