Natural ferrihydrite (Fh), a poorly crystalline iron oxy-hydroxide with high concentrations of As, was investigated with respect to its crystallinity in a remediated mining environment. The Fh crystallinity increases from proto-Fh to a better crystallized 2-line Fh (called better 2-line Fh), while the associated As contents decrease from 7.8 to 1.9 wt %, respectively. This evolution of crystallinity correlated with decreasing As suggests that As is more likely coprecipitated in the Fh structure than adsorbed onto the surface.

The evolution of Fh crystallinity is related to (i) aqueous transformations for samples continuously submerged and controlled by water composition (pH, ionic strength and redox potential) and (ii) dry thermal transformations for samples in a seasonally humid area with variations of temperature, humidity, agglomeration and compaction. The evolution of Fh crystallinity is more pronounced during dry thermal transformation than during aqueous transformations. Although Fh evolution is observed on the field, no stable form (goethite, hematite) is detected as usually noted during dry thermal transformation. This may be explained by incorporated cations such as Si and Al that can inhibit the transformation of Fh to a more stable form. To understand this transformation in a natural environment, the discussion in this study focuses on two main points: (i) the relationship between spatial mineralogical distribution and As content and (ii) the importance of Fh evolution as a function of transformation processes observed in the field.

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