Oxalate-Promoted Trace Metal Release from Crystalline Iron Oxides under Aerobic Conditions

Iron oxides are common in soils and sediments and bind trace metals, such as nickel, through adsorption and structural incorporation. Recent work has shown that incorporated metals can be released during recrystallization catalyzed by aqueous Fe(II), which is generally stable only under anoxic conditions. This paper investigates the effects of the organic acid oxalate on the fate of nickel substituted into goethite and hematite to evaluate potential nonredox pathways of iron oxide recrystallization. Nickel-substituted hematite and goethite were synthesized and then reacted with 1 mM oxalate at pH 3–7 for ≤15 days. At all pH values, the presence of oxalate led to substantial nonstoichiometric release of nickel into solution compared to oxalate-free systems, with the extent of release increasing with decreasing pH. The rate of nickel release also increased with increasing oxalate surface coverage, and nonstoichiometric release of nickel was observed even under conditions producing no net mineral dissolution. The results demonstrate that iron oxide recrystallization may be a widespread phenomenon occurring in aerobic soils and aquatic systems, substantially altering contaminant fate and micronutrient availability in the environment.