THE EFFECTS OF PH AND TEMPERATURE ON OXALATE-PROMOTED HEMATITE DISSOLUTION

Mineral dissolution plays a central role in many important geochemical processes, including global elemental cycling, long-term climate effects, and biochemical mechanisms for nutrient acquisition. The rate of mineral dissolution is often regulated by organic ligands, of which oxalate is one of the most important due to its frequent occurrence as an exudate from plant roots and microbial activity, its high aqueous concentrations, and its large mineral dissolution activity. Oxalate adsorbs to mineral surfaces and regulates the rate of dissolution. In the current study, oxalate ligand-promoted dissolution of hematite is investigated under non-reductive conditions. Replicate flow-through reactors (1 mM oxalate) are employed to determine the dependence of the dissolution rate on pH at a constant temperature. The results show the oxalate-promoted dissolution rate decreases with increasing pH from pH 1 to 6, all other factors being equal. The pH dependence is distinct from the proton-promoted rate alone and suggests a synergistic interaction in the dissolution mechanism between protons and adsorbed oxalate. The temperature dependence of the dissolution rate between 10 and 30 degrees C is also investigated. From these experimental results, the dependence of the ligand-promoted dissolution rate on pH and temperature is explored and our understanding of the dissolution mechanism is improved.