High affinity lanthanum doped iron oxide nanosheets for phosphate removal

Abstract

• New synthesis route leading to nano La(OH) 3 cluster interlayered iron oxides. • Sorbent affinity for phosphate increases with La content with partitioning coefficients up to 10 5 L/kg. • La doped sorbents show high selectivity for phosphate in presence of competing solutes. • Sorption of phosphate dominated by inner sphere La-O-P complexation. Phosphorus induced eutrophication of freshwaters is one of the great global challenges. As the critical threshold concentration for eutrophication is very low, there is a strong need for development of phosphate sorbents with high affinity and selectivity. Single sheet iron oxide (SSI), a nanomaterial prepared from oxidation and exfoliation of layered iron(II)-iron(III) hydroxide (green rust), is a fast reacting and promising sorbent. Phosphate sorption affinity and selectivity may be improved by incorporation of lanthanum (La) in the structure. Lanthanum was added during SSI synthesis resulting in La-SSIs with 0 to 22.5 wt% of La content. XPS and EXAFS showed all La was present as La(OH) 3 nanoclusters between iron hydroxide layers and on the surface of SSI. The sorbent showed fast phosphate sorption with 90% completion within 30 min and high stability with minimal La leaching. All La-SSI nanomaterials showed better sorption affinity than non-doped SSI, and the La-SSI with the highest La content showed an extreme affinity with K d of 10 5 L/kg at solution concentration of 0.1 mg P/L. The sorption affinity was not seriously affected by pH. The La-SSIs showed high selectivity for phosphate with<10% reduction in phosphate sorption in presence of co-existing solutes (Cl – , NO 3 – , SO 4 2- , HCO 3 – and humic acid). Similar high affinity and selectivity was seen for phosphate sorption in real natural waters. The main phosphate sorption mechanism is attributed to inner sphere Fe-O-P and La-O-P surface complexation. In conclusion La doping turns SSI into a high-affinity and selectivity sorbent that has potential for polishing low-phosphate yet eutrophying natural waters such as drainage and lake waters.