Formation and Morphology Evolution from Ferrihydrite to Hematite in the Presence of Tartaric Acid

Hematite, with ferrihydrite as the common precursor, is the most stable iron oxide in soils and sediments and has many applications in environmental systems. As a common reducing agent in soils, tartaric acid (L-TA) can reduce Fe³⁺ to Fe²⁺ and template the formation of hematite from ferrihydrite. Here, the formation of hematite in the presence of L-TA was investigated under different L-TA concentrations, initial suspension pH, and aging time. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and high- resolution transmission electron microscopy (HRTEM). Both the transformation process and the particle morphology of hematite were affected by the initial suspension pH_i at which the L-TA was added to the suspension. Optimal pH_i values at a L-TA/Fe(III) molar ratio of 1.0% and an aging time of 10 h at 100 °C were pH_i 7 and pH_i 11. At pH_i 7, the optimal L-TA/Fe(III) molar ratio for the transformation was 1.0% and aging at 100 °C was completed after about 20 h. The transformation occurred through a dissolution-crystallization process. Crystalline corn-like particles (84 m²/g) were obtained through an oriented attachment mechanism. At a L-TA/Fe(III) molar ratio of 3.0% the ferrihydrite surface was saturated with L-TA and the transformation was inhibited. At pH_i 11, L-TA/Fe(III) of 1.0% and aging for 10 h (100 °C), subrounded crystalline particles (24 m²/g) were obtained by solid-phase transformation, oriented attachment, and Ostwald ripening mechanism.