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Description
The recovery of phosphate from wastewater is essential for a healthy aquatic ecosystem. One of the most commonly used methods for recovering phosphate from wastewater is chemical precipitation. In this process Fe(III) reacts with phosphate to form ferric phosphate that settles as Fe-P particles. However, the molecular mechanisms that govern iron-phosphate interactions under different chemical conditions are not well understood. In this study, nuclear inelastic scattering experiments were conducted to identify iron species via their the vibrational properties. We used Nuclear Inelastic Scattering (NIS) of 57Fe enriched iron(III) solutions at pH 1, 2, 7, and 11. NIS data of iron(III)-phosphate solutions were also measured under the same pH conditions, with phosphate solutions serving as a model wastewater systems. Comparative analyses between iron(III) and iron(III)-phosphate solutions were performed to investigate the influence of phosphate on the coordination environment and vibrational characteristics of the iron species.
Quantum chemical density functional theory (DFT) calculations were performed with the aim to relate vibrational bands observed in the NIS experiments to molecular structures of iron species formed. In water solutions at very low pH values, iron is generally present predominantly as monomeric species. However, as the pH approaches around 2, the formation of small polymeric species becomes more feasible, like heptameric or Keggin-type iron clusters. Neutral (pH~7) and basic (pH~11) solution promote the formation of large polymeric iron oxo species.
