Transformation of natural As-associated ferrihydrite downstream of a remediated mining site

Natural ferrihydrite, a poorly crystalline iron oxyhydroxide with a high As content, has been studied with respect to its crystallinity in a remediated mining environment in the Saint-Yrieix Au mining district in France. The crystallinity increases from proto-ferrihydrite to a better crystallized 2-line ferrihydrite, while the associated As contents decrease from 7.8 to 1.9 wt.%; this correlation of increasing crystallinity with decreasing As suggests that As is more likely to be coprecipitated in the ferrihydrite structure than adsorbed onto the surface. The evolution of ferrihydrite crystallinity is related to 1) aqueous transformation for samples continuously submerged and controlled by water composition (pH, ionic strength and redox potential) and 2) dry thermal transformations for samples in a seasonally humid area with variations in temperature, humidity, agglomeration and compaction. The evolution of ferrihydrite crystallinity is more pronounced during dry thermal transformation than during aqueous transformations. Although the evolution of ferrihydrite is observed, no stable form (goethite, hematite) is detected as usually noted during dry transformation. This may be explained by incorporated cations such as Si and Al that can inhibit transformation of ferrihydrite to a more stable form. To understand this transformation in a natural environment, the discussion focuses on two main points: 1) the relationship between spatial mineralogical distribution and As content and 2) the importance of ferrihydrite evolution as a function of transformation processes observed in the field.