  Superparamagnetic {\gamma}-Fe2O3 nanoparticles (5 nm diameter) were
synthesized in water. The bare particles exhibit good colloidal stability at ~
pH 2 because of the strong electrostatic repulsion with a surface charge of +25
mV. The polyacrylic acid (PAA)-coated particles exhibit remarkable colloidal
stability at ~ pH 7 with abundant free carboxyl groups as reactive sites for
subsequent functionalization. In this work, we used zeta potential analysis,
transmission electron microscopy, small angle X-ray scattering, and Inductively
coupled plasma mass spectrometry to investigate the adsorption behavior of U
(VI) on bare and coated colloidal superparamagnetic nanoparticles at pH 2 and
pH 7. At pH 2, uranyl ion (UO22+) absorbed on the surface of the bare particles
with decreasing particle surface charge. This induced particle agglomeration.
At pH 7, uranyl ion (UO22+) hydrolyzed and formed plate-like particles of
uranium hydroxide that were ~ 50 nm in diameter. The PAA-coated iron oxide
nanoparticles absorbed on the surface of these U (VI) hydroxide plates to form
large aggregates that precipitate to the bottom of the dispersion. At both pH 2
and pH 7, the resulting U (VI)/nanoparticle complex can be easily collected and
extracted from the aqueous environment via an external magnetic field. The
results show that both bare and polymer-coated superparamagnetic {\gamma}-Fe2O3
nanoparticles are potential absorbents for removing U (VI) from water.
