Characterization of hematite dissolution affected by oxalate coating, kinetics and pH

Iron-complexes on the surface of minerals may play an important role in Fe dissolution in acidic cloud water containing certain organic ligands, and dissolved Fe serves as a critical nutrient in biogeochemical cycles in certain aquatic systems. As the first step to explore this issue, laboratory experiments were conducted to investigate the effects of oxalate on the dissolution of hematite, with leaching of Fe from oxalate-coated hematite in comparison with pure hematite in oxalate solution. The dissolution of oxalate-coated hematite was measured as a function of dissolution time, loading amount of oxalate and pH. The amount of oxalate adsorbed on hematite at pH 2.4 is greater than that at pH 5, while the amount of adsorption increases with increasing oxalate equilibrium concentration in solution. Adsorption of oxalate on hematite follows the Freundlich adsorption model. The amount of Fe dissolved at pH 2.4 is much more than that at pH 5. In low concentration oxalate solution, the amount of Fe dissolution from hematite is independent of oxalate loading on the surface of hematite. In high concentration oxalate solution, however, a relatively high oxalate loading on the hematite surface releases more Fe relative to low oxalate adsorption density on the surface of hematite when the system reaches equilibrium, suggesting that the high content of Fe(III)–oxalate complexation promotes Fe dissolution. Ferric ion dissolution and adsorbed oxalate leaching in solution as a function of pH are two co-existing processes, and pH not, vert, similar2.5 is a critical turning point relating the two processes that occur simultaneously. The fitting of the experimental data from this work to a model indicates that Fe–(oxalate)+ and image are predominant species in solution in the pH range of 1.5–4.5 during oxalate-coated hematite dissolution in background electrolyte.