Efficiency of organic ligands in adsorptive dissolution and photoreductive dissolution of hematite

Organic ligands, especially oxalate, play an important role in iron dissolution from iron-containing minerals. To study the effects of organic acid ligands on the dissolution of iron-containing minerals, the dissolution kinetics of hematite in the presence of oxalate, acetate, and formate were studied under ultraviolet radiation with varying ligand concentrations (10–3 mM). The results indicate that for adsorption dissolution, oxalate is the dominating ligand for producing soluble iron (III) from hematite; for photoreductive dissolution under ultraviolet radiation and in oxic conditions, the production of iron (II) is highly proportional to the concentrations of oxalate, whereas the effects of varying concentrations of formate and acetate are not significant. At low oxalate concentrations (10–500 µM), the photoreductive dissolution of iron (II) is substantially low, while at high oxalate concentrations (3 mM), oxalate is equally effective as formate and acetate for producing photoreduced iron (II) from hematite. Combining with field data from other works, it is likely that the ratios of oxalate to total iron need to be higher than a threshold range of ~1.2–5.5 in order for oxalate to effectively produce photoreduced iron (II) from hematite. This study demonstrates that the iron (II) yield from photoreduction of hematite is significantly lower when the hematite surface is pre-coated with organic ligands versus when it is exposed to ultraviolet radiation instantaneously.