Stability and Transformation of Ferrihydrite Coprecipitated with Metals: Dependence of Choice of Iron Salt Precursor

Coprecipitation of environmentally important metals with iron oxyhydr(oxides) may significantly change the metal mobility and, in return, affect the transformation and stability of the formed iron minerals. Ferrihydrite is one of the common amorphous iron oxyhydr(oxides) that has been extensively studied and synthesized using various iron precursor salts. In this study, the effects of the choice of iron precursor salts (Fe₂(SO₄)₃ and Fe(NO₃)₃) on the formation of pure and heavy-metal coprecipitated ferrihydrites and their subsequent transformation and stability are investigated. First, pure and heavy-metal ions [Cr (VI), Cr(III), Ce(III), V(V), and Mn(II)] at three different heavy-metal-to-iron ratios (0.0025, 0.025, and 0.25) coprecipitated with ferrihydrite were synthesized from Fe₂(SO₄)₃ (SFh) and Fe(NO₃)₃ (NFh) and subjected to thermal transformation by heating at 450 °C. Unlike NO₃^-, SO₄^2- in the synthesizing medium significantly impacted the formation of pure and coprecipitated ferrihydrites and their subsequent thermal transformation. At room temperature, the coprecipitation of heavy-metal ions did not affect the mineralogy of SFh and NFh. The thermal transformation of coprecipitated SFh at most molar ratios and coprecipitated NFh at a high molar ratio was inhibited compared to corresponding pure ferrihydrites during heating at 450 °C. However, the coprecipitated NFh with low and medium heavy-metal contents were completely transformed to hematite during heating at 450 °C. A majority of heavy metals coprecipitated with SFh and NFh are redistributed in the structure of the newly formed mineral, thus reducing their mobility.