Facile upscaled synthesis of layered iron oxide nanosheets and their application in phosphate removal

Abstract

Layered iron oxide nanosheet material is a new efficient adsorbent which is here developed and investigated for removing the eutrophicant phosphate from water. The layered Fe^III nanosheets are synthesized by anion exchanging layered Fe^II/Fe^III hydroxide chloride (green rust; GR) with dodecanoate, followed by solid state oxidation and exfoliation, by which the yield of the product can be easily scaled up. The obtained product exhibits a distinct platy structure with a minimal Fe^III oxide sheet thickness of 1 nm and lateral size of 100 to 200 nm as observed by means of high resolution transmission electronic microscopy, selected area electronic diffraction and atomic force microscopy. The Fe^III nanosheets can be well dispersed and stable in aqueous solution, showing a low tendency of aggregation. The performance of the obtained nanosheets to remove phosphate has been evaluated in kinetic and batch experiments under different environmental conditions (i.e. interfering anions and pH). The kinetic study shows that phosphate removal by the iron oxide quickly reaches a plateau with the rate constant k₂ ranging from 5.2 × 10⁻⁴ to 1.4 × 10⁻³ g (μmol⁻¹ min⁻¹) for the initial phosphate concentrations of 40 and 80 μmol L⁻¹ (1.2 to 2.4 mg P per L) at pH 7. The maximum phosphate adsorption capacity of the layered Fe^III nanosheets is 2540 μmol g⁻¹ (about 77 mg g⁻¹; 4006 μmol P per g Fe) at pH 5, showing excellent potential for phosphate removal from wastewater. Our results also indicate that the performance of the layered Fe^III nanosheets for phosphate removal is selective. X-ray photoelectron spectroscopic analyses of O 1s, Fe 2p and P 2p spectra provide evidence for phosphate removal by forming inner-sphere Fe–O–P surface complexes. The cost-effective, environmentally friendly and highly efficient layered Fe^III nanosheet material is a promising material for the removal of phosphate or other oxyanions in future water treatment technologies.