Phosphate capture by ultrathin MgAl layered double hydroxide nanoparticles

Abstract

Capture of phosphorus from runoff and wastewater is of high priority in order to reclaim phosphorus for food security and to prevent water pollution. Here we report an environmentally friendly method to synthesize ultrathin MgAl layered double hydroxide (LDH) nanoparticles for phosphorus adsorption. Fast co-precipitation of magnesium and aluminum at 25–80 °C in the presence of urea resulted in the desired LDH with variable admixtures of amorphous aluminum hydroxide (16–38%) quantified from solid state 27Al MAS NMR. Freshly synthesized particles appeared as exfoliated single layers that upon drying stacked to form particles with thickness of 3 to 5 nm (four to six LDH layers) and lateral sizes of ~30 nm, as seen by XRD, SEM, TEM, and AFM. Phosphate adsorption on LDH nanoparticles synthesized at room temperature (LDHns-U25) was very fast and reaction reached equilibrium within 15 min at pH 8.5. The freeze-dried LDHns-U25 nanoparticles exhibited phosphate sorption capacity of 98 ± 15 mg P·g−1, which is 55% higher than for conventional LDH. Phosphate was bound to LDH electrostatically and via inner-sphere surface complexation as evidenced from a combination of 31P MAS NMR spectroscopy, surface potential measurements, IR spectroscopy, and ionic strength effects on phosphate sorption. This study demonstrates that urea-facilitated synthesis of LDH nanoparticles provides high capacity phosphate sorbents with potentials for phosphate recovery from waste waters.