Crystal structure-dependent oxidation pathways of metallic arsenic(0): Implications for environmental risk and material handling

Long regarded as an environmental contaminant, arsenic (As) has now been classified as a critical raw material (CRM) due to the increasing use of metallic As(0) in products essential for the transition away from fossil fuels. Despite the growing societal importance of As(0), its stability under oxic conditions, and how this depends on crystal structure, remain poorly understood. In this work, we investigated the stability of crystalline and amorphous As(0) in conditions ranging from mild (air-saturated solutions) to strongly oxidizing (3% H 2 O 2 ) environments. The evolution of dissolved As species was monitored over time, and the solid reaction products were characterized using synchrotron-based X-ray absorption spectroscopy, microdiffraction, and transmission electron microscopy. Remarkably, amorphous As(0) was found to be more resistant to oxidation than crystalline As(0), producing less dissolved As in most cases. Structural analyses revealed that amorphous As(0) largely retained its initial structure, whereas crystalline As(0) transformed primarily to As 2 O 3 . Given that the As(0) oxidation products, As(III) and As(V), are more toxic than As(0), these results provide new insights into the safe-handling and environmental risks of As(0), an understudied and increasingly important CRM. • Stability of crystalline and amorphous As(0) investigated in oxic environments • Amorphous As(0) showed higher oxidation resistance than crystalline As(0) • Structural data showed As 2 O 3 was the dominant product of crystalline As(0) oxidation • Dissolved As release from amorphous As(0) consistently lower than crystalline As(0) • Results clarify environmental risks of As(0), an important critical raw material