Nitrate Reactivity in Iron (Oxy)hydroxide Systems: Effect of pH, Iron Redox State, and Phosphate

Nitrate and phosphate are major components of fertilizers, which upon runoff can lead to the eutrophication of aqueous systems. Iron (oxy)hydroxides present a potential method for removing nutrients via denitrification/adsorption. However, the efficiency by which these contaminants are removed may be impacted by the competitive effects of compounds present in the system. Herein, we conducted anoxic experiments testing the interactions between nitrate (NO₃⁻) and Fe minerals at different pH values, Fe redox state, and with the presence of phosphate (HPO₄²⁻). In our anoxic and alkaline experiments containing 100% Fe²⁺, approximately 20% of the initial NO₃⁻ concentration was reduced, while ∼60 to 80% of Fe(II)-hydroxides were oxidized. The reduction of NO₃⁻ and the oxidation of Fe(II) precipitates formed NH₃/NH₄⁺ and magnetite (Fe₃O₄). Nitrate reduction was not observed under conditions with 100% Fe²⁺ at 6.5 or in experiments containing either 1:1 Fe²⁺:Fe³⁺ or 100% Fe³⁺ at any pH. Upon addition of HPO₄²⁻, nitrate reduction was inhibited, and no redox was observed. Additionally, NO₃⁻ inhibited HPO₄²⁻ adsorption with ferrous iron-containing minerals, although HPO₄²⁻ adsorption was observed in 100% Fe²⁺ experiments. This work demonstrates the challenges with developing treatment mechanisms for nutrient-impacted sites and elucidates how nutrients could further react with Fe (oxy)hydroxides in sediments, should they remain in the system.